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. Although the gas potential of plus Aeronian interval (Fig. 7). These maps have not been the Late Cambrian shales was proved, in neither hole was a converted to depth and thickness maps. Velocity information reservoir interval encountered with more than 2% porosity. is limited (Table 1) and significant uplifts have occurred in a The seismic survey identified two further structural highs with number of areas. Consequently a simple depth conversion closure, but these were not drilled owing to the likely inad- model would need to be applied and the resulting depth maps equacy of the reservoirs. Sovereign relinquished the licences in would be no more informative than the travel-time maps, yet 1992 and no further exploration has been undertaken. All the would contain more significant errors. seismic data are now available through the U.K. Onshore Geophysical Library. Structural geometry of the survey area The regional structure is well represented by a map of two-way time to the top of the Woolhope Limestone (Lower Wenlock). Seismic mapping procedure This map (Fig. 4) reveals faults in three main orientations The seismic data used in this study (Fig. 1) were acquired by striking approximately NW, N and NE, matching those sug- Sovereign in five Vibroseis surveys between 1986 and 1992, gested by the surface geology. However, these fault sets are along with part of a BGS line in the Malverns area (BGS line each variably developed across the survey area. 84-01). The high quality seismic data have been interpreted Block EXL 187 is characterized by a gentle dome cut by using ties to the three wells at Collington, Fownhope and Usk, NW–SE normal and reverse faults with only minor displace- and to the surface outcrop, particularly in the inliers at ments. The Collington-1 well was drilled on a small hanging Woolhope, Ledbury–Malvern, May Hill, Tortworth and Usk. wall high above one of these reverse faults, the Collington The main horizons picked in the interpretation are listed in Fault (Brandon 1989). Two ENE-striking faults (F1 and F2) Table 1 and shown in Fig. 2. The reflectors at the top of the parallel the extrapolation of the major Swansea Valley Distur- Much Wenlock and Woolhope Limestones are strong and well bance (Fig. 1 inset), mapped northeastward to within 20 km of dated. Those within the Llandovery are strong but less tightly this area by Weaver (1975). Fault F2 is close to, but not dated (from acritarchs, Barron & Molyneux 1989a). The coincident with, the Ivington Fault chosen by Brandon (1989) near-top-Aeronian seismic marker followed is slightly dia- as the surface continuation of the Swansea Valley line. To the chronous, being coincident with the Top Aeronian in the east of Collington, the Woolhope Limestone and other reflec- Fownhope and Collington wells, but lying between 341 feet tors are approximately flat lying and unfaulted until steep dips (104 m) and 439 feet (134 m) below the palynologically deter- mark the Malvern Line, beyond which lie the Permo-Triassic mined boundary in Usk (Fig. 2). The near-top-Rhuddanian sequences of the Worcester Graben.
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/209/4887277/gsjgs.154.2.0209.pdf by guest on 03 October 2021 212 A. J. BUTLER ET AL.
Global standard chronostratigraphy Usk -1 Fownhope -1 Collington -1 37.61 km 26.05 km Sea Level
stages await definition
PRIDOLI Top Ludlow
Ludfordian Top Much Wenlock Limestone Gorstian
LUD. Homerian
Sheinwoodian Top Woolhope Limestone
WEN.
Telychian Top Aeronian
Near Top Aeronian seismic marker Lithological Key
SILURIAN SYSTEM 0 0 sandstone Aeronian limestone
Base Silurian feet metres interbedded sands and silts
LLANDOVERY Near Top Rhuddanian Top Cambrian 500 shale Top Precambrian 2,000 conglomerate igneous / metamorphic Rhuddanian basement
Fig. 2. Lithological logs of the three exploration boreholes, with outline lithostratigraphy, correlations, chronostratigraphy and chronometric calibration.
The Woolhope Limestone rises gently southwards into EXL the subsurface show net normal displacements, although some 158, culminating in a small structural high beneath the reversal is apparent, notably on the East Usk Line, which Shucknall Inlier, abutting the major ENE-striking Shucknall shows up to 350 ms TWT of offset in the Usk area. Using Fault (Squirrell & Tucker 1960; Brandon 1989). This fault is interval velocities from the Usk-1 well, this corresponds to a the northeasterly continuation of the Neath Disturbance, one throw of 800 m. Much less significant reversals are seen on of a set of Caledonoid (broadly NE–SW) faults that parallel a small number of other N–S faults in this area. the margin of the Welsh Basin (Fig. 1; Woodcock 1984a). A large structural high is mapped out at the level of the The Woolhope Inlier dominates the central part of EXL Woolhope Limestone in the western part of EXL 080, beneath 158. Its periclinal structure represents a hanging wall anticline the Usk Inlier. This structure was identified by Sovereign, but above a major reversed fault, termed the Woolhope Fault (Fig. was not drilled owing to the poor reservoir parameters encoun- 5), roughly coincident with the Western Boundary Fault tered in the nearby Usk-1 well. Its western edge is bounded mapped at outcrop by Squirrell & Tucker (1960). The approximately by the Usk Line, which runs almost parallel to Woolhope Fault has over 500 m of throw at Woolhope the East Usk Line but shows no reversal. Data quality to the Limestone levels. A second pericline, the May Hill Inlier, west of the Usk Line deteriorates on most seismic lines. It is occurs where the Woolhope Fault meets the Malvern Line at therefore difficult to observe structures in the Early Palaeozoic the western edge of the Worcester Graben (Lawson 1955). The as the reflectors dip beneath the Carboniferous sequences of Woolhope structure is broad to the northwest and narrows South Wales. southeastward towards the May Hill Inlier. The Fownhope- 1 well was drilled in the structurally highest part of the Woolhope Anticline and penetrated the Precambrian (Fig. 3). In the southern part of EXL 158, the Woolhope Limestone Kinematic history in relation to regional geology is at a depth of about 2500 m and shows gentle dips cut by The structural architecture of the Silurian in the southern NW–SE faults with small offsets. These faults increase in Borderland potentially derives from two major orogenic frequency into the north of EXL 080. The limestone rises to a events—the Acadian (Early to Mid-Devonian) and Variscan culmination on the northern (footwall) side of one of these (Late Carboniferous to Early Permian)—and interposed exten- faults (labelled F3, Fig. 4), which has up to 300 ms (about sional episodes in the Early Carboniferous and Late Permian 500 m) of throw. to Triassic. The absence of post-Acadian cover in the survey The southwestern part of the EXL 080 is dominated area makes local discrimination of these events problematic, by N–S-striking faults. Faults of this set are seen at outcrop in and evidence must be sought from the surrounding region. the Silurian of the Usk Inlier (Walmsley 1959; Squirrell & Also important is the possible control on Acadian and later Downing 1969; Barclay 1989). Most of the faults mapped in deformation by pre-formed basement structures.
