Andean Exhumation and Growth of the Subandean-Chaco Foreland Basin, Southern Bolivia: Spatial-Temporal Variations and Implications for Hydrocarbon Exploration

Calle, Amanda Z.; Horton, Brian K.

Evolution of the Neogene Subandean fold-thrust belt since its inception along the eastern margin of the Central Andes of Bolivia has dictated hydrocarbon generation, migration, timing and entrapment in the Subandes and adjacent Chaco foreland basin. Few studies have assessed the linkages among surface processes and threshold responses to climate change, tectonics, and/or catchment-area modification in this thin-skinned fold-thrust belt and coupled foreland basin. Analyses of stratigraphic variations and sediment provenance within two of the proximal, westernmost sections of the Subandean belt (Bartolo and Emborozu) are consistent with an eastward advancing synorogenic fold-thrust system inducing crustal thickening, flexural loading and subsidence in the nearby foreland basin. Results from new Cenozoic detrital zircon U-Pb geochronology, conglomerate clast compositions, sandstone petrography and paleocurrent analyses provide information on the gradual eastward diachroneity of upward-coarsening fluvial megafan deposits. New and published zircon U-Pb data for interbedded Miocene tuffs and an updated magnetostratigraphic study contribute to the interpretation of several 2-D seismic lines and balanced cross sections. Regionally, the Neogene accumulation history shows a gradually subsiding basin until 8 Ma and increasingly coarser facies up to 6 Ma. High accumulation rates after 6 Ma are interpreted for proximal alluvial-fan facies and their distal counterpart facies within the Chaco basin. Evidence for the introduction of progressively younger rocks in the sediment source areas, as documented by conglomerate counts and detrital zircon age populations, lead us to interpret (1) protracted unroofing and sediment dispersal from the Eastern Cordillera-Interandean Zone commencing at ~20-15 Ma (as estimated from new tuff ages and accumulation rates) followed by (2) initial propagation of the deformation front into the Subandean Zone during the late Miocene. A temporal shift from axial to transverse fluvial drainage recorded by paleocurrent data reinforces evidence for an eastward advancing depocenter and structural partitioning of the older segments of the Subandean-Chaco foreland basin, particularly from 8 Ma to present. An integrated, chronostratigraphic assessment of the Oligocene-Quaternary succession emphasizing spatial and thickness variations in key reservoir properties helps minimize petroleum exploration uncertainties and introduces new play concepts.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013