BASIN-WIDE CHANGES REFLECTED BY LINKED GEOCHRONOLOGY AND GEOCHEMISTRY OF TWO STRATIGRAPHIC SECTIONS WITHIN THE EARLY CRETACEOUS ROCAS VERDES BASIN, PATAGONIA

Understanding the causal mechanisms for physical and environmental change in sedimentary basins is necessary to link the sedimentary record to local versus global events in earth’s past. Moreover, constraining the timing of these changes in sedimentary records further aids our understanding of how a basin evolves and why. This investigation focuses on the late Jurassic to early Cretaceous Rocas Verdes Basin (RVB) in the southernmost Andes, where previous studies and field observations have documented substantial changes in depositional environments. In this study, we aim to better reconstruct depositional conditions of the RVB by evaluating new geochemical and geochronological data from mudstones of the Zapata Formation in southern Chile. New results are correlated with previously characterized age-equivalent stratigraphy of the Rio Mayer Formation further North in Argentina.

Sampled sections of the Zapata Formation were selected based on its inferred paleogeographic position, which represents a deeper and less restricted portion of the basin relative to the Rio Mayer Formation. We conducted whole-rock geochemical analyses of 30 samples from the Zapata Formation. We then correlated lithological and geochemical records of the Zapata and Rio Mayer Formations using measured stratigraphic sections, chemostratigraphy, and geochronology. Sections in both study areas show a relatively abrupt increase in Ca wt %, reflecting CaCO3 deposition. This transition is accompanied by the introduction of abundant bioturbation and a lithologic shift from black carbonaceous shale to gray calcareous mudstone indicating a correlatable basin-wide change. This stratigraphic transition also shows a similar increasing trend in Mn concentration and a decrease in concentrations of redox sensitive metals, U and Cr. While the absolute concentrations differ between the Zapata and Rio Mayer formations, they follow similar trends suggesting basin-driven changes with subtle differences in chemical depositional environments. Significant depositional changes in both records supports the hypothesis of an early Cretaceous ocean oxygenation of the RVB. Future work will introduce new zircon U-Pb age data from ash beds within the Zapata Formation and TOC to further constrain timing and assess accuracy of this correlation.