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 213
South North
Top Woolhope
Top Aeronian Top Rhuddanian
Base Silurian Top Wenlock
Top Woolhope
Top Aeronian
Top Rhuddanian
Base Silurian
02km
Fig. 3. Seismic cross section across Woolhope Anticline and Woolhope Fault, illustrating the quality of the seismic reflection data used. Location shown on Fig. 1. The Fownhope-1 well is extrapolated from 1.8 km west of line. Horizontal length of section is 7.8 km.
Published evidence on the kinematic history of the region is Evidence of Late Devonian and Early Carboniferous growth summarized in Table 2. The three main directional sets of on NE-, NW- and N-striking structures comes from thickness structures are considered separately, and the history of the and facies variations. In particular, the Usk Anticline and Malvern Line is distinguished from that of other N–S Woolhope Anticline were rising, the Forest of Dean Synclines structures to the west. (Fig. 4) were subsiding and the Neath Disturbance was suffer- Published evidence of pre-Acadian (Silurian and earlier) ing downthrows of variable polarity suggesting strike-slip activity on the relevant structures is inconclusive. Sub- displacement. The regional kinematics are speculative. East– Llandovery unconformities along and to the west of the west shortening has been invoked to explain the Usk and Malverns suggest Ordovician or earlier uplift and tilting. Woolhope Anticlines (Walmsley 1959; Squirrell & Tucker Thickness variations in the Wenlock and Ludlow hint at 1960), perhaps as hanging-wall anticlines to the reverse East intra-Silurian deformation. Seismic mapping evidence of intra- Usk Line and Woolhope Fault respectively (Fig. 4). East–west Llandovery growth across both NE and NW-striking struc- shortening would also explain proposed dextral displacements tures forms a major part of the present work, to be discussed on the Neath and Severn Estuary lines. Regional dextral below. strike-slip may even have been the predominant control An Acadian (Mid-Devonian) age for reverse faulting along (Wilson et al. 1988). the Malvern Line may explain the erosional absence of Silurian By the Late Carboniferous, South Wales had become a and Lower Devonian rocks to its east and the sub-Upper foreland basin to the northward propagating Variscan thrust Devonian unconformity in the Tortworth Inlier. West of the sheets (Kelling 1988; Jones 1991). The Neath Disturbance Malverns, the Upper Devonian lies with only weak angular acted as a reverse/sinistral-oblique faulted buttress to new discordance on the Lower Devonian. Acadian activity is E–W thrusts in the South Wales Coalfield. NW–SE faults first suggested only by thinning of the Brownstones (Lower acted as strike-slip lateral ramps to these thrusts, then suffered Devonian), towards either the Usk Anticline or arguably normal displacement, perhaps connected with regional dextral towards the E–W Vale of Glamorgan Axis and NE-striking transpression (Gayer & Nemcock 1994). This later Variscan Severn Estuary Fault Zone. episode is the most likely origin of the small normal offsets
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/209/4887277/gsjgs.154.2.0209.pdf by guest on 03 October 2021 214 A. J. BUTLER ET AL.
Collington F1 Fault Two Way Time Map on 600 Woolhope Limestone 650 Collington -1
600
700 ABBERLEYS INLIER (Silurian undivided) Contour Interval = 100 ms F2
0 km 5 650
600 Llandovery Outcrops EXL 187
500 Structural Positions of Silurian Inliers at Usk, Woolhope and Shucknall Malvern Line SHUCKNALL Shucknall Fault INLIER LEDBURY - 400 500 MALVERN 400 300 INLIER
100 0
500 Fownhope -1 Thin Aeronian present 700 (lies on Tremadoc) 600 200 800 WOOLHOPE INLIER EXL 158 900 300 900 400 600 700 1000
800 1100 Woolhope Fault Neath Disturbance
900 1200
EXL 080 MAY HILL Thin Rhuddanian 700 INLIER and Aeronian 500 probably present 600
500 400 400 300
Usk Line 400 East Usk Line F3 600
800
Synclines of Forest of Dean 600 TORTWORTH INLIER
200 F4 Tremadoc outcrop
Usk -1 700 700
800 400 Fig. 4. Time–structure map for the top of 300 Severn Estuary Fault Zone the Woolhope Limestone (murchisoni 600 Biozone, Lower Wenlock). Contours are 800 USK 800 two-way time (ms). Main Silurian inliers are INLIER marked.
observed on NW and WNW-striking faults throughout In summary, published evidence suggests that most of the survey area (Fig. 4). The N–S structures of Usk and the the structures observed in the survey area can be ex- Malverns continued to develop further, presumably above plained by Carboniferous, broadly Variscan, tectonics. reverse faults, and the Forest of Dean tightened and subsided Acadian (Early and Mid-Devonian) displacements pro- after brief Namurian inversion. bably occurred on the Neath, Malvern, Usk and Severn Major post-Variscan (Permian and Triassic) displacements Estuary lines. An earlier ancestry for these structures seem restricted to the Worcester Basin, bounded to its west is probable, but the published evidence is circumstantial. We by the normally reactivated East Malvern Fault. Tertiary now focus on this pre-Acadian history, first through displacements on the Neath line have been hypothesised on isochron maps for selected Silurian intervals, and the basis of offset of erosion surfaces (Owen 1974; Weaver then through subsidence analysis of two of the exploration 1975). wells.
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 215
Fownhope -1
South Woolhope Shucknall Fault North Fault (Neath Disturbance)
0.0
w a Time Way Two Top Ludlow
(seconds) Top Wenlock Top Woolhope Top Rhuddanian Top Aeronian Base Silurian
1.0 Top Cambrian Top Precambrian Position of Figure 3.
05Kilometres
Fig. 5. Interpreted seismic section across the Woolhope Anticline. Location shown on Fig. 1. Rhuddanian and Aeronian (Lower and Middle Llandovery) is stippled.
