The Western Irish Namurian Nasin reassessed

Current basin models for the Western Irish Namurian Basin (WINB) envisage a n elongate trough along the line of the present-day Shannon Estuary that wa s infilled with clastic sediments derived from a hinterland that lay to the W or NW. This paper argues for an alternative basin configuration with sou rce areas to the SW supplying sediment to a basin where deepest water condi tions were in northern County Clare. Rapid subsidence along the present-day Shannon Estuary ponded sediment in this area throughout the early Namurian and, only with the rapid increase of sedimentation rates within the mid-Na murian (Kinderscoutian Stage), were substantial amounts of sediment able to prograde to the NE of the basin. This alternative model better explains th e overwhelming predominance of NE-directed palaeocurrents in the Namurian i nfill, but requires fundamental revisions to most aspects of current deposi tional models. Deep-water black shales (Clare Shale Formation) initially accumulated throu ghout the region and were progressively downlapped by an unconfined turbidi te system (Ross Formation) prograding to the NE. This in turn was succeeded by an unstable, siltstone-dominated slope system (Gull Island Formation) c haracterized by large-scale soft-sediment deformation, which also prograded to the NE. In the northern-most basin outcrops, in northern County Clare, this early phase of basin infill was developed as a condensed succession of radiolarian-rich black shales, minor turbiditic sandstones and undisturbed siltstones. The new basin model envisages the northern exposures of County Clare to be a distal, basin floor succession whereas the traditional model considers it a relatively shallow, winnowed, basin margin succession. Late r stages of basin infill consist of a series of deltaic cycles that culmina te in major, erosive-based sandstone bodies (e.g. Tullig Sandstone) interpr eted either as axial, deltaic feeder channels or incised valley fills genet ically unrelated to the underlying deltaic facies. Within the context of th e new basin model the former alternative is most likely and estimated chann el depths within the Tullig Sandstone indicate that the basal erosive surfa ce could have been generated by intrinsic fluvial scour without recourse to base-level fall. The northerly flowing Tullig channels pass down-dip into isolated channel sandbodies interbedded with wave-dominated strata that sug gest the deltas of the WINB were considerably more wave-influenced than hit herto proposed. The retreat of the Tullig delta during sea-level rise saw t he rapid southerly retrogradation of parasequences, as may be expected if t he basin margin lay to the SW of the present-day outcrops.