INSIGHTS INTO THE EVOLUTION OF THE CENOZOIC CENTRAL ANDES FROM EOCENE WEDGE-TOP STRATA IN SOUTHERN PERU

Deposition in the wedge-top depozone of a retroarc foreland basin system occurs between the mountain belt and flexural foredeep of a Cordilleran orogen, preserving tectonic processes active during the development of high topography. Preservation potential of such strata within the fold-thrust belt is rare; when identified it provides an opportunity to study the conditions influencing basin formation at the foreland-hinterland transition. We investigate the Eocene Anta Formation in southern Peru (14-16°S) through stratigraphic measurement and lithofacies characterization, compositional analysis, and detrital zircon U-Pb laser ablation ICP-MS geochronology. Three exposures of the Anta Formation were investigated: the type section in the SW Anta province, strata near Pomacanchi, Peru, and a newly discovered exposure near Coyabamba, Peru. Although these three exposures were previously mapped as the same formation, they show distinct differences. The Anta type section is 1200 m thick with a basal MDA of ~34 Ma and consists of volcaniclastic sandstones and conglomerates. The Anta Formation at Pomacanchi is ~2200 m thick with a MDA of ~32 Ma and is characterized by a mix of siliciclastic and volcaniclastic sandstones, conglomerates, and lacustrine limestones. The Anta Formation at Coyabamba is ~2500 m thick with a basal MDA of ~43 Ma and is overwhelmingly pebble to boulder conglomerates with subordinate lenticular sandstones; this latter section yields a sedimentation rate of ~170 m/Myr. The type section and Pomacanchi have dominantly unimodal age distributions consistent with Eocene point sourcing of the Andean volcanic arc, whereas the Anta at Coyabamba yields multimodal age distributions consistent with sediment sourced from recycled Paleozoic and Mesozoic strata. These three progressions record distinct processes within the fold-thrust belt with varying degrees of influence within the foreland-hinterland transition. Whereas the Anta type section and Pomacanchi may have been incorporated into the hinterland as nascent piggyback basins, Coyabamba records proximal deposition consistent with shortening of a coupled plate margin experiencing thrust loading and concomitant rapid flexural subsidence. Results highlight the complex nature of wedge-top deposition in a retroarc foreland basin system.