field trip april 5–8 -...

I N T R O D U C T I O N

The geology around the city of Guanajuato is speciallyinteresting because of the quality of the outcrops, thegreat diversity of the rocks, and the large number ofclearly exposed structures. The objective of this field tripis to show the participants some of the most outstandingfeatures in the transitional zone (Figures 1, 2) betweenthe Mesa Central and the Transmexican Volcanic Belt(TMVB), with special emphasis on the record of volcanicactivity in the Guanajuato Mining District and nearbyregions. We hope that during this short visit the partici-pants will obtain an understanding of the tectonic andmagmatic evolution of the area, which is, of course,linked to the complex geologic history of the adjoiningregions.

The field trip will last for four days and consists ofa total of about 27 stops, with variable amounts of timespent at each one.

The first day of the field trip is spent in the MiningDistrict. We provide the participants with an overview ofthe geologic evolution of the southeastern part of theSierra de Guanajuato with the purpose of showing thelithologies and the formations that are important in theDistrict. We include in this day one stop at an outcrop ofthe pre-volcanic basement, since these Mesozoic rocksare an important source of clasts both in the earlyTertiary alluvial fan deposits and in the overlying vol-canic units.

The second day of the trip we return to the MiningDistrict to study in more detail the different facies of theCalderones Formation, which is the principal subject ofour present research in the region. We can see outcrops ofvent structures as well as of proximal, medial and distalfacies of the bedded tuffs.

In the third day of the trip we travel fromGuanajuato to San Miguel Allende, following the bound-ary between the Mesa Central and the TMVB. Near SanMiguel we can observe the remnants of two largeandesitic volcanoes of Miocene age (~12–10 Ma) and wevisit the scarp of the San Miguel Allende Fault.Cretaceous marine sediments, strongly deformed in thislocality both by movement along a late Mesozoic orPaleogene reverse fault and by superposed normal fault-ing during the Miocene (≥11 Ma).

The morning of the fourth day of the trip the wholegroup is taken to see the outcrops of the Mesozoic base-ment complex of the Sierra de Guanajuato along the roadwhich goes from the village of La Valenciana to theMontaña Cristo Rey (Cerro del Cubilete). In the after-noon, those participants who are able to stay with us visitoutcrops of the Comanja Granite, in the central part of theSierra de Guanajuato.

PA RT 1 . OV E RV I EW O F T H E R E G I O N A L G E O L -O G Y B E TW E E N QU E R É TA R O AND L E ÓN

PHYSIOGRAPHIC PROVINCES AND MAJOR ROCK GROUPS

In the region located between Querétaro and León twophysiographic provinces of central Mexico come togeth-er (Figure 1). These provinces are the Mesa Central andthe TMVB. Observed in detail, the boundary between theprovinces is complex and transitional, as can be seen inFigure 2. Immediately north of the boundary, outcrops of

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GUIDEBOOK FOR FIELD TRIPS OF THE 99TH ANNUAL MEETING OF THE CORDILLERAN SECTION OF THE GEOLOGICAL SOCIETY OF AMERICA

Aranda-Gómez, J.J.; Godchaux, M.M.; Aguirre-Díaz, G.J.; Bonnichsen,Bill; and Martínez-Reyes, Juventino, 2003, Continental edge tecton-ics of Isla Tiburón, Sonora, Mexico, in Geologic transects across Cordilleran Mexico, Guidebook for the field trips of the 99th GeologicalSociety of America Cordilleran Section Annual Meeting, Puerto Vallarta, Jalisco, Mexico, April 5–8, 2003: Mexico, Universidad NacionalAutónoma de México, Instituto de Geología, Publicación Especial 1, Field trip 6, p. 123–168.

FIELD TRIP 6: APRIL 5–8

THREE SUPERIMPOSED VOLCANIC ARCS IN THE SOUTHERN CORDILLERA—FROM THE EARLYCRETACEOUS TO THE MIOCENE, GUANAJUATO, MEXICO

José Jorge Aranda-Gómez1,@, Martha M. Godchaux2Gerardo de Jesús Aguirre-Díaz1, Bill Bonnichsen3,

and Juventino Martínez-Reyes1

1Centro de Geociencias, Universidad Nacional Autónoma de México, CampusJuriquilla, 76230 Querétaro, Qro., México.@E-mail address: [email protected] of Geology and Geography, Mount Holyoke College, SouthHadley MA 01075, U.S.A.Present Address: 927 East Seventh Street, 83843 Moscow ID, U.S.A.E-mail address: [email protected] Geological Survey, University of Idaho, 83844 Moscow ID, U.S.A.E-mail address: [email protected]

mid-Oligocene felsic volcanic rocks predominate (Figure3); they are genetically related to the Sierra MadreOccidental (SMO) Volcanic Province of western Mexico(Figure 1). To the south of the boundary, the rocks mostcommonly exposed are late Tertiary and/or Quaternaryandesites considered part of the TMVB (Figures 1, 3). Onthe basis of their lithologic characteristics, depositionalenvironments, styles of deformation and ages, the rockswhich crop out between Querétaro and León can bedivided into two great packages, which we refer to infor-mally as the “basal complex” and the “cover rocks.” The

basal complex is Mesozoic to earliest Tertiary in age andis made up of rocks of marine origin, metamorphosed andintensely deformed by shortening; the complex includesintrusive bodies of diverse compositions and ages (Ortiz-Hernández et al., 1990). The basal complex is exposed ina narrow belt which trends NW-SE, near the transitionzone between the two provinces (Figure 3). Rocks of thisbasal complex are known only in isolated outcrops inadjoining regions situated to the north (in the Zacatecasarea) or to the south (near the Valle de Bravo and thenorthern part of the state of Guerrero). This basal com-plex has been assigned to the tectono-stratigraphicprovince known as the Guerrero Terrane. The GuerreroTerrane has been interpreted as an island arc and the rem-nants of the floor of an ocean basin, both accreted to therest of Mexico during the later part of the EarlyCretaceous, around 100 million years ago (Tardy et al.,1991, 1994).

The Cenozoic cover rests discordantly on the basalcomplex and consists of continental sediments and sedi-mentary rocks, which generally occupy topographicallylow zones, and subaerial volcanic rocks, which are prin-cipally exposed in ranges and higher plateaus. The rocksof the Cenozoic cover have experienced only extension-al deformation and in some places are gently tilted. Theserocks contain the record of the more recent geologic evo-lution of the region.

BASAL COMPLEX OF THE SIERRA DE GUANAJUATO

The basal complex (Chiodi et al., 1988; Dávila andMartínez, 1987) crops out principally in what has beenreferred to as the Sierra de Guanajuato (Martínez-Reyes,1992) and is made up of:1. Weakly metamorphosed rocks, developed mostly

from original limestones, shales, and sandstones.2. Submarine lava flows and pyroclastic rocks, domi-

nantly mafic but occasionally felsic (keratophyres),metamorphosed to lower greenschist facies.

3. Arc-related pre- and syn-tectonic intrusive bodieswhich range in composition from ultramafic(pyroxenites) to felsic (granites, sensu lato).Diorites and tonalites are by far the most commonrock types in this group; locally they are intruded bybasaltic to andesitic dike swarms.

4. Post-tectonic plutonic rocks (granites, sensu stricto)with abundant tourmaline.

ARANDA-GÓMEZ, GODCHAUX, AGUIRRE-DÍAZ, BONNICHSEN, AND MARTÍNEZ-REYES124

UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO, INSTITUTO DE GEOLOGÍA, PUBLICACIÓN ESPECIAL 1 GEOLOGIC TRANSECTS ACROSS CORDILLERAN MEXICO

Figure 1. (a) Morphotectonic provinces in central Mexico (afterSedlock et al., 1993). The location of León (L) and Querétaro (Q) isshown (see Figure 2). (b) Late Cenozoic normal faults of the southernBasin and Range Province in northern and central Mexico. The mostobvious structures occur north of the Trans Mexican Volcanic Belt(TMVB). Some faults south of the TMVB have been interpreted aspart of the same province (Henry and Aranda-Gómez, 1992; Jansmaand Lang, 1997).

The intense deformation exhibited by the rocks ofGroups (1), (2), and (3), which make up the Mesozoicportion of the basal complex, is attributed to two periodsof orogenesis. The first pulse of compressive deformationhappened during the latest part of the Early Cretaceousand is related to the accretion of the Guerrero Terrane tothe North American craton, and the second one is theLaramide Orogeny of early Tertiary time.

In some places we find resting unconformably onthe eroded basal complex a thick sequence of continentalred beds (i.e., the Guanajuato Red Conglomerate;Edwards [1955]). In other places mid-Tertiary intermedi-ate to felsic volcanic rocks, genetically related to theSierra Madre Occidental Volcanic Province, lie directlyon the Mesozoic basement. In still other places andesiticrocks related to the TMVB were deposited atop the base-ment package.

PULSES OF CENOZOIC MAGMATISM

The Cenozoic magmatism in this region tookplace in seven distinct pulses (see Figures 4–6):

Pulse 1. This pre-SMO magmatism took place around 51Ma with the emplacement of the Comanja Granite, a plu-ton with a present-day exposure some 50 kilometers inlength by 20 kilometers in width and a northwesterlytrend roughly parallel to El Bajío Fault, along which therange is uplifted.

Pulse 2. This pre-SMO volcanism was a brief episode ofemission of andesitic lavas at 49 Ma (Aranda-Gómez andMcDowell, 1998), contemporaneous with the accumula-tion of the Guanajuato Red Conglomerate. This magma-tism is seen both as subaerial lavas forming packages of

FIELD TRIP 6: THREE SUPERIMPOSED VOLCANIC ARCS IN THE SOUTHERN CORDILLERA—FROM THE EARLY CRETACEOUS TO THE MIOCENE, GUANAJUATO 125


Figure 2. Digital elevation map of the central portion of the TMVB and the southern part of the Mesa Central. The larger volcanoes of the SanMiguel Allende Volcanic Field are shown: P = Palo Huérfano; J = La Joya; Z = El Zamorano; S = San Pedro. Pliocene volcanoes in El Bajíoplain are Culiacán (C) and La Gavia (G). Sierra del Ocote = O. Cities: León (L); M = San Miguel Allende; D = Dolores Hidalgo; F = SanFelipe.

several flows each and as small shallow intrusive bodieswhich did not quite reach the surface.

Pulse 3. Despite a paucity of radiometric dates on rocksof this pulse, we consider it to be volcanism that corre-sponds to an early phase of SMO activity. Centered inthe Guanajuato Mining District, this intense and pro-longed period of explosive and effusive volcanismoccurred in early Oligocene time. It includes all units

from the Bufa Ignimbrite, along with its preliminarypyroclastic surges, mapped as the underlying LoseroFormation, upward through the Calderones Formationto the andesitic to basaltic lava flows of the CedroFormation.

Pulse 4. This series of eruptions occurred around 30 Ma;it seems to have involved bimodal volcanism, with ratherextensive flows of andesite spatially and temporally asso-



Figure 3. Regional geology of the area between Guanajuato (G) and San Luis Potosí (SLP). Other abbreviations: S, Salinas de Hidalgo; SF,San Felipe; DH, Dolores Hidalgo; L, León; SM, San Miguel de Allende; VM, Veta Madre; AF, Aldana Fault. Note that south of latitude 22°30’N most of the area is covered by Cenozoic volcanic rocks. Most stratified Eocene fanglomerates and Oligocene volcanics are tilted to the NE.Inset shows a rose diagram of orientation of the Cenozoic faults in the Luis Potosí and Guanajuato 1:250,000 quadrangles. Sections A-A’ andB-B’ are diagrammatic and intended only to show the Cenozoic faulting style. Cenozoic volcanic rocks were grouped in a single unit. AfterAranda-Gómez and McDowell, 1998.

ciated with several forms of rhyolite, all with relativelyhigh silica contents and occasionally with tin and topaz.These rhyolites were erupted both as flow-dome com-plexes and as widespread ignimbrites. They appear on themap produced by Martínez-Reyes (1992) as theChichíndaro, Cuatralba and El Ocote Formations. Almostthe entire outcrop area of these three formations is out-side the limits of the Mining District (only theChichíndaro Formation is exposed within the central partof the District). These silicic rhyolites, particularly theCuatralba Formation and other ignimbrites and felsiclava flows, cover an extensive region between theDistrict and the city of San Luis Potosí (Figures 3 and 4).This magmatic pulse belongs to the peak phase of SMOvolcanism.

Pulse 5. This pulse of volcanism occurred between 27and 24 Ma, and it is represented in this region by thelarge dacitic domes of El Gigante Field (early Miocene),by widely distributed but not specially voluminous ign-imbrites (27–24 Ma), and by Miocene basalts. We con-sider this pulse as belonging to the late, waning, phase ofSMO volcanism.

Pulse 6. This pulse includes the volcanism that is trulytransitional between that of the Sierra Madre Occidentaland that of the Transmexican Volcanic Belt (Figure 1). Itis manifested as isolated volcanic domes and ignimbritesof intermediate composition, emplaced/erupted between16 and 13 Ma (Cerca et al., 2000) and as widespreadandesite flows.



Figure 4. Simplified geologic map of the central portion of the TMVB and the southern part of the Mesa Central. The larger volcanoes of theSan Miguel Allende Volcanic Field are shown (PH = Palo Huérfano; LJ = La Joya; EZ = El Zamorano; SP = San Pedro), as well as the fluvio-lacustrine Río Laja Basin (labeled as Csc (RL)), El Bajío plain is a broad flat area located between León (L) and Celaya (C). Compare region-al fault patterns in the Mesa Central and TMVB. Key: SMA= San Miguel Allende; D = Dolores Hidalgo; SF = San Felipe; I = Irapuato; SLP= San Luis Potosí. Chronostratigraphic units: Csc = Continental sedimentary deposits; Csc(RL) = Fluvio-lacustrine sediments of the Río LajaBasin; Qba = Quaternary alkalic basalts; Qtpv = Plio-Quaternary andesites; Tv = Tertiary volcanic rocks; Nb = Neogene andesites; Tof =Oligocene felsic volcanic rocks; K and Ks = Creatceous marine sediments; Mvs = Sierra de Guanajuato basal complex. Modified from Ortega-Gutiérrez et al., 1992.

Pulse 7. The events of this pulse took place between 12and 8 Ma and include the initial products of the TMVB,represented by broad benches covered by andesite lavaflows and by the earliest dacitic to andesitic stratovolca-noes, located a bit to the north of the main part of thisvolcanic province (Figures 3, 4).

HIATUSES BETWEEN PULSES

It is important to point out the hiatuses in magmaticactivity in this region. There was a long hiatus betweenthe activity associated with the mid-Cretaceous vol-cano-sedimentary complex and the Tertiary magmatismin the Sierra de Guanajuato, whose first manifestationsare represented by the Comanja Granite, dated at ~51Ma (Zimmermann et al., 1990). The andesitic volcan-ism around 49 Ma (Aranda-Gómez and McDowell,1998) seems to have followed the emplacement of thebatholith without any important hiatus. After the erup-tion of these 49 Ma lavas, there was a long hiatus lead-ing up to the eruption of the Bufa Ignimbrite around 36Ma (Gross, 1975); it was during this epoch of magmat-

ic quiescence that the Guanajuato Red Conglomeratewas deposited. The next important hiatus occurredbetween the end of ignimbritic volcanism of the SMOtype at 24 Ma and the volcanism transitional to theTMVB type at 16 Ma. During this period there wasongoing deposition of gravels and sands, which gaverise to the Xoconostle Formation, a fluvio-lacustrinedeposit which filled a broad shallow basin between SanMiguel Allende and Dolores Hidalgo. Fluvial depositsare still accumulating in the present-day Río Laja basin(Figure 4). After 16 Ma there has not been any impor-tant hiatus in the volcanic activity. In the early part ofthis time period (pulse 6, 16–13 Ma), the volcanism wassporadic and localized; afterward (pulse 7, 12–8 Ma),the volcanism began to intensify, reaching peak outputbetween 10 and 8 Ma. Intercalated with all of the vol-canic products from 12 to 8 Ma are widespread fluvio-lacustrine deposits, whose broad distribution in the cen-tral part of the TMVB indicates the presence of exten-sive lake systems contemporaneous with the early tomiddle phases of TMVB volcanism (Aguirre-Díaz andCarranza-Castañeda, 2000).



Figure 5. Geology of the southeastern part of the Sierra de Guanajuato (simplified from Martínez-Reyes, 1993).

DETAILED DESCRIPTION OF PULSES

Pulse 1. Pre-SMO magmatism: the Comanja Granite

The Comanja Granite is an intrusive body of possiblybatholithic dimensions, with a surface exposure ofapproximately 50 by 20 kilometers. It was initially datedat 55±4 and 58±8 Ma (K-Ar, biotite: Mugica andAlbarrán, 1983). More recently, Zimmermann and others(1990) obtained more precise ages of 53±3 and 51±1Ma(K-Ar, biotite). Like the majority of circum-Pacificbatholiths, it is probable that this body has a range of agesin its constituent plutons, though these have not beenmapped separately within it. The body is a granite withabundant K-feldspar (locally as megacrysts several cen-timeters in length), quartz, biotite and plagioclase, withtextural variations from a coarse-grained core outward toa more fine-grained marginal facies. Compositional zon-ing is not immediately obvious in outcrop; however, min-eralogical and geochemical studies, which might revealthe presence of some variety of cryptic zoning have not

yet been carried out. A common phenomenon amongPaleogene calcalkaline granites in other parts of theCordillera is the presence of a peripheral ring of verysmall tonalitic bodies of slightly greater age and slightlygreater depth of emplacement than the main granite (e.g.,around epizonal granite bodies in the Eocene Challis sys-tem of Idaho [Earl Bennett, personal communication]). Itis possible that detailed mapping of areas around the mar-gin of the Comanja Granite, and/or of the region to thesouthwest of Cerro El Cubilete might identify such bod-ies. The emplacement of the Comanja Granite post-datesLaramide deformation, since there is no evidence in out-crop of ductile deformation of the granite. As with thequestion of subtle zoning, detailed fabric studies mightreveal some effect of the waning phases of Laramidecompression on the mode of emplacement of the graniticmagma, but such studies have not yet been carried out.The development of the Comanja Granite marks animportant change in the genesis of intrusive rocks of theSierra de Guanajuato, because it was formed by magmarelatively rich in potassium, in sharp contrast to the syn-



Figure 6. Geology of the Guananjuato Mining District (simplified from Buchanan, 1979).

tectonic plagiogranite of Cerro Pelón. Two more charac-teristics make the Comanja Granite notable; one is theabundance of tourmaline and the other is the hints oftungsten mineralization (El Maguey Mine) at its margins,which is the only known case in central Mexico. Thetourmaline is present as a late magmatic phase dissemi-nated in the granite, as a coarse-grained phase crystal-lized in radial clusters in small pegmatitic pockets, and asthe ‘cementing’material in veins both within and beyondthe granite body. A common feature of these veins is thepresence of breccias formed from granite clasts, largelyin jigsaw-puzzle arrangement, with microcrystallinetourmaline as the matrix. Along the widest shear zonesevery style of gradation is present between simple veinsof massive tourmalinite, without granite fragments, andbreccias with large angular fragments of granite separat-ed by veinlets of tourmalinite. The abundance of tourma-line in the above mentioned three paragenetic habits–magmatic, pneumatolytic and hydrothermal- suggestsan unusually high boron content in the magma, whichcould be obtained by some mechanism of pre-concentra-tion of this element. Production and/or contamination ofthe magma through fusion of boron-rich sediments, forexample those sediments accumulated in a fore-arc basinlocated close to the continent, seem to be one possiblemechanism. An alternative mechanism might be theoperation of a long-lived hydrothermal cell in the roofrocks above the magma chamber. Another prominent fea-ture of the Comanja Granite is its external ring of poly-metallic skarn ore prospects, which are specially com-mon around the southern margin of the granite body.These skarn deposits suggest two things: leaching of themetals from the oceanic crust beneath the fore-arc basin,and emplacement of the batholith at shallow epizonaldepths, 2–4 kilometers below the Paleocene surface, withconcomitant development of a complex system ofmesothermal to epithermal veins.

Pulse 2. Pre-SMO volcanism: 49 Ma andesitic lavaswithin the Red Conglomerate

These andesites are intercalated with red beds of theGuanajuato Conglomerate, principally in the lower mem-ber of that unit. The most common type of body is pack-ages of subaerial lava flows that were emplaced on analmost-horizontal surface, on poorly consolidated sedi-ments. In some outcrops there is sparse evidence of inter-

action between the lava and surface water —poorly devel-oped pillows, thin lenses of phreatomagmatic tuffs orhyaloclastites, and small clastic dikes of red mud whichoccupy cracks at the bases of flows that apparently passedover wet ground. All the features of these flow packagesare consistent with an environment of deposition thatincludes alluvial fans and playa lakes. At other localities,bodies which could be hypabyssal intrusives (sills orsmall laccoliths) or invasive lava flows, are found.

