Geology of Tasmania - Geological History - Tertiary - Tectonics

Tectonics

Tasmania finally disconnected from Antarctica 45 million years ago. Several basins were formed by faulting. Faulting was connected with continental breakup. Most faulting was finished by the Eocene, but the Sorell Basin continued into the Oligocene. Tertiary age deposits are found in the northern midlands (Tamar Graben), and south of Macquarie Harbour in the Macquarie Harbour Graben. In the south east are the Derwent Graben and the Coal River Graben. Thick layers of Tertiary rocks are found in the estuary of the Derwent River, D'Entrecasteaux Channel, Sandy Bay, Taroona, Middleton, Craigow Hill, and Spring Bay. The rocks are mostly silstone and clay. The deep estuary rocks are from the Paleocene. Travertine is found at Geilston Bay. Silcrete and laterite from this time is found too.

The Macquarie Harbour graben deposits dating from Palaeocene and Eocene are poorly consolidated sand, and gravel, with some beds of lignite and clay. Sediments are up to 500 meters thick, with the lowest layers consisting of dolerite boulders.

The Tamar Graben was an extension to the south of the Bass Basin onto the Tasmanian island. Sediments started in the graben at the very end of the Cretaceous, and into the Paleocene and Eocene with conglomerate, sandstone, mudstone and lignite. Basalt and conglomerate is buried south of White Hills. There is Eocene carbonaceous silt. The Longford Sub-basin extends inland south of the Tamar Graben, and is filled with 800 m of clay, sand and gravel, with some basalt towards the top layers, mostly from the Eocene.

The Devonport-Port Sorell Sub-Basin was formed in Paleocene with carbonaceous mudstone and sandstone. The Thirlstane Basalt is above at 38 million years ago, an alkali-olivine basalt. Then the Wesley Vale Sand follows, and the Moriarty Basalt is 50 meters thick at 29.5 million years ago.

The Sorell Basin forms the continental shelf off the west coast. It has sub-basins of King Island, Sandy Cape, Strahan, and Port Davey which were formed in the Early Cretaceous. The King Island Basin is terminated on the east by a normal fault. It is south of King Island and north west of Tasmania. It has a basement of the Rocky Cape Group from the Proterozoic. The first sediments are red conglomerate beds for 190 m. mid-Upper Cretaceous sandstone and mudstone follow, the same age as the Sherbrook Group. Then more conglomerate sandstone and mudstone matching the Wangerrip Group up to early Eocene. Quartz sandstone is above this, with marl, mudstone and limestone from Oligocene and Miocene age. There is an unconformity at the Pliocene base. Approximately 4 km of sediment is found in each subbasin.