sedimentary environments and diagenesis of a cretaceous reef complex, eastern mexico

7/28/2019 Sedimentary Environments and Diagenesis of a Cretaceous Reef Complex, Eastern Mexico

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ANALES DEL CENTRO DE CIENCIAS DEL MAR Y LIMNOLOGA

SEDIMENTARY ENVIRONMENTS AND DIAGENESIS OF ACRETACEOUS REEF COMPLEX, EASTERN MEXICO

Trabajo rec ibido el 28 de marzo de 1977 y aceptado para su p ubl icac in el 13 de junio d e

1977.

J. EDUARDO AGUAYO C.

Subdireccin de Tecnologa de Exploracin, Instituto Mexicano del Petrleo. Av. Cien Metros 152,Mxico 14, D. F.

RESUMEN

El complejo arrecifal de la Caliza El Abra se encuentra en la margen oriental de la plataformacretcica Valles-San Luis Potosi, en el Este de Mxico. La edad de la formacin comprende desdeel Albiano hasta el Turoniano, en base a un depsito de foraminferos planctnicos localizados en

la zona de post-arrecife. Se reconocen dos ambientes sedimentarios mayores y cinco sub-ambientes en las calizas de El Abra: 1) El arrecife, caracterizado por una gran diversidad de faunay 2) El post-arrecife, constitudo por calizas micrticas ricas en mililidos y en estructurasestromatolticas. El complejo arrecifal contiene fbricas diagenticas que reflejan los ciclosrepetidos, tanto de sumersin como de emersin de la plataforma cretcica.

Se analizaron geoqumicamente diferentes constituyentes calcreos. Aquellos que forman elarrecife tienen valores menores a 2000 ppm de magnesio. Por otro lado, aquellos de la zona post-arrecifal tienen un promedio mayor a 2 000 ppm del mismo elemento. Por lo tanto, se concluyeque, por medio de litologa, estructuras sedimentarias primarias, palcontologa, fbricasdiagenticas y diferencia en el contenido de magnesio, es posible descriminar los ambientessedimentarios de la Formacin El Abra, asi como los procesos de diagnesis que afectaron alcomplejo arrecifal durante su formacin.

ABSTRACT

The reef complex El Abra Limestone is at the eastern margin of the Cretaceous Valles-San LuisPotosi platform in eastern Mexico. The age of this formation is from Albian to Turonian on the basisof a layer rich in planktonic foraminifera, located at the back-reef zone. Two major sedimentaryenvironments and five sub-environments are present within the El Abra Limestone: 1) The rudist-reef environment characterized by its great faunal diversity, and 2) The back-reef environment thatcontains micritic limestones rich in miliolids and stomatolitic structures. The reef complex containsdiagenetic fabrics showing the repeated cycles of emersions and immersions of the Cretaceousplatform.

Several calcareous constituents were geochemically analized. In reef samples, the magnesiumcontent averaged less than 2000 ppm. On the other hand, back-reef samples averaged more than2000 ppm of the same element. Therefore, by means of lithology, primary sedimentary structures,paleontology, diagenetic fabrics and magnesium concentration, it is possible to discriminate El Abrasedimentary environments and the diagenetic processes which affected the reef complex during itsformation.


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INTRODUCCIN

The purpose of this paper is: 1) to establish the sedirnentary environments of the El Abra Limestone reef complex at its type locality, 2) to establish the diagenetic fabrics in each environ ment,3) to develop a diagenetic model of the area within more detailed facies may be considered infuture work.Quarry exposures of the El Abra Limestone in East Mexico afford an excellent opportunity toexamine a complete suite of platform facies which range from a shallow, protected lagoonfacies tonear-back reef facies and reef facies in an interval of 7 to 8 kilometers, along a trend normal to theedge of the Cretaceous VallesSan Luis Potos Platform. The carbonate sequence is well-preservedand most of the skeletal and non-skeletal grains are not strongly altered. Hence, the lithology andthe faunal distribution are clearly discernable in terms of their position within the reef complex.

A study of this type should provide a better understanding of the influence of the diageneticprocesses taking place at different rates in laterally adjacent sedimentary environments. Attentionwas devoted to mineralogical stabilities and textual properties. This type of study should also shedlight on how various carbonate particles were affected by the processes of dissolution andneomorphism. Other processes, such as biologic disruption, internal erosion, and nontectonicfracturing were also considered.GEOLOGIC SETTING

The type locality of the El Abra Limestone in the Sierra de El Abra is situated at El Abra Stationalong the National Railroad, about 9 kilometers east of Ciudad Valles, San Luis Potosi State (Fig. 5). The Sierra de El Abra is, an elongate carbonate complex on the easternmost side of the SierraMadre Oriental (Fig. 1) It extends about 150 kilometers in the northeast-southwest direction, andabout 7 to 15 kilometers from east to west. The Sierra de El Abra rises abruptly, rough1y 250 to 300meters above the coastal plain in the east and some 100 to 150 meters above the rolling hill countryin the west. It forms the eastern side of a Cretaceous platform known as the Valles-San Luis PotosiPlatform (Carrillo, 1969 and 1971), which divides the Mesozoic Basin of Central Mexico to the westfrom the Tampico Embayment Region to the east.The Tampico Embayment Region is the zone of major negative gravity anomalies in the coastalprovince of eastcentral Mexico. It is bordered to the north by the Burgos Basin and by theTamaulipas Arch, which includes the Sierra de San Carlos and Las Cruillas, and by the associatedeastern San Jose de las Rusias homocline. This Embayment is bounded to the south by the upliftedpre-Mesozoic Teziutlan and Jalapa igneous massifs. The Embayment extends westward to theSierra Madre Oriental and eastward to the Gulf of Mexico.The basement beneath the Tampico Embayment consists of a series of structural highs formed by alarge chain of folded and faulted Paleozoic rocks, which extend across eastern Mexico fromTamaulipas to Yucatan. These features are part of the Yucatan Archipelago (Murray, 1961).STRATIGRAPHY

The Cretaceous Valles-San Luis Potosi Platform consists of a sequence of marine rocks underlainby fluvial-marine Jurassic rocks and by igneous and metamorphic rocks of Paleozoic age. Thestratigraphic sequence of the Platform. This sequence exhibits repetitive vertical patterns of faciesdistributions as a consequence of the discontinuous subsidence of the platform during deposition.The subsurface stratigraphic sequence has a thickness of about 1 800 meters (Carillo, 1971; Moya,personal communication). An average of approximately 50 meters is visible in quarry exposures inthe study area. The El Abra Limestone is laterally adjacent to the "Tamabra" Limestone, a


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transitional fore-reef to basinal deposit. The "Tamabra" interfingers with carbonate pelagicsediments of the Tamaulipas Superior Limestone to the east (Fig. 2) On the basis of lithologic and paleontologic attributes two members are recognized in the El AbraLimestone at the type locality. These are the El Abra and the Taninul Members (KeIlum, 1930; Muir,1936; Bonet, 1952 and 1963).

According to Kellum (1930) and Muir (1936), the El Abra Member is underlain by the TaninulMember. Bonet (1952; 1963) considers the members to be stratigraphic correlatives; the twomembers are therefore contemporaneous and represent only a lateral change of facies. The latteragrees with the observations made during this study.


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Fig. 1. Reference map showing location of the studied area.


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Fig. 2. Generalized stratigraphic column of the easternmost portion of the Valles-San Luis PotosiPlatform in the Tampico Region.