Pulse 3. Early SMO volcanism: early Oligocene explo-sive to effusive volcanism of the Guanajuato MiningDistrict

After a long period of normal faulting and accumulationof red beds (mid-Eocene to the beginning of the earlyOligocene), a series of voluminous and varied eruptionsbegan. Although these volcanic rocks presently have amore or less restricted area of distribution, they arenonetheless of great importance (Figure 7). This rele-vance is not only for the geologic evolution of theDistrict and the Sierra de Guanajuato (and of the entireSMO province) but also for the later emplacement of themajor economic mineral deposits for which the District isfamous. We do not know how extensively these rocksmay have been deposited originally because most of thepresent-day boundaries of the outcrop area are eitherfaults or stratigraphic contacts with thick deposits ofyounger units; however, it is unlikely that they weredeposited in areas far from the District. This pulse beganwith the accumulation of the Losero and BufaFormations. The first of these two formations is princi-pally made up of subaerial pyroclastic surge layers and oftuffs of uncertain eruptive style deposited in (and locallyreworked by) shallow water. The Bufa Formation is a fel-sic ignimbrite with biotite as its mafic phase. This ign-imbrite is in general not highly welded, but owing tomoderate welding and extensive and pervasive silicifica-tion it is a hard rock which forms prominent cliffs east ofthe city of Guanajuato. It locally contains large lithicclasts of various types; many derived from the pre-vol-canic basement.

After the emplacement of the Losero-Bufasequence, there was a hiatus of unknown duration, duringwhich a surface of considerable relief, at least part ofwhich was erosional, was developed on the poorly weld-ed and poorly silicified top of the Bufa. The time period



for its development, however, needs not to have beenspecially long, because syn-volcanic and post-volcanicfaulting produced much of the initial relief. It seemsprobable to us that a caldera was formed as a conse-quence of the Losero-Bufa eruption. There are subtleindications of a caldera-forming episode in the topogra-phy of the region surrounding the District, and in theregional drainage pattern, which suggest the develop-ment of a broad pre-Bufa uplift whose precise form andrelationship to the regional faulting are yet unknown.This supposed caldera associated with the Bufa ign-imbrite must have been profoundly modified by syn-vol-canic and post-volcanic normal faulting, by subsequentvolcanic activity, and by erosion, and it may or may nothave been classically circular or oval in form. We con-clude very tentatively that the formation of this caldera,whatever its shape, produced a closed basin in which theproducts of the following series of eruptions, those whichproduced the dominantly andesitic Calderones and CedroFormations, became trapped. Randall and collaborators(1994) were the first researchers that postulated the ideaof a caldera in the District.

The Calderones Formation is a true encyclopedia ofstyles of eruption and emplacement of volcanic products.It includes low- to medium-grade ignimbrites, deposits ofpyroclastic flows of the block-and-ash type, pyroclasticsurge layers related to phreatomagmatic activity, airfallash-rich tuffs, minor Plinian pumice layers, lahars, debrisflows, reworked tuffaceous layers deposited in water,tuff-breccias, and megabreccias. Ubiquitous and charac-teristic chlorite alteration imparts a green to greenish bluecolor to almost all outcrops of the Calderones, suggestingthat almost the entire formation was deposited in bodiesof shallow water, possibly lakes retained inside the (mod-ified) Bufa Caldera. An alternative interpretation of thealteration may involve processes of hydrothermal circula-tion through the Calderones tuffs immediately after depo-sition, even in the absence of lakes. A third style of alter-ation, propylitic alteration adjacent to veins and dikes, isof local importance in many outcrops. It is possible thatdetailed petrographic and/or geochemical studies of themineral assemblages in many parts of the unit might pro-vide a better assessment of the relative importance of syn-depositional alteration (lakes), immediately post-deposi-tional alteration (intracaldera hydrothermal cells) andlater post-depositional alteration (propylitic alterationadjacent to veins and dikes). Because of the alteration,and also because of the high quantity of accidental frag-ments, it is difficult to determine with precision the orig-inal compositions of the juvenile volcanic products, butwe consider that in general they were andesites anddacites. Echegoyén (1970) distinguished three membersin the Calderones; in a very rough way, we think thatthese members are equivalent to the proximal facies(lower member), the medial facies (intermediate mem-ber), and the distal facies (upper member) of theCalderones pyroclastic sequence. The source vents of theCalderones are located just to the northeast of the deposi-tional basin, in a ring dike which crosses the western ridgeof Cerro Alto de Villalpando, and to the north of the basin,in the Peregrina Dome Field. Given the internal complex-ity and variety of pyroclastic products in the Calderones,we consider it possible that other source vents may exist.

Everywhere within the District, the CalderonesFormation passes upward into the Cedro Andesite, whichis a package of lava flows and associated tuffs ofandesitic to possibly basaltic composition. TheCalderones-Cedro transition consists of an interval ofinterstratification of very fine-grained green tuffs with



Figure 7. Generalized stratigraphic column of the Guanajuato MiningDistrict. Modified after Buchanan (1979).

dark brown tuffs and isolated lobes of water-affected lavaflows. We think that the Cedro Andesite was fed by a sys-tem of dikes with strikes from ENE to NE, which show aroughly radial pattern on the map of Echegoyén (1970).Some of the well-exposed dikes that invaded the distalparts of the Calderones have marginal facies composedof peperites and isolated crude pillows which provideevidence of their interaction with shallow waters and/orwith recently deposited wet tuffs. The uppermost layersof the Calderones resemble phreatomagmatic deposits;they carry abundant autoclasts and have a stratigraphy,which is the reverse of adjacent undisturbed portions ofthe formation down to the level of the underlying poorlyconsolidated top of the Bufa. This phenomenon of“inverse stratigraphy” is interpreted as the result ofdownward coring of the focus of explosive interactionbetween the tip of the dike and its host rocks. The CedroAndesite passes upward from its base, within a few dozenmeters, from mixed tuffs and lobes of pillowed lavas intowidespread, apparently subaerial, lava flows. In all out-crops that we have seen, even the larger andesite flowsnear the base of the Cedro show evidence of interactionwith water; some flows have well-developed spheroidalweathering (which locally mimics pillows), and the asso-ciated pyroclastic deposits contain matrix palagonite.

The Peregrina Dome Complex also belongs tothis third pulse of volcanism. The Peregrina is foundprincipally in a large dome field in which one canobserve diverse and very complex relationships with theCalderones Formation. The range of compositions of thePeregrina complex varies from dacite to fairly high-silicarhyolite. In the field it is clear that there are layers in allmembers of the Calderones which have abundant clastsof all the lithologies observed in the Peregrina domecomplex. It also appears certain that the youngestPeregrina domes cut Calderones layers. According toEchegoyén (1970), there is at least one Peregrina domeemplaced in the Bufa Formation. Because in the lowerpart of the Bufa we find many clasts of a rhyolite withvery delicate flow-banding, there exists the possibilitythat the first-erupted domes associated with this pulseformed a bit before the emplacement of the Bufa or evencontemporaneously with it. We consider it probable thatthroughout this entire pulse of activity domes were beingperiodically emplaced, and that the formation we call thePeregrina is diachronous. Thus, it is impossible to estab-lish a unique age relationship between the Peregrina to

the rest of the volcanic units in the District, with the pos-sible exception of the Chichíndaro rhyolite. Chichíndarodomes and lava flows seem consistently to cross-cutand/or overlie the Peregrina in the few places where theyare seen in contact. Blind dikes of Chichíndaro are com-mon in some underground workings of El Cubo Mine,where they can be seen to cut Peregrina rocks (J. JoséReyes-Martínez, personal communication, 2001).

Although we lack geochemical data on the rocksformed in this pulse, based on the great compositionalchanges observed and on the regional tectonic context,we presently consider as a working hypothesis the fol-lowing model for the evolution of the magmas involved.An original body of andesitic to basaltic magma was gen-erated in a subduction zone along the Pacific Coast ofsouthern Mexico, which dipped to the east or northeast,with the downgoing slab passing beneath the ancientMesozoic suture zone of the Guerrero terrane with thecontinental margin. This magma could have caused par-tial fusion of various low-melting components of thiscomplex portion of the North American continental crust,giving rise to a rhyolitic magma with a fairly highvolatile content. Partly as a result of the ongoing region-al and syn-volcanic tectonic extension, this secondmagma rose to the surface and erupted explosively, pro-ducing the Losero and Bufa formations. Slightly later, theandesitic magma continued its ascent toward the surface,establishing a shallow magma chamber. Processes ofMASH (melting, assimilation, storage and hybridization)may have occurred on a small scale, but it seems likelythat the principal process that modified the magma in theupper part of the chamber was differentiation (also on alimited scale), which produced dacitic liquids.Nonetheless, the most voluminous product of theCalderones and Cedro eruptions was andesite. In some ofits aspects our working hypothesis has a general similar-ity with the model proposed for the Taupo Ignimbrite ofNew Zealand (Freundt et al., 2000).

Pulse 4. Peak phase of SMO volcanism: Sililcic andandesitic volcanism from 32 to 30 Ma

The andesitic lava flows of the Cedro Formation (sensulato) have a broader distribution along the southernboundary of the Mesa Central than do the other volcanicunits of the Guanajuato Mining District. Cerca and others(2000) report 32–30 Ma andesites to the southeast of the



District, which they correlate to Cedro. This broad distri-bution may suggest a drastic change in the style of vol-canism, from predominantly explosive (the Losero, Bufaand Calderones Formations) to largely effusive. Almostcontemporaneous with the outpouring of the youngerCedro andesites there was an important episode of activ-ity which produced a large number of high-silica rhyolitedomes and flows, along with a lesser volume of ign-imbrites of similar composition (Guillermo Labarthe,personal communication, 2001). These rocks have beengrouped under the name Chichíndaro Rhyolite (e.g.,Martínez-Reyes, 1992; Cerca et al., 2000). The distribu-tion of this rhyolite is broad and is similar to that of theCedro Andesite (s.l.), and various sources have been doc-umented for it, both within and outside of the MiningDistrict. Cerca and others (2000) dated the ChichíndaroRhyolite at about 30 Ma. The Cedro and Chichíndaroepisode could be interpreted as a stage of bimodal vol-canism in the Sierra de Guanajuato, which took placeabout 30 Ma. In addition to the Cedro-Chichíndarodomes and flows, voluminous silicic ignimbrites wereemplaced, principally to the north of the District, in theSierra de Santa Rosa (where they are mapped as part ofthe Chichíndaro Formation by Martínez-Reyes, 1992),and to the north of the outcrops of the Comanja Granite.In this latter part of the Sierra de Guanajuato, these pyro-clastic rocks have been mapped as the CuatralbaIgnimbrite, but this large unit in reality is made up of aseries of ignimbrites and intercalated tuffaceous fluviola-custrine sedimentary rocks whose sources have not yetbeen determined. Near the city of San Miguel Allendethere are outcrops of El Obraje Ignimbrite, which has aradiometric (K/Ar) age of ~28 Ma (Pérez-Venzor, 1996)and which is of a distinctly higher grade than the otherignimbrites of this region. For this reason we consider itas the most characteristic example, among the volcanicrocks seen in this field trip, of the ignimbrites of theSierra Madre Occidental. El Obraje Ignimbrite is a thickunit and one that appears to be very extensive, but itssource is still unknown.

Pulse 5. Waning phase of SMO volcanism: large daciticdomes of the El Gigante Field, ignimbrites (~24–22 Ma),Arperos Gabbro, and early Miocene basalts

During the early Miocene there was a change in the vol-canism of the region from widespread bimodal volcanism

to the formation of large domes of intermediate composi-tion, such as the hills named El Gigante and La Giganta,and to the emplacement of extensive ignimbrites whichwe interpret as the final phases of the SMO volcanism(24–22 Ma) in the region. Overlying these are basaltspossibly of early Miocene age. This volcanism was alllocated outside the Guanajuato Mining District, both tothe northwest and to the southeast of it. There are onlytwo published reports that describe the Cenozoic geologyof the area around the District, the map of Martínez-Reyes (1992) and the map of Cerca and others (2000) forthe southeastern portion of the Mesa Central. Martínez-Reyes (1990) groups several ignimbrite units as theCuatralba Formation. However, the youngest of the ign-imbrites turn out to have ages between 24 and 22 Ma, aswas found in the sequence of the Mesa San José deAllende (Cerca et al., 2000), and thus should be consid-ered as events separate from the Cuatralba series ofapproximately 30 Ma exposed north of León. Martínez-Reyes (1990) also documents the presence of a maficintrusive body near the town of Arperos, which he calledthe Arperos Gabbro. We interpret this rock body as thesubvolcanic equivalent of the early Miocene basalts. TheCenozoic volcanic sequence to the north of the town ofArperos is little studied. In that general region areexposed several ignimbrite units covered by olivine-richbasalt flows (not yet dated, but probably early Miocene).Near the town of Arperos it is possible to observe com-plex contact relationships between the feeder dikes of thebasalts and the enclosing ignimbrites. There are numer-ous examples of transition zones with intricate mixturesof both kinds of rock bordering the diabasic dikes andshallow sills of the Arperos.

Pulse 6. Volcanism transitional between the SMO and theTMVB: intermediate lavas and domes

In the period between 16 and 13 Ma isolated domes andlava flows of intermediate composition were formed. Inthe San Miguel Allende Volcanic Field (Pérez-Venzor etal., 1997), close to the volcanoes Palo Huérfano (Figure9) and La Joya (Figure 10), mid-Miocene andesitic anddacitic domes have been documented. These includeCerro Colorado (~16 Ma, K/Ar, biotite: Pérez-Venzor etal. [1997]) and El Maguey Dome, which underlies the~10 Ma andesitic stratovolcano La Joya (Valdez-Moreno et al., 1998). Cerca and others (2000) also men-



tion undated ignimbrites with intermediate composition(Las Pilas ignimbrite) and 14 Ma andesitic domes justnorth of Salamanca, which they interpret as a transi-tional volcanic stage between the southern SMO and theTMVB. The andesitic lava flows that crown the highpoints of the Sierra de Guanajuato that were mapped asthe Cubilete Formation by Martínez-Reyes (1992) weredated at 13.5 Ma by Aguirre-Díaz and others (1997). Itis important to note that these same flows (CubileteFormation) are exposed both on the upthrown block andon the downthrown block of El Bajío Fault, which is themajor structure bounding the southern front of theSierra de Guanajuato. The age of the CubileteFormation provides us with a control on the timing ofthe fault.

Pulse 7. Initial products of the Transmexican VolcanicBelt: stratovolcanoes, domes and lava-capped mesas

During this latest pulse of volcanism, in the periodbetween 12 and 8 Ma, the styles of eruption and emplace-ment once again changed definitively. This was probablyrelated to a change in the type of magma produced afterthe re-organization of tectonic plates in the Pacific Coast.Dominated by andesites, these magmas produced broadflat lava benches and locally formed the first major vol-canoes of the northern part of the TMVB, that are repre-sented by Palo Huérfano (Pérez-Venzor et al., 1997), LaJoya (Valdez-Moreno et al., 1998), and El Zamorano(Carrasco-Núñez et al., 1979). The TMVB is still activetoday, but the active front is located some 150 kilometersto the south of the San Miguel Allende Volcanic Field(SMAVF) and of the boundary between the Mesa Centraland the TMVB (Figures 4, 8).

STRUCTURAL PATTERNS IN THE SOUTHERN PART OF THE MESACENTRAL

The boundary between the Mesa Central and the TMVBis evident not only in the stratigraphy but also in thestructures. It has been argued (Aranda-Gómez et al.,1989; Henry and Aranda-Gómez, 1992, 2000) that in thisregion we see the true boundary between the Basin andRange Tectonic Province and the TMVB (Figure 1b).North of the transitional zone the morphology and struc-ture are controlled by at least two conjugate systems offaults trending respectively NW-SE and NE-SW (Figures

2-3), whereas to the south structures striking ENE to E-W predominate (e.g., Martínez-Reyes and Nieto-Samaniego, 1990; Pasquaré et al., 1986, 1987a,b, 1988).In the zone between Querétaro and San Miguel Allende(Figure 8), both provinces are cut by a less well-studiedsystem of faults, whose orientations range from N-S toNNW, called the Taxco-San Miguel Allende System(Demant, 1978).

The San Miguel Allende Volcanic Field (Figures3, 8–10) is part of the Cenozoic cover. It is locatedbetween San Miguel Allende and the village of Colón(Qro), and immediately north of El Bajío depression.Four larger volcanoes and several smaller centers ofemission, peripheral to the larger ones, stand out in thisvolcanic field. The four large volcanoes are the alreadymentioned El Zamorano, La Joya, Palo Huérfano and SanPedro (Figures 3, 8). These peaks form some of the high-est elevations in the region. Radiometric ages (K-Ar and40Ar-39Ar) published at this time vary from ~12 to ~10Ma. The volcanoes of the San Miguel Allende Field havemorphological features characteristic of volcanic edificesin a moderately advanced state of erosion. Their mor-phology contrasts with that of the younger volcanoes,such as Culiacán and La Gavia (K-Ar ~2.2 Ma; Ban etal., 1992), situated immediately to the southwest in ElBajío depression (Figures 2, 3), whose cones are excep-tionally well preserved.



Figura 8. Approximate limits of the Trans Mexican Volcanic Beltand general distribution of Miocene-Pliocene and Pliocene-Quaternary volcanic rocks in it. Key: SMAVF = San Miguel AllendeVolcanic Field; A = Amealco caldera; NT = Nevado de Toluca; C =Colima; G = Guadalajara; Mo = Morelia; M = Mexico City; P =Pachuca; Q = Querétaro; V = Veracruz. After Pérez-Venzor et al.,1996. Compare orientation of fault patterns with that in the MesaCentral (Figuras 3, 4).

The most outstanding morphological features ofthe volcanoes La Joya and Palo Huérfano (Figsures 9, 10)are the great central depressions in their summit areas,which form semicircular craters with diameters of ~4kilometers and average depths of 200 to 300 meters.Their aspect is similar to that of volcanic calderas of theGalápagos type. However, it is thought that the unusual-ly large size of these depressions (with respect to theoverall sizes of the volcanoes) is the result of differentialerosion associated with intense hydrothermal alterationaround their central vents (Valdez-Moreno et al., 1998).Lack of caldera-related deposits near the volcanoes sup-port this interpretation.

All of the volcanoes of the San Miguel AllendeVolcanic Field are composed predominantly of andesitic

and dacitic lavas. By reason of their ages and composi-tions we consider them to be the oldest large-dimensionvolcanoes in the TMVB (Pérez-Venzor et al., 1997). Theform of the edifices and the high proportion of lavas rel-ative to pyroclastic products in La Joya and PaloHuérfano suggest that they may be structures intermedi-ate between stratovolcanoes and exogenous domes.Typical stratovolcano deposits, such as pyroclastic flowsand tuffs are relatively scarce in both volcanoes (Pérez-Venzor, 1996; Valdez-Moreno et al., 1998). Like theactive volcanoes in the southern part of the TMVB, thevolcanoes of the San Miguel Allende Field owe their ori-gin to magmatism associated with subduction of theCocos plate under the Pacific margin of southern Mexico.It seems likely that both the rate and the exact direction



Figura 9. Generalized geologic map of Palo Huérfano volcano. Key: PH = Palo Huérfano; EP = El Picacho; CC = Cerro Colorado; CE = CerroLa Elvira; CP = Cerro El Pilón; SMA = San Miguel Allende; CB = Cañada Begoña; Pa = Presa Ignacio Allende; SM = San Marcos; Cal =Calderón; RR = Rinconcillo; ER = El Refugio; C = Comonfort; J = Jalpilla; LG = Las Gallinas; PC = Peña Colorada; AB = Agua Blanca; OA= Ojo de Agua; P = Purgatorio; Ja = Jalpa; E = Elvira; DJ = Doña Juana; Ca = Cañajo; A = Alcocer; Es = Estancia. Simplified after Pérez-Venzor et al. (1996).

of subduction of the Cocos Plate differ from those of theearlier subducted Farallon plate that drove the Oligocenemagmatism (Ferrari et al., 1999). These differences mayin turn be responsible for the different bulk compositionsand volatile contents of the Miocene and younger TMVBmagmas, as compared to the earlier SMO magmas.

