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Quaternary International 253 (2012) 80e90

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Quaternary International

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Geomorphology, tectonism and Quaternary landscape evolution of the central Andes of San Juan (30Se69W), Argentina

Laura P. Perucca a,b,*, Liliana M. Martos b a CONICET, Gabinete de Neotectónica, INGEO-FCEFyN-UNSJ, Av. Ignacio de La Roza y Meglioli, 5400 San Juan, Argentina b Departamento Geología, FCEFyN-UNSJ, Av. Ignacio de La Roza y Meglioli, 5400 San Juan, Argentina article info abstract

Article history: The northesouth trending valley of Iglesia is a regional tectonic depression limited to the west by the Available online 23 August 2011 Cordillera Frontal Unit and to the east by Precordillera Occidental Mountain Units. The forms of the resulting landscape in the region are the result of glacial, periglacial, ﬂuvial and alluvial action, aggradational and deggradational processes, as well as neotectonic activity and climatic changes. The generation of large Quaternary alluvial fan aggradational surfaces is related to previous climatic conditions, colder and more humid than the present ones. Abundant snowfalls and rains during the Pleistocene made possible detritus deposition, generating alluvial covers whose thickness increases toward the east. These climatic conditions alternated with arid periods, during which vertical erosion of streams prevailed, forming a landscape of stepped levels. In addition, the presence of faults with Quaternary tectonic activity indicates, a strong structural control in the evolution of the landscape during the PleistoceneeHolocene periods, effectively starting vertical erosion and ﬁnishing a cycle of erosion-accumulation and the beginning of the following one. Ó 2011 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction occasionally overflowing during intense rainfalls in summer, or in those years of higher snow precipitation in the Cordillera Frontal Between 32 and 52S, a wide region in Argentina with arid and area. The action of mechanic weathering and mainly wind action semi-arid climatic conditions is developed with a narrow strip of promotes glaciplanation processes, good development of desert maximum aridity, named the Arid Diagonal (Bruniard, 1982) pavement, desert varnish and cryoclastic phenomena. extending from the Atlantic coast at 44S to the north to 27S along This paper will propose a paleoclimatic analysis aimed at the eastern flank of the Andes. understanding its influence on the evolution of the landscape, Landscape evolution in semi-arid areas of this portion of the adding the control of the active tectonics of the Pleistoce- Andes can take on distinctive characteristics of their own. These neeHolocene as a main factor in such development. The main regions are clearly marked by low annual precipitation, distinctive constraint in the analysis of these mountain paleoenvironments is vegetation, and characteristic ephemeral processes of erosion and/ the lack of previous geological and paleoecological data on the or deposition, which water and wind are the most important Pleistocene/Holocene conditions. Most of the existing studies are driving agents. centered on the glacial chronology of several valleys of the However, deposits and landforms recognized in the region Cordillera, hundreds of kilometers south of the study area, as suggest past climate conditions much different from today’s. general paleoclimatic interpretations, whereas very little is known Different accumulation and erosion landforms identified in the area about the paleoenvironmental conditions at this latitude. There- indicate changes in climatic conditions, leading to greater aridifi- fore, this work is a contribution to current knowledge of the region. cation during the Late Holocene. These changes in turn lead to The methodology applied in the analysis of the area is based on decreased transport capacity of rivers, and to dominating aeolian the interpretation and digital analysis of the geomorphological processes. Fluvial activity is restricted to rivers and streams, features of the land surface. 1:30,000 scale aerial photographs provided by the Secretaría de Minería de la Provincia and Landsat TM images with 30 and 15 m of resolution were used to that effect. * Corresponding author. CONICET, Gabinete de Neotectónica, INGEO-FCEFyN- A slope analysis map was prepared in a GIS environment with UNSJ, Av. Ignacio de La Roza y Meglioli, 5400 San Juan, Argentina. E-mail addresses: [email protected] (L.P. Perucca), lmartos@unsj-cuim. the creation of a digital terrain model (DTM). Altitudes were edu.ar (L.M. Martos). obtained as a result of the partial digitization of topographic charts

