quaternary alluvial terraces in an active tectonic region: the san juan river valley, andean ranges,...

Quaternary alluvial terraces in an active tectonic region:the San Juan River Valley, Andean Ranges,

San Juan Province, Argentina

F. Colomboa,* , P. Busquetsa, E. Ramosa, J. Vergesb, D. Ragonac

aDepartamento de Estratigrafia y Paleontologia, Facultat Geologıá, Universidad de Barcelona, E-08071 Barcelona, SpainbInstitute of Earth Sciences “Jaume Almera”, CSIC. c/Lluı´s Solei Sabarıs s/n, E-08028 Barcelona, Spain

cServicio Geolo´gico Minero Argentino, Avda Julio A. Roca 651, A-1322 Buenos Aires, Argentina

Received 1 November 1998; revised 1 March 2000; accepted 1 June 2000

Abstract

The San Juan River, located in San Juan Province (Argentina), crosses the Precordillera and other geologic units including the Ullumtectonic valley and the La Laja Zone between latitudes 318S and 328S. The San Juan River is antecedent as is suggested by its twoperpendicular segments linked by an almost parallel segment to the main structural trend. Along the Precordillera, the San Juan Rivervalley has many different alluvial fans at the river junctions with its tributaries. The Quaternary alluvial fans display surfaces cut in a series ofsteps which we consider to be alluvial terraces generated by aggradation and repeated incision episodes. The studied sector includes one areawith recent major seismic activity (La Laja Zone), another without major seismic activity (Precordillera area), and a subsident area (Ullumarea) where a large lake was formed 6500 yr BP. The old San Juan River was captured by the Quebrada de Ullum valley by means of a 25-mincision, which resulted in river-gradient headward erosion. The San Juan River gradient shows some irregularities that, although unrelated tothe main structures, are associated with river dynamics, which emphasizes lithologic differences. The main river valley width, the geometryand gradient of each tributary, together with the basement rock lithologies and the size of each local source area are the major factors whichcontrol the alluvial terrace generation processes. In the La Laja Zone, where the uppermost terrace is capped by travertine, dating oftravertine deposits suggests that the maximum incision rate is 0.9–1 mm/yr related to episodic activity on the La Laja Fault.q 2000Published by Elsevier Science Ltd.

Resumen

El rıo San Juan, situado en la Provincia de San Juan (Argentina) cruza la Precordillera y otras unidades geolo´gicas incluyendo la Depresiońde Ullum y la Zona de La Laja, entre las latitudes 318S y 328S. El curso del rıó tiene un cierto caracter antecedente como puede deducirse porsus dos trazas perpendiculares unidas por otra casi paralela a las alineaciones estructurales principales. En la zona de la Precordillera, el valledel rıo San Juan muestra numerosos abanicos aluviales, situados en las zonas de confluencia entre el rıó principal y sus tributarios. Lassuperficies de los abanicos aluviales cuaternarios estan cortadas por una serie de escalones que consideramos como terrazas aluvialesgeneradas por episodios repetitivos de agradacioń y degradacioń. El sector estudiado incluye una zona con una importante actividad sı´smicareciente(La Laja), otra sin una importante actividad sı´smica reciente (Precordillera), y una zona subsidente (Ullum) donde se formo´ un granlago natural hace unos 6500 anõs. El antiguo rıó San Juan fue capturado por el valle de la Quebrada de Ullum mediante una incisioń del ordende 25 m, que implico´ una nueva adecuacioń del gradiente del rıó mediante los efectos de la erosioń remontante. El gradiente del rıó San Juanmuestra algunas irregularidades que, aunque no se presenten relacionadas directamente con las estructuras principales, estan relacionadascon la propia dina´mica fluvial que acentuá la diferenciacioń litologica. La anchura del valle del rıó principal, la geometria y el gradiente decada tributario, junto a las litologias del basamento y a las dimensiones de cada area fuente local, son los factores principales que controlanlos procesos de la generacioń de las terrazas aluviales. En la zona de La Laja, donde la terraza mas alta soporta un nivel de travertino, ladatacion de los depo´sitos travertıńicos proporciona datos como para suponer una tasa de incisioń del orden de 0,9–1 mm/anõ, asociada a laactuacion periodica de la falla de La Laja.q 2000 Published by Elsevier Science Ltd.