The El Abra Member consists of light cream to gray interlayered mudstone and wackestone withwellstratified beds from 1 to 5 meters thick, all rich in miliolids (Nummoloculina) and stromatolitic

layers. Other benthonic organisms in subordinate numbers are: gastropods, pelecypods, ostracods,as well as several planispiral and hiserial benthonic microforaminifera. Collectively, the low faunaldiversity, as well as the lithology and the sedimentary structures observed within the El AbraMember suggest that it was deposited in a back-reef environment.The Taninul Member, located along the eastern border of the Cretaceous platform, consists of acomplex of rudist banks associated with mudstone, wackestone and packstone. Principalcomponents of these banks are rudist shells, colonial and solitary corals, encrustingstromatoporoids, gastropods, pelecypods, and benthonic and planktonic foraminifera within thematrix.Rudists are quantitatively the most important group in the Taninul Member. No biostratigraphiczonation was attempted in this study and only those fauna reported from previous work will be

mentioned.Bonet (1963) cited the following taxa from G. Boehm, E. Bse, and W. S. Adkins: Toucasia texana(Roemer), Eoradiolites aff., E. quadratus (Hill), E. cf. davisoni (Hill), Caprina (sphaerucaprina)occidentalis Conrad, Caprinula cf., C. anguis Roemer. Lamellibranchs include: Pecten cf., P.bonellensis Kuiller, Pecten sp., Lima waconensis Roemer, and Chondrodonta cf., C. munsoniHill.Gastropods include: Tronchus, Cerithium, Turritela, Actaeonella and Nerinea; and brachiopodsinclude: Kingena cf. wacoensis (Roemer).


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Coogan (1973) reported the radiolitid rudist Radiolites abraensis Coogan, Sauvagesia texana(Roemer), Caprinuloidea multitubifera Palmer, other species ofCaprinuloidea, and the new genusMexicaprina. He also reported Pecten roemeri Hill and the stromatoporoid Parkeria sphaericaCarpenter.

On the basis of the presence of Pecten roemeri, Kingena wacoensis and Chondronta cf. munsoni,

Muir (1936) stated that the El Abra Limestone is Albian to lower Cenomanian. Bonet (1963) statedthat the benthonic foraminifera Dictyoconus and Orbitolina are abundant in the Golden Lane, whichis stratigraphically lower than the Sierra de El Abra. Coogan (1973) suggested that the El AbraLimestone is Cenomanian and probably early Cenomanian at the 'Taninul" quarry. He stated thatthe El Abra Limestone contains neither characteristic older middle Albian rudists nor foraminifera.Furthermore, the fauna of the El Abra Limestone is different from the late Albian fauna in southTexas, and the. fauna Pecten roemeri, Radiolites and Dicyclina are not known to appear in strataolder than Cenomanian.During this investigation a thin, wavy, planktonic foraminifera-rich layer 1 to 10 cm thick, was foundin the El Abra Member at outcrops within the back-reef environment, approximately at the middle ofthe measured stratigraphic sections: VI, VII and VIII (Figs. 10, 11, 12 and Plate 21-E, F). Accordingto determinations by Dr. Emile A. Pessagno, Jr. and Mr. Kunio Kanamori (University of Texas at

Dallas) , this horizon is late Turonian. They found the following planktonic foraminiferal assemblage:Heterohelix reussi (Cushman), Marginotruncana canaliculala (Reuss), M. helvetica (Bolli), M.Pseudolinneiana Pessagno, M. sigali(Reichel). These new data suggest that the upper two thirdsof exposed section is late Turonian or younger.The uppermost surface of the Taninul Member has a karst appearance and is distinguished byseveral voids and collapse-breccias formed by leaching during episodes of exposure. Some ofthese voids are filled with rudist-limestone fragments surrounded by a planktonic foraminiferarichargillaceous matrix impregnated with asphalt (Plate 13-A, B) . According to Mr. Kunio Kanamori andDr. Emile A. Pessagno, Jr. (ibid.), the argillaceous matrix contains fauna attributable to the earlyCampanian. The assemblage found is as follows: Heterohelix globulosa (Ehrenberg),Pseudotextularia elegans (Rzehak), Marginotruncana sp. cf. angusticarenata (Gandolfi),Globotruncana arca (Cushman), G. Bulloides (Vogler), G. fornicata (Plummer), G. lapparenti

Brotzen, G. rosetta (Carsey), G. stuartiformis Dalbiez, G. ventricosa White, Rugoglobigerina sp. cf.tradinghousensis Pessagno. Along the easternmost side of the Sierra de El Abra at Taninul Station,the late Campanian "Tamuin" Member of the Mendez Shale dips to the east, according to Aguayoand Kanamori (1976), and rests disconformably on the Taninul Member of the El Abra Limestone.Limestone and marl of the early to late Campanian San Felipe Formation dip to the southwest(ibid.), in thrust fault contact with th e "Tamuin" Member. The Turonian Agua Nueva Formation doesnot crop out in the area under study although it has been reported to crop out, further to the north(Bonet, 1952).PREVIOUS WORK

This paper concerns only the shelf-edge limestone facies of the El Abra Limestone in outcrops ofthe Sierra de El Abra at the type locality. Almost all previous work in this area has been

lithostratigraphic and biostratigraphic in nature (Kellum, 1930; Muir, 1936; Im1ay, 1944; Bonet,1952 and 1963; Rose, 1963; Griffith et al., 1969; Carrillo, 1971; Coogan, 1973; and others). Prior tothis study little attention has been given to diagenesis in this unit. Roehl (unpublished report)describes the diagenesis and porosity in relation to the migration of oil. Perkins (1970) recognizedin the El Abra Limestone several subaerial and related features as evidence of continuousexposure-and resubmergence of the Cretaceous platform during the deposition of the reef complex.METHODSField work included the measurement and description of eight stratigraphic sections in quarry


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exposures. Systematic sampling was performed in selected localities considered to berepresentative of each specific bed, and in those areas containing obvious facies changes. A totalof 650 polished slabs were prepared in order to observe large scale relationships. betweensedmentary structures, lithology, and biota. The conventional petrographic microscope was usedfor the examination of textures, and for identification of the constituents in 380 uncovered thin-sections. Following Friedman's (1959) method for calcitedolomite differentation, 40 thin sections

were treated with alizarin red-s solution. A Nuclide Corporation Luminoscope model ELM-2A wasused on 20 uncovered thin-sections to examine diagenetic textures as well as those fabrics invisibleunder ordinary white or polarized light. The operating conditions ranged above 16 kV, withdischarge currents from 60 microamperes to 0.5 milliamperes.Using the JEOL-JSM-1 scanning electron mi'29 fresh1y fractured, micritic chips croscopel weremagnified to 1000x and 3000x, in order to examine the textural variations and diagenetic alterationsof samples. Prior to examination under the electron microscope the samples had been prepared bycementing then to aluminum mounting plugs, and then vacuum coating them, with gold-palladium(40:60) in order to insure that the surface of the limestone chips would have proper electricalcontact with the plugs.

Fig. 3. Conceptual model of the reef complex: El Abra Limestone showing the distribution of theprincipal lithologic, paleontologic and diagenetic parameters and their relative frequency ofoccurrence in each subenvironment.


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A thin-section was prepared for microprobe analysis in order to examine the trace elementvariations in the neomorphic fibrous calcite and the radiaxial and drusy calcite cements. The thin-section was polished and carbon-coated. The instrument used was an ARL probe, type EMX, withand accelerating voltage of 15 kV and a beam current of 0.1 microampere.

DESCRIPTION OF MAJOR SEDIMENTARY ENVIRONMENTS

Two major and distinctive sedimentary environments are recognized within the El Abra Limestone:1) the reef environment and 2) the back-reef environment (Fig. 3). The first comprises two sub-environments: 1a) the foreslope reef zone and lb) the shelf-edge reef sone. The back-reefenvironment comprises three subenvironments: 2a) the nearback reef/lagoon zone, 2b) the tidal-flat/lagoon zone and 2c) the lagoon zone. Environmental differentiation within the formation isbased upon criteria derived from the study of modern carbonate platforms. Studies of recentcarbonates during the last 15 years have led to the establishment of criteria for differentiatingdepositional environments and diagenetic effects which can reliably be used as analogs for ancientcarbonate sequences. Depositional environments of a modern reef complex can be distinguishedby their fauna, lithology, and primary sedimentary structures. In addition, lateral and vertical facies

relationships and the diagenetic features were used to interpret the sedimentary environments ofthe El Abra Limestone reef complex and its diagenetic history using criteria given by Folk (1965),Dunham (1969), Bathurst (197la) and others. The environmental stratigraphy based upon individualbeds is shown in the eight measured stratigraphic sections (Figs. 4 and 6-12).Two major diagenetic stages are distinguished: an early diagenetic stage, and a late diageneticstage. Both stages are recognized on the basis of their diagenetic fabrics and diageneticsuccessions during the evolution of the reef complex.

