TEMAPÁ HIDALGO: A NEW FOSSILIFEROUS OUTCROP FROM LOWER JURASSIC WITH CRINOIDS AND AMMONOIDS ON EAST CENTRAL MÉXICO. PALEONVIRONMENT INTERPRETATION

The Huayacocotla Fm. has been the subject of several geological and paleontological studies. This work reports a new outcrop with ammonites (Arnioceras abjectum Fucini)and isocrinids, some of them benthic (Isocrinidae) and some pseudoplanctonic (Pentacrinitidae). The presence of this fauna led us to classify the outcrop as belonging to Sinemurian (Lower Jurassic). These taxa, together with some gastropods, and lamelibranches are used to develop and interpret a depositional model of a deep marine bottom, periodically influenced by an adjacent noncarbonate shelf, and also with a predominance of pelagic-mass water. This interpretation is based on microstratigraphical and petrological studies of the thin sections, and on analysis of ammonites size structure to better understand,the characteristics of the sedimentary environement by distinguishing between in situ versus transported deposition of fossils. We also include discussion on the organisms´lifestyle with an taxonomic uniformitarianism criterion. The sedimentary sequence of this outcrop consists of shale with a microstartification,incluiding variable width sandstone layers, typical of the Huayacocotla Fm. The ammonites may be found on both sandstone and shale sediments, but the biggest and most widespread are situated in the finest sediments. The clastic sediments with ammonites and crinoids are only on the coarse grain strata; this is an important fact for the paleoenvironmental interpretation.