The case against a lower palaeozoic geosyncline in Wales

Abstract

AbstractThe author contends that we are in danger of spoiling further a term which has been, in its time, of great service in geology, if we describe as geosynclinal the Lower Palaeozoic rocks in Wales.Their facies and distribution yield no evidence that they were deposited in an elongated trough. At the present day they occupy an almost equidimensional area which is made up of a series of basins of deposition modified by tectonics of the platform type. The “mountains” of Wales (ca. 3000 ft.) are irregularly‐distributed monadnocks on a dissected plateau, and owe their elevation to resistant lavas, fragmental volcanic rocks and basic intrusions. There is no mountain chain of either Caledonian or later age. The rocks are predominantly conglomerates, sandstones, siltstones, mudstones and shales (slates): limestones are thin, scarce and scattered. Ultrabasic intrusives are very rare; and granites insignificant and of doubtful age. No metamorphic rocks (apart from slates) are known younger than Pre‐Cambrian. Shelf and basin facies have been identified at a few horizons and in certain areas, but much work remains to be done in this field. The statement that they are symmetrically distributed, and hence the styling of the geosyncline as the prototype of a new variety, the symmetrical geosyncline, are not warranted.No deep borings from which information about thicknesses could be obtained have been sunk. Estimates from field evidence show that no great thickness of sediments occurs at any particular locality. Breaks in the succession are numerous and widespread. The method of summating “restored” maximum thicknesses of formations and citing the aggregate is of doubtful value in view of the well‐known tendency for the compensatory thickening and thinning of formations as they are traced laterally. Local occurrences of high dips, steep folding and up‐thrusting are not regarded as evidence of orogenesis. It is therefore concluded that the Welsh region consists of a series of gently‐folded basins, in which the facies, thicknesses, trends and structures vary, situated on the south of the Scoto‐Scandinavian geosyncline.