Control on Pleistocene Shelf Drainage by Post-Eocene Stratigraphy of the Gippsland Basin

Abstract

The Jemmys Point Formation — uppermost member of the cool-water carbonate Seaspray Group — records a terrestrial drainage system that traversed the now submerged continental shelf of the Gippsland Basin during the latest ice age. A partial network of large sinuous channels that includes a few diffuse areas of arced broadening (probably recording local slack-water lacustrine conditions), has been imaged by aeromagnetic survey data and now large-scale 3D exploration seismic data. Channels are well resolved in narrow-band quadrature phase exploration seismic data by a surface probe that runs along the first zero crossing of the seismic wavetrain. This corresponds closely with the modern-day bathymetric surface when assuming a seawater acoustic velocity of 1,500 m/s.

The stratigraphy of these channel features has not been ground-truthed despite the basin being a highly mature and productive petroleum province. In fact, it seems that their distribution is a ‘negative’ of the distribution of petroleum field areas. This fact leads to the consideration of two hypotheses: ongoing structural inversion of existing trapping structures at depth pushes channels away; or intervening zones of relatively high differential compaction subsidence pull channels towards them. No evidence was found to confirm the former hypothesis but this may be because the rate of local uplift does not outpace shelf-wide sedimentation (so cannot be resolved in seismic or seabed bathymetry data). By contrast, the loci of drainage channels appear to correspond well with regions of the thickest post-Eocene stratigraphy. This suggests that differential compaction subsidence has continued to hold the lowstand drainage system in place since a pre-cursor canyon head network was developed by the Mid-Miocene inversion maximum.