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The Woolhope and Usk Basins: Silurian Rift Basins Revealed by Subsurface Mapping of the Southern Welsh Borderland

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The Woolhope and Usk Basins: Silurian Rift Basins Revealed by Subsurface Mapping of the Southern Welsh Borderland Journal of the Geological Society, London, Vol. 154, 1997, pp. 209–223, 10 figs, 2 tables. Printed in Great Britain The Woolhope and Usk Basins: Silurian rift basins revealed by subsurface mapping of the southern Welsh Borderland A. J. BUTLER1, N. H. WOODCOCK1 & D. M. STEWART2 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK (e-mail: [email protected]) 2Sovereign Exploration Limited, 60 Grays Inn Road, London WC1X 8AQ, UK Abstract: Subsurface mapping of the southern Welsh Borderland has utilized over 600 km of seismic data and three exploration boreholes to investigate the subsurface of this classic geological area. The survey reveals two Silurian basins, each about 400 km2 in area, in the western part of the Midland Platform. The Woolhope Basin underlies the Silurian inlier of the Woolhope Anticline. The basin fill is bounded to the northwest by faults of the Neath Disturbance, and was also influenced by WNW-striking extensional growth faults. The fill thins eastward towards the Malvern Line. Lower Silurian sequences thin southward before thickening again, particularly across WNW-striking faults. The resulting Usk Basin, underlying the Usk Inlier, has ill-defined margins, but some additional N–S fault control. The Fownhope-1 and Usk-1 wells indicate that rapid subsidence occurred in both basins during Rhuddanian and Aeronian (Early to Mid-Llandovery) time. Modelling of this subsidence, assuming the finite duration lithospheric extension model, yields stretching factors of between 1.3 and 1.6. The later part of the fill to both basins is marine, but red conglomerates, sandstones and mudstones in their Early Llandovery intervals suggest continental fluvial environments in the initial stages of basin development. The Woolhope and Usk Basins show that the western sector of the Midland Platform rifted during Early to Mid-Llandovery time, perhaps during regional dextral transtension. The rifting is synchronous with, and perhaps genetically related to, an episode of mafic volcanism in the southwest part of the platform. However, this rifting apparently predates the Late Llandovery (Telychian) fault-controlled subsidence seen in the Welsh Basin to the northwest. Keywords: Llandovery, Silurian, Welsh Borderland, rift zones, basins. The Early Palaeozoic geology of Wales and its Borderland has have documented small Cambrian to Ordovician rift basins on usually been described on a template of three palaeogeographic the platform edge and a Tremadoc (Early Ordovician) basin areas: the Welsh Basin, flanked to the northwest by the beneath the N–S Worcester Graben. Thorpe et al. (1993) and Irish Sea Platform and to the southeast by the Midland Pharaoh et al. (1993) have described Late Ordovician lampro- Platform (Fig. 1 inset). The basin was an area of more persist- phyre intrusions in the northern half of the platform, consist- ent crustal subsidence, thicker and deeper-marine sediments ent with limited extension and lithospheric melting beneath it. and, during Ordovician time, of voluminous volcanism. The More plentiful Early Silurian volcanics, preserved on the NE-striking faults of the Welsh Borderland Fault System southern half of the platform, have been ascribed to exten- mark a rapid southeastward transition to the thinner, laterally sional tectonics related to the Rheic ocean to its south and spatially discontinuous sedimentary sequences of the (Pharaoh et al. 1991). Midland Platform. The platform reaches eastwards to a con- This study focuses on the western triangle of the platform, cealed NW-striking boundary with further Early Palaeozoic west of the N–S Malvern Line (Fig. 1 inset). In this area, basinal and volcanic terranes beneath eastern England shallow-marine Silurian rocks are exposed in inliers through a (Pharaoh et al. 1987; Woodcock 1991). To the south, it is subhorizontal blanket of marginal marine to continental se- overthrust by the Upper Palaeozoic rocks of the Variscan quences of Prˇı´dolı´ to Early Devonian age. Facies patterns and Orogen (Fig. 1 inset). isopachs reveal a persistent shoal zone along the inboard edge The Midland Platform (or Midlands Microcraton, Pharaoh of the Welsh Borderland Fault System, and suggest an area of et al. 1987) has been regarded as the stable core of the Eastern greater subsidence behind it as far as the Malvern Line Avalonia continent. During Early Palaeozoic time it appar- (Holland & Lawson 1963; Bridges 1975). This subdued basin ently comprised normal thickness continental crust, assembled accumulated modestly enhanced thicknesses of Wenlock and and stabilized during the Avalonian events of the Late Prot- Ludlow (Mid-Silurian) shallow marine sediment (Holland & erozoic (reviewed by Pharaoh & Gibbons 