Regional variations of Tournaisian and Viséan lithostratigraphy and their links to basin and structural evolution in Ireland

This work details the lithological and unit thickness variability across a large portion of the Tournaisian and Viséan of Ireland, using 476 stratigraphically representative boreholes. A particular focus was put on how lithological changes link with their depositional environment and with the evolving rift structural framework. The deposition of carbonates during the Tournaisian in Ireland is characterised by a south to north diachroneity, reflecting a northward transgression, initially on a generally south-dipping overall ramp geometry. The transgression was accompanied by a northward stepping of facies belts in several sub- Waulsortian units across the study area. Compiled micropalaeontological information suggests that early extensional faulting initiated in the early Ivorian, represented by clear syn-sedimentary thickening of the Moathill Fm across faults in the Central Midlands province. Conversely, such relationships are not shown for the underlying units such as the Meath Fm. The character of the Waulsortian Limestone Fm changes dramatically in terms of facies and thickness across the study area. In south-eastern areas, the Waulsortian forms laterally continuous “sheet-reef” growth forms which substantially thicken towards the central Iapetus suture zone. However, towards the north its development becomes less consistent and is evidently more often localized to the immediate proximity of faults and in most northerly areas, the Waulsortian is commonly absent. Some of the reasons for this dramatic change in character are discussed. The Waulsortian Limestone Fm records the transition from a general ramp geometry to an increasingly fault-controlled basin architecture. The Lower Viséan to Arundian is marked by a major change in facies deposition. Continued syn-rift extension localized on fewer but faster moving faults, temporally associated with a complex facies mosaic comprising areas of interfingering coeval deep basinal and shallower water shelf deposition. This was interpreted to coincide with eustatic sea level changes which strongly influenced the sedimentary environment. Using newly compiled biostratigraphical data, and lithological correlations, regional shallowing and (sudden) deepening trends can be recognized which correlate with well documented examples in laterally equivalent basins in Belgium, Wales and Germany.