GEOCHEMICAL WEATHERING CHARACTERISTICS AND SOIL CHRONOSEQUENCE DEVELOPMENT ON BLUE RIDGE DEBRIS FAN SURFACES IN MADISON COUNTY, VIRGINIA

Debris fans in Madison County, Virginia, activated by the June 1995 Rapidan Flood, are characterized by multiple surfaces of varying age that reflect major episodic debris flow events (Kochel et al., 1997). Relative ages of fan surfaces can be distinguished by elevation, but occasionally the volumetric dominance of an individual surface masks this relationship. Daniels (1997) and Kochel et al. (1997) compared indices of rubification and textural development in the Bt horizon to regional soil chronosequences developed in the Virginia Piedmont (Pavich et al., 1989) to map the relative ages of surfaces on several fans. Eaton (1999) dated samples from stratigraphies at fan-head trenches using radiometric techniques, but extension of these units to specific fan surfaces is problematic. Developing a radiometrically calibrated soil chronosequence may enable absolute ages of debris fan surfaces to be estimated. This study uses soil-weathering geochemistry as a proxy for isotopic methods used to date Quaternary fan surfaces. Four surfaces were mapped on two fans along the Rapidan River. Relative ages were assigned using elevation and indices of soil development. Amorphous and crystalline metal hydroxides were removed through sequential extractions to evaluate the chemical signatures of soils representative of each fan surface. The ratio of iron bound to crystalline phases versus iron bound to amorphous phases increases systematically with age. Geochemical information, in combination with other soil attributes, appears useful for the differentiation and relative dating of fan surfaces. Additionally, comparison of the geochemical signatures of soils from debris fans investigated in this study to radiocarbon-dated samples from fan-head trenches of these and neighboring fans permits the absolute ages of fan surfaces to be approximated. Overall, the geochemistry demonstrates reasonable trends and may be used to estimate ages of other landforms in the region.