L.P. Perucca, L.M. Martos / Quaternary International 253 (2012) 80e90 81 provided by the Instituto Geográfico Militar. Field work consisted of Previous studies proposed that the Iglesia basin includes a detailed analysis of the most relevant landforms and natural a Neogene sequence up to 3.5 km thick in its center, thinning to the trenches. east and west, and also to the north and south, and interpreted the basin as a piggyback basin that developed during the eastward 2. Environmental setting of the study area advance of Precordillera thrusts (Beer et al., 1990; Jordan et al., 1993). Siame et al. (2006) found that at 30S both the Iglesia The area under study is located in the central western region of Valley and the Precordillera can be seen as a crustal-scale trans- the San Juan province, between 30 and 30 200S and 69 and 69 pressive zone, whose deformation is distributed with a dextral 300W(Fig. 1a). From east to west, it covers a portion of the western strike-slip along El Tigre Fault zone. This phenomenon is closely border of the Precordillera Occidental, the Iglesia Valley and the related to the Precordillera fold-and-thrust belt. eastern portion of the Cordillera Frontal. Based on the analysis of seismic profiles in the Iglesia Valley and In the Precordillera, the main mountain outcrops trend north- several outcrops in a cross-section along the Jáchal river, Alvarez- esouth, at the Sierra Negra, whose peaks reach 3500 m asl. The Marrón et al. (2006) interpreted a positive flower-type structure most important ranges in the Andean Cordillera are the Colangüil during the Neogene for this region. The authors considered that and the Agua Negra, both exceeding 5000 m asl. these models would not totally reflect the structural arrangement Surface drainage is constituted by the Blanco River and its of fault and thrust belt proposed by others (Allmendinger et al., perennial tributaries, among which the most important are the 1990; Jordan et al., 1993). Colangüil and the Agua Negra creeks both coming from the According to Siame et al. (2006), the Precordillera mountain Cordillera Fontal. To the east, other tributaries coming from the belt, which is nearly 400 km long and 80 km wide, is a thrust-and- Precordillera Occidental, is the Carrizal creek and coming from the fold belt separated from the Cordillera Frontal by an NeS piggyback south, the Iglesia Creek. basin: the Calingasta-Iglesia Valley. Allmendinger et al. (1990) The harshness of the present climate is manifested by extreme suggested that, between 29 and 31S latitude, the Cordillera winter temperatures in the Cordillera (to 30 C), wide tempera- Frontal is uplifted as a ramp-anticline over a mid-crustal ture variations, minimum humidity, winter snow precipitation and décollement. very scarce pluvial precipitation (Minetti et al.,1986). The climate in Outcrops in the Cordillera Frontal area consist of sandstones and the Precordillera Occidental and Iglesia Valley is arid-desert, with siltstones with an Upper CarboniferouseLower Permian age (Agua large daily and annual temperature variations, atmospheric trans- Negra Formation), Permian granites and granodiorites. Mesozoic parency and low humidity. The rainfall regime is continental, with granites of the Batholith of Colangüil, Neogene continental sedi- summer rains and only with very low average frequency of days mentary rocks (Iglesia Group) and PleistoceneeHolocene deposits with rain. According to the classification of Köppen (1936) it is of (Fig. 2) are present in the Valley (Cardó et al., 2000, 2001). type BWK (average annual temperature of 15.7 C in Rodeo, but In Precordillera Occidental, the oldest statigraphic units are above 18 C during the warmest month. January >23 C) (Minetti Ordovician and Devonian sedimentites (Yerba Loca and Punilla et al., 1986). From a climatic viewpoint, this area may be classi- Formations) covered by Upper Paleozoic deposits (Malimán fied as hyper-arid (120 > p > 60 mm) or eremitic (60 > p > 30 mm), Formation), with some acid dikes assigned to the Choiyoi Magmatic depending on the subzone (Minetti et al., 1986; Le Houérou, 1999). Cycle (Lower PermianeLower Triassic) (Llambías et al., 1996). The Prevailing winds come from the southeast, and the Zonda (föehn) PaleogeneeNeogene sequence is represented by the Iglesia Group and north winds are almost constant occurrences during (Wetten, 1975; Contreras et al., 1990), with Lomas del Campanario AugusteSeptember. and Las Flores Formations (Fig. 2). Lomas del Campanario Formation includes andesite and dacite 3. Geologic and tectonic setting rocks, volcanic bombs and tuffs and a higher level with cross- stratified conglomerates and some diatomite layers (Wetten, 1975). The western portion of South America has a complex Las Flores Formation consists of a succession of well laminated morphology, with an active western margin, with topography and siltstones and claystones and interbedded layers of gypsum sheets seismicity reflecting tectonic intersection between Nazca, (Wetten, 1975). The outcrops of these units are located to the west Antarctica and South America plates. This convergence began about of Pismanta and Colangüil (Fig. 2). 200 million years ago with the east dipping subduction of the The Quaternary deposits in the western area of the valley consist oceanic plates beneath the South America Plate (Uyeda and of gravels with greywacke, quartzite, and granodiorite clasts. These Kanamori, 1979). clasts are over 50 cm in diameter. Furque (1979) identified the Between 28 and 32S, the Nazca Plate subducts horizontally Tudcum Formation as distributed to the west of the Iglesia and beneath the South American Plate, about 100 km deep at a rate of Blanco rivers, covering Neogene deposits consisting of fine to 6.3 cm/year (Pardo Casas and Molnar, 1987; Somoza, 1998; medium conglomerates. He estimated the Tudcum Formation Kendrick et al., 2003). This subhorizontalization started between having 50 m in thickness, decreasing to the West. 8 and 10 Ma, in close association with subduction of the ancient Quaternary deposits occupy the lowest topographic position, Juan Fernández Ridge (Jordan and Gardeweg, 1987; Ramos, 1988; forming alluvial sequences with clasts of sandstones, wackes, Kay et al., 1991; Yáñez et al., 2001; Ramos et al., 2002, among shales and a variety of igneous rocks. Playa deposits, made of silt, others). clay and sand are placed in the lowest part of the tectonic Central Chile (about 30S) is characterized by an intermediate depression of the Iglesia Valley and they form the local base levels obliquity of the convergence vector of the Nazca Plate beneath the of the ephemeral rivers. South America Plate (DeMets et al., 1994; Yáñez et al., 2001). This According to Jordan et al. (1993), the Iglesia basin would have mode of oblique subduction affects the deformation distribution evolved to a system of east-verging overthrusts in the Precordillera and the resulting morphology, favoring the occurrence of strike- Central. Movements along the imbricate faults began in the slip faults (Bastías et al., 1990; Siame et al., 1997, 2002). The western Precordillera at 20 Ma. Alvarez-Marrón et al. (2006) geological setting of the Iglesia Valley is the result of complex interpreted in a WeE cross-section of Precordillera along the geodynamic processes as a consequence of the Nazca and South Jáchal river, two major sets of structures revealing differing defor- America Plates convergence. mation styles that have been superimposed during the Paleozoic to Author's personal copy

