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Geomorphology, tectonism and Quaternary landscape evolution of the centralAndes of San Juan (30�Se69�W), Argentina

Laura P. Perucca a,b,*, Liliana M. Martos b

aCONICET, Gabinete de Neotectónica, INGEO-FCEFyN-UNSJ, Av. Ignacio de La Roza y Meglioli, 5400 San Juan, ArgentinabDepartamento Geología, FCEFyN-UNSJ, Av. Ignacio de La Roza y Meglioli, 5400 San Juan, Argentina

a r t i c l e i n f o

Article history:Available online 23 August 2011

a b s t r a c t

The northesouth trending valley of Iglesia is a regional tectonic depression limited to the west by theCordillera Frontal Unit and to the east by Precordillera Occidental Mountain Units. The forms of theresulting landscape in the region are the result of glacial, periglacial, fluvial and alluvial action, aggra-dational and deggradational processes, as well as neotectonic activity and climatic changes.

The generation of large Quaternary alluvial fan aggradational surfaces is related to previous climaticconditions, colder and more humid than the present ones. Abundant snowfalls and rains during thePleistocene made possible detritus deposition, generating alluvial covers whose thickness increasestoward the east. These climatic conditions alternated with arid periods, during which vertical erosion ofstreams prevailed, forming a landscape of stepped levels.

In addition, the presence of faults with Quaternary tectonic activity indicates, a strong structuralcontrol in the evolution of the landscape during the PleistoceneeHolocene periods, effectively startingvertical erosion and finishing a cycle of erosion-accumulation and the beginning of the following one.

� 2011 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

Between 32� and 52�S, a wide region in Argentina with arid andsemi-arid climatic conditions is developed with a narrow strip ofmaximum aridity, named the Arid Diagonal (Bruniard, 1982)extending from the Atlantic coast at 44�S to the north to 27�S alongthe eastern flank of the Andes.

Landscape evolution in semi-arid areas of this portion of theAndes can take on distinctive characteristics of their own. Theseregions are clearly marked by low annual precipitation, distinctivevegetation, and characteristic ephemeral processes of erosion and/or deposition, which water and wind are the most importantdriving agents.

However, deposits and landforms recognized in the regionsuggest past climate conditions much different from today’s.Different accumulation and erosion landforms identified in the areaindicate changes in climatic conditions, leading to greater aridifi-cation during the Late Holocene. These changes in turn lead todecreased transport capacity of rivers, and to dominating aeolianprocesses. Fluvial activity is restricted to rivers and streams,

occasionally overflowing during intense rainfalls in summer, or inthose years of higher snow precipitation in the Cordillera Frontalarea. The action of mechanic weathering and mainly wind actionpromotes glaciplanation processes, good development of desertpavement, desert varnish and cryoclastic phenomena.

This paper will propose a paleoclimatic analysis aimed atunderstanding its influence on the evolution of the landscape,adding the control of the active tectonics of the Pleistoce-neeHolocene as a main factor in such development. The mainconstraint in the analysis of these mountain paleoenvironments isthe lack of previous geological and paleoecological data on thePleistocene/Holocene conditions. Most of the existing studies arecentered on the glacial chronology of several valleys of theCordillera, hundreds of kilometers south of the study area, asgeneral paleoclimatic interpretations, whereas very little is knownabout the paleoenvironmental conditions at this latitude. There-fore, this work is a contribution to current knowledge of the region.

The methodology applied in the analysis of the area is based onthe interpretation and digital analysis of the geomorphologicalfeatures of the land surface. 1:30,000 scale aerial photographsprovided by the Secretaría de Minería de la Provincia and LandsatTM images with 30 and 15 m of resolution were used to that effect.

A slope analysis map was prepared in a GIS environment withthe creation of a digital terrain model (DTM). Altitudes wereobtained as a result of the partial digitization of topographic charts

* Corresponding author. CONICET, Gabinete de Neotectónica, INGEO-FCEFyN-UNSJ, Av. Ignacio de La Roza y Meglioli, 5400 San Juan, Argentina.

E-mail addresses: [email protected] (L.P. Perucca), [email protected] (L.M. Martos).

