LATE-GLACIAL TO EARLY POST-GLACIAL SEDIMENTOLOGICAL EVOLUTION OF SUNSET LAKE; A SMALL, UPLAND BASIN IN CENTRAL VERMONT

Sediment cores were obtained from Sunset Lake, central Vermont. The lake, at 1212’ asl, lies well above the level of glacial Lake Hitchcock. Bedrock in the area is calcareous metasediments. To determine late-glacial to early post-glacial history of the basin, core sediments were analyzed for grain size, loss-on-ignition, X-ray diffraction, and macrofauna, and two radiocarbon dates were obtained. A basal unit of pebbly sand overlain by silt and clay is interpreted as a proglacial deposit that formed when ice sat to the north. Above the proglacial lake deposit is a 15-30+ cm thick, nearly pure calcium carbonate deposit (a marl), which is overlain by a thick organic-rich mud.

The marl contains calcite and aragonite, as matrix and mollusc shell, respectively, and virtually no clay minerals or other silicates. The marl unit contains plant fragments that are not rooted, suggesting they were washed into the basin. Mollusca include the gastropods Viviparidae, Planorbida, and Valvatidae as well as unidentified bivalves. Ostracods are also abundant. A calcite mineralogy and lack of clays in the marl matrix, and a lack of microbial borings on the mollusc shell, indicate that the marl unit is a direct precipitate postdating glacial sedimentation in the basin, but predating the onset of deposition of organic-rich mud.

Two AMS radiocarbon dates were obtained from the basal 10 cm of the marl. Mollusc shell gave a conventional radiocarbon age of 10,440 +/- 50 y.b.p. (Cal BP 12,820-12,060 and Cal BP 12,030-11,960, 2 σ), and plant remains gave a radiocarbon age of 10,010 +/- 40 y.b.p. (Cal BP 11,640-11,270, 2 σ). We suggest that after the ice margin retreated north of the local drainage divide, Sunset Lake was largely fed by groundwater moving through calcareous bedrock. The lake quickly became saturated with respect to CaCO₃ and microcrystalline calcite precipitation commenced, producing 30+ cm of marl. The precipitation of marl ended relatively quickly, marked by sharp contact in cores, and organic-rich mud deposition overwhelmed the basin.

The marl has received virtually no attention in Vermont, but a cursory investigation of upland lakes and wetlands suggests this unit commonly overlies glacial deposits and thus warrants investigation as a source of geological and ecological information for the immediate post-glacial period.