The big oysters of the San Julián Formation, Paleogene of Patagonia (southern Argentina), as a substrate for endolithic and epilithic organisms

CASADÍO, S.; PARRAS, A.

Barcelona

Workshop; Third International Bioerosion Workshop; 2000

Universidad de Barcelona

The Paleocene through Miocene deposits in Patagonia, southern Argentina consist of a thin sedimentary wedge deposited in an epicratonic environment related to the passive continental edge bordering the Atlantic Ocean. These rocks contain rich vertebrate and invertebrate fossil assemblages that stirred the interest of d´Orbigny (1842)and Darwin (1846). Subsequent work has revealed the pivotal role of Patagonia for the resolution of Cenozoic Paleobiogeographic problems in the Southern Hemisphere. Well developed exposures of the lower part of the "Patagoniano" assigned to San Julián Formation, are located in Gran Bajo de San Julián and Cabo Curioso in the province of Santa Cruz, southern Patagonia, where seven sections were studied. Several horizons within the San Julián Formation contain extremely large oysters forming prominent accumulations. The organisms Crassostrea? hatcheri Ortmann, produced valves weighing several kilograms and lived as gregarious masses. The valves show the presence of a well-developed bioerosion sculpture. Borings are dominated by Entobia ispp., Maeandropolydora ispp., and overall by a high diversity of traces made by bryozoans. Among the boring patterns of Bryozoa developed in Crassostrea? hatcheri are Iramena isp., Pennatichnus isp. and Pinaceocladichnus ispp. There are also traces made by encrusting cheilostome bryozoans. They are etchings consisting of small pits described as Leptichnus Taylor, Wilson and Bromley, 1999. It comprises pits which are sub-circular in cross section and are found in either uniserial (Leptichnus dromeus Taylor, Wilson and Bromley, 1999) or multiserial arrangements (L. peristroma Taylor, Wilson and Bromley, 1999)