The Mount Read Volcanics and Cambrian‐Ordovician relationships at Queenstown, Tasmania

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Abstract
The Cambrian Mount Read Volcanics at Queenstown can be subdivided into three major units: (i) a central belt of quartz‐keratophyric lavas, consisting mainly of quartz and feldspar porphyries with lenses of massive potassic lava along the axial zone; (ii) a flanking belt of acid pyroclastic rocks which interfinger with sedimentary sequences of the Dundas Trough to the west and with the central lavas, and which are here termed the Queenstown Pyroclastics; and (iii) an unconformable Late Cambrian sequence of volcanic and sedimentary rocks here defined as the Tyndall Group. Important minor units are quartz‐feldspar porphyry and hornblende‐pyroxene porphyry intrusives and some newly‐discovered granitic bodies. The Tyndall Group includes the Jukes Formation of earlier workers and a lower formation, which is missing in some areas, of acid tuff, breccia, and shale defined as the Comstock Tuff. A fossiliferous limestone occurs at the unconformable base of this group at Comstock, and gives a minimum age for the Jukesian Movement at Queenstown as late Mesial or early Late Cambrian. The group is conformable with the Lower Ordovician Owen Conglomerate in most areas, but near Queenstown itself is truncated unconformably by a correlate of the Upper Owen. Westward restriction of Owen Conglomerate deposition by the Great Lyell Fault until the time of the Haulage Movement in the Upper Owen, and erosion of the Tyndall Group west of the fault during this period, seems the best explanation for this unconformity.

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