Pre-Seminulan Main Limestone of the Avonian Series in Breconshire

Abstract

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.