STRATIGRAPHIC CONTEXT OF THE CARLISLE CLOVIS CACHE IN CENTRAL IOWA

An artifact cache, including 25 bifaces, 12 large flakes, and 6 small flakes, was recovered from the Des Moines River Valley in central Iowa during salvage of a late prehistoric Oneota village for a levee construction project. Although the cache was anomalous by Oneota standards, in the absence of a Clovis point it is easy to understand its initial late prehistoric association. While levee construction erased the find-spot in 1968, we determined the stratigraphic position of the find nearby, thereby reinforcing our initial technological assessment that the cache is actually assignable to the Clovis Complex

The study area is a pasture ~500 m N-NW of the find-spot. The landform here consists of an extensive terrace remnant standing ~7.5 m (25 ft) above the modern low-water Des Moines River at 238.98 m (784 ft). Backhoe Trench 1 exposed ~1.6 m of Holocene alluvium belonging to the Early Gunder Member of the DeForest Fm. The alluvium between depths of 237.78 m and 237.92 m is brown to dark brown (10YR 4/3), which matches the color of the alluvium on which the cache was resting. Our reconstructed position thus places the cache just over 1.0 m below the uneroded terrace surface.

Four OSL ages from Trench 1 are both stratigraphically consistent and within the range of radiocarbon ages previously reported for the Early Gunder Member, with a single exception. Converting OSL sample no. 3 (12,380 ± 870) to a range of uncalibrated radiocarbon ages narrows the age of the cache position to 11,280-9985 B.P. The upper end of this age-range dovetails with current age estimates for the Clovis Complex.

Particle-size analyses of samples from Trench 1 reveal a lithologic discontinuity at 237.60 m that probably reflects a channel avulsion on the aggrading floodplain. The dominance of silt and clay above (elevations 237.60 m to 237.92 m) is typical of a backswamp area away from the active channel. The reconstructed stratigraphic position of the cache is on the surface of these fine-grained deposits. These were then rapidly buried as a result of another channel avulsion. The δ13C data reveal a gradual up-profile trend of values becoming increasingly positive through time, with values near -20‰ at the base and -15‰ at the surface of pre-settlement alluvium, suggesting a gradual conversion from a mixed C3-C4 biotic environment to a predominantly C4 environment.