Keywords: San Juan River, Argentina; Alluvial terraces; Tectonic activity

Journal of South American Earth Sciences 13 (2000) 611–626

0895-9811/00/$ - see front matterq 2000 Published by Elsevier Science Ltd.PII: S0895-9811(00)00050-X

www.elsevier.nl/locate/jsames

* Corresponding author. Tel.:134-93-403-4488; fax:134-93-402-1340.E-mail address:[email protected] (F. Colombo).

1. Introduction

The eastern side of the Andean fold-and-thrust beltbetween latitudes 318S and 328S consists of the CordilleraFrontal, the Precordillera and the Sierras Pampeanas (Fig. 1).These N–S-trending ranges are separated by the Calingasta,the Ullum and the present foreland Tulum basins. TheCordillera Frontal is mainly made up of a suite of Triassicto Miocene volcanic rocks (Ramos, 1995). The Precordil-lera consists of an imbricated thrust system mainly formedby Paleozoic rocks, especially a thick turbiditic Devonianunit (von Gosen, 1992; Ramos et al., 1996). Finally, theSierras Pampeanas are large, uplifted basement blocks(Jordan et al., 1983; Cristallini and Ramos, 1995; Jordan,

1995). The Cordillera Frontal and the Precordillera arebounded by east-directed thrusts whereas the emergentthrusts of the Sierra Chica de Zonda and Sierra de Pie dePalo are west-directed (Figs. 1 and 2).

Late Tertiary compression produced up to 50% shorten-ing within the Precordillera (Allmendinger et al., 1983; vonGosen, 1992; Ramos, 1995). The eastward migration of thedeformation is recorded by growth strata, especially towardthe north of the study area (Zapata and Allmendinger,1996), as well as in the progressive younger age of thebasin infill toward the foreland (Lower to middle Miocenein the Calingasta basin, middle to upper Miocene in thePrecordillera, and Pliocene and Quaternary in the presentforeland).

F. Colombo et al. / Journal of South American Earth Sciences 13 (2000) 611–626612

Fig. 1. Simplified geological map of the Provincia de San Juan showing the Cordillera Frontal, the Precordillera and the Sierras Pampeanas between 308S and328S latitude (Ragona et al., 1995 modif.). Regional cross-section of Fig. 2 is located. Boxes indicate localities described within the text as: F� Tigre Fault,T� Tambolar, S� Sasso tributary, A� Albarracın tributary, U�Ullum basin, L� La Laja area. The dotted pattern in the Ullum and Tulum basinscorresponds to the present-day alluvial fan extension along the San Juan River.

The San Juan River flows from west to east and crossesthe Precordillera, the Ullum and Tulum basins from theCordillera Frontal to the foreland (Fig. 1). The La LajaZone in the Tulum basin is a present-day uplifted areawith seismicity and neotectonism. The Ullum correspondsto the broken-foreland subsident basin, and the Precordillerais made up of an imbricated and eastward-directed thrustsystem. The river course shows a general transverse direc-tion, particularly when crossing the ranges with downcuts ofmore than 1800 m in the Precordillera. However, the riverdisplays an almost N–S flow direction for more than 30 km,from the Tambolar to the Sasso tributary confluence.Concurrently, the river gradient increases abruptly eastwardof the El Tambolar pass, whereas the passage from matureto immature roughly coincides with the El Tigre Fault,which conforms the western boundary of the Precordillera(Fig. 2). Well-preserved incised alluvial terraces along theSan Juan River document late Quaternary river dynamicsand the landscape evolution of the frontal Andes. Thedrainage pattern of the San Juan River has been conditionedby Andean tectonics.

The Ullum area, which is characterized by a thick accu-mulation of more than 1200 m of alluvial sediments(Damiani, 1993), is capped by widespread lacustrine depos-its with some archaeological sites. The major aggradationalepisode could have developed approximately 6500 yr BP, assuggested by the coarse-grained sediments that closed theformer Quebrada de Zonda valley allowing the developmentof the former Ullum Lake. Thus, the very humid weatherconditions attributed to the climatic maximum of the lastinterglacial period (corresponding to 8500–6000 yr BP,Richtie et al., 1983; Porter and Orombelli, 1985) mighthave controlled the development of a widespread aggrada-tion episode. This episode is characterized by a thick accu-mulation of gravelly alluvial fans at the confluences of theSan Juan River tributaries, followed by repeated events of

degradation and aggradation, which were recorded bysuccessive incised alluvial deposits.

