Paleogeography of Miocene Western Amazonia: Isotopic composition of molluscan shells constrains the influence of marine incursions

摘要: Strontium, oxygen, and carbon isotope compositions of well-preserved mollusks (bivalves) indicate a dominantly freshwater depositional setting for the lower Miocene–upper Miocene Pebas Formation in Western Amazonia. Molluscan 87Sr/86Sr ratios identify different sources. Andean runoff was dominant water source Amazonia, though there occasional influx waters from cratonic catchments. At only one stratigraphic level, signals increased (mesohaline) aquatic salinities, concert with clearly more saline molluscan faunal assemblage. Strontium isotope–based salinity estimates are surprisingly low when compared to other paleosalinity based on interpretation (ichno)faunal assemblages sedimentological structures. We propose that these seemingly contrasting observations can be unified if Amazonia occupied by long-lived (lacustrine) wetland system restricted connection, via Los Llanos Basin, Caribbean Sea. Abundant supplied fresh this system, which effectively blocked through marine connection north. Much like modern Lake Maracaibo, such could have been site microtidal currents thus hosted brackish-water fauna system.