Jointing in the jurassic limestones of the Cotswold Hills

Abstract

The gently tilted Jurassic limestones of the Cotswolds are disrupted by two principal systems of normal faults, striking north-south and east-west, and by six sets of systematic joints symmetrically arranged about the north-south regional strike of the beds. Four of the joint-sets are normal to bedding surfaces and two are moderately inclined to the bedding. After restoration of bedding to the horizontal, two of the sets normal to bedding are seen to form a complementary system of horizontally slicken-sided, wrench shear joints striking north-east-south-west (dextral), and north-west-south-east (sinistral), and enclosing a mean angle of 82° out an east-west bisectrix. After restoration, the other two sets normal to bedding form an orthogonal system of vertical extension joints, parallel with the strikes of the normal faults, and bisecting the angles between the wrench joints, which are the older. After restoration the moderately inclined joints strike east-west, and dip north or south at 55°. They are complementary normal shears, parallel with normal faults. They are restricted to the neighbourhood of Painswick. Although the joint system is typically arranged in this way it may be apparently rotated at some localities by up to 25° clockwise or anticlockwise. The stresses responsible for the systematic joints were probably residual. Superficial structures of late-Pleistocene age are younger than the systematic joints.