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  • Publication
    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 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. The focus has now shifted to the key Ballybunion section on the south side of the Shannon, which 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 have suggested that (1) the 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 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 (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 is underway. Together the matched pair of Kerry boreholes with the outcrop section provide a reference section (480 m thick) that can be compared with the Loop composite section. 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. Thereafter hybrid event beds appear not to be as important at Ballybunion. Several of the mass transport units and condensed sections extend across the Shannon and tie the sections.
  • Publication
    Sedimentology of the upper Ross Formation (Pennsylvanian) in borehole GSI 09/05 at Ballybunion, Co. Kerry
    The Pennsylvanian Ross Sandstone Formation is well exposed around the Shannon Estuary, in western Ireland. It forms the deep-water part of a major shallowing-upward succession filling the Clare Basin and it has been extensively used as a turbidite reservoir analogue and for subsurface training.   Since 2009, a major program of behind-outcrop drilling targeting the Ross Sandstone Formation has been undertaken in west Co. Clare (Loop Head) and across the Shannon estuary in Co. Kerry. To date, most of the focus has been on boreholes acquired on Loop Head. These have provided a composite vertical section through the Ross Sandstone Formation and a framework for interpreting bed type variations and overall system evolution. Now, the emphasis is changing to examine lateral variability away from Loop Head using a new core at Ballybunion in the lowermost Ross Sandstone Formation, together with legacy data and two recent GSI slim-holes (at Inishcorker and Ballybunion).   This poster focuses on first results from the GSI 09/05 borehole, located c.4.3 km north of Ballybunion town. This 150 m long slim-hole core intersects the upper Ross Formation as confirmed by correlation with a measured outcrop section on the adjacent cliff c.1.2 km away. Correlation is based on goniatite-rich "marine bands", distinctive thick sandstone units and slump bodies. Three marine bands are identified, the uppermost being unusually thick (c.4.5 m) and provisionally equated with the R. paucicrenulatum band marking the top of the Ross Sandstone Formation on Loop Head. The core records an upward transition from well-bedded sandstones (turbidites and subordinate hybrid event beds) interpreted as stacked lobe deposits to more amalgamated sandstones towards the top of the formation indicating increased channelisation. A major composite slump body (25 m thick) occurs below the central marine band. This remobilised both sand-prone and silty deposits and is significantly thicker than any of the slump units identified in the Loop Head cores and outcrop.