Early Holocene alluvial stratigraphy, chronology, and Paleoindian/Early Archaic geoarchaeology in the Loup River Basin, Nebraska, U.S.A.

摘要

The preservation of alluvium dictates the discovery of archaeological sites in valleys. We address two questions in our study of the Loup River Basin in the midcontinent of North America: (1) What is the alluvial chronology of the Loup River Basin between about 10,300 and 7700 BP?; and (2), What is the potential for preservation and exposure of Paleoindian and Early Archaic sites? Cleaned cut banks, walls of backhoe trenches and archaeological test units, cores collected with a trailer-mounted Giddings Soil Probe, and bucket auger samples were described. Buried A horizons, organic-enriched alluvial strata, peat, and charcoal were sampled for radiocarbon dating. Dating was supplemented by temporally diagnostic cultural materials. We recognize the absence of 14,000-10,300 BP alluvium in the Loup River Basin. Floodplain stability/very slow aggradation occurred from 10,300-10,000 BP. Rapid floodplain aggradation is inferred between 10,000 and 9450 BP. Between 9450 and about 9250 BP erosion initially produced the Kilgore Creek Terrace. The valley-floor landscape stabilized from about 9250 to 9100 BP before an episode of rapid aggradation overwhelmed floodplains and alluvial/colluvial fans. The fans stabilized about 8600-8500 BP, while floodplains in larger valleys (>5th-order) stabilized shortly after about 8400-8200 BP. Renewed valley-bottom aggradation after 8200 was interrupted at about 7750. The absence of alluvium dated between 14,000 and 10,300 BP in the Loup River Basin precludes discovery of Early Paleoindian sites in valleys, and contrasts with the presence of alluvium and buried soils containing Clovis and Folsom Paleoindian sites in other drainage basins in the U.S. Central Great Plains. Although the distribution of the Kilgore Creek Terrace is an excellent proxy of preservation of 10,300-9450 BP alluvium in the Loup River Basin, and thus Late Paleoindian sites, very few remnants of this terrace have been preserved. Two early-Holocene erosional episodes have been documented in the Loup River Basin at 9450-9250 BP, and again immediately before 8300-8200 BP. Alluvium deposited between 10,300 and 9450 contains one archaeological site, and alluvium deposited 9250-8200 contains five other Paleoindian sites. Five Early Archaic sites are documented in three alluvial/colluvial fans and on a terrace remnant in the North Loup River Valley, but have not been discovered in an alluvial strati-graphic context outside of fans. Although stable surfaces are much more likely to accumulate a greater density of artifacts than sedimentary units that are deposited rapidly, we document three Paleoindian components that are in alluvium between buried soils.