MULTI-SCALAR STUDIES OF SOIL GEOMORPHOLOGY AND SOIL PRODUCTIVITY IN PETRIFIED FOREST NATIONAL PARK TO UNDERSTAND PREHISTORIC AGRICULTURAL POTENTIAL

The Petrified Forest National Park in northeastern Arizona is one of the earliest known areas to contain firm archaeological evidence for prehistoric sand dune agriculture, dating from the Pueblo I period (c. A.D. 700-900). The area contains extensive deposits of Quaternary eolian sediments overlying Triassic clay and sandstone. Archaeologists have long inferred the use of the landscape for dry-farmed agriculture during the entire Pueblo sequence (c. A.D. 700-1450), and frequently recognize a correlation between archaeological site locations and sand dunes, suggesting targeted habitation of dunes for agricultural use. However, the extent of dune farming and the productive potential of these eolian sediments for agriculture have never been studied.

This paper uses soil geomorphology, soil chemistry, Optically Stimulated Luminescence (OSL) dating, clay mineralogy studies, micromorphology, and geospatial analyses to better understand the Quaternary landscape history and to document how prehistoric agricultural societies used this landscape. Results demonstrate that eolian sediments were deposited during the late Pleistocene and during the Mid- to Late Holocene, with widespread eolian mobilization and redeposition after c. 2000 BP. Soil geomorphic studies demonstrate high clay content and weak soil development within younger sand dunes. Clay mineralogy and micromorphological studies are consistent with a conclusion that the high clay content is the result of clay inputs from local sources. The proximity to Triassic-age clay sources resulted in increased clay content in sand dunes, improving water holding capacity and agricultural potential. Soil chemistry from both buried and surface soils shows similar results of soil productivity. Some areas of the landscape demonstrate higher productive potential, especially those areas with well-developed, older soils. However, soil chemistry throughout the study area meets minimum nutrient levels required for agriculture, including nitrogen, phosphorus, available phosphate, and soil organic matter, suggesting that the landscape was likely able to support crops. Local spatial studies suggest that micro-environmental factors, such as position relative to semi-stabilized dunes, were more important for prehistoric field locations.