Geology of Tasmania

The north east of Tasmania began to form as part of the Lachlan Orogen with turbidity flows of mud and sand on to the ocean floor.

The continental breakup happened in the Cretaceous and Cenozoic Periods, splitting off undersea plateaus, forming Bass Strait and ultimately breaking Tasmania away from Antarctica.

Previous theories had suggested that Tasmania emerged from central Australia as the supercontinents broke apart, but the recent spectroscopic and radioactive dating evidence contradicts this.

It extends north northeast to Phillip Island, Victoria, and also south 40 km to just off the west coast of Tasmania stopping at the Braddon River Fault[2] In the Rocky Cape Block west of Wynyard and north of Granville Harbour, the Precambrian rocks consist of the Rocky Cape Group from the Stenian period, with Cowrie Siltstone, Detention Subgroup, Irby Siltstone, and Jacob Quartzite.

The Burnie Formation followed in the Tonian period south east of the lineament with greywacke and slaty mudstone, and also some basic pillow lavas.

In the Smithton Synclinorium the Togari Group followed with conglomerate from the Sturtian and Marinoan glaciations and dolomite marking the end of Cryogenian and on into the Ediacaran and Cambrian.

[4] The Sheffield Element extends from Wynyard past Devonport and the Asbestos Range on the north coast and as far south east as Golden Valley.

The Andersons Creek Ultramafic Complex is west of Beaconsfield and east of the inlier with serpentinite, pyroxenite, gabbro and a sliver of oolitic chert introduced as a fault bounded block.

It has a strip exposed on the surface that includes the Florentine Synclinorium, The Adamsfield District, the Jubilee Region, and down to the South Coast at Precipitous Bluff and Surprise Bay.

Near Waratah is a sub-alkaline basalt from an ocean floor, another is a high-magnesium andesite-basalt with chrome spinel and clinoenstatite named boninitic rock after the Bonin Islands.

Thirdly there is a low titanium basalt-andesite with extreme light rare-earth element depletion that produced the layered pyroxenite-peridotite and associated gabbro cumulate.

In the Adamsfield area The Ragged Basin Complex is a broken up formation of chert, sandstone, red mudstone and mafic magma derived rocks.

The Noddy Creek Volcanics extend north of high Rocky Point to Macquarie Harbour with pyroxene and feldspar containing andesite as lava, breccia and intrusives.

The Arthur Lineament was metamorphosed to phyllite, slate and schistose quartzite, The Burnie and Oonah Formation were folded in various ways, and the Rocky Cape Group and the Smithton Synclinorium developed cleavage texture.

Upper Owen Sandstone is found in Queenstown, it formed while the Great Lyell Fault was active, resulting in folding of the lower parts.

Correlated rocks also occur in a syncline south west of Birchs Inlet, around the upper part of the Wanderer River, and in the Dial Range Trough the equivalent unit is called the Duncan Conglomerate.

These units are called the Roland Conglomerate and Moina Sandstone and represent contrasting stratigraphic architecture to that observed in western Tasmania, a reflection of evolution of different rift depocentres.

In the north east of Tasmania the Mathinna Group received its last deposits in the form of turbidites in the Bellingham Formation and Sidling Sandstone containing more feldspar.

Mineralisation from the Heemskirk Granite with cassiterite or tin and tungsten skarn, or silver lead and zinc veins occur in the Zeehan field.

Geophysical exploration and a borehole has revealed a large granite mass a kilometer below Zeehan, Renison, Dundas, Rosebery mineral fields.

An ashfall tuff in the Denison Rivulet area of eastern Tasmania is dated at 214 ± 1 million years ago (Late Triassic).

Henrik Svensen claimed that the magma baked coal and oil shale producing up to 27.4 teratonnes of carbon dioxide, some of which entered the Earth's atmosphere.

About 83 million years ago a rift entered the east coast of Tasmania from the south and split off the Lord Howe Rise.

This East Tasman Plateau microcontinent was originally off the southeast of Tasmania; it is a circular piece of continental rocks surrounded by oceanic crust.

[8] The Macquarie Harbour graben deposits dating from Palaeocene and Eocene are poorly consolidated sand, and gravel, with some beds of lignite and clay.

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 part north of Langdon Point and Ballast Bay consists of serpentinite derived from gabbro, troctolite, and peridotite (dunite, wehrlite, and harzburgite).

The Rocky Cape Element is densely packed with linear textures parallel to the Arthur Lineament, with the Smithton Syncline showing as a Y shape.

William B. Clarke, a geologist and Anglican parson, predicted that gold would be found in Tasmania at 146 degrees east longitude line.

He had become the Government Geologist of Tasmania in 1944 where he organised the understanding of Paleozoic formations in the west coast mineral fields, and introduced the Cenozoic rift valley idea, and the policy of publishing the results of the Geologic Survey.