The structure of the Coal Measures in the Pontardawe-Ammanford Area

Abstract

Summary The tectonic structures described lie in Coal Measures on the northern limb of the major South Wales Syncline. The Coal Measures sequence is divided, for mapping purposes, into seven groups. The earth-movements which have affected the area can be divided into three episodes. The first is referable to the Armorican orogeny, when forces directed from the south gave rise to differential northward movement of the strata, observable along north-north-westerly trending tear-faults. The northerly moving strata were forced against the pre-existing Towy Anticline and west-south-westerly structures were developed in the younger Palaeozoic rocks parallel to that Caledonoid massif, of which the Cwm Twrch and the Ammanford compressional belts are the most important. The compression-belts consist of folds, thrusts and lag-faults. The second episode was a period of tension during which a second system of north-north-westerly faults was developed. These elevated or depressed pre-existing structures but did not displace them laterally. During this period the region was tilted southwards and this tilt gave rise to the west-south-westerly pitch of the folds formed during the compression period. The tear-faults also moved vertically during the period of tensional stress and there are three cases of tensional sags and associated trough-faults developing on the western side of pre-existing tear-faults. Four stages in the development of a fault-trough from a sag are illustrated by sections across the Cwm Clydach trough. In the last tectonic episode, the structures of the two previous episodes were moved laterally by the north-easterly trending Tawe Valley tear-fault.