Brian K. Horton¹, Brian A. Hampton¹, Peter Copeland²
(1) Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA
(2) Department of Geosciences, University of Houston, Houston, TX

Abstract: Revised age of Tertiary foreland basin deposits in the Bolivian Altiplano plateau and implications for subsidence history

New palynological and ⁴⁰Ar/³⁹Ar age data require major revision of the existing Cenozoic stratigraphy for the Bolivian Altiplano plateau. Nonmarine strata of the Potoco Formation and equivalent units (2000-6000 m thick) comprise the majority of Cenozoic basin fill in the Altiplano. Accurate age data for these deposits is essential for analyses of the burial/thermal history of the Altiplano and reconstructions of Andean orogenesis and basin evolution. The Potoco overlies the mid Paleocene Santa Lucia Formation (50-200 m thick, age defined by mammal fossils and magnetostratigraphy), although some workers define uppermost Santa Lucia strata as mid-late Paleocene Cayara Formation. The Potoco underlies early Miocene volcanic deposits (500-4000 m thick) dated by K-Ar and ⁴⁰Ar/³⁹Ar methods. Thus the Potoco Formation must be post-mid-late Paleocene and pre-early Miocene in age. Our new age data includes late Eocene-Oligocene pollen assemblages sampled from the lowermost Potoco and an ⁴⁰Ar/³⁹Ar age of 25.4 ± 0.1 Ma on an uppermost Potoco tuff. These data require (1) a period of non-deposition or very limited deposition from late Paleocene (top Santa Lucia) to late Eocene (lowermost Potoco) time, roughly 20 m.y., and (2) extremely rapid average sediment accumulation rates (up to 400 m/m.y.) during the Oligocene. Low rates of Eocene sediment accumulation are consistent with a 20-100-m-thick section of stacked paleosols in the basal Potoco, possibly indicative of stratigraphic condensation due to forebulge migration through the Altiplano. Eastward migration of the foreland basin system potentially induced a rapidly subsiding foredeep in the Altiplano during Oligocene time.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana