Biostratigraphy and macroinvertebrate palaeontology of the petroleum-rich Belfast Mudstone (Sherbrook Group, uppermost Turonian to mid-Santonian), Otway Basin, southeastern Australia

The hydrocarbon-rich Turonian to Santonian Belfast Mudstone (Sherbrook Group) of the Otway Basin, southern Victoria, Australia, represents transgressive lower delta plain to prodelta conditions in inner to middle shelf depths to slope environments. Comprising relatively homogeneous mudstone that thickens to more than 1 km in the offshore Otway Basin, the biostratigraphic resolution has been enhanced by study here of both macro- and microfossil assemblages. Macrofossils consist of 12 species of bivalves from a relatively low diversity of genera (Nucula s.l., Nuculana, Mesosaccella, Pinna?, Tethyoceramus, Oistotrigonia), gastropods (Cerithioidea, Latiala, Ringiculidae), cephalopods (Eutrephoceras?, Borissiakoceras), and cnidarians (Trochocyathus (Platycyathus?)). Other invertebrate groups such as echinoids and serpulids are too poorly represented and preserved for description. New and probably new taxa described include the bivalve Oistrotrigonia austronana sp. nov. (Trigoniidae), Mesosaccella sp. nov.? (Nuculanidae) and ammonite Borissiakoceras sp. nov.? (Binneyitidae). The probable presence of the inoceramid bivalve and index taxon Tethyoceramus madagascariensis and binneyitid ammonite Borissiakoceras sp. nov.?, corroborated by known ranges of other macroinvertebrates and Foraminifera in the Belfast Mudstone from the offshore Otway Basin, constrain the age in cores 13 and 16 of the well Voluta-1 to the Coniacian Stage (probably equivalent to the Tethyoceramus madagascariensis Zone, lower part of Coniacian, ca. 89–88 Ma, for core 16 and late? Coniacian for core 13). All of the bivalves and gastropods are infaunal and/or semi-infaunal suspension and deposit feeders that preferred fine-grained facies, and these fossils and others support the interpretation of a low-energy hydrodynamic regime in a quiescent, deeper shelf setting.