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Geomorphological evolution of the Tilcara alluvial fan (Jujuy Province, NW Argentina): Tectonic implications and palaeoenvironmental considerations
Journal of South American Earth Sciences 26 (2008) 68–77
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Geomorphological evolution of the Tilcara alluvial fan (Jujuy Province, NW Argentina): Tectonic implications and palaeoenvironmental considerations Carlos Sancho a,*, José Luis Peña b, Felipe Rivelli c, Ed Rhodes d, Arsenio Muñoz a a
Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain Geografía y Ordenación del Territorio, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain c Facultad de Ciencias Naturales, Universidad Nacional de Salta, Avda. Bolivia 5150, 4400 Salta, Argentina d Research School of Earth Sciences and Research School of Pacific and Asian Studies, The Australian National University, Canberra, ACT 0200, Australia b
a r t i c l e
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Article history: Received 16 July 2006 Accepted 11 October 2007
Keywords: Tilcara alluvial fan OSL dating Upper Pleistocene Thrust tectonics Arid environments Andean Cordillera
a r t i c l e
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Article history: Received 16 July 2006 Accepted 11 October 2007
Palabras clave: Abanico aluvial de Tilcara Dataciones OSL Pleistoceno superior Cabalgamientos Ambientes áridos Cordillera de los Andes
a b s t r a c t The development and evolution of the Tilcara alluvial fan, in the Quebrada de Humahuaca (Andean Eastern Cordillera, NW Argentina), has been analysed by using geomorphological mapping techniques, sedimentological characterisation of the deposits and OSL chronological methods. It is a complex segmented alluvial fan made up of five evolutionary stages (units Qf1, Qf2, Qf3, Qf4 and Qf5) developed under arid climatic environments as well as compressive tectonic conditions. Segmentation processes, including aggradation/entrenchment cycles and changes in the location of the depositional lobe, are mainly controlled by climatic and/or tectonic changes as well as channel piracy processes in the drainage system. Alluvial fan deposits include debris flows, sheet flows and braided channel facies associated with high water discharge events in an arid environment. The best mean OSL age estimated for stage Qf2 is 84.5 ± 7 ka BP. In addition, a thrust fault affecting these deposits has been recognized and, as a consequence, the compressive tectonics must date from the Upper Pleistocene in this area of the Andean Eastern Cordillera. Ó 2008 Elsevier Ltd. All rights reserved.
r e s u m e n Se han utilizado técnicas geomorfológicas (cartografía), sedimentológicas (análisis de facies) y cronológicas (dataciones de OSL) para analizar la evolución del abanico aluvial de Tilcara, en la Quebrada de Humahuaca (Cordillera Oriental Andina, NO Argentina). Se trata de un abanico complejo segmentado, en el que se han diferenciado cinco etapas constructivas (Qf1, Qf2, Qf3, Qf4 y Qf5) en un contexto de clima árido y de actividad tectónica compresiva. El encajamiento de las superficies de agradación y las modificaciones en la posición del lóbulo deposicional del abanico están controlados por cambios tectónicos y/o climáticos y procesos de captura en el drenaje del sistema. La etapa Qf2 tiene una edad media ponderada de 84, 5 ± 7 ka BP mediante OSL. Los depósitos incluyen facies de flujos de masas, flujos laminares y canales entrelazados acumulados en eventos de alta descarga hídrica bajo condiciones de aridez. Una falla inversa superpone el Terciario sobre estos depósitos lo que permite prolongar la tectónica compresiva hasta bien entrado el Pleistoceno superior en la Cordillera Oriental Andina. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Quaternary alluvial fans are significant landforms within the Quebrada de Humahuaca landscape (Jujuy Province, NW Argentina) (Fig. 1). Their occurrence is related to the scarps defining the Quebrada and is associated with the sedimentary activity of the Río Grande tributary network under arid environmental condi* Corresponding author. Tel.: +34 976761091; fax: +34 976761106. E-mail address: [email protected] (C. Sancho). 0895-9811/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2008.03.005
tions and in an active compressive tectonic context. Inside the Quebrada de Humahuaca several patterns of alluvial fan evolution (Bull, 1977; Silva et al., 1992; Harvey, 1997; Colombo et al., 2000) can be differentiated based on their morphostratigraphic features. Although the present-day alluvial fan activity, affecting routes, railways and villages, has been analysed by Chayle and Wayne (1995), Solís and Orozco (1996), very few detailed studies concerning geomorphological, sedimentological and chronological aspects of these Quaternary morphosedimentary units have been carried out (Azarevich et al., 1999).
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Fig. 1. Location of the study area and geological setting (after Rodríguez-Fenández et al., 1999).
The Huasamayo River, a tributary of the Río Grande, has built one of the most interesting alluvial fans in the Quebrada de Humahuaca, located at latitude 23–24°S near Tilcara village. Besides the geomorphological and applied interest, this alluvial fan shows spectacular features of compressive tectonic activity during Quaternary times (Salfity et al., 1984; Marret et al., 1994; Rodríguez-Fenández et al., 1999).
The main objectives of this work deal with the study of the geomorphological evolution of the Tilcara fan, the analysis of the sedimentological characteristics of the deposits and the determination of the alluvial fan age by using OSL (Optically Stimulated Luminescence) techniques. In addition, these chronological data are very useful to determine both the palaeoenvironmental and
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palaeoclimatic context of alluvial fan development and the age of the compressional deformation affecting the alluvial fan deposits. In arid regions with active tectonics, dating of alluvial fans is necessary to elucidate the palaeoclimatic meaning of the alluvial fan development and to quantify the tectonic activity (Siame et al, 1997). From a regional point of view, there are interesting contributions on Quaternary alluvial fans conducted by Colombo et al., 1996, 2000; Siame et al. (1997) and Colombo (2005a, 2005b), in different areas of the San Juan Province, and by Colombo (2005a, 2005b), at several locations within the Salta and Mendoza provinces, which may be used for comparison. In addition, Robinson et al. (2005) have applied luminescence techniques to date several deposits within Late Pleistocene alluvial fan sequences, in Jujuy and Salta provinces, reporting an age from the Tilcara alluvial fan.
2. Study area The studied alluvial fan is located near Tilcara village (2459 m a.s.l.) (Figs. 2 and 3). The Tilcara alluvial fan has been built by the Huasamayo River, a tributary of the Río Grande draining the Quebrada de Humahuaca from North to South, within the Bermejo River Basin. The Quebrada is a 150 km long mountain valley, with an asymmetric transverse profile, sited within a multiple basin and mountain ridge system trending from North to South. The highest zone in the area corresponds to the watershed with the Puna area (Nevado de Chañi, 6200 m), while the bottom of the Quebrada ranges in height between 3340 m in the North, near the border with Bolivia, and 1350 m in the South. Quaternary alluvial fans and associated lacustrine deposits related to fan damming processes are very frequent inside the Quebrada. The Arroyo del Medio fan, located near Volcán village (50 km to the south of the Tilcara fan), is probably the most impressive example in the area (Chayle and Wayne, 1995). This circumstance is also ubiquitous in others mountain valleys from the NW Argentina (Colombo, 2005a, 2005b; Robinson et al., 2005). Climate in the Quebrada is mountain arid type with high intensity summer storms, irregularly distributed, with cold dry winters (Capitanelli, 1992). In the central area, mean annual temperature is 12.5 °C, with a maximum of 16.2 °C in December and a minimum of 6.9 °C in July. Both seasonal and daily thermal oscillations are very strong. On the other hand, mean precipitation does not surpass 200 mm inside the Quebrada (mean annual rainfall at Tilcara is 136 mm), increasing towards the North and South borders. The vegetation scarce cover, made up of xerophytes, does not completely cover the ground (Beck et al., 2003), favouring highly effective hydrological processes. From a geological point of view, the Quebrada de Humahuaca is located in the Eastern Cordillera morphotectonic domain of the Andes (Turner et al., 1972; Ramos, 1999), between the Puna and Subandean Ranges domains. At the same time, two main tectonic units can be distinguished in the Eastern Cordillera (Fig. 1) (Rodríguez-Fenández et al., 1999): the Casagrande thrust system in the western part and the Humahuaca thrust system in the East. This last includes the Humahuaca Basin. In this area of the Eastern Cordillera (Fig. 1), the outcropping basement consists of the Upper Precambrian-Lower Cambrian low-grade metamorphic flysch of the Puncoviscana Formation (Turner et al., 1972). These rocks are unconformably overlain by marine clastic materials of the Meson (1000 m in thickness) and Santa Victoria (900 m in thickness) Groups, Cambrian and Ordovician in age, respectively (Ramos, 1999). Overlying Mesozoic is represented by a preorogenic andine sequence, 400 m thick, made up of Upper Cretaceous-Lower Tertiary limestones and sandstones, correspond-
