The geology of the St. Helens-Scamander area, Tasmania

Abstract

The Scamander Slate and Quartzite consist essentially of a succession of stratified slates, quartzites and sub-greywackes with a few siltstone bands, which probably form part of the Mathinna Group. They contain primitive plants (Hostimella (?) and are regarded to be of Siluro-Devonian age. They are intruded by a Devonian (?) quartz monzonite batholith, and possibly at a later date by St. Marys Porphyrite, which may however prove to be a dacitic fiow. At present their structure is confusing; close parallel folding occurs which is overfolded from the west and complicated by numerous minor fault breaks. Major faulting in the area occurred in lower Tertiary, isolating the coastal plain physiographic unit from the hinterland. The Permian System which follows unconformably has been divided into the following six formations:- 1. Sisters Granule Conglomerate. 2. Lohrey's Gully Sandstone and Calcareous Mudstone. 3. Enstone Park Limestone. 4. Binn's Gully Mudstone. 5. German Town Tillite. 6. Rays Hill Arkose. The structure of these sediments is controlled by the Mt. Nicholas dolerite sill which intrudes them, and the concomitant faulting developed at the time of its emplacement in the Jurassic. Lower Tertiary faulting and sedimentation during the Cainozoic resulted in the development of Tertiary wash covering the coastal plain to 350 feet above sea level, and probably the alluvial tin deposits at st. Helens were formed at this time. Economic considerations reviewed include potential tin, tungsten, and copper deposits.