Aeronian (Middle Llandovery) basin structure The Usk Basin Inspection of the seismic profiles, together with subsidence The Aeronian interval thickens again to the south of fault F3 analysis reported below, reveals anomalously thick sections of in EXL 080. To its north, under the palaeo-high, seismic lower Llandovery (Rhuddanian and Aeronian) rocks. Iso- profiles show little character at depth whereas to its south chron maps for these intervals chart the spatial pattern and many parallel reflectors are observed. This fault F3 is therefore structural control of this basin-forming event. The Aeronian interpreted as a major bounding fault to an Usk Basin to the (Middle Llandovery) isochron map represents the interval south. A parallel fault F4, some 3 or 4 km to the south of F3, between the near-top Rhuddanian and near-top Aeronian also shows Aeronian growth. The Aeronian in the southern markers (Fig. 6). The fault pattern has been taken from the half of EXL 080 generally thickens from east to west. How- lower horizon. ever, the N–S faults appear to show little involvement in this thickening, with the exception of a portion of the East Usk Line. This fault exhibits some footwall thinning, but in an The Woolhope Basin overthrust area that may present a resolution problem or local interpretation error. The Aeronian interval has not been mapped north of the Neath Disturbance. Here the Rhuddanian (Lower Llandovery) is absent and the Aeronian becomes too thin to map accu- rately. Strong Aeronian growth across the Neath line is Rhuddanian and Aeronian (Lower and Middle demonstrated by comparing the 1343 feet (409 m) of inter- Llandovery) basin structure preted Aeronian in the Fownhope-1 well with a probable 300 feet (89 m) in the Collington-1 well to the north. The Neath The Woolhope Basin Disturbance marks the northwestern edge of the Woolhope Inclusion of Rhuddanian data gives a more complete picture of Basin. the Woolhope Basin (Fig. 7) and removes uncertainty about The maximum Aeronian thickness mapped in the Woolhope the exact definition of the top Rhuddanian. However the base area (using Fownhope-1 velocity information) is 1842 feet Silurian marker becomes difficult to follow southwards, as (562 m), about 3.5 km northwest of Fownhope. A small the Silurian thickens dramatically and data quality at depth amount of growth is recorded across the Woolhope Fault, deteriorates, requiring jump correlations across faults. The indicating its involvement in Aeronian extensional tectonics. map has not therefore been extrapolated into the Usk Basin. The structural growth across the Neath Disturbance and the To the north of the Neath Disturbance the Rhuddanian and Woolhope Fault is also evident from the interpreted seismic Aeronian show little variation in thickness, being approxi- section (Fig. 5). mately 50 ms thick (365 feet, 111 m, using Fownhope-1 The Aeronian of the Woolhope Basin thins eastwards to- interval velocity averaged over the composite interval). wards the N–S-striking Malvern Line. Aeronian is usually Dramatic thickening is again observed across the Neath taken to be absent in the May Hill Inlier (e.g. Cocks et al. Disturbance. Growth evidently occurred during Rhuddanian 1992), but new palynomorph dating suggests a thin Aeronian as well as Aeronian time. Minor Rhuddanian involvement of section at the base of the Huntley Hill beds (Barron & the Woolhope Fault is again recorded, such that a local Molyneux 1989b). The Aeronian is less than 100 m thick in the depocentre is recorded to the northwest of the Fownhope-1 Ledbury/Malvern Inlier, where it overlies Tremadoc rocks well. Rapid eastward thinning occurs towards the Malvern (Ziegler et al. 1968). The third side to the triangular basin is Line, but seemingly without local fault involvement. The marked by the southward thinning of the Aeronian onto a Rhuddanian is absent in the Ledbury–Malvern Inlier (Cocks et palaeo-high in the northeast of EXL 080. This onlap probably al. 1992) and is thin at May Hill, where acritarchs indicate a continues the trend observed to the east of EXL 158. If so, the Rhuddanian age for the lowest part of the May Hill sequence thick Aeronian in the Woolhope Basin parallels the Neath (Huntley Hill Beds) (Barron & Molyneux 1989b; Bassett et al. Disturbance. 1992, p. 39).
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/209/4887277/gsjgs.154.2.0209.pdf by guest on 03 October 2021 216 A. J. BUTLER ET AL.
Table 2. Timing and nature of deformation on the main sets of structures in south Wales and the Borderland
NE–SW structures (Neath Disturbance ND and Severn Estuary Fault Zone SEFZ) and connected E–W structure NW–SE structures N–S structures Usk (Vale of Glamorgan Axis including Woolhope Axis Anticline (UA), Forest of N–S structures Malvern Age VGA) (WA) Dean Synclines (FDS) Line (ML)
Triassic and Permian Triassic u/c across NNW East down normal faults in South Wales reactivation of East Malvern Fault bordering Worcester Basin (Chadwick & Smith 1988) Permian u/c on Tremadoc in Worcester Basin
Late Carboniferous Westphalian thins to ND, Strike-slip then normal UA uplifted to erosion West-down reverse NW-up sinistral displacement on ‘cross level in later Westphalian faulting uplifts former strike-slip suggested faults’ during Variscan Growth of FDS renewed Palaeozoic floor to (Owen 1971; Kelling thrusting in S Wales in Westphalian (Owen Worcester Basin to 1974) coalfield (Barclay 1989; 1971) erosion level (Chadwick NE–SW structures Cole et al. 1991; Gayer & Absent Namurian & Smith 1988) buttress developing E–W Nemcock 1994) suggests FDS inverted thrusts in South Wales Normal displacement on then coalfield (Jones 1991) ‘cross faults’ in S Wales Thickness contrasts coalfield during suggest growth on UA deposition of throughout Late Westphalian B (Jones Carboniferous (Owen 1991; Cole et al. 1991) 1971)
Early Carboniferous and Thickness and facies Viséan thickness changes Viséan growth of UA Late Devonian u/c on Late Devonian contrasts across ND suggest growth on NNW and FDS suggested by Early Devonian and suggest NW-up slip in faults on north crop of thickness changes (Owen Silurian in Tortworth Tournaisian and SE-up coalfield (Weaver 1975) 1971) Inlier in Viséan (Owen 1971) Viséan sediment in Forest Late Devonian growth of Dextral strike-slip on ND of Dean sourced from UA and FDS suggested (Owen 1974) and SEFZ uplifting WA (Squirrell & by thickness changes (Wilson et al. 1988) Tucker 1960) (Allen 1965) Late Devonian u/c on VGA (Wilson et al., 1988)
Mid-Devonian and Early Uplift of SEFZ and Possible growth of UA West-down reverse Devonian VGA gives thinning of gives thinning of Early faulting uplifts former Early Devonian (Wilson Devonian (Allen 1974) Devonian and earlier et al. 1988) floor to Worcester Basin to erosion level (George 1963; Smith 1987)
Silurian and older Earlier history implied by Llandovery u/c on Wenlock and Ludlow Late Llandovery u/c on protracted Tremadoc or earlier in thin onto N—S Gorsley Camb and Precamb Ordovician-Silurian Collington-1 and axis (George 1963; Earlier displacement of activity on sub-parallel Fownhope-1 Holland & Lawson 1963) uncertain nature Welsh Borderland Fault SW-down growth on System (Woodcock & WNW faults during Early Gibbons 1988) to Mid-Llandovery SE-down growth on ND bounding Usk Basin (this during Early to paper) Mid-Llandovery bounding NE-down growth on the Woolhope Basin (this intrabasinal Woolhope paper) Fault during Mid-Llandovery (this paper)
Bold type indicates evidence in this paper. Italics indicate main unconformities.
