Special Research Center 267
DEFORMATION PROCESSES IN THE ANDES
Freie Universitaet Berlin · Technische Universitaet Berlin · GeoForschungsZentrum Potsdam · Universitaet Potsdam
Sedimentation, tectonics and volcanism in the Salar de Antofalla area, southern Puna (NW Argentina) - Project D1B
Thrust tectonic controls on late Tertiary sedimentation pattern in the Salar de Antofalla area,
southern Puna (NW Argentina)
Memorias del I Congreso Latinoamericano de Sedimentologia, Soc. Venezolana de Geol. Tomo 1, 7 - 13, Noviembre 1997
Since Miocene times the area of the Salar de Antofalla, located east of the present Andean volcanic arc, was an isolated area of continental sedimentation. Compressional tectonics between early Miocene and Pliocene times produced a series of westvergent folds and thrusts. In particular, faulting during the mid-Miocene (Quechua phase) led to a narrow, elongated broken-foreland basin (Salar de Antofalla basin). The development of frontal thrust ramps caused topographic highs along the eastern edge of this sedimentation area and controlled the facies distribution pattern. Coarse-grained alluvial fans were deposited at the basin margin during the initial stages of the basin infill and graded basinwards into sandstones and mudstones of a playa mudflat and sandflat facies. This alluvial fan sedimentation was followed, during a period of tectonic quiescence, by the sedimentation of lacustrine carbonates and sulfates along the border of the fan bodies. Contemporarily, massive halite deposits developed in the center of the partially flooded basin. Finally, the Pliocene Diaguita phase triggered compressional deformation together with another period of alluvial fan sedimentation. These deposits overlay the older, deformed late Tertiary sediments with an angular unconformity.
The Salar de Antofalla area is part of the southern Altiplano-Puna plateau which is located in northwestern Argentina and southwestern Bolivia. Especially the Puna is subdivided into numerous endorheic basins and mountain ranges. The most significant morphological features in the study area are the Salar de Antofalla, the Salina del Fraile and the Salar de Incahuasi (Fig. 1).
Previous studies on the geology of the Salar de Antofalla area of northwestern Argentina largely concentrated on the structural inventory (Palma & Vujovich, 1987; Allmendinger et al., 1989; Marrett et al., 1994) or the volcanism (Coira & Pezzutti, 1976; Coira et al., 1993). The sedimentary record including early Paleozoic, Permian, Jurassic and Tertiary is documented only peripherically (Palma & Irigoyen,1987; Jordan & Alonso, 1987; Palma,1990; Singer et al., 1994, Voss et al., 1996). First detailed studies on the Cenozoic stratigraphy will be presented by Görler et al. (in prep.). Investigations concerning the basin evolution during the late Cenozoic are missing so far.
My study presents a comprehensive view of the basin evolution and its sedimentary strata since the mid-Miocene. The facies development of the Salar de Antofalla basin, and the Miocene-Pliocene compressional basins of the Southern Puna in general, reflects the structural development of the latest stage of Andean orogeny.
Geological setting and stratigraphy
The Puna of northwestern Argentina, together with the Bolivian Altiplano, forms an extended high plateau (Fig. 1A). Located in the central Andes between 15° and 28°S lat., the plateau has an average elevation of over 3 500 m above sea-level (Isacks, 1988) and coincides with an area of increased crustal thickness of about 65 km (Götze et al., 1994). The study area covers the 30°-east-dipping segment of the subducted Nazca plate (Barazangi & Isacks, 1976).
The Andean basins, and related volcanic belts of the Salar de Antofalla area formed on a continental crust composed of a Precambrian to early Paleozoic high-grade-metamorphic basement, early Paleozoic sedimentary and volcanic rocks (Palma & Irigoyen,1987) as well as Permotriassic and Jurassic sediments (Voss et al.,1996) (Fig. 1 B). Andean sedimentary evolution started during the early Tertiary, presumably during the Eocene. Initial deposits consist of reddish detrital sediments of a fluvial-deltaic environment. According to lithostratigraphic analyses the unit can be compared with the Lower Tertiary Geste Formation of the Pastos Grandes area (Turner, 1964). After Sempere (1995) it developed in a broad foreland basin situated east of the late Cretaceous-early Tertiary magmatic arc, in part located in the present Chilean Precordillera (Scheuber et al., 1995). The succession is overlain by sediments of the Oligocene to Lower Miocene Quinuas unit (Erpenstein et al., 1995; Görler et al., in prep.), which were deposited on a broad low elevation alluvial plain (Jordan & Alonso, 1987). A tectonic and sedimentary upheaval occured during this period, involving a rapid change from a foreland to an intramontane basin setting. Due to increased tectonic activity during the Pehuenche (18 - 22 Ma), the Quechua (14 - 10 Ma), and the Diaguita phase (4 - 2 Ma) the Andes grew as a mountain belt and the study area became structurally isolated. Intramontaneous sedimentation was restricted to a variety of narrow, mostly N-S trending compressional basins from early Miocene times on. In the Salar de Antofalla basin itself, sedimentation continued with the Potero Grande (Lower to Middle Miocene)(Erpenstein et al., 1995) and Juncalito units (Upper Miocene to Pliocene). Their sediments were deposited in an arid to semiarid environment and disconformably cover the older strata (Görler et al., in prep.). The abundance of lava flows, ignimbrites and intercalated tuff layers shows a high volcanic activity since the mid-Miocene (Görler et al., in prep.). Volcanic material derived from the adjacent volcanoes or the Andean volcanic arc located in the adjacent Western Cordillera.
