Geology of Tasmania - Geological History - Tertiary - Basalt

Basalt

Volcanic vents opened up 58 to 8 million years ago. Lava flows of basalt up to 20 meters thick were formed. Some volcanoes were explosive with bombs, and pyroclastic tuff. The eruptions are probably from the Oligocene and Miocene. The earliest eruption was at Bream Creek on the east coast at 58.5 million years ago. From Weldborough it is 47 million years ago but mostly eroded.

In the south east, basalt from Sandy Bay dates from 26.5 ± 0.3 million years ago. Campania has an alkali basalt from 24.2 million years ago but it also has younger flows of olivine tholeiites. From near Hobart there is olivine basalt from 23 million years ago.

In north east Tasmania, there are many lava flows from middle Eocene to early Miocene. There are at least four types: alkali olivine basalt, quartz tholeiite 30.7 million years ago, alkaline basalt, and olivine nephelinite. Lava flows in the north east flowed down valleys to the sea.

In the north west, there was so much lava that valleys filled and overflowed. A plain resulted with up to 750 meters thickness, and maximum extent south of Wynyard and Burnie. In the late Eocene and early Oligocene lakes were formed near Waratah. Older alkaline basalt in the north west is from 26.3 million years ago, at Table Cape basanite from 13.3 million years ago and at Stanley basanite is dated to 12.5 million years ago and 8.5 million years ago. Mount Cameron West has olivine basalt from 15.5 and 14.4 Ma.

On the southern part of the Central Plateau, there are olivine melilite nephelinite, olivine nephelinite, quartz tholeiite lava flows. These ran south down tributaries of the Derwent River 24.3 to 22.4 million years ago. On the east side of the Central Pateau an olivine nephelinite is from 24.9 million years ago, and a flow of nepheline hawaiite is from 24.2 million years ago. In the western Midlands there is basalt from 36.3 million years ago, and hawaiites from 25 and 24.3 million years ago.

Around Launceston, igneous rocks were intruded into Tertiary sediments forming dolerite and monzonite.

Read more about this topic:  Geology Of Tasmania, Geological History, Tertiary

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