GEOARCHAEOLOGICAL DATING OF HOLOCENE STREAM TERRACES ALONG THE SAN PEDRO RIVER AND ITS MAJOR TRIBUTARIES, SOUTHEASTERN ARIZONA

This study demonstrates the value of using archaeological data to help correlate and date alluvial terraces when mapping Quaternary deposits. Chronometric data extracted from an archaeological literature search and from archaeological site records maintained by the Arizona State Museum were used to better constrain the ages of Holocene stream-terrace deposits and surfaces along the San Pedro River and its major tributaries in southeastern Arizona. This investigation was part of a larger geologic mapping project conducted by the Arizona Geological Survey (AZGS) for the Arizona Department of Water Resources, the main goal of which was to delineate the extent of Holocene channel and floodplain alluvium. AZGS geologists identified three Holocene terrace surfaces (Qy1r, Qy2r, and Qy3r), the ages of which were initially estimated based on soil development and historical records.

Eight deeply buried archaeological sites found in the fill of the most extensive Holocene terrace (Qy2r) contained a total of ten radiocarbon dates, as well as a handful of temporally diagnostic San Pedro stage artifacts. In addition, numerous temporally diagnostic Hohokam pottery sherds and features were documented at 17 sites on the Qy2r surface. The age constraints provided by these archaeological materials suggest that the Qy2r fill was deposited primarily between 3500 and 1000 BP, with only a thin veneer of flood deposits added between 1000 and 100 BP. This finding is consistent with other alluvial histories in the greater Gila River drainage basin and suggests that the initial, latest Holocene to Historic (2000–100 BP) age estimate for Qy2r can be refined to late Holocene to Historic (4000–100 BP). The archaeologically derived maximum age estimate for Qy2r also provides a minimum age for the topographically higher and older Qy1r terrace and supports narrowing its initial, early to late Holocene age estimate to early to middle Holocene. The post-A.D. 1890 age originally proposed for the Qy3r terrace deposits inset into Qy2r is consistent with the presence of five Historic period surface sites and a concordant absence of surface and buried prehistoric sites.