82 L.P. Perucca, L.M. Martos / Quaternary International 253 (2012) 80e90 present tectonic evolution. These combined effects of initial 4. Faults with Quaternary tectonic activity compression and then transpression make the structural interpretation difﬁcult, since the Neogene deformation would not be Evidence of tectonic activity during the Quaternary is found in suited to a thrust and belt system, but to a ﬂower-type structure. the 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. Author's personal copy

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Fig. 2. Geological sketch map. Author's personal copy

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Angualasto faults) and Precordillera. The El Tigre fault (Bastías et al., a horizontal displacement of about 260 m, and a slip trace between 1984; Bastías, 1985; Siame et al., 1996, 1997; Fazzito et al., 2009)is 1 and 3 mm/year. Siame et al. (1997) identiﬁed a dextral slip of located in the western piedmont of Precordillera Occidental, a maximum value of about 1 mm/year from cosmogenically dating between Jáchal and San Juan rivers. Geomorphological evidence of different alluvial fan levels affected by the fault. The El Tigre fault Quaternary faulting is clear along tens of kilometers, with rivers and has a discontinuous trace, with segments 1e7 km long, affecting alluvial fans offset, sag ponds, escarpments and aligned springs. Neogene and Quaternary deposits. Fault segments have a “horse- The east-facing fault scarps, whose heights vary between 0.80 tail” like arrangement in the northern end (Siame et al., 2006). and 50 m, have a markedly straight trace, which trend N10E. On the eastern piedmont of Cordillera Frontal, sub parallel Bastías (1985) and Bastías and Uliarte (1991) and estimated faults, whose lengths reach several kilometers, affect Pleistocene