The aim of this study is to analyze the characteristics,distribution and geometry of the late Quaternary alluvialterraces along the San Juan River (all the studied areas aremarked on Fig. 1), generated by repeated degradation andaggradation events (Suvires, 1985; Milana, 1994; Colomboet al., 1996; Ruzycki and Paredes, 1996). Although theterraces are distributed along the river area, only incisedterraces are studied in two different tectonic contexts: fill-cut terraces and straths in the La Laja Zone, upstream fromthe surface deformations of the 1944 (Harrington 1945)earthquake which destroyed San Juan City, and fill-cutterraces located on the eastern side of the Precordillera.The terrace distribution and topography along the SanJuan River valley were studied by theodolite surveys.

2. Terrace situation and characteristics

In the Precordillera there are several discontinuous allu-vial fans located at the mouth of the tributaries along the SanJuan River valley. In several cases the confluence areas ofthe major tributaries display varying levels of strath terracesthat are cut across bedrock. The Quaternary alluvial fansshow surfaces cut in a series of steps interpreted as alluvialterraces. These fill-cut terraces are formed by the accumula-tion of poorly sorted gravels (acid volcanics, sandstones andigneous rocks), interfingering with sand and silt deposits.There are fine-grained lacustrine deposits associated withthe alluvial fan deposits. Eastward of the Sierra Chica deZonda, the Quaternary deposits become extensive thinsheets of gravel surrounding active basement highs(Allmendinger et al., 1983; von Gosen, 1992; Ramos,1995). These different types of Quaternary scenarios areaffected by very recent river dynamics.

F. Colombo et al. / Journal of South American Earth Sciences 13 (2000) 611–626 613

Fig. 2. Schematic E–W-trending crustal-scale regional cross-section from the Cordillera Frontal to the Sierra de Pie de Palo (Sierras Pampeanas) intheforeland basin (see location in Fig. 1). The section has been constructed based on von Gosen (1992) and Cristallini and Ramos (1995), and our observations. Inthis work we assume a large west-dipping thrust bounding the Sierra Chica de Zonda at La Laja region based seismicity (Smalley et al., 1993) although themain thrust is located at the western side of the range and dips towards the east. The major thrust underneath the Sierra de Pie de Palo is east-dipping aswell asthe Sierra del Valle Fe´rtil to the east. The Calingasta and Ullum basins are intramountainous basins, whereas the Tulum basin is the foreland basin at the frontof the Andes.


Fig. 3. The Albarracı´n river terraces are arranged from the youngest (T0) to the oldest (T4); 5 is the basement, 6 is the current terrace of the San Juan River and7 is the terrace escarpment. The line A–B corresponds to the cross-section of Fig. 4.

The Albarracıń Alluvial Fan. In the Precordillera zone,two alluvial fans related to the Albarracıń and Tambolartributary rivers were studied along the San Juan Rivervalley. The Albarracıń River has an average gradient of9%, with a source area mainly made up of Devonian shalesand greywackes (Damiani, 1993) and Tertiary red sand-stones and shales, and volcanics. At its junction with theSan Juan River, the Albarracıń River develops an alluvialfan (Fig. 3) which shows five terraces (T0–T4). The succes-sive terraces display a radial distribution from an apexlocated approximately 1800 m upstream from and south-wards of its junction with the San Juan River. The terracesare constituted by gravels and were generated by repeatedincision events after the main period of accumulation repre-sented by the higher terrace. These incision events mighthave been punctuated by an accumulation episode asevidenced by a clast found 60 cm over the basal part ofterrace T1, which corresponds to a fragment (exceeding50 cm in diameter) of an old road bridge wall destroyedduring 1968–1972. The old bridge was placed 50 mupstream of the new bridge of national road N 20 thatcrosses the Albarracıń River. Terrace (T4) shows gravelsinterfingered with levels of very fine materials whichcontain some remains of pollen (Rubiaceae, cf.Nertera;Allionia, sp.; Podocarpus, sp.; Grammitidaceae cf.Grammitis) and fresh water cysts and spores. This associa-tion suggests a period (such as subtropical humid forest)which was warmer and more humid than the present. Thedevelopment of this uppermost terrace during a non-glacial(such as an interglacial) period seems very probable. Across-section of the fan (Fig. 4) displays the convex-upsurfaces of terraces T1–T3 and their relative distribution.This convex-up disposition of the surface terraces suggestthat the terraces T1–T3 are not in geomorphic equilibriumowing to the rapid erosion processes affecting the previouslydeposited coarse-grained materials. The detailed longitudi-nal topographic profiles (Colombo et al., 1996) of the fourlower terraces (Fig. 5) show a non-parallel divergentdisposition, which suggests that these are not in equilibrium

