The Bristol Channel basin was developed as an early Mesozoic half-graben, with the Bristol Channel fault zone (BCFZ) representing the principal down-to-the-south basin controlling structure. The basin was inverted during the Tertiary. This paper illustrates the application of structural analysis to the northern and southern margins of the Bristol Channel basin, where on shore medium- to small-scale structures provide analogues for the BCFZ. Throughout the outcrop four distinct structural mechanisms and associated fracture systems can be recognised. i) The oldest are normal faults, in many cases resulting from the reactivation of underlying WNW-ESE to NE-SW Late Palaeozoic Variscan thrusts. These are linked by N-S transfer faults and associated with WNW-ESE to ENE-WSW extensional veins. Evidence for synsedimentary development of both normal faults and extensional veins has been found in the Triassic and Lower Liassic sequences. Palaeostress analysis suggests an approximately NE-SW oriented σ 3 . ii) During inversion the normal faults acted as buttresses, forming WNW-ESE and younger NW-SE trending folds. Associated thrusts and oblique- to strike-slip faults reactivated the earlier transfer faults. The orientation of σ 1 was NE-SW during this deformation. iii) A regionally consistent system of NE-SW striking extensional veins formed at the close of the strike-slip inversion event. iv) Following primary inversion, lateral escape structures formed against the buttresses, with the development of oblique- and strike-slip faults in a variable and locally controlled stress field. Fracture porosity determined at sites throughout the outcrop show highest levels, up to 20%, formed in association with normal faults during the rifting event, but also significant amounts developed during the inversion event. Veins associated with strike-slip faulting of the latter give average porosities of 6.5%, whilst the later inversion-related extensional veins give average porosities of 0.8%. All these porosities show a high degree of directional permeability. It is argued that the oil generation window was reached during burial in the later stages of the rifting event, and that rapid polyphase fluid discharge from over-pressured fracture-bounded compartments allowed hydrocarbon migration into normal fault-related fracture porosity to form traps. The development of fracture porosity during inversion produced a long-lasting directed permeability and allowed many of the traps to drain. Only those traps associated with normal faults not directly affected by the inversion will have survived. A strategy to discover such a play requires a well targeted and detailed structural study.
The Watchet-Cothelstone-Hatch Fault (WCHF) comprises a system of northwest-southeast-trending basement and cover faults that are traceable for at least 40 km from the Bristol Channel, southeast into the western Wessex Basin. The west-dipping WCHF displays outcrop and seismic evidence of a complex movement history involving early (Variscan) and late (?Cretaceous or Tertiary) strike-slip, separated by phases of normal extension. In the Variscan basement of North Somerset, the WCHF shows a 14 16km dextral offset to Devonian markers and Variscan fold axes, and can be linked to similar trending faults in South Wales. Offsets to early Permian (and possibly late Carboniferous) rifts indicate a dextral transfer of north-south extension from the Crediton and Tiverton troughs, across the WCHF and into the Wessex Basin. In Mesozoic cover, the WCHF swings to become part of the east-west Central Bristol Channel Fault Zone in the north and Lopen Fault Belt (western Wessex Basin) in the south. This geometry reflects a present-day preservation of a later sinistral movement. The structure of juxtaposed Palaeozoic and Mesozoic successions along the WCHF provides an analogue for what might be expected at the basement-cover contact beneath the Wessex Basin. The existence of Mesozoic cover to the WCHF most likely explains why no Cenozoic pull-apart basins were formed, unlike the Sticklepath Fault (in granites and metamorphic rocks) to the southwest.
Experiments are described that test how tide and geological structure affect saline intrusion beneath an area of coastal wetland in Hampshire, southern England. Resistivity tomography and time-dependent ground conductivity surveys were carried out at two closely located survey sites. Resultant maps and sections show clear geophysical anomalies that can be attributed to tidal saline intrusion and to the position of geological structures. Results suggest that migration of saline groundwater is, in places, out of phase with the tidal cycle, and is concentrated along sub-surface tidal channels, gravel aquifers and depressions in Chalk bedrock topography. We conclude that the seawater-fresh-water boundary in the studied wetland environment is complicated by shallow-level saline flow along geologically definable routes within the Quaternary deposits above the less permeable Chalk bedrock.
The Upper Carboniferous Kent Coalfield lies concealed beneath various Mesozoic formations, its southern areas lying about 20 km north of the commonly accepted position of the main Variscan Deformation Front. However, despite intense intra-coal deformation, the existing literature is ambivalent about compressional Variscan features in Kent, the general view being that coal deformation is largely the product of the depositional environment. The main deformation is interpreted here as the result of Variscan compression, the structural style being imposed by the sandstone-dominated lithology. This conclusion is necessitated by the regularity of deformational structures revealed by mine workings, and supported by coal sequence irregularities suggestive of thrusting, especially in the lower Westphalian strata, all of which is paralleled in parts of the South Wales Coalfield. The Kent data indicate that, as in South Wales, a zone of thrusting many tens of kilometres wide lies in advance of the main deformation front. Structural trends are consistent with an overall swing in the front from east–west across much of central-southern England, to more northwest–southeast across northeastern France. This swing may represent a transpressional transfer zone, within which stress deflection and block rotation produced thrust vergence oblique to the overall direction of maximum compression.
The deep structure of the Vale of Glamorgan is investigated using recently acquired seismic data. Reflection sections from a commercial seismic survey in the Vale enable the Carboniferous Limestone, Old Red Sandstone and Silurian sequences to be mapped in the subsurface. The Old Red Sandstone sequence thins westwards under the southern part of the Vale, and in the southwest corner of the Vale the base of the Old Red Sandstone is interpreted to overstep across the underlying Silurian sequence in an analogous manner to its regional overstep in west Wales. The Precambrian basement surface is not clearly represented on the sections but may coincide with the base of a reflective sequence observed in places along one of the reflection lines. Basement depth estimates from the reflection sections are compared with estimates derived from a new time term analysis of data from long seismic refraction lines. Basement depths remain uncertain to 1–2 km but appear to be greater under the central part of the Vale than under the western part and along the south coastal zone. A linked system of Variscan forethrusts is traced to outcrop in the core of the Cowbridge anticline but none involves major displacement. The Ty'n-y-Nant-Moel Gilau fault system of the South Wales coalfield is shown to be a component of the linked fault system and its current net extension is attributed to Mesozoic reactivation of a Variscan thrust involving only limited displacement. The individual thrusts appear to connect to a basal thrust that may extend northwards into or under the South Wales coalfield. The amount of displacement along the basal thrust cannot be determined reliably but it may exceed 10 km and involve large scale repetition of Palaeozoic sequences under the Vale.
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