A late Middle Cambrian shallow-water trilobite fauna from the MT Read volcanics, northwestern Tasmania

Abstract

Near Middlesex Road, 9 km northeast of the Hellyer mine, northwestern Tasmania, a mineral exploration hole intersects a polymict volcaniclastic conglomerate of the Southwell Subgroup of the Mt Read Volcanics. At depths of 353 m and 364 m, fossiliferous limestone clasts within this conglomerate contain trilobites, including an unassigned agnostoid cephalon, Amphoton sp., Liopeishania sp., Menocephahlites(?) sp., Lisania(?) sp. and an unassigned member of the Dorypygidae, thus suggesting an age of Goniagnostus nathorsti Zone to early Lejopyge laevigata Zone (i.e. Late Middle Cambrian) range. This shallow-water fauna is unusual for the Tasmanian Cambrian, in which most faunas are found in deeper water shales and siltstones. The horizon from which the fossils come is stratigraphically close to a rhyolite with a SHRIMP zircon age of 503.2 ± 3.8 Ma; the usefulness of this figure in terms of the Cambrian time-scale is questioned.