Cheshire Basin
The Cheshire Basin is a late Palaeozoic and Mesozoic sedimentary basin extending under most of the county of Cheshire in northwest England. It extends northwards into the Manchester area and south into Shropshire. The basin possesses something of the character of a half-graben as its deepest extent is along its eastern and southeastern margins, where it is well defined by a series of sub-parallel faults, most important of which is the Red Rock Fault. These faults divide the basin from the older Carboniferous rocks of the Peak District and the North Staffordshire Coalfield. In terms of its architecture, the basin is sometimes considered to be divided into two sub-basins: the more southerly Wem–Audlem Sub-basin and the Sandbach–Knutsford Sub-basin to its northeast. The basin fill is mainly Permian and Triassic sandstones and mudstones, but it also incorporates economically important halite beds. The sequences referred to the Sherwood Sandstone Group and the overlying Mercia Mudstone Group are the thickest in England. An isolated outlier of Jurassic rocks occurs within the basin at Prees in north Shropshire. The basin is just one part of a wider complex of basins that includes the Worcester Basin, the Stafford Basin and the East Irish Sea Basin. These basins developed while the crust of this region was experiencing east–west tension during the Permian period. It is traversed by a series of largely north–south aligned normal faults, some of which help to define the Mid Cheshire Ridge and Alderley Edge, two upstanding areas within the Cheshire Plain. Tectonics The Permo-Triassic rocks of the Cheshire Basin are heavily faulted. The longer axis of the basin trends towards the NNE, being flanked on the west by the Carboniferous rocks of north Wales and to the east by the Pennine foothills. The dips in the Permo-Triassic rocks reflect the steady swing of the beds round the north-east edge rim of the Cheshire basin, except to the north of Alderley Edge, where a gentle anticlinal fold centred on Wilmslow plunges westward and is intersected by a number of north–south tension faults. Crustal extension controls the tectonic accommodation space available for sediments in rift settings and may be defined by the structural and depositional geometry of sedimentary successions observed on seismic data and the rate of subsidence through time as represented by the accommodation of sediment. The characteristic features of each are dependent on three variables: the time taken for deposition; the interplay between tectonics and eustasy and the lithology (thus facies) of the succession observed. The Sherwood Sandstone Group has been considered to represent a syn-rift phase of fluvial deposition throughout Europe, with the overlying Mercia Mudstone Group interpreted as the succeeding phase of deposition in an evaporitic seaway during post-rift thermal subsidence. More recently, however, there has been the recognition that it is the Mercia Mudstone Group which is seen to thicken markedly into faults imaged on seismic data rather than the Sherwood Sandstone Group. This work demonstrates the Mercia Mudstone Group to be a syn-rift phase of deposition, with the