LATE QUATERNARY RECORD OF HYDROLOGIC AND ENVIRONMENTAL CHANGE FROM A SMALL KETTLE IN CHAMPAIGN, ILLINOIS, USA

In 2005, an exposure of a ca. 0.4 ha kettle on the Pesotum Moraine was briefly available for study during construction of a flood control basin (40.010° N, -88.327° W). A monolith of fossiliferous materials from this site was collected and frozen for future investigations. Resting above till and outwash of the last glaciation, the 1.4-m lacustrine succession included 64 cm of interbedded diamicton and silt loam (Unit A); 30 cm of silt loam (Unit B), and 46 cm of laminated silt loam penetrated by krotovina (Unit C). Approximately a meter of organic-rich soil had been removed.

Unit age range and key fossils include: Unit A (21.8-20.3 ka); Dryas integrifolia, Candona candida, Succinea sp.; Unit B (12.8-11.6 ka); abundant sedge, rare Fabaeformiscandona rawsoni and Heterocypris sp. and, Unit C (3.9-0.54 ka); abundant ostracode valves (Candona paraohioensis, C. ohioensis, Cyclocypris ampla, Cypridopsis vidua, Limnocythere verrucosa). Notable characteristics of texture and clay mineralogy include: Unit A; sand and gravel, abundant illite and chlorite, little smectite; Unit B, vermiculite and illite; Unit C, mostly expandable clays (smectite); no sand.

Collectively, Units A and B represent two pulses of sedimentation into a shallow hydrologically open pond induced by landscape instability. Unit C was deposited during a period of stable hydrologic conditions. The high sediment accumulation rate of Unit A is attributed to slope instability induced by ablating permafrost. A 4 cm layer of sand and fine gravel separates Units A and B marking a ca. 6 ka hiatus. The boundary between units B and C is unremarkable but represents a ca. 10 ka disconformity. Occurrence of a permanent pond (that persisted for at least 4 ka) in a hydrologically open system suggests damming of the outlet by beavers or humans. The age of Unit C, and dominance of smectite, indicates that the source of the latter is from erosion of very-poorly drained soil in which smectite had (neo)formed. There is a surprising lack of charcoal in the record.

Other resource-rich depressions occur locally and throughout the region. Examination of additional “wet spot” depressions may yield discontinuous records that collectively reveal patterns of land- and hydroscape evolution.