Contents lists available at SciVerse ScienceDirect

Quaternary International

journal homepage: www.elsevier .com/locate/quaint

1040-6182/$ e see front matter � 2011 Elsevier Ltd and INQUA. All rights reserved.doi:10.1016/j.quaint.2011.08.009

Quaternary International 253 (2012) 80e90


provided by the Instituto GeográficoMilitar. Field work consisted ofa detailed analysis of the most relevant landforms and naturaltrenches.

2. Environmental setting of the study area

The area under study is located in the central western region ofthe San Juan province, between 30� and 30� 200S and 69� and 69�

300W (Fig. 1a). From east to west, it covers a portion of the westernborder of the Precordillera Occidental, the Iglesia Valley and theeastern portion of the Cordillera Frontal.

In the Precordillera, the main mountain outcrops trend north-esouth, at the Sierra Negra, whose peaks reach 3500 m asl. Themost important ranges in the Andean Cordillera are the Colangüiland the Agua Negra, both exceeding 5000 m asl.

Surface drainage is constituted by the Blanco River and itsperennial tributaries, among which the most important are theColangüil and the Agua Negra creeks both coming from theCordillera Fontal. To the east, other tributaries coming from thePrecordillera Occidental, is the Carrizal creek and coming from thesouth, the Iglesia Creek.

The harshness of the present climate is manifested by extremewinter temperatures in the Cordillera (to �30 �C), wide tempera-ture variations, minimum humidity, winter snow precipitation andvery scarce pluvial precipitation (Minetti et al.,1986). The climate inthe Precordillera Occidental and Iglesia Valley is arid-desert, withlarge daily and annual temperature variations, atmospheric trans-parency and low humidity. The rainfall regime is continental, withsummer rains and only with very low average frequency of dayswith rain. According to the classification of Köppen (1936) it is oftype BWK (average annual temperature of 15.7 �C in Rodeo, butabove 18 �C during the warmest month. January >23 �C) (Minettiet al., 1986). From a climatic viewpoint, this area may be classi-fied as hyper-arid (120> p> 60 mm) or eremitic (60> p> 30 mm),depending on the subzone (Minetti et al., 1986; Le Houérou, 1999).Prevailing winds come from the southeast, and the Zonda (föehn)and north winds are almost constant occurrences duringAugusteSeptember.

3. Geologic and tectonic setting

The western portion of South America has a complexmorphology, with an active western margin, with topography andseismicity reflecting tectonic intersection between Nazca,Antarctica and South America plates. This convergence began about200 million years ago with the east dipping subduction of theoceanic plates beneath the South America Plate (Uyeda andKanamori, 1979).

Between 28� and 32�S, the Nazca Plate subducts horizontallybeneath the South American Plate, about 100 km deep at a rate of6.3 cm/year (Pardo Casas and Molnar, 1987; Somoza, 1998;Kendrick et al., 2003). This subhorizontalization started between8 and 10 Ma, in close association with subduction of the ancientJuan Fernández Ridge (Jordan and Gardeweg, 1987; Ramos, 1988;Kay et al., 1991; Yáñez et al., 2001; Ramos et al., 2002, amongothers).

Central Chile (about 30�S) is characterized by an intermediateobliquity of the convergence vector of the Nazca Plate beneath theSouth America Plate (DeMets et al., 1994; Yáñez et al., 2001). Thismode of oblique subduction affects the deformation distributionand the resulting morphology, favoring the occurrence of strike-slip faults (Bastías et al., 1990; Siame et al., 1997, 2002). Thegeological setting of the Iglesia Valley is the result of complexgeodynamic processes as a consequence of the Nazca and SouthAmerica Plates convergence.

Previous studies proposed that the Iglesia basin includesa Neogene sequence up to 3.5 km thick in its center, thinning to theeast and west, and also to the north and south, and interpreted thebasin as a piggyback basin that developed during the eastwardadvance of Precordillera thrusts (Beer et al., 1990; Jordan et al.,1993). Siame et al. (2006) found that at 30�S both the IglesiaValley and the Precordillera can be seen as a crustal-scale trans-pressive zone, whose deformation is distributed with a dextralstrike-slip along El Tigre Fault zone. This phenomenon is closelyrelated to the Precordillera fold-and-thrust belt.

