SEDIMENTOLOGY OF LATE-GLACIAL CLAYS IN LACUSTRINE BASINS, CENTRAL IRELAND

Distal bottom sediments of a large ice marginal (interdomal) lake that covered most of the central part (midlands) of Ireland during the las t deglaciation were poorly known. Recent work centred on two raised-bo g covered basins confirmed relatively thick distal glaciolacustrine se diments. Undisturbed samples taken in 100 mm cores enabled thin sectio ning and micromorphological analysis to be carried out. The upper part of the fine sediment (mostly clay), which is apparently homogenous at the macroscopic level, is shown to have been bedded or laminated and subsequently homogenised by load-casting. With depth, bedding or lamin ation becomes more common until the whole deposit is very finely and r hythmically laminated. Turbidites, a consequence of density underflows , together with coarse sediment rain-out and dropstones confirm a like ly ice marginal environment. The finer sediment towards the top of the sequence reflects increasing distality. Synsedimentary folding and fa ulting, load-casting and water-escape structures point to rapidly accu mulating sediment in an unstable sedimentary environment. The lack of bioturbation is not surprising and apparent minor traces towards the b ase of the rhythmites seems most likely to relate to synsedimentary fa ulting. (C) 1997 Elsevier Science Ltd.