CONICET | Buscador de Institutos y Recursos Humanos

The La Pastosa beds constitute a nice example of sediments deposited in thehighly meandering reach of the fluvial-tidal transition within an estuary, developedat the top of the ?Rionegrense? stage, a marine-estuarine sequence of late Mioceneage from Patagonia. Sedimentary facies include channel lags rich in mud intraclasts;cross-bedded sandstones with mud drapes and ?set-climber? ripples; heterolithicand herringbone bedding; and inclined heterolithic stratification (IHS), and suggesta strong tidal influence. Tidal facies together with paucity of bioturbation andmarine fossils indicate that sedimentation took place mainly in brackish tidal-fluvialchannels, bounded by deposits formed in transgressive conditions at the base andat the top of the succession. Channel lag intraformational conglomerates are productof collapse of the cutbank due to erosion in the active margin of lateral migratingmeandering channels that eroded adjacent muddy tidal flats and salt marshes. Crossbeddedsandstones were accumulated above the conglomerates, forming subtidalsand dunes in the deeper parts of the channels and IHS was formed in point bars.Discontinuities at the base of the channels are the result of the entire channel-systemlateral migration, whereas discontinuities capped by small mud pebbles at the baseof large IHS sets are the result of seasonally increased run-off and widening of thechannels. Alternation of sandy and muddy IHS suggests periodical changes in theposition of the turbidity maximum due to seasonal variation of fluvial discharge,in good agreement with the seasonal climate in Patagonia during the late Miocene.IHS sets dipping alternatively to the north and south indicate lateral migration ofthe east-west running, high-sinuosity channels in the tightly meandering reach ofthe tidal-fluvial transition. The well preserved vertebrate fauna of Huayquerianmammal age is dominated by the hydrocherid Cardiatherium patagonicum togetherwith whistling ducks and loricariid fishes. This fauna inhabited out-of-channel,freshwater, low-energy restricted environments, like marshes and ponds where thecarcasses accumulated and underwent diagenetic fossilization. Repeated lateralmigration of meandering channels caused erosion of these deposits and the boneswere exhumed and incorporated into the channels, and finally concentrated inchannel lags after short transport. Combination of fossilization in a low-energyenvironment and concentration after short transport in meandering channels makethe fluvial-tidal transition ?first class? deposits for prospecting vertebrate fossils. In spite of exhumation and transport, vertebrate fossil association closely reflects theenvironments adjacent to the channels. Excellent preservation is more related torapid burial than to the energy level of the environment of final accumulation.

enviar mensaje