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 217
AERONIAN ISOCHRON MAP
Collington-1
ABBERLEYS INLIER (Silurian undivided)
0 km 5
Contour Interval = 50 ms EXL 187 EXL 158 Llandovery outcrop
WOOLHOPE 150 LEDBURY - BASIN Shucknall Fault 100 MALVERN 200 INLIER
? Fownhope-1 200 150
200 Thin Aeronian present (lies on Tremadoc) 100
150 100
200 150
50 200 200 150 100 EXL 080 Thin Rhuddanian 100 MAY HILL and Aeronian INLIER probably present 150 100 USK BASIN 100 150
450 200 F3 300 350
500 600 F4 600 350 450 TORTWORTH 500 INLIER Usk-1 Tremadoc outcrop 400 400 350
300 600 500 Rhuddanian Fig. 6. Isochron map for the interval and Aeronian 600 between the top Rhuddanian and top 500 absent Aeronian markers. Contours are two-way time (ms). 600
Rhuddanian fault displacements are evident on some of the Inlier. The Woolhope Basin then appears as an elongate WNW–ESE faults in the south of EXL 158, such that a second feature parallel to, and probably controlled by, the Neath local depocentre is produced. This feature ceased to grow in Disturbance. the Aeronian. Large Rhuddanian growth is also seen in the hanging wall of a small NE–SW fault in the south west corner of EXL 158. However, this growth is seen on only one seismic line and should be viewed with caution. The Usk Basin A palaeohigh is again mapped in the northeast corner of The dramatic Rhuddanian growth to the south of fault F3 has EXL 080. Rapid southward and southeastward thinning oc- not been mapped out. In the Usk-1 well, the combined curs onto it. This thinning is uninterrupted by the normal Aeronian/Rhuddanian thickness is over 4000 feet (1200 m). On faults in the area, indicating their post-Mid-Llandovery origin. the palaeohigh north of fault F3, this interval is about 1200 It is postulated that the palaeohigh connects with the May Hill feet (360 m) thick.
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/209/4887277/gsjgs.154.2.0209.pdf by guest on 03 October 2021 218 A. J. BUTLER ET AL.
Collington Fault AERONIAN AND F1 RHUDDANIAN Collington-1 ISOCHRON MAP 50 50 ABBERLEYS INLIER (Silurian undivided) 0 km 5 F2
Contour Interval = 50 ms 50 EXL 187 EXL 158 100 Llandovery outcrop
LEDBURY - WOOLHOPE 300 MALVERN BASIN Shucknall Fault INLIER 250
150
400 350 400 400 ? Fownhope -1 Thin Aeronian present 350 (lies on Tremadoc) 400 350
250
350 150 300 300
350 400
300 250 150
600 400 400
EXL 080 350 MAY HILL Thin Rhuddanian 300 INLIER and Aeronian probably present 350
250
350 150
F3
TORTWORTH INLIER Usk-1 Tremadoc outcrop
Rhuddanian Fig. 7. Isochron map for the interval and Aeronian absent between the base Silurian and top Rhuddanian markers. Contours are two-way time (ms).
Subsidence analysis of the Usk-1 and Fownhope-1 wells thickness of water. The method and its assumptions are detailed by Wooler et al. (1992), whose model parameters we Theory have used (their table 2). The results (Fig. 8) plot water-loaded Subsidence analysis of the Usk-1 and Fownhope-1 wells has subsidence against time. These plots effectively show the rate at been attempted, to define better the timing and magnitude of which the basement would have subsided without any sedi- basin formation. The well sections have been backstripped ment load. Steep segments of the curve indicate periods of (Steckler & Watts 1978), to remove the subsidence component rapid subsidence, potentially corresponding to rift episodes. caused by sediment loading, and to isolate that due to a Theoretical subsidence curves, based on the finite-duration tectonic driving force. The backstripping method incremen- lithospheric extension model of Jarvis & McKenzie (1980), tally decompacts each layer to recover its depositional thick- have been fitted to the observed subsidence profiles in the ness, then replaces this thickness of sediment by the same Usk-1 and Fownhope-1 wells (Fig. 9). This second phase of
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 219
(a) Llandov. Ordov. RhAe Tl Wen. Ludlow Pr Devon. Cambrian Ordovician Silurian D. 0.0 0.0 (a)
β = 1.30 unconformity 1.0 1.0
Subsidence (km) Fownhope -1 Subsidence (km) β = 1.40 Fownhope -1 2.0 550.0 500.0 450.0 400.0 2.0 440.0 420.0 400.0
(b) Llandov. Llandov. Ordov. Wen. Ludlow Pr Devon. Ordov. Wen. Ludlow Pr Devon. 0.0 Rh Ae Tl 0.0 RhAe Tl (b)
1.0 β 1.0 = 1.48
Subsidence (km)
Subsidence (km) Fownhope -1 Usk -1 β = 1.60 2.0 2.0 440.0 420.0 400.0 440.0 420.0 400.0 Time (Ma) (c) Fig. 9. Modelled subsidence curves fitted to the Fownhope and Usk Llandov. data, assuming the finite duration lithospheric extension model. Ordov. Wen. Ludlow Pr Devon. 0.0 Rh Ae Tl Multiple curves are for different values of stretching factor â. Increment for Fownhope is 0.02 and for Usk 0.04.