and were controlled by local base-level variations (Bull,1991).

The Tambolar Alluvial Fan.The Tambolar alluvial fanwas formed at the mouth of a ravine which cuts a sourcearea mainly constituted by shales and greywacke rocks. Thewidth of the San Juan River valley at this site is approxi-mately 100–150 m, and the average gradient of the tributaryravine is 15%. There are five (Fig. 6) terrace levels: the twolower (T0 and T1) are fluvial from the San Juan River, andthe three upper (T2–T4) are alluvial from the Tambolar fan.The difference in altitude between the upper (T4) and thecurrent channel of the San Juan River (T0) attains 30.8 m.The white materials displayed on terrace T1 correspond to asmall recent mudflow episode (Fig. 7).

The Ullum Depression.The Ullum Depression is filledwith 1200 m of coarse alluvial sediments (Punta Negrasystem) with a surface longitudinal gradient of 0.8%(Milana, 1994), which (toward the N and NNE) interfingersdistally with fine-grained sediments (Fig. 8), containingsome remains of pollen (Chenopodiaceae;Suaeda divari-cata; Ephedra triandra) and undetermined Pteridophyta.This association suggests climatic conditions that weremore humid than the present, such as a subtropical humidforest. In the surroundings of the Quebrada de Zonda Dam,15 m above the present level of the sedimentary filling of theUllum Depression, there are fine-grained materials withPteridophyta, pollen remains (Grammitidaceae cf.grammi-tis) and uncertain specimens. Within the same levels thereare many ostracoda (Cypridopsiscf. intermedia; Darwinulacf. africana brasiliensis; Chlamidotheca incisa; Limno-cythere sp.) and charophyta remains (Musacchio, 1988).All the fossil specimens suggest deposition in a lacustrineenvironment. A major enlargement of the surface of the oldUllum Lake is evidenced by the distribution of the lacustrineterraces, and an increase in its depth resulted from theQuebrada de Zonda valley closing event due to the progra-dation of major alluvial fans from the south. It seems prob-able that the Ullum Lake developed approx. 6500 yr BP, assuggested by prehistoric remains (Uliarte et al., 1990). The


Fig. 4. The convex-up disposition in the cross-section of the Albarracıń main terraces is noticeable and suggests very rapid erosion processes.K corresponds tothe San Juan River gradient in the junction area. The triangle corresponds to the San Juan River local base level at its junction with the Albarracıń River.


Fig. 5. The downstream divergent distribution of the longitudinal profiles of the Albarracı´n terraces T3, T2, T1, and T0, is noticeable, suggesting that they werecontrolled by local base-level variations.

Fig. 6. The Tambolar Alluvial Fan distribution of the main terraces arranged from the youngest (T0) to the oldest (T4).

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Fig. 7. Picture of the Tambolar Fan where the terrace distribution and relationships are displayed. The white materials placed in the central part correspond to a recent mudflow episode. More explanations inthe text.

aggradational character of the Punta Negra system, directlyrelated to the subsidence of the Ullum Depression, wasmodified by a subsident-degradational stage associatedwith a fall in the local base level. This fall was producedby the capture of the Punta Negra system by the Quebradade Ullum, which implies the abandonment of the Quebradade Zonda valley and a generation of 25 m total incision.

