6. Clay-dominated rhythmites

1. 6. Clay-dominated rhythmites 109. Glacilacustrine Sediments in the Irish Midlands and the Potential for a Late Weichselian Varve Chronology for the British-Irish Ice Sheet Cathy Delaney, Department of Environmental and Geographical Sciences, Manchester Metropolitan University, John Dalton East, Chester St, Manchester M1 5GD. Email: c.delaney@mmu.ac.uk 4. Silt-Dominated Rhythmites 5. Silt-Clay Rhythmites These rhythmites have coarse units on the same order of thickness as fine units. Coarse units consist of 1-3 laminae, which can vary in thickness. These rhythmites are characterised by coarse (silt) units which are much thicker than fine (clay) units (white arrows). These are similar in appearance to silt-dominated rhythmites, but have thinner coarse units (usually <20mm) and erosional contacts are rare. Coarse units thin upwards in the cores. 1. RATIONALE Clastic varves formed due to seasonally-controlled changes in meltwater discharge and sediment flux can be used to construct long-term integrated chronologies by correlating varve thicknesses across hydrologically linked proglacial lake basins. Such chronologies are of use in reconstructing deglaciation timing and dynamics and in correlating events on land with ice core and marine records, improvingunderstanding of interactions between terrestrial ice sheets, meltwater discharge, ocean circulation and climate change[1,2,3]. Kn4.80-5.30m Kn 6.05-6.55m Kn 5.30-5.80m Coarse units are characterised by combined underflow and interflow/overflow deposition. Erosional contacts (red arrows above), cross-laminated sediments and thin (<2mm) coarse silt/sand laminae are common, indicating operation of underflows[10]. Soft sediment deformation structures, including loading and flame structures, indicate relatively continuous deposition involving loading of fines with high pore water contents. Normally graded laminae without internal lamination most likely reflect suspension deposition from episodic interflows/overflows[11]. The uppermost lamina in the coarse unit is a thin coarse silt/sand laminae and may represent aeolian deposition[12] (see silt-clay rhythmites). Coarse units are dominated by normally graded laminae (up to 10 per unit, but usually 5 or less) with no internal stratification, indicative of suspension deposition from sporadic inputs such as increased meltwater flow or rainfall[11,14]. Fine sand/coarse silt laminae occur within the coarse unit, but are usual immediately below the fine unit, as in silt-dominated rhythmites. In some grains SEM analysis shows upturned plates, characteristic of aeolian transport. Units consisting of three laminae commonly exhibit a coarse-fine-coarse pattern associated with two separate meltwater inflow maxima - the late spring/early summer snowpack melt, and the late summer glacial melt[15]. Proglacial glaciolacustrine deposits formed during recession of the British-Irish Ice Sheet (BIIS) after 22 Ka BP are found across the Irish Midlands (Fig. 1)[4,5]. Rhythmic lamination in these sediments[4,6], is similar in appearance to proven varved sediments in Scandinavia and New England. Potentially, the deposits could be used to date the recession of the BIIS. Coarse units consisting of single normally graded laminae or unstructured, matrix poor coarse silt laminae may represent summer deposition, or may reflect non-seasonal, sporadic deposition from surge currents or by wind. Erosional contact Soft sediment deformation Yellow arrows indicate fine units Upturned plates on quartz grains within coarse unit are indicative of aeolian transport[16]. Rare hollows on surfaces of finer sediments may indicate bioturbation 2. SITE DESCRIPTION AND SAMPLING The sediments described here were formed within a proglacial lake, Glacial Lake Riada, which extended eastwards from the Shannon across the Irish Midlands during deglaciation[5,7]. Cores were retrieved from sites north of a readvance limit dated between 17-11ka BP[8]. Cores sites are c.600m apart and lie parallel to the ice margin, c. 600m south and 1600m (Kn) and 1000m (Ro) west of deltas and eskers formed by point discharge of meltwater into the lake. Two offset cores were taken at each site so that a complete undisturbed sequence was retrieved. Cores were air-dried in the lab to improve laminae visibility prior to sediment logging and sampling. Cores were described to submillimetre resolution; selected laminae were prepared for S.E.M. microfabric analysis[9]. Sand intraclast cross-lamination Normal grading Many normally graded laminae contain multiple clay domains (packages of parallel clay grains) with multiple edge-to-edge contacts between clay domains,indicating deposition partly by flocculation[9]. Fine units have sharp lower contacts consistent with a hiatus in deposition between cessation of summer meltwater input and autumn lake overturning[10]; internal normal grading most likely reflects suspension deposition without further sediment input during winter[10]. Thin coarse silt laminae within fine units are interpreted as surge currents due to periodic slumping[13]. Strong preferred orientation of clay grains towards the top of these laminae is consistent with grain-by-grain suspension deposition[9]. Clay domains in normally graded laminae indicate minor flocculation during deposition. 