Timing of Changes in Sea-Level and Currents along Miocene Platforms on the Marion Plateau, Australia
The question of global synchroneity of sea-level changes and their role in the formation of coeval unconformities on continental margins in different ocean basins remain major research topics in sequence stratigraphy. The Ocean Drilling Program (ODP) has addressed questions surrounding sea level in several drilling legs. One of these drilling efforts was ODP Leg 194, which drilled two platform-to-slope transects of the Miocene carbonate platforms on the Marion Plateau, situated just seaward from the Great Barrier Reef, NE Australia. The seismic and core information of this leg are used for an assessment of eustasy by determining the ages of seismic sequence boundaries on the Plateau and comparing them to sequence boundaries in the Atlantic. In addition, we evaluate the influence of current changes on the current-swept Plateau and its effect on the sequence architecture and the drowning of these cool, subtropical carbonate sequences. The two platforms, Northern and Southern Marion Platforms, are built by cool, subtropical faunal assemblages and have an asymmetric geometry. Four previously defined megasequences (A-D) are subdivided into 14 unconformity-bounded sequences. The early to middle Miocene sequences are prograding and aggrading sequences that responded mostly to the fluctuating sea level. From the late middle Miocene onward, the sea-level changes are coupled with increased activity of the southwardflowing East Australian Current. As a result, the sequences developed a characteristic mounded geometry in the basinal area where large drift deposits accumulated. Changes of current strength and position produced unconformities within the drift successions that are identified by downlap, onlap terminations and, locally, erosional truncation. These drift unconformities are observed along reflections that in the proximal position are onlap unconformities and sequence boundaries. The coeval nature of the two types of unconformities indicates that changes in sea level and currents occurred in concert. The interplay between sea level and currents also produced hardgrounds that record long hiatuses at sequence boundaries. The two most prominent sequences boundaries are drowning unconformities. An older one covers the top and flanks of the Northern Marion Platform, which drowned during the sea-level rise at 11.1 Ma. The Southern Marion Platform survived this event only to drown at the end of the late Miocene at approximately 7 Ma. A thin and varied drowning succession documents that the platform was intermittently active in the Pliocene. The top of the drowning succession is still a hardground surface on the modern seafloor, whereas Pliocene to Recent drift deposits overlie the hardgrounds on the slopes. It is likely that the combined effect of sea-level rise and subsequent sweeping of the platform by strong currents prevented the re-establishment of carbonate production on the bank and aided in the demise of the platforms. The timing of the sea-level changes is assessed by determining the ages of the sequence boundaries based on revised age models relying on biostratigraphy and Sr-isotope dates from cores at ODP Leg 194 drill sites. The age of each sequence boundary is remarkably similar at each site along the drilled transects. The age consistency along the seismic reflections corroborates the hypothesis that seismic reflections follow depositional surfaces and have chronostratigraphic significance. Furthermore, the timing of many Neogene sequence boundaries on the Marion Plateau (ODP Leg 194) coincides with the timing of sequence boundaries on the Queensland Plateau (ODP Leg 133) and along the Bahamas Transect (ODP Leg 166). The similar ages of the sequence boundaries and associated sea-level changes on the Pacific northeast Australian margin and the Atlantic Bahamian margin indicate a global synchroneity of third-order sea-level changes in the Neogene.