HOLOCENE ALLUVIUM IN A THIRD-ORDER STREAM ON THE IOWAN SURFACE

Holocene-age alluvial deposits were studied in a small perennial stream with a drainage basin of less than 45km2. This stream is located on the Iowan Surface, where loamy Wisconsinan sediments overly Pre-Illinoian glacial till. Alluvial deposits were identified as members of the DeForest Formation.

A 66m surveyed transect across Dry Run Creek Valley near the University of Northern Iowa revealed the presence of a Late-Wisconsinan surface and a Holocene floodplain. The Holocene-age deposits are found along the 31m nearest the stream. Three holes, numbered 1, 2, and 3 starting at the stream, were cored into Holocene alluvium at regular intervals along the transect using a 4-inch bucket auger. The holes were cored down to coarse gravel deposits. Gravel was encountered at depths of 110cm in core hole #1, 130cm in core hole #2, and 160cm in core hole #3. The gravels in core hole #3 were deeper due to the presence of an abandoned channel. Minimal pedogenesis was seen in the alluvium from core hole #1; in the remaining two cores, soil developed in the alluvium consisted of humus enrichment in the upper 20cm. There is no evidence of B horizon development. All soil colors were 10 YR in the Munsell Color Book. The local water table was never reached in any of the core holes, so no radiocarbon-datable material was recovered to obtain absolute ages for the alluvial depositional units. Particle size and organic carbon analyses were performed to help characterize properties of the alluvium and soils.

The purpose of this study was to discover if the Holocene-age alluvial units in Dry Run Creek Valley on the Iowan Surface are similar to the Holocene deposits found in other valleys in eastern Iowa. Our preliminary interpretations of the loamy sediments show two, and possibly three units of the Roberts Creek Member of the DeForest Formation. No Gunder Member is present along the valley cross-section. A thin layer of Camp Creek Member overlies the floodplain of the two holes adjacent to the stream. Overall, our results indicate that alluvium in Dry Run Creek Valley is similar to alluvium in other small valleys in eastern Iowa.