estimating the depth of water in which each sedimentary unit was deposited. The depth ranges used are 100–500 m for turbidites and other sediments below storm wave base, 1.0 0–100 m for shallow marine sediments and 0 to "50 m for non-marine deposits. Uncertainty in the Early Palaeozoic chronometric timescale also affects the shape of the curve. The Subsidence (km) scale of Harland et al. (1990) is used here. Curves on the scale Usk -1 of Tucker & McKerrow (1995), with its considerably earlier base to the Cambrian, are not significantly different in shape 2.0 over the Silurian interval of importance here. 440.0 420.0 400.0 Time (Ma) Fig. 8. Back-stripped water-loaded subsidence plots for the Results Fownhope-1 and Usk-1 wells. Timescale is from Harland et al. The subsidence curve for the Fownhope-1 well has two distinct (1990). parts. In the first part (Fig. 8a), rapid subsidence in the Late Cambrian and Tremadoc is followed by an unconformity through the remainder of the Ordovician. The second part analysis is justified by the strong indication from the seismic (enlarged in Fig. 8b) comprises rapid subsidence through the data of an extensional component during formation of the Usk Rhuddanian and Aeronian, decaying during the Telychian and Woolhope Basins. The parameters used to generate these and followed by gentle subsidence through the Wenlock and curves are those of Wooler et al. (1992), who also detail the Ludlow. This second phase of subsidence in Fownhope-1 is technique. closely matched by the subsidence profile for the Usk-1 well There are significant uncertainties on the subsidence curves (Fig. 8c). The apparently later start of subsidence in Usk-1 is in both depth and time. The vertical bars show the errors in an artefact of the uppermost Rhuddanian limit to penetration
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/209/4887277/gsjgs.154.2.0209.pdf by guest on 03 October 2021 220 A. J. BUTLER ET AL.
of this well. Seismic interpretation shows a thickness of Rhud- danian compatible with a base Silurian start to subsidence. The v Early - Mid-Silurian volcanics Usk-1 curve also hints at enhanced subsidence in the Early Generalised basin form lines Active Faults during Shoreline after Devonian. Early - Mid-Llandovery sedgwickii promontory (late Aeronian) The Silurian subsidence curves can be interpreted in terms of Shelve - Longmynd transgression (late Telychian) transgression rifting and subsequent thermal decay (post-rift) subsidence, as emergent until predicted by lithospheric extension models (e.g. Jarvis & WELSH BASIN Malvern (marine throughout McKenzie 1980). The rift model is supported by the exten- Llandovery) sional growth faulting seen on the seismic profiles affecting the Pontesford
Collington-1 griestoniensis Rhuddanian to Aeronian interval, synchronous with the steep Rhuddanian / early segments of the subsidence curves. Theoretical subsidence Aeronian shoreline curves have been calculated for different values of the stretch- ing factor (â), with rifting constrained to the Rhuddanian and WOOLHOPE Tywi BASIN Aeronian (Fig. 9). v Netherton The Fownhope-1 data fit best with â between 1.30 and 1.40. F -1
The Usk-1 data fit best with stretching factors of between 1.48 postulated and 1.60. The negative value of initial subsidence results from dextral Neath the earliest data in this well being uppermost Rhuddanian. In strike-slip the Usk-1 well, it is clear that rifting finishes before the end of the Telychian.
USK Usk -1 BASIN v Tortworth v Maesteg Sedimentary fill to the Woolhope and Usk Basins
Lithological information is derived from the well records at edge of Early to Mid-Llandovery rift depocentre - probably non-marine during Collington-1, Fownhope-1 and Usk-1 (Fig. 2), from the Silu- Rhuddanian then marine during Aeronian. rian outcrop in the Woolhope Inlier (Telychian and later) and the Usk Inlier (Wenlock and later), and by comparison with 0 km 20 Mendips bordering inliers at Tortworth, May Hill, Ledbury–Malvern, v and the Abberley Hills. Fig. 10. Early to Mid-Llandovery palaeogeographic map showing the relationship of the Usk and Woolhope Basins to the Welsh Basin and to records of Early Silurian volcanic rocks. Upper Llandovery, Wenlock and Ludlow Facies and thicknesses in Wenlock and Ludlow units in the age. This interval contains abundant fragments of mica schist three wells closely match the outcrop records. They confirm or granite, possibly derived from the Malverns or similar that shallow marine mudstones, calcareous siltstones and Precambrian terranes. limestones covered the whole area during most of mid-Silurian By contrast, the Aeronian in Fownhope-1 and Usk-1 com- time (Holland & Lawson 1963). Increase in the thickness prises predominantly marine sandstone and shale. A whole and completeness of Ludlow sequences from May Hill to core in the Aeronian at Usk consisted of laminated, light Fownhope-1 may reflect enhanced thermal subsidence over the grey-brown medium to coarse sandstone in beds 30 cm to 2 m site of the Woolhope Basin. Underlying Upper Llandovery thick, interbedded with units of very fine to medium sandstone (Telychian) sequences are also shallow marine, mostly shales, and shale. Sandstone beds are commonly graded, with ero- with sandstones at Fownhope and Collington comparable with sional bases, load casts and internal cross-lamination and those in the Wych Beds in the Malverns and Abberleys convolutions. The framework grains are mostly quartz, with respectively (Ziegler et al. 1968). minor lithic fragments, predominantly glassy tuffs and basalts. Shell fragments, crinoid ossicles and a rich acritarch assem- blage imply a marine environment. These sediments have been Middle Llandovery interpreted as the products of medium to low concentration The Middle Llandovery (Aeronian) sequences show much turbidity currents (J.C.M. Taylor; Sovereign internal report stronger facies contrasts between the basinal wells (Fownhope 1989). On this interpretation, a significant topographic con- and Usk) and other areas, consistent with the strong sub- trast had developed by Aeronian time between the Woolhope/ sidence of the Woolhope and Usk basins at this time. This Usk Basins and their fringing platforms to the east and contrast is summarised on a palaeogeographic map (Fig. 10). northwest (Fig. 10). Aeronian rocks are probably absent north of the Neath Disturbance and east of the Malvern line until late in Aeronian time, when the sedgwickii transgression (Bassett et al. 1992) left Lower Llandovery patches of marginal marine or shallow marine sandstone and This platform to basin topography is equally evident in the conglomerate across this area. These are represented by the Rhuddanian record. Few rocks of this age occur around Cowleigh Park Beds (90–100 m thick) in the Malverns and the basin margin and direct evidence comes mainly from the Abberleys (Ziegler et al. 1968). The 90 m of conglomeratic Fownhope and Usk wells. Here, the fossiliferous grey sedi- sandstone at the base of the Collington-1 well (Fig. 2) probably ments of the Aeronian give way downwards to mostly unfos- records this transgression, a suggestion supported by the siliferous reddened sediments. A 7.6 m core from the base of absence of underlying reflectors on seismic profiles. The sand- the Usk-1 well comprises red conglomerate in beds 0.5–2.5 m stones here may however be of mid- rather than late Aeronian thick, with rare thin interbeds of red micaceous shale. Feint
