Late Quaternary loess stratigraphy, soils, and environments of the Shaw Creek Flats Paleoindian sites, Tanana Valley, Alaska

摘要

The late Quaternary stratigraphy of three Paleoindian archaeological sites, located on the edge of Shaw Creek Flats in the middle Tanana Valley, Alaska, consist of up to 2 meters of calcareous eolian sand, loess, and buried paleosols. Two Paleoindian occupations at the Broken Mammoth, Swan Point, and Mead sites date from 11,800 to 11,000 yr B.P. and from 10,800 to 9500 yr B.P. Well-preserved faunal remains, worked mammoth ivory, stone and organic tools, and at Swan Point, the early occurrence of microblades dating to 11,700 yr B.P., are associated with buried paleosols at the base of the loess. Stratigraphic and radiocarbon-chronological correlations between the sites, and at similar geological sections, suggest the presence of a regionally-correlative, eolian stratigraphy consisting of three main units: (1) a basal gray eolian sand, overlying a deflated, ventifacted, bedrock surface, was deposited as bluff-top sand sheets probably during the Birch Period transition from periglacial steppe-tundra environments to shrub tundra about 12,000 to 14,000 yr B.P. (2) An overlying lower loess unit contains three paleosol complexes, classified as Typic Cryorthents, which consist of a series of cumulative Abk horizons overlying Ck loess parent material. The lower paleosol complex dates to 11,800 to 11,000 yr B.P. as is associated with the initial occupation of the sites. The middle paleosol complex dates from 10,800 to about 9500 yr B.P. and is associated with the second Paleoindian occupation. The upper paleosol complex is weakly developed, contains no cultural material, and reflects an increase in loess deposition rates. Abundant pedogenic carbonate features indicate dry, warm, alkaline soil conditions. Faunal remains, soil characteristics, and regional palynological studies indicate a warm, dry, open parkland of poplar-willow scrub forest during the deposition of the lower loess, paleosol formation, and Paleoindian occupations. (3) An upper loess unit, up to a meter thick, lacks paleosols, has been leached of carbonate, and has a late Holocene Alfic Cryochrept soil developed on its upper surface. Rapid, coarse-grained loess deposition occurred from about 9000 to 6000 yr B.P. By 4500 yr B.P., slow, fine-grained loess deposition began under boreal forest conditions and continues today.