A model of subaqueous sedimentation at the margin of the Late Midlandian Irish Ice Sheet, Connemara, Ireland, and its implications for regionally high isostatic sea-levels

Abstract

The geomorphology and sedimentology of a series of Late Midlandian glacigenic sites in Connemara, western Ireland, are described and a set of commonly occurring sedimentary assemblages identified. These form components of a simple model of ice-contact subaqueous sedimentation involving progressive down current transition from subglacial esker tunnel fill, through proximal tunnel-mouth fans, proximal and distal subaqueous fans into basin floor sediments. In two of the sites aggradation to the water-line passes into fan-delta topsets and foresets. All sites abut into inferred water bodies to their west and imply minimum water-surface heights of between 35 and 65 m Irish OD or, in the case of the two deltaic sites, shoreline heights of between 75 and 80 m. These high water levels, very close to, and facing directly into, the immediately adjacent open Atlantic Ocean are difficult to explain as ice-dammed glaciolacustrine water bodies without invoking improbable ice-sheet marginal geometries. They may, however, be explained as a set of successive ‘fjord-head’ grounding-line depositional systems that trace temporary still-stands in the phased retreat of the Irish Ice Sheet from its offshore maximum. This possibility, not yet confirmed by the discovery of in situ marine fauna, implies a degree of Late Midlandian isostatic depression and subsequent response unexplained by currently applied crustal response models but supported by previously constructed glacio-sedimentary models.