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 221
parallel and cross lamination occur. The conglomeratic clasts 1996) and voluminous turbidite influx from a rejuvenated are predominantly volcanic, mostly glassy tuff and lava. There collisional source area (Soper & Woodcock 1990). The earlier are subordinate sandstone clasts and quartz grains. All grains Rhuddanian onset of rifting in the Woolhope and Usk Basins have hematite coatings, indicating an arid diagenetic environ- implies either a different driving mechanism, or an earlier date ment. These sediments are interpreted as the products of for hard collision of Avalonia and Laurentia (Pickering et al. braided streams on a coastal plain (J.C.M. Taylor; Sovereign 1988; Pickering & Smith 1995). internal report 1989), although one acritarch and one grain of glauconite warn of a possible marine influence. The sediment source was almost entirely volcanic. The only rocks of this age Extensional consequences known on the margins of the Woolhope and Usk Basins come Stretching (â) factors estimated using the finite duration litho- from the Huntley Quarry Beds of the May Hill Inlier. Here, spheric extension model are 1.3–1.4 in the Woolhope Basin volcanic flows are interbedded with red bed sediments. This and about 1.5–1.6 at Usk. These factors are high enough to unit was previously thought to be Precambrian in age (Lawson explain the observed facies transition within the basins, from 1955; Ziegler et al. 1968) but recent work indicates the presence early non-marine sequences to later marine sequences devel- of Rhuddanian age palynomorphs (Barron & Molyneux oped as subsidence outpaced accumulation and the basins 1989b). The Rhuddanian Usk and Woolhope Basins must have suffered marine incursion. occupied a continental rift, within the angle formed by the The â-factors are as high as those encountered in the flanking uplifts across the Malvern and the Neath lines (Fig Mesozoic North Sea Basin, and high enough for the develop- 10). ment of volcanics sourced from partial melting of the sub- lithospheric mantle (e.g. White & Latin 1993). No volcanic Interpretation: Early Silurian rift basins rocks are encountered where they might be expected in the deeper parts of the Woolhope or Usk Basins. However the Kinematics presence of Rhuddanian volcanic rocks below the penetration The origin of the Woolhope and Usk Basins by a component depth of the Usk-1 well cannot be ruled out, and a local of Early to Mid-Llandovery lithospheric stretching is sup- volcanic source, possibly the Tortworth area, is implied by the ported by the observed growth across normal faults within composition of the Rhuddanian conglomerates and sandstones both basins. However, it cannot be assumed from these at Usk. observations that the regional tectonic setting was purely Any such volcanic centre would be analogous to those extensional. Other evidence points to a component of regional evidenced elsewhere along the southern edge of the Midland strike-slip displacement across Early Silurian Wales. Platform (Fig. 10). Volcanic rocks, mostly basic, are exposed The major bounding fault along the Neath Lineament is in the Mendips (middle Wenlock), at Tortworth (Telychian) probably part of a deeply rooted steep fault belt which was and on Skomer Island in west Wales (Aeronian or older, well-oriented at least for Carboniferous strike-slip (Owen & Thorpe et al. 1989). Intermediate tuffs of probable Early Weaver 1983). The Neath Disturbance is one of a suite of Silurian age were encountered in boreholes at Maesteg-2, Caledonoid (NE–SW) lineaments, some with an earlier strike- Bicester-1 (east of the survey area) and Netherton-1 (Pharaoh slip history. Most relevant is the evidence for pre-Telychian et al. 1991). The Bicester and Netherton volcanic rocks are tied dextral strike-slip along the Pontesford Lineament bounding to a prominent reflector near the base of the Silurian, imaged the Welsh Basin (Woodcock 1984a, 1987; Lynas 1988; Wood- more widely across the southern part of the Midland Platform cock & Gibbons 1988). A similar sense of Early to Mid- (Pharaoh et al. 1991; Chadwick & Smith 1988). The Skomer Llandovery displacement along the Neath line would withdraw Volcanics have a geochemistry that suggest within-plate rift- a wedge of lithosphere from the angle between the Neath and ing, with only minor subduction components (Thorpe et al. Malvern lines and promote its extension and subsidence (Fig. 1989). Pharaoh et al. (1991) have postulated a phase of Early 10). Formation of NW- or NNW-striking normal growth Silurian rifting along the southern edge of the platform, faults is kinematically compatible with this model. Comp- perhaps related to developments of the Rheic Ocean to the lementary strike-slip, probably sinistral, on the Malvern Line south. might be expected. Field evidence for the early structural history of the Malverns has been much debated (Chadwick & Smith 1988; Phipps & Reeve 1969 and references therein) and Southward continuation of the Usk Basin? remains inconclusive. The extent and deteriorating quality of the seismic data in the southwest of the survey area prevent the delineation of the Usk Basin in this direction. Two main possibilities exist. The first is Timing that the basin could be closed on its south side, perhaps by an The formation of the Usk and Woolhope Basins during Early E–W continuation of the Severn Estuary Fault Zone into the Silurian transtension matches the hypothesis that the larger Bristol Channel. This hypothesis would define a triangular Welsh Basin developed under the influence of regional strike basin, much as portrayed for the Mid-Llandovery by Bassett slip (Woodcock 1984b, 1990). Post-Caradoc/pre-Telychian et al. (1992, maps S1b, S2a). The Usk Basin would be flanked strike-slip within the Eastern Avalonian continent has been along the southern rim of Eastern Avalonia by the emergent ascribed either to Caradoc/Ashgill interaction of the Iapetus area of Pretannia, and would require a narrow westward ocean ridge with the Avalonian trench (Woodcock 1990; marine connection with the Welsh Basin. Pickering & Smith 1995), or to Llandovery impingement of The second possibility is that the Usk Basin continued to Eastern Avalonia with Laurentia (Soper & Woodcock 1990). deepen southward and was continuous with the southern The second of these mechanisms is dated in the Welsh Basin by continental margin of Eastern Avalonia. Evidence for the rapid Telychian fault-controlled subsidence (Woodcock et al. Silurian and earlier position and nature of this margin has
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/209/4887277/gsjgs.154.2.0209.pdf by guest on 03 October 2021 222 A. J. BUTLER ET AL.