The early diagenetic stage is here defined by the physical and chemical changes occurring to thesediment during the transition from deposition to lithification at the sediment-water inter face or justbelow it. The early diagenetic stage includes constructional processes, resulting. from sedentary,frame-building organisms, addition of internal sedimentation and cementation. Destructional

processes resulted from burrowers, borers, grazers, browsers and predators, and by mechanicalbreakdown.The late diagenetic stage is defined by the physical and chemical changes occurring to thesediment after lithification but before metamorphic changes. According to Schmidt (1965) latediagenesis is not influenced by the environment of deposition or by the physico-chemical conditionsof the supernatant water. Late diagenesis may take place in the subsurface, in the subaerial orvadose zone, in the subaqueous or phreatic zone, or in surface exposures in which weathering isone the most important processes.FORE-SLOPE REEF ZONE

The fore-slope reef zone is exposed along the foot hills of the easternmost part of the Sierra de ElAbra at "Cementos Anahuac" quarry (Fig. 4), 9 kilometers northeast of the Taninul Station (Fig. 5).This zone is distinguished by its complex overlapping of lenses, layers and wedges of biomicriticlithofacies and scattered colonies of rudist bioherms 3 to 8 meters high. Some rudists are still intheir growth position (Plate 1-B) , and are attached to the substrate by their conical lower valve bymeans of encrusting organisms such as red coralline algae and stromatoporoids, or by acombination of several generations of neomorphic fibrous calcite cement and intemal sedimentwhich lithified the framework. Simultaneously, boring and burrowing organisms and mechanicalbreakdown caused by hydrodynamic forces partially destroyed the framework. Such processesformed numerous cavities which were subsequently filled with syngenetic cement and internal


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sediment that protected the biologic framework from total destruction.

Fig. 5. Stratigraphic cross-section in quarry exposures of the reef complex: El Abra Limestone,showing the lithofacies arid the faunal distributions across (he Sierra de El Abra. Location:

approximately 9 kilometers east of Ciudad-Valles, San Luis Potosi on Mexican Highway 70.


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SHELF-EDGE REEF ZONEThe shelf-edge reef zone is exposed at the "Taninul" quarry, on the easternmost side of the Sierrade El Abra (Fig. 5), adjacent to the Taninul Station, 14 kilometers east of Ciudad Valles, San LuisPotos.Sedimentary Facies-Description and Interpretation

The sedimentary environments in the shelfedge reef zone were influenced by the tectonic settingwhich controlled the rate of submergence and emergence of the Cretaceous platform. This resultedin the formation of a complex biologic framework composed of frame-builders and repeated cyclesof internal sedimentation, cementation, bioturbation and leaching. The biota was zoned ecologicallywithin the frarnework due to local changes in the sedimentary environment during a submergentstage.Four prominant sedimentary facies can be recognized in the shelf-edge reef zone on the basis oftheir biofacies, lithofacies, and stratigraphic position. From bottom to top these are: 1) monopleurid-requieniid-coral biolithite facies, 2) monopleurid-caprinid-requieniid biolithite facies, 3) caprinid-radiolitid biolithite facies, and 4) the shelf-margin calcarenite facies. Two additional facies,

interlayered within the above facies, are composed of unsorted rudist biomicrudite and biomicrite ofthe inter-reef and fore-reef facies (Plate 5-A). These facies form a very complex overlapping ofbanks, reef-cores, lenses, layers, and wedges. They vary from a few centimeters to several metersin thickness and lateral extent.1) the monopleurid-requieniid coral biolithite facies consists of small colonies of rudists and coralsoccurring in lenses 1 to 2 meters high and 8 to 30 meters in length. Layers and wedges of unsorted,commutated, mollusk-rich biomicrudite, coraline algae, echinoid fragments, encrustingstromatoporoids, and other miscellaneous organisms are also present (Fig. 6; Plates 6 and 7-A).Monopleurid rudists are abundant within this facies. Some shells still remain attached to thecalcarenitic substrate and in growth position (Plate 6-A). Requiend shells are locally abundantalthough, like colonial corals composed of Cladophyllia, they are subordinant components of thebiostromes (Plate 6-B, E). Gastropods and pelecypods are also locally abundant. Encrusting

organisms and calcite cement played an important role in the lithification of these biostromes. Themost abundant encrusting forms are coralline algae, stromatoporoids while bryozoans are rare.Thin-shelled ostracods and other benthonic: organisms are present with the micrite and biomicriteinterna] sediment deposited in the numerous cavities of the framework. Hence, the monopleurid-requieniid-coral biolithite facies is interpreted to represent a protected shallow-water marineenvironment of deposition.


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Fig. 6. Measured stratigraphic section II at "Taninul" quarry, showing the distribution patterns of themain lithofacies and biofacies forming the reecore, the fore-slope reef, and the shelf-margincalcarenite facies. Location: approximately 14 kilometers east of Ciudad-Valles, on railroadadjacent to the Taninul Station.

2) A transition zone is represented by the biocoenosis consisting of the monopleurid-caprinid-requieniid biolithite facies (Fig. 6) in which colonial corals have been replaced by isolated caprinidrudists. This facies consists of lenses of small rudist colonies. Monepleurids and caprinids are the

most prominent rudists withing biostromes that are 1 to 3 meters high and about 10 meters or morein lateral extent. Sofitary corals, gastropods and pelecypods are locally abundant. Also, encrustingred coralline algae and stromatoporoids are present, either attached to rudist shells or as detritalsediments. These form part of the calciruditic inter-reef and forereef layers. Other organisms suchas ostracods and biserial and uniserial microforaminifera are cornmon within the biomicritic andpelmicritic internal, cavity-filling sediments. In this facies, monopleurids dominate over the othergroups. The disappearance of colonial corals can be explained by the competition between coralsand rudists in this habitat, and/or the physical changes occurring in the environment in favor of therudist group. All these observational facts suggest a protected and shallow marine environment ofdeposition.3) The caprinid-radiolitid biolithite facies overlies the other two previously-described facies. Itconsists of biostromes of colonial caprinids and scattered radiolitids (Fig. 6). Some caprinid shells

remain in their growth positions (Plate 5-B, C). Monopleurids, when present at all, are reworked.Gastropods and pelecypods are common. An abundance of encrusting forms such as corallinealgae and stromatoporoids, combined with several generations of fibrous cements alternating withinternal micrite sediments containing planktonic: and benthonic microfossi1s suggest anoverlapping of unprotected and protected, shallow, normal marine environments.4) The shelf-margin calcarenite facies consists of biosparite, biosparrudite, and poorly washedintraclastic biosparite (Fig. 6). Bedding is well-developed, and low angle cross-bedding is presentmainly in those biosparites that are texturally well-sorted and rounded at the top of the stratigraphic


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section (Plate 5-B) . This facies contains essentially the same organisms as the blostromes fromwhich they were derived. In addition, reworked stromatoporoids (Parkeria sp.) are also present insome layers (Plate 5-F) . The beds are laterally discontinuous along the shelf edge and vary from afew centimeters to 4 meters in thickness.Diagenetic Description and Interpretation

