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 3 53 m and 364 m, fossiliferous limestone clasrs within this conglomerate contain trilobites, including an unassigned agnostoid cephalon, Amphoton sp., Liopeishania sp., Menocephafites{I) 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. Lare 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 strarigraphically close to a rhyolire with a SHRIMP zircon age of 503.2 ±3.8 Ma; the usefulness of this figure in terms of the Cambrian rime-scale is questioned.

The Global Boundary Stratotype Section and Point (GSSP) of the Guzhangian Stage (Cambrian) in the Wuling Mountains, Northwestern Hunan, China

“…This estimate accords well with a mean SHRIMP age on zircons of 503.2 ± 3.8 Ma (Perkins and Walshe, 1993) for an interval probably equivalent to the Goniagnostus nathorsti Zone through the basal part of the L. laevigata Zone in the Southwell Sub-group of the Mt. Read Volcanics, Tasmania (Jago and McNeil, 1997). Encarnación et al (1999) provided dates from volcanic tuffs in the Taylor Formation, Antarctica, that provide broad support for an age close to 503.0 Ma for the base of the Guzhangian Stage.…”

Geochronology of the Cambrian: a precise Middle Cambrian U–Pb zircon date from the German margin of West Gondwana

“…Perkins & Walshe (1993) reported a SHRIMP 206 Pb– 238 U and an 40 Ar– 39 Ar date of 502.6±3.5 Ma that they termed ‘upper Middle Cambrian’ from the Tasmanian Mount Read Volcanics. However, this date is actually a composite based on a number of samples, which have been noted to lack a precisely reported stratigraphic or geographic provenance (Jago & McNeil, 1997, p. 87).…”

A middle Cambrian shallow water trilobite fauna from the Comstock Formation, near Queenstown, western Tasmania

“…It contains both agnostoid and polymerid trilobites from the Lejopyge laevigata Zone (Laurie et al 1995, p. 5). In the immediately underlying Southwell Subgroup, limestone clasts from within a polymict volcaniclastic conglomerate contain trilobites that suggest an age of Goniagnostus nathorsti Zone to low in the L. laevigata Zone (Jago & McNeill 1997), thus providing a lower limit for the age of the Tyndall Group in this area (Corbett & Vicary 2014).…”

“…suggest that the lower part of the Radfords Creek Group is equivalent to the Tyndall Group. Trilobite faunas from Sugarloaf Gorge described by Jago (1974Jago ( , 1976 have an age of low in the L. laevigata Zone (Laurie et al 1995).…”

“…The only exceptions are the late middle Cambrian trilobites described from limestone clasts within a volcaniclastic conglomerate north of the Hellyer Mine (Jago & McNeill 1997) and an undescribed late Cambrian assemblage from near New River Lagoon on the south coast of Tasmania. The fossils from the limestone clasts and those from the limestone lens at Comstock represent shallow water faunas, in contrast to the deeper water faunas from the shales and siltstones (Jago & McNeill 1997). Jago et al (1972) first reported these fossils in a preliminary note that contained no confirmatory taxonomic data.…”