ing to the Salta Group. It has been interpreted as a synrift fill related to the Cretaceous extensional tectonic event (Ramos, 1999). Pliocene-Quaternary sandstones and pyroclastic rocks representing the Maimará Formation (35 m in thickness as a minimum) (Salfity et al., 1984), included in the Humahuaca Group, 300 m thick, represent the top of the regional stratigraphic section and the end of the Tertiary synorogenic sequence. These rocks represent the sedimentary filling of a piggy-back basin (Humahuaca Basin) related to out-of sequence thrusts (Heredia et al., 1999). In addition, Quaternary sediments of alluvial fans are spread out on the bottom of the Quebrada. Geological structure in this sector of the Quebrada de Humahuaca (Ramos, 1999; Rodríguez-Fenández et al., 1999) is characterised by the occurrence of elongated North–South trending slices related to East-verging thrusts (Fig. 1), involving the basement and corresponding to a deformation style of thick-skinned fold and thrust belts. Some of the reverse faults result from inversion of previous Cretaceous extensional faults. These Andean compressional structures were created during a main event of thrust sheet emplacement, between 13 and 1 Ma associated with a WNW–ESE shortening (Marret et al., 1994). Nevertheless, Rodríguez-Fenández et al. (1999) distinguish two phases in the compresional event at 17–11 Ma (Eastern Cordillera emplacement) and 8.5–1.8 Ma (Subandean Ranges emplacement with out-of sequence thrust in Eastern Cordillera). After this stage, strike-slip and extensional faulting occurred during the Quaternary (Marret et al., 1994), still active at the present time, in a transtensional kinematics context with some local compressive events until 1 Ma (Rodríguez-Fenández et al., 1999). 3. Methods Detailed geomorphological mapping of alluvial fan morphosedimentary units (fan stages) in the Tilcara area was obtained from 1:30,000 scale aerial photographs and fieldwork recognition and surveillance. As a consequence, a morphostratigraphic sequence, made up by several stages (Qf1, Qf2, Qf3, Qf4 and Qf5), was established. Topographic and geometric features of alluvial fan segments were controlled using altimeter and GPS measurements. At the same time, observation and description of alluvial fan facies was carried out. In addition, a very detailed search of alluvial fan deposits was made in order to identify sand and silt lens suitable for luminescence-dating techniques. Taking into account both morphostratigraphic characteristics and deformation features observed in the Qf2 segment, four samples for OSL dating determinations were taken using black plastic tubes (20 cm long and 4 cm in diameter) forced into freshly exposed sediment to minimize sample exposure to sunlight. Sample preparation and OSL measurements were carried out at the Luminescence Dating Laboratory of the Research School of Earth Sciences (The Australian National University). Sand-sized quartz was extracted using the methodology described by Rhodes (1988), which includes sieving, concentrated hydrofluoric acid treatment and density separation of heavy minerals using sodium polytungstate solution. OSL dating was based on a single aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000), using a Risø TL-DA-15 automated luminescence reader. Beta and gamma dose rate were estimated using neutron activation analysis (NAA) of sediment U, Th and K content, and cosmic dose rates were calculated using the equations of Prescott and Hutton (1994).
4. Characteristics of the Tilcara alluvial fan 4.1. Geomorphological features The Tilcara alluvial fan area is nearly 5 km2 and the drainage catchment area is almost 120 km2, with a North–South elongated shape. The Huasamayo River constitutes the feeder channel,
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Fig. 2. Geomorphic features of the Tilcara alluvial fan area. (A) Geomorphological mapping (see location in Fig. 1). (B) Schematic cross section showing the differentiated alluvial fan segments. Vertical scale is approximate.
12 km long, which collects the waters from the Chilcahuada, Alfarcito and Casa Colorada rivers, near Alfarcito village. The differ-
ence in height between the watershed of drainage area in the mountains (4615 m in the Ovejería-Suncho Norte hills) and the
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Fig. 3. Aerial oblique photograph of the Tilcara alluvial fan.
fan apex (2500 m) is close to 2000 m. As a consequence, the mountain basin channels feeding the Tilcara fan have a short longitudinal profile with high mean slope (11°). In a preliminary analysis, the Tilcara alluvial fan is characterised by a complex segmented development pattern. According to the relative morphotopographic location of different segment remains, a sequence made up of five evolutionary stages has been recognized (Figs. 2 and 3): (a) The oldest segment (Qf1) has the highest topographic location. A very extensive remnant dipping to the West is well preserved in the northern sector of the fan, resting directly against the eastern mountain front of the Quebrada de Humahuaca (Figs. 2 and 3). (b) Fan surface Qf2 is built beyond the former fan stage (Qf1). At present, there is a segment clearly separated from the mountain front in the southern sector, although other small rem-
nants can be recognized, related to the front scarp in the northernmost sector. Tectonic movements affect this unit giving steepened surfaces (Figs. 2–4). According to its geomorphological location and significance, it is possible to recognize this stage, at a regional scale, inside the Quebrada de Humahuaca, representing the main stage of alluvial fan sedimentation. (c) Alluvial fan stage Qf3 is well represented in the southernmost sector fan occupying a lower fringe between Qf2 segments and the mountain front (Figs. 2 and 3). This surface dips to the South, indicating an important change in the fan drainage from the West to the South. This fact can be explained by the partial blockage of the Huasamayo River by the scarp of the Qf2 segment, developed by thrust fault movement. At the present time, it constitutes a hanging paleovalley. At the same time, this fan surface appears more or less continuously preserved in the eastern side of the
Fig. 4. Thrust fault affecting deposits of the Qf2 stage and location of samples used for luminescence dating (see location in Fig. 2).
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Quebrada. On the other hand, another segment of this surface with opposite slope has been recognized in the Pucará of Tilcara site (Figs. 2 and 3). That points to an origin related to the discharge of the Huichaira River in the West side of the Quebrada. This is also justified by the lithological nature of the deposits, different from those observed in the segments associated with the Huasamayo River, and also by the paleocurrent directions obtained from sedimentary structures (clasts imbrication). The geomorphological location of these segments with opposite origin indicates that the Río Grande was probably dammed at this time. (d) Fan surface Qf4 is entrenched within the previous surfaces. It occupies a highly continuous, narrow trench parallel to the feeder channel, presenting a surface dissected by the Huasamayo River (Figs. 2 and 3). Other Qf4 remnants can be recognized in the northernmost fan sector, along the front mountain. Gemorphological occurrences of this Western trending segment, compared to the distribution of the previous Qf3 stage (trended to the South) is best explained by channel piracy near the fault outcrop. (e) The youngest fan surface (Qf5) corresponds to the active fan segment trending to the northwest. It displays a telescopic pattern in relation to the previous Qf4 surface (Figs. 2 and 3). This Qf5 fan surface can be correlated with the lowest terrace of the Río Grande which forms an abandoned meandering-like channel around the Pucará hill. The Río Grande is
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entrenched within the Qf5 surface, forming a wide braided active channel which may be geomorphologically correlated with the active Qf6 surface. On the Qf5 fan-shaped surface, the Tilcara village has been settled in a dangerous geomorphological position, partially mitigated by the construction of artificial levees. As a consequence, the active depositional lobe (Qf6) is constrained to a narrow sector between Tilcara village and the Pucará hill site (Figs. 2 and 3). The activity of this lobe is ephemeral, with large sediment discharges giving debris and mud flows in response to brief but intense summer storms. The resulting floods often affect village houses, camping sites and sections of route number 9. A record of several recent events of high magnitude has been compiled by Azarevich et al. (1999). The debris and mud flow triggered in 1984 broke the man-made levees, affecting more than 30 neighbouring houses. Other documented events took place during 1998 and 1999. The first one accumulated 3–5 m of sediments near to the Huasamayo-Grande confluence and the transit of vehicles along the route number 9 was temporally interrupted. During summer 1999 three extreme events were recorded. Resulting sediments occupied farming areas and blocked the Rio Grande, flooding the camping site. As a consequence of this recent sedimentary aggradation, the possibility of changes in the position of the active depositional lobe (Qf6), with potential hazardous effects for Tilcara village, is not negligible.
Fig. 5. Different aspects from the Tilcara fan facies. (A) General view of the conglomeratic facies. The clasts are poorly sorted. (B) Detail of the conglomeratic facies. The coarsening-upward evolution of this deposit is related to a lobe deposit. (C) General aspect of the conglomeratic and sandy facies. In the conglomeratic facies, tabular layers and abrupt grain size variations are observed, as well as lenticular geometry with channelized bases in the sandy layers. (D) Detail of the sandy facies showing the multiepisodic filling of a channel and cross-stratification.