Facies associations during the Mid-Miocene to Pliocene
Sedimentation of the Mid-Miocene to Pliocene Juncalito unit was concentrated on an elongated basin exposed directly east of the present Salar de Antofalla. Along most of the eastern edge of this basin, the mid-Miocene Quechua deformation produced a series of westvergent reverse faults. The early Paleozoic basement was deformed and uplifted together with older Tertiary strata. These rocks, exposed in the Sierra Calalaste (Fig. 1B, 2), formed the erosional source from which detritus was shed into the adjacent basin. Its western margin is not exposed. Analyses in the area between the present Salar de Antofalla and the Salina del Fraile (Fig. 1B) indicate that this part was uplifted. However, significant mid-Miocene-Pliocene displacement is missing. The existing thrust systems, shown in Fig. 1C, developed earlier during the early Miocene tectonic event. The western part was presumably a tectonically passive, low relief upland, from which no significant amounts of detritus were delivered.
The mid-Miocene to Pliocene age of the Juncalito unit is indicated by intercalated tuff layers. Tuff beds near the base of the exposed section yielded a biotite K-Ar age of 11,2 ± 0,3 Ma. Higher in the section other tuffs yielded biotite K-Ar ages between 9,3 ± 0,2 Ma and 3,2 ± 0,1 Ma, respectively. The deformed sediments of the Juncalito unit in the south of the study area are disconformably overlain by ignimbrites. Their oldest known age is dated with 3,6 ± 0,1 Ma (data from Görler et al., in prep.).
Several sediment dispersal patterns have been recognized in the study area. Two major facies associations are defined: (a) alluvial fan deposits and (b) playa sediments.
Alluvial fan deposits
The remnants of the late Tertiary alluvial fan sediments are found along the eastern margin of the accumulation area. The fan bodies formed directly adjacent to the topographic high induced by the contemporary thrust belt (Fig. 2). They merge to form wedge-shape bajada deposits with a lateral extension of 2 to 5 km. The deposits are mainly composed of conglomerates and subordinated conglomeratic sandstones, reaching thicknesses up to several hundred metres. They generally contain boulders (ranging from 0.2 to 1 m in diameter), which originate from adjacent early Paleozoic and Paleogene strata.
Several mechanisms of sediment transport, that depend on the water supply, were observed. The sheet-like shape and the minimal evidence of cut banks and bar formations suggest that the dominant mode of deposition of the conglomerates and gravelly sand and siltstones was from debris or mud sheet flows which spread over large areas of the fan surface. They are matrix supported, poorly sorted and display an unordered fabric and lack of internal bedding structures. Some of the conglomerates, displaying flat erosional bases and normal graded bedding, are considered as low-viscosity debris flow deposits (after Steel & Gloppen, 1980). Subordinate coarse-grained units, which are composed of horizontally stratified conglomerates, are interbedded with cross-bedded pebbly sand and siltstones. The conglomerates, which display imbricated gravel and low-angle cross-stratifications, are interpreted as longitudinal channel fill, gravel bar deposits on modern proximal fan zones (after Collinson & Thompson, 1982).
The playa facies comprises an assemblage of several lithofacies. Three lithofacies, each reflecting different conditions within the playa environment, are recognized: mud and sandflat deposits, saltpan deposits and lacustrine carbonates and sulfates.
Playa mud and sandflat deposits. Most typical features are red mudstones/siltstones which are commonly massive and more rarely display mm-scale parallel laminae. Desiccation cracks and interbedded gypsum/anhydrite and halite layers are frequently observed. Subordinated are coarse-grained sheet sandstones showing horizontal to low-angle cross stratification and a sharp erosional base contact. The sheet geometry, large lateral extent, and relatively uniform thickness suggests that the silt and sandstones represent distal sheetflows in a playa mud and sandflat environment. The intercalated evaporites indicate that parts were formed as ephemeral efflorescent crusts on saline mudflats.
Saltpan deposits. This facies is characterized by an abundance of saline minerals. The deposits are mainly cyclic and consist of alternating millimeter- to decimetre-scale layers of halite and mud. They represent saline lakes producing successive layers of salt by repeated flooding and subsequent reprecipitation. In the central part of the basin the salt pan deposits can reach a thickness of 50 m. The maximum spatial distribution was achieved during periods of tectonic stability due to a decreasing clastic input.
