Joints in the Mesozoic sediments around the Bristol Channel Basin

Analysis has been carried out at four locations on the edges of the Bristol Channel Basin to illustrate the later phases of deformation of a sedimenta ry basin, and to illustrate the control on joint patterns of subtle changes in the stress system. The characteristics of the joints are described and influences on joints are determined, including the roles of faults. folds a nd beds. There is a low coefficient of correlation between joint spacing an d bed thickness, except in very thin limestone beds, which have a high dens ity of joints. The lengths and spacings of earlier joint phases are usually greater than those of later phases. Later joints normally abut against ear lier joints. The joints abut the latest faults but are nor displaced by the m, so the joints post-date the main Alpine contraction. The joints formed in five main phases during reduction of the Alpine stress es. Phase I joints are subparallel to the regional compression direction (1 60-180 degrees). Phase 2 joints are perturbed by faults, often curving towa rds points of stress concentrations along the faults. Phase 3 joints are su b-parallel to the earlier E-W-striking fold axes. Phase 4 joints are cross- joints, and phase 5 joints form polygonal patterns within joint-bound block s. Phases 2 and 3 do not occur in the absence of faults and folds, and corr espond with a reduction in horizontal compression and an increase in the im portance of local factors. Phases 4 and 5 occur at all locations. (C) 1998 Elsevier Science Ltd. All rights reserved.