Larger Foraminiferal Biostratigraphy of Sites 815, 816, and 826, Leg 133, Northeastern Australia

Abstract

The Marion Plateau is a large carbonate platform off northeastern Queensland. Three sites (815, 816, and 826) were drilled on this platform and form the basis for this study. Larger benthic foraminifers, together with rare planktonic forms from the shallow-water carbonates that form the main part of the platform sequence, were studied to establish a biostratigraphy. The presence of Lepidocyclina (Nephrolepidina) howchini sensu lato and Ladoronia vermicularis, together with Globowtalia (Globorotalia) praemenardii and Orbulina, indicate an early middle Miocene (N9-N12) age (i.e., lower Tf stage) for these carbonates. Dolomitization has destroyed much of the original fabric of these carbonates, making study of the larger foraminifers difficult. Sites 815 (forereef location) and 826 (backreef, lagoonal setting) provide the best faunas. However, at all sites nodular coralline algae and Halimeda are the major bioclasts; coral fragments form a major component at Sites 816 and 826. The middle Miocene neritic sequence is separated from the overlying hemipelagic sequence by an unconformity that spans much of the middle and late Miocene. At Site 815, which is in a forereef situation, the overlying hemipelagic sequence contains a Zone N17A fauna, but at Site 816, higher on the platform, a similar sequence contains a Zone N19 fauna. The faunas indicate that the platform was built up during the early middle Miocene and remained at fairly constant water depths and temperatures during this period. It was then exposed prior to subsiding rapidly during the late Miocene and Pliocene to depths similar to those of the present day.