The stratovolcano Palo Huérfano is located to thesouth of the prominent scarp of the San Miguel AllendeFault, and it covers this structure without being affectedby it (Figure 9). Based on geologic and geophysic data,Arzate and others (1999) report that this fault continuesto the south buried under younger deposits for over 80km and passes through Tarimoro and reaches Parácuaro.In northern flank of Palo Huérfano lavas from this vol-cano are displaced by normal faults striking approxi-mately N80E (Figure 9); the orientation of these faultssuggests that they are related to the tectonics of theTMVB (Figures 1, 8). In the highest part of the Río LajaBasin, in the area bounded by San Miguel Allende,Dolores Hidalgo and San Felipe, there are extensivedeposits of gravel and sand (Figures 2-4). Near theIgnacio Allende Dam (Figure 9) these same gravels and

sands are intercalated with lake sediments and with sev-eral volcanic units. The precise age of this stratigraphicsequence which forms the filling of the Río Laja Basin isuncertain, and the only sites where precise detailed workhas been carried out are to the north of San MiguelAllende, in Blancan-Hemphillian fossiliferous localitiesstudied by Carranza-Castañeda (1987). The sediments inthese localities have been dated (fission tracks in zirconand/or 40Ar-39Ar in sanidine) between 3.5 and 5 Ma(Kowallis et al., 1998). On the other hand, in the regionaround the village of Xoconostle (Figure 3) Nieto-Samaniego and others (1996) documented similar gravelsintercalated with a rhyolitic ignimbrite whose radiomet-ric age (K-Ar) is ~25 Ma.

We attribute the origin of this fluviolacustrinebasin to the interaction between normal faulting on thethree fault systems of San Miguel Allende, Alcocer-LaEstancia and El Bajío (see Part II of this manuscript) andthe lava flows emitted by Palo Huérfano, which blockedthe outlet of the hydrologic basin in the region presentlyoccupied by the mouth of the San Miguel AllendeReservoir. The fossil vertebrate fauna in the Río Laja



Figure 10. Simplified geologic map of La Joya volcano (after Valdez-Moreno et al. 1998).

Basin deposits is very important, as it is the oldest local-ity in which it is possible to recognize fossils of animalswhich came out of South America mixed with typicallyNorth American fossils (Oscar Carranza-Castañeda andAguirre-Díaz, 2001). The migration of faunas betweenthe two Americas dates from the early Pliocene and isassociated with the closing of the Isthmus of Panama byvolcanic activity.

At the base of the Palo Huérfano VolcanicComplex (Figure 9) rocks of the basal Mesozoic complexare exposed (Chiodi et al., 1988; Ortiz-Hernández et al.,2002), as is the rhyolitic El Obraje Ignimbrite of mid-Oligocene age (K-Ar, 28.6±0.7 Ma). To the west of thevolcanic center occur outcrops of Miocene basalticandesites (i.e., the Allende Andesite, K-Ar, 11.1±0.4 Ma;Figure 9). Lying above them are the products of the PaloHuérfano stratovolcano.

PART I I . A BRIEF REVIEW OF THE GEOLOGIC AND TECTON-IC EVOLUTION OF THE SOUTHEASTERN PART OF THE SIERRADE GUANAJUATO

GENERAL FEATURES

The Sierra de Guanajuato is an orographic feature thatextends in a continuous manner over a distance of some80 kilometers, with an orientation N45W. The southwest-ern front of the range (El Bajío Fault Zone, Figure 5) isan important boundary which separates two physiograph-ic provinces in central Mexico (Figure 1a). South of ElBajío Fault (Figures 3, 5, 8) is the TMVB and north of thefault is the Mesa Central (Aranda-Gómez et al., 1989),which is considered as an integral part of the Basin andRange extensional province (Henry and Aranda-Gómez,1992, 2000). The present-day morphology of the Sierrade Guanajuato was caused by this Cenozoic extensionaltectonism. Erosion products of the uplifted Sierra werecarried both to the northeast and to the southwest, accu-mulating in El Bajío depression (Figure 5), at the foot ofthe mountains, as well as in the inner part of the Río LajaBasin (Figures 3, 8). These deposits of gravels, shalesand claystones, all poorly consolidated, contain verte-brate fossil faunas of Pliocene-Pleistocene age, but thereare also some Miocene deposits. Similar deposits arefound in other regions of the southern portion of theMesa Central, in the states of Hidalgo, Jalisco and

Guanajuato (Carranza-Castañeda et al., 1996). In theregion of San Diego de la Union (Figure 3) these depositsare covered by flows of Quaternary alkali basalt whichcontain mantle xenoliths (Aranda-Gómez et al., 1989).

The rocks exposed in the Sierra de Guanajuatocan be divided into the two great groups referred to: (1)the basal complex, which includes both Mesozoic rocks—volcanic and plutonic rocks of the Guanajuato Arc (inturn part of the larger Guerrero Terrane) and sedimentaryrocks of the Arperos (fore-arc) Basin— and early Tertiaryintrusive rocks (i.e., the Comanja Granite), and (2) theCenozoic sedimentary and volcanic cover (Figure 5). Thesequence in the basal Mesozoic complex includes intru-sive rocks of different ages (K-Ar, ranging from 157down to 108 Ma; Ortiz and Martínez-Reyes, 1993) and avariety of compositions (ultramafic to felsic) and low-grade metamorphic rocks (derived from volcanic andsedimentary protoliths of oceanic origin). The basal com-plex was intensely deformed by two compressive events.The first took place at the end of the Early Cretaceous(Neocomian), when the Guanajuato Arc and its accom-panying fore-arc (Arperos) basin were accreted to theNorth American continent (Figure 12), and the secondoccurred during the Paleocene Laramide Orogeny(Quintero-Legorreta, 1992). The Cenozoic cover packagein the Sierra de Guanajuato consists of Eocene continen-tal red beds and a thick sequence of volcanic rocks ofOligocene to Miocene age (Figures 5, 6), predominantlyfelsic to intermediate in composition, with the earlier fel-sic rocks having markedly greater volume than the gen-erally later intermediate rocks.

STRATIGRAPHY, AGE DATES, AND GEOLOGIC EVOLUTION

The basal Mesozoic complex and the early TertiaryGranite

The pre-Tertiary stratigraphy of the Sierra de Guanajuatoconsists of two major associations, a volcano-plutonicassociation, which comprises the volcanic and sub-vol-canic rocks of the long-lived and possibly multiple arcand its oceanic crustal substrate, and a volcano-sedimen-tary association, which comprises the sedimentary rocks(Figure 11) and intercalated tuffs of the fore-arc basin(Monod et al., 1990; Ortiz et al., 1992; Lapierre et al.,1992). These rocks range in age from the oldest plutonicunits (Late Jurassic) to the youngest fore-arc basin com-



ponents (Early Cretaceous). A small outcrop of shallow-water limestone (the Albian-Aptian La Perlita Formation;Martínez-Reyes [1992]) rests discordantly on the meta-morphosed Mesozoic rocks north of the city of León(Figure 3).

Near the Guanajuato Mining District, the vol-cano-plutonic association (Figure 11) includes: (1) athick succession (>1000 meters) of basaltic pillow lavasand massive submarine flows (K-Ar, 108.4±56 Ma;Monod et al., [1990]), with relatively scarce basaltictuffs; (2) a group of closely-related diabase dikesemplaced in gabbros (K-Ar, 112 Ma; Lapierre etal.[1992]), diorites, quartz-diorites and tonalites; (3) amassive diorite pluton (K-Ar, ~120-122 Ma; Lapierre etal. [1992]) with hornblende-rich pegmatitic segregations,locally cut by basaltic dikes; (4) an intrusive body com-posed of trondhjemite and leucotonalite (K-Ar, ~143-157Ma; Lapierre et al. [1992]) and other plutons, also intrud-

ed by swarms of diabase dikes. All these units of thebasal complex are found piled one on top of another on aseries of thrusts (Figurres 5, 11). In many outcrops thethrusts appear almost horizontal and undeformed; how-ever, there are other outcrops in which the thrusts areclearly folded. In some places the folding of the thrustsappears to be drag-folding adjacent to Cenozoic normalfaults, while in other places it appears to be unrelated toCenozoic structures. In the field, the lowermost unit isthe one containing the pillow lavas (1), which is in turncovered by the heterolithologic plutonic unit containingthe dike swarm (2), followed by the massive diorite (3),and then the trondhjemite-leucotonalite (4). Ortiz andMartínez-Reyes (1993) have proposed an idealizedreconstruction of the original sequence (Figure 11). Thisvolcano-plutonic association of the Sierra de Guanajuatohas been interpreted as the upper crust of an intra-ocean-ic volcanic arc (i.e., the Guanajuato Arc of the so-called



Figure 11. Reconstructed stratigraphy of the Guanajuato Arc and the Arperos Basin. Φ = Tectonic contact (thrust fault). After Ortiz andMartínez-Reyes (1993).

Guerrero Terrane). If indeed the apparent age range ofthese arc-related rocks (157 to 108 Ma; nearly 50 millionyears) is real, this portion of the Guerrero Terrane may bemade up of more than one arc system; however, moredetailed dating and geochemical work would be neededin order to establish or refute this contention.

Even younger ages have been obtained for somecomponents of the volcano-plutonic association, in therange of 66 to 108 Ma. In our opinion the value of theseradiometric dates is a bit uncertain, given that most of theMesozoic rocks in this area have experienced regionalgreenschist facies metamorphism, and some of themhave also been subjected to heating related to theemplacement of the Eocene Comanja Granite and/or tohydrothermal activity associated with the mid-Tertiaryvolcanism. Any or all of these factors could have modi-fied the argon content of the rocks, rendering the radio-metric dates insignificant as to the true age of theMesozoic arc magmatism. On the other hand, it is possi-ble that these anomalous young ages actually are signifi-

cant, and that they provide evidence of a second majorperiod of arc volcanism, following a change in the polar-ity of the subduction zone (from west- to east-dipping),before and during the emplacement of the ComanjaGranite. In summary, the volcano-plutonic associationconsists of an intra-oceanic arc or arcs of Late Jurassic toEarly Cretaceous age (Ortiz and Martínez-Reyes, 1993).

The volcano-plutonic association describedabove is thrust over a volcano-sedimentary sequence,which in places is strongly deformed. The rocks of thisassociation are pelagic in character and consist principal-ly of dark laminated limestone and of thin-bedded blackshale, chert, sandstone and siltstone (Figure 11). Pillowedbasalts, hyaloclastites and basaltic tuffs intercalated withthe sedimentary rocks are found at several localities. K-Ar ages for these volcanic rocks are in the range from 85to 93 Ma (Cenomanian to Santonian, or Late Cretaceous;Ortiz and Martínez-Reyes [1993]). As with the similarlyyoung ages for certain rocks of the volcano-plutonicassociation, these radiometric dates also could be some-



Figure 12. Early Cretaceous tectonic evolution of the Guerrero terrane. Key: Guanajuato (Alisitos-Teloloapan) arc; BHCP = Central MexicoMesozoic Basin; PVSLP = Valles-San Luis Potosí calcareous platform; TG = Guerrero terrane. Lapierre et al., 1991

what untrustworthy. Poorly preserved radiolarian fossils,recovered by Dávila-Alcocer and Martínez-Reyes (1987)from rocks of this association give uncertain ages fromValanginian to Turonian (roughly 135–90 Ma, or Early toMiddle Cretaceous). Nannofossils collected by Corona-Chávez (1998) from layers of limestone indicate agesfrom Tithonian to Berriasian (roughly 150 to 140 Ma, orLate Jurassic). On the basis of these data, the volcano-sedimentary association is considered roughly contempo-raneous with the volcano-plutonic association (Ortiz andMartínez-Reyes, 1993). An idealized reconstruction ofthe volcano-sedimentary association is shown in Figure11. This association is interpreted as sediments accumu-lated in a long-lived oceanic basin (the Arperos Basin)located between the volcanic island arc(s) and the car-bonate platforms, which bordered the Mexican subconti-nent (e.g., the Valles-San Luis Potosí Platform). The tec-tonic juxtaposition of the volcano-plutonic associationand the volcano-sedimentary association is thought to berelated to the closing of the Arperos Basin and to theaccretion of the volcanic arc to the southern edge ofNorth America during mid-Cretaceous time (Tardy et al.,1991; Figure 12 of this field guide). Neither the originalwidth of the Arperos Basin nor the distance between theedge of the continent and the center of the arc is knownfor any time period during the long history of the sub-duction system; however, the Arperos Basin seems tohave had a significant contribution of clastics derivedfrom the continent, emplaced as turbidites and/or by pro-cesses of off-scraping from the downgoing slab. We spec-ulate that these sediments in turn may have served assources of potassium and boron during the period of for-mation of the Comanja Granite magma.

A bit less than 15 kilometers northeast of the cityof León, shallow-water limestones of Aptian-Albian age(roughly 120 to 100 Ma, or late Early Cretaceous; shownas the La Perlita Limestone in Figure 5) crop out restingdiscordantly on older, metamorphosed rocks of the vol-cano-sedimentary association (Chiodi et al., 1988;Quintero-Legorreta, 1992). These limestones includeoolites and calcareous breccias with abundantammonites, brachiopods and gastropods. The presence ofCeritium bustamantii and Psilothyris occidentalis indi-cates ages of Neocomian to Aptian (roughly 130 to 112Ma, or Early Cretaceous). A brachiopod (Peregrinellasp.) indicates a Hauterivian age (132 to 127 Ma, or EarlyCretaceous).

The Comanja Granite (K-Ar ~51±1.3 Ma; Steinet al. [1993]) is exposed in the core of the Sierra deGuanajuato, forming a chain of outcrops more than 50kilometers long (Figure 3). It is emplaced in rocks of theMesozoic basement complex after compressional defor-mation of the region had largely ended. The Comanja isa medium- to coarse-grained calcalkaline granite withlarge subhedral to euhedral; Carlsbad-twinned K-feldsparphenocrysts set in a generally medium-grained ground-mass of light-colored plagioclase, quartz and biotite. Thepluton is in places cut by dikes of pegmatite and apliteand by a well-developed network of tourmaline veinlets.Along its contacts the calcareous sediments of the vol-cano-sedimentary association were transformed to skarnsby contact metamorphism; in some places the primarytextures were completely wiped out. Brittle shear zonesare very common in the vicinity of the intrusive contacts;some of these zones are present several tens of metersinward from the margin of the pluton. The textures ofthese shear zones are varied, with granite and/or tactiteclasts of varying sizes and angularities firmly cementedby abundant tourmalinite.

The Cenozoic volcanic and sedimentary cover

Guanajuato Conglomerate

The basal complex is separated by a major angularunconformity from the overlying formations (Figure 7).In the Guanajuato Mining District, immediately abovethe unconformity, there is a sequence of continental redbeds (1,500 to 2,000 meters thick; Edwards [1955]). Thissequence consists of boulder and pebble conglomerates,sandstones and siltstones, with sorting that varies some-what rhythmically from poor to good and bedding thatvaries from massive to thinly layered. Near the base ofthe sequence there are intercalated andesitic lavas (K-Ar~49 Ma; Aranda-Gómez and McDowell [1998]). Basedon the lithology of the deposits and the vertebrate fauna,Edwards (1955) concluded that the Guanajuato RedConglomerate consists principally of sediments accumu-lated in alluvial fans situated at the base of block-faultmountains that were rapidly uplifted during the mid-Eocene and Early Oligocene. Near the top of theGuanajuato Conglomerate there is a series of thin fine-grained layers with ripple marks and stream cross-bed-ding, which suggests that by that time the rate of move-



ment on the faults bounding the depositional basin haddiminished. A statistical analysis of the dips of layers inthe stratified units of the cover indicates a complex his-tory of normal faulting. Extension in this area beganaround the middle of the Eocene and continued at leastuntil the later part of the Oligocene (Aranda-Gómez andMcDowell, 1998).

Losero Formation

Resting unconformably on the Guanajuato Conglomeratelies the Losero Formation, which is a mixed deposit thatpasses upward from a well-sorted sandstone with thin tomedium bedding and dark red color to an interval inwhich red layers of sedimentary origin are interstratifiedwith green pyroclastic surge layers. In the upper part ofthe Losero Formation, the green pyroclastic surge layersare predominant. This transitional sedimentary-to-vol-canic package indicates the change from a sedimentaryregime to a dominantly volcanic one (Figure 7). TheLosero Formation has been little studied, but it is impor-tant in the interpretation of the volcanic sequence thatwas deposited on top of it. Its thickness varies from 0 to55 meters, but it is widespread in the District, generallybeing 10 to 20 meters thick. Structures such as ripplemarks, cross-bedding and graded bedding, cut-and-fillstructures and raindrop impressions in the Losero havebeen interpreted uniquely as sedimentary features; how-ever, volcanic structures such as very-low-angle (surge)cross-bedding and accretionary lapilli (?) are equallycommon in these deposits, especially in the upper part ofthe unit. The depositional environment is interpreted as ashallow lake (Edwards, 1955), although our observationssuggest that the upper, surge-dominated, layers weredeposited subaerially.

Bufa Ignimbrite

An erosional surface separates the Losero Formationfrom the overlying unit, the Bufa Formation (K-Ar,37>0±3.0 Ma; Gross [1975]), which is an ignimbrite withless than 25 percent by volume of phenocrysts of quartz,sanidine and plagioclase, and small euhedral biotite com-monly replaced by opaque minerals. Dispersed in thedeposit are lithic clasts of andesite and rhyolite. Near itsbase, the ignimbrite contains clasts derived from thelower units and abundant fragments of a delicately flow-

banded rhyolite of unknown origin and affinity. Thethickness of Bufa varies from 350 m in the vicinity of theLas Torres Mine to less than 10 m on Sirena Hill, lessthan 5 kilometers from Las Torres. In the southeasternpart of the District, the Bufa ignimbrite has crude colum-nar jointing, possibly formed during cooling in a zonethat is more densely welded and/or silicified than is typi-cal of the unit.

Calderones Formation

The Calderones Formation is a complex unit thatincludes an indeterminate number of andesitic to daciticignimbrites and layers of volcaniclastic material thataccumulated in a shallow lake. This formation rests on aneroded and faulted surface developed on the BufaIgnimbrite (Figure 7). Calderones commonly fills pale-ochannels, especially in the proximal and medial areas.Some of these channels appear to have been formed bystream erosion prior to the deposition of the Calderones,while others may have been formed, or at least deepened,by the passage of the surges and density currents whichgave rise to the basal layers of the Calderones. There areplaces where angular lithic fragments of (metamor-phosed?) chloritized andesite (derived from the basalcomplex?) make up ~75 per cent of the deposit. In otherplaces fragments derived from sedimentary rocks of theMesozoic basement are more abundant than the juvenilevolcanic materials. In still other places, clasts derivedfrom the growth and/or destruction of one or more domesin the Peregrina Dome Field (Figure 6) constitute animportant component of the deposit. The CalderonesFormation is medium- to coarse-bedded, and the grainsize of the accidental clasts ranges from fine sand to peb-bles and cobbles, although the most common are pebblesand small boulders. There are some layers with well-rounded clasts, but the majority of the layers has angularclasts. In the vicinity of El Cubo Mine, Calderones con-tains pyroclastic flow deposits. These tuffs range fromthin (3 m) to moderately thick (20-30 m), and the base ofeach flow is marked by a horizon rich in boulders of pur-ple latite or dacite (probably derived from the PeregrinaDomes) in a vitroclastic and chloritized matrix. Abovethis basal horizon there are welded tuffs which displaycollapsed pumices that are entirely replaced by chlorite.In some places there are delicately laminated layers offine-grained material that we interpret as pyroclastic



surge deposits associated with emplacement of the over-lying ignimbrites or as deposits produced in localphreatomagmatic events. The total thickness ofCalderones Formation (Figure 7) has been estimatedbetween 200 and 250 m (Buchanan, 1979). However,there is not sufficient information about the thickness ofthe unit in the block bounded by El Cubo and La LeonaFaults for us to be certain that we know the true maxi-mum thickness.

Cedro Andesite

Resting on the Calderones Formation and interstratifiedwith its upper layers is the Cedro Formation, which ismade up of grey to black andesite lava flows, in placeswith interbeds of pyroclastic materials. The total thick-ness varies from 100 to 640 m. As in the case of theCalderones Formation, the age of the outcrops of theCedro within the District is not known. Cerca and others(2000) dated (K-Ar, 30.5±0.5 Ma) an andesite which theyconsidered as part of the Cedro Formation in the volcanicsequence of the Mesa de San José de Allende, locatedbetween the cities of Guanajuato and San Miguel Allende(Figure 3).

Dike Complex

In its lower portion, the volcanic sequence of the Districtis characterized by the presence of dikes whose composi-tions are similar to those of the nearby Cedro andesiteflows. These structures are overwhelmingly most abun-dant, most elongate and widest where they are exposedcutting outcrops of the Calderones Formation. Many ofthese dikes cut across the La Leona Fault (Figure 6),which separates surface exposures of Calderones east ofthe fault trace from surface exposures of the Bufa west ofit. Invariably these dikes terminate a few meters or tensof meters into the Bufa block. Elsewhere, Cedro dikes dopersist for long distances in outcrops of the Bufa, espe-cially in the region between the village of Calderones andthe city of Guanajuato, but they are relatively narrow. Afew dikes were mapped by Echegoyén (1970) as cuttingoutcrops of Cedro flows, but most dikes are overlain bythese flows. The reason for the paucity of dikes exposedwithin the block of Bufa Ignimbrite and GuanajuatoConglomerate bounded by the La Leona Fault and theVeta Madre is not entirely clear. We consider these struc-

tures as feeder dikes for the Cedro lava flows. There isevidence that locally the dikes had phreatomagmaticinteraction with the uppermost (generally but not exclu-sively distal) layers of the Calderones Formation. Thisactivity produced small lahars that appear to be interca-lated with lenses of tuffaceous materials characterized bytheir massive nature and by the abundance within them ofandesite clasts derived from the dikes.

