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Subject: geochemistry × Subject: petrology × Subject: Victoria × Author: Duddy, Ian Ross × Start date: 1983 × End date: 1983 ×

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    The geology, petrology and geochemistry of the Otway formation volcanogenic sediments
    Duddy, Ian Ross ( 1983)
    The geology, petrology and geochemistry of the Early Cretaceous Otway Formation have been investigated in detail and used to determ ine the nature of the source rocks and to develop a model for the diagenetic and low-grade metamorphic readjustments. The fluviatile Otway Formation was deposited in continental rift grabens that stretched some 1000 km along the southern coast of Australia during the Early Cretaceous. The main areas of deposition in the Otway, Gippsland and Bass Basins contain an estimated 100,000 cubic kilometres of detritus. The major part of this detritus was derived from pyroclastic material which has been shown by the fission track dating studies to have been derived from contemporaneous volcanism. The pile of volcanogenic material comprising the Otway Formation is at least 3 to 4 km thick in the main basins. The sediments are entirely non-marine and were deposited by large scale multichannel streams cut in extensive floodplains. The streams deposited thick multistorey channel sandstones in sheet-like bodies and a diverse spectrum of overbank mudstones and fine-grained sandstones. The complex channel sandstones fine upwards but have numerous erosional breaks indicating repeated flood cycles. Whereas the channel deposits have internal features consistent with braided stream channels the overall system has a large proportion of floodplain which was been considered in the past to have been a feature of meandering channels. The oversupply of volcanogenic detritus is considered to have been responsible for the development of the multiple channel depositional system in a climate of high seasonal rainfall. Whole rock chemical analyses of all lithologies in the sedimentary suite, recalculated i.nto a set of normative minerals, have proved useful in the distinction and description of sedimentary rocks in general. P20S was found to be useful for the identification of Early Cretaceous soil forming processes. The study of the chemical composition of detrital minerals has demonstrated the usefulness of this approach in the identification of the nature of the source magmas of volcanogenic sediments. For the Otway Formation, analyses of clinopyroxenes, amphiboles, feldspars and sphene in particular, have shown that high potassium dacitic to shoshonitic volcanism dominated during Early Cretaceous rifting. The new data on the geology and mineralogical and chemical features of the Otway Formation have application to the study of diagenesis and low-grade metamorphism in volcanogenic sediments in general. (From Abstract)