A NEW ALLUVIAL CHRONOLOGY FOR THE LOWER SALT RIVER, ARIZONA: IMPLICATIONS FOR BRAIDED CHANNEL PROCESSES AND ARCHAEOLOGICAL PRESERVATION

Recent geoarchaeological investigations along the Lower Salt River west of Phoenix resulted in the identification of an archaeological site containing burned pit features that AMS ¹⁴C dated 5700-7200 cal yr BP. Field description, bulk sediment analysis (granulometry, organic carbon, and CaCO₃), and thin-section analysis of alluvial soils support an early-to-mid Holocene age for deposits containing the features. Combining these results with data from a previous geoarchaeological study yields a total of 12 AMS ¹⁴C and four OSL ages on alluvial stratigraphy from the site and indicates the presence of an “island” of latest-Pleistocene to mid-Holocene alluvium surrounded by late-Holocene deposits within the geological floodplain of the Salt River. Previous to this discovery, there had been a notable absence of early-to-mid Holocene landforms and Archaic archaeology along the Lower Salt River and adjacent rivers. The preservation of this site is linked to channel avulsion associated with braided stream channel dynamics and provides additional evidence that archaeological site patterning in the Lower Salt River valley is strongly influenced by fluvial processes. Other patches of older alluvium containing Archaic archaeology may occur along the Lower Salt River, but predicting their location is hampered by 1) the stochastic nature of channel avulsion in braided streams, 2) limited topography within the geological floodplain, and 3) historic/modern surface disturbances. Braided channel systems like the Lower Salt River contain a complex mosaic of alluvial deposits and are not well suited for long distance correlation of alluvial stratigraphy and past floodplain changes driven by climate change.