CONICET | Buscador de Institutos y Recursos Humanos

Repeated lateral migration and vertical incision of channels in the Puerto Madryn Formation deposits produced complex patterns of channel-fills mainly developed between highstand and lowstand shorelines (Figure 2). Preserved deposits are mostly channel bars and lags of different scales (Scasso et al., 2014) overlying erosion surfaces. Erosion surfaces are frequently cut into older channel deposits and produced a complex succession of channel fills (Figure 2), whereas inter-channel deposits are rare or absent. Fining upward channel fills are composed of bioclastic lags with broken and abraded shells, intraformational conglomerates rich in mud blocks and clasts. Bioclastic lags show cross-bedding and imbrications, and large-scale trough cross-bedding from subaqueous dune migration is common. In the tidal-fluvial transition tracts of the channels remains of fresh water vertebrates like capibaras are common (Scassoet al., 2012). Paleocurrent reversal is common. The rest of the channel fill may be formed by cross-bedded sands with mud drapes, inclined heterolithic stratification (IHS), heterolithic beds with rippled sands and mud, and tuffaceous muds. Ash driven by the rivers after large explosive eruptions on-land resulted in high input of sediment into the system, rapid accumulation, and ?freezing? of the paleorelief. The fine-grained facies mayshow well-defined tidal rhythmites. Large bars with IHS are straight or arcuate (point bars) and also show rhythmic sedimentation but sedimentary structures may be obliterated by bioturbation. Oyster banks grow on channel bars and form part of the channel system. Channel migration and consequent erosion of bars produce large sedimentologic, monospecific oyster accumulations in channel lags related to tidal ravinement surfaces.