Mesozoic Sedimentology, Provenance, and Subcrop Mapping of the Eastern Cordillera and Western Subandean Zone: Insights for the Extensional Record of Southern Bolivia

Abstract

An integral geologic assessment of the eastern margin of the Eastern Cordillera and western Subandean Zone in southern Bolivia unravels a partitioned pattern of Triassic – Cretaceous extensional basin evolution of the distal segment of the proto-Pacific convergent margin. Pilot studies along two regional transects (19.5, 21.5°S) targeting remnant Permian-Cretaceous successions elucidate the dynamics of pre-Andean structures and sediment dispersal patterns, and serves as an analogue for extensional basins postdated by sag basins, and foreland basins. New sedimentologic and provenance studies coupled to novel geochronology and subcrop mapping shed light on the >1.2 km-thick Triassic – Cretaceous chronostratigraphy, depositional systems, paleocurrents, U-Pb detrital geochronology, sandstone and conglomerate compositions, and structure. Mixed siliciclastic and carbonate rocks of the Triassic Serere, and Jurassic-Cretaceous? Tacuru Groups include deposition on: (1) alluvial, fluvio-lacustrine and eolian systems with basalt flows, followed by (2) restricted, potentially tidally-influenced coastal plain and fluvio-eolian systems, and (3) extensively fluvial and tidally-influenced fluvial and eolian systems. The Cretaceous Puca Group characterizes for deposition on ephemeral basin-center and lake margin deltaic systems, and widespread fluvial systems. Triassic-Jurassic rocks at both transects depict variable paleoflows, from ESE-directed to N, S, and W-directed paleocurrents. Conversely, a W-SW-directed paleocurrent reversal is documented in the late Cretaceous. Paleocurrent reorganizations are coupled to clastics input and sandstone modal variations from litharenites to subarkoses. Although the distribution of U-Pb ages suggests protracted recycling of Pampean-Brazilian grains (500-700 Ma), scattered input of Famatinian-Ocloyic (400-500 Ma) and Choiyoi-Mitu (200-300 Ma) grains constrain local middle-late Devonian sources, and syndepositional igneous sources, respectively. Contrasting regional stacking patterns, sediment routing systems, igneous ages, and growth stratal relationships define a time-transgressive, partitioned extensional basin, with Triassic and Cretaceous extension located in the Eastern Cordillera, and Jurassic extension focused in the Subandean Zone. Significantly, determining the Mesozoic sedimentation and tectonic dynamics will improve evaluations of exploratory plays in the Subandean Zone and Chaco basins.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016