Terrestrial-marine transition in the Cambrian rocks of Kangaroo Island, South Australia |
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| Authors: | B DAILY P S MOORE B R RUST |
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| Institution: | Department of Geology, University of Adelaide, Adelaide, S. Australia 5001;Department of Geology, University of Ottawa, Ottawa K1N 6N5, Canada |
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| Abstract: | The upper part of the Lower Cambrian succession in northeast Kangaroo Island comprises three interbedded facies associations. The fine-grained association is composed of siltstone, mudstone and minor sandstone. It contains flat lamination and abundant ripple cross-lamination which shows bipolar palaeocurrents, and occurs in combinations of flaser bedding, lenticular bedding and wavy lamination. Although body fossils are relatively rare, trilobite traces and desiccation cracks are common, and the association is interpreted as a predominantly subtidal to intertidal deposit. The conglomerate facies association contains horizontally bedded cobble to boulder conglomerate, with subordinate trough cross-stratified coarse sandstone to granule/pebble conglomerate. Fabrics and structures in the coarse conglomerates are consistent with alluvial transport (stream and debris flow), but not beach deposition. The conglomerate association is attributed to tectonic uplift and erosion of a Precambrian-Lower Cambrian succession developed adjacent to the present north coast of Kangaroo Island. Southward progradation of an alluvial fan complex occurred across east-west oriented tidal flats on which limited wave activity reworked sand and fine gravel, but not coarser material. The sandstone facies association mainly comprises trough cross-stratified and plane-laminated sandstone, the latter with current lineation predominantly sub-parallel to the east-west shoreline. Trough cross-stratification is ascribed to onshore waves and longshore currents, and current lineation to predominantly shore-parallel tidal currents, augmented by longshore drift and storm surge. Tectonic movements gave rise to cycles of transgression and regression as tidal and alluvial processes dominated alternately. |
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