Inferences concerning the distribution and composition of pre-Carboniferous rocks in southeastern Tasmania

Interpretation of gravity and magnetic data in southeastern Tasmania indicates that blanketing post-Carboniferous rocks generally lie unconformably on late Precambrian rocks. Supporting evidence is provided by clasts of basement within overlying pyroclastics and till, and three boreholes.
Near Hobart, west of the Huon River and south of Bruny Island, deep troughs were filled with (?) Cambrian sequences with a significant volcanic component. Some ultramafics are present along the western margins. Relatively thin wedges of Ordovician-Devonian rocks are also implied, especially west of the Huon River. All rock-units are comparable with those exposed in western Tasmania.
The boundary between eastern and western Tasmania terranes occurs near Sorell and granitoids are dominant east of it. Jurassic dolerite feeder locations have been controlled by basement structure, and doming at Cygnet is related to a thick syenite intrusion. Many modem topographic, or Tertiary structural patterns may be recognised in deep structures.