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
Basin development in the Southern Puna:
sedimentary record from the Salar de Antofalla area, NW Argentina
Dirk Adelmann and Konrad Görler
Memorias del X Congreso Latinoamericano de Geología, Buenos Aires, Noviembre 1998
The area of the Salar de Antofalla in NW Argentina forms the southernmost part of the Altiplano-Puna plateau. The salar is one of the most significant endorheic basins of this part of the high plateau. Its Tertiary strata preserve the sedimentological record of the change from a coherent foreland basin to a broken foreland basin.
After a period of exhumation in Cretaceous and Early Tertiary times sedimentation started during the Late Eocene with nonmarine clastic sediments of the Quiñoas unit. Playa mud and sandflat as well as fluvial deposits represented by an up to 750 m thick sequence of reddish pelites and sandstones (Quiñoas subunits I & II) were deposited widespread across the Southern Puna. The detritus presumably derived from the west from the erosional remnants of the Incaic mountain belt located in the Chilean Precordillera (Jordan & Alonso 1987). Along the basinward flank this orogenic belt was probably formed during the Incaic shortening phase around 38 Ma. The relatively monotonous sedimentation implies the persistence of this basin configuration for at least 10 Ma suggesting that no significant cratonward migration of the deformation occurred during this period.
A fundamental change in basin geometry took place in late Oligocene time when large amounts of coarse clastics (Quiñoas subunit III) were shed into the basin. This unit is composed of conglomerates and medium- to coarse-grained sandstones of a proximal fan environment overlain by a significant horizon of aeolian sandstones. In the central part of the study area it reaches a thickness of 350 m. Paleocurrent and petrographical data indicate that the coarse-grained deposition is accompanied by reverse faulting and bivergent thrusting whereby basement rocks were uplifted and eroded. This thick-skinned tectonism caused the segmentation of the former coherent foreland basin and led to the broken foreland development in the Southern Puna.
During the Miocene (Potrero Grande unit) ongoing tectonism and uplift caused further segmentation and broken-foreland basin development. Facies distribution, paleocurrent and petrographical data point to the existence of a significant depocenter bounded by an eastvergent reverse fault system in the west and a westvergent reverse fault system in the east. Syntectonic sedimentation is evidenced by basal unconformities related to the installation of fault systems. Partially, at the fault contact conglomerates display an offlap-onlap disposition with a fan-like geometry. Sedimentation of the Mid-Miocene to recent Juncalito unit was concentrated on an elongated basin like the basin today. Facies distribution patterns were significantly controlled by reverse and thrust faulting in the adjacent Sierra de Calalaste in the east. As a consequence the basin shape is asymmetric and the facies distribution is unidirectional and wedge-shaped. Proximal parts are characterized by alluvial fan sedimentation reaching a maximum during periods of higher tectonic activity. More distal parts are characterized by playa mudflat and saltpan deposition (Adelmann 1997). Continuation of compressive tectonic activity initiated final deformation followed by the development of the modern alluvial fans and salt flats.
Adelmann, D., 1997. Thrust tectonic controls on late Tertiary sedimentation pattern in the Salar de Antofalla area, southern Puna (NW Argentina). Memorias I Congreso Latinoamericano de Sedimentología, 1: 7-13.Jordan, T.E. and R.N. Alonso, 1987. Cenozoic stratigraphy and basin tectonics of the Andes mountains, 20°-28° south latitude. American Association of Petroleum Geologist, Bulletin 71: 49-64.
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