USING THE ACCUMULATION OF CBD-EXTRACTABLE IRON AND CLAY TO ESTIMATE SOIL AND LANDFORM AGE, FRONT RANGE, COLORADO

In transport-limited environments, the color (oxidized Fe) and clay content of regolith, particularly soil horizons, provide a basis for estimating the exposure age of surficial deposits and associated landforms. We were interested in quantifying soil properties at several classic Front Range glacial and hillslope localities; here we use extractable iron and clay content to estimate soil age or residence time at undated sites. We measured citrate-buffered dithionite (CBD)-extractable Fe (Fed) concentrations in ~ 140 samples of fresh rock, saprolite, morainal and colluvial materials and soil horizons from the Boulder Creek catchment and nearby areas; clay content was measured on ~90 samples. Fresh rocks contain < 1% Fed and < 5% clay. As bedrock and surficial deposits age, Fed and clay accumulate from weathering and dustfall. Holocene regolith contains small amounts of Fed and clay; soils developed on early Holocene cirque deposits locally contain 1.5% Fed and 8% clay. Concentrations mainly are higher in soils on older glacial deposits (Pinedale and Bull Lake age) and are still higher on hillslopes beyond the glacial limit, where colluvium may be >130 kyr. Clay and Fed contents are positively correlated (r²=0.46), but scatter in the data likely reflects the influence of variable parent material, local climate, dustfall and landscape stability. Maximum profile concentration and the total mass of Fed in soil and in an underlying buried soil in Lefthand Canyon suggest that colluvium has accumulated episodically over >100 kyr. Another red soil developed in colluvium on an old surface near Ward appears even older. In contrast, at Gordon Gulch, one of the Boulder Creek CZO catchments developed on what appears to be a stable surface, Fed and clay concentrations suggest that soils mainly are of Pinedale age. In the absence of radiometric ages, the accumulation of Fed and clay in various soils provides a useful chronofunction for 10³ to ~10⁶ yrs with a strong dependence on climate.