~ t Pergamon PII: S0277-3791(97)00022-X Quaternary Science Reviews, Vol. 16, pp. 779-791, 1997. © 1997Elsevier ScienceLtd. All rights reserved. Printed in Great Britain 0277-3791/97 $32.00 SEDIMENTOLOGY OF LATE GLACIAL CLAYS IN LACUSTRINE BASINS, CENTRAL IRELAND JAAP J.M. VAN DER MEER* and WILLIAM P. WARRENt *Fysisch Geografisch en Bodemkundig Laboratorium, University of Amsterdam, Nieuwe Prinsengracht 130, 1018 VZ Amsterdam, The Netherlands (E-mail: j.j.m.meer@frw.uva.nl) tGeological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland Abstract - - Distal bottom sediments of a large ice marginal (interdomal) lake that covered most of the central part (midlands) of Ireland during the last deglaciation were poorly known. Recent work centred on two raised-bog covered basins confirmed relatively thick distal glaciolacustrine sediments. Undisturbed samples taken in 100 mm cores enabled thin sectioning and micro- morphological 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 lamination becomes more common until the whole deposit is very finely and rhythmically laminated. Turbidites, a consequence of density underflows, together with coarse sediment rain-out and dropstones confirm a likely ice marginal environment. The finer sediment towards the top of the sequence reflects increasing distality. Synsedimentary folding and faulting, load-casting and water-escape structures point to rapidly accumulating sediment in an unstable sedimentary environment. The lack of bioturbation is not surprising and apparent minor traces towards the base of the rhythmites seems most likely to relate to synsedimentary faulting. © 1997 Elsevier Science Ltd QSR INTRODUCTION The ice sheet of the last glaciation in Ireland was composed of a number of ice-domes which began as individual ice caps, coalesced to form an icesheet covering almost the whole of Ireland and finally re- emerged as individual ice bodies during deglaciation (Fig. 1). The two largest of these, the Northern Dome and the Central Dome, sustained between them an expanding interdomal multibasin lake as they shrank back towards their respective centres. Initially the lake was supported almost entirely by ice margins and its depth at this stage cannot be ascertained. As deglacia- tion progressed the lake became increasingly supported by land and its surface level was controlled by this. Although delta surfaces have not been confirmed, probable washing surfaces and the maximum height achieved by subaqueous fans together with the level of the emerging terrestrial lake margins suggest that lake depths were quite shallow (of the order 30 m at maximum). Although contemporaneous ice margins for the separating domes have not been determined a schematic representation of the expanding lake is presented in Fig. 2, The midlands of Ireland are characterised by glaciolacustrine deposits and eskers. Ice contact subaqueous fans abound, both in association 779 with the eskers and in isolation and the bog basins are known to be underlain by lake bottom sediments The postglacial landscape is one of peat-filled lake basins, esker ridges and subaqueous fan mounds. In the course of a multidisciplinary study of two raised bogs in Central Ireland we mapped and examined the infilling of the bog basins in relation to the regional (hydro-)geology (Warren et al., in press). The two bogs studied, Clara Bog and Raheenmore Bog are located in County Offaly, central Ireland (Figs 1-3). The topogra- phy of the midlands is characterised by alternating raised- bog basins, sand/gravel ridges and mounds, and till mounds. The sand/gravel features comprise eskers and subaqueous ice marginal fans which were deposited in close association with one-another (Warren and Ashley, 1994). The fans represent the proximal glaciolacustrine deposits of an extensive inter-domal lake which spread across the midlands during deglaciation of the ice sheet of the last glacial event and into which much of the finer sediment that was carried through the subglacial (esker) tunnels was vented. The proximal fan sediments are well exposed in gravel pits throughout the area but the distal lake-bottom sediments are not commonly exposed. Fine clays and silts were known, from augering, to line the bog basins beneath the peat, but they had never been closely examined. The objective of this paper is, in describing the