Eocene exhumation and basin development in the Puna of northwestern Argentina

Abstract

The Puna is part of the larger Puna‐Altiplano Plateau (also known as the Central Andean Plateau), characterized by high elevation, low relief, and aridity, located in the central Andes of Bolivia and Argentina. Tertiary sedimentary rocks preserved within the Puna contain a unique archive of information regarding the paleogeography, depositional environments, and timing of sediment source exhumation during the early stages of Andean mountain building. The Eocene Geste Formation in the Salar de Pastos Grandes area (within the central Puna of northwestern Argentina) consists of deposits that are the result of confined to unconfined flows in a sandy to gravelly, braided fluvial system and alluvial fans proximal to the source terrane. Paleocurrent data document an overall eastward flow direction. Up‐section coarsening of the Geste Formation suggests that topographic relief in the source area increased through time, possibly owing to enhanced tectonic activity and source terrane unroofing. Sandstone petrography and conglomerate clast‐count data document quartzose and phyllitic compositions typical of Ordovician rocks preserved just west of the Salar de Pastos Grandes area. Paleocene‐Eocene detrital apatite fission track age populations (P1: ∼35–52 Ma; P2: ∼52–65 Ma) of the Geste Formation and their consistent trends up‐section suggest moderate to rapid (∼0.4 mm/a to >1 mm/a) exhumation of western sediment sources during the early to mid‐Tertiary stages of Andean mountain building. Sedimentation rates increase up‐section from ∼0.1 mm/a to 1 mm/a. Our data, when combined with other structural, stratigraphic and seismic evidence from surrounding regions, suggest that the Geste Formation was deposited in response to crustal shortening and resulting erosion and sedimentation, which started as early as Cretaceous in the Chilean Cordillera de Domeyko and in the Salar de Pastos Grandes area by Eocene time. The Geste Formation could be interpreted either as a local wedge‐top accumulation on the eastward propagating central Andean orogenic wedge, or as a local intermontane basin. The similarities between wedge‐top deposits preserved in Bolivia and Eocene deposits in northwestern Argentina, south of ∼25°S, lead us to favor the wedge‐top scenario for the Geste Formation. If correct, this implies that the deformation front of the Andean orogenic wedge incorporated both thin‐ and thick‐skinned structures as it migrated, possibly unsteadily, from the Cordillera de Domeyko during the Cretaceous‐Paleocene to areas within the Puna and Eastern Cordillera by mid‐late Eocene time. Contemporaneously, a regional‐scale foreland basin system developed over an along‐strike distance of at least 650 km.