Drilling during Ocean Drilling Program (ODP) Leg 194 on the Marion Plateau offshore the Great Barrier Reef in northeast Australia documented the depositional history of a carbonate-dominated shelf (Isern, Anselmetti, Blum, et al., 2002). Sedimentation on this subsiding basement started after rifting in the Cretaceous and a transgression during the Oligocene–early Miocene. Two major carbonate platforms were established, the Northern and Southern Marion Platforms (NMP and SMP, respectively), which grew to the middle and late Miocene, respectively, after which both edifices drowned (Isern, Anselmetti, Blum, et al., 2002). The NMP became buried after drowning under a thin veneer of deeper water deposits; the SMP today still stands out topographically with moderate relief above the surrounding seafloor.
Today, the shelf lies in a water depth between 300 and 500 m (Fig. F1). The sedimentation pattern is strongly influenced by ocean currents that have been responsible for local and periodic winnowing, causing condensed or absent sediments in some areas and focusing of the sedimentation in others. Leg 194 drilling revealed a series of unconformities characterized by the presence of submarine hardgrounds at many of the sites, indicating major hiatuses in the sedimentary record. These surfaces can also be identified on seismic data (Fig. F2) and in sediment petrographic, biostratigraphic, and petrophysical data (logging data) (Isern, Anselmetti, Blum, et al., 2002; Isern et al., 2004). Using seismic stratigraphy as well as biostratigraphy, these unconformities and associated hiatuses were dated and put into an overall depositional/stratigraphic context (Isern, Anselmetti, Blum, et al., 2002). New isotope geochemical analyses of the hardgrounds that represent the hiatuses and mainly consist of ferromanganese encrustations are presented in this study with the goal of validating and refining the stratigraphic interpretations and ages and understanding the genesis and growth rates of the ferromanganese crusts. Deep-sea hydrogenetic ferromanganese crusts represent condensed chemical sedimentary sections, which reflect the radiogenic and Be isotope composition of ambient deep water. The evolution of the radiogenic isotope composition at particular locations over time has been used successfully to reconstruct changes in deep ocean circulation and weathering inputs over as long as the past 60 m.y. (see Frank, 2002, for a review). Geochemical data from ferromanganese nodules grown in shallow water have been used to reconstruct environmental changes (Hlawatsch et al., 2002).