Richard W Howe

Integrated uppermost Campanian–Maastrichtian calcareous nannofossil and foraminiferal biostratigraphic zonation of the northwestern margin of Australia

During the latest Campanian–Maastrichtian the northwestern Australian margin was situated between the cool-water Austral Province to the south and the warm-water Tethyan Province to the north. The transitional nature of calcareous microfossil assemblages on the margin makes application of Tethyan biostratigraphic zonation schemes awkward, as many marker-species are missing or have different ranges. This study presents an integrated uppermost Campanian–Maastrichtian calcareous microfossil zonation based on two Ocean Drilling Program (ODP) holes on the Exmouth Plateau and eight petroleum exploration wells from the Vulcan Sub-basin. The zonation is refined and revised from the previously unpublished KCN (nannofossils), KPF (planktonic foraminifera), KBF (benthonic foraminifera) and KCCM (composite nannofossil and planktonic foraminifera) zonations, which are commonly used for petroleum exploration wells drilled on the northwestern margin. Revision of the zonations has highlighted a major Upper Campanian to lower Upper Maastrichtian disconformity on the Exmouth Plateau, which went largely unnoticed in previous examinations of the ODP material, but had been recorded previously elsewhere on the northwestern margin. The duration of the disconformity in the Vulcan Sub-basin is unclear, since intervals of the succession may be condensed in this area.

Chronostratigraphy of proposed Turonian-Coniacian (Upper Cretaceous) stage boundary stratotypes: Salzgitter-Salder, Germany, and Wagon Mound, New Mexico, USA

Abstract A knowledgeable choice for a stage boundary stratotype is dependent upon obtaining high-resolution stratigraphic data. Detailed analyses conducted for the two potential Turonian-Coniacian stage boundary stratotypes that were considered at the Second Cretaceous Stage Boundary Symposium provide both positive and negative insights for consideration. The Salzgitter-Salder Quarry in central Germany (which was recommended by the symposium) contains abundant bivalve fossils, including the recommended boundary datum: the lowest occurrence of the inoceramid bivalve Cremnoceramus deformis erectus. Foraminifera are also abundant, but extensive diagenetic recrystallization seriously degrades nannofossil and palynomorph recovery and limits the potential of the section for stable isotope stratigraphy and radiometric dating. Furthermore, palaeoenvironmental analysis indicates that much of the Salzgitter stage boundary interval has resulted from allochthonous sedimentation, indicating that the well-developed lithological cyclicity between limestone and marlstone that occurs in the section is largely autocyclic. The orbital cyclostratigraphic potential of the section is therefore also in question. The Wagon Mound outcrop in northeastern New Mexico, USA, has good recovery and biostratigraphic control for all three microfossil groups, but the base of C. deformis erectus occurs above the section, by definition placing the section entirely in the Upper Turonian Well-preserved ammonites and inoceramid bivalves are also recoverable in over half of this section. Facies have not been recrystallized and represent continuous autochthonous sedimentation with sharply defined lithological cyclicity between limestone/marlstone couplets on a fine stratigraphic scale. In addition, a number of bentonites with proven datability occur in the section. Thus the bio- and chemostratigraphic dating potential, as well as the radiometric dating potential, of the section are good. However, much of the section is composed of carbonaceous, dysoxic facies with abnormal marine micro- and macrofossil assemblages, limiting study of the stratigraphic or palaeoecological trends leading up to the boundary. The absence of the datum at Wagon Mound is puzzling, because microfossil biostratigraphy suggests that the section is substantially coeval with the Salzgitter-Salder section. The C. deformis erectus datum may thus be diachronous. Until the suitability of the recommended boundary datum is addressed, a reasoned choice of a section for the boundary stratotype is not possible. In any case, the absence of C. deformis erectus and the abnormal facies in the lower part of the Wagon Mound section, and the extensive diagenesis and partly allochthonous nature of the Salzgitter-Salder section, are serious enough problems to warrant rejection of both sections as stratotypes.