35-15 Origin of Deposits in Nasmith Brook Valley, Central Vermont; Glacio-lacustrine Sediments Overlain by Thick Till

Session: Ice sheets, glaciers, and landscapes, oh my! (Posters)

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Abstract:

During retreat of the Late Wisconsinan Laurentide Ice Sheet through central Vermont, several proglacial lakes existed that eventually merged to form Glacial Lake Winooski (Larsen 1987). Deposits in these lakes are characterized by extensively distributed bedded sands and silt-clay varves with coarser sediments present in deltas. However, in the upper Winooski basin, the tributary valleys of Jail Branch, Great Brook, and Nasmith Brook contain thick Pleistocene sedimentary sections that lie at elevations up to and significantly above Lake Winooski level (Dunn and Springston 2019; Wright et al. 2024). The deposits are thick and characterized by rapid vertical changes in bedding character and grain size, and a variety of soft-sediment and glacio-tectonic features are common. Additionally, each location is capped by up to 10 m of dense, compact silty diamict. The lower part of the deposits has been interpreted as glacio-lacustrine, having formed in a proximal, proglacial position marked by fluctuating energy and sedimentation rates, as evidenced by the common occurrence of sediment gravity flow deposits and abundant soft-sediment deformation. The glacio-lacustrine deposits were in turn overridden by readvancing ice (Larsen 2001), which produced glacio-tectonic features and a thick till (Dunn et al. 2015, 2019).

Recent landslides have exposed eleven sections in Nasmith Brook valley, and these were investigated for comparison and correlation to previously mapped deposits. Nasmith deposits are dominated by well-sorted sand and silt, with local occurrences of sand-silt rhythmites or coarse imbricated gravels. Some sections display ice-contact deposition, and soft-sediment deformation is fairly common. All sections are capped by an 8-10 m thick silty, compact diamict. The uppermost part of the bedded sand-silt deposits exhibits folding or dismemberment, and it is marked by an erosional unconformity with the overlying diamict. Bedded sand-silt sediments are commonly sheared or rafted into the base of the diamict. In general, the sections represent glacio-lacustrine deposition, in some cases with a rapid rate of deposition, followed by glacio-tectonism and erosion to an unknown depth by overriding ice which then deposited a thick till. This study demonstrates that ice readvanced in the Winooski basin at least as far east as East Barre and Marshfield.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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