Spatial and temporal patterns of Midcontinent USA colluvial landscapes during the Holocene; interpreting the archaeological record; Geological Society of America, 2000 annual meeting

Alluvial fans and colluvial slopes were used extensively by prehistoric inhabitants of the midcontinent USA. The sediments comprising these landforms harbor some of the most well preserved and continuous records of human habitation on the continent. Geoarchaeological investigations have revealed spatial and temporal patterns of sedimentation that control regional and local visibility, preservation, and distribution of archaeological deposits in colluvial slope and alluvial fan settings. Regional colluvial slope and alluvial fan sedimentation began c.a. 8,500 B.P. Two major fan and slope building episodes are represented in most areas; 8,500-6,500 and 6,000-2,000 B.P. A well expressed buried soil, formed during the period c.a. 6,500-5,000 B.P., separates sediments of the two major aggradation episodes. Within each of the major aggradation episodes sedimentation was episodic, with shorter intervening periods of reduced sedimentation rate and soil formation. In parts of the region where middle Holocene drying was delayed, alluvial fan and colluvial slope building did not begin until after 5,500 B.P., and the early aggradation episode is absent. Periods of colluvial slope and alluvial fan aggradation correspond to degradation periods in the tributary valleys and bordering valley margin slopes. During the late Holocene, c.a. 2,500-2,000 B.P., fan-head trenches developed and most tributary- and slope-derived sediment bypassed colluvial slopes and alluvial fans. The focus of sedimentation then shifted to distal fan lobes. Late Holocene shifting of feeder stream trenches removed earlier apex and mid-fan fan sediments, formed a complex series of cut-and-fill fan-trench sequences, and deposited a series of coalescing distal fan lobes. Archaeological deposits older than about 8,500 B.P. pre-date aggradation of Holocene alluvial fans and colluvial slopes and are not found in these landforms. Archaeological deposits younger than 2,000 B.P. are shallowly buried on colluvial slopes and alluvial fans except in filled fan-head trenches and distal fan lobes. Fan-head trench areas are sedimentologically and stratigraphically very complex, with later archaeological components having a greater preservation potential.