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Back to Bed Types – New Constraints on the Distribution of Hybrid Event Beds in the Pennsylvanian Ross Formation, Western Ireland

Abstract

‘Behind-outcrop’ cores from the 490 m thick deep-water Ross Formation show that hybrid event beds (HEBs) make up a significant component of this system, comprising 27% (by thickness) overall, alongside turbidites (30%) and amalgamated sandstones (23)%. The remainder is made up of mass-transport units (7%) and background siltstones (12%). In this respect, the Ross resembles a number of other sandy deep-water systems in the subsurface (Forties, Wilcox) where HEBs are also known to be important and where they can impact on reservoir quality. The types, distribution and context for the Ross HEBs have been documented in a fully cored composite vertical section tied using condensed sections and marker slump horizons. HEBs dominate the lower Ross (89%) but remain significant components in the mid (24%) and upper Ross (16%), despite the progressive upward change to a more proximal setting. In the lower Ross, variable HEB character tracks the wider stratigraphic position. Ross sand deposition was preceded by stacked thin-bedded and mud-prone HEBs, before the arrival of unusually thick (up to 4.4 m) sandy HEBs that extend for at least 18 km down dip to Ballybunion. A system reset straddling a condensed section was then followed by the initiation of the main Ross system. The larger-volume initial flows were turbulence damped but mudclasts were rare in contrast to the outsized event beds beneath. In the mid and upper Ross, HEBs occur as (1) upward-sandying units immediately beneath amalgamated sheet or channel elements; (2) HEB-dominated bed stacks 0.25 to 25 m thick alternating with units dominated by conventional turbidites, (3) thin bedded HEB bundles and (4) rare isolated outsized stand-alone HEBs. HEB abundance and character away from the base of the Ross does not appear to mirror externally-forced cycles of fan activity. Instead, HEBs are interpreted to record local perturbations related to repeated channel switching and erosional extension of channels across the fan surface. The dominance of HEBs in the lower Ross may reflect significant initial disequilibrium, higher drop heights for the flows that would have promoted erosion and the presence of extensive clay-draped slopes prior to the arrival of sand-bearing flows. The new bed inventory shows the net ‘clean’ sand in the system as a whole is c. 50%, discounting what would be poor quality clay-prone divisions in the hybrid event beds. The mid-Ross is confirmed as the sandiest part of the system.