Fig. 3. Geomorphological map of the Iglesia Valley. Author's personal copy

L.P. Perucca, L.M. Martos / Quaternary International 253 (2012) 80e90 85 alluvial fan levels. Main fault segments with activity during the has also been recognized within Tudcum normal faulting, dipping Quaternary (Fig. 3), are distributed along the middle and distal to the east and affecting sediments from the Las Flores Formation portion of the Cordillera Frontal piedmont, such as Colangüil- and Pleistocene deposits. Guañizuil (Fig. 4a), Pismanta-Las Flores-Bella Vista and Angua- Pérez and Costa (2006) noted that scarps of different fault lasto faults, all of them trending NeS. There are minor faults segments are discontinuous and distributed with a general “V” trending northwest to southeast and northeast to southwest. pattern in plain view. The apex is located in the Cerro Negro de These are reverse faults dipping with high angles to east or west, Iglesia, with two main branches trending north-northeast and as it is observed in Guañizuil where Pleistocene (Q1 level) alluvial north-northwest. deposits overlies lacustrine deposits of possible upper Pleistoce- neeearly Holocene age. Lacustrine outcrops are very small and 5. Discussion have yellowish pink tones, where alternate sandy banks in a lower sequence, and clayey silt at the upper levels, reaching The longitudinal Iglesia Valley is a regional tectonic depression a thickness of 4.5 m. The reverse fault dips 82 E and trends N30 E (Heredia et al., 2002), limited by a western mountainous unit that is (Fig. 4a). composed by ranges with elevations exceeding 5000 m asl The rectilinear trace of the faults, the inversion of the upthrown (Cordillera Frontal), and an eastern unit showing elevations of and downthrown sides of the escarpment and the high inclination around 4000 m asl (Fig. 1a). The valley is crossed by trending NeS of the fault plane, suggest a prevailing strike-slip component. This thrust 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 the northwest 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. Author's personal copy

