THE GIPPSLAND BASIN TOP LATROBE UNCONFORMITY, AND ITS EXPRESSION IN OTHER SE AUSTRALIA BASINS

Abstract

The Early/Middle Eocene was an important time for developing the present configuration of the Indo- Australian plate, with the onset of fast spreading beginning in the Southern Ocean, and the commencement of northwest directed compression in the Gippsland Basin. Significant unconformities developed during this time including the Top Latrobe Unconformity (Top Latrobe) within Gippsland, and similar unconformities in the Torquay and Otway Basins.

On seismic over uplifted highs, (and where close spaced well data exists), a low angular unconformity exists between interbedded sand/shale/coal facies of the Latrobe Group and the Seaspray Group. The Marlin and Flounder channels eroded up to 600 m into the earliest Eocene deformed surfaces, and their infill in turn has been eroded at a top-Latrobe group unconformity where tectonic deformation and the resultant variable tilting produced an angular unconformity up to 5°. Missing biostratigraphic zones occur below the unconformity and many faults terminate at the Top Latrobe. The Top Latrobe is also characterised by resistant sandstone strike-ridges that created a varied topography. In areas of uplift where interbedded sandstone/shale units occur in the Top Latrobe subcrop, strike ridges are common. Where thick shale units occur at the Top Latrobe subcrop, topographic troughs or valleys are more common.

A study of 50 key offshore wells across the Gippsland Basin suggests that the best correlation between the seismic/synthetic Top Latrobe, and the lithobiostratigraphic Top Latrobe occurs in the upper part of the Middle Eocene. This date can be constrained between 40 and 44 Ma based on the ages of Marlin and Flounder channeling and infill and the Gurnard Formation. In the onshore part of the Gippsland Basin, the Top Latrobe can be located as a disconformity within coal measure units along the top of the Middle Eocene Traralgon–2 coal seam. In the Torquay Basin the only exposed example of this Eocene event is preserved in the Anglesea coal mine as a low angle unconformity between the A group coal seam and the overlying Boonah Formation. Low angular unconformities in seismic data are evident in the offshore Torquay and Otway basins at this time indicating the widespread nature of this unconformity in the southeastern Australian coastal basins.