The occurrence of the several overlapping diagenetic fabrics and, biologic communities in the reefzone reflects the complex relationship between "short periods of organically controlled growth inshallow water alternating with brief periods, of subaerial exposures and early diagenesis" (Perkins,1970). Different diagenetic processes occurred simultaneously at different sites of a calcareousconstituent. This is due to differences in the microenvironment; parts of the surfaces were boundedby algae and other organisms, and others were encrusting-free surfaces. Ginsburg et al. (1971) andSchroeder (1972 a, b) related cement differences to the substrate of a given specimen, concludingthat variations in the substrate were a mechanism for micro-environmental change.In the shelf-edge reef zone of the El Abra Limestone the diagenetic sequence usually occurred inthe following manner: a) one of the first vestiges of an early submarine cementation was that ofencrusting organisms binding rudist shells and other biotic constituents (Plate 9-D).The borings presented micro-environments in which micritic alteration rind form around the skeletalparticles. b) Crusts of fibrous calcite cement, and micritic rind succeeded, partially filling primaryvoids and cavities. c) The crusts of fibrous calcite cement lining voids were partially leached orreplaced by radiaxial fibrous calcite by dissolution of aragonite with precipitation of calcite. d) Aclear drusy calcite cement then grew with the same curved cleavage of the substrate (Plate 3-E; 7-D, E; 12-A, B, C). e) The remaining voids were filled with vadose silt and with bocky sparry calcitecement forming geopetal fabrics (Plate 7-C). f) During later periods of submergence, encrustingorganisms interrupted the process of vadose and phreatic sedimentation and cementation. Onceagain, marine cementation and internal sedimentation lithified the framework temporarily until theaction of burrowers, borers, predators, mechanical breakdown, internal erosion, nontectonicfracturing and leaching acted as natural destroyers of the biologic framework. Final preservation ofthe skeletal frame resulted from the complex dynamic interplay between several combinations of

processes and cycles such as constructional or depositional processes, and destructional orerosional processes. g) Later intense and permanent wave action swept the surface sediments,continuously reworking and sorting them to form the shelf margin calcarenite facies, generallycomposed of biosparite, with well developed bedding and low angle cross-bedding. h) In the finalemergence of the platform during later periods of exposure, the limestones were extensively andnonselectively dissolved, creating karst topography, and the remaining voids were partially filledwith vadose sediment, collapse breccias, and by drusy and blocky, sparry calcite cements.Three calcite cements were examined microscopically under cathodoluminescence (Plate 3-E, F) :1) the neomorphic fibrous calcite cement, 2) the radiaxial fibrous calcite cement and 3) the drusymicrestalactitic calcite cement.The neomorphic fibrous calcite cement shows as orange luminiscence; on the other hand, the

radiaxial fibrous calcite cement does not luminesce. The latter was described by Bathurst (1959,1971) as being characterized by curved, twin lamellae with optic axes that converge in the samedirection. Also, the drusy microstalactitic calcite cement does not luminesce. Sippel and Glover(1965) and Meyers (1974), stated that the orange cathodo-luminescence of calcite is due to thepresence of divalent manganese lons within the calcite lattice. The absence of luminescence incalcite crystals in thought to be due to an enrichment, commonly in ron, in the lattice, or simplyabsence of manganese. Therefore, the three calcite fabrics were examined with the electron-microprobe (Plate 4) in order to document the distribution pattems of manganese, ron andstrontium. All three cements are poorly enriched in manganese and ron. Hence, neither the type of


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activator ion responsible for producing orange luminescence in the neomorphic fibrous calcitecement, nor the ions for inhibiting luminescence in other calcite crystals were evident in this study.However, strontium is always distributed quite uniform1y in all three calcite mosaics. According to,Bathurst (1971), the strontium content in limestones decreases during diagenesis to commonvalues which range from 160 to 380 ppm; similar concentrations were found for the severalcarbonate constituents analyzed here from different environments (Fig. 13).NEAR-BACK REEF/LAGOON ZONE

The near-back reef/lagoon zone is represented by the stratigraphic sections III and IV (Figs. 7, 8),12.5 and 11.5 kilometers east, respectively, of Ciudad Valles, San Luis Potos, on Mexican Highway70.Sedimentary Facies-Description and Interpretation

The near-back reef/lagoon zone is composed of a wide range of texturally angular and unsortedcalcarenite and calcirudite derived from the peripheral reef-flat area and thrown back by ware actionand during storms. These are interlayered and laterally interfingered with the lagoonal sedimentscomposed of pelmicrite, micrite, and stromatolitic layers. The various sedimentary accumulations

were controlled by normal tide levels and by storm washovers or by storm tides. Three tidal zonesare distinguished on the basis of sediment type, faunal content, and sedimentary structures: 1)subtidal zonesediments deposited below low tide; 2) intertidal zone-sediments deposited betweennormal low and normal high tide; 3) supratidal zonese-diments deposited above normal high tidebut within the range of spring and storm tides.

Fig. 7. Measured stratigraphic section III, showing the lithology, the primary sedimentary structures,the biota and the environments of deposition in the near-back reef/lagoon zone. Location:aproximately 12.5 kilometerseast of Ciudad-Valles. San Luis Potosi on Mexican Highway 70.


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Fig.8. Measured stratigraphic section IV, showing the lithology, the primary sedimentary structures,the biota and the enviroments of deposition in near-back-reef/lagoon zone. Location: approximately11.5 kilometers east of Ciudad-Valles, San Luis Potosi on Mexican Highway 70.

The rocks in the subtidal zone are composed of interbedded massive, dark and light gray biomicriteand pelmicrite with nodular aspect attributable to burrowing. Most of the burrows are horizontallyoriented, roughly parallel to bedding. Burrowers destroyed the primary laminations. The biomicriteincludes abundant miliolids (Nummoloculina), scarce ostracods, and planispiral and biserialmicrofossi1s (Plate 14-F).The intertidal zone includes interbedded and wedged light to cream biomicrite, pelmicrite, andintramicrite with a wide range of texturcs depending on local conditions of bioturbation and flowregime. Elsewhere, primary laminations generally are lacking due to burrowing organisms. Themost noticeable structures are root hairlike structures similar to those described by Shinn et al.(1969) from the modern tidal-flat of Andros Island, Bahamas. Weathered intraclasts, as weIl asreworked rudist shells, red coralline algae, bryozoans, echinoids and corals, are found interlayeredwithin the lagoonal sediments. These were probably deposited during washover stages of by actionof storm tides. The indigenous fauna consist of requieniids forming banks, gastropods such asNerinea (Plate 15-D), scattered pelecypods, miliolids, ostracods, and other benthonic microfossils.Stromatolites and pelmicrite are interlayered in the upper intertidal zone.The stratigraphic sequence in the supratidal zone includes a wide textural range of intramicrite andbiomicrite, commonly associated with weathered intraclasts, and weathered surfaces (Plates 14-B,15-A, C). The lightcolored biosparitic layers, probably of storm origin, alternate with dark-coloredmicrite, which could have been deposited originally in shallow and stagnate water. Such dark color


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may be the- result of the presence of organic matter causing reducing conditions in localsubenvironments. The fauna is restricted to scattered, gastropod and pelecypod shells, weatheredand reworked rudist shells, corals, red-coralline algae, and stromatolites of probable blue-greenalgal origin.Diagenetic Description and Interpretation

Most of the processes that acted during diagenesis in the near-back reef /lagoon zoneaccomplished the following four operations: bioturbation, dissolution, internal sedimentation, andcementation. They either alternated or acted simultaneously during and after the deposition of thestratigraphic sequence.Bioturbation by burrowers was the first mechanism to destroy primary laminations; lime mud wasexpelled by the organisms and then removed by local currents. The burrows acted as sedimenttraps; therefore, they were filled with finer particles than those in the surrounding matrix. Birdseyevoids occurring between algal laminations also served as sediment traps. Calcareous constituentssuch as miliolids, pellets, and intraclasts were held by the filamentous framework while theremaining interparticle porosity was fully cemented. The cavities in both cases were first filled withfine-grained internal sediment and later with drusy and blocky sparry calcite cement (Plate 16-A, B).Biological erosion by borers including plantroots and some mollusks are commonly observed. Plant-roots penetrated into the cemented substrate and into partially lithified sediments. The resultingbores were filled with the overlying sediment after decay of the organic matter, and those whichremained empty were lined with drusy sparry calcite cement and later filled with asphalt (Plate 14-B).During short periods of subaerial exposure, fresh water partially dissolved existing carbonatesediments. The resulting voids were formed with little discrimination between one part of the rockfabric to another. During another period of submergence the cavity walls were lined with fibrouscement, and the floor was lined with micrite and pelmicrite internal sediment con taining thin-shelledostracods and other benthonic microorganisms Alternating neomorphic fibrous calcite cement,internal sediment, drusy sparry calcite cement, vadose silt and blocky sparry calcite cement are

commonly observed. These formed during cycles of emergence and submergence of theCretaceous platform. The final configurations of such vugs are digitate tops and flat-laminated floorsdescribed as "stromatactis" by Lowenstam. (1950), Bathurst (1959), Schwarzacher (1961), Shinn(1968c), Heckel (1972), and others.Two theories have been formulated to explain the origin of such structures: 1) the biologic theorystates that stromatactis are recrystallized remains of framebuilding sediment trappers or the decayof soft organisms, 2) the inorganic theory explains stromatactis as being due to a non-uniformcompaction and collapsin of loosely-packed lime mud sediment. An intermediate point of view isgiven by Shinn (1968c). He suggested that stromatactis formed by selective leaching of burrow-filling after lithification.Stromatactis-like structures may have different origins in different places. These in the nearback

rcef/lagoon zone probably formed by dissolution of previously lithified carbonate sedments.Originally the voids were formed by dissolving the limestone indiscriminately, irrespective of thetexture or the primary environment of deposition. They were observed in unsorted intramicrudite ofthe supratidal environment (Plate 14-C, E); in pelmicrite of the intertidal environment (Plate 14-D,G); and in miliolid biomicrite of the subtidal environment (Plate 14-F).

