Integrated diagenetic and sequence stratigraphy of a late Oligocene–early Miocene, mixed-sediment platform (Austral Basin, southern Patagonia): resolving base-level and paleoceanographic changes, and paleoaquifer characteristics

Abstract

A condensed (~ 20-m-thick) marine transgressive-highstand succession comprises the upper San Julián Formation (upper Oligocene–lower Miocene) of the northern retroarc Austral Basin, southern Patagonia. Mixed-sediment facies identify a shelf-interior setting, part of an overall warm-temperate regional platform of moderate energy. Giant oyster-dominated skeletal–hiatal accumulations along the maximum flooding surface and forming high-energy event beds in the highstand succession preserve relict micrite in protected shelter porosity, and identify periods of reduced sediment accumulation. The stratigraphic distribution of marine-derived glaucony and diagenetic carbonates is spatially related to sequence development. Depositional siderite coincides with prominent marine transgression, defining transient mixing of marine and meteoric waters across coastal-plain deposits. Chemically evolved autochthonous glaucony coincides with periods of extended seafloor exposure and transgressions that bracket the marine succession, and within the oyster-dominated skeletal accumulations. Seafloor cement, likely once magnesian calcite, formed in association with an encrusting/boring biota along the maximum flooding surface in concert with incursion of cool (11–13 °C) water. The cement is present locally in skeletal event beds in the highstand succession suggesting a possible association with high-order base-level change and cooler water. As the highstand succession coincides with elevated global sea level in the late Oligocene–early Miocene, the locally marine-cemented glauconitic skeletal event beds in the highstand succession may identify higher order glacio-eustatic control. Local stratal condensation, however, is best explained by regional differences in basement subsidence. In the burial realm, carbonate diagenesis produced layers of phreatic calcrete coincident with skeletal-rich deposits. Zeolite (clinoptilolite-K) cement is restricted to the lowermost marine transgressive interval probably due to initial elevated metastability of reworked weathered silicates. Clay (illite)-cement is restricted to siliciclastic-rich intervals wherein skeletal carbonate did not buffer pore-water pH. Diagenetic carbonate geochemistry (Sr, Na, and δ18O and δ13C) shows that, with burial, the transgressive and highstand system tracts developed as distinct paleoaquifers resulting from different proximities to meteoric recharge zones.