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

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.

Architectural analysis of a Triassic fluvial system: The Sherwood Sandstone of the East Midlands Shelf, UK

“…Channel fills with erosive bases are accompanied by lateral and mid-channel bar forms showing a mixture of lateral and downstream accretion. A braided river interpretation is concordant with observations by other authors for the lower part of the Sherwood Sandstone elsewhere in the UK and Ireland: Northwest England (Burley, 1984;Jones and Ambrose, 1994), East Irish Sea Basin (Cowan, 1993;1993b;Meadows and Beach 1993a), Southern…”

Chirotherium barthii Kaup 1835 from the Triassic of the Isle of Arran, Scotland

“…The base of the Mercia Mudstone Group in Arran may be isochronous with part of the Sherwood Sandstone Group in the Vale of Eden and Solway Firth (Lovell 1983). This is consistent with a northward migration of the St Bees Sandstone Formation (Jones andAmbrose 1994, Holliday et al 2001). Correlation between the Triassic basins of Scotland, England and Ireland is very difficult due to the range in thicknesses and lack of continuity of lithological units (Taylor et al 1971), however, Lovell (1983 showed the Rathlin, Arran and Ulster Basins as being interconnected.…”

Heavy-mineral, mineral-chemical and zircon-age constraints on the provenance of Triassic sandstones from the Devon coast, southern Britain

“…This northward palaeocurrent appears to extend north even as far as the Cumbrian coast, on the NW margin of the East Irish Sea Basin (Jones & Ambrose, 1994).…”