Lacustrine carbonates and sulfates. This facies is dominated by gypsum/anhydrite horizons, grey marls and white limestones which contain predominantly stromatolites and oolites. Mud and siltstones and fine- to coarse-grained, gravelly sandstones as well as several up to 10 m thick volcanoclastic layers are interbedded. Horizons of symmetrical wave-ripples are common in both mudstones and siltstones. Bioturbation is common, consisting of vertical burrows and grazing traces on bedding planes. The lacustrine facies interdigitates with the alluvial fans, the saltpan and the mudflat deposits. Its spatial distribution is predominantly influenced by the location of the deposited alluvial fan systems. Mainly the carbonates and sulfates hem around their front. In areas with low clastic input, they additionally occur in the wedge between alluvial fans directly at marginal parts.
Structural control on sedimentary evolution
In the eastern part of the Salar de Antofalla (Fig. 2) the late Tertiary tectonic activity culminated during the mid-Miocene Quechua phase. Compressional events produced a series of westvergent folds and thrusts stretching along the entire length of the present Salar de Antofalla. Thrust-related frontal ramps produced topographic highs in the Sierra Calalaste area and led to the development of a narrow, highly elongated area of sedimentation stretching parallel to the mountain chain. The western basin margin was tectonically passive and formed a flat upland, from which no significant amounts of detritus were delivered. Due to the basin asymmetry the facies distribution was wedge-shaped and unidirectional.
According to Jordan (1995) this sedimentary basin can be described as a broken-foreland basin. Its development is based on the differentiation of the broad Eocene-Oligocene (retroarc) foreland basin. Due to basement-cored uplifts, numerous narrow basins with internal drainage appeared in the Southern Puna.
The facies distribution patterns of the Juncalito unit was controlled as follows (Fig. 3): Thrust-tectonic induced sedimentation started with coarse alluvial fan sediments, deposited directly at the eastern basin margin. This first conglomerate unit unconformably overlies the previously deformed older strata. At the contact with the thrust, they display an offlap-onlap disposition, as in the model proposed by Riba (1976), with a fan-like geometry open towards the west. The alluvial fan conglomerates pass basinward into playa mud and sandflat deposits. Ceased tectonic activity between 10 and 4 Ma terminated the coarse-grained sedimentation. Lacustrine carbonates and sulfates developed next to the border of the fan bodies. This calcareous lacustrine environment is thought to be linked to a stage of relative tectonic quiescence. In the center of the partially flooded basin gypsum and up to 50 m thick halites deposited simultaneously. After this period, renewed shortening (Diaguita phase) with folding and thrusting of the entire succession resumed. This tectonic event triggered another basinward progradation of alluvial fan bodies. Their conglomerates and gravelly sandstones cover the older playa and lacustrine sediments. A widespread exposed angular unconformity is significant. Continuation of the compressive tectonic activity initiated the final deformation, followed by the development of the modern alluvial fan complexes and the present salt flats.
(1) The Juncalito unit in the Salar de Antofalla area represents the syntectonic infill of the Salar de Antofalla basin which originated during the mid-Miocene. Sedimentation patterns were significantly affected by the position of the frontal ramps induced by compressive tectonic activity. Two phases of syndepositional deformation, concentrated on the eastern margin of the basin, were coeval with major alluvial fan sedimentation directly at the basin margin. An interval of tectonic quiescence between 10 and 4 Ma interrupted the alluvial fan sedimentation. Due to the decreased clastic input, this period is marked by deposition of lacustrine carbonates and sulfates.
(2) The basin evolution in the Salar de Antofalla area in late Tertiary times can be described by a compressional environment, an intramontane setting and a thrust tectonic-induced sedimentation. Due to its asymmetric shape, the facies distribution is wedge-shaped and unidirectional. Further features are the narrow and highly elongated shape. According to basin models (e.g. Jordan, 1995) the basin can be classified as a broken-foreland basin.
(A) Location of the Salar de Antofalla in the Central Andes. Shaded areas represent the Altiplano-Puna plateau.
(B) Generalized geological map of the Salar de Antofalla area showing distribution of the Pre-Tertiary rocks, the Tertiary strata and volcanics as well as the modern salt flats and the alluvium.
(C) Strike-normal section through the central part of the study area. Location is given in (B)
Facies distribution of the basal part of the Miocene-Pliocene Juncalito unit, deposited in the eastern part of the Salar de Antofalla basin. The western part is not exposed. The source area included early Palaeozoic low-grade metamorphic rocks and Eocene to Early Miocene sediments, which were effected by the westvergent thrust-and-fold tectonics.
Tectonosedimentary chart of the Mid-Miocene to Pliocene Juncalito unit in the eastern part of the Salar de Antofalla basin during the uppermost Cenozoic.
This study was supported by the German Research Foundation (DFG) through the Special Research Program 267 - Deformation Processes in the Andes. The help and cooperation of Prof K. Görler and Dr E. Kiefer is gratefully acknowledged. I would like to thank Dr D. Mertmann, K. Fiedler and M.R. Haschke for valuable discussions
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