Abstract - Fault and fracture systems are the most important store and pathway for groundwater in Ireland’s bedrock aquifers either directly as conductive flow structures or indirectly as the locus for the development of dolomitised limestone and karst. Through the quantitative analysis of fault and fracture systems in the broad range of Irish bedrock types, this project is designed to develop generic conceptual models for different fault/fracture systems in different lithologies and at different depths, linking them to observed groundwater behaviour. In this talk we briefly describe the geometrical characteristics of the full range of post-base Carboniferous fault/fracture systems controlling groundwater flow in different Irish bedrock types from field observations at more than 60 outcrop, quarry and mine localities. The structures range from Lower Carboniferous normal faults through to Variscan-related faults and veins, with the most recent structures including Tertiary strike-slip faults and ubiquitous uplift-related joint systems. A variety of attributes are described, including fracture orientations, densities, spacing/clustering, sizes (e.g. aperture/thickness), scaling, connectivity and depth, all of which are critical determinants of the flow behaviour of such systems: differences in attributes within certain lithological sequences are attributed to mechanical controls. The geometrical characteristics of different fault/fracture systems combined with observations of groundwater behaviour in both quarry and mine localities, can be linked to general flow and transport conceptualisations of Irish fractured bedrocks, but most importantly they also provide a basis for relating groundwater flow to particular fault/fracture systems and their expression with depth. Although uplift-related joints always contribute to flow and storage, Variscan faults and veins and Tertiary strike-slip faults, in particular, are often the most permissive Carboniferous or younger structures.
Abstract of talk given to:
Irish Geological Research Meeting, University of Ulster, March 2013.