The Late Cambrian to Early Ordovician sequence on the Denison Range, southwest Tasmania

Abstract

A well-exposed middle Late Cambrian to Early Ordovician clastic sequence, herein referred to as the Denison Subgroup, unconformably overlies Middle Cambrian rocks on the Denison Range and is conformably overlain by Ordovician limestone. Four formations are recognized. The basal Singing Creek Formation (720 m) consists of interbedded siltstone, quartzwacke turbidites, siliceous fine conglomerate, and slump sheets, and contains an abundant Franconian trilobite-brachiopod fauna. It grades upwards into the Great Dome Sandstone (510 m), a shallow marine-deltaic-fluvial sequence of crossbedded quartz sandstone, siltstone and fine conglomerate, with a sparse fauna of probable Late Cambrian age. Above this the Reeds Conglomerate (1560 m) comprises largely non-marine pink siliceous conglomerate and cross-bedded sandstone, probably deposited on alluvial fans. This grades into the marine Squirrel Creek Formation, comprising a lower sandstone member (150 m) with worm burrows and gastropods, a siltstone- limestone member (150 m) with an abundant Early Ordovician shelly fauna, and an upper sandstone member (300 m). The Denison Subgroup is approximately equivalent to the pre-limestone part of the Junee Group in the 'original area of Lewis, and to the Owen Conglomerate of western Tasmania.