Complex Seismic Morphology on the Slope of NW Australia: Successive Episodes of Fossil Pockmarks in the Lower Tertiary

This reference is for an abstract only. A full paper was not submitted for this conference. The Tertiary slope deposits of the Australian NW shelf show a complex imbrications of apparently erosional surfaces. Observed in 3D, these surfaces appear as essentially made up of coalescent craters. This morphology indicates that they do not correspond to canyons or channels as could be suggested by cross-sectional views only. Detail seismic correlation between the infill of the craters and the surrounding areas shows that the craters actually formed below sedimentary cover, in other terms correspond to sediment dissolution at depth and not erosion at the seafloor. Individual craters typically measure about 1 to 2 km in diameter and are 100 to 300 m deep. They can also be strongly elongated and at some levels coalesce to make very irregular surfaces over several hundreds of km2. Craters affect most of the Palaeocene and Eocene of the studied area, i.e. a thickness of about 1 km. The lithology of the interval is dominated by carbonates and claystones. The stratigraphic relationships with the adjacent shelf indicates that the area was already in the deep domain at the time of deposition. This rules out a formation process by continental karst for the craters. At the same time, the conical geometry is more reminiscent of pockmarks or conical injections than of collapsed karsts. Recently published work in the Gulf of Guinea has evidenced seafloor craters associated with the progressive dissolution of gas hydrates, with sizes comparable to what is observed in the NW shelf area. The complex seismic morphology observed in the Tertiary of the studied area is therefore tentatively interpreted to reflect successive episodes of growth and dissolution / dissociation of km-scale hydrate bodies in shallow subsurface. The episodes of dissolution may be related to the pressure fluctuations associated with relative sea-level changes. Whatever the exact dissolution mechanism, this represents a significant departure from classical "erosional" models in deep water, and should be considered in presence of highly irregular reflectors to avoid erroneous conclusions about the reservoir potential of affected series for instance.