86 L.P. Perucca, L.M. Martos / Quaternary International 253 (2012) 80e90 asymmetric profile, crossed by the NeS Blanco River, that locates Polanski (1963) described some fanglomerate levels from toward the eastern edge of the depression. Cordillera Frontal that overlie a Neogene surface in the depression The valley fill consists of Neogene sediments, corresponding to of Tunuyan, Mendoza Province (Fig. 1b). These alluvial deposits materials deposited in a basin of similar morphology to the present were assigned to Los Mesones (Lower Pleistocene), La Invernada valley. They rest on a palaeorelief composed of Paleozoic rocks and (early Upper Pleistocene) and Las Tunas (Upper Pleistocene) are overlain by slightly consolidated to unconsolidated deposits Formations. La Tunas Formation is the upper surface of the bajada assigned to the PleistoceneeHolocene, disposed in angular downstream and disappears beneath the loessic plain sediments of unconformity. These deposits correspond to large alluvial fans from La Estacada and Zampal Formations, of an Upper Pleistoce- the eastern front of the Cordillera Frontal, with lengths between 33 neeHolocene age (Zárate and Mehl, 2008). Polanski (1963) related and 38 km. This broad piedmont has controlled the longitudinal to the four Aggradational Cycles (I, II, III and IV) with the Principal, extension of the western piedmont from Western Precordillera Posthumous, Final and Minor uplift Neotectonic Phases respec- formed by telescopic alluvial fans, whose lengths vary between 13 tively (Table 1). and 6 km (Fig. 3). Martos (1995, 2008) related the Quaternary stepped levels on The mountainous front of Cordillera Frontal is sinuous. This, the eastern piedmont of Precordillera Oriental (Fig. 1b) with the accompanied by glaciplanation processes are signs of periods with main neotectonic uplifts. She noted that during the absence of tectonic stability. However, there are at least three alluvial levels tectonic forces, climatic morphogenesis prevailed (Table 1). affected by Quaternary faults. The effects of neotectonics are in most In the Iglesia Valley, NeS trending thrust faults affect the middle cases accompanied by spring waters, some of them thermal (40 C). and lower portions of alluvial fans. These faults have either east or The main rivers of Cordillera Frontal, that trend NNWeSSE, west vergence. The neotectonic effects have led to topographic evacuate debris from the mountain areas, Colangüil and Agua ledges, in most cases accompanied by hot and temperate springs. Negra rivers being the higher order streams. These permanent Some minor piedmont hills are recognized in the middle and rivers significantly excavate deep gorges in the Andean mountains, eastern portion of the valley (Figs. 1a and 2), uplifted by upper which in some cases exceed 800 m depth. Precordillera Occidental Pleistocene tectonic activity (Fig. 4a). Neogene hills show is drained by numerous ephemeral trending NW streams, which a badlands landscape, linked to past semi-arid climatic conditions flow down to the Blanco River only during torrential summer rains. (annual rainfall from 250 to 500 mm/year), while present condi- Large volumes of sediment tend to be transported short distances tions show an arid climate, with annual average rainfall varying during these storm rainfall events. These ephemeral rivers are in between 39 and 81 mm/year from east to west (Minetti et al., 1986). some cases fed by springs. In the western piedmont of Sierra Negra, pseudo-karstic Another main feature of regional significance is the occurrence features of considerable size (caves) and some karst pipes are of an extensive erosion surface overlying the Neogene rocks which developed within the Neogene sediments. Other phenomena slopes gently from the mountain front of the Cordillera Frontal to recognized in the area are stalactites, stalagmites, and sinkholes. the east, at an average gradient of 12. Over much of the area, the In the middle portion of the eastern piedmont, at least two plain is covered by a thin veneer of alluvial origin. Inselbergs and levels of lacustrine deposits are recognizable. These deposits can be ridges, built of granite and Paleozoic sandstones rise above the correlated tentatively with the Holocene lacustrine sediments plain. Their height varies from a few tens of meters up to described in surrounding areas, along the Jáchal River (Colombo 200e300 m. The fine-grained, pink colored Neogene sediments et al., 2000)(Fig. 1b). These authors analyzed gastropod and paly- and sedimentary rocks have a badland topography, an intricately nomorph samples. They noted the formation of those temporary rilled and barren terrain with an extensive network of convoluted lakes during wet episodes, possibly related to ENSO variations, in rills and gullies. a context of prevailing aridity. In the piedmont of Cordillera Frontal, three generations of Quaternary alluvial fans (Q1,Q2 and Q3), and a present accumula- 6. Results tion level (Q4) with a telescopic array can be recognized (Fig. 3). Neotectonic activity in the valley affects the development of the The landscape is the result of climatic variations during the alluvial fans. The longitudinal topographical profiles of those fans Quaternary. Although these may have not been very extreme, they show several segments with decreasing or increasing slopes were significant, as indicated by the carving of the surface features respectively. The thickness of the Quaternary alluvial cover varies (Perucca and Martos, 2009). There was an alternation of colder and from 10 cm to 3 m in the proximal to medium zone, to 10 m in the wetter conditions in mountainous areas and arid and periglacial distal portion. The alluvial cover overlies the leveled surface of conditions in the piedmonts with warmer and drier periods, where Neogene sediments. North of the Colangüil creek, the alluvial levels vertical erosion prevailed, creating a stepped landscape. The pres- are strongly incised and have a thick detrital cover. Between ence of Quaternary faults also indicates a strong structural control Colangüil and Agua Negra creeks, the alluvial levels show an even on landscape evolution during the Upper PleistoceneeHolocene, slightly tilted surface, barely affected by the streams (Fig. 3). Even which would have 2 the end of a cycle of erosion-accumulation, though there are no numerical ages for these alluvial levels, it is with regional and vertical erosion at the beginning of the possible to assign some tentative chronologic constraints by following cycle in which a new alluvial level would form at a lower correlating them with alluvial deposits located at the piedmont altitude than the previous one. area of the El Tigre range, south of the study area (Fig. 1b), where The generation of large alluvial fans during the Pleistocene is Siame et al. (1997) did radiometric dating in several alluvial fan related to colder and wetter climatic conditions than during the surfaces. They determined that alluvial fans remaining as relics Holocene, with a significant generation of debris in the moun- were deposited roughly 770,000 years ago, and the minimum tainous area, resulting from landslides and cryoclastic phenomena. exposure for the youngest alluvial surface dates at 41,000 Cryoclastic activity occurred during the most coldest and humid a(Table 1). They considered that this stepped topography origi- periods of the Pleistocene, with elevated rates of detrital produc- nated due to variations in the load and unload of river flows and/or tion, easy removal from the upper portion of slopes by different the fall of the base level linked to a regional tectonic uplift and transportation agents (water, gravity) and consequent thick accu- hydrological changes due to climate changes (Schumm et al., 1987; mulation at the foot slope. Heavy rain and snow precipitation Ritter et al., 1993). during the Pleistocene, together with water melt from glaciers, Author's personal copy

