CONICET | Buscador de Institutos y Recursos Humanos

La Pastosa beds constitute a nice example of sediments deposited in the highly meandering reach of the of fluvialtidal transition (Dalrymple and Choi, 2007; van den Berg et al., 2007) within an estuary developed at the top of the Rionegrense, a marineestuarine sequence of Late Miocene age in northeast Patagonia (Scasso and del Río, 1987; Scasso et al., 2001; Dozo et al., 2010). Sedimentary facies like channel lags rich in rip-up boulders and mud intraclasts, crossbedded sands with mud drapes and setclimber ripples, inclined (HIS)and horizontal heterolithic stratification, herringbone bedding and tidal rhythmites, together with paucity of bioturbation and marine fossils, indicate that sedimentation took place in tidal channels subjected to strong tidal influence bounded by deposits formed in transgressive conditions at the base and at the top of the succession (Figure 1). Channel lag intraformational conglomerates are product of collapse of the cutbank due to erosion in the active margin of meandering channels. Crossbedded sands accumulate in deeper parts of the channel and IHS formed in point bars. Discontinuities at the base of the channels and at the base of large IHS sets are the result of the migration of the whole channelsystem and seasonally increased runoff and widening of the channel, respectively. Thick mud drapes and mud pebbles point to high suspendedsediment concentration and subsequent erosion by peak currents. Mud pebbles and blocks were also formed by lateral migration of the channels that eroded adjacent muddy tidal flats and salt marshes. Alternation of sand-rich and muddy IHS suggests periodical changes in the position of the turbidity maximum due to seasonal variation of fluvial discharge, in good agreement with the seasonal climate in Patagonia during the Late Miocene. Heterolithic bedding preserves neapspring tidal cycles interrupted by periods of erosion occurred during spring tides or increases in the fluvial discharge, and no sand deposition occurred during neap tides. IHS sets dipping in N - S opposite directions indicate recurrent migration of high sinuosity channels in the tightly meandering reach. Grain size analyses of successive sand-mud layers in heterolithic bedding allow distinction of a part of a tidal cycle. Layers of currentripple laminated sands with bipolar palaeocurrents directions show a consistent asymmetry in mean grain size, with coarser grained west (flood) oriented layers and finer grained east (ebb) oriented ones. The east oriented layers tend to disappear during neap tides. Repeated lateral migration of meandering channels caused erosion of the adjacent freshwater, lowenergy restricted environments, including a well preserved vertebrate fauna concentrated in channel lags after short transport.

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