Evolución paleoambiental de la formación Río Mayer, cretácico inferior, cuenca austral, provincia de Santa Cruz, Argentina

the marine deposits of the río mayer formation are part of the initial filling of the austral basin, which is developed in the southernmost part of south america, including the south of chile and argentina (southwestern patagonia, santa cruz and tierra del fuego provinces, figs. 1, 2). the study area is located between the san martín and argentino lakes in the santa cruz province (fig. 3). the río mayer formation (fig. 2) was studied in three main localities, where detailed sedimentary logging was undertaken (figs. 4, 5), taking into account the lithology, the primary sedimentary structures, the palaeontological content, rock bodies geometry, their orientation and the hierarchy as well as the contacts with underlying and overlying units. eighteen sedimentary facies were defined (figs. 6, 7; table 1), which were grouped into three facies association representing different sedimentary palaeoenvironments, like marine outer platform (af 1), marine outer platform influenced by a deltaic system (af 2) and prodelta (af 3) (fig. 8, tables 1, 2). the facies of the outer platform palaeoenvironment corresponds to the rocks traditionally assigned to the río mayer formation, according to the available bibliography, and deposited by hemipelagic and authigenic processes interrupted by episodic sedimentation. the deposits of the outer platform influenced by a deltaic system palaeoenvironment are similar to the above mentioned but presents continental fossils that indicate a deltaic source. finally, af 3 of a prodelta palaeoenvironment is dominated by sandy/heterolithics levels associated with turbidity currents and hyperpycnal flows originated in a delta front. the best and most complete exposures of the unit were observed in the río guanaco locality, where palaeoenvironmental interpretations were performed, allowing the division of the 350 m-thick unit in three sections (fig. 9). the lower section is composed mainly by black shales with both tabular and concretional marls levels (