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4.2. Characteristics of the deposits and facies analysis The recognition of sediments corresponding to different outcrops from the stages Qf1, Qf2, Qf3 and Qf4, has allowed us to differentiate two main types of facies: (a) Conglomeratic facies (Fig. 5A–C). Clast supported conglomerates, locally matrix supported, are characterised by low roundness, poorly sorted cobblestones and mainly siliceous boulders. The matrix of the conglomerates is sandy and they are only weakly cemented. The conglomeratic facies are arranged in crudely developed horizontal bedding of around one metre thickness, truncated by planar erosive surfaces with slight irregularities. In this facies, alignments of pebbles and larger blocks, as well as grain size evolutions, are recognized. Imbrication structures are locally observed. Clast size distributions and the geometry of the layers indicate flash floods, whereas the fining-upward sequences correspond to a loss of energy of the depositional currents (Wells and Harvey, 1987; Hartley et al., 2005). The presence of coarsening-upward sequences in the distal sectors of the fan corresponds to gravel lobes deposited at the channel toes. The matrix supported conglomerates appear to be debris flow deposits. (b) Sandy facies (Fig. 5C and D). Medium to coarse grain sands laid down in centimetre to metre thick lenticular beds with channelized bases. Channel lag deposits over erosive surfaces, as well as cross-stratification, climbing ripples, horizontal and cross-lamination structures are recognized in these facies. The lenticular geometry of the sandy beds shows multiepisodic filling of channels and indicates the existence of stream flows with sporadic reactivation in their energy. The presence of cross-stratification and cross-lamination allows us to infer the existence of megaripples and ripples migrating in the channels, indicating a lower flow regime (Arzani, 2005). In addition, climbing ripples would indicate that high rates of sedimentation took place during discharge events. Conglomeratic and sandy facies are grouped in fining-upward sequences of c. 1 m thickness. The sequences begin with a flash flood that erodes the substrate and deposits its sedimentary load as the flow energy decreases. In the final stages of these torrential episodes the flow channelizes and allows the sedimentation of sands with the above mentioned bedforms. Stream flows can also occur between two torrential episodes, during low water episodes. In the distal zones of the fan, the stream flows truncate the sedimentary surface and deposit their sedimentary load abruptly, giving rise to deposits with coarsening-upward evolution and fan geometry corresponding to sand and gravel lobes.
Rodríguez-Fenández et al. (1999). The Maimará Formation (Salfity et al., 1984), which is composed of sandstones and mudstones, alternating with conglomerates and volcanic pyroclastic rocks, and contains Miocene-Pliocene micromammal fossils (Marshall et al., 1982), overlies Quaternary alluvial deposits of the Qf2 segment of the Tilcara fan. The strike of the fault plane trends almost North–South and the dip measured in the lower part of the outcrop is 40 degrees to the West, changing to nearly horizontal in the upper part (ramp and flat geometry). According to Salfity et al. (1984), the resulting calculated vertical displacement along the fault is close to 15–20 m. As a consequence, a marked cliff affecting the fan stage Qf2 was generated and a small trench between the fault scarp and the mountain front was developed (Figs. 2–4). On the other hand, a tentative calculation of the horizontal displacement would be close to 40 m. The Huasamayo River flowed along this trench towards the South and the bottom became covered by alluvial deposits corresponding to the Qf3 fan stage. Later, the portion of the Qf2 unit in the footwall was covered by Qf4 sediments. Both Tertiary and Quaternary deposits display brecciation in proximity to the fault plane. An apparent eastward tectonic transport seems evident (Salfity et al.,1984), although Marret et al. (1994) point out the occurrence of fault striae developed in Quaternary alluvial sediments, indicating a right-lateral movement with subhorizontal NE–SW shortening and NW–SE extension for the Quaternary faults. In addition, two sets of normal faults trending 060° and 170°, respectively, and with centimetre displacement have been measured in Quaternary sandy sediments, which are consistent with an ENE–WSW extension in a transtensional tectonic context (Marret et al., 1994). 4.4. Chronological data Four samples for luminescence analysis were collected in adequate sandy sediments (Fig. 5D) of the fan stage Qf2 at the Tilcara site (Fig. 4) after a careful inspection of facies. They are located in both hanging and foot wall sides of the thrust fault and they probably could be related to the same stratigraphic level. Three samples (TIL-2, TIL-3 and TIL-4) have yielded consistent dating (93.8 ± 7.9, 88.0 ± 7.4, 74.8 ± 6.7 ka). However, sample TIL-1 has provided an estimated age of 44.3 ± 6.9 ka, which is inconsistent with the others from a stratigraphic point of view. These data (Table 1) suggest that the sample was either exposed during sampling and shipping or the sediments have been affected by reworking processes after sedimentation. Based on the results of the other three samples, it is possible to assign an age range between 94 and 75 ka, with a mean age of 84.5 ± 7 ka BP, for stage Qf2 of the Tilcara alluvial fan. Unfortunately, after a detailed search of many outcrops for suitable deposits, it was not possible to collect additional samples from younger and older fan segments to draw a more robust chronostratigraphic framework.
4.3. Thrust fault As has previously been indicated, fan stage Qf2 is affected by tectonic deformation. Specifically, near Tilcara a spectacular thrust fault outcrops (Fig. 4). This fault was previously studied by Salfity et al. (1984), Marret et al. (1994), Mena (1997) and
5. Discussion As result of the differential topographic movements of blocks, related to the emplacement of several imbricated thrust
Table 1 OSL chronological data Field code
Laboratory code
Depth (m)
De (Gy)
Dose rate (mGy/a)
Age estimate code
Age (years BP)
TIL-1 TIL-2 TIL-3 TIL-4
K0165 K0166 K0167 K0168
35 35 10 10
139 ± 20 341 ± 22 251 ± 16 251 ± 18
3.14 ± 0.17 3.64 ± 0.19 2.85 ± 0.15 3.35 ± 0.18
ANUOD1599 ANUOD1600 ANUOD1601 ANUOD1602
44,300 ± 6900 93,800 ± 7900 88,000 ± 7400 74,800 ± 6700
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Fig. 6. Schematic diagrams showing different stages of the Tilcara alluvial fan development. (A) Deposition of the Qf2 fan segment, entrenched within Qf1 fan surface, which displays a telescopic pattern. The age of the Qf2 segment is 75–94 ka. (B) Deformation of the Qf2 segment affected by a thrust fault almost North–South in trend. (C) Entrenchment and deposition of the Qf3 fan segment isolating central remains of the Qf2 segment. The Qf3 segment is mainly fed from the East (Huasamayo River), but also from the West (Huichaira River). (D) Entrenchment and deposition of the Qf4 segment. (E) Deposition of the Qf5 segment and the lowest terrace of the Río Grande. (F) Active depositional lobe (Qf6 segment).
sheets during the compressional Andean cycle (Marret et al., 1994; Rodríguez-Fenández et al., 1999), a North–South trending system of trenches and mountain ridges was developed in the Andean Eastern Cordillera area (NW Argentina). The Quebrada de Humahuaca constitutes one of these elongated tectonic depressions. The gradient between watershed and depositional
areas has favoured alluvial fan development under arid conditions. In this context, the Tilcara fan shows a complex segmentated pattern. New geomorphological, sedimentological and chronological data have allowed us to improve the knowledge of the development and evolution of this segmented fan (Fig. 6).
