Petrology and Structure of Some Lower Carboniferous Reefs in Northwestern Ireland

Abstract

Some Carboniferous knoll-reefs in Ireland have been mapped in detail. The reef bodies are 30-500 feet in thickness and several miles in diameter. All reefs contain many small-scale unconformities which dip in the same way as the sedimentary bedding, radially outward from the reef centers. No distinct core facies has been found. Paleocurrent directions run parallel with the length of the reefs. The reef limestone was found to be a clotted fine-grained calcareous mud (bahamite) which commonly contains larger mud pebbles. The only organic remains which might have petrological significance are bryozoans; their bulk contribution to the rock however is small. Early during diagenesis a cavity system was formed probably due to sliding movements on the reef slope. This was soon filled with calcite and dolomite crystals. It can be demonstrated that the dolomite formed at a stage when the reef was still a mound on the sea floor. Lithification set in at a later stage, it led to a preferred orientation of calcite grains and is tentatively explained by solubility gradients in the migrating pore solutions. An inorganic origin for the reefs seems likely; algae may have helped precipitating the calcite mud, but there is no evidence of this.