STRATIGRAPHIC AND PEDOLOGICAL ANALYSIS OF A PREHISTORIC ANTHROPOGENIC “SILT FIELD” IN PHOENIX, ARIZONA

Discontinuous accumulations of fine-textured alluvium located on gravel bars along the middle reach of Cave Creek in northern Phoenix are associated with Hohokam (AD 600-1400) agricultural sites. Archaeologists have debated whether or not these “silt fields” are natural deposits that attracted Hohokam farmers for their relatively fertile and water-retentive soils, or if they were intentionally produced by the Hohokam through diversion of seasonal water and capture of relatively fine-textured sediments in order to increase arable land along this gravelly ephemeral desert stream. Archaeological excavations conducted in 2010 at site AZ T:4:30 (ASM) allowed for a stratigraphic and pedological analysis of one of these “silt fields”. Covering an area of 2.1 ha, this site contains an accumulation of ca. 19,000 m³ of alluvial sand, silt, and clay overlying a gravel bar within the Cave Creek floodplain. The site is streamlined parallel with Cave Creek and contains a low ridge down its axis. Ceramics and five ¹⁴C dates on charcoal indicate that the sediment was deposited sometime within the period AD 750-1150. Horizontal textural gradients in the lower stratigraphy suggest that sediments carried by overbank floods were captured in an area of increased hydraulic roughness. A paucity of rock alignments that are common to nearby “silt fields” suggest that vegetation may have been used to capture sediment, a strategy documented ethnographically in northern Mexico. The upper stratigraphy is more bioturbated and linked to repeated irrigation and sedimentation from a previously documented prehistoric canal identified at the edge of the site. The geoarchaeological evidence supports an anthropogenic origin for the fine-textured alluvium at AZ T:4:30 (ASM). The Hohokam intentionally created arable land out of gravel bars located along Cave Creek through water control and soil management, thus supporting the northward expansion of population from the Lower Salt River Valley ca. AD 750-1150. By AD 1350, settlements along Cave Creek were abandoned.