Sedimentology of the Cretaceous Greensand, Quadrants 48 and 49, North Celtic Sea Basin: a progradational shoreface deposit

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Abstract
The Aptian-Albian Greensand interval is the main reservoir unit in the North Celtic Sea Basin. The Greensand comprises three units: (1) the Lower Claystone, (2) the ‘A’ Sand, and (3) the Gault Claystone. Sedimentological and wireline log studies of the Lower Claystone and ‘A’ Sand units within Irish Quadrants 48 and 49 indicate an overall 35 to 50 m thick, coarsening-upward profile from inner shelf mudstones to middle and possibly upper shoreface fine-grained sandstones. Both the mudstones and sandstones contain glauconite, marine fauna and marine ichnofauna. The ‘A’ Sand can be sub-divided into three coarsening-upward packages which show a progressive upward increase in higher-energy facies and thickness. Previous interpretations have suggested that the Greensand interval represents a series of tidal sand bars formed during transgression. However, the wide lateral extent and the pulsed, progradational nature of the coarsening-upward units indicates deposition in a (wave-dominated?) shoreface environment during a punctuated regression. The overlying Gault Claystone represents an abrupt return to open marine shelf conditions. Thickness variations across the study area suggest that faults were active to the south of the basin during Lower Claystone deposition but that activity had largely ceased by ‘A’ Sand times. Facies and isopach maps indicate that shoreface sandstones prograded west, north and east from a source lying somewhere to the south of the present-day position of the Kinsale Head field. Sediment may have been supplied from the uplifted South Celtic Sea Basin/Pembrokeshire Ridge.