AUTOCHTHONOUS BIOFACIES IN THE PLIOCENE LORETO BASIN, BAJA-CALIFORNIA-SUR, MEXICO

The present paper examines the molluscan and/or echinoid assemblages r ecovered from two lithostratigraphic units (Piedras Rodadas Sandstone and Arroyo de Arce None Sandstone) outcropping in the Pliocene Loreto Basin, Baja California Sur, Mexico. Ten biofacies have been identified , i.e. Trachycardium procerum-Trachycardium senticosum Biofacies, Chio ne compta-Transennella modesta Biofacies, Laevicardium elenense-Chione kelletii Biofacies, Xenophora sp. 1-Strombus subgracilior Biofacies, Crassostrea californica osunai Biofacies, Myrakeena angelica Biofacies , Vermetid-Nodipecten Biofacies, Argopecten abietis abietis Biofacies, Aequipecten dallasi Biofacies and Encope Biofacies. The first four bi ofacies have been defined on the basis of statistical analyses (cluste r analysis, MDS). The other six, which are monospecific or definitely low-diversity, were already identified during field work. The deduced paleoecological bearing of biofacies, largely relying upon the compari son to their closest modern counterparts, provides the basis for the p aleoenvironmental reconstruction. The latter also considers sedimentol ogical evidence and is framed within the tectonic and sedimentary cont ext recently proposed by American workers. Biofacies point toward envi ronments differing in terms of substrate texture, presence/absence of vegetal cover, energy level, variously distributed within the low tide mark-40 m bathymetric range.