LINKING THE SEDIMENTOLOGY AND GEOCHEMISTRY OF THE EARLY CRETACEOUS ROCAS VERDES BASIN TO THE OPENING OF THE SOUTH ATLANTIC OCEAN, INSIGHTS FROM THE PATAGONIA ANDES

The Rocas Verdes Basin in the Southern Patagonian Andes (48°-57°S) was a subduction- related backarc basin that underwent extension during the Late Jurassic until the Early Cretaceous, associated with the breakup of Gondwana. This period of extension is represented in the rock record by the Zapata Formation, which archives the stratigraphic evolution throughout the opening and closure of the Rocas Verdes ocean basin. Subsequent coarse clastic deposition coincides with a transition from basin extension to basin compression, with the compressional regime ultimately inverting the basin and incorporating the Zapata Formation black shales into the Southern Patagonian fold-and-thrust belt. Prior to inversion, the Rocas Verdes Basin experienced episodes of dysoxia to anoxia associated with elevated organic carbon burial, which we hypothesize were linked to oxygenation and deoxygenation patterns in the incipient South Atlantic Ocean. However, the timing and duration of this organic carbon burial remain unresolved in the Rocas Verdes Basin. In this study, we are investigating the Zapata Formation outcrop by integrating sedimentology, geochronology, and geochemistry to constrain periods of organic carbon deposition and heightened dysoxia-anoxia. Measurements of total organic carbon content in collected mudstone samples are used to identify organic carbon-rich successions in the sedimentary record, while trace element redox proxies help identify successions deposited under oxygen-limited bottom water environments. Radiometric U-Pb zircon dating of intercalated, arc-related ash layers provides new age constraints on these successions. Intervals with elevated organic carbon burial show limited evidence for bioturbation and coincide with elevated concentrations of redox-sensitive trace metals, which we interpret as periods of dysoxic-anoxic bottom-water conditions. The first of these occurs in the basal member of the basin from 150 to 144 Ma and is correlative with stratigraphic successions elsewhere in the basin. Further investigation into the timing, duration, and magnitude of organic carbon burial will yield insights into the role that the developing Rocas Verdes ocean basin played in the Cretaceous global carbon cycle, and into the linkages between the Rocas Verdes ocean basin and the burgeoning South Atlantic Ocean.