Based on the analysis of seismic profiles in the Iglesia Valley andseveral outcrops in a cross-section along the Jáchal river, Alvarez-Marrón et al. (2006) interpreted a positive flower-type structureduring the Neogene for this region. The authors considered thatthese models would not totally reflect the structural arrangementof fault and thrust belt proposed by others (Allmendinger et al.,1990; Jordan et al., 1993).

According to Siame et al. (2006), the Precordillera mountainbelt, which is nearly 400 km long and 80 kmwide, is a thrust-and-fold belt separated from the Cordillera Frontal by an NeS piggybackbasin: the Calingasta-Iglesia Valley. Allmendinger et al. (1990)suggested that, between 29� and 31�S latitude, the CordilleraFrontal is uplifted as a ramp-anticline over a mid-crustaldécollement.

Outcrops in the Cordillera Frontal area consist of sandstones andsiltstones with an Upper CarboniferouseLower Permian age (AguaNegra Formation), Permian granites and granodiorites. Mesozoicgranites of the Batholith of Colangüil, Neogene continental sedi-mentary rocks (Iglesia Group) and PleistoceneeHolocene deposits(Fig. 2) are present in the Valley (Cardó et al., 2000, 2001).

In Precordillera Occidental, the oldest statigraphic units areOrdovician and Devonian sedimentites (Yerba Loca and PunillaFormations) covered by Upper Paleozoic deposits (MalimánFormation), with some acid dikes assigned to the Choiyoi MagmaticCycle (Lower PermianeLower Triassic) (Llambías et al., 1996). ThePaleogeneeNeogene sequence is represented by the Iglesia Group(Wetten, 1975; Contreras et al., 1990), with Lomas del Campanarioand Las Flores Formations (Fig. 2).

Lomas del Campanario Formation includes andesite and daciterocks, volcanic bombs and tuffs and a higher level with cross-stratified conglomerates and some diatomite layers (Wetten, 1975).

Las Flores Formation consists of a succession of well laminatedsiltstones and claystones and interbedded layers of gypsum sheets(Wetten, 1975). The outcrops of these units are located to the westof Pismanta and Colangüil (Fig. 2).

The Quaternary deposits in thewestern area of the valley consistof gravels with greywacke, quartzite, and granodiorite clasts. Theseclasts are over 50 cm in diameter. Furque (1979) identified theTudcum Formation as distributed to the west of the Iglesia andBlanco rivers, covering Neogene deposits consisting of fine tomedium conglomerates. He estimated the Tudcum Formationhaving 50 m in thickness, decreasing to the West.

Quaternary deposits occupy the lowest topographic position,forming alluvial sequences with clasts of sandstones, wackes,shales and a variety of igneous rocks. Playa deposits, made of silt,clay and sand are placed in the lowest part of the tectonicdepression of the Iglesia Valley and they form the local base levelsof the ephemeral rivers.

According to Jordan et al. (1993), the Iglesia basin would haveevolved to a system of east-verging overthrusts in the PrecordilleraCentral. Movements along the imbricate faults began in thewestern Precordillera at 20 Ma. Alvarez-Marrón et al. (2006)interpreted in a WeE cross-section of Precordillera along theJáchal river, two major sets of structures revealing differing defor-mation styles that have been superimposed during the Paleozoic to

L.P. Perucca, L.M. Martos / Quaternary International 253 (2012) 80e90 81


present tectonic evolution. These combined effects of initialcompression and then transpression make the structural inter-pretation difficult, since the Neogene deformation would not besuited to a thrust and belt system, but to a flower-type structure.

4. Faults with Quaternary tectonic activity

Evidence of tectonic activity during the Quaternary is found inthe piedmonts of both Cordillera Frontal (Colangüil, Pismanta and

Fig. 1. a) Overview of the studied area, b) overview of San Juan and Mendoza provinces. Boxes locate the different compared zones.

L.P. Perucca, L.M. Martos / Quaternary International 253 (2012) 80e9082


Fig. 2. Geological sketch map.



