Large scale, cross-bedded sandstone bodies in a Cenozoic, tide-dominated embayment

CUITIÑO, J.I.; SCASSO, R.A.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

The Late Oligocene – Early Miocene “Patagonian” transgression covered a huge area in Patagonia. It is represented by shallow marine to coastal deposits exposed in several localities in a large area of Santa Cruz province, Argentina, in which conspicuous, large scale cross-bedded sandstone sets, 3 to 10 meters thick, are found intercalated with numerous oyster-rich beds. Two episodes of sedimentation are recognized within the “Patagonian” transgression. The first one of Late Oligocene age, was restricted to the eastern part of the studied area, and the second one of Lower Miocene age, covered a much larger area. Large scale (5 to 10 meters thick), cross-bedded sandstones where observed at San Julián area, with a clearly coarsening upward trend and an internally complex arrange of low angle master bedding planes and superimposed medium scale cross-beds. Palaeocurrent analysis showed south-directed downstream accretion, with few reversals indicating tidal action, and fossil content indicate normal marine conditions. These beds are composed of fine, well-sorted sands at the base,grading upwards to coarse sands with increasing proportion of biogenic particles. A bioclastic dominated cross-bedded set culminate the succession, which is followed by deeper shelf, fine-grained sediments. About 200 kilometers to the west, at Lago Cardiel area, two superposed large sandstone bodies were recognized. The lower one is composed of simple, tabular cross-bedded fine sandstones up to 7 m thick, with evidences of slack-water periods and dipping to the SSE. The upper, 5 m thick body is composed of coarse bioclastic sandstones with sigmoid master bedding planes and internal decimeter scale cross-beds. All these planes dip in the same direction (SE) pointing to downstream accretion. This body reaches about 2 kilometers long, and can be traced up to 20 kilometers. No reversal neither slack-water evidences were found. Southward directed large-scale bedforms are widespread in Lago Argentino area. They consist of simple to composite cross-beds up to 3 meters thick, composed of coarse sandstones with abundant biogenic particles. They lie above a discontinuity which can be traced about 30 km and according to the stratigraphic analysis it is inferred to represent a low order sequence boundary. The entire succession lying above the discontinuity shows a prograding trend from shallow marine-estuarine to fluvial setting. Sedimentary structures such as herringbone cross- stratification, mud drapes and heterolithic bed- ding indicate that tidal currents played an important role in sedimentation. Additionally, adjacent sets of cross-stratified sandstones show opposite directions of migration. Although all these deposits are probably not time equivalent, it is evident that tidal currents played a primordial role in the formation of these cross-bedded units as well as in the sedimentary history of the basin. The outcrop distribution and palaeocurrent analysis, together with the paleoenvironmental and paleogeographic reconstructions, and the lack of evidence of channel confinement, indicate that a wide embayment with tidal amplification was the most suitable setting for the origin of these large-scale bedforms, and they reflect tidal dynamics at the embayment scale. The bedforms were preserved almost complete suggesting high sedimentation rates within a wide, shallow embayment, deep and energetic enough to allow sandwaves to develope. The marked difference in energy between these cross-beds and the underlying sediments, and the high amount of biogenic particles in some bodies, suggest erosion prior to development of sandwaves (tidal ravinement surface?). The reworking of previous deposits is probably related to the instauration of strong tidal currents enhanced by the shape of the basin and triggered by high-order relative sea level rises.