WEDGE-TOP STRATIGRAPHY OF SOUTHERN PERU: KEY TO UNDERSTANDING FORELAND BASIN EVOLUTION OF THE CENOZOIC CENTRAL ANDES

Although the individual depozones that compose a complete Cenozoic foreland basin system in southern Peru (13–14°S) have been identified, only the foredeep, forebulge, and back-bulge have all been mapped, measured, and dated with modern geochronological methods. Remaining is the wedge-top depozone, the commonly conglomeratic section of a foreland basin system which deposits within the frontal part of the orogenic wedge that is rarely preserved due to the destructive nature of the propagating fold-and-thrust belt and progressive migration of the foreland basin system. The Anta Formation offers a glimpse into the depositional systems of the Cenozoic Andean wedge-top: it has been previously documented to extend ~400 km northwest-southeast along orogenic strike, although measurable exposures are rare. We performed detailed measurement, compositional analysis, and detrital zircon U-Pb geochronology of the Anta Formation, as well as an examination of its provenance signature and sediment source regions using Monte Carlo-based mixture modeling. Results show the Anta Formation is a >2 km thick package of non-marine siliciclastic strata comprised of medium to coarse grained sandstones and thick intervals of pebble and cobble-boulder conglomerate. Preliminary geochronology suggests the Anta Formation was deposited in the late Eocene, coeval with the previously interpreted foredeep deposits of the adjacent Soncco Formation to the east. Like the Soncco Formation, the Anta is distinctive in that sediment provenance analysis suggests dominant Ordovician sediment source, whereas most foreland basin deposits in the region follow a more classic unroofing sequence through Cretaceous to Jurassic quartzite sediment sources. We anticipate further investigation into the Anta Formation in the context of Cenozoic foreland basin system evolution in the region will yield critical insights into the geodynamic processes active during the Cenozoic construction of the Andean Plateau we see today.