Turonian inoceramids and biostratigraphy of the Sergipe Basin, northeastern Brazil: an integrated study of the Votorantim and Nassau quarries

A detailed biostratigraphical survey was carried out in the Turonian (Upper Cretaceous) of the Sergipe Basin, northeastern Brazil, on the basis of bed-by-bed collecting of inoceramids, ammonites and other macrofossil groups from the Votorantim (locality Retiro 26) and Nassau (locality Mata 11) quarries in the Laranjeiras–Nossa Senhora do Socorro area. The inoceramids are described systematically with the aim of providing a reliable basis for correlation with the international biostratigraphic “standard” zonation. Additional localities were investigated in order to obtain additional material for a re-evaluation of the genera Rhyssomytiloides Hessel, 1988, Sergipia Maury, 1925 and Didymotis Gerhardt, 1897 and to locate the Turonian–Coniacian boundary on the basis of inoceramids. The palaeobiogeography and palaeoecology of the inoceramids are briefly discussed. The carbonate successions exposed in the two quarries contain a relatively abundant and diversified bivalve and ammonite fauna, with inoceramid bivalves representing the dominant group. Twenty-nine species of the genera Mytiloides Brongniart, Rhyssomytiloides Hessel, Inoceramus J. Sowerby, Didymotis Gerhardt, and Cremnoceramus Cox are described and illustrated. Re-evaluation of the genus Sergipia Maury shows it to be a junior synonym of Didymotis Gerhardt, 1897. Rhyssomytiloides Hessel, 1988 is considered to be a justified genus; its similarity with Cladoceramus Heinz, 1932 is interpreted as a case of homeomorphy. Although less abundant than the inoceramids, the ammonites are represented by 28 species. Nine successive inoceramid interval zones are recognized in the two quarry sections and calibrated with the local ammonite zonation. The lower Turonian is subdivided into the Mytiloides puebloensis, M. kossmati–M. mytiloides and M. labiatus zones, the middle Turonian into the M. subhercynicus and M. hercynicus zones, and the upper Turonian into the M. striatoconcentricus, M. incertus, M. scupini and Cremnoceramus waltersdorfensis waltersdorfensis zones. The Turonian–Coniacian boundary is not present in the quarry sections but exposed in the Nossa Senhora do Socorro area, east of the Nassau (Mata 11) quarry. The stage boundary is marked by the first occurrence of C. deformis erectus (Meek), which is firmly correlated with the corresponding level in the proposed candidate Global boundary Stratotype Section and Point (GSSP), in the Salzgitter-Salder quarry, Lower Saxony, Germany. On the basis of a large number of common elements, the Turonian inoceramid zonation of Sergipe is reliably correlated with the European and U.S. Western Interior zonation. However, at least three inoceramid zones appear to be missing between the Retiro 26 and Mata 11 sections, corresponding to the upper middle Turonian to basal upper Turonian interval. Whether the missing zones are represented by a stratigraphical gap or concealed in the unexposed interval between the two sections is a question that cannot be resolved on the basis of available data. Study of the ammonite fauna has allowed a refined, though as yet provisional ammonite zonation. The inoceramid and ammonite zonations are integrated with previously established zonations based on planktonic and benthic foraminifers and calcareous nannofossil.