The stratigraphy, structure and sedimentation of the Old Red Sandstone of the Comeragh Mountains and adjacent areas, County Waterford, Ireland

Abstract

Summary The Old Red Sandstone of the Comeragh Mountains, probably all of the upper division of the British succession, is subdivided as follows : 1, Coumshingaun Conglomerate Group ; 2, Comeragh Conglomerate-Sandstone Group; 3, Nier Sandstone Group; 4, Kiltorcan Sandstone Group. These groups are described. The Old Red Sandstone rests unconformably on Lower Palaeozoic rocks composed mainly of slates and felsites, but in one area the underlying rocks comprise conglomerates, formerly mapped as Old Red Sandstone. These are provisionally designated “Croughaun Beds” and their stratigraphical position discussed. Lower Carboniferous rocks follow in apparent conformity. The Old Red Sandstone here is unfossiliferous apart from poorly preserved plant fragments. The area described forms part of a gently folded broad-crested anticline with steep limbs to north and south and a strong wastward plunge. Several minor folds, as well as important faulting, mainly N.-S., are present. Jointing, especially of the massive conglomerates, is well developed, but cleavage of the finer beds is not pronounced The detailed petrography of the sedimentary rocks is described; the heavy mineral content is poor. The lowest rocks were laid down as coarse piedmont fans, the detritus coming probably from a highland area to the east largely of Lower Palaeozoic volcanic and minor intrusive rocks. The finer sandstones appear to have been playa-lake deposits near the eastern end of a rapidly subsiding trough of sedimentation. Highly felspathic and micaceous clastic constituents are not infrequent and these again were probably derived from the east or south-east, from a more distant region of granite or Pre-Cambrian basement, or both. The Kiltorcan Group is probably humid-lacustrine, with uniform conditions over a wide area, finally invaded by the Lower Carboniferous sea.