218-10 A Multi-Phase Record of Basin Restriction and Oxygenation during the Breakup of Southern Gondwana: Sedimentology, Chemostratigraphy, and Geochronology of the Early Cretaceous Rocas Verdes Basin

Abstract:

Paleogeographic reconstructions of the south Atlantic opening during Gondwana breakup remain poorly resolved, limiting understanding of Early Cretaceous Southern Hemisphere hydrographic connections. The Rocas Verdes backarc basin (RVB), at the southern tip of South America, opened during the latest Jurassic–Early Cretaceous and occupied a critical paleogeographic position between the Pacific and developing Atlantic oceans, yet the timing and extent of marine exchange between them remain uncertain. At the Tyndall Glacier locality in Torres del Paine National Park, a ~1 km mudstone succession of the Zapata Formation preserves a record of basin evolution between 150 and 110 Ma and contains numerous volcanic ash horizons providing chronostratigraphic control. Whole-rock major- and trace-element geochemistry, integrated with sedimentologic observations and a Bayesian stratigraphic age model constrained by 11 zircon CA-TIMS and LA-ICPMS U-Pb ash-bed ages, records a multi-phase history of redox evolution, basin connectivity, and sediment provenance. We evaluate Mn enrichment as a proxy for marine connectivity because exchange with oxygenated marine waters promotes delivery and preservation of particulate Mn(III/IV) oxides, whereas persistent restriction and anoxic bottom waters favor reduction to soluble Mn(II), resulting in Mn depletion. RVB Mn chemostratigraphy records episodic marine exchange throughout basin evolution. At the base of the section, organic-rich carbonaceous shale with elevated TOC and depleted Mn records an initially restricted, anoxic basin, after which a transition to calcareous shale accompanied by increased Mn enrichment marks the onset of exchange with oxygenated marine waters. The transition from calcareous to overlying siliceous mudstone reflects basin deepening below the carbonate compensation depth, increased terrigenous dilution, or both, while persistent Mn enrichment indicates continued marine exchange. In the middle Zapata Formation, depleted Mn coincides with rhythmically interbedded siliceous-calcareous slump and debris-flow deposits interpreted as submarine canyon fill, recording renewed restriction and anoxic bottom waters. This restricted interval and an overlying disconformity may reflect tectonically driven basin reorganization, including basin narrowing or sill formation during incipient compression and backarc closure. Above the disconformity, siliceous debris-flow deposits coincide with renewed Mn enrichment, documenting reestablished marine exchange before grading upward into siliceous mudstone with declining Mn enrichment, consistent with progressively reduced interaction with the open ocean. This study illustrates how combining chemostratigraphy, sedimentology, and high-precision geochronology can identify transient marine connections in backarc basin successions that are otherwise cryptic.

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