SEDIMENTOLOGY OF ESKER SYSTEMS AND ASSOCIATED LANDFORMS IN THE IRISH MIDLANDS: PROCESSES, CONTROLS AND IMPLICATIONS FOR FACIES MODELS

This paper compares two eskers systems within the Irish Midlands. A major east-west trending system consists of five sub-parallel groups of ridges extending over 65km, which are dendritic at their western end and converge at their eastern end. These ridges have been interpreted as interlobate moraines (Warren and Ashley, 1994), but are associated with ice-push ridges and corrugated moraine indicating ice-flow was parallel to the dominant esker orientation. A second, younger NW-SE trending system to the north consists of smaller, shorter ridges arranged parallel to each other at 3-10km intervals, are associated with hummocky moraine are interpreted as time-transgressive eskers composed of overlapping beads formed within an ice-marginal stagnation zone. Eskers in both systems are composites of three morpho-sedimentary elements - conduit/ice-walled channel elements, delta elements, and subaqueous outwash fan elements - each characterised by suites of distinctive facies associations. However, the relative importance of individual mega-scale elements differs between the two systems, as does the proportion of different facies associations within the morpho-sedimentary elements. In particular, conduit/ice-walled channel facies associations containing matrix-rich material characteristic of hyperconcentrated flows and debris flows are of much greater significance in the NW-SE trending eskers, while subaqueous outwash fans and deltas are more prominent in the E/W system. The differences between the two systems have implications for groundwater flow and are suggested to be controlled by two factors: the presence of standing water at the ice margin, and the mode of subglacial water storage immediately upstream from the conduits.

References Warren, W.P., Ashley, G.M. 1994. Origins of the ice-contact stratified ridges (eskers) of Ireland. J. Sedimentary Research, A64, 433-449.