Abstract

The Chaco foreland basin marks the eastern edge of the Andean orogenic system and contains up to 7.5 km of mostly nonmarine siliciclastics, thus providing an opportunity to study the detrital record of late Cenozoic Andean geodynamics and paleogeography. Sandstones from the southern Bolivian Chaco foreland basin testify to variable contributions from the craton and the Andean foreland fold-and-thrust belt in the context of an eastward-advancing orogen. Sandstone and conglomerate petrography of the Oligocene to Recent strata exposed in the Chaco Basin show an overall trend towards reduced mineralogical maturity, corresponding to diminishing transport distance, increasing topographic gradient and changing depositional environment from arid continental plain (Oligocene to middle Miocene Petaca sandstone) through marginal marine (middle Miocene to late Miocene Yecua sandstone) and braided fluvial settings (late Miocene Tariquia sandstone) to semihumid alluvial-fan environments (late Miocene to Recent Guandacay and Emborozú formations).

Paleocurrent patterns of pre-tectonic sediments record westward transport. The late Miocene transitional-marine Yecua Formation sediments show a complex shoreline-related pattern. Syntectonic strata (Tariquia, Guandacay, and Emborozú formations) filling the foreland basin indicate dominant east-directed sediment transport from the advancing Andean orographic front. Because the Eastern Cordillera provided a drainage barrier to Altiplano erosion, sandstones of the Chaco Basin do not directly record Andean volcanism or Altiplano uplift; however, climatic inferences from changing facies and petrographic composition suggest major uplift and formation of orographic barriers between 14 and 7 Ma.

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