Geology of Alderley Edge - Tectonics

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 fine grained nature of the sedimentary record at this time controlled by the prevailing arid climate. Such conditions were not conducive to the large-scale and rapid movement of sediments from the hinterlands raised by relative footwall uplift, thus the sediments are fine grained. The minor thickening of the Sherwood Sandstone Group into faults is interpreted to be a combination of minor extension in the early Triassic superimposed on thermal subsidence inherited from the important regional phase of extension in the early Permian. Analysis of the timing of fault growth indicates a larger proportion of fault-controlled, synsedimentary movement occurred during the mid-to-late Triassic (Mercia Mudstones) rather than the early Triassic (Sherwood).