A kinematic analysis of meso-fractures from Studland Bay, Dorset

Abstract

Meso-fractures (faults, joints and stylolite seams) in the gently dipping beds of the Upper Cretaceous Chalk and Tertiary sandstones of Studland Bay, Dorset have been analysed in an attempt to determine the tectonic history of the region since the Upper Cretaceous. The study area forms part of the down-thrown, gently dipping limb of the Purbeck-Isle of Wight monocline. Thirteen sets of meso-fractures have been recognised and in the sandstones of the area they include east-west striking thrusts and normal faults and less commonly north-south striking steeply dipping conjugate shear joints. In the Upper Chalk the fractures are more discrete and include four sets of strike-slip faults, two dextral striking 305° and 330° and two sinistral striking 028° and 055° and bedding plane faults. Joints are well developed and can be grouped into seven sets. These are bedding joints, two sets of vertical extension joints striking north-south and east-west and two pairs of conjugate shear joints, one pair striking north-south and the other east-west. Sub-vertical, east-west striking stylolite seams also occur. By comparing the results of the fracture analysis of the study area with those obtained from other parts of the gently dipping downthrown limb of the monocline it can be shown that the fractures of the area are the result of the regional north-south compression which generated the monocline combined with movements on local macro-faults (the Ulwell and Ballard faults).