Coquinas and shelf deposits of the San Julian Formation (Upper Eocene-Lower Oligocene) southern Patagonia, Argentina

MANASSERO, M.; GRIFFIN, M.; PASTORINO, G.

Revista de la Asociación Geológica Argentina

Asociación Geológica Argentina

Lugar: Buenos Aires; Año: 1997 vol. 53 p. 286 - 296

0004-4822

The San Julián Formation is a Paleogene shallow marine unit exposed near the coast of southern Patagonia,Argentina. It includes both siliclastic and calcareous sediments and shows three upward-shoaling cyc1es developed in asand-dorninated shelf sequence. This paper describes three main types of coquinas on the basis of their geometry and faciesassociations, e.g. storm coquinas, channel coquinas and swell coquinas. They are related to the lithofacies arid show a paleobathymetricalcontrol in their development. The main origin of the coquinas seems to be sudden death caused by uprootingagents, such as large waves and storms. Storm and wave action is recorded in the facies associations observed, enablingthe recognition of three main subenvironments: transition-off shore (FI+Hc), lower shore face (Srn--St) and upper shore face(St+Sp+Gm+Gms).The storm deposits show a high preservation potential with many bivalves having articulated, sornewhatunbroken and non abraded shells. Three cycles were distiguished within the Upper San Julián Formation, well exposedin the four sections. Cycles 1 and 2 are relatively similar, both begining with massive sandstones with channel coquinas,and evolving into cross-bedded sandstones with extended sheet storm coquinas. Cycle 3, is the best exposed along the borderof the San Julián Depression, showing thick cross-bedded sandy bodies. The variety, abundance and good preservationof the coquinas in this unit, make this type locality unique ancl underline the importance of further sedimentological andpaleoecological studies along the San Julián Depression ancl surrounding areas of southern Patagonia.