Geology and structure of the middle Derwent Valley.

A geological study has been made of 130 square miles of the Middle Dement Valley between New Norfolk and Hamilton, Tasmania. A Permian sub-greywacke type sequence consists of some 1300 feet of sandstone, siltstone, mudstone and limestone, deposited on a stable to mildly-unstable shelf. Disconformably overlying the Permian are rocks belonging to the Triassic ystem which have been grouped tentatively into three formations. The Permian and Triassic are extensively and complexly injected by dolerite and mineralogical variations from the normal tholeiitic dolerite to granophyre have been traced. Liquid immiscibility is present towards the base of the sill. The Jurassic structure Is interpreted as cauldron subsidence faulting accompanied by dolerite intrusion as transgressive to concordant sills, lopolithic near the root zone. Various Tertiary faults combine to form a stepped graben, the axis of which trends NW - SE on echelon, resulting in the formation of a chain of lakes and lowlands. In late Tertiary times, large outpouring of olivine basalt and associated tuffs at least 350 feet thick overran the lakes and filled the valley to a depth of some hundreds of feet. The basalts have been mapped into several flows. Five periods of volcanism have been recognized, the inter-basaltic periods being marked by either lake sediments or fossil forests. The graben faulting controlled the drainage pattern both during the pre-basaltic times and subsequently.