La Laja Area.The La Laja Zone is located to the eastand northeast of the Ullum Depression, to the east of theSierra Chica de Zonda-Villicum ranges and to the west ofthe La Laja Fault (L area on Fig. 1). This zone is made upof a Cambrian basement unconformably covered byNeogene continental sediments. There are some fluvialQuaternary terraces made up of Cambrian and Neogeneclasts covering the Neogene and Paleozoic materials. TheLa Laja Zone is affected by considerable neotectonic activ-ity (Bastıas et al., 1990) with common reverse faults that

thrust the Quaternary terraces (Fig. 9). Associated with ther-mal springs along the faults (Sa´nchez et al., 1986) aretravertine deposits that overlie (Fig. 10) recent Quaternaryterraces (T4) displaying five different topographic levels(T0–T4). The present bed of the Damiani ephemeral river(T0), which constitutes the main tributary in the La LajaZone, cuts the older terraces (T1–T4), the travertine whichfossilizes T4 and also the Neogene basement rocks. Near thegully the maximum incision value reaches 38.5 m (Fig. 11).The terrace distribution close to the overthrust plane of theLa Laja Fault shows an upstream divergence (Fig. 12). Abed of travertine fossilizes the uppermost terrace (T4) of theLa Laja Zone and has yielded an age of 42,000^ 1000 yrBP (U–Th). The maximum topographic difference of theterraces (T0–T4) attains 43.8 m giving an average incisionrate of 0.9–1 m each 1000 yr, a rather high value. The highincision values may be attributed to recent tectonics


Fig. 8. The Ullum basin is filled with a relatively thick Quaternary unit constituted by coarse alluvial deposits to the west and fine lacustrine sediments to theeast impinging on the Sierra de Zonda. The minimum incision of the San Juan River is 25 m.

Fig. 9. The La Laja section scheme show a northwest-directed thrust fault which cuts the Quaternary terrace with a vertical displacement of 5 m.

associated to the seismic activity in the La Laja Zone(Bastıas et al., 1990).

3. Terrace genetic model

The fact that the tributaries of the San Juan River aremore confined and have greater topographic gradients(Fig. 13A) than the main river accounts for our geneticmodel. After a high water discharge as a flash flood, a fanis formed at the tributary junction with the San Juan River(Fig. 13B) damming the main river valley and favoring thedevelopment of a temporary lake. When these processes arerepeated from time to time, the successive natural damsproduce local increases in the base level related to the allu-vial system aggradation periods (Fig. 14). These were regis-tered as silty lacustrine deposits placed in the San Juan

River valley and located upstream from the intersectionswith the tributaries, laterally interfingering with fluvialand alluvial deposits. Subsequently, the main river cutsacross the fan deposits of the tributary and, by lateral shift-ing of the fluvial course in the active channel, a lateral scarpis generated by erosion of the previously formed alluvial fan(Fig. 13C). The lateral erosive front generated in the alluvialfan suggests that the tributary channels are increasinglyincised given the considerable topographical differencebetween the initial and final local base levels related tothe main river. The successive fill-cut terraces (Fig. 13Dand E) are generated by repetitive aggradation and degrada-tion episodes. These can be of a high frequency, assuggested by the remains of the old road bridge wall(destroyed during 1968–1972 time span) and found as clastsin the Albarracı´n Fan T1 terrace.

4. San Juan River longitudinal profile variability

Generally the longitudinal profile of the river is sensitiveto the uplift, and thus yields information about activetectonic structures. The steady state of the river suggeststhat erosion is approximately equivalent to the uplift(Schumm, 1993). The abrupt changes in slope along theriver profile suggest that active faults cross the river. Incontrast, the river drainage pattern is not easily affectedby tectonics when the river channels are entrenched indeep valleys. A river in a deep gorge tends to maintain itscourse despite tectonics (Seeber and Gornitz, 1983). Thus,the river pattern has no direct relationship with subsequenttectonic activities. This antecedent character of the river isshown where the San Juan River orthogonally crosses themain tectonic structures of the Precordillera (Fig. 1).

We calculated (Fig. 15) the stream gradient�SL� index inorder to check the river profile variations. This is the rela-tionship between the channel slope (S) at a given point andits channel length (L), which is measured along the longestchannel situated above the point where the calculation ismade (Seeber and Gornitz, 1983). The gradient index ofthe river (K), which is considered to be the river equilibriumgradient (Hack, 1973), is calculated from the initial and finaltopographic elevations of the river and the initial and finaldistances to the drainage divide. Tectonic events, climate


Fig. 10. La Laja area. Relationships between travertine deposits (3) and T4

Quaternary terrace (2) overlying Neogene materials (1).