been entirely lost by overthrusting in the Variscan Belt. On this J, J.A. 1991. A mountain front model for the Variscan deformation of the second hypothesis, the Silurian connection to the southern South Wales Coalfield. Journal of the Geological Society, London, 148, 881–891. margin was short-lived. Wenlock and, more clearly, Ludlow K, G. 1974. Upper Carboniferous sedimentation in South Wales. In: facies appear to shoal southwards from the Usk Inlier to the O, T.R. (ed.) The Upper Palaeozoic and post-Palaeozoic rocks of Wales. Cardiff Inlier (Bassett et al. 1992), presaging a landmass in the University of Wales Press, Cardiff, 185–224. Bristol Channel that became even more prominent during —— 1988. Silesian sedimentation and tectonics in the South Wales Basin. In: Devonian time (Cope & Bassett 1987). B,B.M.&K, G. (eds) Sedimentation in a synorogenic basin complex: the Upper Carboniferous of northwest Europe. Blackie, Glasgow and London, 38–42. The authors thank Sovereign Oil and Gas plc. and its partners L, J.D. 1955. The geology of the May Hill Inlier. Quarterly Journal of the (Mobil North Sea Ltd, Union Texas Petroleum Ltd, Brabant Geological Society of London, 111, 85–116. Petroleum Ltd, Amerada Hess Ltd and Total Oil Marine plc) L, B.D.T. 1988. Evidence for dextral oblique-slip faulting in the Shelve for releasing the seismic and well data on which this study is based, Ordovician Inlier, Welsh Borderland: implications for the south British Caledonides. Geological Journal, 23, 39–57. and R.A. Waters for valuable discussion of the geology of South O, T.R. 1971. The relationship of Carboniferous sedimentation to structure Wales. A.J.B. gratefully acknowledges NERC research studentship in South Wales. In: Sixth Congress on Carboniferous Stratigraphy, 3, GT4/93/130/G. 1305–1316. —— 1974. The Variscan orogeny in Wales. In: O, T.R. (ed.) The Upper Palaeozoic and post-Palaeozoic rocks of Wales. University of Wales Press, References Cardiff, 285–294. ——&W, J.D. 1983. The structure of the main South Wales coalfield and A, J.R.L. 1965. Upper Old Red Sandstone (Farlovian) palaeogeography in its margins. In: H, P.L. (ed.) The Variscan fold belt in the British South Wales and the Welsh Borderland. Journal of Sedimentary Petrology, Isles. Adam Hilger, Bristol, 47–73. 35, 167–195. P,T.C.&G, W. 1994. Precambrian rocks in England and Wales A, J.R.L. 1974. The Devonian rocks of Wales and the Welsh Borderland. south of the Menai Strait Fault System. In: G,W.&H, A.L. In: O, T.R. (ed.) The Upper Palaeozoic and post-Palaeozoic rocks of (eds) A revised correlation of Precambrian rocks in the British Isles. Wales. University of Wales Press, Cardiff, 47–84. Geological Society, London, Special Reports, 22, 85–97. B, W.J. 1989. The geology of the South Wales Coalfield, Part II. The ——, B,T.S.&W, P.C. 1993. Subduction-related magmatism of country around Abergavenny (3rd Edition). Memoirs of the British Geologi- Late Ordovician age in eastern England. Geological Magazine, 130, 647– cal Survey, sheet 232. 656. B,H.F.&M, S.G. 1989a. Final well report: stratigraphic paly- ——, M, R.J., E, J.A., B, T.S., W,P.C.&S, N.J.P. nology of sovereign Usk A. Report of the British Geological Survey, 1991. Early Palaeozoic arc-related volcanism in the concealed Caledonides PD/89/87. of Southern Britain. Annales de la Société Géologique de Belgique, 114, —— & —— 1989b. Palynological investigation of outcrop samples from the 63–91. Huntley Quarry Beds, May Hill Inlier, Gloucestershire. Report of the British ——, ——, W,P.C.&B, R.D. 1987. The concealed Caledonides of Geological Survey, WH/89/299C. eastern England: preliminary results of a multidisciplinary study. Proceed- B, M.G., B, B.J., C, R., H,C.H.&L, J.D. 1992. ings of the Yorkshire Geological Society, 46, 355–369. Silurian. In: C, J.C.W., I,J.K.&R, P.F. (eds) Atlas of P,C.B.&R, F.A.E. 1969. Structural geology of the Malvern, Abberley palaeogeography and lithofacies. Geological Society, London, Memoirs, 13, and Ledbury Hills. Quarterly Journal of the Geological Society of London, 37–56. 125, 1–37. B, A. 1989. Geology of the country between Hereford and Leominster. P,K.T.&S, A.G. 1995. Arcs and backarc basins in the Early Memoirs of the British Geological Survey, sheet 198, 1–62. Paleozoic Ocean. The Island Arc, 4, 1–67. B, P.H. 1975. The transgression of a hard substrate shelf: the Llandovery ——, B,M.G.&S, D.J. 1988. Late Ordovician-Early Silurian (Lower Silurian) of the Welsh Borderland. Journal of Sedimentary Petrol- destruction of the Iapetus Ocean: Newfoundland, British Isles and ogy, 45, 79–94. Scandinavia—a discussion. Transactions of the Royal Society of Edinburgh, C,R.A.&S, N.J.P. 1988. Evidence of negative structural inver- 79, 361–382. sion beneath central England from new seismic reflection data. Journal of S, N.J.P. 1987. The deep geology of central England: the prospectivity of the the Geological Society, London, 145, 519–522. Palaeozoic rocks. In: B,J.