A SOIL CHRONOSEQUENCE ON FLUVIAL TERRACES THAT SUPPORTS OBSIDIAN HYDRATION AGE ESTIMATES OF PALEO-INDIAN OCCUPATION

We use soil development on fluvial terraces located within the Cache Creek Natural Area (CCNR), 10-15 km east of Clear Lake, California, to establish a chronosequence that is useful in determining relative ages of geomorphic features, particularly the fluvial terraces. The CCNR has been the focus of detailed archeological research for well over a decade (e.g. DeGeorgey, 2002 and references therein), and well established occupational periods have been delineated for the fluvial terraces. The ages of the landforms which have been the focus of archeological investigations have been less well documented. This study has been initiated to help investigate to what extent geologic events have influenced human occupation/abandonment of sites.

A well defined suite of three stream terraces occupy the Cache Creek drainage near the confluence with the North Fork Cache Creek, and each of these terrace remnants has extensive records of human occupation. The oldest terrace (T3) is ca. 35 m above the present stream, the intermediate terrace (T2) is ca. 20 m above the present stream, and the lowest terrace is ca. 5 m above the present stream. Times of ccupation of these surfaces have been estimated using obsidian hydration techniques to be ca. 10-13 ka (T3), ca. 7-9 ka (T2), and ca. 0.3-2 ka (T1) (DeGeorgey,2002; Burcell, 2004).

Thus far, a limited number of soils have been studied, and calibration of local development rates is not well established. However, soils on T3 surfaces have weak Bt horizons whereas soils on T2 surfaces have only Bw horizons. Maximum clay %'s from the T3 soils is as much as 18%, and from T2 soils as much as 12%. Soil colors, structure, and other pedologic properties are not significantly different between T3 and T2 aged soils. Calculation of Profile Development Indexes has allowed age estimates when compared to similar study areas; maximum age estimates for the T3 surface are consistent with a latest Pleistocene age, and T2 soils seem to support a mid-late Holocene age. The incipient soil profiles of the T1 surfaces are consistent with an age of a few hundred years.

Thus, the age estimates of the geomorphic surfaces appears to be in general agreement with the age estimates of the times of human occupation , suggesting a potential tie between geologic events and human occupation/abandonment of sites.