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.