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(a) Alluvial fan evolution. Fan segmentation implies both the channel entrenchment and/or avulsion of the active sedimentation lobe. These processes are controlled by the relationships between rates of uplift, fan sedimentation and channel incision (Bull, 1977). Thus, segmentation models include the external influence of tectonic and/or climatic events, in addition to intrinsic complex response, as factors controlling fan evolution (Cooke et al., 1993). In the case of the Tilcara fan, it is difficult to establish the causes of segmentation. This is very frequent in alluvial fan evolution because the role and relative importance of tectonics and climate are far from being resolved (Ritter et al., 1995). As a first attempt, the regional occurrence of segmented alluvial fans along the Quebrada could indicate an evident climatic influence in fan development. Nevertheless, the location of segments parallel to the mountain front, the high thickness of deposits and deformation structures affecting deposits also suggest a possible tectonic control in the development of Qf1, Qf2 and Qf3 fan segments. On the other hand, the location orthogonal to Quebrada and the terraced arrangement of Qf4 and Qf5 fan stages could indicate a minor influence of tectonism. (b) Alluvial fan chronology and tectonic activity. An age of 75– 94 ka has been obtained from stage Qf2 Tilcara fan sediments using OSL. The ages obtained may be supported by a luminescence age estimate determined by Robinson et al. (2005) (83.1 ± 4.7 ka). However, we do not know the fan segment sampled by these authors. This dating is very useful to constrain chronologically the thrust fault observed in the outcrop produced by the Huasamayo River downcutting, near Tilcara village. This fault is a thrust with tectonic transport to the East (Salfity et al., 1984) and represents a small back-thrust of the andine orogenic wedge. The propagation of this fault indicates that in the recent andine evolution, zones in compression existed locally until the Upper Pleistocene, under generalized strike-slip kinematics (Marret et al., 1994). (c) Palaeoenvironmental conditions. It is difficult to infer the palaeoclimatic conditions during alluvial fan stages based only on the sedimentary record of the alluvial fan. In addition, it is necessary to consider that, rather than climatic characteristics, the bedrock lithology and the drainage basin morphology (particularly the slope) are the factors controlling the kind of alluvial fan processes and sediments (Blair and McPherson, 1994). The short longitudinal layout with high slope and associated sedimentary products (flash flood, debris flows and lobe deposits) displayed by the Tilcara alluvial fan indicate ephemeral stream flows. These features are typical of dry fans (Blair and McPherson, 1994). In addition, other characteristics recording fluvial processes related to alluvial fans with high effectiveness of sediment transport (Colombo, 1989) have been observed. In addition to the difficulty of deducing the palaeoclimatic significance of fan deposits, it is necessary to consider the scarcity of available palaeoenvironmental information from a regional point of view. Other morphosedimentary records indicate the predominance of arid conditions during the period of activity corresponding to the fan stage Qf2 (75–94 ka BP). In the San Juan Province, Siame et al. (1997) deduce an alluvial fan development stage with a minimum age of 100 ± 21 ka associated to the end of the interglacial isotopic stage 5. Sancho et al. (2004) indicate that the accumulation of gypsiferous crust speleothems (90.2–64.3 ka) in the Caverna of Brujas (Province of Mendoza) clearly show arid conditions. On the other hand, Bobst et al. (2001) establish a period of high aridity at 106.1–75.7 ka in the Salar of Atacama. In
addition, the development of the Qf2 fan stage would take place before both the humid Minchin phase of extensive paleolakes, in the Bolivian Altiplano between 73 and 30 ka (Fornari et al., 2001), and the relatively mild and humid conditions of a Last Glaciation Interstadial period, between 60 and 30 ka, which has been identified in several Andean areas (Rabassa and Clapperton, 1990). Under these arid conditions, the Tilcara fan sedimentary dynamics could be controlled by ephemeral stream flows, whose high effectiveness of sediment transport was due to large water discharges (Colombo et al. 2000), and probably related to heavy rainfall associated with El Niño events occurring throughout the Late Pleistocene. At a regional scale in the Andes an evident relationship between historical alluvial fan activity and ENSO has been observed (Vargas et al., 2000; Keefer et al., 2003; Hartley et al., 2005; Colombo, 2005b).
Acknowledgments This contribution has been supported by Ibercaja-Universidad de Zaragoza (2003) and Acción Especial de la Universidad de Zaragoza (2004) grants. The authors thank Prof. Ferran Colombo and Dr. Nemesio Heredia for a review of the first draft, and Norman Hill for laboratory assistance. We are grateful to reviewers Prof. Adrian Harvey and Dr. Jaume Vergés for their helpful comments. References Arzani, N., 2005. The fluvial megafan of Abarkoh Basin (Central Iran): an example of flash-flood sedimentation in arid lands. In: Harvey, A.M., Mather, A.E., Stokes, M. (Eds.), Alluvial Fans: Geomorphology, Sedimentology, Dynamics, Special publications 251. Geological Society, London, pp. 41–59. Azarevich, M., Novara, M., Armella, M., Deus, R., Sánchez, M.C., 1999. Geomorfología de la región de Tilcara, Jujuy. XIV Congreso Geológico Argentino, Actas II, 31–34. Beck, S., Paniagua, N., Yevara, M., 2003. Flora y vegetación: las áreas de Rodero, Tilcara y Volcán. In: Reboratt, C. (Ed.), La Quebrada. Geografía, historia y ecología de la Quebrada de Humahuaca. Editorial La Colmena, Buenos Aires, pp. 47–70. Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes and facies assemblages. Journal of Sedimentary Research A64, 450–489. Bobst, A.L., Lowenstein, T.K., Jordan, T.E., Godfrey, L.V., Ku, T.L., Luo, S., 2001. A 106 ka paleoclimate record from drill core of the Salar de Atacama, northern Chile. Palaeogeography, Palaeoclimatology, Palaeoecology 173, 21–42. Bull, W.B., 1977. The alluvial fan environment. Progress in Physical Geography 1, 222–270. Capitanelli, R.G., 1992. Loa ambientes naturales del territorio argentino. In: Roccatagliata, J.A. (Ed.), La Argentina. Geografía general y los marcos regionales. Editorial Planeta, Buenos Aires, pp. 73–143. Chayle, W., Wayne, W.J., 1995. Impact of erosion, mass wasting, and sedimentation on human activities in the río grande basin, jujuy province, Argentina. Environmental & Engineering Geoscience 1, 403–416. Colombo, F., 1989. Abanicos aluviales. In: Arche, A. (Ed.), Sedimentología. Colección Nuevas Tendencias 1. C.S.I.C., Madrid, pp. 143–218. Colombo, F., 2005a. Abanicos aluviales telescópicos cuaternarios. Andes argentinos. Geogaceta 38, 103–106. Colombo, F., 2005b. Quaternary telescopic-like alluvial fans Andean Ranges, Argentina. In: Harvey, A.M., Mather, A.E., Stokes, M. (Eds.), Alluvial Fans: Geomorphology, Sedimentology, Dynamics, Special publications 251. Geological Society, London, pp. 9–84. Colombo, F., Anselmi, G., Busquets, P., Cardó, R., Ragona, D., Ramos-Guerrero, E., 1996. Aterrazamientos en el río Albarracín, cuenca del río San Juan: génesis y significado geodinámico (Provincia de San Juan, Argentina). Geogaceta 20, 1112–1115. Colombo, F., Busquets, P., Ramos, E., Vergés, J., Ragona, D., 2000. Quaternary alluvial terraces in an active tectonic region: the San Juan River Valley, Andean Ranges, San Juan Province, Argentina. Journal of South American Earth Sciences 13, 611– 626. Cooke, R., Warren, A., Goudie, A., 1993. Desert Geomorphology. UCL Press, London. pp. 526. Fornari, M., Risacher, F., Feraud, G., 2001. Dating of paleolakes in the central Altiplano of Bolivia. Palaeogeography, Palaeoclimatology, Palaeoecology 172, 269–282. Hartley, A.J., Mather, A.E., Jolley, E., Turner, P., 2005. Climatic controls on alluvial-fan activity, Coastal Cordillera northern Chile. In: Harvey, A.M., Mather, A.E., Stokes, M. (Eds.), Alluvial Fans: Geomorphology, Sedimentology, Dynamics, Special publications 251. Geological Society, London, pp. 5–116.
C. Sancho et al. / Journal of South American Earth Sciences 26 (2008) 68–77 Harvey, A., 1997. The role of alluvial fans in arid zone fluvial systems. In: Thomas, D.S.G. (Ed.), Arid Zone Geomorphology. Wiley, Chichester, pp. 231–259. Heredia, N., Rodríguez Fernández, R., Seggiaro, R.E., González, M.A., 1999. Estructuras de inversión tectónica en la Cordillera Oriental de los Andes, entre 23° y 24°, Provincia de Jujuy, NO de Argentina. Acta Geológica Hispánica 32, 93–101. Keefer, D.K., Moseley, M.E., deFrance, S.D., 2003. A 38,000-year record of floods and debris flows in the Ilo region of southern Peru and its relation to El Niño events and great earthquakes. Palaeogeography, Palaeoclimatology, Palaeoecology 194, 41–77. Marret, R.A., Allmendinger, R.W., Alonso, R.N., Drake, R.E., 1994. Late Cenozoic tectonic evolution of the Puna Plateau and adjacent foreland, northwestern Argentine Andes. Journal of South American Earth Sciences 7, 179–207. Marshall, L.G., Butler, R.F., Drake, R.E., Curtis, G.H., 1982. Geochronology of type Uquían (Late Cenozoic) land mammal age, Argentina. Science 216, 986–989. Mena, R., 1997. Estudio cinemático de fallas menores en la Cordillera Oriental (65°300 , 65°150 longitud oeste, 23°230 , 23°400 latitud sur), Provincia de Jujuy, Argentina. VIII Congreso Geológico Chileno, Actas I, 173–177. Murray, A.S., Wintle, A.G., 2000. Luminiscence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurement 32, 57–73. Prescott, J.R., Hutton, J.T., 1994. Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long term time variations. Radiation Measurements 23, 497–500. Rabassa, J., Clapperton, Ch.M., 1990. Quaternary glaciations of the southern Andes. Quaternary Science Reviews 9, 153–174. Ramos, V., 1999. Las provincias geológicas del territorio argentino. In: Caminos, R. (Ed.), Geología argentina. Servicio Geológico y Minero Argentino-Instituto de Geología y Recursos Minerales, Buenos Aires, pp. 41–96. Rhodes, E.J., 1998. Methodological considerations in the optical dating of quartz. Quaternary Science Reviews 7, 395–400. Ritter, J.B., Miller, J.R., Enzel, Y., Wells, S.G., 1995. Reconciling the roles of tectonism and climate in Quaternary alluvial fan evolution. Geology 23, 245–248. Robinson, R.A.J., Spencer, J.Q.G., Strecker, M.R., Richter, A., Alonso, R.N., 2005. Luminescence dating of alluvial fans in intramontane basins of NW Argentina. In:
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Harvey, A.M., Mather, A.E., Stokes, M. (Eds.), Alluvial fans: Geomorphology, Sedimentology, Dynamics, Special publications 251. Geological Society, London, pp. 53–168. Rodríguez-Fenández, L.R., Heredia, N., Seggiaro, R.E., González, M.A., 1999. Estructura andina de la cordillera oriental en el área de la Quebrada de Humahuaca, provincia de Jujuy, NO de Argentina. Trabajos de Geología 21, 321– 332. Salfity, J.A., Brandán, E.M., Monaldi, C.R., Gallardo, E.F., 1984. Tectónica compresiva cuaternaria en la Cordillera Oriental Argentina, Latitud de Tilcara (Jujuy). IX Congreso Geológico Argentino, Actas II, 427–434. Sancho, C., Peña, J.L., Mikkan, R., Osácar, C., Quinif, Y., 2004. Geomorphological evolution of the Brujas cave system (Southern Andean Range, Argentine). Geomorphology 57, 367–384. Siame, L.L., Bourlès, D.L., Sébrier, M., Bellier, O., Castaño, J.C., Araujo, M., Pérez, M., Raisbeck, G.M., Yiou, F., 1997. Cosmogenic dating ranging from 20 to 700 ka of a series of alluvial fan surfaces affected by the El Tigre fault, Argentina. Geology 25, 975–978. Silva, P., Harvey, A., Zazo, C., Goy, J.L., 1992. Geomorphology, Depositional style and Morphometric relationships of Quaternary alluvial fans in the Guadalentin Depresion (Murcia, Southeast Spain). Zeitscrift für Geomorphologie 36, 325– 341. Solís, N., Orozco, O., 1996. Riesgo geológico en la localidad de Tilcara, provincia de Jujuy, República Argentina. XII Congreso Geológico de Bolivia, Memorias I, 927– 934. Turner, J.C.M., 1972. Cordillera Oriental. In: Leanza, A.F. (Ed.), Geología Regional Argentina (II Simposio). Academia Nacional de Ciencias, Córdoba, pp. 117– 142. Vargas, G., Ortlieb, L., Rutllant, J., 2000. Aluviones históricos en Antofagasta y su relación con eventos El Niño/Osilación del sur. Revista Geológica de Chile 27, 157–176. Wells, S.G., Harvey, A.M., 1987. Sedimentologic and geomorphic variations in storm-generated alluvial fans, Howgill Fells, northwest England. Geological Society of America Bulletin 98, 182–198.
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Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames
Geomorphological evolution of the Tilcara alluvial fan (Jujuy Province, NW Argentina): Tectonic implications and palaeoenvironmental considerations Carlos Sancho a,*, José Luis Peña b, Felipe Rivelli c, Ed Rhodes d, Arsenio Muñoz a a
Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain Geografía y Ordenación del Territorio, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain c Facultad de Ciencias Naturales, Universidad Nacional de Salta, Avda. Bolivia 5150, 4400 Salta, Argentina d Research School of Earth Sciences and Research School of Pacific and Asian Studies, The Australian National University, Canberra, ACT 0200, Australia b
a r t i c l e
i n f o
Article history: Received 16 July 2006 Accepted 11 October 2007
Keywords: Tilcara alluvial fan OSL dating Upper Pleistocene Thrust tectonics Arid environments Andean Cordillera
a r t i c l e
i n f o
Article history: Received 16 July 2006 Accepted 11 October 2007
Palabras clave: Abanico aluvial de Tilcara Dataciones OSL Pleistoceno superior Cabalgamientos Ambientes áridos Cordillera de los Andes
a b s t r a c t The development and evolution of the Tilcara alluvial fan, in the Quebrada de Humahuaca (Andean Eastern Cordillera, NW Argentina), has been analysed by using geomorphological mapping techniques, sedimentological characterisation of the deposits and OSL chronological methods. It is a complex segmented alluvial fan made up of five evolutionary stages (units Qf1, Qf2, Qf3, Qf4 and Qf5) developed under arid climatic environments as well as compressive tectonic conditions. Segmentation processes, including aggradation/entrenchment cycles and changes in the location of the depositional lobe, are mainly controlled by climatic and/or tectonic changes as well as channel piracy processes in the drainage system. Alluvial fan deposits include debris flows, sheet flows and braided channel facies associated with high water discharge events in an arid environment. The best mean OSL age estimated for stage Qf2 is 84.5 ± 7 ka BP. In addition, a thrust fault affecting these deposits has been recognized and, as a consequence, the compressive tectonics must date from the Upper Pleistocene in this area of the Andean Eastern Cordillera. Ó 2008 Elsevier Ltd. All rights reserved.
r e s u m e n Se han utilizado técnicas geomorfológicas (cartografía), sedimentológicas (análisis de facies) y cronológicas (dataciones de OSL) para analizar la evolución del abanico aluvial de Tilcara, en la Quebrada de Humahuaca (Cordillera Oriental Andina, NO Argentina). Se trata de un abanico complejo segmentado, en el que se han diferenciado cinco etapas constructivas (Qf1, Qf2, Qf3, Qf4 y Qf5) en un contexto de clima árido y de actividad tectónica compresiva. El encajamiento de las superficies de agradación y las modificaciones en la posición del lóbulo deposicional del abanico están controlados por cambios tectónicos y/o climáticos y procesos de captura en el drenaje del sistema. La etapa Qf2 tiene una edad media ponderada de 84, 5 ± 7 ka BP mediante OSL. Los depósitos incluyen facies de flujos de masas, flujos laminares y canales entrelazados acumulados en eventos de alta descarga hídrica bajo condiciones de aridez. Una falla inversa superpone el Terciario sobre estos depósitos lo que permite prolongar la tectónica compresiva hasta bien entrado el Pleistoceno superior en la Cordillera Oriental Andina. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Quaternary alluvial fans are significant landforms within the Quebrada de Humahuaca landscape (Jujuy Province, NW Argentina) (Fig. 1). Their occurrence is related to the scarps defining the Quebrada and is associated with the sedimentary activity of the Río Grande tributary network under arid environmental condi* Corresponding author. Tel.: +34 976761091; fax: +34 976761106. E-mail address: [email protected] (C. Sancho). 0895-9811/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2008.03.005
tions and in an active compressive tectonic context. Inside the Quebrada de Humahuaca several patterns of alluvial fan evolution (Bull, 1977; Silva et al., 1992; Harvey, 1997; Colombo et al., 2000) can be differentiated based on their morphostratigraphic features. Although the present-day alluvial fan activity, affecting routes, railways and villages, has been analysed by Chayle and Wayne (1995), Solís and Orozco (1996), very few detailed studies concerning geomorphological, sedimentological and chronological aspects of these Quaternary morphosedimentary units have been carried out (Azarevich et al., 1999).
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Fig. 1. Location of the study area and geological setting (after Rodríguez-Fenández et al., 1999).
The Huasamayo River, a tributary of the Río Grande, has built one of the most interesting alluvial fans in the Quebrada de Humahuaca, located at latitude 23–24°S near Tilcara village. Besides the geomorphological and applied interest, this alluvial fan shows spectacular features of compressive tectonic activity during Quaternary times (Salfity et al., 1984; Marret et al., 1994; Rodríguez-Fenández et al., 1999).
The main objectives of this work deal with the study of the geomorphological evolution of the Tilcara fan, the analysis of the sedimentological characteristics of the deposits and the determination of the alluvial fan age by using OSL (Optically Stimulated Luminescence) techniques. In addition, these chronological data are very useful to determine both the palaeoenvironmental and
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palaeoclimatic context of alluvial fan development and the age of the compressional deformation affecting the alluvial fan deposits. In arid regions with active tectonics, dating of alluvial fans is necessary to elucidate the palaeoclimatic meaning of the alluvial fan development and to quantify the tectonic activity (Siame et al, 1997). From a regional point of view, there are interesting contributions on Quaternary alluvial fans conducted by Colombo et al., 1996, 2000; Siame et al. (1997) and Colombo (2005a, 2005b), in different areas of the San Juan Province, and by Colombo (2005a, 2005b), at several locations within the Salta and Mendoza provinces, which may be used for comparison. In addition, Robinson et al. (2005) have applied luminescence techniques to date several deposits within Late Pleistocene alluvial fan sequences, in Jujuy and Salta provinces, reporting an age from the Tilcara alluvial fan.
