Stratigraphy, volcanology and sedimentology of the Cambrian Tyndall Group, Mount Read volcanics, western Tasmania

The Tyndall Group is a Cambrian, dominantly submarine, volcano-sedimentary succession that occurs in the upper part of the Mount Read Volcanics, western Tasmania. The Tyndall Group comprises a relatively complex assemblage of lithofacies including crystal- and lithic-rich volcaniclastic breccia, conglomerate and sandstone, welded ignimbrite, rhyolite lava and/or intrusions, laminated mudstone and carbonate. Problems with the previously defined stratigraphic nomenclature have prompted development of a new stratigraphic scheme, based on detailed mapping and facies analysis of major Tyndall Group exposures in the central Mount Read Volcanics. The Tyndall Group is herein divided into two formations, the Comstock Formation and the overlying Zig Zag Hill Formation. The Comstock Formation is further subdivided into the Lynchford Member and the overlying Mount Julia Member. This stratigraphic scheme is based on regional lithological variations, which largely reflect different provenance characteristics. Volcanogenic sedimentary lithofacies are abundant in the Tyndall Group and were largely deposited from low- to high-density turbidity currents, implying that the depositional setting was dominantly subaqueous and below storm wave base. An in situ limestone unit containing abundant shallow marine fossils (Jago et al. 1972) occurs at the base of the Comstock Formation at Comstock, and indicates that at least part of the group was deposited in water less than a few hundred metres deep. Sources of the volcaniclastic components in the Tyndall Group are not exposed or have been eroded away. However, syn-eruptive volcaniclastic facies in the Comstock Formation provide a record of the character and setting of volcanic activity in the source. The high proportion of juvenile pyroclasts in these facies (e.g. crystals, crystal fragments, shards, pumice) indicates the occurrence of voluminous explosive magmatic and/or phreatomagmatic eruptions in the source, and that the source areas were in subaerial to shallow marine environments. Pyroclasts were probably transported to the marine setting by pyroclastic flows which transformed into water-supported sediment gravity flows after sufficient interaction with water, forming crystal-rich volcaniclastic sandstone. Occurrences of welded ignimbrite in the Comstock Formation represent relicts of primary deposits from pyroclastic flows that did not interact with water. Although the welded ignimbrites could be deposits from hot, gas-supported submarine pyroclastic flows, two other emplacement mechanisms are suggested: 1) a thick delta of crystal-rich volcaniclastic sand, generated by interaction of voluminous subaerial pyroclastic flows with sea water, caused temporary shallowing and allowed subsequent pyroclastic flows to deposit welded ignimbrite across the shallowly submerged (or sub aerial?) top of the delta; 2) large allochthonous blocks of welded ignimbrite were transported in giant submarine sediment gravity flows, forming units of ignimbrite-block-bearing breccia. Rhyolite lava dome complexes also built up on the sea floor during the Comstock Formation stage. The lower part of the Comstock Formation (Lynchford Member) has an andesitic to dacitic provenance and the overlying Mount Julia Member is more silicic in character. Distinctive bedding-parallel alteration banding in the crystal-rich volcaniclastic sandstone facies is thought to have originated during diagenetic alteration and compaction of the vitric-rich ash matrix. In contrast to the Comstock Formation, the overlying Zig Zag Hill Formation records post-eruptive erosion and reworking of the subaerial to shallow marine source areas, resulting in the influx of wellrounded, polymict, epiclastic and reworked pyroclastic aggregates to the marine setting. The palaeogeographic setting for the Tyndall Group comprises a subaerial to shallow marine volcanic terrain adjacent to the sea. The central North Island and offshore Bay of Plenty in New Zealand, and the Grenada Basin and Lesser Antilles arc, are considered to be two modem geographic and volcanic analogues. The source magmas for the Comstock Formation were probably derived from melting of Proterozoic crust (Crawford and Berry 1992), and the Zig Zag Hill Formation probably formed in response to tectonic uplift in the source. In summary, the Tyndall Group is the submarine record of active volcanism (Comstock Formation) and subsequent erosion (Zig Zag Hill Formation) of a subaerial to shallow marine volcanic terrain that is not preserved.