Paleogene synorogenic sedimentation in the Altiplano plateau and implications for initial mountain building in the central Andes

Sedimentologic data and palynological ages from Paleogene elastic deposits of the northern and central Altiplano plateau suggest foreland basin develo pment in the central Andes by mid-Paleocene time. The nonmarine Potoco Form ation (3000-6500 m thick) constitutes the majority of Cenozoic basin fill. The Potoco overlies the Santa Lucia Formation (50-300 m thick), previously dated as mid-Paleocene by mammal fossils and magnetostratigraphy. New geoch ronologic data for the Potoco Formation include late Eocene to Oligocene pa lynomorph assemblages recovered throughout lower to upper stratigraphic lev els. These ages and published Ar-40/Ar-39 and K-Ar ages from overlying uppe r Oligocene-lower Miocene volcaniclastic rocks indicate (1) nondeposition o r greatly reduced deposition (average sediment-accumulation rates < 10 m/m. y.) from mid-Paleocene to middle Eocene time (top Santa Lucia to lowermost Potoco Formation), followed by (2) rapid deposition (average sediment-accum ulation rates up to 500 m/m.y.) throughout late Eocene and Oligocene time ( majority of Potoco Formation). Lithofacies and paleocurrent data confirm both depositional phases. A 20-10 0-m-thick interval of superimposed paleosols in the basal Potoco supports r educed sediment accumulation and low rates of subsidence during mid-Paleoce ne to middle Eocene time. The overlying main body of the Potoco contains fa cies assemblages and stratigraphic architecture (dominantly nonerosive fluv ial sheet sandstone interbedded with overbank mudstone) consistent with an aggradational fluvial system in a rapidly subsiding late Eocene-Oligocene e nvironment. An upsection reversal in paleocurrents across the Santa Lucia-P otoco contact, from west-directed to cast-directed patterns, also indicates a major change in depositional systems. The switch from low to high rates of subsidence and reversal in paleocurren ts can be explained by eastward migration of a foreland basin system. Basin migration may have induced a change from a mid-Paleocene-middle Eocene, br oad-wavelength, low-amplitude forebulge to a late Eocene-Oligocene, rapidly subsiding foredeep, Advance of such a forebulge-foredeep pair would requir e a topographic load to the west that propagated eastward through time. Alt hough synchronous loading east of the Altiplano is plausible, paleocurrent data suggest a greater influence of sediment dispersal systems derived from a belt to the west. In general, the Paleogene sedimentary history of the A ltiplano is compatible with shortening and crustal thickening in an eastwar d-propagating contractional belt to the west in the modern are and forearc regions of westernmost Bolivia and Chile.