2. Study area The studied alluvial fan is located near Tilcara village (2459 m a.s.l.) (Figs. 2 and 3). The Tilcara alluvial fan has been built by the Huasamayo River, a tributary of the Río Grande draining the Quebrada de Humahuaca from North to South, within the Bermejo River Basin. The Quebrada is a 150 km long mountain valley, with an asymmetric transverse profile, sited within a multiple basin and mountain ridge system trending from North to South. The highest zone in the area corresponds to the watershed with the Puna area (Nevado de Chañi, 6200 m), while the bottom of the Quebrada ranges in height between 3340 m in the North, near the border with Bolivia, and 1350 m in the South. Quaternary alluvial fans and associated lacustrine deposits related to fan damming processes are very frequent inside the Quebrada. The Arroyo del Medio fan, located near Volcán village (50 km to the south of the Tilcara fan), is probably the most impressive example in the area (Chayle and Wayne, 1995). This circumstance is also ubiquitous in others mountain valleys from the NW Argentina (Colombo, 2005a, 2005b; Robinson et al., 2005). Climate in the Quebrada is mountain arid type with high intensity summer storms, irregularly distributed, with cold dry winters (Capitanelli, 1992). In the central area, mean annual temperature is 12.5 °C, with a maximum of 16.2 °C in December and a minimum of 6.9 °C in July. Both seasonal and daily thermal oscillations are very strong. On the other hand, mean precipitation does not surpass 200 mm inside the Quebrada (mean annual rainfall at Tilcara is 136 mm), increasing towards the North and South borders. The vegetation scarce cover, made up of xerophytes, does not completely cover the ground (Beck et al., 2003), favouring highly effective hydrological processes. From a geological point of view, the Quebrada de Humahuaca is located in the Eastern Cordillera morphotectonic domain of the Andes (Turner et al., 1972; Ramos, 1999), between the Puna and Subandean Ranges domains. At the same time, two main tectonic units can be distinguished in the Eastern Cordillera (Fig. 1) (Rodríguez-Fenández et al., 1999): the Casagrande thrust system in the western part and the Humahuaca thrust system in the East. This last includes the Humahuaca Basin. In this area of the Eastern Cordillera (Fig. 1), the outcropping basement consists of the Upper Precambrian-Lower Cambrian low-grade metamorphic flysch of the Puncoviscana Formation (Turner et al., 1972). These rocks are unconformably overlain by marine clastic materials of the Meson (1000 m in thickness) and Santa Victoria (900 m in thickness) Groups, Cambrian and Ordovician in age, respectively (Ramos, 1999). Overlying Mesozoic is represented by a preorogenic andine sequence, 400 m thick, made up of Upper Cretaceous-Lower Tertiary limestones and sandstones, correspond-
ing to the Salta Group. It has been interpreted as a synrift fill related to the Cretaceous extensional tectonic event (Ramos, 1999). Pliocene-Quaternary sandstones and pyroclastic rocks representing the Maimará Formation (35 m in thickness as a minimum) (Salfity et al., 1984), included in the Humahuaca Group, 300 m thick, represent the top of the regional stratigraphic section and the end of the Tertiary synorogenic sequence. These rocks represent the sedimentary filling of a piggy-back basin (Humahuaca Basin) related to out-of sequence thrusts (Heredia et al., 1999). In addition, Quaternary sediments of alluvial fans are spread out on the bottom of the Quebrada. Geological structure in this sector of the Quebrada de Humahuaca (Ramos, 1999; Rodríguez-Fenández et al., 1999) is characterised by the occurrence of elongated North–South trending slices related to East-verging thrusts (Fig. 1), involving the basement and corresponding to a deformation style of thick-skinned fold and thrust belts. Some of the reverse faults result from inversion of previous Cretaceous extensional faults. These Andean compressional structures were created during a main event of thrust sheet emplacement, between 13 and 1 Ma associated with a WNW–ESE shortening (Marret et al., 1994). Nevertheless, Rodríguez-Fenández et al. (1999) distinguish two phases in the compresional event at 17–11 Ma (Eastern Cordillera emplacement) and 8.5–1.8 Ma (Subandean Ranges emplacement with out-of sequence thrust in Eastern Cordillera). After this stage, strike-slip and extensional faulting occurred during the Quaternary (Marret et al., 1994), still active at the present time, in a transtensional kinematics context with some local compressive events until 1 Ma (Rodríguez-Fenández et al., 1999). 3. Methods Detailed geomorphological mapping of alluvial fan morphosedimentary units (fan stages) in the Tilcara area was obtained from 1:30,000 scale aerial photographs and fieldwork recognition and surveillance. As a consequence, a morphostratigraphic sequence, made up by several stages (Qf1, Qf2, Qf3, Qf4 and Qf5), was established. Topographic and geometric features of alluvial fan segments were controlled using altimeter and GPS measurements. At the same time, observation and description of alluvial fan facies was carried out. In addition, a very detailed search of alluvial fan deposits was made in order to identify sand and silt lens suitable for luminescence-dating techniques. Taking into account both morphostratigraphic characteristics and deformation features observed in the Qf2 segment, four samples for OSL dating determinations were taken using black plastic tubes (20 cm long and 4 cm in diameter) forced into freshly exposed sediment to minimize sample exposure to sunlight. Sample preparation and OSL measurements were carried out at the Luminescence Dating Laboratory of the Research School of Earth Sciences (The Australian National University). Sand-sized quartz was extracted using the methodology described by Rhodes (1988), which includes sieving, concentrated hydrofluoric acid treatment and density separation of heavy minerals using sodium polytungstate solution. OSL dating was based on a single aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000), using a Risø TL-DA-15 automated luminescence reader. Beta and gamma dose rate were estimated using neutron activation analysis (NAA) of sediment U, Th and K content, and cosmic dose rates were calculated using the equations of Prescott and Hutton (1994).
4. Characteristics of the Tilcara alluvial fan 4.1. Geomorphological features The Tilcara alluvial fan area is nearly 5 km2 and the drainage catchment area is almost 120 km2, with a North–South elongated shape. The Huasamayo River constitutes the feeder channel,
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Fig. 2. Geomorphic features of the Tilcara alluvial fan area. (A) Geomorphological mapping (see location in Fig. 1). (B) Schematic cross section showing the differentiated alluvial fan segments. Vertical scale is approximate.
12 km long, which collects the waters from the Chilcahuada, Alfarcito and Casa Colorada rivers, near Alfarcito village. The differ-
ence in height between the watershed of drainage area in the mountains (4615 m in the Ovejería-Suncho Norte hills) and the
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Fig. 3. Aerial oblique photograph of the Tilcara alluvial fan.
fan apex (2500 m) is close to 2000 m. As a consequence, the mountain basin channels feeding the Tilcara fan have a short longitudinal profile with high mean slope (11°). In a preliminary analysis, the Tilcara alluvial fan is characterised by a complex segmented development pattern. According to the relative morphotopographic location of different segment remains, a sequence made up of five evolutionary stages has been recognized (Figs. 2 and 3): (a) The oldest segment (Qf1) has the highest topographic location. A very extensive remnant dipping to the West is well preserved in the northern sector of the fan, resting directly against the eastern mountain front of the Quebrada de Humahuaca (Figs. 2 and 3). (b) Fan surface Qf2 is built beyond the former fan stage (Qf1). At present, there is a segment clearly separated from the mountain front in the southern sector, although other small rem-
nants can be recognized, related to the front scarp in the northernmost sector. Tectonic movements affect this unit giving steepened surfaces (Figs. 2–4). According to its geomorphological location and significance, it is possible to recognize this stage, at a regional scale, inside the Quebrada de Humahuaca, representing the main stage of alluvial fan sedimentation. (c) Alluvial fan stage Qf3 is well represented in the southernmost sector fan occupying a lower fringe between Qf2 segments and the mountain front (Figs. 2 and 3). This surface dips to the South, indicating an important change in the fan drainage from the West to the South. This fact can be explained by the partial blockage of the Huasamayo River by the scarp of the Qf2 segment, developed by thrust fault movement. At the present time, it constitutes a hanging paleovalley. At the same time, this fan surface appears more or less continuously preserved in the eastern side of the
Fig. 4. Thrust fault affecting deposits of the Qf2 stage and location of samples used for luminescence dating (see location in Fig. 2).
