Sedimentary Structures Associated With Hybrid Event Beds From the Gull Island Formation (Pennsylvanian) Of North Clare, Ireland

Abstract

A previously unreported assemblage of sedimentary structures, including several types of ripple, chevron marks, ridge and furrows, plant-rich laminations, found in association with hybrid event beds, with related muddy debrites and the trace fossil Kouphichnium, is described from a short 10m section through the middle of the Gull Island Formation [GIFm] (Pennsylvanian) of northern County Clare, Ireland. The section exposes a sequence of thickening and coarsening-upwards gravity flow deposits and interbedded mudstone layers. The upper part of the section is dominated by thick, massive and amalgamated sandstone beds; the middle section contains thinner sandstone beds with debritic mudstone caps, while the lowermost section comprises thin beds with a range of sedimentary structures produced by low density, transitional flows influenced by mud content, including mud ripples. ‘High-density’ turbidity currents operating in a delta front setting are inferred to have transformed to hybrid event beds with dilute but mud-rich suspensions travelling further distally to emplace thin beds, with evidence for clay-induced transitional flow and an assemblage of sedimentary structures not previously described from the GIFm. Smallscale ripple foresets from the upper part of the lower section indicate a southerly component of palaeocurrent flow direction. Chevron marks show size and palaeo-flow direction variation, suggesting temporal changes in current flow. Ridge and furrows have previously been interpreted as being formed under the head of turbidity current; however, the scale of the ridge and furrows described here suggest that interpretation is unlikely and some other unknown mechanism is involved. The presence of Kouphichnium trackways is significant as Carboniferous examples of this ichnofossil are usually recorded from very shallow marine or marginal marine environments. This part of the GIFm in north Clare is interpreted as a shallowing, delta front lobe environment exhibiting an upward transition from marginal or distal lobe to proximal or axial lobe sub-environment. This study highlights the influence of flow transformation in a muddy delta front environment with a high level of sediment supply favouring sediment remobilisation and the generation of a distinctive suite of sedimentary structures.