Fig. 11. Relative distribution of the terrace levels along the main incised ravine in La Laja area. The travertine deposit (3) shown in Fig. 10 is located overterrace T4.


Fig. 12. Terrace longitudinal profiles in the La Laja area show a marked downstream convergence, suggesting that these might have been affected by tectonicmovements during their generation.

Fig. 13. A model for terrace generation. (A) The initial position of the San Juan River valley and a tributary ravine are shown in low base-level conditions(black triangle). (B) An alluvial fan from the lateral tributary produces a natural dam, a temporary lake and an elevation of the base level (white triangle). (C)The incision of the San Juan River produces terraces and a fall of the base level (black triangle). (D) Another alluvial fan can be developed generatinga newtemporary lake and another increase in the base level (white triangle). A new incision (E) results in new terraces and a fall in the base level (black triangle).

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Fig. 14. The interfingering of the alluvial fan conglomeratic deposits (dark levels) and the terrigenous stratified lacustrine sediments (white levels) can be observed. On the right there is a person for scale.

and the geomorphic history should be taken into considera-tion when using the relationship�SL=K� between the streamgradient and the river gradient indexes. Thus, whereSL=K ,1 the segment is less steep than the equilibrium; whereSL=K � 1 the segment is in equilibrium; whereSL=K . 1the segment is steeper than the equilibrium (Seeber andGornitz, 1983).

The values ofSL=K seem to be determined by the areacrossed by the river and where the main structural accidentsoccur. The calculation of theSL=K index for the zone of theCordillera Frontal and for the upper part of the CalingastaValley shows anomalous values. This type of anomaly is notuncommon in the upper parts of rivers where the topographyis very abrupt and where the mountain slopes have veryactive dynamics (Seeber and Gornitz, 1983). The valuesof the Cordillera Frontal are below 1 and can be explainedby the proximity of this area to the upper part of the drainagedivide, which is usually not in equilibrium (Seeber andGornitz, 1983) and is controlled by different lithologies.The values of theSL=K index used in this paper were calcu-lated taking into account the pattern of the San Juan River,which is parallel to the Andean structures, when it crossesthe Calingasta Valley. When the San Juan River enters thePrecordillera it has an antecedent character since it cutsthe main tectonic structures almost orthogonally. In the

Calingasta Valley, the values are higher, reaching 3.02.This suggests that the profile is steeper than the equilibriumand that the high value might be related to the confluence ofthe major tributaries, which are able to carry large amountsof gravel bed load. When the river crosses the Precordillerathe values are also high, suggesting a gradient increase.

The gradient upstream from the Tambolar Pass (Fig. 16)reaches 1.3, while downstream from the pass it attains 3.4.This means that the bedrock of the area constitutes a localbase level for upstream sedimentation, whereas downstreaman increase in the gradient is generated. The Tambolar Zonecorresponds to a local base level, which determines the riverprofile. The San Juan River longitudinal profile in this areais more mature than in the Calingasta Valley. The San JuanRiver, in the zone between the Sasso River and the Tambo-lar area, has a consequent character since its pattern is paral-lel to the main tectonic structures. In this sector the slope ofthe San Juan River is more marked than in the previoussector and this becomes more pronounced until the PuntaNegra area. The junction with the Sasso River shows valuesreaching 2.3, given the large coarse-grained bed loadsupplied by the Sasso River. Near the Punta Negra Damthe values reach 3.4 because the Albarracı´n tributarysupplies large amounts of coarse-grained bed load to themain San Juan River.


Fig. 15. The longitudinal profile irregularities of the San Juan River are magnified when the segment gradient index related to the whole river gradientindex�SL=K� is calculated for each point. See text for discussion.

In the Ullum basin there are values that can reach 5.4,which can be explained by an important upstream incisionas a result of the capture of the old San Juan River by theQuebrada de Ullum valley. The sector of the San JuanRiver located downstream from the deeply incised Ullumvalley shows a very high longitudinal profile. This highslope value corresponds to a lack of maturity of the SanJuan River longitudinal profile as a consequence of thecapture of the old Ullum Lake by an ancient tributary ofthe San Juan River that eroded the Sierra Chica de Zonda.The backward erosion currently constitutes a smoothingprocess of the San Juan River longitudinal profile in thissector.