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Quebrada. On the other hand, another segment of this surface with opposite slope has been recognized in the Pucará of Tilcara site (Figs. 2 and 3). That points to an origin related to the discharge of the Huichaira River in the West side of the Quebrada. This is also justified by the lithological nature of the deposits, different from those observed in the segments associated with the Huasamayo River, and also by the paleocurrent directions obtained from sedimentary structures (clasts imbrication). The geomorphological location of these segments with opposite origin indicates that the Río Grande was probably dammed at this time. (d) Fan surface Qf4 is entrenched within the previous surfaces. It occupies a highly continuous, narrow trench parallel to the feeder channel, presenting a surface dissected by the Huasamayo River (Figs. 2 and 3). Other Qf4 remnants can be recognized in the northernmost fan sector, along the front mountain. Gemorphological occurrences of this Western trending segment, compared to the distribution of the previous Qf3 stage (trended to the South) is best explained by channel piracy near the fault outcrop. (e) The youngest fan surface (Qf5) corresponds to the active fan segment trending to the northwest. It displays a telescopic pattern in relation to the previous Qf4 surface (Figs. 2 and 3). This Qf5 fan surface can be correlated with the lowest terrace of the Río Grande which forms an abandoned meandering-like channel around the Pucará hill. The Río Grande is
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entrenched within the Qf5 surface, forming a wide braided active channel which may be geomorphologically correlated with the active Qf6 surface. On the Qf5 fan-shaped surface, the Tilcara village has been settled in a dangerous geomorphological position, partially mitigated by the construction of artificial levees. As a consequence, the active depositional lobe (Qf6) is constrained to a narrow sector between Tilcara village and the Pucará hill site (Figs. 2 and 3). The activity of this lobe is ephemeral, with large sediment discharges giving debris and mud flows in response to brief but intense summer storms. The resulting floods often affect village houses, camping sites and sections of route number 9. A record of several recent events of high magnitude has been compiled by Azarevich et al. (1999). The debris and mud flow triggered in 1984 broke the man-made levees, affecting more than 30 neighbouring houses. Other documented events took place during 1998 and 1999. The first one accumulated 3–5 m of sediments near to the Huasamayo-Grande confluence and the transit of vehicles along the route number 9 was temporally interrupted. During summer 1999 three extreme events were recorded. Resulting sediments occupied farming areas and blocked the Rio Grande, flooding the camping site. As a consequence of this recent sedimentary aggradation, the possibility of changes in the position of the active depositional lobe (Qf6), with potential hazardous effects for Tilcara village, is not negligible.
Fig. 5. Different aspects from the Tilcara fan facies. (A) General view of the conglomeratic facies. The clasts are poorly sorted. (B) Detail of the conglomeratic facies. The coarsening-upward evolution of this deposit is related to a lobe deposit. (C) General aspect of the conglomeratic and sandy facies. In the conglomeratic facies, tabular layers and abrupt grain size variations are observed, as well as lenticular geometry with channelized bases in the sandy layers. (D) Detail of the sandy facies showing the multiepisodic filling of a channel and cross-stratification.
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4.2. Characteristics of the deposits and facies analysis The recognition of sediments corresponding to different outcrops from the stages Qf1, Qf2, Qf3 and Qf4, has allowed us to differentiate two main types of facies: (a) Conglomeratic facies (Fig. 5A–C). Clast supported conglomerates, locally matrix supported, are characterised by low roundness, poorly sorted cobblestones and mainly siliceous boulders. The matrix of the conglomerates is sandy and they are only weakly cemented. The conglomeratic facies are arranged in crudely developed horizontal bedding of around one metre thickness, truncated by planar erosive surfaces with slight irregularities. In this facies, alignments of pebbles and larger blocks, as well as grain size evolutions, are recognized. Imbrication structures are locally observed. Clast size distributions and the geometry of the layers indicate flash floods, whereas the fining-upward sequences correspond to a loss of energy of the depositional currents (Wells and Harvey, 1987; Hartley et al., 2005). The presence of coarsening-upward sequences in the distal sectors of the fan corresponds to gravel lobes deposited at the channel toes. The matrix supported conglomerates appear to be debris flow deposits. (b) Sandy facies (Fig. 5C and D). Medium to coarse grain sands laid down in centimetre to metre thick lenticular beds with channelized bases. Channel lag deposits over erosive surfaces, as well as cross-stratification, climbing ripples, horizontal and cross-lamination structures are recognized in these facies. The lenticular geometry of the sandy beds shows multiepisodic filling of channels and indicates the existence of stream flows with sporadic reactivation in their energy. The presence of cross-stratification and cross-lamination allows us to infer the existence of megaripples and ripples migrating in the channels, indicating a lower flow regime (Arzani, 2005). In addition, climbing ripples would indicate that high rates of sedimentation took place during discharge events. Conglomeratic and sandy facies are grouped in fining-upward sequences of c. 1 m thickness. The sequences begin with a flash flood that erodes the substrate and deposits its sedimentary load as the flow energy decreases. In the final stages of these torrential episodes the flow channelizes and allows the sedimentation of sands with the above mentioned bedforms. Stream flows can also occur between two torrential episodes, during low water episodes. In the distal zones of the fan, the stream flows truncate the sedimentary surface and deposit their sedimentary load abruptly, giving rise to deposits with coarsening-upward evolution and fan geometry corresponding to sand and gravel lobes.
Rodríguez-Fenández et al. (1999). The Maimará Formation (Salfity et al., 1984), which is composed of sandstones and mudstones, alternating with conglomerates and volcanic pyroclastic rocks, and contains Miocene-Pliocene micromammal fossils (Marshall et al., 1982), overlies Quaternary alluvial deposits of the Qf2 segment of the Tilcara fan. The strike of the fault plane trends almost North–South and the dip measured in the lower part of the outcrop is 40 degrees to the West, changing to nearly horizontal in the upper part (ramp and flat geometry). According to Salfity et al. (1984), the resulting calculated vertical displacement along the fault is close to 15–20 m. As a consequence, a marked cliff affecting the fan stage Qf2 was generated and a small trench between the fault scarp and the mountain front was developed (Figs. 2–4). On the other hand, a tentative calculation of the horizontal displacement would be close to 40 m. The Huasamayo River flowed along this trench towards the South and the bottom became covered by alluvial deposits corresponding to the Qf3 fan stage. Later, the portion of the Qf2 unit in the footwall was covered by Qf4 sediments. Both Tertiary and Quaternary deposits display brecciation in proximity to the fault plane. An apparent eastward tectonic transport seems evident (Salfity et al.,1984), although Marret et al. (1994) point out the occurrence of fault striae developed in Quaternary alluvial sediments, indicating a right-lateral movement with subhorizontal NE–SW shortening and NW–SE extension for the Quaternary faults. In addition, two sets of normal faults trending 060° and 170°, respectively, and with centimetre displacement have been measured in Quaternary sandy sediments, which are consistent with an ENE–WSW extension in a transtensional tectonic context (Marret et al., 1994). 4.4. Chronological data Four samples for luminescence analysis were collected in adequate sandy sediments (Fig. 5D) of the fan stage Qf2 at the Tilcara site (Fig. 4) after a careful inspection of facies. They are located in both hanging and foot wall sides of the thrust fault and they probably could be related to the same stratigraphic level. Three samples (TIL-2, TIL-3 and TIL-4) have yielded consistent dating (93.8 ± 7.9, 88.0 ± 7.4, 74.8 ± 6.7 ka). However, sample TIL-1 has provided an estimated age of 44.3 ± 6.9 ka, which is inconsistent with the others from a stratigraphic point of view. These data (Table 1) suggest that the sample was either exposed during sampling and shipping or the sediments have been affected by reworking processes after sedimentation. Based on the results of the other three samples, it is possible to assign an age range between 94 and 75 ka, with a mean age of 84.5 ± 7 ka BP, for stage Qf2 of the Tilcara alluvial fan. Unfortunately, after a detailed search of many outcrops for suitable deposits, it was not possible to collect additional samples from younger and older fan segments to draw a more robust chronostratigraphic framework.
4.3. Thrust fault As has previously been indicated, fan stage Qf2 is affected by tectonic deformation. Specifically, near Tilcara a spectacular thrust fault outcrops (Fig. 4). This fault was previously studied by Salfity et al. (1984), Marret et al. (1994), Mena (1997) and
5. Discussion As result of the differential topographic movements of blocks, related to the emplacement of several imbricated thrust
Table 1 OSL chronological data Field code
Laboratory code
Depth (m)
De (Gy)
Dose rate (mGy/a)
Age estimate code
Age (years BP)
TIL-1 TIL-2 TIL-3 TIL-4
K0165 K0166 K0167 K0168
35 35 10 10
139 ± 20 341 ± 22 251 ± 16 251 ± 18
3.14 ± 0.17 3.64 ± 0.19 2.85 ± 0.15 3.35 ± 0.18
ANUOD1599 ANUOD1600 ANUOD1601 ANUOD1602
44,300 ± 6900 93,800 ± 7900 88,000 ± 7400 74,800 ± 6700
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Fig. 6. Schematic diagrams showing different stages of the Tilcara alluvial fan development. (A) Deposition of the Qf2 fan segment, entrenched within Qf1 fan surface, which displays a telescopic pattern. The age of the Qf2 segment is 75–94 ka. (B) Deformation of the Qf2 segment affected by a thrust fault almost North–South in trend. (C) Entrenchment and deposition of the Qf3 fan segment isolating central remains of the Qf2 segment. The Qf3 segment is mainly fed from the East (Huasamayo River), but also from the West (Huichaira River). (D) Entrenchment and deposition of the Qf4 segment. (E) Deposition of the Qf5 segment and the lowest terrace of the Río Grande. (F) Active depositional lobe (Qf6 segment).
sheets during the compressional Andean cycle (Marret et al., 1994; Rodríguez-Fenández et al., 1999), a North–South trending system of trenches and mountain ridges was developed in the Andean Eastern Cordillera area (NW Argentina). The Quebrada de Humahuaca constitutes one of these elongated tectonic depressions. The gradient between watershed and depositional
areas has favoured alluvial fan development under arid conditions. In this context, the Tilcara fan shows a complex segmentated pattern. New geomorphological, sedimentological and chronological data have allowed us to improve the knowledge of the development and evolution of this segmented fan (Fig. 6).