Chichíndaro Rhyolite

The youngest volcanic unit in the Guanajuato MiningDistrict is a rhyolite porphyry that forms large domes,tholoids and lava flows, along with associated ign-imbrites and volcanic breccias. Its type locality isChichíndaro Hill (Figure 6), where a large altered domeof uncertain age and affinities lies between two branchesof the Veta Madre. Somewhat similar volcanic structuresare exposed at the summit of Cerro Alto de Villalpandoin the northeastern part of the Mining District, in otherplaces to the northeast of the District, and in the northernpart of this end of the Sierra de Guanajuato. In places,such as the Sierra del Ocote (Figure 2), the rhyolitedomes contain disseminated tin and vapor-phase cavity-filling topaz distributed along the flow foliation. Gross(1975) reported K-Ar ages of 32±1 Ma for theChichíndaro Rhyolite, at its type locality. Nieto-Samaniego and others (1996) obtained two K-Ar (sani-dine) ages on rhyolitic domes that they considered part ofthe Chichíndaro Formation, one in the La SaucedaGraben (Figure 3), south of the District (30.8±0.8 Ma),and the other north of the town of Santa Rosa (Figure 3),north of the District (30.1±0.8 Ma).

Cubilete Andesite

At the summit of Cerro del Cubilete, resting directly onMesozoic metasedimentary rocks, there is a sequence ofgravel deposits topped by an andesitic lava flow (Figure5). This gravel, shown on the map of Martínez-Reyes(1992) as El Capulín Gravel, accumulated in what origi-nally were low zones and/or channels that were thecourses of major streams. Later extensional tectonicactivity is responsible for their presence at an elevationabout 600 m higher than the valley known today as ElBajío Plain. This gravel is composed principally of smallrounded boulders derived from the mid-Tertiary volcanic



sequence and, in much lesser proportion, of clasts withprovenance in the basal Mesozoic complex. The thick(presumably intracanyon) andesite(s) produced markedthermal-alteration effects in the underlying sediments,and the flow bases are autobrecciated. Sets of verticalfractures create rough columns in the middle portion ofthe flow(s). In the uppermost part of the andesite outcrop,there is an interval of subhorizontal fracturing, andbetween the upper and lower columnar-jointed portionsthere is a second surface of platy jointing. This type offracture pattern could be developed in a single thick flowwith colonnade-and-entablature structure, or it couldequally well be developed in two thinner lava flows. Thisunit is also found in El Bajío Plain, south of El Cubilete,at an elevation 600 m below the Cubilete summit, whereit commonly forms the tops of benches held up by eitherthe Cuatralba Ignimbrite or El Capulín gravel. Thus theandesite, like the gravel (but note that there are broadareas of much younger gravels, not belonging to ElCapulín, but included with it for mapping purposes, in ElBajío Plain south of El Cubilete), crops out both on theupthrown and on the downthrown blocks of El BajíoFault Zone. The precise age of the true El Capulín grav-els is not known, but the andesite has an age of about 13.5Ma (Aguirre-Díaz et al., 1997), while the youngest iden-tifiable clasts in El Capulín gravels are those derivedfrom ~30 Ma ignimbrites. Assuming that the displace-ment on El Bajío Fault took place between the mid-Miocene and the present, its long-term rate of verticaldisplacement would be on the order of 0.04 millimetersper year. Of course, the actual period during which faultmovement took place may have been much briefer than13.5 Ma, but we have no stricter field constraints on it atpresent.

Structure

The main structural trend in the southern part of theSierra de Guanajuato has a NW-SE orientation, which isdefined by: 1) the schistosity in the low-grade metamor-phic rocks, 2) the average attitude of fold axial planes inrocks of the basal complex, 3) the outcrop pattern of theMesozoic basal complex and the Comanja Granite, and4) the trends of some of the larger Tertiary faults (e.g., ElBajío Fault and the Veta Madre). Detailed analysis of thetrend and plunge of microfold axes in the metasedimentsalso shows a less evident NE-SW trend. Tertiary normal

faults, such as the Villa de Reyes Graben (Tristán-González, 1986) and the Aldana fault are also orientedNE-SW (Figures 3, 5).

The contacts between the principal stratigraphicsequences (i.e., the volcano-plutonic association(Guanajuato Arc) and the volcano-sedimentary associa-tion (Arperos Basin), and even the contacts between cer-tain lithologic units within each of these two associationsof the metamorphic basement, are persistent subhorizon-tal mylonite zones of little thickness. Monod and others(1990) considered these contacts as low-angle thrusts ofmid-Cretaceous age. Overprinted on this first event ofcompressive deformation, which caused the formation ofan early foliation, with a distinctive and penetrativenorth-south oriented crinkle lineation, is a second com-pressive deformation of Laramide age (Paleocene-earlyEocene), which produced somewhat larger folds withaxial planes that have northwesterly strikes and north-easterly dips.

The cover rocks display structures associatedwith extensional tectonism. There are at least two conju-gate systems of faults in the Mesa Central (Figures 2, 3),which in the Sierra de Guanajuato affect both the basalcomplex and the cover. The most important trend in theSierra de Guanajuato is the NW-SE trend discussedabove, but there are also important structures orientedNE-SW and ENE-WSW. Examples of the later are theVilla de Reyes and La Sauceda grabens, respectively, anda linear feature between the villages of Los Mexicanosand Santa Rosa which has been considered a graben bysome authors (e.g., Martínez-Reyes, 1992) and a paleo-valley by others (e.g., Guillermo Labarthe, personal com-munication, 2001). This feature, whatever the origin of itsboundaries is, apparently was filled by early Tertiary con-glomerates and mid-Tertiary volcanic rocks (Figure 5).

There is indirect evidence of extensional tecton-ism in the Sierra de Guanajuato during the Eocene, con-temporaneous with deposition of the continental red bedsof the Guanajuato Conglomerate. Aranda-Gómez andMcDowell (1998), based in a statistical analysis, havesuggested that variations in the degree and direction ofdip of both the red beds and the volcanic sequence areconsistent with tilting associated with normal faultingactive during the accumulation of these layers. Themajority of the normal faults has its downthrown blockson their southwestern sides; an important exception is theLa Leona Fault (Figure 6), which dips to the northeast



and has its northeastern side downthrown. The signifi-cance of this exception to the general rule is not yet clear.There is a possibility that it is related in some way to theeruption of the Bufa Ignimbrite and the formation of atleast a part of the system of closed basins which a bit laterserved to entrap the volcanic materials which make upthe Calderones Formation. The principal northwest-trending faults developed before the mineralization of thevein systems of the District, because virtually all the oredeposits were emplaced along these structures. In theregional setting (the southern portion of the MesaCentral, Figure 3), it is not possible to establish consis-tent crosscutting relationships between the NW-trendingfaults and the NE-trending ones (Aranda-Gómez et al.,1989). It is therefore concluded that the pattern of fault-ing in this broad region cannot be attributed to a singleperiod of extension in which the orientation of the prin-cipal stresses was constant (Aranda-Gómez, 1989). It ispossible that the present-day fault pattern evolvedthrough successive periods of extension, each takingplace under the influence of a different set of forces(Aranda-Gómez and McDowell, 1998).

Mineralization

By a considerable amount, the most important mineral-ization in the Guanajuato District consists of epithermalveins of silver and gold whose age of formation has beendated as 27.4±0.4 Ma (Buchanan, 1975). The District hasa history of mining activity of more than 450 years. Thefirst of the present underground mines was developed bythe Spaniards in 1548, although there is some evidencethat the indigenous peoples of the area had been extract-ing gold and silver from near-surface deposits for manyyears before that date. It has produced ~130 tons of goldand ~30,000 tons of silver, making it one of the mostimportant silver districts in the history of precious-metalmining worldwide. The production is derived from threeprincipal vein systems (the La Luz, Veta Madre and LaSierra Systems, Figure 6) of quartz, adularia and calcite,emplaced both in rocks of the basal complex and in thoseof the Cenozoic cover (Figure 6). Concentrations of pre-cious metals are present in isolated packets (known asbonanzas, or “spikes”) distributed vertically and laterallybetween non-mineralized segments of the veins. Thereare three principal levels of production at 2,100-2,350,2,200-1,700, and <1,700 m a.s.l. The mineral associa-

tions in the upper- and middle –level bodies are: acanthite+ adularia + pyrite + electrum + calcite + quartz. In thelower-level bodies they are: chalcopyrite + galena +sphalerite + adularia + quartz + acanthite. This suggeststhat the mineralization was produced by fluids of two dif-ferent compositions (Buchanan, 1979). The veins occupywhat originally were normal faults. The Veta Madre canbe followed on the surface for about 20 km, it dips from35 to 55 degrees to the SW and it has measured displace-ments of around 1,200 meters near the Las Torres Mineand 1,700 meters near La Valenciana Mine.

In addition to the epithermal veins, nearGuanajuato small deposits of stratabound massive sul-fides (e.g., Los Mexicanos, Figure 5) have been reportedin the Mesozoic volcano-sedimentary association.Similarly, there is gold mineralization in the ComanjaGranite, and in its contact aureole small tungsten depositshave been found. In the Tertiary volcanic rocks, princi-pally in the topaz rhyolites, there are small tin prospects.Near Cerro del Cubilete, there are tabular bodies wherekaolinite is being quarried. These bodies possibly arehydrothermally altered Tertiary dikes emplaced in thebasal complex. Finally, the finely laminated Losero bedsand some of the Calderones ash flow tuffs have beenextensively quarried and used for construction (mainly asa facing stone on large buildings and for construction ofcolumns and smaller buildings).

A C KNOWL E D GM EN T S

Throughout the years our research in the transitionalregion between the Mesa Central and the TMVB hasbeen financed by different agencies. J. Aranda gratefullyacknowledges grants form CONACYT (37429-T) andDGAPA PAPIIT (INI114198); M. Godchaux gratefullyacknowledges support provided by Mount HolyokeCollege for her work in the Sierra de Guanajuato andadjoining regions; and G. Aguirre thanks CONACYT andDGAPA PAPIIT for financial support through the grants33084-T and IN-120999, respectively.

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DAY 1 : AN OV E RV I EW O F T H E G E O L O GY O F

T H E GUANA J U ATO M I N I N G D I S T R I C T

Refer to Figures 5 and 6 for locations of stops.

Km 0.0: We begin at the Hotel Parador San Javier.Heading north on the paved road toward Dolores Hidalgo(Federal Highway 110), the highway passes through theTertiary redbeds of the Guanajuato Conglomerate. Km 1.9: A short distance before the town of LaValenciana, looking toward the east we can see apanoramic view of an outcrop of the Veta Madre fault-vein. The structure was mined at the surface and the oldworks resemble triangular facets. Closer to the road, inthe stream, are the tailing ponds of the Cooperativa SantaFe de Guanajuato.Km 2.4: La Valenciana town. To the right is an imposingsixteenth century church, which was built on the trace ofthe NE-trending Aldana Fault, which puts into contactthe Eocene redbeds of the Guanajuato Conglomerate andthe Mesozoic Formations (Figure 5). Once we pass thetown, the road continues uphill through a diabasic dikecomplex emplaced in dioritic to tonalitic host rocks(shown in Figure 5 as La Palma Diorite), part of theMesozoic volcanoplutonic sequence. The outcrops of thecomplex are located west of the road.Km 3.2: Near the Camino de Guanajuato Hotel, the roadcrosses the trace of the Veta Madre fault, which puts intocontact rocks of the volcanoplutonic sequence of theGuanajuato Arc and pelagic sediments of the ArperosBasin. From this point on, the highway was built onintensely deformed rocks of the Arperos Basin, mostlyslates and thin-bedded limestones.Km 3.5: To the left is the road that leads to the LaEsperanza Dam and the Los Insurgentes (DIF) camp-ground. As we go uphill on Highway 110, a panoramicview of the city of Guanajuato and the town of LaValenciana can be seen to the right.Km 4.45: To the left, there is a small flat area where wewill pull off the road and have our first stop. From thislocality we can obtain a broad overview of the most rel-evant rock units and Cenozoic structures exposed in theregion.

STOP 1-1. INTRODUCTION TO THE GEOLOGY OF THE

GUANAJUATO MINING DISTRICT, PANORAMIC VIEW OF THE

CITY, LITHOLOGIES AND CONTACT RELATIONSHIPS IN THE BASE-

MENT: THE JURASSIC-CRETACEOUS VOLCANIC ARC AND ITS

ADJACENT (FORE-ARC) SEDIMENTARY BASIN (UTM14Q0266176; 2328677)

The most important regional features seen in thisoverview are as follows:—To the south, the city of Guanajuato was built in abasin occupied by the redbeds of the GuanajuatoConglomerate. The southeastern part of the city isflanked by near-vertical cliffs where the lowermostTertiary volcanic formations overlie the GuanajuatoConglomerate (Figs. 5, 7). Behind those mountains is thewide valley known as El Bajío. The depression in whichGuanajuato is limited to the west by the NE-trendingAldana Fault and to the north by the NW-trending VetaMadre fault (Figure 6). Southeast of Stop 1-1 are the hillsCerro Chichíndaro and Cerro de Sirena, with their north-eastern flanks displaced by the Veta Madre fault, whichin that place marks the limit between the Mesozoic andCenozoic units. From this point it is also worthwhile toobserve the marked changes in thickness of some of themid-Tertiary volcanic units, such as the Bufa Ignimbrite,which in the vertical cliffs south of Stop 1-3 exceeds 300meters and in Cerro de Sirena is ≤10 meters. We believethis dramatic northward decrease in thickness indicatesactive erosion and/or caldera-wall faulting at the time ofBufa volcanism, or possibly deposition of the Bufa ash-flow tuff on a surface made very irregular by regionalfaulting.—To the east, we see the mountain range known as theSierra de Santa Rosa, and it is partially covered byTertiary volcanic units which dip gently (~12 degrees)toward the northeast. Compared with the sharp south-western limit of the Sierra, determined by the El Bajíofault, the northeastern boundary is poorly defined, andthe volcanic rocks gradually merge with the thickdeposits of gravel that partially fill the Río Laja Basin.The overall geomorphologic picture of the region sug-gests that the Sierra is located at the southern end of alarge block tilted to the northeast during the lateCenozoic.—To the north, in the background are exposures of theCerro Pelón Tonalite (Figure 5), which stands out as thewhite ground without vegetation. In the foreground isCerro El Plomo, which is made up of a terrigenous flyschsequence; between that hill and us, in the bottom of thestream valley, is La Esperanza Dam.



—To the west, in the background, is Cerro El Cubilete,crowned by a shrine topped with a large statue of Christ.The church was built on top of Late Cenozoic andesiteand gravel, which in turn overlie the Mesozoic vol-canoplutonic rocks of the Guanajuato Arc. Between ElCubilete and Stop 1-1 is an unforested hilly country withextensive outcrops of the dike complex (shown in Figure5 as La Palma Diorite) and metalavas and metatuffswhich form parts of both the volcanoplutonic association.In that area there are also extensive outcrops of the vol-canosedimentary association (Figure 5).

Description of the outcrops at Stop 1-1

In the road cuts there are examples of the complex con-tact relationships among the various volcanic compo-nents of the Mesozoic basement, common in all parts ofthe Sierra de Guanajuato. They also show some of thecharacteristic lithologies of the sedimentary basin nearthe volcanic arc (Tardy and others, 1991). The outcropincludes interbedded pelitic and carbonate sediments ofthe Esperanza Formation (Echegoyén, 1970), submarineandesite flows and tuffs, and tonalitic intrusives. In thisoutcrop the calcareous-to-argillaceous sedimentary rocksare intensely deformed, displaying a well-developedschistosity. In the argillaceous rocks, crenulations strik-ing approximately N-S, associated with a pre-Laramidedeformation, are observed. During Laramide deforma-tion these structures were compressed, producing micro-folds which are clearly observed along the outcrop. Theaxial planes of these microfolds have an average strike ofN35-40W and an average dip to the northeast of less than45 degrees. Other prominent structural features includeincipient shear zones in the cores of some of the largerfolds, and lenses or boudins of weakly metamorphosedmarls dispersed among the more phyllitic argillaceouslayers. Another notable feature is the presence of near-vertical extension fractures. These may be associatedwith mid-Tertiary Basin-Range extension, or possiblywith a stress field related to the accretion of the Arperosforearc basin sequence to the Guanajuato Arc during finalassembly of this portion of the Guerrero Terrane, justprior to its obduction onto the edge of North America.

Volcanic rocks metamorphosed to greenschistfacies are found both in tectonic contact and in deposi-tional contact with the calcareous metasediments. Theprotoliths of these rocks probably were principally

andesitic lava flows; however, metatuffs are also abun-dant. The metatuffs form sequences of thin layers inwhich basal crystal concentrations are easily recogniz-able. Some layers also have a marked gradation in parti-cle size from lapilli-rich bases to ash-rich tops. At thissite it is possible to observe one of the many thrust con-tacts in the Mesozoic sequence. We speculate that it wasfolded, probably during the Laramide Orogeny. Alongthis thrust an allochthonous slice of intrusive rock, appar-ently a leucotonalite, which now has a mylonitic folia-tion, was transported over rocks of the volcanosedimen-tary association described previously. Several Tertiarynormal faults displace the thrust; in some places adjacentto these later faults, the thrust and the rocks of the roadcut in general are drag-folded into nearly vertical posi-tions.

The basement-rock lithologies observed in thisroadcut will be easily identifiable as clasts in theGuanajuato Red Conglomerate and also as accidentallithics in the volcanic rocks of the Mining District.

We will retrace our route from this stop, returning tothe city of Guanajuato. Before arriving at our base hotel,we will take the Panorámica Highway heading southwestuntil we reach the dirt road near the School of Mines ofthe University of Guanajuato.

STOP 1-2: GUANAJUATO RED CONGLOMERATE AND ~49 MALAVAS EXPOSED IN A CUT ON A DIRT ROAD LOCATED CLOSE TO

THE SCHOOL OF MINES (UTM 14Q0264773;2326644)

At this site are outcrops of greenish andesitic lavas inter-calated with the Guanajuato Red Conglomerate (GRC).The conglomerate, with attitude N4W, 30SW, showsnoticeable variations in grain size, sorting, thickness ofindividual strata, and nature of the clasts. Near thePanorámica is an alternation of conglomeratic coarse-grained sandstones with fissile shales in strata from10 to30 centimeters in thickness. As we move to the west, thegrain size increases considerably and the thickness of thelayers goes up to about a meter, and in some layers nor-mal grading and/or the presence of paleochannels isapparent.

In this outcrop one also observes at least four lavaflows, each with an autobrecciated base and a vesiculartop. The flows vary from the massive type to the blockytype. The rock is generally fine-grained, with scarce phe-nocrysts of plagioclase and altered mafic minerals. At the



base of the sequence of flows we can see a tuff a little lessthan a meter thick, composed principally of coarse to fineash and oxidized mafic minerals. The tuff is concordantwith the underlying GRC. The lavas are completelypropylitized, turning them into green rocks very similarin aspect to the basement metalavas that we saw at Stop1-1. Aranda-Gómez and McDowell (1998) obtained a K-Ar age (whole-rock) of ~49 Ma on one of the outcrops ofandesite intercalated in the GRC. This age, together withthe paleontological ages reported by Edwards (1955) andthe K-Ar age of the Bufa Ignimbrite (~37 Ma; Gross,1975), permits us to know the age of deposition of theGRC, and therefore to have an idea of the time at whichthe Guanajuato Basin formed (Middle Eocene to EarlyOligocene). The GRC, which overlies these lavas, is richin large boulders of andesite, similar to the underlyingandesite or to the metalavas of the basal complex. It alsocontains clasts of felsic volcanic rocks of unknownprovenance and some fragments of limestone.

STOP 1-3: SUBSTATION CFE AND LA CUEVA. CONTACT OFTHE GRC WITH THE LOSERO FORMATION AND THE BASE OF

THE BUFA IGNIMBRITE (UTM 14Q0266028; 2323640)

We will return to the base hotel and from there we willtake the road toward the center of the city. We will crossGuanajuato heading toward the Silao exit. Before arriv-ing at the exit we will take the overpass to Pozuelos, inorder to get onto the Panorámica in its southern part. Wepass the ISSSTE Clinic and stop at the CFE electricalsubstation.