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allowed for the removal of debris, creating an alluvial cover with

) ) ) thickness increasing from west to east. ) Zárate During the mid-Holocene, an increase in temperature and C(

14 aridity in southern South America has been inferred from pollen 21 38 78 8 records (Grimm et al., 2001). In this area, the Arid Diagonal is extended and has been studied from a geomorphologic, pedologic 700.000

Siame et al., 1997 Siame et al., 1997 Siame et al., 1997 and paleoecological point of view (Garleff et al., 1991; Veit, 1994, 1400 + 130 Absolute ages and Mehl, 2008 37.000 100.000 ( 180.000 380.000 ( < ( 1996; Gil et al., 2005). Based on the compilation of all records for the central-western Wurn Argentina, Rojo et al. (2010) proposed a regional change that is ¼ Riss

¼ evident at ca. 3 ka BP, indicating more water availability prior to that time, probably in all ﬂuvial systems that drain the cordillera. Zárate ) After 3 ka BP, present conditions were established. Wisconsin Miindel ¼ ¼ The present climate in the Iglesia Valley is hyper-arid, with Wurn I ed from

¼ rainfall below 50 mm/year. In these climatic conditions, erosional fi processes are most conspicuous, linked to the main permanent Postglacial and Mehl, 2008 Correlation with northern hemisphere (modi Wisconsin Interstadial Wurn s.l Interglacial Sangamon Sangamon Jowan Yarmouth rivers from Cordillera Frontal and also to ephemeral streams and flash floods during torrential rainfall. Rivers affect the mid- proximal areas, with water tending to drain radially in the distal Siame ) sector of the eastern Piedmont. However, very deep dry channels found in the mid-distal portion could be perhaps linked to a fall of the base level that at a regional scale would be related to the Recent alluvial level El Tigre Western Piedmont ( et al., 1997 Q6 level Q3 level Q2 level Q1 level tilting of the valley to the east. Segmentation on alluvial fans is an indicator of recent tectonic activity (Bull, 1968, 1977). Thus, reverse faults affecting this sector

) favor retro-wedge erosion in the elevated blocks. Retrocedent erosion 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 Cordillera Martos, 1995 Q4 recent alluvial level Q3a alluvial level Precordillera Oriental Piedmont ( Q3 level Q0 level Mogna Fm. and equivalents Q2 and Q2a levelsQ1 and Q1a levels Q5 level Q4 level Frontal and in the western piedmont of the Precordillera. Based on the different topographic position (stratigraphic relationships), the morphology of the upper surfaces of alluvial fans in terms of )