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(a) Alluvial fan evolution. Fan segmentation implies both the channel entrenchment and/or avulsion of the active sedimentation lobe. These processes are controlled by the relationships between rates of uplift, fan sedimentation and channel incision (Bull, 1977). Thus, segmentation models include the external influence of tectonic and/or climatic events, in addition to intrinsic complex response, as factors controlling fan evolution (Cooke et al., 1993). In the case of the Tilcara fan, it is difficult to establish the causes of segmentation. This is very frequent in alluvial fan evolution because the role and relative importance of tectonics and climate are far from being resolved (Ritter et al., 1995). As a first attempt, the regional occurrence of segmented alluvial fans along the Quebrada could indicate an evident climatic influence in fan development. Nevertheless, the location of segments parallel to the mountain front, the high thickness of deposits and deformation structures affecting deposits also suggest a possible tectonic control in the development of Qf1, Qf2 and Qf3 fan segments. On the other hand, the location orthogonal to Quebrada and the terraced arrangement of Qf4 and Qf5 fan stages could indicate a minor influence of tectonism. (b) Alluvial fan chronology and tectonic activity. An age of 75– 94 ka has been obtained from stage Qf2 Tilcara fan sediments using OSL. The ages obtained may be supported by a luminescence age estimate determined by Robinson et al. (2005) (83.1 ± 4.7 ka). However, we do not know the fan segment sampled by these authors. This dating is very useful to constrain chronologically the thrust fault observed in the outcrop produced by the Huasamayo River downcutting, near Tilcara village. This fault is a thrust with tectonic transport to the East (Salfity et al., 1984) and represents a small back-thrust of the andine orogenic wedge. The propagation of this fault indicates that in the recent andine evolution, zones in compression existed locally until the Upper Pleistocene, under generalized strike-slip kinematics (Marret et al., 1994). (c) Palaeoenvironmental conditions. It is difficult to infer the palaeoclimatic conditions during alluvial fan stages based only on the sedimentary record of the alluvial fan. In addition, it is necessary to consider that, rather than climatic characteristics, the bedrock lithology and the drainage basin morphology (particularly the slope) are the factors controlling the kind of alluvial fan processes and sediments (Blair and McPherson, 1994). The short longitudinal layout with high slope and associated sedimentary products (flash flood, debris flows and lobe deposits) displayed by the Tilcara alluvial fan indicate ephemeral stream flows. These features are typical of dry fans (Blair and McPherson, 1994). In addition, other characteristics recording fluvial processes related to alluvial fans with high effectiveness of sediment transport (Colombo, 1989) have been observed. In addition to the difficulty of deducing the palaeoclimatic significance of fan deposits, it is necessary to consider the scarcity of available palaeoenvironmental information from a regional point of view. Other morphosedimentary records indicate the predominance of arid conditions during the period of activity corresponding to the fan stage Qf2 (75–94 ka BP). In the San Juan Province, Siame et al. (1997) deduce an alluvial fan development stage with a minimum age of 100 ± 21 ka associated to the end of the interglacial isotopic stage 5. Sancho et al. (2004) indicate that the accumulation of gypsiferous crust speleothems (90.2–64.3 ka) in the Caverna of Brujas (Province of Mendoza) clearly show arid conditions. On the other hand, Bobst et al. (2001) establish a period of high aridity at 106.1–75.7 ka in the Salar of Atacama. In
addition, the development of the Qf2 fan stage would take place before both the humid Minchin phase of extensive paleolakes, in the Bolivian Altiplano between 73 and 30 ka (Fornari et al., 2001), and the relatively mild and humid conditions of a Last Glaciation Interstadial period, between 60 and 30 ka, which has been identified in several Andean areas (Rabassa and Clapperton, 1990). Under these arid conditions, the Tilcara fan sedimentary dynamics could be controlled by ephemeral stream flows, whose high effectiveness of sediment transport was due to large water discharges (Colombo et al. 2000), and probably related to heavy rainfall associated with El Niño events occurring throughout the Late Pleistocene. At a regional scale in the Andes an evident relationship between historical alluvial fan activity and ENSO has been observed (Vargas et al., 2000; Keefer et al., 2003; Hartley et al., 2005; Colombo, 2005b).
Acknowledgments This contribution has been supported by Ibercaja-Universidad de Zaragoza (2003) and Acción Especial de la Universidad de Zaragoza (2004) grants. The authors thank Prof. Ferran Colombo and Dr. Nemesio Heredia for a review of the first draft, and Norman Hill for laboratory assistance. We are grateful to reviewers Prof. Adrian Harvey and Dr. Jaume Vergés for their helpful comments. References Arzani, N., 2005. The fluvial megafan of Abarkoh Basin (Central Iran): an example of flash-flood sedimentation in arid lands. In: Harvey, A.M., Mather, A.E., Stokes, M. (Eds.), Alluvial Fans: Geomorphology, Sedimentology, Dynamics, Special publications 251. Geological Society, London, pp. 41–59. Azarevich, M., Novara, M., Armella, M., Deus, R., Sánchez, M.C., 1999. Geomorfología de la región de Tilcara, Jujuy. XIV Congreso Geológico Argentino, Actas II, 31–34. Beck, S., Paniagua, N., Yevara, M., 2003. Flora y vegetación: las áreas de Rodero, Tilcara y Volcán. In: Reboratt, C. (Ed.), La Quebrada. Geografía, historia y ecología de la Quebrada de Humahuaca. Editorial La Colmena, Buenos Aires, pp. 47–70. Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes and facies assemblages. Journal of Sedimentary Research A64, 450–489. Bobst, A.L., Lowenstein, T.K., Jordan, T.E., Godfrey, L.V., Ku, T.L., Luo, S., 2001. A 106 ka paleoclimate record from drill core of the Salar de Atacama, northern Chile. Palaeogeography, Palaeoclimatology, Palaeoecology 173, 21–42. Bull, W.B., 1977. The alluvial fan environment. Progress in Physical Geography 1, 222–270. Capitanelli, R.G., 1992. Loa ambientes naturales del territorio argentino. In: Roccatagliata, J.A. (Ed.), La Argentina. Geografía general y los marcos regionales. Editorial Planeta, Buenos Aires, pp. 73–143. Chayle, W., Wayne, W.J., 1995. Impact of erosion, mass wasting, and sedimentation on human activities in the río grande basin, jujuy province, Argentina. Environmental & Engineering Geoscience 1, 403–416. Colombo, F., 1989. Abanicos aluviales. In: Arche, A. (Ed.), Sedimentología. Colección Nuevas Tendencias 1. C.S.I.C., Madrid, pp. 143–218. Colombo, F., 2005a. Abanicos aluviales telescópicos cuaternarios. Andes argentinos. Geogaceta 38, 103–106. Colombo, F., 2005b. Quaternary telescopic-like alluvial fans Andean Ranges, Argentina. In: Harvey, A.M., Mather, A.E., Stokes, M. (Eds.), Alluvial Fans: Geomorphology, Sedimentology, Dynamics, Special publications 251. Geological Society, London, pp. 9–84. Colombo, F., Anselmi, G., Busquets, P., Cardó, R., Ragona, D., Ramos-Guerrero, E., 1996. Aterrazamientos en el río Albarracín, cuenca del río San Juan: génesis y significado geodinámico (Provincia de San Juan, Argentina). Geogaceta 20, 1112–1115. Colombo, F., Busquets, P., Ramos, E., Vergés, J., Ragona, D., 2000. Quaternary alluvial terraces in an active tectonic region: the San Juan River Valley, Andean Ranges, San Juan Province, Argentina. Journal of South American Earth Sciences 13, 611– 626. Cooke, R., Warren, A., Goudie, A., 1993. Desert Geomorphology. UCL Press, London. pp. 526. Fornari, M., Risacher, F., Feraud, G., 2001. Dating of paleolakes in the central Altiplano of Bolivia. Palaeogeography, Palaeoclimatology, Palaeoecology 172, 269–282. Hartley, A.J., Mather, A.E., Jolley, E., Turner, P., 2005. Climatic controls on alluvial-fan activity, Coastal Cordillera northern Chile. In: Harvey, A.M., Mather, A.E., Stokes, M. (Eds.), Alluvial Fans: Geomorphology, Sedimentology, Dynamics, Special publications 251. Geological Society, London, pp. 5–116.
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