Waulsortian Limestone: Geology and Hydrogeology

Waulsortian limestone is extensively developed in Ireland, and it represents a phase of submarine bank development during the Tournaisian (Mississippian, Carboniferous). These carbonate buildups are rich in (now lithified) lime mud, contain a shelly marine fossil fauna and generally lack any sign of a rigid supporting framework. Complex cavity systems commonly developed and these were subsequently infilled with a variety of geopetal muds and sparry cements. As Waulsortian banks grew and developed they generated a complex topography on the seafloor. Lateral facies variation is common as a result: the centre, or core, of individual mounds is typically represented by massive, or poorly-bedded bank facies. This may pass laterally through flank facies (around the periphery of mounds) and into laterally equivalent off-bank facies, which is generally composed of interbedded argillaceous limestones, shales and cherts. Hydrogeologically, the Waulsortian limestone has generally been considered unproductive, however, potential for groundwater flow does exist. The massive bank (or core) facies is not spatially or stratigraphically ubiquitous, and the laterally equivalent off-bank bedded facies might present better opportunities for promoting flow. The very fine-grained carbonate lithology is also relatively pure and, in the right circumstances, amenable to dissolution and karst development. It is also susceptible to Mg replacement and dolomitisation, which can further enhance permeability. Brittle faults and fractures are also extremely important in providing conduits and pathways for groundwater in the subsurface. Some of these deep-seated fault structures played a key role in the generation of many Pb Zn deposits in the past, and now provide potential conduits for deep groundwater. Waulsortian banks originally developed in a dynamic and heterogeneous environment on the seafloor and an appreciation, or awareness, for its capacity for lateral facies variation is essential when investigating it. An integrated and holistic approach is recommended to maximise understanding of this carbonate system, using all geological and hydrogeological information available.