Apparently the voids originally had an irregular configuration. During successive stages ofernergence and submergence, the voids were filled with cement and internal sediment in the threediagenetic zones: marine, phreatic and vadose zones. The resulting structures are characterized by


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flatlaminated floors and irregular roofs. They contain several cycles of sedimentation andcementation patterns, according to their stratigraphic position. At lower stratigraphic levels, severalcements and internal sediments were deposited in the solution voids formed from the marine zoneto the phreatic and vadose zones due to vertical fluctuations of the water-table. At upperstratigraphic levels the vadose zone only sporadically became the phreatic zone when the level ofthe water-table rose during short periods of submergence. Vadose internal sediment was overlain

by fibrous calcite cement and internal sediment containing benthonic microorganisms. Theremaining voids were filled later with microstalactitic drusy calcite and with blocky calcite cement(Plate 16-C).The cements and internal sediments deposited in the stromatactis-like structures were examinedunder catholuminescence. The neomorphic f ibrous calcite cement shows luminescence similar tothe neomorphic fibrous calcite cement interpreted to be of marine origin in the foreslope reef zone(Plate 3-A, C) . In both cases, the fibrous cement shows interruptions during growth stages (Plate17C, D and E, F). Finally, a clearer crystalline sparry calcite cement developed with a curvedcleavage which is characteristic of the radiaxial-fibrous mosaic. Excluding the fibrous cement, allothers lack luminescence. Therefore, from these results and by their petrographic association it issuggested that the fibrous cementation occurred during the early submarine diagenetic stage,alternating with micrite internal sediment containing benthonic microfossils. During the late subaerialdiagenetic stage the other cements and the associated reddish-vadose internal sediment and thevadose calcite silt were deposited in the phreatic and vadose environments, which were controlledlargely by repeating emergence and submergence of the Cretaceous platform.TIDAL-FLAT AND LAGOON ZONES

The tidal-flat and the lagoon zones are represented by the stratigraphic sections V, VI, VII and VIII(Figs. 9, 10, 11, 12) 11.0, 10.5, 10.0 and 9.6 kilometers east, respectively, of Ciudad Valles, SanLuis Potos, on Mexican Highway 70.

Fig. 9. Measured stratigraphic section V, showing the lithology, the primary sedimentary structures,the biota and environments of deposition in the tidal-flat lagoon zone.


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Fig. 10. Measured stratigraphic section VI, showing the lithology, the primary sedimentarystructures, the biota and the enviroments of deposition in the tidal-flat/lagoon zone. Location:apprximately 10.5 kilometers east of Ciudad-Valles, San Luis Potosi on Mexican Highway 70.


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Fig. 12. Measured stratigraphic section VIII, showing the lithology, the primary sedimentarystructures, the biota and the enviroments of deposition in the tidal-flat/lagoon zone. Location:approximately 9.6 kilometers east of Ciudad-Valles, San Luis Potosi on Mexican Highway 70.

Sedimentary Facies-Description and Interpretation

The tidal-flat zone is composed of a stratigraphic sequence of pelmicrite, biomicrite and stromatoliticlayers in association with subaerially formed features and bioturbated horizons. The sedimentsdeposited in this environment are interlayered and laterally discontinuous. The lagoon zone iscomposed of pelletal-biomicrite and biomicrite rich in miliolids and other scattered benthonicmicrofossi1s. The rocks are gray because their high content in organic matter causing reducingconditions in local subenvironments, or in part, because of asphalt impregnation. The bedding isquite uniform, reflecting continuous sedimentation, which was only disrupted by wide-spreadbioturbation and sporadic storm disturbances (Plate 20-F).In the tidal-flat zone, three environments are distinguished: 1) the subtidal, 2) the intertidal, and 3)the supratidal. The subtidal rocks consist, of pelmicrite and miliolid biomicrite interlayered with otherbeds ranging from the intertidal to supratidal environments. The bedding is not well developed;instead it is wedged or lens-shaped. Again, bioturbation is apparent in every layer (Plate 19-A);

therefore, primary laminations are absent due to burrowing organisms.The sediments deposited in the intertidal environment consist of light cream colored pelmicrite,biomicrite and intramicrite, with a wide range of textures. They are wedgeshaped and bedded.Laterally, the rocks show continuous changes of lithofacies, probably reflecting local variations inwater conditions.The biota, at the lower intertidal zone, is composed of small banks of Toucasia (Plate 19-D),scattered accumulations of gastropod and pelecypod shell fragments broken and unsorted to


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various degrees due to bioturbation. Other benthonic organisms are commonly associated withthem, such as thin-shelled ostracods and miliolids. Also, mixtures of planktonic microfauna of thenormal marine environment, and miliolids of the back-reef environment were deposited in a thin andwavy layer 1 to 10 cm thick (Plates 19-F and 21E, F). This layer perhaps was deposited duringspringhigh tide washover. The upper intertidal zone is composed of laminated stromatolitesinterlayered with biomicrite, intramicrite and pelmicrite (Plates 18-A, F and 19-E), associated with

birdseye vugs.

The supratidal environment consists of stromatolitic crusts, and thin layers of biomicrite, lightcolored, poorly washed biosparite, and intrasparite which perhaps represent storm deposits. Therocks are commonly associated with birdseye vugs, root-like structures, weathered surfaces, mud-cracks, and other subaerially formed features (Plates 18-C, D, E and 20-A, D). The supratidalhorizons are laterally discontinuous and interfingered with rocks deposited in the intertidal andsubtidal environments.

The biota is restricted to scattered gastropods and other reworked and weathered mollusk shells,echinoids, red coralline algae, bryozoan and rudist fragments. These were transported from theperipheral reef flat area and thrown back during spring tides or during storm washovers.

Diagenetic Description and Interpretation

Sediments and primary sedimentary structures in the tidalflat and lagoon zones were modified andpartially destroyed initially by bioturbation, early compaction and desiccation and later by leaching. Most parts of the tidal-flat and lagoon layers were high1y bioturbated by bottom-dwelling organismsdestroying almost any vestige of primary laminations (Plate 19-A). Root-like structures inassociation with weathered stromatolitic crusts are evident. They show a preferential verticalorientation (Plate 18-C).Early compaction is noticed in sediments of the intertidal environment (Plate 20-E). Toucasia shells.were crushed in place after partial filling with micrite internal sediment. During later periods oflithification, the remaining voids were filled with drusy and blocky sparry calcite cements.Desiccation features are common in the upper intertidal and supratidal environments. Polygonalmud-cracks are present in pelmicrite and in biomicrite containing miliolids interlayered withstromatolites of the upper intertidal zone (Plates 18-A, B, F). Weathered stromatolite crustsinterlayered with discontinuous micrite laminations, lacking organisms, and layers of intramicrite andpoorly washed biosparite, which probably represent storm. deposits, are common depositionalfeatures of the supratidal environment (Plates 18-D, E and 20-A). Primary diagenetic features,common in both environments, include birdseye vugs cemented early in diagenesis after previousdeposition of micrite internal sediment and vadose calcite silt. lt is presumed that the vadose siltwashed down through the vadose zone and accumulated within the pore space (Dunham, 1969).The remaining voids were filled with drusy and blocky sparry calcite cements (Plates 20-D and 21-

