ALBERT

Description: 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.