THE INITIAL MARINE TRANSGRESSION IN THE GIPPSLAND BASIN, VICTORIA

Abstract

The Gippsland Basin of south-eastern Victoria is considered as the depositional area of Tertiary sediments lying beneath the Gippsland Plains and extending southwards beneath the Tasman Sea. This area is bounded by Mesozoic and Palaeozoic rocks.

The initial marine transgression across the landward extent of the basin appears to have commenced in uppermost Eocene to lowermost Oligocene times. The nature of the initial marine Tertiary sedimentation is controlled by:

four regional lower Tertiary structures—the Woodside-Seaspray Deep, the Baragwanath Anticline, the Lake Wellington Trough, and the Lakes Entrance Platform; and

the nature of the rocks upon which these sediments were deposited.

Because of the progressive onlap in some areas, particularly on the Baragwanath Anticline, the initial marine transgression is diachronous.

The initial marine Tertiary sediments—constituting the Lakes Entrance Formation (as redefined in this paper)—can be divided into two broad lithological units, a lower sandy one and an upper marly one. Oil traces have been recorded in the basal sands throughout the basin, particularly in the Lakes Entrance area where minor production was undertaken during the 1930s.

The offshore areas of the basin appear to have the greatest oil potential. The prospective reservoir beds would be the offshore extensions of the hasal marine Tertiary sands, or else offshore marine equivalents of the Latrobe Valley Coal Measures (which underlie the marine sediments in all but the Lakes Entrance Platform area)—provided that these beds have not been flushed by artesian waters.