Early-Middle Tertiary Deposition in the Corque Syncline, Altiplano Plateau, Bolivia

• Main Author: Hampton, Brian A
• Other Authors: Brian Horton
• Format: Others
• Language: en
• Published: LSU 2002
• Subjects: Geology and Geophysics
• Online Access: http://etd.lsu.edu/docs/available/etd-0612102-151846/

Tertiary age rocks are exposed along north-south trending structures throughout the hinterland Altiplano plateau, central Andes. The east limb of the Corque syncline (SW Bolivia) contains the thickest and most continuous successions of late EoceneOligocene age non-marine strata (Potoco Formation) on the Altiplano. The Potoco is up to ~6500 m thick and has continuous exposure >10³ km² making it the thickest and most extensive remnant of the mid-Tertiary Altiplano basin. Basin fill during late EoceneOligocene time remains the most rapid and sustained period of deposition since Andean orogenesis, recording a long-term sediment accumulation rate of ~0.5 mm/my. The Potoco consists of fine-grained sandstone, mudstone, and gypsum deposits and lithofacies indicate deposition during periodic flooding in broad, poorly confined channels, and floodplain and playa lake environments. Deposits coarsen upward and channel deposits are thicker, less extensive, and more lenticular upsection. The thickness, extent, and ephemeral depositional style exhibited by the Potoco are attributed to rapid sediment accumulation facilitated by sheetflow-dominated rivers in a region undergoing rapid subsidence and arid climate conditions. Provenance data from the succession, including paleocurrent indicators and sandstone composition, document a paleoflow reversal accompanied by a distinct variation in composition. The basal ~4200 meters coarsens up and was deposited by east-directed rivers containing lithic sandstone, shistose, gneissic, granitic, and volcanic fragments. The overlying succession coarsens up and contains lithic sandstone, shistose, and volcanic fragments deposited by west-directed rivers. Provenance trends indicate two source areas west and east of the basin during late EoceneOligocene time. Thrust faults, active west and east of the basin during mid-Tertiary time, substantiate applying flexural modeling to determine if crustal shortening can account for basin development. Models incorporating effective elastic thickness west (20 km) and east (13.5 km) of the deepest portion of the basin and basin depth (~4200 meters east-directed; ~3000 meters west-directed) and length (120 km east-directed; 75 km west-directed) reveal that subsidence from shortening, while non-unique, is one viable mechanism for basin formation. Potoco sedimentology, stratigraphy, provenance trends, and model results provide a testable tectonic model to explain evolution of the Altiplano Basin during mid-Tertiary orogenesis.