DELTAIC SEDIMENTATION IN THE ROACHES GRIT AND ASSOCIATED SEDIMENTS (NAMURIAN R 2 b) IN THE SOUTH-WEST PENNINES

Abstract
SUMMARY: The Roaches Grit and related underlying sediments form a thick deltaic sequence which accumulated in north Staffordshire during the late Marsdenian. The sequence represents the fourth and last major stage of fill in the Namurian north of England basin and the first widespread phase of shallow-water delta top conditions in north Staffordshire. The delta prograded from the south-east, but the sediments were of northerly derivation, having been diverted into the basin to the east of the present area of outcrop. The sequence is subdivided into three facies associations representing deep-water turbidite fan, delta slope and delta top environments. The deep-water association is restricted in distribution and outlines the limits of the deep-water basin existing prior to the incoming of the delta. Faulting on the upper delta slope allowed down-slope escape of sediment, some eventually reaching the basin floor in turbidity currents, the rest being trapped on the downthrow side of growth faults. River-generated density currents also flowed down the slope depositing rippled sands with bivalve resting traces. The delta top is dominated by the fills of large, mutually-erosive fluvial channels forming sheet sandstones of wide lateral extent. At the Roaches, systematic offsetting of younger channels towards the north suggests tectonic control of sedimentation. Within the channels a variety of bedforms existed and structures within their deposits suggest a regime with a markedly fluctuating discharge.