Near the alignment of La Laja Fault the value reaches 3.3as a result of recent repeated seismicity. Finally, the SanJuan River has anSL=K value of 0.4 near the Lagunas deGuanacache. The regional base level of the San Juan Rivercorresponds to the Lagunas de Guanacache. This is an over-filled basin which has a large number of small lagoonsproduced by overflooding due to spreading processeswhen the channels, which are located in a very flat region,lose their confinement.

On the assumption that no major changes have occurredin the river water discharge or sediment transport, the mainfactors that control river gradient variations are lithologiccontrast and tectonic activity (Seeber and Gornitz, 1983).Since the gradient anomalies in theSL=K values of the SanJuan River do not always coincide with the lineaments of

the main structures, they could be attributed to more recentvariations in water discharge and bed load, or controlled bydifferent bed rock types. Although in a number of cases thecoincidence of theSL=K anomalies with the areas affectedby neotectonics could be possible, the non-deformedalluvial terraces placed in these areas suggest that the recenttectonic activity is older than terrace generation.

5. Discussion and conclusions

In a fluvial context, the generation of terraces has oftenbeen attributed to significant climatic changes such as alter-nating arid and humid events (Smith and Battuello, 1990;Bull, 1991). When the water discharge decreases, the debristends to remain in the upper parts of the drainage basins,contributing to an increase in the gradient. In contrast, inmore humid periods the gradient decreases because of thegreater effectiveness of sediment transport due to largewater discharges. Terrace development might be favoredby high water discharges during deglaciation periods(Pierce and Colman, 1986) or during other relatively impor-tant changes in weather conditions (Starkel, 1993). Theincrease in water discharges suggests a major reworkingof sediments (Nash, 1994) and therefore the generation ofa variety of accumulation terraces. During periods ofgeneral low water discharge a local base-level drop canoccur resulting in erosive processes. Finally, erosion of


Fig. 16. Longitudinal profile of the San Juan River. Changes in gradient between the Tambolar and Sierra Chica de Zonda zones are noticeable. Minorirregularities in the river gradient in the Punta Negra and La Laja areas can be noted in the inset. From source maps at scale 1:50,000.

previously deposited coarse-grained materials may produceincised terraces (Bull, 1991).

Tectonic factors tend to modify the source area geometryas well as that of the accumulation area, and the river equi-librium profile (Schumm, 1993). The increase in the gradi-ent favors incision. Similar tectonic movements will resultin repetitive erosive phenomena of the same type since theriver tends to reach its equilibrium profile. Although thelongitudinal profile of the San Juan River has a tendencyto be concave, there are some irregularities (Figs. 15 and16), that become more evident when the river crosses thecentral part of the Precordillera. Thus, the river longitudinalprofile shows an abrupt gradient increase East of Tambolarand the maturity profile undergoes a change from mature toimmature at the western boundary of the Precordillera. It hasbeen suggested (Ruzycki and Paredes, 1996) that the Precor-

dillera underwent uplift when the San Juan River hadalready occupied its present course, and therefore the rivershows an antecedent course pattern.

The tectonic activity of the Precordillera could have beenaccompanied by a regional uplift generated by a deepcrustal-scale ramp (Jordan et al., 1983; Cristallini andRamos, 1995; Jordan, 1995), as interpreted from seismicitystudies. A regional uplift as a block could generate a set ofapproximately parallel longitudinal profiles (Fig. 17) of theterraces (Amorosi et al., 1996) along the main river valley.However, since the San Juan River alluvial terraces do notdisplay a parallel arrangement, the origin of these terracesdoes not seem to be controlled by tectonics. But if theregional uplift had occurred before the development ofthe Quaternary alluvial terraces along the San Juan River,the aggradation and degradation events would have beenyounger than 6500 yr BP, and no significant major tectonicactivity would have occurred in the Precordillera area in thattime span. In the absence of major evidence of contempor-ary tectonic activity, the irregularities in the river gradientcould be attributed to its own fluvial dynamics. Thus, afterthe capture of the San Juan River by the Quebrada de Ullum,the base level suddenly dropped some 25 m, producing agreat step in the river profile, which today is being smoothedout by headward erosion. Bearing in mind that the incisionwas produced in a recent Quaternary period (from 6500 yrBP to present) when the river was not in equilibrium but in areaccommodation phase (Schumm, 1993), the river nowcontinues to show a gradient profile with convex-up markedirregularities.

