MULTI-PROXY MID-HOLOCENE FLUVIAL PROCESSES AT TWO LOCALITIES IN CENTRAL IOWA

The Palace site (13PK966) is a buried Middle Archaic site in the Des Moines River valley near the city of Des Moines in central Iowa. Several relatively tightly stratified and stratigraphically separated occupation surfaces were inhabited ca. 7,000 BP. Houses were present, and a human burial was also encountered. The archaeological deposits are buried in a silty clay loam overbank facies that veneers a former point bar. The overbank veneer grades laterally to a clay to silty clay channel-fill that was perhaps a wetland, occupying a nearly-filled-in paleochannel at the time of occupation.

Other than an occasional coarse-textured splay or chute deposit, the overbank deposits exhibit no macroscopically visible stratigraphy. Soil thin sections, however, reveal micromorphological evidence for sedimentary bedding in the form of changes in size and sorting of sand grains, for short-lived periods of surface stability in the form of channel microstructures created by bioturbation. d¹³C ratios for the 250 cm thick overbank facies are primarily between -23 and -20 o/oo, values that are intermediate between warm- and cool- season plants. There is, however, a significant excursion in C4 grasses in a 50-cm-thick zone, with ratios of -15 to -17 o/oo. This zone immediately overlies the 7,000-year-old cultural horizon. It may therefore, be the signature of a mid-Holocene warming episode. Pollen records from other central-Iowa sites indicate that the prairie expanded across central Iowa in this approximate time range.

Other recent work by the author on the Middle Raccoon River, about 50 km west of the Palace site, show evidence of a major fluvial discontinuity at ca. 7,000 BP, based on radiocarbon ages from channel and channel-fill facies at the bottom of a 4-5 m thick upward-fining sequence. Large-scale sedimentary structures observe in core and cutbank profiles indicate high energy deposition between ca. 7,000 and 6,500 BP.