Seasonal Controls on Deposition of Late Devensian Glaciolacustrine Sediments, Central Ireland

Abstract

Laminated proglacial glaciolacustrine sediments dating from the Late Devensian (22-10 Ka BP) from central Ireland were examined using a combination of detailed logging and Scanning Electron Microscope (SEM) microfabric analyses. The sediments are rhythmically laminated and consist of coarser, pale silt layers which alternate with darker clay layers containing occasional thin laminae of fine sand and coarse silt. The pale silt layers contain single or multiple normally graded laminae, erosional surfaces and soft sediment deformation structures, indicating deposition from multiple high density underflows. The dark clay layers have sharp upper and lower contacts and an internal fabric consistent with deposition from a combination of flocculation and grain-by-grain deposition. Silt laminae within the clay layers are interpreted as sporadic turbidity underflows. The sediments are interpreted as annually laminated (varved). Varves deposited close to the ice margin showed considerable spatial variation in thickness and adjacent sequences could not be correlated; however sequences of medial varves separated by 500 m were correlatable by thickness. As glaciolacustrine deposits are widespread throughout the Irish midlands, it is likely that a varve chronology could be constructed.

Electrical resistivity and Ground Penetrating Radar for the characterisation of the internal architecture of Quaternary sediments in the Midlands of Ireland

“…Moreover, glaciolacustrine deposits described in the Irish Midlands (Delaney, 2002) envisaged a glacial lake (Glacial Lake Riada) with a maximum water table at 92 m ordnance datum (OD) in the study area during deglaciation times. This is supported by mushroom stones mapped by Feehan and Dunne (2003) indicating presence of long term shorelines of a glacial or post glacial lake in the Irish Midlands and by varved sediments deposited during Late Weichselian times (22-10 Ka BP) in a proglacial lake environment (Delaney, 2008).…”

“…Diamicton overlying the bedrock was deposited under subglacial conditions; these deposits are overlain by two main features, the esker ridge and the fan. The existence of Glacial Lake Riada with an approximate water level at 92 m OD (Delaney, 2002(Delaney, , 2008 requires these features to form either under subglacial or subaqueous conditions. The formation of subaqueous outwash fans associated with eskers can develop either, (i) subglacially within side cavities adjacent to the main tunnel formed in areas with high hydrostatic pressure (Gorrell and Shaw, 1991), (ii) near the ice margin, as ice-walled channel deposits, defined by the ratio of their length to their width and defined as long bead eskers by Warren and Ashley (1994), or (iii) at the ice margin snout as short bead eskers or ice marginal subaqueous fans (Warren and Ashley, 1994).…”

Irish Ice Sheet dynamics during deglaciation of the central Irish Midlands: Evidence of ice streaming and surging from airborne LiDAR

“…At its maximum extent the lake drained through a col at 82 MOD, 10 km east of Tullamore. This is also the height of many ice-contact deltas around the lake basin, indicating a relatively stable lake surface level (Delaney, 2002(Delaney, , 2007Pellicer et al, 2012). Further recession of ice westward is presumed to have allowed drainage of the lake southward around the western margin of the Slieve Bloom uplands.…”

Karst subsidence as a control on the accumulation and preservation of aeolian deposits: A Pleistocene example from a proglacial outwash setting, Ebro Basin, Spain

“…These small proglacial lakes developed in the temporarily inactive areas of the outwash plain and episodically received large volumes of sediment‐laden runoff (cf. Sturm & Matter, 1978; Eyles & Eyles, 1992; Delaney, 2007). In some cases lutites integrate fining‐upward laminae and could represent summer–winter cycles but, alternatively, it is possible that they resulted from individual events.…”

“…In some cases lutites integrate fining‐upward laminae and could represent summer–winter cycles but, alternatively, it is possible that they resulted from individual events. Coarser‐grained laminae interbedded with rhythmically laminated lutites correspond to underflow currents entering the lakes during periods of high sediment‐laden runoff, whereas finer‐grained laminae are due to settling from suspension thereafter (Gustavson, 1975; Sturm & Matter, 1978; Eyles & Eyles, 1992; Delaney, 2007; Osleger et al. , 2009).…”