From this saddle (UTM 14Q0266028; 2323640),we can take in a panoramic view of Cerro de La Bufa, theplace from which the name of the Bufa Ignimbrite comes,on one side (south). On the other side (north), we can seethe city of Guanajuato from a point opposite to that ofStop 1-1. From this point we see the contact between theGRC and the overlying units, the Losero Formation andthe Bufa Ignimbrite. The contact is very clearly exposed,given the scarce vegetation and the marked color contrastbetween the units, dark red for the GRC and light greenand yellowish green for the Losero-Bufa package. Fromhere we can also observe the change in dip of the upperlayers of the GRC with respect to the lower layers. Thedip becomes gradually gentler upward, and at the top iseven almost concordant with the subhorizontal layers(~16 degrees) of the Losero Formation. This gradual

change in the dip is interpreted as a rollover fold in theTertiary sequence by Aranda-Gómez and McDowell(1998). These authors argue that the GRC and the vol-canic sequence were accumulated at the same time thatintense normal faulting was going on in the region.

From here we will go up to the contact area of thethree units (UTM 14Q0266471; 2323485). Then, we willwalk along the contact between the GRC and the Loseroto a place known as La Cueva (UTM 14Q0265958;23230009), that is a somewhat tabular excavation madein order to extract sheets and blocks of the LoseroFormation. Losero by reason of its characteristic greencolor and finely laminated layers with graceful dune bed-forms has been used as an ornamental stone in construc-tions around the region. The quarrying of the Losero is nolonger going on at La Cueva, and now it is a small chapeltraditionally visited during the Holy Week holidays.Owing to the workings of this quarry, the Losero isunusually well exposed, and it is possible to observedetails of the sequence of surge deposits in the Losero onmutually perpendicular surfaces, as well as the contactbetween the Losero surge beds and the Bufa Ignimbrite.

The GRC-Losero Transition: At this locality (UTM14Q0266157; 2323097) we have a well-exposed sectionwhich shows the transition from the GRC to the LoseroFormation and the contact between the Losero Formationand the Bufa Ignimbrite. The upper layers of the GRC arestrata of fine gravel and coarse sands with rather thinbedding (10-20 centimeters) which give way upward tored siltstone. The sequence continues with a mixed inter-val of green and red siltstone with cross-bedding, whichchanges in an almost imperceptible way to the entirelygreen layers of the Losero. Therefore the contact betweenthe GRC and the Losero is transitional and concordant.

Characteristics and origin of the Losero: Along the out-crops on this road, the Losero has a thickness that is vari-able, from 0 to 10 meters, and it consists principally of arhythmic sequence of surge layers with intercalations ofepiclastic deposits of well-sorted sands and silts. Thesedeposits form fine laminations with thicknesses from afew millimeters up to 3 centimeters. The surge layershave both dune and antidune forms (very low -amplitudeand long-wavelength dunes with well-developed stoss-side accretion and lee-side erosion of material). Thesefeatures mostly indicate transport from SE to NW,



assuming that the temperature of the pyroclastic currentfrom which they were deposited was above the boilingpoint of water, so that the crests of successive dunesupward in each duneform retreat toward the source. Thesurge deposits are composed of pyroclastic material thatvaries from very fine ash to lapilli. The uppermost part ofthe Losero includes some deposits of very fine ash inwhich normal grading can be seen. We think that many ofthe pyroclastic deposits described above accumulated inwater, possibly in ephemeral shallow lakes. This inter-pretation is based on the presence of layers of well-sort-ed sandstone and siltstone, and because we also occa-sionally observe traces of surge features in some of thesestrata. This interpretation is congruent with the lithologyof the uppermost portion of the GRC; however, it is alsopossible that these apparently detrital layers are in factthe planar facies corresponding to the sandwave faciesdescribed above.

The Losero-Bufa Contact: The Bufa Ignimbrite in manyplaces seems to overlie the Losero concordantly, but inothers it is easy to see layers of the Losero truncated bythe base of the ignimbrite. One can also see surfaces withraindrop pits in the upper layers of the Losero, whichimplies that there was a time lapse between the activitythat produced the Losero and that which produced theBufa Ignimbrite. On the other hand, there are noreworked deposits or paleosoils between the two units.Thus we infer that the Bufa Ignimbrite was emplacedshortly after the accumulation of the Losero surge layers.In fact, it is possible that the Losero and the Bufa are bothproducts of the same source and that the Losero repre-sents the initial phases of the paroxysmal eruption thatproduced the Bufa. The contact forms a planar surfacewith an average attitude of N30W, 18NE. The lack ofsoft-sediment load structures in the Losero suggests thatit already had considerable bearing strength at the timethat the great weight of the Bufa was deposited on top ofit.

Characteristics of the lower portion of the BufaIgnimbrite: In these outcrops at La Cueva the Bufa isapproximately 400 meters thick. We will only be able tostudy the lowermost part of the unit. The features dis-played by the Bufa in this section are those of an ign-imbrite with a great deal of kinetic energy, but little ther-mal energy, since the degree of welding is in general fair-

ly low. In the lower part of the Bufa, just above the con-tact with the Losero, is a zone approximately 2 metersthick which is rich in lithics and has flattened pumices orgreen fiamme. It changes gradually upward to a partiallywelded, lithic-poor zone several meters thick. This zoneis overlain by a zone with abundant hollow pits, whichrange in size from golf balls to soccer balls. These pitsresult from differential erosion of pumice clasts withrespect to matrix, the pumices in this case being moreeasily eroded. Above the pitted layer, the ignimbritechanges to a highly silicified zone gray in color withblack spots and patches of iron oxide. Silicification ispervasive in this zone, transforming it into a highly ero-sion-resistant rock which projects out over the lower, lesssilicified part making a prominent overhang. The originaltexture of the silicified zone was totally obliterated, andsecondary quartz is abundant. As primary minerals theignimbrite contains fairly abundant euhedral biotite, sani-dine and quartz.

As was mentioned at Stop 1-1, the Bufa Ignimbritehas great lateral variations in thickness within theDistrict, and outside of the District it seems to be absent.Within a horizontal distance of less than 5 kilometers itsthickness varies from a maximum of 400 meters here atLas Cuevas to a minimum of 0–10 meters at Cerro deSirena. It is possible that one or more of the curvilinearfaults that separate this outcrop from Cerro de Sirena (theAmparo and San Clemente faults and the northeasternbranch of the Veta Madre) form part of the northern mar-gin of a caldera associated with the Bufa eruption,although the source(s) of the Bufa are not known. Ashflows exposed near the intersection of the Aldana and ElBajío faults are thought to correlate with the CuatralbaIgnimbrite, dated at 30 Ma, and not with the Bufa.According to the work of Martínez-Reyes (1992), theCuatralba Ignimbrite is widely distributed west of theVilla de Reyes graben, as well as along the downthrownblock of the El Bajío fault (including two small outcropswithin the La Sauceda graben, SSE of La Cueva).Likewise the Calderones Formation, which has a verydistinctive lithology, has not been observed in areas out-side the block bounded by the Rodeo-Yerbabuena fault,the La Sauceda graben, and the La Sierra vein system.

We will return to the vehicles and take the PanorámicaHighway toward the La Olla Dam (to the east). This con-struction dates from the 1700’s, and for many years it was



the only source of water for the city. It also played animportant role in controlling floods (the city wasdestroyed several times by flooding). Upon arriving atthe dam, we will continue a bit further toward the monu-ment to Miguel Hidalgo and the square at the SanRenovato Dam.

Km 0.0: We will take the cobblestone road (which laterturns into a dirt road) toward El Cubo. Km 1.0: Contact between the GRC and the LoseroFormation. A few meters up the road the Bufa Ignimbritecomes into view. These contacts are exposed on bothsides of the road and in the cuts on the other side of theSan Renovato Dam. As we go further up the road we canlook down into the canyon on the right. At the bottom ofthis canyon you can see the green layers of the Loserounder the Bufa Ignimbrite. The road crosses a zone wherethe Bufa has an interval of crude columnar jointing. Here,as everywhere, the Bufa has been eroded to form spec-tacular cliffs.Km 1.4: The road crosses the Arroyo de Los Rieles. Wewill park in a spot in which the road widens next to a tinychapel.

STOP 1-4: CONTACT BETWEEN THE UPPER PART OF THE BUFAAND THE CALDERONES FORMATION. DISTAL FACIES OF THE

CALDERONES SEQUENCE IN THE ARROYO DE LOS RIELES(UTM 14Q0268222; 2324110)

We will walk upstream in the arroyo for a short distancein order to be able to observe the uppermost part of theBufa Ignimbrite and the lower part of the Calderonespyroclastic sequence.

The Bufa Ignimbrite here at the top of the unit ispink (color intensifies upward in the uppermost part) andvery fine-grained. It is weakly welded, but a bit indurat-ed due to silicification. Some white pumice clasts can beseen, but both pumicees and lithics (red aphyric rhyolite)are scarce. The contact between the Bufa and theCalderones is poorly exposed, but we will see it later inthis same stop, in a large roadcut uphill from the curvewhere the vehicles are parked.

The base of the Calderones (UTM 14Q0268369;2324216) is a well-stratified deposit, with individual lay-ers ranging from 5 to 30 centimeters in thickness. Thebottom 3 meters are characterized by relatively thick bed-ding (up to 0.3 meters) and by the presence of abundant

angular fragments of pale reddish purple dacite. Thesedistinctive clasts were probably derived from the domesof the Peregrina Dome Field, which we will visit in thelast stop (1-7) of the day. Other recognizable clastsinclude those from the GRC and some andesite chips.The high content of lithics and their angularity impart aroughness to exposed surfaces in this part of theCalderones sequence. We interpret this part of thesequence as the distal deposits of several thin pyroclasticflows. Above the clast-rich basal beds, the unit is com-posed of generally finer-grained and more finely lami-nated green layers (less than 30 centimeters thick, onaverage), with prominent cross-bedding. In these layersdune forms with stoss-side accretion of laminae are clear-ly exposed. Some of the dunes have pebble trains, anddune-regression patterns are compatible with a NE-to-SW transport direction. We interpret these beds as surgelayers; they are interbedded with layers that lack cross-bedding but are graded with respect to clast size, whichwe interpret as probable fall deposits. The colors of indi-vidual layers in this interval include both purplish red andgrayish green, with green predominating. One exception-ally fine-grained, planar-bedded interval about a meterthick may be a planar-facies surge deposit or a falldeposit composed of fine ash; its orientation is N45W,10NE. Further upstream are more pyroclastic flowdeposits with cross-bedding, and some thicker (0.5meters to several meters) ignimbrites. One such unit con-tains a feature typical of the Calderones distal facies, awedge of massive tuff occupying a paleochannel cut intostratified deposits. At this point in the arroyo, the pack-ages of ignimbrites form high ledges (3-5 meters) whichmake it difficult to climb further upstream. The entireCalderones section at this point is bright green, the char-acteristic color imparted to the unit by pervasive chloriti-zation of all original glass.

We will return to the El Cubo road and walk up itfor a couple of hundred meters in order to study a cut,which has a good exposure of the contact between Bufaand Calderones.

At UTM 14Q0268457; 2324077 we can see that thepyroclastic flows at the base of the Calderones filled abroad paleochannel developed in the upper part of theBufa Ignimbrite. Therefore the contact between theseunits is locally an erosional disconformity. Unlike mostchannel fillings, this one does not have any of the typicaldeposits, such as gravels and sands, nor is there any evi-



dence of a paleosoil on top of Bufa. The Calderones pyro-clastic flow deposits rest directly on the Bufa Ignimbrite;they consist of a series of thin relatively coarse-grainedlayers, slightly wavy, with cross-bedding and with lithicsfrom the Peregrina Dome and the GRC, similar to the lesswell-exposed basal beds in the arroyo. This initialsequence, which is seen here thinning and becomingfiner-grained toward the channel margins, probablyresulted from the initial blasts and/or surges related to thechannel-filling pyroclastic flow(s) which overlie it. Aboutmidway between the bottom and the top of the roadcut,there is a layer of large clasts at the base of a thicker (morethan 5 meters) pyroclastic flow. In total the Calderonessequence exposed in this cut is approximately 12 metersthick in the central part of the channel filling.

Km 3.9: Junction with the road to the Las Torres Mine.This mine is one of the most recent discoveries (Gross,1975) along the Veta Madre. It is also one of the largestand most modern operations in the District.

STOP 1-5: CONTACT BETWEEN THE CALDERONES PYROCLASTICROCKS AND THE CEDRO ANDESITE (UTM 14Q0268833;2323324)

About 100 meters west of the bus stop, the contactbetween the two units is exposed. In an interval of about12 meters, it is possible to see that at the top of theCalderones is a sequence (~3 meters thick) of thin layerscomposed of relatively crystal-rich tuffs with highlyvesiculated and intensely palagonitized glassy matrixmaterial. These yellowish brown layers are interstratifiedwith the more typical fine-grained green layers in thelower part of the transitional interval. Just at the base ofthe Cedro lava flows is a horizon of very thin layers withwell-formed dessication cracks. Above these layers restsan andesite flow with well-developed spheroidal weath-ering and small pillows (?), which changes graduallyupward into massive andesite. We interpret the observedfeatures at the flow base as evidence that it interactedwith water. In a small quarry located to the north of thebus stop it is possible to see that the first andesite flow atthe base of the Cedro is overlain by another flow withcharacteristics similar to those described above.

Km 4.5: Looking eastward from this point you can seethe village of El Cedro in the bottom of the canyon; the

name of the Cedro Andesite was taken from this location.In the same direction, on the other side of the valley, isthe Sierra de La Leona (i.e. the hill with the cross on topat the northern end of a long ridge). The ridge is com-posed of red layers of the GRC at its base and of LoseroFormation and Bufa Ignimbrite along the summit. TheCenozoic sequence is repeated by the Veta Madre fault,which here has considerable throw (hundreds of meters),bringing the Cedro Andesite down against the GRC (Stop1-6).

Km 4.7: Immediately west of the road there is anembankment of material excavated in the CedroAndesite. The material quarried from here is used to con-tain the tailings ponds behind the adjacent dam.Km 5.0: Here we will park at the side of the road toinspect the Veta Madre fault zone.

STOP 1-6: THE VETA MADRE FAULT AND THE RELATIONSHIP

BETWEEN CALDERONES AND CEDRO FORMATIONS (UTM14Q026497; 2324366)

The Veta Madre crops out here along the road cut. In thisregion the Veta Madre branches, surrounding the ridgecapped by the Chichíndaro Rhyolite. In the road cut thefault has an approximately E-W orientation, but on theregional scale it strikes NW-SE. Walking along the road-cut, you can see tectonic contacts between the GRC andCalderones, and Calderones with Cedro, even though therocks in this outcrop are intensely altered and brecciated.

Km 5.8: North of this point we have Cerro Chichíndaro,which is crowned by a dome or domes of rhyolitic lavawith a K-Ar age of ~32 Ma (Gross, 1975), and it is thetype locality of the Chichíndaro Rhyolite. The age of theVeta Madre fault is bracketed between the age of theChichíndaro Rhyolite and the age of the mineralizationalong it (K-Ar, ~29-27 Ma; Gross, 1975).Km 5.7: We cross the pass between Cerro Chichíndaroand the hill to the east. The Las Escobas fault crosses thispass; this structure puts the basal Mesozoic complex intectonic contact with the redbeds of the GRC.Km 6.6: Intersection of road to El Cubo with road northto Peregrina. We turn left onto the road that goes to thePeregrina Mine. Km 9.2:We will stop here, pulling off the road near thetop of the rise.



STOP 1-7: THE PEREGRINA DOME COMPLEX AND ITS RELA-TIONSHIP TO THE CALDERONES SEQUENCE (UTM14Q0271496; 2326250)

From this vantage point we can see, first, a wicked goodpanoramic view of the Peregrina Dam and the northwest-ern part of the Peregrina Dome Complex. Second, out-crops of the dacitic to latitic phase of the Peregrina and ofa small block-and-ash flow derived from the collapse ofone of the domes. Study of these outcrops reveals adynamic scenario of growth and destruction of smalldomes, repeatedly, which produced several block-and-ash flows that filled paleochannels and/or a paleograbenjust to the south of these domes. The contact betweenthese two units, Peregrina domes and Calderones tuff-breccias and other proximal deposits, seems to be a com-plex and repetitive one, as the pyroclastic flows producedby the Peregrina domes are intercalated with, and indeedform part of, the Calderones sequence. South of here,many of the flow units observed in the Calderones arerich in angular lithic fragments of this dacitic phase of thePeregrina, as we will see in the Arroyo de Los Silvestres,later in the trip (Stop 2-4).

Looking toward the Peregrina Dam, you can seetwo small hills on the skyline. In the sides and tops ofthese hills, flow foliation typical of domes can be seen;that is to say, there is a concentric pattern of pseudostrat-ification that gradually becomes steeper from the exteri-or to the interior of the dome. This flow-banding is near-ly vertical around the Peregrina Dam.

Where we are standing, we can see in detail the con-tact relationships between the external part of one of thedomes and the associated pyroclastic deposits. We can seethat the rock making up the internal part of the dome is com-posed of a pale grey porphyritic dacite, with well-developedflow-banding. It has phenocrysts of plagioclase and quartzin a devitrified groundmass. The outer part of the dome con-sists of a totally devitrified carapace made up of the brec-ciated equivalent of the grey porphyry in the interior. A fewmeters beyond the outcrop of the intact dome rocks, theproximal facies of the dome collapse, with large clasts of theporphyry, passes transitionally into a block-and-ash flowwhich in turn passes into laterally into more typical pyro-clastic deposits of the Calderones sequence.

DAY 2: PYROCLASTIC SEQUENCE OF THE CALDERONESFORMATION AND POSSIBLE SOURCE VENTS.

We will go out of the base hotel headed for the La OllaDam and will take the road to El Cubo. Refer to Figure 6for locations of stops.

Km 0.0: Beginning of the road to El Cubo.Km 6.6: Road intersection. We take the road to the left,headed for Peregrina.Km 9.5: Gate to the Peregrina Mine.Km 10.1: Road intersection. Turn left, toward Cerro Altode Villalpando.Km 10.4: Road intersection. Turn to the right, going upa steep hill.Km 11.4:We will park as far off the road as possible andwalk a short distance up the road for an overview of theDistrict.

STOP 2-1: CERRO ALTO DE VILLALPANDO. RING DIKE WITH

CALDERONES VENT FACIES AND A PANORAMIC VIEW OF THE

CALDERONES SEQUENCE FROM ITS PRINCIPAL POINT OF ORI-GIN. (UTM 14Q0273434; 2326116)

In this long roadcut just beneath the summit of the hill,we can observe a large ring dike which is composed ofCalderones tuff and tuff-breccia. The dike cuts a daciticto rhyolitic dome that forms the greater part of Cerro Altode Villalpando and that we consider part of the Peregrinadome complex. Overlying the Peregrina dome, and like-ly cutting it is the Chichíndaro Rhyolite, which forms thehighest part of the hill. The road cuts obliquely across thering dike, affording us an excellent exposure of its con-tacts and its interior along a traverse of several hundredmeters; the true width of the dike is at least 50 meters.The contact near where we left the vehicles is subverticaland irregular, with well-developed shear surfaces bothwithin the dike and in the host rock. In other locations wehave seen breccias and cataclastic rocks in zones up to 8meters wide along the dike margin. These zones are madeup of well-preserved fragments of the Peregrina domerocks in matrices of Calderones tuff. The fragments in theinterior of the dike are heterolithologic, including clastsderived from the Mesozoic basement, such as phyllite,argillite, quartzite, meta-andesite and calcareous rocks, aswell as clasts derived from the Cenozoic cover, such asthe GRC and altered rhyolitic to dacitic rocks, presum-ably from the Peregrina domes. All these clast typesexhibit considerable size variation, ranging from a fewmillimeters to several meters in diameter. Extremely



large blocks of phyllite of the Esperanza Formation arepresent in the dike, suggesting that this formation is pre-sent at shallow depths below the surface; one such blockcan be seen behind the small white building. This pyro-clastic dike probably served ad the principal ventingstructure for the Calderones Formation, and it can beinterpreted as a partial-ring fracture bounding a caldera.Because its outcrop is limited to the northeastern quad-rant of the putative circular boundary, a partial or trapdoor, morphology is suggested for the Calderonescaldera.

After examining the dike, we will avail ourselves ofthe excellent panoramic view, looking westward, of theCalderones sequence. Immediately below the road is anarea with a noticeable reddish brown color, which formsa band of low, rounded hills in the vicinity of the Tiro deSan Lorenzo. These hillocks correspond to the intra-caldera facies of the Calderones sequence, which consistsof a collapse megabreccia made up of enormous clasts ofEsperanza Formation (phyllites and schists) in a scantmatrix of Calderones tuff. In addition to the more abun-dant Esperanza-dominated megabreccia, there are isolat-ed outcrops of Peregrina-dominated megabreccia. Wewill be looking at these outcrops at the next stop. In arough way, the map pattern of this megabreccia unit isparallel to the outcrop of the ring dike, having the samecurvature.

