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1 School of Geosciences, Queen’s University of Belfast, Belfast BT7 INN, UK (e-mail: a.ruffell{at}qub.ac.uk)
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 dependant 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 Mst Gp which is seen to thicken markedly into faults imaged on seismic data rather than the Sherwood Sandstone Group. This work demonstrates the Mercia Mst Gp 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 Sst Gp 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 Mst Gp) rather than the early Triassic (Sherwood Sst Gp).
Key Words: Triassic • palaeoclimate • rift basins • subsidence
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