Triassic sandy braidplain and aeolian sedimentation in the Sherwood Sandstone Group of the Sellafield area, west Cumbria

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Abstract
SUMMARY: During the Permo-Triassic the Sellafield area formed part of the eastern margin to the East Irish Sea Basin. Basin-margin alluvial fan, shallow marine and coastal mudflat sedimentation created a locally thick Permian succession. Triassic sediments are represented by the Sherwood Sandstone Group, which comprises the St Bees, Calder and Ormskirk sandstone formations. The basal few metres of the St Bees Sandstone are characterized by sheetflood sandstones, interbedded with playa-mudflat deposits. These pass upwards into thick, multistorey fluvial sandbodies, formed on sandy braidplain. Rare aeolian units are interbedded with the channel facies. The top of the St Bees Sandstone is sharply defined and represents the regional abandonment of the fluvial system. The overlying Calder Sandstone Formation is coarser grained and dominated by aeolian sediments, with dune and interdune facies recognized. These sands were blown into the area from the east. Minor episodes of fluvial reworking punctuated aeolian deposition and the top of the Calder Sandstone is represented by a fluvially deposited unit. The lower part of the overlying Ormskirk Sandstone is of aeolian facies and can be distinguished from the Calder Sandstone by its finer grain size. The two formations can also be differentiated using geophysical logs, as aeolian and fluvial sandstones have diagnostic geophysical log characteristics allowing recognition of different sedimentary facies associations.