ABSTRACT

Lithofacies variability, lithofacies relationships, and sedimentary structures indicate that well-exposed Late Pleistocene glaciogenic sequences in south County Down, Northern Ireland formed in subaqueous environments. Regional considerations of ice-flow patterns and isostatic depression indicate that the sediments accumulated in a glaciomarine basin. Four major lithofacies associations are identified. The first consists of muds, diamictic mud, and stratified sand/gravel beds deposited by dispersed and concentrated meltwater sources. The second occurs as a thick apron of massive and stratified diamictons formed by rainout from debris-rich icebergs and resedimentation seaward of tidewater grounding zones. The third association is preserved as fills in large, subaqueous channels eroded nto the diamicton apron. These ills are characterized by slumping and debris flow (interbedded sand, gravel, diamicton, and mud) from channel flanks; interactions between underflows and overflows (rhythmically bedded mud) along channel axes, and traction-current activity (prograded sand aprons) at the top of the sequence. The fourth comprises a late-glacial, emergent-beach facies (sand and gravel) that truncates the earlier deposits along the entire exposure.

Abrupt variations in dominant clast type, erratics, and matrix carbonate content within the sediment pile reflect indirect glacial input from three distinct geological provenances into the glaciomarine basin. Variability of this type should not be used for stratigraphic correlations in place of direct glacial stratigraphy. This type of situation may be more common than hitherto realized in the late Pleistocene, mid-latitude forelands characterized by complexly interbedded diamicton sequences which formed when isostatic depression exceeded sea-level fall.