Variable character and diverse origin of hybrid event beds in a sandy submarine fan system, Pennsylvanian Ross Sandstone Formation, western Ireland

Hybrid event beds comprising both clean and mud-rich sandstone are important components of many deep-water systems and reflect the passage of turbulent sediment gravity flows with zones of clay-damped or suppressed turbulence. ‘Behind-outcrop’ cores from the Pennsylvanian deep-water Ross Sandstone Formation reveal hybrid event beds with a wide range of expression in terms of relative abundance, character and inferred origin. Muddy hybrid event beds first appear in the underlying Clare Shale Formation where they are interpreted as the distal runout of the wakes to flows which deposited most of their sand up-dip before transforming to fluid muds. These are overlain by unusually thick (up to 4.4 m), coarse sandy hybrid event beds (89% of the lowermost Ross Formation by thickness) that record deposition from outsized flows in which transformations were driven by both substrate entrainment in the body of the flow and clay fractionation in the wake. A switch to dominantly fine-grained sand was accompanied initially by the arrest of turbulence-damped, mud-rich flows with evidence for transitional flow conditions and thick fluid-mud caps. The mid and upper Ross Formation contain metre-scale bed sets of hybrid event beds (21 to 14%, respectively) in: (i) upward-sandying bed set associations immediately beneath amalgamated sheet or channel elements; (ii) stacked thick-bedded and thin-bedded hybrid event bed-dominated bed sets; (iii) associations of hybrid event bed-dominated bed sets alternating with conventional turbidites; and (iv) rare outsized hybrid event beds. Hybrid event bed dominance in the lower Ross Formation may reflect significant initial disequilibrium, a bias towards large volume flows in distal sectors of the basin, extensive mud-draped slopes and greater drop heights promoting erosion. Higher in the formation, hybrid event beds record local perturbations related to channel switching, lobe relocations and extension of channels across the fan surface. The Ross Sandstone Formation confirms that hybrid event beds can form in a variety of ways, even in the same system, and that different flow transformation mechanisms may operate even during the passage of a single flow. This article is protected by copyright. All rights reserved.