Patterns in Holocene colluvium and alluvial fans across the prairie–forest transition in the midcontinent USA

Abstract The sediments composing alluvial fans and colluvial slopes harbor some of the most well‐preserved and continuous records of human habitation on the North American continent. Geoarchaeological investigations in the midcontinent of North America 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 ca. 8,500 yr B.P. Two major fan and slope building episodes are represented in most areas: 8500–6500 and 6000–2000 yr B.P. Within each of the major aggradation episodes, sedimentation was episodic, with shorter intervening periods of reduced sedimentation and soil formation. 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 approximately 8500 yr B.P. pre‐date aggradation of Holocene alluvial fans and colluvial slopes and are not found in these landforms, while archaeological deposits younger than 2000 yr B.P. are shallowly buried on colluvial slopes and alluvial fans except in filled fan‐head trenches and distal fan lobes. The challenge for interpreting and managing cultural resources in alluvial fan and colluvial slope settings is to properly evaluate how geologic processes have modified cultural patterns and affected the visibility of the archaeological record. © 2003 Wiley Periodicals, Inc.