In the middle distance, beyond the megabreccia, wecan see a prominent ridge with several summits held upby thick, apparently massive, layers. The largest andhighest of these summits is Cerro de La Loca, where wewill go for the third stop of the day. The summits are sep-arated from each other by minor faults striking approxi-mately NE-SW, with their northwestern sides down-thrown. Cerro de La Loca itself, like the entire La Locaridge, is made up of the proximal facies of the Calderonessequence, with a series of relatively thick and volumi-nous ignimbrites at the top. The ridge beyond the LaLoca ridge is the La Leona Ridge, whose summit iscapped by a thick section of the Bufa Ignimbrite. TheBufa is in tectonic contact with the Calderones along theLa Leona normal fault, whose trace follows the base ofthe dip slope of the ridge. Unlike most of the normalfaults in the District, it dips to the NE, toward us; the lowhills and ridges on the downthrown block are underlainby the Calderones and Cedro Formations. We will seethis part of the Calderones sequence (the medial facies),

bounded by the La Leona and El Cubo faults, at thefourth stop of the day, on a traverse following the Arroyode Los Silvestres. Even further in the distance we can seea series of small tilted mesas on the other side of the LaLeona ridge. These mesas contain the distal facies of theCalderones, near the village with the same name. The hillknown as Cerro Coronel and the crags called Las DosComadres stand out as landscape features, and we willvisit them at the fifth and sixth stops of the day.

We will return by the same road to the exit from the mine.

Km 12.8: At the first intersection we turn left.Km 15.2: Mill and flotation plant of the El Cubo Mine.We take the road to the left, toward the valley where theTiro de San Lorenzo is located.Km 16.3:We will park at the side of the road near a smallditch.

STOP 2-2: TIRO DE SAN LORENZO: COLLAPSE MEGABRECCIA(UTM 14Q0273297; 2325698)

At this stop there are outcrops next to the road and in thesmall ditch which show features that we interpret as theintracaldera collapse megabreccia related to the paroxys-mal eruption of the Calderones sequence. The brecciahere includes fragments of various sizes, from at least 10meters down to a few centimeters; some blocks make upentire outcrops. By far the most common lithology at allfragment sizes is phyllite of the Esperanza Formation.The matrix is difficult to see, because the deposit isdeeply weathered and the fine-grained fraction has most-ly been converted to a yellowish brown to reddish brownsoil. Upon careful inspection, however, one can observethat this soil consists of materials of the same type as thelarger blocks, pulverized to the size of sandy grit or finegravel. The deposit as a whole is quite altered by theaction of hydrothermal solutions that permeated it, pro-ducing abundant cross-cutting tiny veinlets of quartz andrendering it susceptible to intense weathering. All of thelarger blocks are pervasively fractured and some fracturedomains show a jigsaw-puzzle pattern. Blocks of theEsperanza Formation here have a somewhat better devel-oped schistosity than that observed at Stop 1-1. In oneespecially large block we see the typical stratigraphy ofthe Esperanza Formation: metalava (greenschist), sand-stone (quartzite), and metamorphosed shale (schist).



Another large block, internally a phyllite, has a cataclas-tic margin at its contact with Calderones tuff. Near theflotation plant that we passed on the road below this stop,we find another type of component of the megabreccia,which is a megablock approximately 10 meters long offinely flow-banded dacitic or rhyolitic Peregrina domerock. This block shows a great internal complexity, per-haps inherited from the original dome, which includes azone of jigsaw-puzzle fracturing, a highly sheared zone,and a zone of fault gouge in contact with a clastic (?) orpyroclastic (?) dike with fragments of many differenttypes in a matrix of Calderones tuff. Above this dike, inits apparent hanging wall, we see Peregrina dome brecciaand very fine-grained Peregrina dacite or rhyolite, whichappears to be a devitrified glass.

From this stop we leave the area of the Peregrina Mineand pass through the same control gate that we crossedearlier. We take the road back toward Guanajuato, but atthe intersection with the road to the El Cubo Mine weturn to the left. 0.9 km beyond the turn we will stop nextto Cerro de La Loca in order to climb up it and look at asection of the Calderones sequence.

STOP 2-3: CERRO DE LA LOCA: PROXIMAL FACIES OF THE

CALDERONES SEQUENCE. TRANSITION FROM THIN IGNIMBRITESAND INTERBEDDED BRECCIAS UPWARD TO THICK CAPPING IGN-IMBRITES. INTERACTION BETWEEN DIKES AND BEDDED

DEPOSITS (UTM 14Q0272690; 2324213)

At the base of the section exposed here is a package ofpale green thin-bedded (5-15 centimeters) deposits com-posed of fine-grained and well-sorted clastic materialwith a considerable ash component. The strata have wavyshapes, suggestive of gentle folding, draping, or differen-tial compaction over an irregular surface that does notcrop out at the surface, and they lack cross-bedding.Some beds have rather poorly developed normal gradedbedding, while other beds exhibit flow texture. We thinkthese beds are best described as a rhythmic sequence ofmixed fall and surge deposits, which were deposited invery shallow water. The sequence passes upward into aseries of thin to medium-thickness (15–30 centimeters)pyroclastic flow deposits, which include surges, minorignimbrites and breccias of uncertain origin, possiblymedial portions of block-and-ash flows. Here, as in otherparts of the Calderones Formation, almost all the deposits

contain fiamme converted to dark green chlorite. Thereare many interesting color variations in these beds – insome layers only the shard-rich matrix and the fiammeare green; other layers have green fragments in a grayishmatrix, and still other layers have both green lithic frag-ments and green fiamme/matrix. This series of beds con-tinues for several meters, until a contact is reached witha package of thin cross-bedded layers accumulatedalmost exclusively from pyroclastic surges. The surgelayers are overlain by a series of green thinly-beddedsandstones which are in turn overlain by another packageof surge-bedded tuffs, rich in large (up to 35 centimeters)angular lithic fragments of various types (GRC, veinquartz, granite, etc.).

The pyroclastic rocks that make up the summit ofCerro de la Loca are a group of thick ignimbrites whichform the culminating sequence of this part of Calderones.We interpret this entire package as a single cooling unit,accumulated in at least four pulses of emplacement orchanges in the nature of the pyroclastic density current.The degree of welding is comparable to the strongestwelding observed in any part of the Calderones; thin sec-tions reveal the presence of spherulites surrounding thelithic fragments, embedded in a matrix with well definedeutaxitic texture. Each of the emplacement units has dif-ferent lithic fragments; for example, the lower most unitcontains sparse small fragments of limestone as well asmany slightly larger angular fragments of Peregrinadacite, while the upper three units have fragments ofphyllite but lack limestone. The third emplacement unit,up from the base, has scarce lithics of small size. Theuppermost unit is characterized by an abundance of verysmall lithics that varies little from base to top. It is per-haps the unit with the highest lithic content of the fourunits that make up this composite ignimbrite. The lithicsare principally of reddish-brown lavas and glassy whitelavas. Pumice is not apparent, but it may have been com-pletely masked by secondary silicification. This upper-most unit has two interesting physical features that arenot present in the lower ones. A prominent pseudo-strati-fication seems to reflect progressive aggradation result-ing from instability of the pyroclastic current (Branneyand Kokelaar, 1992). The second interesting feature is thepresence of broad shallow pits in the uppermost surface,arranged in a regular grid, which may be the tops of fos-sil fumaroles. In total, the thicknesses of the four ign-imbrite emplacement units add up to about 50 meters.



Our interpretation is that these ignimbrites were eruptedduring the paroxysmal phase of the eruptions that formedthe Calderones sequence. The lithic-bearing surgedeposits at the base of these summit ignimbrites corre-spond at least in part to Layer 1 of Sparks and others(1973); some breccia sheets intercalated with the lowermost of the obvious surge deposits may represent a dif-ferent mechanism of transport and deposition.

A subtle Layer 2a, with shearing features and deple-tion of larger grains, can be observed at the base of thefirst emplacement unit, and Layer 2b makes up the rest ofthe unit. Layer 3 deposits appear at the top of the firstemplacement unit as a thin zone of laminated layers,although it is difficult to confirm its identity because ofthe effects of secondary processes, and because it is prin-cipally exposed in the middle of the summit cliff. In addi-tion, the putative Layer 3 of the lower unit is directlyoverlain by an interval of thin cross-bedded layers, whichalmost certainly constitute the Layer 1 surge deposits ofthe next emplacement unit. We have not determined theprecise compositions of any of the emplacement units inthis culminating sequence, but given the relatively high-sodium content of the plagioclases inferred form petrog-raphy, they could well be dacites.

In many places Calderones is cut by andesitic dikesthat are thought to be equivalents, and probably feeders,of the Cedro Andesite. The interaction between the dikesand the apparently still water-bearing pyroclasticdeposits of the Calderones locally gave rise to a secondgeneration of pyroclastic products of phreatomagmatic tostrombolian type. We can observe this phenomenonabout in the middle of the section, where deposits of ashand breccias lie unconformably over the Calderonessequence adjacent to an andesitic dike. The dike also pro-duced thermal alteration in the surrounding layers ofCalderones, indurating them and making them moreresistant to erosion than the parts not altered by the dike.The result of this process is that tabular erosional formsof the baked Calderones stand up above ground level onboth sides of the deeply eroded dikes.

At this locality we will also see a deposit that weinterpret as a lahar. This curious layer covers the strati-fied layers of Calderones sequence with considerable dis-cordance.

The lahar apparently was emplaced long after theoriginal emplacement of the Calderones (possibly even inRecent times), and was possibly caused by the conjunc-

tion of intense faulting and abundant rain in this region.Boulders of the purple dacite of the Peregrina domeswere caught up in remobilized non-indurated falldeposits or other fine-grained pyroclastic layers, formingmudflows that flowed along small paleochannels thatmay have resulted from ground cracking during smallearthquakes.

After inspecting the Calderones section and other fea-tures exposed at Cerro de La Loca, we will take our lunchbreak and enjoy the views in all directions, and then willreturn to the vehicles.

STOP 2-4: ARROYO DE LOS SILVESTRES: BOULDER BEDS,PYROCLASTIC FLOW LAYERS AND SURGE LAYERS IN THE MEDIAL

FACIES OF CALDERONES

At this location we are a bit downstream from yesterday’slast stop (1-7), on an elongate fault block situatedbetween two large normal faults with opposite dips. Aswe face downstream (south), the SW-dipping El Cubofault is to our left, and the NE-dipping La Leona fault isto our right. Thus we are in a major graben which mayhave formed before or during the Calderones eruptions,although both faults have had further displacement afterthe Cedro lava flows were emplaced. Evidence for suchmovement can be seen in the drag folding of theCalderones beds along the La Leona fault and in the factthat the Cedro Andesite was tilted to the NE along the ElCubo fault. We will walk a short distance down stream inorder to look at a representative portion of the medialfacies of Calderones. There are a number of repetitions ofthe typical sequence, which contains (from the baseupward): surge layers of slightly coarser grain size andmore pronounced, larger amplitude duneforms than aregenerally seen in Layer 1 deposits. These are overlain bythick layers (up to three meters) with large boulders invariable amounts of tuffaceous matrix. These, in turn, aresucceeded by ash-flow deposits with abundant conspicu-ously flattened and ramped pumices and abundant smalllithics of various rock types; the sequence is capped by apackage of very fine-grained surge layers with unusuallylow-angle cross-bedding. As we go downstream (up-sec-tion), the boulder layers vary from nearly monolitholog-ic accumulations of Peregrina dome rocks to mixedassemblages containing more and larger GRC bouldersand a significant quantity of andesite (Mesozoic La Luz



meta-andesite and/or GRC hydrothermally alteredandesite). Near the point where we enter the stream wecan see a number of interesting features associated withcomplex branching Cedro Dikes.

STOP 2-5: CALDERONES VILLAGE: DISTAL FACIES OF THE

CALDERONES FORMATION AND EVIDENCE OF ITS

PHREATOMAGMATIC INTERACTIONS WITH CEDRO DIKES

In this locality, far from its known sources, the compo-nents of the Calderones sequence are more varied thanthose we have seen in its proximal and medial facies(stops 2-1 thru 2-4). The section contains ignimbrites thatare thinner and generally finer-grained (there are someexceptions) than their counterparts closer to the sourcearea. There are complicated interactions with the Cedrodikes; we will inspect the peperites and other interactionfeatures along the contacts between the rising dikes andthe uppermost, still water-bearing, tuffs of theCalderones. There are lenses of phreatomagmatic tuffsinterbedded with magmatic tuffs, as well as someandesite bodies that could be invasive lava flows, nearthe dikes.

STOP 2-6: CERRO CORONEL: ENIGMATIC CAPPING UNIT OF

THE CALDERONES FORMATION AND A SUMMARY OF THE MODELFOR THE DEVELOPMENT OF THE VOLCANIC PACKAGE WITHIN

THE DISTRICT

We park the trucks close to the Humboldt shaft and wewill walk upslope looking at the upper part of theCalderones section. Numerous ignimbrites with spectac-ularly flattened fiamme in ashy matrix material areinterbedded with increasingly thicker intervals of surgedeposits. This section probably resulted from depositionin distributive channels and pyroclastic fans in the distalrunout region of the flow currents, and the pyroclasticmaterial was accumulated in water. The thicknesses ofthe layers become smaller near the top of the section. Thefinal two meters of these deposits beneath the summitcliff of Cerro Coronel display a wide variety of structuresand a delicate style of the lamination. These layers are aseries of deposits of high-energy surges, that precededthe emplacement of a large ignimbrite, which caps thesequence and form pronounced cliffs. These capping ign-imbrites form at least two flow units, with notoriousfluid-escape channels with centimeter-scale spacing at

their tops. Another characteristic is the high lithic contentof this deposit (more than 30% of the deposit in someplaces). As for the lithologic nature of those fragments(the overwhelming majority are of a moderately to high-ly welded [?] felsic rock with fine flow banding). Thematrix of the ignimbrite is generally poorly indurated ashnow completely devitried, and very little pumice. Wehave an alternative explanation for this capping layer.With respect to its position along the ‘phreatomagmatic-magmatic spectrum,’ it is possible that it was formed byphreatomagmatic eruptive processes rather than by pure-ly magmatic processes.

Tentative summary of the model for the formation of thevolcanic sequence in the District.

If any one thing is clear, it is that the Calderones is defi-nitely a volcanic unit. It is not a sandstone or a conglom-erate made up of detrital fragments derived from olderunits, which were transported into the depositional basinfrom sources outside the Mining District, solely bystream action, as was originally proposed when it wasstudied by Echegoyén (1970). Precisely how much re-working there was of primary volcanic deposits remainsan important question. Our present model has several ele-ments:

From the first surges of Losero to the final andesiticflows of Cedro, the volcanic products of the District wereprobably associated to a shallow magma chamber.Magma formed as a consequence of subduction and roseto a high crustal level in the region during a part of theearly Tertiary. At the time of the volcanism the rate ofregional extension remained high.

The eruptions that produced the Calderones were ofseveral different volcanological types. They include thegrowth and collapse of domes, high-mass-flux eruptionsfrom the ring dike, and minor phreatomagmatic eruptionsrelated to the rise of the Cedro dikes into the recentlydeposited tuffs of the upper member of the Calderones.Because of the wide variety of eruptive mechanisms, aswell as the variety of environments of transport anddeposition, the deposits are varied with respect to grainsize, fragment shape and angularity, relative proportionsof accidental and juvenile components, textures, struc-tures, thickness of layers, and other parameters.

It seems probable that the Calderones eruptionsbegan with the rise of the small dacitic domes in the



northwestern part of the Peregrina Dome Field.Successive collapses of these domes produced block-and-ash flows that were emplaced both around the domesand within recently formed grabens and/or river valleys.Events in the northeastern part of the Peregrina DomeField followed these early collapses without much of anintervening quiet interval. Initially, some larger and moresilicic domes were emplaced thru and atop rocks thatincluded the Esperanza Formation, the GRC and the BufaIgnimbrite. It is possible that one or more of these domescontained large roof pendants of the EsperanzaFormation. The “interior” portion of this complex ofdomes, wallrocks and roof pendants (interior with respectto the incipient caldera) collapsed along a fracture coin-cident with or parallel to the presently exposed ring dike,producing the megabreccia with its zones rich inEsperanza fragments or Peregrina fragments, respective-ly. One good candidate for the scarp produced in this col-lapse might be the Veta Falla de Villalpando (J.Echegoyén, pers. comm., 2002). It seems clear that thering dike was a very important source of the ignimbritesof the Calderones sequence, especially those along theCerro de La Loca ridge. It is also possible that zonesalong the trace of the ring dike formed initially as openfissures of the extensional type, related to the principalfracture along which the caldera wall collapsed to formthe megabreccia. Calderones tuffs erupted from othervents might have filled these zones from above.

Almost all the deposition of the CalderonesFormation must have occurred in shallow waters.Conversion of all the glassy materials to chlorite musthave been caused by the original heat of the pyroclasticfragments, rather than occurring much later, as a conse-quence of hydrothermal alteration of cold pyroclasts byrising hot fluids. It is difficult to know for certain howmany of the individual eruptions of Calderones tuffswere phreatomagmatic in nature; some of the thin-bed-ded and very fine-grained deposits may be the result ofphreatoplinian eruptions.

Some important things to investigate in the futureinclude the nature of the southeastern portion of the ringdike, the nature of the chloritization (isochemical or allo-chemical) of the glassy fragments, the timing of dis-placement along the major faults of the District, especial-ly the La Leona Fault, and the details of intrusive struc-tures, compositions and relative ages of individual bodieswithin the Peregrina Dome Field.

DAY 3: THE FINAL STAGES OF THE SIERRA MADREOCCIDENTAL ARC (LATE OLIGOCENE) AND THE BEGINNING

OF THE TRANSMEXICAN VOLCANIC BELT (MIOCENE) IN THEREGION BETWEEN GUANAJUATO AND SAN MIGUELALLENDE

Refer to Figures 8 and 9 for locations of some of thestops.

We will leave from the lobby of the base hotel.From here we will hear toward the exit to JuventinoRosas and San Miguel Allende. The measured distance inkilometers begins at the Santa Fe de Guanajuato Glorieta(traffic- circle/rotary/roundabout) in front of the HolydayInn Express hotel.

Km 0.0: Glorieta Santa Fe de Guanajuato. We take thehighway to Juventino Rosas and San Miguel Allende.Km 20.3:We stop at a prominent roadcut. The road herehas narrow shoulders; please be especially careful aboutthe traffic, which is both fast and heavy.

STOP 3-1: FLOW FOLDS IN THE CHICHÍNDARO RHYOLITE

In this road cut we see intense folding in the mid-Tertiary rhyolites. This deformation is not of tectonicorigin; its origin is syngenetic with the emplacement ofthe lava. Because of its high silica content, this lavamust have been very viscous, and upon moving acrossthe surface of the earth, it must have been deforming ina complex way. Almost all the original glass is devitri-fied and hydrothermally altered zones are common.Tension cracks in fan-like arrangement are sporadical-ly visible in the crests and troughs of some of the flowfolds. Vapor-phase topaz occurs in some outcrops ofthe Chichíndaro rhyolite (but not in this particular one)along some of the flow-bands. These folds and the sub-vertical flow foliation are very common close to thecenters of emission of the lava, which in this regioncommonly forms large endogenous domes and tholoidsor coulees. This particular type of rhyolite is commonand abundant in the extreme southern part of the MesaCentral. Along with low to medium grade felsic ign-imbrites they constitute the principal products of SMOvolcanism in the region (Aranda-Gómez and others,1983). Labarthe and others (1982) have estimated athickness of ~1,000 meters for the mid-Tertiary vol-canic sequence (K-Ar: 30–27 Ma) near the city of SanLuis Potosí.



Km 39.0: Intersection of the highway to San MiguelAllende. We turn north and park the vehicles some fivehundred meters beyond the intersection.

STOP 3-2: PANORAMIC VIEW OF THE VOLCANIC SEQUENCE OF

THE MESA DE SAN JOSÉ DE ALLENDE (LATE OLIGOCENE TOMIOCENE). HIGH-STAND PORTION OF THE FLUVIOLACUSTRINEBASIN OF THE RÍO LAJA AND DRAG FOLDING OF BASIN CON-GLOMERATES ASSOCIATED WITH NORMAL FAULTING (UTM14Q0289371; 2311238)

At this locality we will discuss the sequence exposed inthe Mesa de San José de Allende, which can be seen inthe distance to the southeast of us. We will also take aclose look at the highway cut, which exposes sedimentsaccumulated in the marginal portion of a wide fluviola-custrine basin that extends from here across a broad areain central Mexico (Figure 3).

