Variscan structures in the opencast coal sites of the South Wales Coalfield

The structures revealed by the ongoing excavation of coal in the opencast sites of the South Wales Coalfield enable analysis of both the detailed geometry and the deformational age relationship of the different structures. Field surveys have shown that the earliest structures to be formed were syn-depositional, gravity-driven extensional faults which influenced sedimentation of the Lower and Middle Coal Measures. In some cases these structures take the form of listric gravity slides and, in general, downthrow southward. The earliest compresssional deformation was the development of thrusts and thrust-related folds with a general E-W trend. Displacements range from less than 10mm to over 200m with more intense deformation developed in the western half of the coalfield. Thrusts verge to the north in the northern half and to the south in the southern half of the coalfield. ENE-WSW trending disturbances show evidence of thrusting and later folding with the development of oblique slickenfibres in the fold limbs. In some sites thrusts and folds trend NNW-SSE with ENE vergence. The thrusts are cut by NW-SE and N-S cross-faults which show extensional geometry. In some cases there is evidence that these cross faults were also active during deposition of the Middle Coal Measures. It is suggested that the geographic control on structural style in the basin is partly related to: 1) reactivation of basement structures to produce the ENE-WSW belts of disturbance, and 2) to the effect of buttressing against the NE-SW trending Caledonian massif to the north resulting in greater compressional deformation in the west than the east of the coalfield. The strong influence of stratigraphic control on compressional deformation is thought to be due to changes in the rheology of different lithologies within the Middle Coal Measures.