Perucca development of desert pavement and varnish, the presence or lack of calcrete, the percentage of fragments affected by cryoclastic activity, the glaciplanation of surfaces associated with the degree of dissection of the landforms slopes, the alluvial fans can be classified with three levels of accumulation, Q1,Q2, and Q3, besides the present Lacustrine eastern piedmont Iglesia Valley ( and Martos, 2009 Lacustrine Q3level Q2 level Q1 Level accumulation-erosion surface (Q4), where Q1 is the oldest Quater- nary alluvial level and Q4 the youngest. The glaciplanated alluvial fans Q1,Q2 and Q3, have slope debris, indicating past semi-arid conditions (Fig. 4b). Q1 and Q2 alluvial levels have desert varnish Holocene Q4 recent alluvial level Upper Pleistocene Lower Pleistocene Middle Pleistocene Pliocene coating clasts. These manganese-rich black varnishes are characteristic of moderately arid, near-neutral environments (Staley et al., 1991) and forms as moisture from rain, fog, dew, and snow interacts with detrital materials on rock surfaces (Perry et al., 2006). Several nebkas and climbing dunes on the slopes of the oldest alluvial fans have been observed near Tudcum, related to current Tectonic movements Age Cordillera Frontal Phase (general uplift) Neotectonic Phase Main Neotectonic Phase climatic conditions where the action of winds prevails (Fig. 4c). Runoff influence is paralyzed under the present climatic environment. ) Desiccation cracks observed in dry river beds (Q4) show significant evidence of wind deflation, indicating a considerable time without runoff or rainfall. Even the sides of polygons are well rounded because of deflation, which in turn widens the cracks Polanski, 1963 ( IV Aggradation Cycle Light uplift Aggradation events Tunuyan Mendoza I Aggradation Cycle III Aggradation Cycle II Aggradation Cycle Posthumous (Fig. 4d). Another evidence of aridification in recent times comes from settlers’ comments, which describe the strong decline in the

) water 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 glaciplanated surfaces, they relate to earlier periods with abundant rainfall. In mountain deserts and depressions, Mabbutt (1977) noted that conditions vary considerably, and that the contribution of water from rainfall, melting snow or ice is important. In these cases the Zárate and Mehl, 2008 Volcanica Postglacial ( Pumicea VolcanicaPaleopleistocena piedmont receives substantial contributions of materials and Peat bogs Asociacion Piedmont/Tunuyan depression El Zampal Formation La Estacada Formation Final Neotectonic Las Tunas Formation Asociacion Piroclastica La Invernada Formation Asociacion Los Mesones Formation

Table 1 Comparative stratigraphic scheme between theOriental. eastern piedmont of Cordillera Frontal (Iglesia Valley), Tunuyan depression, western piedmont of Precordillera Occidental (El Tigre range) and eastern piedmont of Precordillera water from allochthonous streams, draining into the basin. Author's personal copy

