Facies and internal stratigraphic variability in the Ross Sandstone Formation (Pennsylvanian), western Ireland - new borehole data from south of the Shannon Estuary

The 500 m thick Ross Sandstone Formation is well exposed in sea cliffs facing the Atlantic and along the 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 Sandstone Formation has been undertaken, focussing primarily on the Loop Head peninsula in west Clare. This has provided a full composite Ross cored section that underpins a new understanding of bed-scale variability and the wider evolution of the system. The focus has recently shifted to the key Ballybunion section on the south side of the River Shannon, obliquely down-dip from the Loop Head area (c. 18 km from the tip of the Loop) and is important in that previous outcrop studies have inferred that (1) the distinctive character of the lower Ross here with its abundant hybrid event beds may reflect a marginal position; and (2) extra sandy section may be present in the uppermost Ross due to offset stacking of the youngest sandy lobes. Two new cores are now available ¿behind¿ the Ballybunion section - a 200 m PQ borehole straddling the lower Ross and the upper part of the underlying Clare Shale Formation (12-CE-UCD-09), and a 151.5 m long, cored slimhole with associated wireline log data acquired by the Geological Survey of Ireland (GSI 09/05). The latter is 1.1 km along strike from coastal exposures of the upper Ross and the study reported here focusses on the relationship of the section acquired in this borehole to the local cliffs and to upper Ross outcrops on the north side of the River Shannon at Kilcredaun, some 4.6 km away. Correlation is based on goniatite-rich ¿marine bands¿, legacy biostratigraphic data (new determinations are underway), and a number of laterally extensive slump bodies which form distinctive marker beds.   The GSI 09/05 core contains three thin goniatite-rich levels, and a fourth candidate level, each interpreted as marine bands. These separate sand-prone packages, interpreted as stacked isolated to amalgamated lobe units, and at least two mass-transport units (MTDs), the lower and thickest of which is 25.5 m thick (true thickness). In the local cliffs to the west, all four marine bands can be identified, as well as the two MTDs. In addition, a third MTD is more obvious in the cliff. The lobe sandstones are dominated by deposits of high-density turbidity currents; amalgamated sandbodies become more abundant upwards. Hybrid event beds are rare (<10%) compared to lower in the formation. At least 50% of the sandbodies extend from the core to the adjacent outcrop without change; the remainder show a change from deposition from high- to low-concentration flows or vice versa. Overall, the Ballybunion Ross section is 480 m thick, broadly similar to the thickness established by drilling on the Loop. At longer length scales, all but the upper marine band are found at Kilcredaun. Correlatives of the two MTD units also occur in the core here, although the thickest slump has become thinner and muddier. Nonetheless this MTD unit can be traced widely across the Loop as a distinctive couplet. As correlated, the Ballybunion outcrop and core suggest there may not be an additional younger sandbody in this area, however the location appears axial and down-dip rather than marginal in character overall.