Carbonate Diagenesis in a Geologically Complex Region – Lower Carboniferous of the Southern Irish Midlands

Jay M. Gregg¹, Kevin L. Shelton², Ian D. Somerville³, Wayne R. Wright⁴, Aaron W. Johnson⁵, Zsolt R. Nagy⁶, and Stephen P. Becker^7
1 Oklahoma State University, Stillwater, OK
² University of Missouri-Columbia, Columbia, MO
³ University College Dublin, Dublin, Ireland
⁴ Texas Bureau of Economic Geology, Austin, TX
⁵ University of Virginia's College at Wise, Wise, VA
⁶ Schlumberger Data and Consulting Services, Houston, TX
⁷ Virginia Polytechnic Institute & State University, Blacksburg, VA

Complex extensional/compressional tectonics affected Lower Carboniferous rocks in Ireland north of the Variscan Front. These rocks were deposited on Devonian Old Red Sandstone overlying igneous and metamorphic terrains. Sedimentation patterns are expressed as complex facies mosaics consisting of juxtaposed basinal and shallow marine shelf environments. Carbonate facies include deep ramp argillaceous limestones, moderately deep water mud banks (Waulsortian facies), shallow water platform limestones, peritidal carbonates and evaporites.

Initial diagenesis consisted of compaction, marine cementation and lithification of limestones. Petrographic, geochemical, and modeling studies indicate that regional replacement dolomitization of the Waulsortian and platform carbonates resulted from early regional circulation of “seawater” and reflux of evaporite brines originating in the peritidal region adjacent to the Leinster Massif.

On substrates of earlier dolomite, large-scale fault-associated brecciation, carbonate cementation, sulfide precipitation, and hydrocarbon emplacement occurred. Fluid inclusion, stable isotope and other geochemical data indicate that at least three saline fluids ranging from 70° to 270°C existed during this cataclastic episode. Halogen compositions of included fluids are indicative of a seawater origin, with evaporation beyond halite precipitation for one of these fluids. Trace element and Sr isotope compositions indicate some interaction of these fluids with underlying basement rocks. The hot saline fluids, originating as Carboniferous seawater and evaporite brines, were entrained into and migrated within several units and upon further heating rose buoyantly and mixed with a regional, lower temperature, seawater-derived brine. The presented paleo-diagenetic history is a result of a complex interaction of lithology, fluid migration/chemistry, paleogeography, and structure.