LATE PLEISTOCENE MORAINAL BANK FACIES AT GREYSTONES, EASTERN IRELAND - AN EXAMPLE OF SEDIMENTATION DURING ICE MARGINAL RE-EQUILIBRATION IN AN ISOSTATICALLY DEPRESSED BASIN

A late Pleistocene morainal bank is sited in a depocentre to the lee o f a major rock ridge, near Greystones, in the western Irish Sea Basin. During deglaciation the ridge provided a pinning point during tidewat er wastage northwards. Sedimentation patterns and palaeocurrent data s how morainal bank growth by discharge from a single basal efflux locat ed to the east or south-east of the ridge during ice marginal re-equil ibration. The four lithofacies associations which are recognized from the western part of the formerly more extensive apron are related larg ely;to variable jet and plume sedimentation. At the base of the 1 . 6 km long exposure, Lithofacies association 1 (massive mud, muddy diamic t and laminated mud) was deposited from turbid plumes, variable ice ra fting and traction current activity. Lenticular units of gravels withi n this mud bank record high energy pulses and sediment fluxes from the efflux jet. Lithofacies association 2 (sands, laminated muds and mudd y diamict) is discontinuous and occurs within basins along a marked er osion surface cut in Lithofacies association 1. It is associated with a decrease in jet strength, traction currents and suspension sedimenta tion. Lithofacies association 3 is a tabular body of interbedded diami cts and gravels which is present along the entire section. It document s the decay phase of re-equilibration as the ice margin disintegrated catastrophically and released large volumes of heterogeneous sediment which was resedimented by quasicontinuous mass flow. Lithofacies assoc iation 4 consists of stratified and massive gravels within distributar y channels cut into underlying facies and represents the last phase of meltwater activity. Sediment geometries, particularly sedimentary con trasts representing erosion surfaces at a variety of scales and abrupt textural contrasts are attributed to jet switching. Lithofacies assoc iation 1 (60%) and Lithofacies association 3 (30%) are the dominant fa cies. In favourable topographic settings this stratigraphic couplet is a signature for re-equilibrated ice margins in isostatically depresse d basins dominated by tidewater fronts, rapid ice flux and high relati ve sea level. Morainal banks document rapid environmental change and i n the Irish Sea Basin they form part of a deglacial event stratigraphy related to unstable tidewater margins and high relative sea level. De glaciation was therefore controlled primarily by high relative sea lev el rather than climatic forcing. Facies variations should therefore no t be used for stratigraphic correlations in place of direct stratigrap hy. This type of situation may be more common than hitherto realized i n Late Pleistocene, mid-latitude shelves where most of the preserved s tratigraphy is characterized by complex, interbedded sequences formed when isostatic depression exceeded sea-level fall.