The role of fluids in the evolution of the South Wales coalfield foreland basin

The South Wales coalfield developed as a Silesian flexural basin in response to Variscan tectonic loading in the Bristol Channel and SW England. Subsequent northward propagation of thrust deformation into the coal basin is recorded in the later stages of the foreland basin evolution. The temperatures required for the development of the anthracite field, in the NW of the coalfield, are unlikely to have developed by burial within a normal continental geotherm. It is argued that the style of thrust deformation, in which several thrust detachments parallel to coal seams moved simultaneously, producing a progressive easy-slip thrust (PEST) style, probably required fluid over-pressuring in the coal seams for its development. Preliminary study of mineralisation, developed in pre- and syn-thrusting fractures within the coal, reveals: pyrophyllite, stable at temperatures between 225-275°C in the presence of methane; and the hydrothermal minerals, harmotome, galena and sphalerite. Together, these minerals suggest the presence of hot fluids passing through the coal, resulting in a perturbation of the geotherm to generate anthracite, and over-pressuring of the coal seams to trigger PEST deformation. It is suggested that these fluids may have originated within the Variscan tectonic wedge to the south, and may have been driven by a combination of gravity flow and thrust load expulsion forwards and upwards into the coal basin.