HOLOCENE ALLUVIAL GEOLOGY OF THE EAGLE TREE ARCHAEOLOGICAL SITE (48CO2920), CONVERSE COUNTY, WYOMING

Archaeological data recovery performed by GCM Services at the Eagle Tree archaeological site (48CO2920) in Converse County, Wyoming facilitates study of alluvial stratigraphy. Investigations document aggradation of Kaycee Formation alluvium and stabilization of the terminal Kaycee Formation floodplain within the central axis of a tributary of Antelope Creek. Alluvial fill consists of a ~ 7 m thick, fining-upward sequence of basal channel gravel, transitioning to fine-bedded channel-to-channel marginal sand and silt, overlain by overbank mud engulfed by a surface soil containing a Bw horizon. Thin section microscopy by Paul Goldberg at Boston University, supports an overbank origin for the surficial mud. Chronometric control includes an age on an underlying hearth feature at 7.6 mbs dating to ~11,304 ka, an OSL date of ~5660 ka at 1.1 mbs, and Late Prehistoric era hearths at 0.1 mbs that range in age from ~984-1669 ka. Results indicate that the Kaycee terrace tread is underlain by a ~7 m thick sequence of Holocene-era Kaycee Formation alluvium which compares well to the Leopold and Miller model and fits well with John Albanese’s late Holocene age estimate for terminal Kaycee Formation alluviation.