The Mesa de San José de Allende is located in thesouthernmost part of the Sierra de Guanajuato. In thispart of the sierra, the southern boundary of the SierraMadre Occidental province and the TransmexicanVolcanic Belt province overlap, so that it is possible tofind volcanic rocks of both provinces in this sector. Cercaand others (2000) mapped and dated the units that makeup this mesa in particular and the southern part of theSierra de Guanajuato in general. The units exposed in themesa section unconformably overlie the Cedro Andesite(sensu lato), which crops out in small windows at theedge of the mesa. A K-Ar age of 30.6 Ma was obtainedon andesite collected at this site. Above the Cedroandesite is a sequence of ignimbrites typical of the SMO;that is to say, large-volume felsic ignimbrite sheets ofbroad lateral extent. Within the mesa section two of theseignimbrites are exposed, the lower one having a K-Ar ageof 23.1 Ma and the upper one yielding an Ar40/Ar39 ageof 22.4 Ma. The mesa is crowned by andesitic lavaswhose ages (13.2- 13.8 Ma) has been interpreted as theinitial eruptive phases of the Transmexican Volcanic Belt(Cerca et al., 2000).

In the roadcut you can see a normal fault which putsthe basin gravels into contact with the Cedro andesite.These gravels are in the hanging-wall block of the fault;they are relatively coarse-grained and clast-supported.They show an open synclinal form, which we interpret asa big drag fold the downthrown block of a down-to-the-basin normal fault. The clasts are derived from the

Oligocene volcanic sequence; principally they are rhyo-lites and andesites ranging from subangular to subround-ed in shape. Clasts of andesite predominate close to thefault trace. Farther away from the fault the most commonlithology, present as large (0.3 meters) to small (0.05meters) subrounded clasts, is a reddish, nearly aphyricfelsic ignimbrite. Small subangular green chert clasts arealso common.

At this site we are in the highest part of the Río Lajafluviolacustrine basin (Figs. 2, 3), far from the basin axis,possibly in a depositional environment transitionalbetween piedmont-type alluvial fans and high-standlakeshore zones. We think the gravels were derived byrapid erosion of the footwall block of the fault and weredeposited very close to their source, at the foot of thefault scarp.

Km 49.5: Turnoff toward the village of Peña Blanca. Wewill park by the roadside.

STOP 3-3: SUCCESSION OF GRAVEL AND SAND DEPOSITS ACCU-MULATED IN ALLUVIAL FANS IN THE HIGHER PART OF THE RÍOLAJA FLUVIOLACUSTRINE BASIN (UTM 14Q0297588;2314632)

Intercalated gravel and sand deposits are well exposedalong the road cuts of the highway. These deposits werederived principally from the Oligocene volcanicsequence. Approximately 50% of the sequence isformed by lenses of gravel, some of which appear tohave been deposited in well-defined fluvial channels.Clast size is, on average, about 5 centimeters, consider-ably less than that observed at Stop 3-2, and clast shapeis more rounded. These possibly represent fluvialdeposits accumulated in braided distributary streams,which transported distal alluvial-fan deposits downtoward the central part of the basin. The gravels occu-pied shallow channels, and the sands and silts weredeposited in adjacent pools. Looking in the direction ofPeña Blanca, we can see a resistant unit with crudecolumnar joints. This unit is the San Nicolás Ignimbrite(K-Ar, sanidine, 24.8+/- 0.6 Ma; Nieto-Samaniego andothers, 1996), which is found here intercalated withgravels similar to those in the road cut.

We continue on the highway to San Miguel Allende. Aswe descend toward the Ignacio Allende Dam, we can



observe that the average grain size in the gravel horizonsis continually diminishing. At the same time, the relativeamount of sand in the deposit is increasing notably.

Km 63.0: Village of La Ciénega (UTM 14Q0306846;2311514). Along this part of the highway there is a smallroadcut where true lake sediments are exposed. West ofthe village there are more extensive outcrops of this sameunit. These rocks are made up of fine-grained felsic pyro-clastic material and appear as thin-bedded white silts andclays, in layers 5–10 centimeters thick. Some layers dis-play concentrations of small pumice fragments (lapilli).Concretions and wavy layers of light brown to yellowishbrown chalcedony are common in this sequence. Thestratigraphic position of these layers in the larger fluvio-lacustrine sequence of the Río Laja Basin is uncertain.Some lacustrine sediments are known to underlie and beinterdigitated with products of the Palo HuérfanoVolcano (Stop 3-8) while other lacustrine layers areknown to contain an early Pliocene fossil fauna(Kowallis and others, 1998). Therefore we conclude thatin the Río Laja sequence, lacustrine deposits must occurat several levels with different ages. This is what onewould expect, given that the basin was formed as a resultof tectonic activity in several known pulses of differentages. It is logical to suppose that large lakes periodicallyappeared and were destroyed, perhaps in different partsof a large complex basin.Km 73.2: Ignacio Allende Dam. The curtain of the damwas constructed in a narrow canyon excavated in asequence of unusually thick andesite lavas. In the roadcuts are outcrops of the Allende Andesite (K-Ar, whole-rock, 11.1+/- 0.4 Ma, Pérez-Venzor and others, 1996).In the canyon of the Río Laja the Allende Andesite iscut by andesitic domes. According to Cerca and others(2000), the domes do appear to be in intrusive contactwith the Allende Andesite, so they must be younger than~11 Ma.Km 77.2: Intersection of the highways San MiguelAllende-Comonfort and San Miguel Allende-Guanajuato. Eastward from this point, in the hill withthe microwave towers, the Allende Andesite uncon-formably underlies products of the Palo HuérfanoVolcano.Km 81.5: We will park on the right-hand shoulder of theroad for an overview of part of the San Miguel AllendeVolcanic Field.

STOP 3-4: VIEW OF PALO HUÉRFANO VOLCANO; PRODUCTSOF PALO HUÉRFANO ABOVE ALLENDE ANDESITE (~11–12 MA)AND INFERRED AGE OF THE SAN MIGUEL ALLENDE FAULT.

From this point looking south from the east side of thehighway, one can enjoy a good view of the PaloHuérfano Volcano and the primary dip of its productstoward the west. Immediately to the right of the high-way you can see a mesa tilted to the east. The base ofthis mesa contains rocks of the basal Mesozoic com-plex. The rimrock at the top is the Allende Andesite.The Allende Andesite was dated by Pérez-Venzor andothers (1996) at about 11 Ma (K-Ar) and by Cerca andothers (2000) at 12.3 ± 0.3 (K-Ar). The andesite is amicroporphyritic rock with a very fine-grained ground-mass. The phenocrysts account for less than 5% of therock; they are hypersthene and augite, which rarelyreach even a millimeter in length. This contrasts greatlywith the texture and mineralogy of the products of thePalo Huérfano Volcano, which are always porphyriticand invariably contain phenocrysts of plagioclase sev-eral millimeters long. The precise location of the vent(s)from which the Allende Andesite was erupted is as yetunknown.

The interpretation of Pérez-Venzor and others(1996) is that the Allende Andesite is independent of thePalo Huérfano Volcano and pre-dates the latest move-ment on the San Miguel Allende fault. Similar fine-grained andesite mesas underlie the La Joya volcano,which is just to the east of Palo Huérfano. The dip of themesa surface suggests that there is a listric componentto the fault motion, which caused eastward rotation ofthe andesite along the N-S-striking fault plane. In thehill with the microwave towers near the village ofCalderón, an unconformable contact between theAllende Andesite and the overlying Palo Huérfano rocksis exposed.

Km 89.6: Glorieta/Roundabout at the entrance to SanMiguel Allende. We turn to the east, following the free-way toward Querétaro and Dr. Mora.Km 93.2: Ignacio Allende Glorieta/Roundabout. Wecontinue on the highway toward Querétaro.Km 94.8: Intersection of Highway 111 and the highwayto Dr. Mora. We go straight, toward Querétaro.Km 110.0:We park here for an excellent overview of thecentral part of the San Miguel Allende Volcanic Field.



STOP 3-5: JUNCTION OF THE ROAD TO GUADALUPE TAMBULA:PANORAMIC VIEW OF THE VOLCANOES PALO HUÉRFANO, LAJOYA, AND THE CERRO COLORADO DOME, AND ALCOCER-LAESTANCIA FAULT SYSTEM.

From this point on the edge of the highway, lookingtoward the southeast, we have a good panoramic view ofthe northeastern flank of the Palo Huérfano Volcano andthe Cerro Colorado Dome (Figure 9). The most notablefeature is that part of the lava flows from Palo Huérfanodip to the south, back toward their source vent area.Pérez-Venzor and others (1996) show on their map a sys-tem of normal faults with average N80E strike and blockssuccessively downthrown toward the north (Figure 8).These authors think that movement along listric faultplanes, which forced the layers to rotate counterclock-wise (as seen from the east), caused the anomalous dip ofthe lava flows. The maximum age of the N80E fault sys-tem is given by the age of the volcano (~11 Ma). Thesefaults are parallel to the system of faults known as theChapala-Tula system, which crosses the TMVB south ofthis area (Figure 2).

The Cerro Colorado dome is a volcano that predatesPalo Huérfano. It is composed of dacites and has a radio-metric age (K-Ar, plagioclase) of 16.1 ± 1.7 Ma (Pérez-Venzor et al., 1996). The age relationship is apparent onaerial photographs and on the geologic map, becauseCerro Colorado acted as a topographic barrier, whichdeflected the lava flows emitted from the crater of PaloHuérfano.

Looking toward the SE we see the neighboring vol-cano, La Joya, which is very similar in age and morphol-ogy to Palo Huérfano. Valdéz-Moreno and others (1998)provide a good report of the geologic evolution of thevolcano, as well as two K-Ar ages (Figure 10). The his-tory of La Joya is similar to that of Palo Huérfano andbegins with an andesitic dome called El Maguey.Although this dome was not dated radiometrically, itseems likely that it was contemporaneous with the CerroColorado dome that underlies Palo Huérfano. In fact, thedomes are very close to each other. Later fine-grainedandesitic lavas were emplaced in the area northeast of LaJoya; these lavas resemble the Allende Andesite petro-graphically and may well be equivalent to it. Valdéz-Moreno and others (1998) called these lavas “OlderAndesite.” The products erupted by La Joya weredeposited on top of these older andesites. The initial

flows of La Joya package are andesitic porphyries whosemost outstanding characteristic is the presence of lightgreen enclaves which are interpreted as glomerophe-nocryst clusters that formed part of the ‘crystal mush’attached to the walls and/or roof of the subvolcanicmagma chamber. These lavas yield an age of approxi-mately 10 Ma. Above these enclave-bearing flows aremore andesite lavas, which built up much of the lowerpart of the volcano. The lavas that form the principalbody of the La Joya Volcano are dacites with K-Ar agesaround 9.9 Ma. Finally, the southern flank of the volcanowas partially covered by mafic andesites of 6.2 Ma, pro-duced by a field of cinder cones located to the south ofthe volcano. Interesting features of La Joya and PaloHuérfano are the broad depressions in their summitregions. These physiographic forms are the result of ero-sion of the volcanic craters to a broad circular to ellipti-cal depression roughly 3 kilometers wide and 5 kilome-ters long. This deep erosion was favored by intense alter-ation resulting from fumarolic activity (possibly continu-ing for a long time after the final stages of construction ofthe volcano). Further along the route we will have oppor-tunities to observe the dacitic flows, and we will go upinto the eroded ‘cirque-like’ summit area of La Joya.

Km 121.0: Village of La Monja. North of the highwaythere is a small ledge of material from which andesite hasbeen quarried. We will park here for our next stop.

STOP 3-6: VILLAGE OF LA MONJA: BASE OF LA JOYAVOLCANO AND OUTCROP OF THE LOWER ANDESITE WITH GREENENCLAVES

Valdéz–Moreno and others (1998) considered these rocksas lava flows entirely pre-dating La Joya, but now webelieve that in reality they are the first flows to come outfrom the volcano. From this locality Valdéz-Moreno andothers (1998) obtained an Ar40-Ar39 age of 10.4 to 10.9.The rock is dark gray to black on a fresh surface, withporphyritic texture and aphanitic groundmass. The phe-nocrysts form 15-20% by volume of the rock; they areplagioclase and orthopyroxene. One of the most notice-able features of the outcrop is the presence of fracturesthat divide the lava flow into rough and irregular“sheets.” In other locations, such as the village of SantaInés, the spacing between the fractures is so regular thatit permits quarrying of this rock type as a construction



material which is used as an ornamental facing stone onbuildings and as a fancy paving stone. Seen at closerange, the andesite contains yellow and green enclaves,which resemble medium-grained sandstones. Their sizesvary from 5 to 30 centimeters in diameter, and their formis noticeably rounded. We think these inclusions wereonce part of the crystal mush attached to the walls/roof ofthe magma chamber; such features are common in vis-cous lavas such as those that form domes. The enclavesare composed of crystals of pyroxene and plagioclase inan interlocking texture, with intersertal glass (Valdéz-Moreno and others, 1998). Lavas at a higher level on thevolcano (the Tambula Dacite of Valdéz-Moreno and oth-ers, 1998) contain similar enclaves.

At La Monja we will take the road toward La Barreta andLa Joya in order to go up to the summit of the volcano.We will be ascending through several dacite flows withautobrecciated bases. The dacite is light grey, with phe-nocrysts of plagioclase and mafic minerals. The sequencebegins with pyroxene andesite at the base (e.g., La MonjaAndesite), and changes upward to hornblende dacite atthe summit (i.e., the Pinalillo Dacite). The lavas of thisvolcano certainly were very viscous; they were emplacedas lobate flows with very high aspect ratios. Apparentlythe viscosity was increasing with time, culminating withshort flows of the coulee type.

Km 129.4 Summit of La Joya volcano.

STOP 3-7: EROSIONAL ‘CALDERA’ AT THE SUMMIT OF LA JOYAVOLCANO

Once we reach the summit we can see the broad cirque-like erosional form, which was developed from the orig-inal eruption crater. The original rock has been totallyargillized to a slippery, fragile, easily eroded yellowishclay. In a few places vestiges of the original dacite canstill be seen, although it is highly altered by hydrothermalactivity close to the conduit of the volcano. Bit by bit theoriginal crater was widened and degraded by the erosionof this argillized material until it ended up as the broaddepression we now see.

From this point we begin the trip back to the city ofGuanajuato. In San Miguel Allende we will make the lasttwo stops of the day.

Km 165.6: Ignacio Allende glorieta/Roundabout. Theplace where Highway 111 joins the freeway south of SanMiguel Allende. On one side of the mall where theGigante supermarket is located, a paved road takes off.We turn to the right and follow the signs leading to thepark called El Charco del Ingenio. Km 167.6: Entry gate of the botanical garden El Charcodel Ingenio. We will leave the vehicles and follow thepathway that leads to the canyon located downstreamfrom the El Obraje Dam.

STOP 3-8: CHARCO DEL INGENIO: EL OBRAJE IGNIMBRITE(~29 MA) AND LAHAR PRODUCED BY PALO HUÉRFANOVOLCANO RESTING ON ASH AND/OR EPICLASTIC/VOLCANICMATERIAL DEPOSITED IN A LAKE (UTM 14Q0320281;2314024)

We follow the trail toward the wall of the dam and thecanyon. At the mouth of the dam, and along the northwall of the canyon the rhyolitic El Obraje Ignimbritecrops out (Pérez-Venzor et al., 1996). This is an extensivelithologic unit, exposed principally to the north of PaloHuérfano, in the footwall block of the San MiguelAllende fault. At the base of the volcano itself, only twosmall outcrops of this ignimbrite have been recognized(Figure 9), resting unconformably on the basal Mesozoiccomplex.

In this locality we estimate that the ignimbrite has aminimum thickness of ~100 meters. It shows a roughzonation, evidenced by a change in the fracture density,and the degree of welding, as well as the presence of sev-eral zone s of flattened lithophysae partially filled byquartz and chalcedony. The texture of the rock is por-phyritic, with 25–30% of phenocrysts (quartz, sanidine,sodic plagioclase, and totally altered (?)hypersthene) setin an aphanitic matrix. We think that this unit is a goodcandidate for an ignimbrite that conforms to the progres-sive aggradation model of emplacement; i.e., it is com-posed of several packages which are separated by shearsurfaces, presumably reflecting periodic changes in theflow regime during an essentially continuous eruption.Discrete shear surfaces may reflect sudden, temporaryincreases in boundary-layer shear between particulateand non-particulate components of the flow (Branneyand Kokelaar, 1992, and Branney, in press). Althoughthis unit has neither abundant lithics nor obviouspumices, it does have some layers characterized by sub-



tle development of ramp structures whose orientationssuggest a transport direction from north-northwest tosouth-southeast.

Pervasive devitrification of original glass and localsilicification partly obscure original textures and struc-tures within this unit.

The El Obraje Ignimbrite is very similar to volcanicrocks with radiometric (K-Ar) ages of ~32-27 Ma, whichform the majority of the outcrops in the region locatedbetween San Miguel Allende and San Luis Potosí.

Along the path situated on the south wall of thecanyon are some outcrops of an epiclastic/volcanicdeposit which rests on crudely stratified white pumice-rich siltstones derived from a tephra fall or some othervolcanic source. On the basis of their appearance andcomposition (andesitic), these aquagene pyroclasticdeposits are thought to come from Palo HuérfanoVolcano (called the San Miguel Allende “tuff” by Pérez-Venzor and others, 1996). The fine-grained sedimentshere are very similar to clastic material of lacustrine ori-gin (see description of the sedimentary sequence exposedin roadcuts near the village of La Ciénega), although wehave no certainty that they are correlatable, nor even thatin this locality they were deposited in a lake of any depth.

Overlying this deposit is a thick (several tens ofmeters) lahar with large andesitic clasts in a muddymatrix, probably produced by Palo Huérfano Volcano.Exposures of the base of this lahar and the top of thewhite silt provide numerous examples of soft-sedimentdeformation in the lower unit; asymmetric load casts andshear features suggest that the lahar was emplaced fromsouth to north.

The marked contrast in the thickness of the ElObraje Ignimbrite in the regions located to the north andto the south of the N80W system of faults can be inter-preted as evidence that this system could have been activebefore the formation of the Palo Huérfano Volcano. Theactivity is constrained to be in the interval between theemplacement of the ignimbrite (~29 Ma) and the age ofthe volcano (≤11Ma). This presupposes that the ign-imbrite also was deposited in the zone presently occupiedby the volcano, but that after the faulting took place it wasalmost totally eroded from the upthrown (southern)block. An alternative interpretation is that at the time ofemplacement of the ignimbrite, the zone now covered bythe volcano constituted a basement high, which could notbe surmounted and covered by the pyroclastic flow.

We return to the vehicles to continue the excursion.

Km 169.6: Ignacio Allende glorieta/roundabout. We takethe freeway south from San Miguel Allende. At the road-cuts in the high stretch of this highway you can see vol-caniclastic deposits of andesitic composition. Thesecame from Palo Huérfano and overlie rocks of the basalcomplex and isolated remnants of the El ObrajeIgnimbrite.Km 171.6:We will pull off the road and park in a smallturnout, which affords an excellent view, to the north, ofthe scarp of the San Miguel Allende fault.

STOP 3-9: HIGHWAY ROADCUT ON THE FREEWAY: MESOZOICMARLS AND CALCAREOUS SHALES, MINOR THRUSTS, AND THE

AFOREMENTIONED PANORAMIC VIEW OF THE SCARP OF THE

SAN MIGUEL ALLENDE FAULT (UTM 14Q033318796;2311984)

At this locality we have several cuts that show in a spec-tacular way some intensely deformed marine sedimentsof the basal Mesozoic complex. The sequence exposedhere consists of marls and argillaceous limestones,weathered to brownish yellow and reddish brown hues.In some places we can see the color of the fresh rock,which varies from medium grey to black, possiblybecoming darker as the presence of organic materialbecomes more abundant. Also present here are numeroussmall veinlets of calcite and gypsum. The sequence iswell-stratified, but in many places the lateral continuityof layers is interrupted. Also, the rocks display a densefracture pattern of tectonic origin (fissility, or spacedcleavage), that cuts across the layering at a relatively lowangle. Looking closely at some parts of this cut, you cansee overturned (many fully recumbent) folds on the orderof a few meters, with subhorizontal axial planes. In gen-eral, the cleavage is axial-planar to these nearly isoclinalsimilar folds, and there are numerous examples of cleav-age refraction between layers. In some layers originalsedimentary (fluvial?) cross-bedding is preserved, beingonly slightly deformed during the development of thefracture cleavage. In addition to the meter-scale folds,there are zones of microfolds (decimeter-scale) andcrenulations (intense folding at the centimeter scale orless) in the clay-rich layers. Gypsum veinlets commonlyare parallel to the fracture cleavage, and they show sometendency to be especially close-spaced in the hinge zones



of the larger folds; calcite veins typically form perpen-dicular to the axial-planar cleavage, along extension frac-tures, and in many places can be seen to cut the veinletsof gypsum.

