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

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The Salar de Antofalla area 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 sedimentary 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 and fluvial sediments represented by a 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 fold-and-thrust belt was formed during the Incaic shortening phase around 38 Ma. Subsidence probably resulted from the flexure of the craton by thrust loading. The relatively monotonous sedimentation implies the persistence of this basin configuration for at least 10 Ma suggesting that no significant cratonward migration of the thrust load occurred during this period.

A fundamental change in basin geometry took place in late Oligocene time when large amounts of conglomerates of a proximal fan environment (Quiñoas subunit III) were shed into the basin.  Paleocurrent and petrographical data indicate that deposition is accompanied by reverse faulting whereby basement rocks were uplifted and eroded. This tectonism caused the segmentation of the former coherent foreland basin and initiated the broken foreland development.
During the Miocene (Potrero Grande unit) ongoing tectonism and uplift led to further segmentation. Facies distribution, paleocurrent and petrographical data point to the existence of a 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.

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 faulting in the adjacent Sierra de Calalaste in the east. The basin shape is asymmetric and the facies distribution is unidirectional and wedge-shaped (Adelmann, 1997). Proximal parts are characterized by alluvial fan sedimentation reaching a maximum during periods of higher tectonic activity.  Continuation of compressive tectonism initiated final deformation followed by the development of the modern alluvial fans and salt flats.

References
Adelmann, D. (1997): Thrust tectonic controls on late Tertiary sedimentation pattern in the Salar de Antofalla area, southern Puna (NW Argentina). - Mem. I Congr.Lat.Sed., Soc.Ven.Geol., 1: 7 - 13.
Jordan, T.E. & Alonso, R.N. (1987): Cenozoic stratigraphy and basin tectonics of the Andes mountains, 20°-28° south latitude. - Bull.AAPG, 71 (1): 49-64.

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Salar de Antofalla