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Facies Trends and Large-Scale Architecture of the Pennsylvanian Ross Formation, Western Ireland – New Insight from Cores South of the Shannon


The Ross Formation is well exposed in sea cliffs facing the Atlantic and Shannon estuary in western Ireland. It forms the sandy deep-water part of a major shallowing-upward late Mississippian to Pennsylvanian succession. Over the last four years, a major behind-outcrop drilling program targeting the Ross Formation has focussed primarily on the Loop Head peninsula in west Clare. This has provided a composite Ross cored section (490 m thick) that underpins a new understanding of bed-scale variability and the wider vertical evolution of the system in this area. The focus has now shifted to a re-analysis of the key Ballybunion section on the south side of the Shannon, a location that sits obliquely down-dip (to the east) of the Loop Head area (c. 18 km away). This area is important in that previous outcrop studies here have suggested that (1) the distinctive character of the lower Ross with its abundant hybrid event beds may reflect a marginal fringe position; (2) an extra sandy section may be present in the uppermost Ross due to offset stacking of the youngest sandy lobes and (3) some of the upper Ross mass transport units may extend across the estuary from Clare. Two new cores are now available ‘behind’ the Ballybunion cliff section: a 200 m long PQ borehole straddling the lower Ross and the upper part of the underlying Clare Shale Formation (12-KY-UCD-09), and a 151.5 m long slimhole core acquired by the Geological Survey of Ireland (GSI 09/05). In addition, a re-analysis of the biostratigraphy using both the outcrops and core is underway. Together the pair of Kerry boreholes and the outcrop section to which they can be matched provide a reference section (480 m thick) that can be compared with the Loop composite section. There are obvious parallels and some key lateral changes. Both sections have a distinctive precursor cycle involving first stacked thin mudflows and then outsized and coarse grained hybrid event beds. The muddier make-up of the latter at Ballybunion is consistent with a down-dip position based on trends in other basins. The onset of the main Ross system that follows is sandier at Ballybunion than at Loop suggesting the former was more axial at this time. Although hybrid event beds also occur in the mid and upper Ross at Ballybunion, they appear not to be as important here. Several of the mass transport units and condensed sections extend across the Shannon and tie the sections.