THE WOOLHOPE AND USK BASINS - SILURIAN RIFT BASINS REVEALED BY SUBSURFACE MAPPING OF THE SOUTHERN WELSH BORDERLAND

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 km(2) in area, in the western part of the Midland Platform. The Woolhope Basin underlies the Silurian inlie r of the Woolhope Anticline. The basin fill is bounded to the northwes t by faults of the Neath Disturbance, and was also influenced by WNW-s triking extensional growth faults. The fill thins eastward towards the Malvern Line. Lower Silurian sequences thin southward before thickeni ng again, particularly across WNW-striking faults. The resulting Usk B asin, underlying the Usk Inlier, has ill-defined margins, but some add itional N-S fault control. The Fownhope-1 and Usk-1 wells indicate tha t rapid subsidence occurred in both basins during Rhuddanian and Aeron ian (Early to Mid-Llandovery) lime. Modelling of this subsidence, assu ming the finite duration lithospheric extension model, yields stretchi ng 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 t heir Early Llandovery intervals suggest continental fluvial environmen ts in the initial stages of basin development. The Woolhope and Usk Ba sins show that the western sector of the Midland Platform rifted durin g Early to Mid-Llandovery time, perhaps during regional dextral transt ension. The rifting is synchronous with, and perhaps genetically relat ed to, an episode of mafic volcanism in the southwest part of the plat form. However, this rifting apparently predates the Late Llandovery (T elychian) Fault-controlled subsidence seen in the Welsh Basin to the n orthwest.