Pre-Seminulan Main Limestone of the Avonian Series in Breconshire

Abstract

Summary The Avonian Main Limestone underlying the Seminula Zone in Breconshire has hitherto been referred to the Zaphrentis , Lower Caninia , and Upper Caninia zones in apparently conformable but attenuated sequence, lithologically not greatly different from that of the South Crop in Glamorgan. A remapping of the eastern outcrops, however, shows that a widespread development of dolomites, supposedly equivalent in part to the “ laminosa Dolomites”, is misleading as a guide to stratigraphy; and that thick oolites, variously ascribed to the “ Seminula Oolite” and the “ Caninia Oolite”, belong to neither of those zonal limestones. The succession consists of two major rock groups : an Oolite Group of Zaphrentidan age below, and a Calcite-Mudstone Group of Upper-Caninian age above. The Lower Caninia Zone is not represented. The Oolite Group is in primary lithology a generally uniform series of almost quartz-free, light-grey, massive crinoidal oolites resting directly on the Lower Limestone Shales. At the plane of the sub-Viséan unconformity it thins rapidly westwards through overstep by the Calcite-Mudstone Group. Most of the beds of the Calcite-Mudstone Group are quartz-rich finely fragmental algal and foraminiferal limestones, pellet-rocks, and calcite-mudstones. Apart from a prominent band of gritty oolite they consist mainly of organic detritus, the finest grades probably being algal muds. Variations in thickness of the Calcite-Mudstone Group, especially notable in the neighbourhood of the Vale of Neath disturbance, are partly internal, but arise mainly because of Seminulan overstep. The greater part of the Upper Caninia Zone is everywhere missing. The Calcite-Mudstone Group is completely transgressed in