Abstract

At Portballintrae, Northern Ireland, a well-preserved emergent facies sequence formed around 17 kyr B.P. as the Late Devensian ice sheet withdrew from the continental shelf. Three main lithofacies assemblages are recognised: (1) At the base of the sequence a massive muddy diamict was formed by resedimentation of an unstable sedimentary apron deposited in a tidewater setting. (2) Rhythmically-bedded sand and mud units contain a wide range of wave diagnostic features and are separated from the underlying diamict by both transitional and wave-eroded planar contacts. Palaeowave calculations suggest deposition in a coastal tract open to Atlantic waves. The. preservation and range of boreo-arctic foraminifera present are also typical of a lower shoreface, storm-influenced setting. (3) Interbedded gravel and sand facies are separated from the underlying rhythmically-bedded facies by a planar wave-cut surface. This facies occurs as crudely organised beds within cross-cutting, shallow scours which are associated with off-shore directed, storm-induced sediment pulses. Overall the coarsening-upwards sequence is explained by tidewater sedimentation during ice wastage, rapid isostatic rebound and isostatic deceleration in the late glacial. An isostatic-tectonic model comprising a forebulge on the outer continental shelf and a peripheral trough on the inner shelf is presented to resolve the apparent discrepancies between onshore and offshore sea-level data.