Geochronology and Geomorphology of the Pioneer Archaeological Site, Upper Snake River Plain, Idaho, USA

Abstract

Pioneer is an open‐air, stratified, multicomponent archaeological site located in the upper Snake River Plain of southeastern Idaho, USA. Block excavations provided an opportunity to contribute to the Late Quaternary geomorphic history of the Big Lost River drainage and provide geochronological context of archaeological components at the site. The stratigraphic sequence is interpreted as reflecting multiple depositional episodes and five soil‐formation periods beginning pre‐7200 cal. yr B.P. and lasting to the historic period. The stratigraphic sequence contains an archaeological component dated to ∼3800 cal. yr B.P. and several other components post‐800 cal. yr B.P. Major site formation processes include fluvial deposition and erosion, pedogenesis (accumulation of secondary carbonates), and bioturbation. Periods of increased deposition at Pioneer and elsewhere along the Big Lost River are inferred to have occurred between ∼8400–6500 cal. yr B.P. and ∼2700–400 cal. yr B.P., potentially related to cooler/wetter episodes of the mid‐to‐late Holocene, including increased precipitation during the Medieval Climatic Anomaly (post‐750 cal. yr B.P.). There is also evidence of a high‐energy erosional event at ∼3800 cal. yr B.P. indicating a large middle Holocene flood. Pioneer provides an example of the archaeological and paleoclimatic value of studying alluvial buried soil stratigraphic sequences in arid environments.

Landscape response to hydroclimate variability shown by the post-Bonneville Flood (ca. 18 ka) fluvial-geomorphic history of the middle Snake River, Idaho, USA

“…The age of incision and stable channel conditions also coincides with high-energy fluvial erosion at ca. 3.8 ka in the upper Snake River Plain (Keene, 2016).…”

“…8B), as well as increased fluvial deposition in the upper Snake River Plain after ca. 0.75 ka (Keene, 2016). Evidence of increased discharge in the Snake River during the LIA is from the date of detrital charcoal in overbank (lithofacies 6b) deposits on the Qt5 terrace with a median probability 14 C age of ca.…”