Angualasto faults) and Precordillera. The El Tigre fault (Bastías et al.,1984; Bastías, 1985; Siame et al., 1996, 1997; Fazzito et al., 2009) islocated in the western piedmont of Precordillera Occidental,between Jáchal and San Juan rivers. Geomorphological evidence ofQuaternary faulting is clear along tens of kilometers, with rivers andalluvial fans offset, sag ponds, escarpments and aligned springs.

The east-facing fault scarps, whose heights vary between 0.80and 50 m, have a markedly straight trace, which trend N10�E.Bastías (1985) and Bastías and Uliarte (1991) and estimated

a horizontal displacement of about 260 m, and a slip trace between1 and 3 mm/year. Siame et al. (1997) identified a dextral slip ofa maximum value of about 1 mm/year from cosmogenically datingdifferent alluvial fan levels affected by the fault. The El Tigre faulthas a discontinuous trace, with segments 1e7 km long, affectingNeogene and Quaternary deposits. Fault segments have a “horse-tail” like arrangement in the northern end (Siame et al., 2006).

On the eastern piedmont of Cordillera Frontal, sub parallelfaults, whose lengths reach several kilometers, affect Pleistocene

Fig. 3. Geomorphological map of the Iglesia Valley.



alluvial fan levels. Main fault segments with activity during theQuaternary (Fig. 3), are distributed along the middle and distalportion of the Cordillera Frontal piedmont, such as Colangüil-Guañizuil (Fig. 4a), Pismanta-Las Flores-Bella Vista and Angua-lasto faults, all of them trending NeS. There are minor faultstrending northwest to southeast and northeast to southwest.These are reverse faults dipping with high angles to east or west,as it is observed in Guañizuil where Pleistocene (Q1 level) alluvialdeposits overlies lacustrine deposits of possible upper Pleistoce-neeearly Holocene age. Lacustrine outcrops are very small andhave yellowish pink tones, where alternate sandy banks ina lower sequence, and clayey silt at the upper levels, reachinga thickness of 4.5 m. The reverse fault dips 82�E and trends N30�E(Fig. 4a).

The rectilinear trace of the faults, the inversion of the upthrownand downthrown sides of the escarpment and the high inclinationof the fault plane, suggest a prevailing strike-slip component. This

has also been recognized within Tudcum normal faulting, dippingto the east and affecting sediments from the Las Flores Formationand Pleistocene deposits.

Pérez and Costa (2006) noted that scarps of different faultsegments are discontinuous and distributed with a general “V”pattern in plain view. The apex is located in the Cerro Negro deIglesia, with two main branches trending north-northeast andnorth-northwest.

5. Discussion

The longitudinal Iglesia Valley is a regional tectonic depression(Heredia et al., 2002), limited by awesternmountainous unit that iscomposed by ranges with elevations exceeding 5000 m asl(Cordillera Frontal), and an eastern unit showing elevations ofaround 4000 m asl (Fig. 1a). The valley is crossed by trending NeSthrust faults, generally verging to the east, and has a markedly

Fig. 4. a) East dipping Colangüil reverse fault located north of Guañizuil, that thrusts fanglomerates (Pleistocene?) over lacustrine sediments (Early Holocene?), b) view to thenorthwest showing Q2 alluvial level affected by neotectonics, c) Nebkas and climbing dunes, d) desiccation cracks (south of Las Flores), e) distal to middle portion of Q2 alluvial level,with deposits associated with sheet floods and channelized flows, f) pavement and desert varnish in Q2 alluvial level.



asymmetric profile, crossed by the NeS Blanco River, that locatestoward the eastern edge of the depression.

The valley fill consists of Neogene sediments, corresponding tomaterials deposited in a basin of similar morphology to the presentvalley. They rest on a palaeorelief composed of Paleozoic rocks andare overlain by slightly consolidated to unconsolidated depositsassigned to the PleistoceneeHolocene, disposed in angularunconformity. These deposits correspond to large alluvial fans fromthe eastern front of the Cordillera Frontal, with lengths between 33and 38 km. This broad piedmont has controlled the longitudinalextension of the western piedmont from Western Precordilleraformed by telescopic alluvial fans, whose lengths vary between 13and 6 km (Fig. 3).