The fact that the San Juan River has not reached thebedrock beneath the alluvial fans, together with the presenceof widespread non-deformed Quaternary sediments,suggests a short-term non-tectonic origin for the incised allu-vial fans along the Precordillera. The longitudinal profiles ofthe fill-cut and fill complex response terraces of the Albarra-cın tributary (Colombo et al., 1996) show an upstreamconvergence (Figs. 5 and 17A) suggesting an origincontrolled by repeated base-level variations (Begin et al.,1981). Thus, the generation of alluvial terraces is directlyrelated to repeated phases of degradation and rapid aggrada-tion, which are attributed to changes in the river base level.For these models, tectonism and significant climatic changesare not necessary in accounting for terrace generation. In thesilty materials accumulated as lacustrine sediments charac-terized by charophyta and ostracoda associations, there aresome remains of palynomorphs, suggesting a hot and veryhumid period punctuated by short periods of climatic aridity(Sole de Porta, N., personal communication, 1997). Thiscould be associated with an interglacial period when thewarmer climatic conditions coincided with the maximumlevel of aggradation along the San Juan River area. Thelong-term fluvial incision dynamics produced during moun-tain uplift are demonstrated by the successive uplifted strathterraces which can be identified along the San Juan Rivervalley, for example in its junction with the Sasso River.


Fig. 17. Conceptual distribution of longitudinal profiles of alluvial terraces.(A) Downstream divergence of the terraces controlled by local base-levelrepeated fall episodes, as in the case of the Albarracı´n Fan. (B) Parallelarrangement of the terraces controlled by tectonics in a compressivecontext, as described for the Po basin (Amorosi et al., 1996). (C) Down-stream convergence of the terraces controlled by repeated tilting, as inter-preted for the La Laja area. The terraces are counted from basal andyounger (T0) to highest and older (T3). The white triangle corresponds tothe base level at each time. The black triangle corresponds to the presentbase level.

Although the alluvial fill-cut terraces are always incisedwithin alluvial deposits in the Precordillera, the La Lajaterraces cut Tertiary bedrock yielding straths which arecapped by gravels from a renewed aggradational event.These terraces show an upstream divergence (Fig. 17C),suggesting that they were affected by local tectonics. TheLa Laja Zone lies in the epicentral area of the earthquakethat destroyed San Juan City in 1944 (7.4 Richter degrees),resulting in a ground step of 40 cm areally extended formore than 10 km (Harrington, 1945). If the La Laja areatectonic exhumation is associated with the incision of itsQuaternary terraces, it is possible to accept values of atectonic rise of 43.8 m for a 43,000 1000 yr time span.According to the travertine age data, an average exhumationrate of approximately 0.9–1 mm/yr in the area affected bythe La Laja Fault earthquake activity can be estimated.

It is not easy to determine the sedimentological implica-tions of local base-level variations given the local effects oftectonic activity, but local base-level variations couldaccount for the control over the terrace generation. Thestudy area, which is placed more than 1100 km from theAtlantic Ocean, is probably not affected by sea-level varia-tions because the Lagunas de Guanacache, located southeastof San Juan City, constitute the regional base level.

Acknowledgements

The comments of referees L. Spalletti, V. Ramos, G.G.Bonorino and J.M. Sayago have considerably improvedthe manuscript. We are indebted to V.H. Sańchez andO. Damiani of CRAS and F. Bercowski of the San JuanUniversity, for geological and geophysical data. We grate-fully acknowledge the support of Hugo de Los Rios, Direc-tor of the Delegacioń de San Juan del Servicio Geolo´gicoMinero Argentino. Dr C. Cingolani of La Plata Universitygave us valuable information about the fossil remains of theUllum area. Dr Nuria Sole de Porta, of the Universitat deBarcelona gave us very valuable information on the vegeta-tion and pollen remains. Dr R. Julia, of the Institut de Cien-cies de la Terra “Jaume Almera”, CSIC, of Barcelonacarried out the U–Th analysis. To R. Pelichotti in memor-iam. This work corresponds to the DGICYT PB 94-0871and PB 98-1189 Spanish Projects of Ministerio de Educa-cion y Ciencia, and was partly supported by the Comissionatper Universitats i Recerca, Generalitat de Catalunya, Grupde Qualitat GRQ94-1048.

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