Upper oligocene conglomerates of the Altiplano, Central Andes: The record of deposition and deformation along the margin of a Hinterland basin

The Altiplano Plateau is a high-elevation, internally drained basin located in the hinterland of the Central Andean fold-thrust belt of Bolivia. Cenozoic strata exposed along the margins of the basin provide a unique record of deposition and deformation in this region and can also be applied to understanding synorogenic sedimentation in other fold-thrust belts. We examined Oligocene conglomerate units deposited along the margin of the nascent Altiplano in an effort to better understand how this large hinterland basin has evolved, and examine the relationship between upper crustal deformation and sedimentation in the interior of the Central Andean fold-thrust belt. Facies associations indicate initial deposition occurred in alluvial-fan and braidplain settings, and growth strata in these units record syndeformational deposition. Facies associations in overlying units contain a greater proportion of fine-grained material and were deposited in isolated fluvial channels surrounded by well drained-floodplain deposits. These units are less deformed relative to the alluvial-fan and braidplain deposits, suggesting that deformation waned as deposition continued. Clast counts and paleocurrents indicate that much of the Oligocene sediment was locally derived from Paleozoic strata and deposited in semi-isolated basins located between major thrusts and folds. Uranium-lead ages of detrital zircons from the conglomerate beds correspond to those from surrounding Paleozoic strata, supporting the hypothesis that most of sediment in the Oligocene conglomerate beds were derived directly from these older units. The youngest population of detrital zircon ages in the conglomerate beds are identical to the inferred ages of the deposits themselves. Collectively, the data indicate that the eastern margin of the Altiplano during Oligocene time was dominated by active upper crustal deformation, with alluvial-fan and braidplain deposition occurring in topographic lows. As deformation waned, deposition shifted from alluvial fans to isolated fluvial channels surrounded by extensive floodplains. In the regional setting, Upper Oligocene sediments exposed along the eastern margin of the Altiplano represent the remnants of a feeder zone to large fluvial distributary systems that occupied and infilled the center of the Altiplano.