7. SUMMARY Clay laminae within all rhythmites have features characteristic of deposition after thermocline overturning and are interpreted as winter layers. Occasional thin laminae of coarse silt and sand are interpreted as winter turbidity currents[13]. Silt-dominated rhythmites are interpreted as proximal varves; coarse units contain features consistent with melt season deposition from suspension and current deposition, reflecting the influence of both snowpack and glacial meltwater, as well as wind transport and turbidity current deposition due to lake margin collapse, episodic floods due to rainfall or release of meltwater stored within the ice sheet[10, 11]. Erosional contacts indicate that some winter layers may have been removed and summer layer thicknesses altered. Silt-clay rhythmites are interpreted as medial varves. Melt season deposition is dominated by suspension deposition, reflecting snowpack, glacial meltwater and probably rainfall inputs; current erosion/deposition is rare. Clay-dominated rhythmites with coarse-fine-coarse patterns in coarse units are interpreted as distal varves, summer layers dominated by two meltwater peaks from snowpack and glacial melt. However, coarse units formed of single laminae which could represent either a single, episodic input from wind or surge deposition in any season, or represent a full summer season’s deposition, so a distinct annual signal cannot be identified Thin, fine sand/coarse silt lamina immediately under the winter layer are a distinctive feature in almost all proximal and medial varves. Surface textures indicate these are most likely deposited by wind immediately before autumn freeze. Evidence of limited deposition by flocculation is present in many normally graded laminae. Flocculation in proglacial and cold freshwater environments can be caused by bioflocculation [17] or electrochemical flocculation[18]. It is associated with high sediment concentration and subsequent clearing of the water column due to wind-induced turbulence[19]. 3. STRATIGRAPHY Sediments consist of c0.35m peat overlying c.2.2m of calcareous lacustrine silts and clays (marls) before passing downwards into alternating organic and inorganic, diffusely laminated clay-silts, thought to be Lateglacial deposits. These overlie 1.1-1.2m of laminated silts and clay-silts which lack a distinct rhythmic signal and are interpreted as surge deposits, possibly associated with lake drainage and slumping of the lake margins. The contact between these and the underlying rhythmically laminated sediments is erosional.The final 1.4m(Kn) – 2.3m(Ro) of each core consists of rhythmically laminated silts and clays. Three main facies types were identified. Fine unit with sharp contacts and internal grading consistent with winter deposition • 9. CONCLUSIONS • Varved sediments are present at both sites. • Proximal rhythmites contain a clear annual signal and are formed from a combination of underflows, interflows or overflows and aeolian deposition. However post-depositional erosion and deformation has removed some winter layers and locally high deposition from surge currents has created large variation in summer layer thickness, so that varves cannot be correlated over short distances. • Medial rhythmites are formed from interflow/overflow deposition, with some aeolian transport, contain a clear annual signal and exhibit consistent lamination patterns and thicknesses over 500m parallel to the ice margin. However it isdifficult to distinguish surge currents and wind-blown material from summer layers in thinnest rhythmites. • Errors: between-site comparison of medial varves showed that there are 4 possible false varves present in the 57 varves used for correlation in the Kn core, so that deposition commenced between 1-5 years later than at the Ro site. This implies a likely between-site error of c. 7% in an integrated chronology. However, comparison over several cores is likely to reduce this error. • Within-site error, as indicated by erosion and post-depositional disturbance, is 43 years at Ro and 9 years at Kn. • The evidence suggests that construction of an integrated chronology should be possible for the Irish Midlands. Criteria for site selection and core retrieval are suggested below: • Locate core sites away from likely meltwater discharge points to minimise underflow erosion • Select core sites with reference to lake bottom topography and sediment thicknesses to minimise likelihood of disturbance • Take multiple cores at each site to assist identification of erosional contacts • Locate cores site within 500m of each other, so that visual correlation can be used to support varve thickness • Use microfabric analyses to identify winter layers. • Base correlation on medial varves only. 8. CORRELATION Silt-clay varves and thicker clay-dominated varves could be correlated using thickness or visual comparison. Four rhythmites present in Kn cores but absent from Ro cores are considered false varves as the coarse in each is a single normally graded silt lamina and no fine sand/coarse silt lamina occurs below the fine unit. A fifth rhythmite has an erosional upper contact in the Kn cores, and correlates with an erosional contact in the Ro cores. No correlation in thickness was found for varves <1mm thick. Acknowledgements I thank Roy Gibson and Eamonn Delaney for help with fieldwork. This research was partly funded by the Manchester Geographical Society. Varve thickness is most variable in silt-dominated rhythmites. Individual varves were difficult to identify within offset cores taken less than 5m apart; thickness variations of up to 75mm were measured. In the Ro cores five of the first six winter layers present in one core were absent from the offset core taken 4m away. Correlation of thickness between the two cores sites was not possible. Varve thicknesses in both cores decrease rapidly upwards, indicating that distance from the receding ice margin was the primary control on sediment flux. Total varve numbers were 166 at Knocknanool and 170 at Rooskagh, indicating a similar time period between ice recession and cessation of varve formation. References [1] Andrén, T., Björck, J., Johnsen, S., 1999. J. Quatern. Sci.14, 361-371 [2] Lindeberg, G., Ringberg, B., 1999. GFF121, 182-186 [3] Boulton, G.S., Dongelmans, P., Punkari, M., Broadgate, M., 2001. Quat. Sci. Rev. 20, 591-62. [4] Van der Meer, J.J.M., Warren, W.P., 1997. Quat. Sci. Rev.16, 779-791 [5] Delaney, C. 2002. Sed. Geol.149, 111-126 [6] Long, M.M., O’Riordan, N.J., 2001. Géotechnique51, 293-309 [7] Delaney, C. in press. In: Glacial Sedimentary Processes and Products (Eds. M. Hambrey, P. Christoffersen, N. Glasser, P. Janssen, B. Hubbard, and M. Siegert) Spec. Pub. I.A.S., Blackwells, Oxford. [8] Knight, J., Coxon, P., McCabe, A.M., McCarron, S.G. 2004. In: Quaternary Glaciations - Extent and Chronology. Part 1: Europe (Eds. J. Ehlers, P.L. Gibbard), 183-191. [9] O’Brien, N.R., Pietraszek-Mattner, S. 1998. J. Sed. 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