CONTINENTAL DRIFT AND BASIN DEVELOPMENT IN SOUTH EASTERN AUSTRALIA

Abstract

The Gippsland, Bass and Otway Basins in southeastern Australia are filled with sediments ranging in age from earliest Cretaceous to Recent. These basins were formed from the processes and forces which fragmented the Pacific margin of Gondwanaland. Their sedimentary histories and tectonic styles locate and date the movement of those continental masses now detached.An Early Cretaceous rift valley extended from the Otway Basin through the Gippsland Basin and on to Lord Howe Rise, (a part of eastern Australia at that time). A transform fault separated Tasmania and Antarctica resulting in the continuing detachment of Antarctica.During the Late Cretaceous, Australia drifted from Antarctica sufficiently to allow the Southern Indian Ocean to invade the Otway Basin and the build-out of river deltas. The Otway continental crust was further stretched, resulting in normal faulting and block rotation. Crustal tension continued in the Gippsland Basin and opened the Bass Basin. Fluvial sediments were deposited in both basins. The Tasman Basin was opened.During the Early Tertiary, crustal tension continued to shape the Bass and Gippsland Basins, where thick, fluvial sediments were deposited. The first marine indicators in Gippsland are Paleocene in age and, during the Eocene, two large submarine valleys were cut and filled and a regional unconformity was developed. Early Tertiary marine elastics prograded into the Southern Indian Ocean.Mid-Tertiary marine shales, marls and limestones formed the continental margin of the Otway Basin and gently filled the Bass Basin. In the Gippsland Basin the same -aged sediments unconformably truncated and sealed Early Tertiary sandstones and contructed the continental slope into the Tasman Sea, while a right lateral shear formed the producing structures.