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Cyclicity in the Permian Kennedy Group, Carnarvon Basin, Western Australia

Helen Lever (Earth Sciences)

The purpose of this project is to analyse the sediments in the Permian Kennedy Group exposed in the Merlinleigh sub-basin of the Carnarvon Basin in the Kennedy Range, Western Australia. The Kennedy Group sediments are sandstones and mudstones, which displays cyclicity of facies within the three formations that make up the Kennedy Group. The aim is to quantify this variation, and to try to determine the frequencies of the observed cyclicity, using detailed measured sections of the Kennedy Group exposed in the Kennedy Range.

A model of sedimentation and the causes of facies variation will be developed to aid in interpretation of cyclicity, and to seek correlations between the observed patterns of cyclicity and known causes of cyclicity, such as sediment source changes, sea-level changes, and Milankovitch driven climate cycles. An end result will be a summary of relative sea-level fluctuations through the deposition of the Kennedy Group. Part of this analysis will require examination of the effectiveness of various methods of analysing stratigraphic sequences for periodic changes. Many methods have been developed by other workers in the field of cyclostratigraphy, and several different methods will be used in this project. A detailed examination of the facies that occur in the Kennedy Group is needed to determine the environments of deposition, and especially to quantify deposition rates and, if possible, the amplitude of sea-level variations. Palynological and paleontological criteria will be used to provide age constraints on the sections. The mapping of eustatic and relative sea level cycles through time has the potential to be highly useful for dating sedimentary successions, and for correlating sections across lenghty distances.

The data used in this thesis was obtained by detailed measurement of sections through the outcrop of the Kennedy Group in the Kennedy Range, Western Australia. Twenty-nine sections were measured in the recent field season. Parameters measured or noted on the sections included grain size, structures, traces, colour, composition and strength of gamma radiation. These parameters will be used to develop a model of facies and therefore environments of deposition for the Kennedy Group. Graphs of the variation of grain size, facies type (a proxy for of depth of deposition) and gamma logs will be analysed as time series, to determine cyclic components. Spectral analysis, time-frequency analysis, and wavelet analysis have been used to analyse the sections. As well as outcrop measurements, available well logs and cores held by the Western Australian Geological Survey were examined, the collected well logs will be digitised and analysed as for the outcrop data. The cores held by the Survey are not in good enough condition to measure accurately, or are missing. This situation is unlikely to improve over the next few years as the Core Stores are undergoing a major reshuffle. Palynology has proven useful in the Carnarvon Basin for correlation, as macrofossils are rare in the sequence. The few macrofossils samples taken will be used for age dating if possible, and palynological samples were taken from muddy sections (where palynomorphs will be more concentrated) and will be analysed to try and constrain the age of the sequence.