SEDIMENTOLOGY AND TIMING OF INITIAL BASIN DEVELOPMENT IN THE NORTHERN ALTIPLANO RECORDED BY THE SYNOROGENIC PEÑAS AND ARANJUEZ FORMATIONS, CENTRAL ANDES, BOLIVIA

The Cordillera Real forms the boundary between the Altiplano basin and the westernmost Eastern Cordillera of the central Andes in northern Bolivia. This mountain range defines a region where Cenozoic upper-crustal shortening of Paleozoic metasedimentary rocks was accommodated principally by northeast-dipping thrust faults of the Huarina backthrust belt. These thrusts were active during the late Oligocene to early Miocene, although the exact timing of initial deformation is poorly constrained. Sedimentation in the adjacent Altiplano basin was the product of denudation in the Cordillera Real and is expressed in the alluvial-fan and fluvial deposits of the Peñas and Aranjuez formations. Paleocurrent measurements (cross-bedding and trough axes) indicate sediment transport to the west-southwest, consistent with an eastern source in the growing Huarina backthrust belt. The provenance of sandstones and conglomerate clasts also suggests an eastern sediment source based on the presence of eroded Paleozoic metasedimentary detritus. Growth strata in these formations suggest that deposition was contemporaneous with thrusting on the Huarina backthrust belt and uplift of the Cordillera Real. Preliminary 40Ar/39Ar analyses of interbedded tuffs near the base of the Peñas and Aranjuez formations suggest a minimum age of 30.1 ± 1.2 Ma and 33.13 ± 0.80 Ma, respectively, for initial basin development. These dates are older than previously dated tuffs in the eastern Altiplano, indicating an earlier onset of basin development in the region. These dates are also consistent with the cooling history of the Cordillera Real, which suggests a period of rapid cooling from 45-40 Ma likely caused by initial deformation in the Huarina backthrust belt.