A mid-Frasnian marine incursion into the southern part of the Munster Basin: evidence from the Foilcoagh Bay Beds, Sherkin Formation, SW County Cork, Ireland

Abstract

Abstract The Munster Basin of southern Ireland contains a thick (7 km +) succession of Old Red Sandstone sediments interpreted as the product of various alluvial processes. The present study presents a preliminary sedimentological and palynological analysis of a grey succession informally known as the Foilcoagh Bay Beds, which is the lowest unit of the Sherkin Formation exposed in the southern part of the basin. Sedimentological analysis of the succession suggests that it is the product of sinuous distributary channels, flanked by permanently flooded overbank areas that endured occasional crevasse splay floods. These conditions evolved into a protected lagoon or lake that received periodic high energy floodings from an adjacent marine environment. Palynological study has refined the age of the Foilcoagh Bay Beds as mid-Frasnian time. Palynofacies analysis has provided direct evidence of marine influence as revealed by the presence of marine microfossils and abundant amorphous organic matter at some dark grey mudrock levels. This suggests deposition in a well-established lacustrine or lagoonal environment in which anoxic conditions prevailed at intervals and which was subjected to a period of marine incursion. The recognition of a marine influence in the Munster Basin at an early stage in its history has several important implications, including the following (1) previous models for the basin that suggested an enclosed centripetally draining entirely non-marine system have to be re-evaluated; (2) the drainage pattern and direction of the marine incursion were probably controlled by localized subsidence along an east-east direction; initiation of subsidence associated with the development of the east-west-trending South Munster Basin may have commenced much earlier than previously considered; (3) the occurrence of marine conditions has been tentatively correlated with the Rhinestreet mid-Frasnian sea-level highstand; (4) the recognition of marine conditions early in the known history of the basin provides a pathway for fish to have entered the basin; (5) the basin may have had a marine connection throughout much of Late Devonian time, opening the possibility for base-level control of the alluvial systems within the basin by external eustatic factors.