Viewed at a distance, the cut shows the traces ofseveral faults with relatively low dips. This is evidentfrom the fact that the layering is cut and displaced.Following with one’s gaze along these fault traces, it ispossible to observe that they do not cut all the layersexposed in this outcrop, but are lost from view by passinginto bedding-plane-parallel surfaces, or, alternatively,they may be buried by overlying layers which are appar-ently not offset. It is not possible to make any visual cor-relation of the displaced layers so as to establish whetherthe movement on the faults is actually normal or reverse.The interpretation that this is a zone of thrusting withapparent offset toward the northeast is in agreement withthe type of compressive deformation that the rocks inthese outcrops show as a whole. We think that the faulttraces are “lost” in this cut because the motion wasaccommodated in part along the bedding planes. The verynature of these rocks, extremely clay-rich, and the pres-ence of small quantities of gypsum in the sequence,would facilitate this type of mechanical response to acompressive stress at high crustal levels. Viewed in amore regional way, this outcrop appears to be the footwallblock of a major thrust, since rocks of the Guanajuato Arccrop out in the arroyo located immediately to the south ofthis cut. They are possibly covered by later marine sedi-mentary rocks; Chiodi and others (1988) recovered anAptian-Albian ammonite from this locality.

From this overlook we have a view of the historiccenter of the city of San Miguel Allende, which is con-structed on the fault scarp of the same name. This is anotable topographic feature, which can easily be seen inthe region north of the Palo Huérfano Volcano. From thepoint where we made Stop 3-5 (the intersection with theroad to Guadalupe de Tambula) Highway 111 is con-structed on a small mesa which terminates abruptly uponreaching San Miguel. The base of the Ignacio AllendeDam is located on the downthrown block. The SanMiguel Allende fault has an approximately N-S strike,the sense of motion is normal, and its trace is buried bythe products of Palo Huérfano. Thus, the age of its latestmovement has to be earlier than 11 Ma.

End of the road log for Day 3. We return to Guanajuato.

DAY 4: GEOLOGY OF THE BASAL MESOZOIC COMPLEX

We will leave from the lobby of the Hotel Parador SanJavier, travelling north on the highway toward DoloresHidalgo. Refer to Figure 5 for locations of stops.

Km 3.5: Road to the face of the Esperanza Dam and theLos Insurgentes campground. We turn left. About 250meters in from the highway is the dam, which was con-structed around 1894 by Ponciano Aguilar, a well-knownengineer from Guanajuato (aguilarite, a silver selenoidthat was first described here in the Guanajuato District, isnamed in his honor). This reservoir is used to supplywater to a good part of the cit. At the south end of the damcan be seen submarine lavas of andesitic composition,which are intercalated with the limestone and pelitic sed-iment sequence of the Esperanza Formation.Km 4.6:At the entrance to the campground we will leavethe vehicles and will walk along the road for about 300meters.

STOP 4-1: CONTACT ALONG THE FAULT (VETA MADRE)BETWEEN ROCKS OF THE VOLCANOSEDIMENTARY AND VOL-CANOPLUTONIC SEQUENCES OF THE SIERRA DE GUANAJUATO.THE LA PALMA DIORITE: AN EXAMPLE OF THE INTERNAL COM-PLEXITY OF THE BASEMENT UNITS (UTM 14Q0265112;2329247 TO 14Q0264645; 2329081)

The road cuts at the beginning of this traverse exposerocks of the volcanosedimentary sequence of the ArperosBasin. These rocks, which belong to the member less richin limestone of the Esperanza Formation of Echegoyén(1970), has a more chaotic style of deformation than thatobserved at Stop 1-1, with disharmonic folding andchevron-style folds. This outcrop has veinlets of quartzand/or calcite filling extension fractures, and well-devel-oped shear zones which separate individual packages offolds. We attribute these fractures and zones of extensionto the proximity of this outcrop to the great normal faultnow occupied by the Veta Madre, which in this part of itstrace is a very broad zone of offset and mineralization. Aband of cataclasites or fault breccias, strongly oxidized,separates the volcano-sedimentary sequence from the LaPalma Diorite, which is one of the units of the vol-canoplutonic sequence (Figure 11).

La Palma Diorite is a very complex lithologic unit,which at this locality is dominated by massive microdi-



orites and a group of dikes, some of them felsic tonalitesand others diabases. In other outcrops of the La PalmaDiorite we see magmatic breccias and zones of peg-matitic diorite. The great abundance of dikes with chilledmargins and the variable textures of the host rocks aredistinctive features of this unit.

We will return to the village of La Valenciana and fromthere we will take the road to Cerro El Cubilete.

Km 0.0: Church at La Valenciana. The road to ElCubilete begins here.Km 0.25: On the left is the road to the La ValencianaMine, which is one of the most famous bonanzas of theDistrict. A few hundred meters from the road junction itis possible to observe a majestic old building, which isthe ruin of the great benefaction plants constructed by theSpaniards. The road here crosses the La Palma Diorite.From this point on, until we arrive at Cerro El Cubilete,the route provides many cuts with excellent exposures ofthe rocks of the Guanajuato Arc. Km 0.6: Guadalupe Mine. This building, with its but-tresses in the form of stylized elephants, is in the processof reconstruction, with a high priority being placed onpreserving as much as possible of the historic materialsand construction.Km 3.0: On the left is the village of Llanos de Santana,and some 200 meters to the right is the shaft of the SanElias Mine. The shaft is in the hanging wall block of theVeta Madre Fault.Km 4.7: On the right is the road that leads to the LaCebada Mine, a property of the Peñoles Group. It is thefarthest west of the active mines of the Veta Madre sys-tem. From this point, the road follows approximatelyalong the contact between the dike complex emplaced inthe La Palma Diorite and the Cerro Pelón Tonalite.Km 6.6: On the right is the road which leads to the vil-lage of Mesa Cuata, and which passes through the sum-mit of Cerro Pelón, the type locality of the unit of thesame name. In this locality there is a plagioclase granitecut by dikes. Along the road one can see alternatively out-crops of the granite and outcrops of the La Palma Diorite.In some places hydrothermal alteration can be noted, aswell as intense weathering. Despite these features, evi-dence of deformation of these rocks can be seen as well.Km 11.0: The road crosses the Cerro Pelón Tonalite. Onthe left, in the distance, is El Cubilete.

Km 12.3: To the west is the old mining town of La Luz.At this site was the first discovery of gold and silver inthe Sierra de Guanajuato, which dates from the six-teenth century. The road continues on the Cerro PelónTonalite.Km 12.6: We will park on the shoulder to inspect thisoutcrop.

STOP 4-2: CERRO PELÓN TONALITE CRISS-CROSSED BY DIA-BASE DIKES; FAULTING AND ASSOCIATED DRAG FOLDS (UTM14Q0261867; 2331021)

In this roadcut the exposure shows the tonalite and dia-base dikes which make a spectacular geometric pattern.These dikes are similar in lithology to the diabasesobserved cutting the La Palma Diorite at Stop 4-1. Theyare interpreted as feeder dikes for the abundant pillowlavas of the volcanoplutonic sequence. However, thenearly horizontal position of these mafic bodies seemsmore consistent with a group of deformed sills. At thislocality both the sills and their host rocks are cut by sev-eral small faults, and one can try to use the geometry ofthe drag folds and other features to decipher the sense ofmotion on the faults.

Km 13.9: We will pull off the road for a brief inspectionof the outcrop.

STOP 4.3: CERRO PELÓN TONALITE CUT BY DIABASE DIKES INUNDEFORMED CONJUGATE SETS (UTM 14Q0260277;2331960)

In this road cut we again see the Cerro Pelón Tonalite, cutby a good number of diabase sills, but the structural styleof these is quite different. In contrast to the sills at Stop4-2, where we see drag folds and irregular margins, thisgroup has planar margins, without evidence of folding.They commonly form conjugate pairs around sigma-1.These two stops, with their contrasting structural styles,suggest that the deformation, which affected the CerroPelón Tonalite after the emplacement of the diabasicdikes and sills was heterogeneous on a small scale, beingbrittle in some places and brittle-ductile in others.

Km 14.2: On the right is the church of the village of LaLuz, and in front of us is the astronomical observatory ofthe University of Guanajuato.



Km 14.8: At the bottom of the gorge, on the right, is theBolañitos Mine, a property of the Peñoles Group. It isone of the few active mines on the La Luz vein system.Km 15.5: On the left is the abandoned GolondrinasMine, also a property of Peñoles. Some 200 meters southof the turnoff is the observatory. At this site we have thecontact between the La Palma Diorite, with its dike com-plex, and the pillow lavas of the volcanoplutonicsequence. From this point on, the road crosses both pil-lowed and massive lavas.Km 16.1: On the right is the road to the Bolañitos Mine;in the distance, in the same direction is Cerro El Gigante,which is covered by mid-Tertiary volcanic rocks.Km 17.2: The road we are following joins the old road tothe village of La Luz.Km 17.7: On the right is the road to the Asunción Mine(Peñoles); close to us is a small ridge on top of which aresome antennas. The hill is composed of rocks consideredto belong to the La Palma Diorite (the dike complex isprominent here). This outcrop is interpreted as a klippe.The dike complex rests on the pillow lavas of the vol-canoplutonic sequence, and it is unlikely that the sectionis overturned. The klippe is cut by the master fault of theLa Luz vein system, which crosses the road at this point.Km 18.8: On the left is the cemetery of La Luz, which islocated on another small klippe. In front of us is Cerro ElCubilete, and on the right is the El Bajío Plain ofGuanajuato. The road continues on volcanic rocks of theLa Luz Formation (basalts and andesites), which arealmost completely free of vegetation.Km 20.3: We will stop to observe the low outcrops onthe left.

STOP 4-3: SUBMARINE LAVAS OF THE VOLCANOPLUTONIC

SEQUENCE(UTM 14Q0256236; 2328009).

The objective of this stop is to show the submarine lavaswith pillowed basalts of the La Luz Formation(Echegoyén, 1970), their normal stratigraphic position(evidenced by the geometry of the pillows) and the vari-able nature of the deformation which affects them. Ingeneral, deformation is controlled partly by the lithologyand partly by discrete shear zones. A this stop, the pene-trative deformation is well developed in the originallyhyaloclastitic matrix between the pillows of lava.Walking along the road toward the village of La Luz, onecan see that the basalts were also affected by deforma-

tion, which converted them into chlorite schists. In someplaces the basalt is better preserved within a matrix ofmylonitic schist. In the less deformed pillows one can seevesicles around their margins. Although their lithology isin broad aspect similar to that of the lavas and tuffs inter-calated in the Esperanza Formation (Stop 1-1.) in the vol-canosedimentary complex, these lavas of the La LuzFormation are thrust over the volcanosedimentary rocks.We think their environment of accumulation was closerto the arc axis, and therefore constitute a separate forma-tion.

Km 20.5: On the right there is a dirt road heading towardthe village of Los Lorenzos. We can still see Cerro ElCubilete in front of us. From this point on, the LaValenciana-El Cubilete road passes through one of themost evolved facies (in terms of magma composition) ofthe whole Mesozoic volcanic sequence. This sequenceincludes dacites and rhyodacites (including some kerato-phyres), as well as pyroclastic rocks of the same compo-sition.Km 23.3: We will pull off the road here to take a look atthe long outcrop on the right.

STOP 4-4: PYROCLASTIC ROCKS METAMORPHOSED TO CHLO-RITE SCHISTS AND INTENSELY DEFORMED (UTM14Q0254905;2325895).

Along several tens of meters the road here crosses chlo-rite schists which are interpreted as metamorphosedpyroclastic rocks (probably andesitic to basaltic tuffsbefore metamorphism). They were subjected to multipleepisodes of deformation. The early schistosity was fold-ed by a later compressional event. Most of the microfoldsin this area have axial planes that strike NW-SE. If thissection has not been overturned, the schistosity indicatesright-lateral shear, or north-over-south. The most spec-tacular and elegant feature in this outcrop is the markedcontrast in deformational style between two types oftuffs. The mafic ash-rich tuff developed a close-spacedcleavage, crinkle-folds and small anastomosing veinletsof quartz in the cores and hinge zones of the microfolds.Its neighbor, on the other side of a small thrust, is a tuff(probably andesitic) with relatively abundant crystals. Itresponded differently to the deformation, with a parallelstyle of folding rather than the similar-fold style of thefine-ash tuff.



Km 24.8: Road crossing. On the left is the road that goesdown to the highway at Silao; to the right is the road thatclimbs to the summit of Cerro El Cubilete and the sanc-tuary of Cristo Rey.Km 24.8: The road crosses unconsolidated gravels andsands of Tertiary age, later than the Late Oligocene andearlier than the mid-Miocene, and goes up toward themonument of Cristo Rey through andesites of mid-Miocene age (the El Cubilete Andesite, K-Ar, whole-rock, ~13.5 Ma: Aguirre-Díaz and others, 1997).Unfortunately a stone wall constructed along the edgeof the road prevents our seeing the gravels. We willvisit an outcrop (Stop 4-6) of these gravels on the roadwhich goes to Silao. At the very top of the hill a chapeland the Cristo Rey monument were constructed. Thissanctuary is considered an offering in honor of thosewho died during the Cristera Revolution, whichoccurred en the decade between 1920 and 1930 in thisregion.

STOP 4-5: CERRO EL CUBILETE: A PANORAMA OF THE SIERRADE GUANAJUATO AND ITS SURROUNDINGS (UTM14Q0253836; 2325061).

Views around the Sanctuary of Cristo Rey. From thispoint, at 2590 meters above sea level and 700 metersabove the El Bajío Plain, one can see the principal fea-tures of the Sierra and its surrounding areas:To the WNW is El Bajío, with the city of León at its west-ern end. More to the right is the NW part of the Sierra deGuanajuato; there the vocanoplutonic sequence of theGuanajuato Arc and the volcanosedimentary sequence ofthe Arperos Basin have been intruded by an extensiveearly Tertiary (53+/- 3 and 51+/- 1 Ma, K-Ar, biotite:Zimmerman and others, 1990) batholith known as theComanja Granite. Close to us are outcrops of a massivediorite exposed near the village of Tuna Mansa. Theseplutonic rocks were thrust over the submarine lavaswhich we saw at Stop 4-3, near Cerro El Cubilete. Thistectonic contact is visible in the downthrown block of theVilla de Reyes graben.To the north are the two hills El Gigante and La Giganta,which are crowned by andesites of Miocene age, simi-lar(?) to those here at El Cubilete, but of different ages.The depression to the west of El Gigante and La Gigantais the Villa de Reyes graben, a mid-Tertiary structurewith a N45E orientation. The graben has about 150 kilo-

meters of length and in the Sierra de Guanajuato termi-nates against the El Bajío Fault (Figure 3). The masterfault on the east side of the graben puts into contact themassive diorite and the submarine lavas. At the center ofthe graben is the village of Arperos, and farther north,inside the graben, is the Sierra El Ocote, a rhyolite domewith tin and topaz (Figure 2). The Villa de Reyes grabenis the northwestern tectonic limit of the Veta Madre.To the east of Cubilete is the city of Guanajuato, whichwas constructed in a depression bounded by the threemajor faults of La Aldana, Veta Madre and El Bajío. Thehigh area to the northeast of the city of Guanajuato is theSierra de Santa Rosa, covered principally with mid-Tertiary felsic volcanic rocks. Beyond the Sierra is thePalo Huérfano Volcano, which we saw yesterday. SanMiguel Allende sits north of the volcano. In the samedirection is a depression oriented ENE-WSW, known asthe La Sauceda graben, a late Cenozoic structure whichforms the southeastern boundary of the Veta Madre sys-tem and also the southeastern boundary of the Sierra deGuanajuato.To the southeast on a clear day one can see the oppositeend of the El Bajío Plain and some of the volcanoes of theTransmexican Volcanic Belt, such as La Gavia, Culiacánand La Batea. To the south bordering the El Bajío depression is the faultzone of the same name. The trace of the fault is close tothe base of the mountains, near the village of La Ermita.This fault was active in the late Cenozoic and caused rel-ative sinking of the Bajío block relative to the Sierra deGuanajuato. This is demonstrated by the sequence ofgravels and the andesite of El Cubilete, which has beendisplaced ~600 meters upward with respect to its coun-terpart on the downthrown El Bajío block. An estimate ofthe rate of displacement on the fault, assuming that the600 meters were accumulated in the last 13.5 millionyears is 0.04 millimeters per year.

STOP 4-6: CERRO EL CUBILETE: GRAVELS AND ANDESITIC

LAVAS OF TERTIARY AGE CROWNING THE MESOZOIC BASEMENTOF THE SIERRA DE GUANAJUATO

At the base of the Cristo Rey monument are exposedunconsolidated fluviolacustrine deposits composed prin-cipally of clasts of volcanic rocks derived from the mid-Tertiary units of the Sierra de Guanajuato, which includeignimbrites, rhyolitic dome and flow rocks, and



andesites. Fragments derived from the basal Mesozoicsequence are relatively rare. All the clasts in this depositare well rounded, and they are supported by a matrix ofcoarse sand, fine gravel and silt. The deposit has a poor-ly developed stratification, marked by changes in grainsize in the finer-grained clastic deposits, and by lenses ofcoarse gravel. These deposits reach an aggregate thick-ness of 150 meters in their thickest part, but they thinagainst the Mesozoic rocks, indicating that this is the fill-ing of a paleochannel formed by erosion of the Mesozoicrocks. No fossils have yet been reported in this sedimen-tary sequence, but it is inferred to be early to middleMiocene, based on the fact that the youngest lithic claststhat it contains are from late Oligocene (around 27–30Ma), and that this deposit is covered by andesite lavas ofmid-Miocene age.

Resting unconformably on the fluviolacustrinedeposits is a thick subaerial andesitic lava flow, whichcaused hydrothermal alteration at the contact with thegravels and sands. The base of the flow is an autobrecciawhich changes upward to a zone with well-developedcolumnar joints. Above this zone, without a marked inter-ruption other than a prominent horizontal joint, is anotherthick andesite, very similar in texture to the earlier one.This one has a base with intense subhorizontal platy joint-ing which passes upward into a thick columnar zone in themiddle part, and culminates with a zone of subhorizontalplaty jointing in the uppermost part. In total the andesitesums up to 70 meters of thickness and we consider that itcould be formed by two flows. An alternative interpreta-tion is that the El Cubilete Andesite is actually one verythick (intracanyon?) flow with complex colonnade-and-entablature structure. Apparently the lava flowed down apaleochannel similar to that in which the underlying grav-el and sand were deposited, but we do not know from thissmall remnant whether or not the lava flow itself was con-fined by canyon walls. We think the El Cubilete Andesite,like the volcanoes of the San Miguel Allende VolcanicField (Day 3), is associated with the earliest phases of vol-canism of the Transmexican Volcanic Belt.

From this stop we will head for the Bajío Airport, wheresome members of the group will board their flights to

return home. Those participants who can remain inGuanajuato will be able to visit outcrops of the ComanjaGranite with us.

STOP 4-7: THE COMANJA GRANITE: A PALEOCENE BATHOLITHWITH ABUNDANT TOURMALINE IN MAGMATIC, PNEUMATOLYTICAND HYDROTHERMAL PARAGENESES (UTM 14Q0246333;2338599)

In the vicinity of El Rancho Los Alamos, we will visitan outcrop of the Comanja Granite. This part of thebatholithic body is characterized by many large phe-nocrysts of alkali feldspar with prominent Carlsbadtwinning; the coarse-grained groundmass has abundantquartz, a bit of plagioclase (probably albite), and a traceof biotite. There are some parts of this outcrop that haverough alignment of the phenocrysts, as if they were flowdomains alternating with static domains, or possibly thisis a secondary structure imposed upon the granite. Thereis a rhombic pattern of fractures, some of which arefilled by aplite dikes. There are also enclaves a few cen-timeters in diameter of very fine-grained material with-in the coarser-grained granite. Tourmaline is present inmiarolitic cavities, in which it has the common doubly-terminated form; in the pegmatitic domains within thegranite it has other habits. This mineral is also present asa magmatic phase, in acicular form, and in hydrothermalbreccias in massive microcrystalline to cryptocrystallineform. One can see, about halfway up the slope of thishill, a wide brittle shear zone, which has various mix-tures of massive tourmaline with granite fragments ofdifferent sizes and degrees of comminution. These mix-tures range from jigsaw-puzzle arrangement of largergranite fragments and relatively little tourmaline, to adistinctive “rosette” pattern of rounded granite clasts ina matrix of tourmaline and very finely comminutedgranitic material and secondary silica, to massive tour-maline with little or no granite. From time to time tracesof pyrite and other sulfides can be seen in these shearbands. It seems probable that a careful search of thisoutcrop and its surroundings might reveal some smallroof pendants of rocks of the volcanosedimentary com-plex.



field trip april 5–8 -...

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