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The paleoforms Q1,Q2 and Q3 correspond to alluvial fans affecting the piedmont. Under these conditions, Q2 and Q3 alluvial generated under these conditions. Even taking into account the fan levels were generated. topographic elevation of the mountains of Precordillera Occidental Landscape evolution is changing very slowly or is nearly para- (3500 m asl) it is possible to infer periglacial and/or nival climatic lyzed today, and alluvial level Q4 corresponds mainly to sporadic conditions during the Pleistocene. These conditions favored the channel fills. Winds prevail in the region, becoming the main generation of mechanical weathering and mass wasting processes morphogenetic agent and landscape modeler in the Iglesia Valley. with the subsequent evacuation of large quantities of debris, leading to a narrow western piedmont. Even when climatic changes 7. Conclusions would have less contrast, alternating periods of cold and humid climates, with periods of less cold and more arid conditions, can be Large alluvial fans fed from the Cordillera Frontal basins are distinguished during the Quaternary, which are responsible for owing to glacial and periglacial periods that favored the large modeling the landscape, together with a significant contribution of volumes of debris caused by weathering processes and landslides neotectonic events. and of abundant water volumes produced by melting of ice, snow Alluvial covers Q1,Q2 and Q3 overlie erosion ramps carved at the and rainfall. These materials were mobilized and deposited in expense of crumbly Neogene sediments. Their genesis could be depressed areas with <10 slopes, originating a piedmont con- linked to tectonic stability that favored its extensive development sisting on several generations of stepped alluvial fans. Climate in under semi-arid climatic conditions during Pleistocene, some mountainous areas in different periods of the Pleistocene was cold wetter than Late Holocene and present climate. and wet, while today in the Iglesia Valley, semi-arid conditions are Detrital material evacuated from the mountain areas generated present. glaciplanation in the proximal and middle portions of piedmonts, In the mountainous areas of Precordillera Occidental, with lower accumulating materials in distal areas. The increase of detritus topographic elevations, the climate would not have been so severe, volumes from the mountainous areas subject to colder and wetter but enough to sustain snow or a periglacial domain. These times conditions leads to an increasing accumulation nearer to the were in coincidence with periods of cooling of the planet (Hansen, mountain front. In this way, the oldest alluvial fan Q1 is generated 2006). from ephemeral streams, with frequent debris flows associated The large Cordillera Frontal eastern piedmont controlled the with torrential rainfall or rapid snow melting in the mountainous extent of the minor Precordillera Occidental piedmont, due to its areas. Such flows are related to areas of high slopes, in cold larger fluvial basins, and uplift during the Cenozoic when the mountains, where detritic material was originated mainly by orogenic front migrated to the foreland (Ramos et al., 1996). That physical weathering, by cryoclastic action and mass wasting, added facilitated the tilting of the Iglesia basin to the east. This caused to tectonic activity that elevated the slopes. Moreover, seismic distal detritic material accumulation from the west, generating events also favor the availability of detrital material (landslides and different alluvial fan levels, whose deposits are wedge-shaped, avalanches) on the slopes. with decreasing thickness to the west. When mountain uplift and aggradation are older than the The piedmont formation consisting on telescopic and stepped incision of the channel, its slope increases and sedimentation is alluvial fans is bound to tectonic causes and/or climatic variations, larger close to the mountain front (Bull, 1968). When differences in and it is difficult to estimate their individual contribution into the relief created by tectonic causes gradually decrease, fluvial landscape modeling. However, by considering that during times of processes, sheet floods and channelld flows dominate. increased global warming, similar climatic conditions to the Debris flow deposits are located close to the apical portion of the present prevail, with a paralysis of the morphogenetic processes alluvial fans, whereas in the middle and distal portion channel associated with surface water, and a major action of the wind, flows layers can be recognized (Fig. 4e). Silt and clay deposits are under conditions of extreme aridity. It is possible to infer a struc- interbedded with sand lenses, which have been deposited in the tural control at the beginning of vertical erosion. The stream distal floodplain, and playa environment. The sandy layers corre- channels tended to become entrenched into the fanhead as valley spond to channel fill or ephemeral streams deposits, associated fan floor continued to lower the stream channel in the mountains. with torrential rainfalls. The uplift of the piedmont along faults or folds counteracts this These sequences occupy different topographic positions, indi- tendency to entrench the fan as was described by Bull (2007). These cating their relationship with the different processes of accumu- regional events of vertical erosion closed the alluvial accumulation lation Q1,Q2 and Q3, which might be partly lacustrine events, often processes. The alluvial level Q1 became in relics, more or less show strongly cemented carbonates, indicating pedogenic isolated. processes associated with tectonic stability. During the Upper Pleistocene, with the establishment of The oldest alluvial fan (Q1) morphogenesis, tentatively assigned a tectonic calm period, and after the installation of widespread to middle Pleistocene, was completed by tectonic processes that lateral erosion processes, glaciplanation began, favored by the favored incision, leaving it exposed to erosion and then to lateral return to cold and wet climatic conditions in mountainous areas erosion that progressively decreased its areal distribution, being and a semi-arid climate in the valley. Accumulation of alluvial fan presently highly dismembered (Fig. 2). The top surface of this level Q2, topographically below Q1, starts as well. Then, the Q3 level alluvial fan suffered glaciplanation processes during tectonic originated, which shows greater axial length, and less exposure to stability periods in the region under semi-arid climatic conditions, lateral erosion processes. somehow more humid than Holocene climate. Desert pavement, At present, the establishment of hyper-arid climatic conditions wind-faceted pebbles (ventifacts), desert varnish and fractured in the Iglesia Valley has caused the end of almost all morphogenetic rocks by cryoclastism can be recognized at the glaciplanated top processes. Wind is the most important process, together with the surface of this alluvial level (Fig. 4f). few permanent rivers and sporadic flash floods. Morphogenetic processes repeated during Pleistocene times, with the influence of climatic changes with alternating wet and Acknowledgments cold conditions in the mountainous areas and semi-arid climate in the valley; with some warmer and dry, arid and hyper-arid times, The authors are indebted to anonymous reviewers for together with tectonic activity as evidenced by Quaternary faults thoughtful reviews and constructive comments. Thanks to Nicolás Author's personal copy

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