The initiation of Waulsortian buildups in Western Ireland

The contact zone at the base of the Waulsortian (Upper Tournaisian) carbonate mud‐bank complex in western Ireland has been investigated at four localities spaced over a distance of 120 km. At all localities, a transition facies up to 3 m thick, characterized by several types of grumous (clotted and/or peloidal) carbonate muds, immediately underlies the Waulsortian facies. These muds show a developmental sequence provisionally interpreted as a necessary precursor to the formation of Waulsortian polymuds. Such pre‐bank precursors produced thin (a few decimetres) units of transition facies. The same mud types also persisted as an aureole around growing banks (mud‐mounds). Migration of the aureole during bank progradation produced thicker units of transition facies. The distribution of skeletal grain types in the Waulsortian banks, the transition facies and the ‘background’ argillaceous bioclastic limestones show two trends: one regional and one local. The regional trend is expressed by progressive north–south attenuation and, in some cases (for example, plurilocular foraminifera), the disappearance of organism groups. It parallels changes in Waulsortian Phases (defined by skeletal grain‐type assemblages) and is thought to indicate a southerly increase in water depth. The local trend, which occurs only in the two southern localities (deeper water), expresses differences between the skeletal grain content of the various lithofacies. These differences result partly from increased sensitivity to substrate texture by organism groups suffering southward attenuation (notably gastropods, hyalosteliid sponges, aoujgaliids, Earlandia and kirkbyacean ostracods) and partly from selective colonization, particularly of the transition facies, by tabulate corals and stick/ramose bryozoans. However, the developmental sequence of precursor carbonate muds is the same at all localities, indicating that the mud‐making process (probably microbial) was independent of water depth.