The mountainous front of Cordillera Frontal is sinuous. This,accompanied by glaciplanation processes are signs of periods withtectonic stability. However, there are at least three alluvial levelsaffected byQuaternary faults. The effects of neotectonics are inmostcases accompanied by spring waters, some of them thermal (40 �C).

The main rivers of Cordillera Frontal, that trend NNWeSSE,evacuate debris from the mountain areas, Colangüil and AguaNegra rivers being the higher order streams. These permanentrivers significantly excavate deep gorges in the Andean mountains,which in some cases exceed 800 m depth. Precordillera Occidentalis drained by numerous ephemeral trending NW streams, whichflow down to the Blanco River only during torrential summer rains.Large volumes of sediment tend to be transported short distancesduring these storm rainfall events. These ephemeral rivers are insome cases fed by springs.

Another main feature of regional significance is the occurrenceof an extensive erosion surface overlying the Neogene rocks whichslopes gently from the mountain front of the Cordillera Frontal tothe east, at an average gradient of 12�. Over much of the area, theplain is covered by a thin veneer of alluvial origin. Inselbergs andridges, built of granite and Paleozoic sandstones rise above theplain. Their height varies from a few tens of meters up to200e300 m. The fine-grained, pink colored Neogene sedimentsand sedimentary rocks have a badland topography, an intricatelyrilled and barren terrain with an extensive network of convolutedrills and gullies.

In the piedmont of Cordillera Frontal, three generations ofQuaternary alluvial fans (Q1, Q2 and Q3), and a present accumula-tion level (Q4) with a telescopic array can be recognized (Fig. 3).Neotectonic activity in the valley affects the development of thealluvial fans. The longitudinal topographical profiles of those fansshow several segments with decreasing or increasing slopesrespectively. The thickness of the Quaternary alluvial cover variesfrom 10 cm to 3 m in the proximal to medium zone, to 10 m in thedistal portion. The alluvial cover overlies the leveled surface ofNeogene sediments. North of the Colangüil creek, the alluvial levelsare strongly incised and have a thick detrital cover. BetweenColangüil and Agua Negra creeks, the alluvial levels show an evenslightly tilted surface, barely affected by the streams (Fig. 3). Eventhough there are no numerical ages for these alluvial levels, it ispossible to assign some tentative chronologic constraints bycorrelating them with alluvial deposits located at the piedmontarea of the El Tigre range, south of the study area (Fig. 1b), whereSiame et al. (1997) did radiometric dating in several alluvial fansurfaces. They determined that alluvial fans remaining as relicswere deposited roughly 770,000 years ago, and the minimumexposure for the youngest alluvial surface dates at 41,000a (Table 1). They considered that this stepped topography origi-nated due to variations in the load and unload of river flows and/orthe fall of the base level linked to a regional tectonic uplift andhydrological changes due to climate changes (Schumm et al., 1987;Ritter et al., 1993).

Polanski (1963) described some fanglomerate levels fromCordillera Frontal that overlie a Neogene surface in the depressionof Tunuyan, Mendoza Province (Fig. 1b). These alluvial depositswere assigned to Los Mesones (Lower Pleistocene), La Invernada(early Upper Pleistocene) and Las Tunas (Upper Pleistocene)Formations. La Tunas Formation is the upper surface of the bajadadownstream and disappears beneath the loessic plain sediments ofLa Estacada and Zampal Formations, of an Upper Pleistoce-neeHolocene age (Zárate and Mehl, 2008). Polanski (1963) relatedto the four Aggradational Cycles (I, II, III and IV) with the Principal,Posthumous, Final and Minor uplift Neotectonic Phases respec-tively (Table 1).

Martos (1995, 2008) related the Quaternary stepped levels onthe eastern piedmont of Precordillera Oriental (Fig. 1b) with themain neotectonic uplifts. She noted that during the absence oftectonic forces, climatic morphogenesis prevailed (Table 1).

In the Iglesia Valley, NeS trending thrust faults affect the middleand lower portions of alluvial fans. These faults have either east orwest vergence. The neotectonic effects have led to topographicledges, in most cases accompanied by hot and temperate springs.