A, B).Stromatactis-like structures are found in rocks of the subtidal erivironment (Plates 18-A and 19-B)They are not as abundant and not as complex internally as those from the near-back reef/lagoonzone. The peripheral configuration of the stromatactis-like structures is a flat-laminated floor anddigitate top. However, these structures described herein are flatter and smal ler than in the near-back reef/lagoon zone. Their walls were rimmed with a first generation of fibrous cement. Duringdiagenesis, the fibrous crystals became neomorphically radiaxial-fibrous cement. At the top of thecavity a drusygravity and clear sparry calcite cement grew after radiaxial crystals with similar curved


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cleavege of the substrate. On the floor of the cavity the radiaxial-fibrous cement was overlain byvadose calcite silt. Remaining voids were filled with clear blocky sparry calcite cement (Plate 21-C).Finegrained sediments were modified by leaching of micrite aggregates. Resulting pores are largerthan the original grain size (Plate 22).Pressure-solution features are characteristic in both the lagoon zone and tidal-flat zone. Basical, ly,

two systems of stylolites are evident; one cutting transverse to the bedding, and the other parallel tothe bedding. Late diagenetic features are represented by solution caverns and collapsebrecciationwhicli developed during the latest period of surface exposures, forming a karst-like surface.

DIAGENETIC SUMMARY OF THE EL ABRA LIMESTONE REEF COMPLEX

Submarine and subaerial diagenesis occurred during, the reef-growth period. These were controlledlargely by repeating emergence and submergence of the Cretaceous platform. A summary of thepetrographic criteria considered most important for distinguishing between and early submarinediagenesis and a late subaerial diagenesis in the several sedimentary environments of the El AbraLimestone are stated in Table 1.

TABLE 1 PETROGRAPHIC CRITERIA FOR DISTINGUISHING BETWEEN AN EARLY SUMARINEDIAGENESIS AND A LATE SUBAERIAL DIAGENESIS DIAGENISIS IN THE EL ABRALIMESTONE


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GEOCHEMICAL ANALYSIS

A total of 105 samples of - different carbonate constituents from the El Abra Limestone reefcomplex were analyzed for concentrations of the minor elements magnesium and strontium, andthe concentration of the major element calcium. A Perkin-Elmer 303 atomic absorption

spectrophotometer was used for the chemical determinations; the precision is considered reliable to 5%.Sample Preparation

Samples were extracted from the matrix with a dental drill and dried in an oven for about two hoursat 110 C. After drying, the samples were placed in a desiccator for approximately 30 minutes. Then0.5 g of each sample was weighed and dissolved in 1N hydrochloric acid. The insoluble residuewas removed by filtering through preweighed fiber glass filters, which were dried for about twohours at 110C and then placed in a desiccator for cooling. The filters were weighed again, and theweight percent insoluble residue was calculated.The determination of strontium and magnesium in carbonates by atomic absorptionspectrophotometry is interference~free (Angino and Billings, 1967). The method of analysis wasthat described in the Perkin-Elmer handbook. Standard solutions were prepared and diluted to theproper concentrations for the detection limits of the instrument, then analyzed together with thesamples.Results

The results of the samples analized are plotted in Figure 13.Carbonate constituents from both reef and back-reef environments were analyzed for calcium,magnesium and strontium. No significant correlation is found among the three ions in a particularcarbonate constituent collected in diferent environments. The magnesium content in most reef

samples is generally less than 2 000 ppm. In back-reef samples magneslum generally exceeds 2000 ppm. The blocky calcite cements have a greater enrichment in calcium (38.5 percent average)than other carbonate constituents in both reef and back-reef environments. Strontium ranges from76 ppm to 458 ppm in both reef and back-reef samples. It does not exhibit diagnostic trends thatwould permit the identification of either major depositional environment.Weight-percent of insoluble residue obtained from samples of both major sedimentary environmentswas. insignificant in terms of total weigth-percent (1.5 weigth-percent average). Asphalt residue wasthe major contributor.Discussion of Results

Although five sub-environments are identified on the basis of stratigraphic position, lithofacies,

biofacies, primary sedimentary structures, and diagenetic fabrics, only the two major sedimentaryenvironments (the reef environment and the back-reef environment) are distinguishable by meansof their chemical composition. The back-reef environment has higher magnesium content (> 2 000ppm) than the reef environment (< 2 000 ppm) . Strontium concentrations display no diagnosticpatterns that would permit the discrimination of either major sedimentary environment. Bathurst(197la) and Kinsman (1969) stated that ancient limestones have normal strontium concentrationsranging from 160 to 380 ppm; however, some range from 70 to 630 ppm. The El Abra Limestonesamples did, in fact, range from 76 to 458 ppm.


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There are three possibilities for the present distribution of magnesium in the two major sedimentaryenvironments of the El Abra Limestone: 1) the magnesium content in both environments wasinitially similar, 2) the magnesium content -was initially higher in the reef environment than in thebackreef environment, 3) the magnesium content was initially higher in the back-reef area.The third possibility seems to be the best explanation because the back-reef environment was

shallow, protected and somewhat restricted, as evidenced by the type of sedimentary structures,micritic and pelmicritic sediments, abundance of miliolids, and stromatolites of probable blue-greenalgal origin. The back-reef environment was not hypersaline as indicated by the lack of dolomitecrusts and evaporites. It was, however, shallow enough to facilitate formation of mud-cracks,birdseye vugs and other envidences of exposure.


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Fig. 13. Variations of magnesium and strotium contents in various carbonate constituents, from thereef and back-reef environments. In reef samples, the magnesium content generally averagedbelow 2000 ppm while back-reef samples averaged more than 2000 ppm. Strontium does not


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exhibit variations in concentration sufficiently diagnostic to permit discrimination of either majordepositional environment.

Initial concentration of magnesium in stromatolites could occur by alternate wetting during tidal

flooding and drying during subaerial exposure. The gelationus layers of blue-green algae aresuggested to precipitate high magnesian calcite containing 15 to 20 mole percent of MgCO3 inmodern carbonate platforms (Shinn et al., 1965; Gebelein and Hoffman, 1971; 1973).

Another possible means for concentratinomagnesium in the carbonate sediments includes theaccumulation of benthonic organisms. Miliolids in modern environments, in fact, are composed ofhigh magnesian calcite. Blackmon and Todd (1959) and Chave (1954) noted that foraminiferaincrease their magnesium content when water temperature increases. Generic factors may alsoplay a significant role in determining the magnesium content at any given temperature as noted byPilkey and Hower (1960); Lowenstam (1961); and Dodd (1965).

According to Schroeder (1969), the magnesium and strontium contents in invertebrate shells fromboth marine and fresh water environments are controlled by the availability of these elements,

skeletal mineralogy, phylogenetic effects, and the temperature and salinity of the environment. Hestated that shells change chemically as soon as the organism dies, establishing a chemicalequilibrium with the surrounding water. This probably occurred in the biologic community in theback-reef environment of the El Abra Limestone. However, it is also expected that a generaldecrease of magnesium occurred in the skeletal constituents during diagenesis (Lowenstam, 1961;Schroeder, 1969; and others).This decrease is directly related to the degree of shell alteration occurring after the death of theorganism (Pilkey, 1964; Schroeder, 1969; and others). On the other hand, Griffith et al. (1969)estimated that the reef environment of the El Abra Limestone contains abouth 40 percent rudistshells, 20 percent void-filling calcite cement, 10 percent each of coralline algae andstromatoporoids, and 5 percent corals. Although no quantitative analysis was made in this study,their estimations are considered reasonable on the basis of field and hand specimen observations.Due to the scarcity of major suppliers of magnesium in this environment, including coralline algae,echinoids, stromatoporoids (?) , and benthonic foraminifera, and also because of the abundance ofmollusk shells and neomorphic fibrous calcite cement probably composed originally of aragonite, itis suggested that in the reef environment the magnesium content was initially lower than in theback-reef environment. Since the magnesium and strontium contents of high magnesian andaragonitic constituents tend to be reduced during diagenesis, it is unlikely that the concentrationsdetermined (Fig. 13) for the different carbonate particles represent the original concentrations.

