Sedimentation of the southern part of the Grindslow shales and the Kinderscout grit

The aim of the research has been to determine the environments of deposition of the Grindslow Shales and Kinderscout Grit in an area north of the Derbyshire Dome. These units overly the Mam Tor Sandstones and Shale Grit, which are thought to be turbidites showing increasing proximity to scource. The Grindslow Shales had been interpreted as the slope down which the turbidity currents were generated. Little was known of the depositional environment of the Kinderscout Grit, though shallow water conditions had been suggested. The sequence, from the base of the Mam Tor Sandstones to the top of the Kinderscout Grit, falls within the zone R1c of the Namurian.
The outcrops of the Kinderscout Plateau, Derwentdale and Longendale have been mapped and numerous sections measured.
In all, 17 facies have been recognised, described and interpreted in hydrodynamic terms on the basis of their internal evidence. Three silts tone facies and one of silty sandstone are thought to be largely deposited from suspension, while fine sandstone, showing ripples and cross-lamination, is interpreted as showing lower flow regime conditions. Parallel laminated fine sandstone is interpreted as an upper flow regime deposit. Facies 8, Massive Bedded Coarse Sandstone, is not, in the first instance, interpretable in hydrodynamic terms. Facies 9, Parallel Bedded Coarse Sandstone, is interpreted as an upper flow regime deposit, while Facies 10, Medium Scale Cross-Bedded Coarse Sandstone is attributed to the lower flow regime along with Facies 11, Medium Scale Cross-Bedded Fine Sandstone. Facies 12, Large Scale Cross-Bedding, consists of single tabular sets of cross-bedding up to 40 m. thick, mainly of very coarse sandstone. These are ascribed to the advance of delta-like sedimentation units into bodies of standing water. Other facies, of minor importance,are Sharp-Based Clean Sandstone Beds, Orthoconglomerates, Seat Earths and Coals, Mudstone Melange and Turbidite-like Sandstones.
Three types of trace fossil have been recognised, though it is concluded that they are probably of little environmental significance.
Five facies associations are recognised, in which facies succeed one another with variable predictability.
The Major Channel Association involves Facies 8, 9 and 10, Massive, Parallel and Cross-Bedded Coarse Sandstones, resting in channels up to 40 m. deep. Other details of channel geometry are not clear, though widths of 300-400 m. have been calculated. There is a preferred upwards facies sequence within the channels of Erosion Surface &rightarrow; Facies 8 &rightarrow; Facies 9 &rightarrow; Pacies 10. From hydrodynamic considerations of the sequence, it is suggested that the Facies 8 Massive Bedded Sandstone was deposited by currents flowing in the upper part of the upper flow regime, the whole channel sequence representing a waning flow.
The Large Scale Cross-Bedding Association involves Facies 12 and 10, the medium scale cross-bedded sandstone being separated from the underlying Large Scale set by an erosion surface. It is suggested that the Association represents delta cones which advanced into bodies of standing water, subsequent to sudden deepening.
The Fluviatile Channel Association is a fining upwards sandstone sequence with a horizontal erosion surface at its base. Facies 10, Medium Scale Cross-Decided Coarse Sandstone forms the bulk of the association, though thin beds of Orthoconglomerate and mudflake conglomerate may occur above the erosion surface. In the top of the association, the cross- bedded sandstone passes into parallel or ripple laminated fine sandstone and sometimes into seat earths and coals. The association is interpreted as the result of the lateral migration of a fluviatile channel, probably of the meandering type.
The Slope Association, which has limited stratigraphical position, is a broadly coarsening upwards silts tone and silty sandstone sequence. Thin turbidites occur in the base and burrows become abundant towards the top. It has occasional large channels, filled mainly with turbidites. The association is interpreted as the prograding slope of a coastline receiving abundant sediment supply.
The Interdistributary Complex Association is a random sequence of predominantly fine grained facies, showing a variety of flow conditions. It is interpreted as forming in shallow water conditions, probably in interdistributary areas of a delta top.
An analysis of the relationships of the facies associations throughout the area gives the following generalised sequence:-
'Gracile' Marine Band.
Fluviatile Channel Association
Slope or Interdistributary Complex
Upper Kinderscout Grit
Butterley Marine Band
Fluviatile Channels and
Interdistributary Complex Association
Large Scale Cross-Bedding
Lower Kinderscout Grit
Major Channels
Interdistributary Complex Association
Grindslow Shales
Slope Association
From a consideration of this sequence, it is concluded that the Grindslow Shales represent the main southwards advance of the delta slope and delta top environments across the area. The major channels which erode into the Interdistributary Complex Association sediments of the top of the Grindslow Shales were non-migrating distributaries which underwent frequent diversion and cut-off. They had high flow regime currents and were possibly associated with a period of eustatically falling sea level. A subsequent eustatic rise led to the ponding up of sediment on the delta, top with the formation of delta cones. Above the delta cones, meandering river channels and interdistributary areas were the main features of the palaeogeography. The Butterley Marine Band and its overlying unit of Slope Association suggest a further eustatic rise of sea level. The Upper Kinderscout Grit is thought to be the deposit of laterally migrating rivers.
Comparison with the succession further north gives additional evidence of eustatic changes of sea level and suggests ways in which the palaeogeography might have responded to these. It is suggested that the turbidity currents in the basin possibly originated as high flow regime traction currents in the major channels on the delta top, at times of flood.