Some minor piedmont hills are recognized in the middle andeastern portion of the valley (Figs. 1a and 2), uplifted by upperPleistocene tectonic activity (Fig. 4a). Neogene hills showa badlands landscape, linked to past semi-arid climatic conditions(annual rainfall from 250 to 500 mm/year), while present condi-tions show an arid climate, with annual average rainfall varyingbetween 39 and 81 mm/year from east towest (Minetti et al., 1986).

In the western piedmont of Sierra Negra, pseudo-karsticfeatures of considerable size (caves) and some karst pipes aredeveloped within the Neogene sediments. Other phenomenarecognized in the area are stalactites, stalagmites, and sinkholes.

In the middle portion of the eastern piedmont, at least twolevels of lacustrine deposits are recognizable. These deposits can becorrelated tentatively with the Holocene lacustrine sedimentsdescribed in surrounding areas, along the Jáchal River (Colomboet al., 2000) (Fig. 1b). These authors analyzed gastropod and paly-nomorph samples. They noted the formation of those temporarylakes during wet episodes, possibly related to ENSO variations, ina context of prevailing aridity.

6. Results

The landscape is the result of climatic variations during theQuaternary. Although these may have not been very extreme, theywere significant, as indicated by the carving of the surface features(Perucca and Martos, 2009). There was an alternation of colder andwetter conditions in mountainous areas and arid and periglacialconditions in the piedmonts with warmer and drier periods, wherevertical erosion prevailed, creating a stepped landscape. The pres-ence of Quaternary faults also indicates a strong structural controlon landscape evolution during the Upper PleistoceneeHolocene,which would have 2 the end of a cycle of erosion-accumulation,with regional and vertical erosion at the beginning of thefollowing cycle in which a new alluvial level would form at a loweraltitude than the previous one.

The generation of large alluvial fans during the Pleistocene isrelated to colder and wetter climatic conditions than during theHolocene, with a significant generation of debris in the moun-tainous area, resulting from landslides and cryoclastic phenomena.Cryoclastic activity occurred during the most coldest and humidperiods of the Pleistocene, with elevated rates of detrital produc-tion, easy removal from the upper portion of slopes by differenttransportation agents (water, gravity) and consequent thick accu-mulation at the foot slope. Heavy rain and snow precipitationduring the Pleistocene, together with water melt from glaciers,



allowed for the removal of debris, creating an alluvial cover withthickness increasing from west to east.

During the mid-Holocene, an increase in temperature andaridity in southern South America has been inferred from pollenrecords (Grimm et al., 2001). In this area, the Arid Diagonal isextended and has been studied from a geomorphologic, pedologicand paleoecological point of view (Garleff et al., 1991; Veit, 1994,1996; Gil et al., 2005).

Based on the compilation of all records for the central-westernArgentina, Rojo et al. (2010) proposed a regional change that isevident at ca. 3 ka BP, indicating more water availability prior tothat time, probably in all fluvial systems that drain the cordillera.After 3 ka BP, present conditions were established.

The present climate in the Iglesia Valley is hyper-arid, withrainfall below 50 mm/year. In these climatic conditions, erosionalprocesses are most conspicuous, linked to the main permanentrivers from Cordillera Frontal and also to ephemeral streams andflash floods during torrential rainfall. Rivers affect the mid-proximal areas, with water tending to drain radially in the distalsector of the eastern Piedmont. However, very deep dry channelsfound in the mid-distal portion could be perhaps linked to a fall ofthe base level that at a regional scale would be related to thetilting of the valley to the east.

Segmentation on alluvial fans is an indicator of recent tectonicactivity (Bull, 1968, 1977). Thus, reverse faults affecting this sectorfavor retro-wedge erosion in the elevated blocks. Retrocedenterosion progresses until the stream captures a higher order river.This facilitates the genesis of telescopic alluvial fans (Bowman,1978), as observed both in the eastern piedmont of CordilleraFrontal and in the western piedmont of the Precordillera. Based onthe different topographic position (stratigraphic relationships), themorphology of the upper surfaces of alluvial fans in terms ofdevelopment of desert pavement and varnish, the presence or lackof calcrete, the percentage of fragments affected by cryoclasticactivity, the glaciplanation of surfaces associatedwith the degree ofdissection of the landforms slopes, the alluvial fans can be classifiedwith three levelsof accumulation,Q1,Q2, andQ3, besides thepresentaccumulation-erosion surface (Q4), where Q1 is the oldest Quater-nary alluvial level and Q4 the youngest. The glaciplanated alluvialfans Q1, Q2 and Q3, have slope debris, indicating past semi-aridconditions (Fig. 4b). Q1 and Q2 alluvial levels have desert varnishcoating clasts. These manganese-rich black varnishes are charac-teristic of moderately arid, near-neutral environments (Staley et al.,1991) and forms asmoisture from rain, fog, dew, and snow interactswith detrital materials on rock surfaces (Perry et al., 2006).

