Genesis and Sedimentation of an Ice-Walled Lake Plain in Northeastern Illinois

ice walled lake plain;Illinois glacial geology;radiocarbon dating;deposition model

Petras, Justine ; Best ; James L.

关键词: ice walled lake plain;    Illinois glacial geology;    radiocarbon dating;    deposition model;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/16797/1_Petras_Justine.pdf?sequence=2&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Ice-walled lake plains (IWLPs) form along glacial margins in areas of stagnating ice. In Illinois, IWLPs tend to have low relief (<7m) and were formed during the last deglaciation of the Lake Michigan lobe (20,800 to 16,000 cal. yr BP).Past research on IWLPs in Illinois has focused on developing the chronology of ice stagnation by obtaining radiocarbon ages of tundra plant fossils deposited within the IWLP sediments.The goals of the present study were to: i) determine the rate of expansion of a single IWLP, ii) detail the sedimentology of the IWLP and iii) propose a depositional model for development of the IWLP.The study was conducted on a single IWLP in Newark, Illinois, located in the Marseilles Morainic System.Seventeen radiocarbon ages of tundra plant macrofossils were obtained from cores sampled across the lake basin.The age range verifies the values obtained from an adjacent IWLP; the mean pooled ages indicate the basin developed at about 21,150 cal. yr BP and was drained shortly after 20,760 cal. yr BP.The ages obtained from samples of the basal lacustrine facies were statistically similar across the lake basin, implying that the lake expanded to near its maximum diameter within 106 years (the sigma one error of the mean pooled ages).Coring, particle size analysis, clay mineralogy and study of the sedimentology revealed five distinctive facies, with facies one being the oldest and facies five the youngest. Facies one is a matrix-supported silt loam diamicton that contains abundant illite and few expandable clay minerals.Facies one is draped by a lag of sand and gravel (facies two) that was deposited by a combination of the calving and melt-out of debris-rich ice that formed the basin sides.Facies three is lacustrine in origin as inferred from its rhythmic bedding and fossil lacustrine ostracodes.The laminae were mainly deposited through grain settling from hypopycnal density currents, with the particle settling velocities revealing that the laminae were deposited in a short period of time (perhaps weekly or monthly), while cumulative settling times reveal longer periods of non-deposition.These rhythmically-bedded laminae, that have previously been thought to be annual varves, appear to have clay layers that are too thin and inconsistent for annual varves.The fourth facies is also considered lake sediment for the same reasons as facies three, but facies 4 also contained thicker sand beds.Bedforms and scours within these sands were formed from hyperpycnal density currents whilst the laminae within facies four are attributed to hypopycnal flows.Facies five caps the lake basin and is the loess that was deposited at the end of the Wisconsin glaciation. A depositional model for the development of the ice walled lake (IWL) is proposed that conjectures the lake may have originated within a remnant moulin within a crevasse.