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.

Bottomset Architecture Formed In the Troughs of Dunes and Unit Bars

“…The Roaches Grit, a 15-90-m-thick deposit in the North Staffordshire Basin, UK, consists of coarse-grained feldspathic fluvial channel bodies and finer interchannel deposits that formed in a paralic setting near the Namurian equator (Jones 1980;Golonka 2002). The fluvial deposits overlie finer siltstones and mudstones, some of which are turbidites, formed in a delta slope setting (Jones 1980).…”

“…The fluvial deposits overlie finer siltstones and mudstones, some of which are turbidites, formed in a delta slope setting (Jones 1980). The North Staffordshire Basin formed in the Late Devonian to early Carboniferous by north-south extension, and was in a phase of thermal subsidence by the Namurian (Leeder 1982(Leeder , 1988Waters et al 2008).…”

“…4). Jones (1980) records 11 channel fills in erosional contact, one being at least 600 m wide. They have been interpreted as forming in low-sinuosity, 1-km-wide channels in delta-top settings (McCabe 1977;Jones 1979Jones , 1980Jones and McCabe 1980).…”

“…Jones (1980) records 11 channel fills in erosional contact, one being at least 600 m wide. They have been interpreted as forming in low-sinuosity, 1-km-wide channels in delta-top settings (McCabe 1977;Jones 1979Jones , 1980Jones and McCabe 1980). The fluvial sandstone bodies consist of three major lithofacies: faintly laminated coarse sandstone (up to 10 m thick), ''giant'' cross-stratification (sets up to 20 m thick), and overlying those, co-sets of thinner tabular cross-stratification.…”

“…The fluvial sandstone bodies consist of three major lithofacies: faintly laminated coarse sandstone (up to 10 m thick), ''giant'' cross-stratification (sets up to 20 m thick), and overlying those, co-sets of thinner tabular cross-stratification. The tabular sets, described below, are 0.5-2 m thick and are tens to hundreds of meters long, arranged in co-sets 10-35 m thick (Jones 1980). These tabular co-sets are interpreted by McCabe (1977) and Jones (1979) as ''sand wave'' deposits that formed on bar tops (the bars forming the ''giant'' cross-bed sets) and in shallow channel areas.…”