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Cyclic sedimentation in the shallow marine Upper Permian Kennedy Group, Carnarvon Basin, Western Australia

2004; Elsevier BV; Volume: 172; Issue: 1-2 Linguagem: Inglês

10.1016/j.sedgeo.2004.08.004

1879-0968

Helen Lever,

Geological and Geophysical Studies

The Upper Permian Kennedy Group of the Carnarvon Basin, Western Australia, was deposited while the Merlinleigh Sub-basin was undergoing thermal subsidence, and the global climate was warming from the Carboniferous–Permian glaciation to the Mesozoic Greenhouse conditions. The Kennedy Group comprises siliciclastic sedimentary rocks, dominated by sandstones in the lower part, with coarsening-up cycles from mudstones to very coarse sandstones and granule conglomerates in the upper part. Cycles were observed and logged in the field, and although many types of cycles were found, three main motifs describe most of the cycles. Cycle motifs are defined based on the dominance of the various facies: Mooka motif cycles are dominated by fine-grained and bioturbated facies (interpreted as deposition during relative sea-level highstand) with only thin laminated or cross-bedded sandstones (interpreted as deposited during relative sea-level fall); Binthalya motif cycles are dominated by laminated and cross-bedded sandstones; and Coolkilya motif cycles consist of alternating laminated and bioturbated beds. A hierarchy of cycles was observed in the field, and this correlated well with spectral analysis of logged parameters. Spectral analysis of section log data and data sets corrected for the effects of compaction and sedimentation rates detected regular cyclicity. Geochronology of the Kennedy Group is not well enough constrained to allow the cycle periods in time to be calculated, but ratios of the different scale cycle thicknesses correlate well with ratios of the Milankovitch orbital cycles that have been calculated for the Permian. The presence of regular cyclicity and even bed thicknesses across large distances are not consistent with tectonic or autocyclic models of cycle formation. The cycles are more likely to have been caused by fluctuating eustatic sea levels, perhaps enhanced by changing amounts or seasonality of precipitation. Both sea levels and climatic fluctuations would ultimately be controlled by the Milankovitch orbital cycles.