&G, K. (eds) Petroleum geology of C, L.R.M., H,C.H.&R, R.B. 1992. A revised correlation of northwest Europe. Graham & Trotman, London, 217–224. Silurian rocks in the British Isles. Geological Society, London, Special Reports, 21, 1–32. ——&R, A.W.A. 1993. Cambrian and Ordovician stratigraphy related to structure and seismic profiles in the western part of the English C, J.E., M, M., F, K., G, R.A., G, P.A., Midlands. Geological Magazine, 130, 665–671. H,A.J.&W, S.C. 1991. Variscan structures in the opencast coal sites of the South Wales Coalfield. Proceedings of the Ussher Society, S,N.J.&W, N.H. 1990. Silurian collision and sediment dispersal 7, 375–379. patterns in southern Britain. Geological Magazine, 127, 527–542. C, J.C.W. & B, M.G. 1987. Sediment sources and Palaeozoic history of S,H.C.&D, R.A. 1969. The geology of the South Wales the Bristol Channel area. Proceedings of the Geologists’ Assocation, London, Coalfield, Part I. The country around Newport (Mon.) (3rd Edition). 98, 315–330. Memoirs of the Geological Survey of England and Wales, sheet 249. D E, 1978. Collington No. 1, Hereford. U.K. Land Well ——&T, E.V. 1960. The geology of the Woolhope Inlier (Herefordshire). records HMSO. Quarterly Journal of the Geological Society of London, 116, 139–185. D T I, 1995. Usk-1, Gwent. U.K. Land Well S,M.S.&W, A.B. 1978. Subsidence of the Atlantic-type conti- records HMSO. nental margin off New York. Earth and Planetary Science Letters, 41, G,R.A.&N, M. 1994. Transpressionally driven rotation in the 1–13. external Variscides of south-west Britain. Proceedings of the Ussher Society, T, R.S., G,J.W.&H, P.J. 1993. Composite Ordovician 8, 317–320. lamprophyre (spessartite) intrusions around the Midlands Microcraton in G, T.N. 1963. Palaeozoic growth of the British Caledonides. In: J, central Britain. Geological Magazine, 130, 657–663. M.R.W. & S, F.H. (eds) The British Caledonides. Oliver & Boyd, T, R.S., L, P.T., B,R.E.&H, D.J. 1989. Late-orogenic Edinburgh, 1–33. alkaline/subalkaline Silurian volcanism of the Skomer Volcanic Group in H, W.B., A, R.L., C, A.V., C, L.E., S, A.G. & the Caledonides of south Wales. Journal of the Geological Society, London, S, D.G. 1990. A geologic time scale 1989. Cambridge University Press, 146, 125–132. Cambridge. T,R.D.&MK, W.S. 1995. Early Paleozoic chronology: a review H,C.H.&L, J.D. 1963. Facies patterns in the Ludlovian of Wales in light of new U–Pb zircon ages from Newfoundland and Britain. and the Welsh Borderlands. Geological Journal, 3, 269–288. Canadian Journal of Earth Sciences, 32, 368–379. J,G.T.&MK, D.P. 1980. Sedimentary basin formation with finite W, V.G. 1959. The geology of the Usk Inlier (Monmouthshire). extension rates. Earth and Planetary Science Letters, 48, 42–52. Quarterly Journal of the Geological Society of London, 114, 483–521.
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 223
W, J.D. 1975. The structure of the Swansea Valley Disturbance between —— 1991. The Welsh, Anglian and Belgian Caledonides compared. Annales de Clydach and Hay-on-Wye, South Wales. Geological Journal, 10, 75–86. la Société Géologique de Belgique, 114, 5–17. W,N.J.&L, D.M. 1993. Subsidence analyses from the North Sea ‘triple —— &G, W. 1988. Is the Welsh Borderland Fault System a ter- junction’. Journal of the Geological Society, London, 150, 473–488. rane boundary? Journal of the Geological Society, London, 145, 915– W, D., D, J.R., S,M.&W, R.A. 1988. Structural controls 923. on Upper Palaeozoic sedimentation in south-east Wales. Journal of the ——, B, A.J., D,J.R.&W, R.A. 1996. Sequence stratigraphical Geological Society, London, 145, 901–914. analysis of Late Ordovician and Early Silurian depositional systems in the W, N.H. 1984a. The Pontesford Lineament, Welsh Borderland. Journal Welsh Basin: a critical assessment. In: H,S.P.&P, D.N. of the Geological Society, London, 141, 1001–1014. (eds) Sequence Stratigraphy in British Geology. Geological Society, —— 1984b. Early Palaeozoic sedimentation and tectonics in Wales. Proceedings London, Special Publications, 103, 197–208. of the Geologists’ Assocation, London, 95, 323–335. W, D.A., S,A.G.&W, N. 1992. Measuring lithospheric stretch- —— 1987. Kinematics of strike-slip faulting; Builth Inlier, Mid-Wales. Journal of ing on Tethyan passive margins. Journal of the Geological Society, London, Structural Geology, 9, 353–363. 149, 517–532. —— 1990. Sequence stratigraphy of the Welsh Basin. Journal of the Geological Z, A.M., C, L.R.M. & MK, W.S. 1968. The Llandovery Society, London, 147, 537–547. transgression of the Welsh Borderland. Palaeontology, 11, 736–782.
Received 1 July 1996; revised typescript accepted 12 September 1996. Scientific editing by Alex Maltman.
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/209/4887277/gsjgs.154.2.0209.pdf by guest on 03 October 2021 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/209/4887277/gsjgs.154.2.0209.pdf by guest on 03 October 2021