DIFFERENTIATION OF IRON PHASES THROUGH SEQUENTIAL EXTRACTIONS TO IMPROVE AGE ESTIMATES OF BLUE RIDGE DEBRIS FAN SURFACES, MADISON COUNTY, VA

Four Quaternary surfaces were mapped on two debris fans activated by the 1995 Rapidan Flood in Madison Co., VA. As in previous studies (Daniels, 1997; Kochel et al., 1997), soil stratigraphy and indices of soil development were compared to a regional soil chronosequence for the VA Piedmont (Pavich et al., 1989) to determine the relative ages of fan surfaces and bracket the absolute ages of soils on these surfaces. Eaton (1999) dated samples from stratigraphies at fan-head trenches using radiometric techniques. Soil weathering geochemistry was investigated in this study as a proxy for isotopic methods used to date fan surfaces. Crystalline and amorphous phases of metal hydroxides were removed from soils through sequential extractions. Crystalline phases are more thermodynamically stable in soils and therefore should increase relative to amorphous phases with time (Drever, 1997).

Extraction results show that the ratio of Fe bound to crystalline phases vs. Fe bound to amorphous phases increases systematically with the age of a soil. This ratio was used as a chronofunction to determine the relative ages of soils and distinguish soils of different ages showing similar levels of geomorphic development. Extraction results were calibrated by geochemically analyzing samples from 14C-dated stratigraphies. Comparison of the geochemical signatures of soils from debris fan surfaces investigated in this study to radiocarbon-dated samples allowed the absolute ages of fan surfaces to be approximated.

Geochemical age estimates of younger fan surfaces (Qf4 and Qf3) correlate well to estimates based on soil chronosequence comparison. However, dates approximated for much older surfaces (Qf1) based on geochemical analysis underestimate those determined from comparison of soil indices to published chronosequence studies. Soils dated using 10Be methods in a related study are currently being geochemically analyzed using extraction methods. Results from these cosmogenically-dated samples may improve estimates of fan surface ages beyond the range of radiocarbon dating. Overall, the geochemical methods herein are useful for estimating ages of Quaternary landforms and will further the development of a soil chronosequence for debris fans in the Appalachian Blue Ridge.