STRATIGRAPHY AND PALAEOVOLCANOLOGY OF THE CAMBRIAN TYNDALL GROUP, MOUNT-READ VOLCANICS, WESTERN TASMANIA

The Tyndall Group is a Cambrian, dominantly submarine, volcano-sedimen tary succession that occurs in the upper part of the Mt Read Volcanics , western Tasmania. The internal stratigraphy of the Tyndall Group is relatively complex, comprising a wide variety of lithofacies including crystal- and lithic-rich volcaniclastic breccia, conglomerate and san dstone, welded ignimbrite, rhyolite, laminated mudstone and carbonate. Problems with the previously defined stratigraphic nomenclature have prompted development of a new stratigraphic scheme, based on detailed facies analysis of major Tyndall Group exposures in the central Mt Rea d Volcanics. The Tyndall Group is divided into two formations, the Com stock Formation and the overlying Zig Zag Hill Formation. The Comstock Formation is subdivided into the Lynchford Member and the overlying M t Julia Member. This stratigraphic scheme is based on regional litholo gical variations, which largely reflect different provenance character istics. In both formations, volcaniclastic lithofacies deposited by lo w- to high-density turbidity currents are abundant, and imply that the depositional setting was subaqueous and below storm wave base. Howeve r, in situ welded ignimbrite and a fossiliferous limestone unit sugges t that part of the group was deposited in proximity to subaerial envir onments. Many of the volcaniclastic units in the Comstock Formation, i n particular the crystal-rich volcaniclastic facies, were ultimately s ourced from subaerial or shallow-marine explosive volcanic eruptions p roducing pyroclastic flows which fed into the sea. The pyroclastic flo ws interacted with water, transforming into water-supported mass flows . Allochthonous blocks and in situ occurrences of welded ignimbrite in the Comstock Formation represent relicts of primary deposits from pyr oclastic hows that did not interact with water. Rhyolite lava dome com plexes also built up on the sea floor at this stage. In contrast to th e Comstock Formation, which is dominated by syn-eruptive volcaniclasti c facies, the Zig Zag Hill Formation records post-eruptive erosion and reworking of the subaerial to shallow-marine source areas that result ed in the influx of well-rounded, polymict, epiclastic debris to the m arine basin.