Assessment of Hirnantian synglacial eustacy and palaeogeography in a tectonically active setting: the Welsh Basin (UK)

Abstract

AbstractThe Hirnantian stratigraphy of Wales is critically assessed against the global evidence for two major Hirnantian synglacial eustatic lowstands. The evidence for two separate lowstands of relative sea level is locally suggestive in shelf sequences but generally only permissive in the basin. Nowhere can the possible candidate lowstands be separated biostratigraphically and thus basinwide correlation that might rule out local tectonic control is impossible to prove. Consistent with palaeolatitude evidence and a situation far removed from the Gondwana margin, there is no facies evidence (e.g. dropstones) for local glacial conditions and the diamictites are of mass-flow origin. Sequence stratigraphic reasoning proves helpful in the correlation of the basinal succession in Central Wales with the shelf sequences along the Tywi Lineament and the Variscan Front, and suggests that the first appearance ofNormalograptus persculptusin Wales does not correspond with the base of thepersculptusBiozone. Extensive new palaeocurrent data and sedimentological restudy is combined with review of earlier knowledge to argue for significant revisions to previous palaeogeographic maps for ‘generalized lowstand conditions’, notably along the Bala Lineament, the Variscan Front and the Central Wales Inliers.