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

Abstract

Sedimentologic data and palynological ages from Paleogene clastic deposits of the northern and central Altiplano plateau suggest foreland basin development in the central Andes by mid-Paleocene time. The nonmarine Potoco Formation (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 geochronologic data for the Potoco Formation include late Eocene to Oligocene palynomorph assemblages recovered throughout lower to upper stratigraphic levels. These ages and published 40 Ar/ 39 Ar and K-Ar ages from overlying upper Oligocene‐lower Miocene volcaniclastic rocks indicate (1) nondeposition or 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-accumulation 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 20‐100-mthick interval of superimposed paleosols in the basal Potoco supports reduced sediment accumulation and low rates of subsidence during mid-Paleocene to middle Eocene time. The overlying main body of the Potoco contains facies assemblages and stratigraphic architecture (dominantly nonero