DOLOMITIZATION OF THE EL ABRA LIMESTOME IN THE SUBSURFACE

The El Abra Limestone in the subsurface is dolomitized (Carrillo, 1971; Moya, 1974). A possiblemechanism for such dolomitization during a late stage of diagenesis is suggested herein. Harmon(1971) determinated magnesium and other ions in the groundwater of the Sierra de El Abra. Hefound that most waters were saturated with respect to calcite and undersaturated with respect todolomite. Therefore, the process of leaching limestone with meteoric water, vadose percolation, andgroundwater reflux does not appear to be the only mechanism involved in dolomitization of the El

Abra Limestone, because a requirement for regional dolomite formation is a large supply ofmagnesium ions. Hanshaw et al. (1971) found that most carbonate aquifers in the Bahamas andYucatan platforms are lacking sufficient magnesium to cause extensive dolomitization. They


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proposed that dolomitization could occur in the brackish water zones. Such water comes frommixing of fresh water with subsurface brines or ocean water. Land (1973) stated that dolomitizationis favored in the zone of mixing between sea water and meteoric water. He suggested that seawater was the source of magnesium ions to dolomitize Pleistocene limestones. Deffeyes et al.(1965) suggested a mechanism of seepage reflux. A brine produced by evaporation is denser thansea water and flows downward into the carbonate sediments and dolomitizes the permeable

sediments. Fisher and Rodda (1969) proposed that dolomitization in the Edwards Limestoneresulted from metasomatism due to the action of magnesium-enriched brine on calcium carbonatesediment. The source of magnesium ions is an extensive evaporite facies located at the center of aformer lagoonal area, a process which probably also happened to the El Abra Limestone. Carrillo(1971) reported a maxium thickness of 3 000 meters on Neocomian-Aptian evaporites located inthe central portion of the Cretaceous Valles-San Luis Potosi Platform. He stated that such athickness may be exaggerated due to intense folding. However, magnesium could have beenremoved from such evaporites by dissolution from percolating meteoric waters moving into thesubsurface. In this manner groundwater reflux, rich in magnesium ions, partially dolomitized therocks of the El Abra Limestone, "Tamabra" Limestone, and Tamaulipas Superior Limestone in thesubsurface.

GEOLOGIC EVOLUTION OF THE SIERRA DE EL ABRA

The basement in eastern Mexico consists of Precambrian and Paleozoic igneous and metamorphicrocks, which were folded and faulted in Late Triassic and Early Jurassic times, resulting in a chainof horst and graben structures. The uplifted blocks were irregularly peneplained by the processes ofsubaerial erosion as the Jurassic sea transgressed the alluvial peneplains. These were slowlymodified by subaerial processes to form a set of broad hills bounded by shallow carbonate shelves.One of these shelves is known as the Cretaceous Valles-San Luis Potosi Platform (Carrillo, 1971).Early carbonate sedimentation was strongly influenced by the influx of terrigenous sediments andfresh water, forming a brackish, restricted marine environment and also evaporite zones on theinterior of the platform, while oolite bars and calcarenite blankets were deposited at the seawardmargin. Deposition of shallow marine sediments was controlled largely by a series of transgressions

and minor regressions. Reef complex growth was intimately related to normal faulting. The El AbraLimestone was deposited on an upfaulted block, while on the downfaulted block the "Tamabra"Limestone, a transitional fore-reef to basinal pelagic sediments of the Tamaulipas SuperiorLimestone to the east through late Aptian (?) -Albian and Cenomanian time (Fig. 14). DuringTuronian time the Agua Nueva Formation was deposited, interfingering with the "Tamabra"Limestone to the west.

The reef complex was exposed and a karst surface formed during Coniacian-Santonian time whiledeposition of the San Felipe Formation began to the east. The Valles-San Luis Potosi Platform wassubjected to renewed subsidence in early Campanian time. The karst surface of the El AbraLimestone was covered with pelagic sediments, which filtered into the solution voids. At this time,due to oil migration, the finegrained pelagic sediments filling the cavities were impregnated withasphalt.Throughout late Campanian time the platform was tilted slight1y eastward. Those sediments restingon the platform slid eastward under the influence of gravity (Carrillo, 1971). As a result, a distalturbidite, the "Tamuin" Member, was deposited along the eastern margin of the Valles-San LuisPotosi Platform. The "Tamuin" Member interfingers with the uppermost San Felipe Formation andwith the middle Mendez Shale.The upper Mendez Shale and the Chicontepec-Velasco Formations were deposited throughMaestrichtian and Eocene time, respectively. Later the stratigraphic sequence was intensively


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deformed by Laramide stresses (Fig. 15), giving rise to faulted synclines and anticlines of the SierraMadre Oriental. The overall depositional history in this arca, in brief, has been one of regressionwith minor transgressions since the Laramide orogeny.

Fig. 14. Generalized geologic evolution of the easternmost portion of the Valles-San Luis Potosiplatform in the Tampico Region.


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Fig. 15. Three generalized stratigraphic cross-sections of the eastern margin of the CretaceousValles-San Luis Potosi Platform at different sites; based on surface and subsurface data.

Conclusiones

1) The El Abra Limestone reef complex was deposited on the Cretaceous Valles-San Luis PotosiPlatform, and is underlain by rocks of Upper Jurassic and Lower Cretaceous age. The El AbraLimestone was deposited on an upfaulted block, while the "Tamabra" Limestone and theTamaulipas Superior Limestone were deposited on the downfaulted block to the east.2) The continuous subsidence of the Cretaceous platform caused the El Abra Limestone to bedeposited as repeated shallow marine carbonate facies, reaching about 1800 meters thickness atthe eastern edge of the platform.3) The El Abra Limestone is Albian to late Turonian or younger in age. A younger age for the uppertwo thirds of the exposed El Abra Limestone on the basis of a thin and wavy layer 1 to 10 cm thickcontaining planktonic foraminifera. This diagnostic bed is interlayered with biomicrite containingmiliolids present in quarry exposures in the back-reef environment.


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4) Two major sedimentary environments are defined within the El Abra Limestone: 1) the reefenvironment and 2) the back-reef environment. These are subdivided into five sub-environments:la) the fore-slope reef zone, lb) the shelf-edge reef zone, 2a) the near-back reef/lagoon zone, 2b)the tidalflat/lagoon zone, and 2c) the lagoon zone. Each zone is recognized by means ofstratigraphic position, lithofacies, biofacies, primary sedimentary structures, and diagenetic fabrics.

5) The fore-slope reef zone is distinguished by its complex overlapping of biomicrudite andscattered colonies of rudist bioherms. Caprinids and scarce radiolitids are present and areencrusted by several generations of fibrous and micritic cements and internal sediment, containingplanktonic and benthonic microfossi1s deposited in the interior of the voids. The shelf-edge reefzone is distinguished by a greater diversity of skeletal forms than the other subenvironments. Thebiota is zoned ecologically according to local changes in the sedimentary environments. Theinteraction between biologic communities and sediments, and subsequent diagenesis of theseresulted in the formation of a complex biogenic framework. exhibiting repeated cycles of internalsedimentation, cementation, bioturbation, and leaching. The near-back reef/lagoon zone iscomposed of unsorted biomicrudite and intramicrudite derived from the peripheral reefflat arca andthrown back by wave action and during storms. These are interlayered and laterally interfingeredwith the lagoonal sediments composed of biomicrite and stromatolitic layers. The diageneticprocesses were largely controlled by the repeated emergence and submergence of the platform.The tidal-flat zone and lagoon zone are composed of stromatolitic layers, pelmicrite and biomicritecontaining abundant miliolids. These subenvironments were subjected to normal tidal fluctuationsand storm washovers which resulted in a complex interlayering of sediments deposited in thevarious local subenvironments. Most of the prominent diagenetic features are attributable tobioturbation, early cementation and early compaction, which alternate with other subaerial featuresassociated with lowered stands of sea level.

6) Although two major sedimentary environments and five sub-environments are recognized on thebasis of their physical and paleontological attributes, only the reef environment and the back-reefenvironment have recognizable differences from measurements of their chemical composition. Ofthe three elements analyzed only magnesium exhibited significant concentration differences in thetwo environments. The magnesium content of most carbonate constituents is generally less than2000 ppm in the reef environment, and greater than 2000 ppm in the back-reef area. Variations inthe strontium concentration do not permit its use as a discriminating criteria between these twomajor environments.7) The metastable carbonate constituents, aragonite and high magnesian calcite, convertedneomorphically to calcite. Magnesium and strontium were lost by pluvial leaching and vadosepercolation through the El Abra Limestone.8) The El Abra Limestone has been dolomitized in the subsurface because a generous supply ofmagnesium ions was available from evaporites in the central portion of the Cretaceous platform.The magnesium in solution was carried downward by groundwater reflux.Agradecimientos

The author offers special thanks to Richard M. Mitterer, David E. Eby, Emile A. Pessagno ir., andMark Landisman from The University of Texas at Dallas; to Jerry Namy from Baylor University; toJames L. Wilson from Rice University; to Paul Enos from New York State University at Binghamton,and Baldomero Carrasco V. from the Instituto Mexicano del Petrleo for critically reading themanuscript; to James L. Carter and James Toney for their gracious help with the use of thescanning electron microprobe; to W. Thomas Rothwell for his valuable he1p with the use of thescanning electron microscope.The author also acknowledge to Fernando Moya from Petrleos Mexicanos for suggestions related


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to the work; to Alfredo Laguarda-Figueras and Alejandro Yez Arancibia from the Centro deCiencias del Mar y Limnologa, UNAM, for giving facilities to publish this work. Finally, the authorextends thanks to his wife for her patience and understanding, and to Rosamara Hefferan whotyped the final copy. This project was supported in part by the Institute for Geosciences of TheUniversity of Texas at Dallas and by the Consejo Nacional de Ciencia y Tecnologa de Mxico.

LITERATURA

AGUAYO C., J. E., Sedimentary environments and diagenetic implications of the El Abra Limestoneat its type locality, East Mexico. Ph. D. Dissertation. The University of Texas at Dallas 1975 159

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ALEXANDERSON, T. Micritization of carbonate particles; processes of precipitation and dissolutionin modern shallowmarine sediments. Geol. Instn. Univ. Uppsalla N. L. 3, H. 1972 201-236

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Plate 1. A. Taninul Member at "Cementos Anahuac" quarry (S-I). Notice wedges and lenses ofinter-reef biomicrudites with steep dip due to fault. Scale bar-12 m. B. Cross-sectional view of acolony of caprinids in growth position forming the reef frarnework at "Cementos Anahuac" quarry(S-I). Scale bar-10 cm. C. Reef-core facies (S-I). Notice rudist shells and multiple stages ofneomorphic fibrous calcite cementation and internal pelletal sediments containing microfossils. Cut

and fill microchannel and synsedimentary disconformities are shown by arrow. Scale bar-2 cm(polished slab). Sample I-195. D. Reefcore facies (S-I). Notice multiple generations of fibrouscement and micritic rind rimming rudist shells, which are impregnated with asphalt. See arrow.Scale bar-2 cm (polished slab). Sample I-209. E. Reefcore facies (S-I). Rudist shell (s) rimmed withseveral generations of micritic rind and neomorphic fibrous calcite cement. Scale bar-0.3 mm (thin-section, crossed nicols). Sample I-27. F. Reef-core facies (S-I). Interference of neomorphic fibrouscalcite cements filling interpartide porosity. Notice micrite rind rimming leached rudist shell(s). Scalebar-0.3 mm (thin-section, crossed nicols). Sample I-209.


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Plate 2. A. Reef-core facies (S-I). Alternating neomorphic fbrous cacite cement and internalpelletal sediment bearing microfossils, filling interparticle porosity. Scale bar-0.5 mm (thin-section,crossed nicols). Sample I-208. B. Reef-core facies (S-I). Geopetal structure formed by pelmicrite,pelmicrosparite, and blocky sparry calcite cement, filling rudist void. Scale bar-1 mm (thin-section,crossed nicols). Sample I-7. C. Reef-core facies (S-I). Rudist shell (s) rimmed with neomorphicfibrous calcite, which is overlain with pelmicrite bearing microfossi1s. Scale bar-0.5 mm (thin-

section, crossed nicols). Sample I-18. D. Reef-core facies (S-I). Rudist shell (s) encrusted by anunidentified algae(a), then rimmed with neomorphic fibrous calcite crust, which is overlain withpelmicrite internal sediment. Scale bar-0.5 mm (thin-section, crossed nicols). Sample I-18. E. Reef-core facies (S-I). Notice degrading neomorphism on top of neomorphic fibrous calcite crystals,shown by arrows, then overlain by pelmicrite internal sediment. Scale bar-0.2 mm (thin-section,crossed nicols). Sample I-18. F. Reef-core facies (S-I). Notice degrading neomorphism on top ofneomorphic fibrous calcite crystals, Pelmicrite internal sediment filling inter particle porosity. Scalebar-0.2 mm (thin-section, crossed nicols). Sample I-18.


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Plate 3. A. Reef-core facies (S-I). On top, neomorphic fibrous calcite. Below right, pelmicrite fillinginterparticle porosity. Notice degrading neomorphism at the contact o the neomorphic fibrouscalcite and pelmicrite internal sediment. Scale bar-0.5 mm (thin-section, crossed nicols). Sample I-18. B. Same as above, photographed by luminescent light revealing 6 growth bands. The lightareas correspond to bright orange luminiscence, and the dark areas are lacking luminiscence.Reference points shown by arrows. Scale bar-0.5 mm (thin-section). C. Reef-core facies (S-I). Ontop, neomorphic fibrous calcite. Below right, pelmicrite filling interparticle porosity. Notice degradingneomorphism at the contact of neomorphic fibrous calcite and internal sediment. Scale bar-0.5 mm(thin-section, crossed nicols). Sample I-18. D. Same as above, photographed by luminescent light

revealing 5 growth bands. Reference points shown by arrows. Scale bar-5 mm (thin-section). E.Shelf-edge reef zone (S-II). Neornorphic fibrous calcite cement (f) lining voids became radiaxialfibrous calcite (r) in early diage nesis, then clear drusy sparry calcite (d) growth in continuity withsubstrate. The zones A, B, and C were analyzed with electron microprobe, see Plate 4. Scale bar-1mm (thin-section, crossed nicols) Sample II-11. F. Same as above, neomorphic fibrous calcitecement shows orange luminescence (light areas). Radiaxial and drusy sparry calcites are lackingluminescence (dark areas) . Reference points shown by arrows. Scale bar-1 mm (thin-section,luminescent light photograph).


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Plate 4. Electron beam scanning photographs of neomorphic fibrous calcite (A), radiaxial calcite (B)and clear drusy sparry calcite (C), lining a void. Notice enrichment in strontium, which shows quitenuniform distribution in the three different calcite cements, and poor enrichment in manganese andiron in the cements. See Plate 3-E for reference. Scale bar-90 microns (thin-section).


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Plate 5. A. Wedges and lenses of biomicrudite in the fore-reef facies. Taninul Hotel. See man forscale. B. Caprinid-lenses overlain by low angle cross-bedded calcarenitic blankets. "Taninul" quarry(S-II). Scale bar-20 m. C. Close-up of a caprinid biolithite (S-II). Notice rudist colony in growthposition. See hammer for scale. D. Shelf-margin calcarenite facies (S-II). Synsedimentary fracturesfilled with permicrite internal sediment which bears marine microfossils. See hammer for scale. E.

Shelf-margin calcarenite facies (S-II). Solution void in unsorted mollusk biomicrudite. Floor of thecavity is lined with neomorphic fibrous calcite cements (f) o

sedimentary environments and diagenesis of a cretaceous reef complex, eastern mexico

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