1994). It is presumed Lawson 1963) and its Llandovery (Early Silurian) manifes- to have been spared the tectonic excitement of the surrounding tation has been tentatively labelled an intra-shelf basin (Bassett basins; the extensional basin-forming events of the Cambrian et al. 1992, p. 41). to Silurian, the arc-related volcanism of the Ordovician, In this paper, we employ subsurface mapping to document and the compressional deformation of the Acadian (Late two sub-basins, named the Woolhope and Usk Basins, in this Caledonian) orogeny. western triangle of the Midland Platform. The basins show Recent discoveries have cast doubt on the unblemished substantial Early Silurian (Llandovery) extensional subsidence stability of the Midland Platform. Smith & Rushton (1993) followed by later Silurian thermal sag. The basins attest to the 209 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/209/4887277/gsjgs.154.2.0209.pdf by guest on 03 October 2021 210 A. J. BUTLER ET AL. 340 000 60 0 km 5 BGS line 84-01 260 000 60 Leominster Collington -1 EXL 187 50 Line of 50 Section (Fig. 5) 40 Hereford 40 Location of Seismic Fownhope -1 Section (Fig. 3) Ledbury 30 30 Ross-on- Wye EXL 158 20 EXL 080 IRISH SEA Abergavenny PLATFORM Monmouth 10 MIDLAND WELSH PLATFORM BASIN ? Usk -1 00 WBFS Fig. 1. Location of seismic lines and ND ML exploration boreholes within licences EXLs SVD 080, 158 and 187. Inset shows block VF locations in relation to main tectonic 40 0 50 structures of the Midland Platform and km SEFS Welsh Basin. instability of this part of the Silurian platform, and provide Exploration history clues to its role in contemporary regional tectonics. The Woolhope and Usk Basins illustrate the danger inherent Data used in this study were acquired as part of a programme in assuming the stability of cratonic platforms. Even in the of hydrocarbon exploration in the Welsh Borderland. Gas historically well-documented area of the Welsh Borderland, prospectivity along the border of the Midland Platform has two rift basins are barely recognizable in surface outcrops of been recognized for some time, with the identification of a sequences deposited only 20 Ma after cessation of active potential source in shales of Late Cambrian age. Reservoir faulting. The results of this study urge caution on the assump- intervals of Cambrian or Silurian age were sought in structural tion of long-term stability for platforms elsewhere, particularly traps ascribed to the Variscan deformation (Late Carbonifer- those which core microcontinental fragments. ous to Early Permian). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/209/4887277/gsjgs.154.2.0209.pdf by guest on 03 October 2021 WOOLHOPE AND USK BASINS, SUBSURFACE 211 Table 1. Depth–time correlations and interval velocities in the Usk, Fownhope and Collington wells Usk-1 Fownhope-1 Collinton-1 Depth TWT Velocity (ft s"1) Depth TWT Velocity (ft s"1) Depth TWT Velocity (ft s"1) (ft) (s) (ft) (s) (ft) (s) Horizon Interval Average Interval Average Interval Average Top Ludlow "2160 0.320 15 300 13 500 "1439 0.225 15 555 13 111 Top Much Wenlock "3101 0.443 15 623 14 000 "2599 0.369 15 205 14 087 Limestone Top Woolhope "4343 0.602 17 354 14 429 350 "3899 0.540 15 625 14 441 Limestone Near Top Aeronian "6009 0.794 15 440 15 136 "610 0.122 13 724 10 000 "4949 0.718 13 786 Near Top "9344 1.226 15 243 "1953 0.318 15 509 12 283 Rhuddanian Base Silurian (Top "3248 0.485 15 326 13 394 Tremadoc) Top Cambrian "5453 0.773 14 983 14 109 Top Precambrian "5895 0.832 19 524 14 171 TD "9920 1.280 "6101 0.853 14 302 "5242 c. 0.755 Exploration was first carried out in the area by Safari seismic marker is strong, but marks a lithological change and Oil Ltd. In 1968 they drilled Collington-1, 20 km west of the exact biostratigraphic level of the boundary is unclear, as Worcester, on an anticlinal structure above a NW-striking palynological control is poor and probably environmentally reverse fault (Department of Energy 1978). This was plugged sensitive. The base Silurian reflector corresponds to the uncon- and abandoned, and exploration activity declined until the formity with the top of the Tremadoc (Early Ordovician) in the 1980s, when the Early Palaeozoic gas play was reinvestigated Fownhope well. It deteriorates southwards, and the age of by Sovereign Oil and Gas plc. Between 1986 and 1989, the subjacent strata is unknown. An example seismic line from Sovereign, on behalf of a consortium of companies, was the dataset (Fig. 3) crossed the Woolhope Anticline and awarded three exploration licences (EXLs 080, 158 and 187). illustrates the high quality of acquisition. Some 607 km of seismic data were acquired (Fig. 1) and two A structure map has been produced on the Woolhope further wells were drilled; Usk-1 (1988–9, Department of Limestone (Fig. 4), together with isochron (two-way interval Trade and Industry 1995) and Fownhope-1 (1992). Both wells time) maps for the Aeronian interval (Fig. 6) and Rhuddanian were abandoned with gas shows.
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