Tidal sandbank evolution in the Lower Cambrian Salterella Grit

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Abstract
Synopsis: The Lower Cambrian Salterella Grit is a 0–15 m thick sequence of quartzarenites, siltstones and mudstones and represents an inner shelf tidal sandbank and sandwave complex. The bank facies consists of a coarsening-upward unit up to 15m thick, containing metre-scale cross-bedded sets enclosed by bounding surfaces that dip gently in a direction perpendicular to the palaeocurrent direction. Bank apron sandstones are thinner bedded with a greater proportion of mud-stones, an increased faunal diversity and bimodal palaeocurrents. The Salterella Grit banks were initiated following an abrupt regressive phase, the sediment being derived from local erosion of the underlying Fucoid Beds. During the subsequent transgression the shelf deepened, mean tidal velocities were reduced and the banks became moribund. Moribund bank facies are characterised by thin, highly burrowed sandstones which were deposited on the flanks of banks and in troughs between the banks. Further reduction in tidal currents resulted in the deposition of a thin sandstone sequence of storm origin in the troughs between banks. Nearshore facies are represented by smaller cosets which also show a vertical increase in bioturbation and decrease in thickness.