Axial-trace-fractures and Deformed Concretionary Rods in South Pembrokeshire

Abstract

AbstractAxial-trace-fractures are defined as joint or fracture cleavage surfaces which intersect bedding surfaces parallel to adjacent fold axes, and which meet stratigraphically higher bedding surfaces at acute angles closing towards adjacent anticlinal hinges. In the Upper Palaeozoic rocks of South Pembrokeshire these fractures are developed equally on fold limbs, in regions of low dip, and around fold hinges. Characteristically they are interrupted at bedding surfaces and vary in frequency with lithology. The value of the angle they subtend with bedding surfaces varies with lithology and dip.Calcareous concretionary rods, thought to have been originally normal to depositional surfaces, are now orientated parallel to axial-trace-fractures and at right angles to fold axes.The axial-trace-fractures of South Pembrokeshire are considered to have been formed immediately before or at the start of the Armorican folding, and then to have been rotated and sheared during the folding.Other joints associated with axial-trace-fractures are described and briefly discussed.