Sedimentologia de la Formación Chenque (Oligoceno-Mioceno) en Comodoro Rivadavia. Argentina

Abstract

The Oligocene-Miocene marine rocks of the San Jorge Basin (Argentine Patagonia) were studied in the proximity ofComodoro Rivadavia. The basal transgression deposited a residual lag composed ofshoreface sediments on the Sarmiento Formation (Eocene-Oligocene). Shelf sediments above the basal transgression have bioclastic lenses suggesting strong currents and transport of sediment under the fair-weather base level. A progradational complex was formed showing coarsening-upward cycles in shoreface-inner foreshore setting, which include some channels with bioclastic and clastic in-fill, and little evidence of tidal action. The presence of carbonaceous concretions, hardground and glauconitic mineral deposits on the top of this section suggests the abandonment of the,system. A new transgressive cycle accumulates shelf clay and siltstone intercalated with bioclastic material. This progradational system was covered with shoreface and foreshore facies, showing tidal and wave action. Bioclastic and clastic materials, accumulated on a major discontinuity surface by meandering tidal channels show progradacional pattern stacking and fining upwards successions. These materials grades, by means of medium sandstone with bioclastic lenses, to mudstone deposited in a estuarine bay (or lagoon) with very little bioturbation. Some bioclastic and clastic materials intercalated with the mudstones, represent storm deposits in a restricted area. The unit may be interpreted as having been deposited in a clastic shelf with tidal and wave influence