LANDSCAPE EVOLUTION AND GEOARCHAEOLOGICAL MODELING, MIDDLE SAN JOAQUIN RIVER, FRESNO AND MADERA COUNTIES, CALIFORNIA

The San Joaquin Valley in northern California has witnessed dramatic changes over the last 22,000 years since the Last Glacial Maximum (LGM), resulting in a landscape completely different from that encountered by the region’s first human inhabitants around 12,000–13,000 years ago. By understanding the timing and extent of geoclimatic processes shaping the landscape, it is possible to assess the “geologic potential” for buried archaeological sites. We conducted a geoarchaeological study within one reach of the middle San Joaquin River. We began by using a buried archaeological site model that estimates the potential for a given area to contain cultural material of certain ages based on mapped distribution of Quaternary-age landforms and distance to perennial water source. We then evaluated 66 soil cores to identify buried soils, i.e. former stable land surfaces that have an elevated potential to contain in situ archaeological sites. Radiocarbon ages determined on soil organic matter range from 549 to 5532 cal BP, capturing three general periods of landscape stability, the first beginning at the end of the middle Holocene around 5,500 years ago, one roughly between 2,400 and 1,000 years ago, and one from about 500 to 800 years ago. These buried soils overlie thick alluvial packages of upward-fining sand, silt, and clay; 2 charcoal samples from these alluvial packages date to 18,718 and 18,580 cal BP, or the final stage of the LGM. Thus, thick late-Pleistocene alluvial deposits too old to contain archaeological material underlie late Holocene alluvium. Buried soils, where present, are not laterally continuous in or near the active river channel. However, buried soils, representing several periods of stability within the last 5,500 years, do occur within the upper 4-5 m of the land surface. The results of soil-stratigraphic description and radiocarbon age-dating allow us to evaluate the influence of landscape evolution on early human settlement patterns and to assess the underlying assumptions of the geoarchaeological model, allowing us to make better project management decisions for archaeological testing across broad landscapes.