SEDIMENTOLOGY AND PROVENANCE OF THE SYNOROGENIC PEÑAS AND ARANJUEZ FORMATIONS, CENTRAL ANDES, BOLIVIA: IMPLICATIONS FOR THE TIMING OF EASTERN CORDILLERA DENUDATION AND INITIAL BASIN DEVELOPMENT IN THE NORTHERN ALTIPLANO

The middle Tertiary Peñas and Aranjuez formations are exposed in northwest-trending outcrop belts located in the northeastern Altiplano of Bolivia. Sedimentological data and lithofacies analysis suggest that these coarse-grained siliciclastic units were deposited in alluvial-fan axial braided-fluvial environments, with facies associations representing distal to proximal streamflow-dominated alluvial fan, mass-flow-dominated alluvial fan, and braided-fluvial channel to overbank depositional settings. An upsection change in facies from primarily fine-grained sandstone to cobble conglomerate suggests progradation of the alluvial fan system due to increased proximity of the sediment source with time. Paleocurrent analysis indicates that flow in the alluvial fan facies associations was predominantly west-southwest-directed. Based on provenance data from conglomerate clast compositions and sandstone petrofacies suggesting derivation from recycled quartz-rich metasedimentary and sedimentary rocks, the sediment source for the Peñas and Aranjuez formations was likely dominated by Paleozoic strata of the Eastern Cordillera.

The increased proximity of the sediment source, paleoflow directions, and Paleozoic sediment provenance suggest that deposition of the Peٌñas and Aranjuez formations occurred in prograding alluvial-fan systems related to the uplift of the Eastern Cordillera through forward propagation of the southwest-directed Huarina fold-thrust belt. Upsection provenance trends indicate a progressively greater contribution from younger Paleozoic strata, possibly due to activation of new thrust faults during advance of the Huarina fold-thrust belt toward the Altiplano. Growth strata observed in the Aranjuez Formation indicate that contractional deformation was syndepositional. The ages of interbedded ash-fall tuff beds in the Aranjuez Formation further suggest that denudation of the Eastern Cordillera had commenced by late Eocene to early Oligocene time, consistent with thermochronologic studies that indicate a period of rapid cooling possibly related to tectonically induced exhumation around 45-40 Ma. This age is earlier than previous studies suggesting a late Oligocene initiation of Eastern Cordillera uplift and west-directed sedimentation in the Altiplano.