Shallow marine and fluvial environments of Quaternary deposits in Pehuen-Có Beach, Buenos Aires, Argentina

Abstract

The studied ou tcrops are approximately located in the present intertidal area of Pehuen-Có Beach, at about 10 km to the SE of the city of Bahia Blanca (lat. 39° S and long. 61° 35 ‘W). Deposits appear discontinuedl y, 4 to 5 km long and 100 to 150 m wide, and their thickness is less than 1 m. Lateral facies relationships, small vertical sequences and paleoflow data were described with certain accuracy in a plane table map at 1:2000 scale. Ten facies have been determined: 1 Cross-bedded quartzitic conglomerate, migrating to the S and SE. 2 Clayey sandstone with subordinated quartzitic clasts. 3 Pelites with dessication cracks and bird ichnites. These units correspond to a longitudinal bar and a sequence of alluvial plain facies from fluvial deposits. The remaining facies are: 4 Planar cross-bedded sandstone, migrating to the W. 5 Trough cross -bedded sandstone, migrating to the W and subordinately to the E and S. 6 Pelites (mudstone and siltstone), locally a mudstone with ripple symmetrical to the top (E-W oriented ridges). 7 52Parallel laminated sandstone. 8 Sandstone with truncated wave-ripple laminae, migrating to the NW and E. 9 A conglomerate of calcareous sandstone, remains of biogenic structures, quartzites, clayey sandstone, coquines and remnants of shelly sands. 10) Sandstone, partially conglomerated, with low angle cross bedding, plunging 4° to 8° S. It corresponds to 3 facies sequences generated in a shallow marine environment, partially represented by deposits which range from foreshore (facies 7 and 10), high shoreface (facies 4, 5, 9), low shoreface (facies 8) and through offshore (facies 6). These sediments were mainly generated by the action of high energy waves. The arrangement (from base to ceiling) of facies sequence is: b, facies 9 + 4 + (7) + 5 + 8 + 6 facies 6 + 4 + 5 and D: facies 9 + (10) + 4 + 5. The evolution of the integrated stratigraphic column indicates the existence of two relative sea level rise (sequence B and D) and a relative sea level lowering during the deposition of sequence C in the area.