Stratigraphy and palaeovolcanology of the Cambrian Tyndall Group, Mt Read Volcanics, western Tasmania

The Tyndall Group is a Cambrian, dominantly submarine, volcano-sedimentary 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 sandstone, 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 Read Volcanics. The Tyndall Group is divided into two formations, the Comstock Formation and the overlying Zig Zag Hill Formation. The Comstock Formation is subdivided into the Lynchford Member and the overlying Mt Julia Member. This stratigraphic scheme is based on regional lithological variations, which largely reflect different provenance characteristics. In both formations, volcaniclastic lithofacies deposited by low- to high-density turbidity currents are abundant, and imply that the depositional setting was subaqueous and below storm wave base. However, in situ welded ignimbrite and a fossiliferous limestone unit suggest that part of the group was deposited in proximity to subaerial environments. Many of the volcaniclastic units in the Comstock Formation, in particular the crystal-rich volcaniclastic facies, were ultimately sourced from subaerial or shallow-marine explosive volcanic eruptions producing pyroclastic flows which fed into the sea. The pyroclastic flows 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 pyroclastic hows that did not interact with water. Rhyolite lava dome complexes also built up on the sea floor at this stage. In contrast to the Comstock Formation, which is dominated by syn-eruptive volcaniclastic facies, the Zig Zag Hill Formation records post-eruptive erosion and reworking of the subaerial to shallow-marine source areas that resulted in the influx of well-rounded, polymict, epiclastic debris to the marine basin.