Several nebkas and climbing dunes on the slopes of the oldestalluvial fans have been observed near Tudcum, related to currentclimatic conditions where the action of winds prevails (Fig. 4c).Runoff influence is paralyzed under the present climaticenvironment.

Desiccation cracks observed in dry river beds (Q4) showsignificant evidence of wind deflation, indicating a considerabletimewithout runoff or rainfall. Even the sides of polygons are wellrounded because of deflation, which in turn widens the cracks(Fig. 4d). Another evidence of aridification in recent times comesfrom settlers’ comments, which describe the strong decline in thewater flow from the springs in the last 200 years.

Gutiérrez Elorza (2001) estimated that in hyper-arid areas,water absence stops hydric erosion, and if there are glaciplanatedsurfaces, they relate to earlier periods with abundant rainfall. Inmountain deserts and depressions, Mabbutt (1977) noted thatconditions vary considerably, and that the contribution of waterfrom rainfall, melting snow or ice is important. In these cases thepiedmont receives substantial contributions of materials andwater from allochthonous streams, draining into the basin.Ta

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The paleoforms Q1, Q2 and Q3 correspond to alluvial fansgenerated under these conditions. Even taking into account thetopographic elevation of the mountains of Precordillera Occidental(3500 m asl) it is possible to infer periglacial and/or nival climaticconditions during the Pleistocene. These conditions favored thegeneration of mechanical weathering and mass wasting processeswith the subsequent evacuation of large quantities of debris,leading to a narrowwestern piedmont. Evenwhen climatic changeswould have less contrast, alternating periods of cold and humidclimates, with periods of less cold and more arid conditions, can bedistinguished during the Quaternary, which are responsible formodeling the landscape, together with a significant contribution ofneotectonic events.

Alluvial covers Q1, Q2 and Q3 overlie erosion ramps carved at theexpense of crumbly Neogene sediments. Their genesis could belinked to tectonic stability that favored its extensive developmentunder semi-arid climatic conditions during Pleistocene, somewetter than Late Holocene and present climate.

Detrital material evacuated from the mountain areas generatedglaciplanation in the proximal and middle portions of piedmonts,accumulating materials in distal areas. The increase of detritusvolumes from the mountainous areas subject to colder and wetterconditions leads to an increasing accumulation nearer to themountain front. In this way, the oldest alluvial fan Q1 is generatedfrom ephemeral streams, with frequent debris flows associatedwith torrential rainfall or rapid snow melting in the mountainousareas. Such flows are related to areas of high slopes, in coldmountains, where detritic material was originated mainly byphysical weathering, by cryoclastic action and mass wasting, addedto tectonic activity that elevated the slopes. Moreover, seismicevents also favor the availability of detrital material (landslides andavalanches) on the slopes.

When mountain uplift and aggradation are older than theincision of the channel, its slope increases and sedimentation islarger close to the mountain front (Bull, 1968). When differences inrelief created by tectonic causes gradually decrease, fluvialprocesses, sheet floods and channelld flows dominate.

Debris flow deposits are located close to the apical portion of thealluvial fans, whereas in the middle and distal portion channelflows layers can be recognized (Fig. 4e). Silt and clay deposits areinterbedded with sand lenses, which have been deposited in thedistal floodplain, and playa environment. The sandy layers corre-spond to channel fill or ephemeral streams deposits, associatedwith torrential rainfalls.

These sequences occupy different topographic positions, indi-cating their relationship with the different processes of accumu-lation Q1, Q2 and Q3, which might be partly lacustrine events, oftenshow strongly cemented carbonates, indicating pedogenicprocesses associated with tectonic stability.

