Basin evolution of late Cambrian – Ordovician sediments, Southern West Coast Range, Tasmania, Australia.

Abstract

The stratigraphic architecture and evolution of the Late Cambrian–Early Ordovician Owen Group is documented in this research project. An integrated approach was used to ascertain the basin evolution,with emphasis on the stratigraphic build-up of the Owen Group in the Mts Jukes–Darwin-Sorell-Strahan area of the southern West Coast Range, Tasmania, Australia. This included field mapping,measurement of detailed stratigraphic sections, sedimentary facies analysis, petrography, and Scanning Electron Microscopy, along with Energy Dispersive X-ray Spectroscopy and geochemical analyses. The thesis presents an overall source to sink understanding from regional to micro-nano scale processes involved in the deposition, stratigraphic build-up and diagenesis of the Owen Group. The depositional settings during the Late Cambrian–Early Ordovician were markedly different from their present day analogues, and the Owen Group sediments provide excellent exposures to facilitate a study of pre-vegetation, warm, humid climate, braided fluvial depositional settings. In the area studied the group is comprised of conglomerate, sandstone and minor mudstone successions that are markedly different compared to other areas in western and northern Tasmania. The sediments were deposited in half-grabens formed during Late Cambrian extensional tectonics, with each depocenter containing a unique stratigraphy reflecting different depositional styles. The most remarkable feature of the Jukes-Darwin area stratigraphy is the occurrence of two, fining up, alluvial–marine cycles, separated by an unconformity. The new, two fold classification presented here divides the siliciclastics into lower and upper sequences. The overall stratigraphy is subdivided into 17 informal mappable units, 13 units (L1–L13) in the lower sequence and 4 units (U1–U4) in the upper sequence. Each sequence begins with sediments deposited in a braided fluvial system and ends with sediments deposited in an intertidal to shallow marine environment. However, in the Mt Sorell–Mt Strahan depocenter the Owen Group is comprised only of a thick, monotonous, conglomerate-dominated sequence deposited by sheet flow processes. The sediments are mainly comprised of quartz, mica, lithic grains (volcanic, metamorphic and sedimentary), clays, zircon, tourmaline, leucoxene, rutile, glauconite and chert grains. These were supplied by a Proterozoic Tyennan metamorphic and a Middle Cambrian Mount Read Volcanic provenance and have undergone severe diagenesis, including quartz overgrowth, iron coating, and haematite, chlorite and sericite alteration. Multi-phase haematite-related diagenesis is common and late stage hydrothermal fluid alteration assemblages have severely altered and coloured the sediments. Geochemical analyses of the Owen Group show that the average composition of sandstones includes very high SiO₂ (92.72%), low Al₂O₃ (3.34%) and Fe₂O₃ (1.71%), very low K₂O (0.90%) and MgO (0.15%), while CaO and Na₂O are extremely low (