DEVELOPING A GEOARCHAEOLOGICAL DECISION SUPPORT TOOL FOR IOWA ARCHAEOLOGISTS

A basic principle that Wakefield Dort imparted to students was that the subsurface is the key to a landscape's geomorphology. In the Midwest, landform sediment assemblages (LSAs) proved a valuable tool for mapping the alluvial landscape because their definition also includes the underlying lithostratigraphy. LSAs allow archaeologists to anticipate the relative age of sites to be expected in project areas, prior to survey.

The volume of LSAs with archaeological potential is intimidating. In a database compiled from Iowa DOT bridge borings, Holocene alluvium is up to 35 m thick, but in 95% of borings is >19 m thick (mean 7.7 m). Much of this thickness, however, is high energy, bottom stratum sediments deposited in channels not susceptible to site occupation or preservation. Buried archaeological sites are most likely to occur in low energy, top stratum deposits. In Iowa DOT borelogs, mean top stratum thickness is 3.3 m (s.d. = 3.0 m).

Overbank aggradation typically slows through time, with soils forming late in the cycle as surfaces stabilize. Buried sites should be most abundant and detectable in soils formed in the top strata. In 868 alluvial soil profile descriptions from Iowa geoarchaeological literature, solum thickness averages 128 cm thick (standard devation, 71 cm).

These considerations can be applied in archaeological survey areas to target buried site prospection to specific depth intervals. Field applications in Iowa suggest that a relatively few cores can provide archaeologists with sufficient geologic data to plan and implement subsurface testing projects. Estimates of top stratum, solum, and historic alluvium thickness are perhaps as important to archaeological survey design as LSA maps and detailed stratigraphic descriptions.

As part of its Landscape Model for Archaeological Site Suitability (LANDMASS), the University of Iowa Office of the State Archaeologist, LANDLogs is an on-line database and geographic information system that compiles descriptions of Holocene alluvium in Iowa from published and unpublished geoarchaeological sources. Stratigraphy is encoded according to LSA and lithostratigraphic units using lithofacies and weathering zone terminology. LANDLogs is an innovative way to organize and disseminate data important to the human habitability and archaeological preservation of alluvial landforms and sediments.