The oldest alluvial fan (Q1) morphogenesis, tentatively assignedto middle Pleistocene, was completed by tectonic processes thatfavored incision, leaving it exposed to erosion and then to lateralerosion that progressively decreased its areal distribution, beingpresently highly dismembered (Fig. 2). The top surface of thisalluvial fan suffered glaciplanation processes during tectonicstability periods in the region under semi-arid climatic conditions,somehow more humid than Holocene climate. Desert pavement,wind-faceted pebbles (ventifacts), desert varnish and fracturedrocks by cryoclastism can be recognized at the glaciplanated topsurface of this alluvial level (Fig. 4f).

Morphogenetic processes repeated during Pleistocene times,with the influence of climatic changes with alternating wet andcold conditions in the mountainous areas and semi-arid climate inthe valley; with some warmer and dry, arid and hyper-arid times,together with tectonic activity as evidenced by Quaternary faults

affecting the piedmont. Under these conditions, Q2 and Q3 alluvialfan levels were generated.

Landscape evolution is changing very slowly or is nearly para-lyzed today, and alluvial level Q4 corresponds mainly to sporadicchannel fills. Winds prevail in the region, becoming the mainmorphogenetic agent and landscape modeler in the Iglesia Valley.

7. Conclusions

Large alluvial fans fed from the Cordillera Frontal basins areowing to glacial and periglacial periods that favored the largevolumes of debris caused by weathering processes and landslidesand of abundant water volumes produced by melting of ice, snowand rainfall. These materials were mobilized and deposited indepressed areas with <10� slopes, originating a piedmont con-sisting on several generations of stepped alluvial fans. Climate inmountainous areas in different periods of the Pleistocene was coldand wet, while today in the Iglesia Valley, semi-arid conditions arepresent.

In themountainous areas of Precordillera Occidental, with lowertopographic elevations, the climate would not have been so severe,but enough to sustain snow or a periglacial domain. These timeswere in coincidence with periods of cooling of the planet (Hansen,2006).

The large Cordillera Frontal eastern piedmont controlled theextent of the minor Precordillera Occidental piedmont, due to itslarger fluvial basins, and uplift during the Cenozoic when theorogenic front migrated to the foreland (Ramos et al., 1996). Thatfacilitated the tilting of the Iglesia basin to the east. This causeddistal detritic material accumulation from the west, generatingdifferent alluvial fan levels, whose deposits are wedge-shaped,with decreasing thickness to the west.

The piedmont formation consisting on telescopic and steppedalluvial fans is bound to tectonic causes and/or climatic variations,and it is difficult to estimate their individual contribution into thelandscape modeling. However, by considering that during times ofincreased global warming, similar climatic conditions to thepresent prevail, with a paralysis of the morphogenetic processesassociated with surface water, and a major action of the wind,under conditions of extreme aridity. It is possible to infer a struc-tural control at the beginning of vertical erosion. The streamchannels tended to become entrenched into the fanhead as valleyfan floor continued to lower the stream channel in the mountains.The uplift of the piedmont along faults or folds counteracts thistendency to entrench the fan as was described by Bull (2007). Theseregional events of vertical erosion closed the alluvial accumulationprocesses. The alluvial level Q1 became in relics, more or lessisolated.

During the Upper Pleistocene, with the establishment ofa tectonic calm period, and after the installation of widespreadlateral erosion processes, glaciplanation began, favored by thereturn to cold and wet climatic conditions in mountainous areasand a semi-arid climate in the valley. Accumulation of alluvial fanlevel Q2, topographically below Q1, starts as well. Then, the Q3 leveloriginated, which shows greater axial length, and less exposure tolateral erosion processes.

At present, the establishment of hyper-arid climatic conditionsin the Iglesia Valley has caused the end of almost all morphogeneticprocesses. Wind is the most important process, together with thefew permanent rivers and sporadic flash floods.

Acknowledgments

The authors are indebted to anonymous reviewers forthoughtful reviews and constructive comments. Thanks to Nicolás



Vargas for his understanding help with field works. This study wassupported by CONICET (PID 6267) and CICITCA (21/E 850) from theUniversidad Nacional de San Juan.

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