EVOLUTION OF SOILS DERIVED FROM WEATHERING OF SAPROLITIC BEDROCK AND COLLUVIUM, BETASSO CATCHMENT, FRONT RANGE, COLORADO

Field and laboratory analysis of soils formed on saprolitic bedrock and on midslope colluvium in the Betasso catchment, suggests that granitic regolith weathers slowly along the dry eastern margin of the Front Range. The bedrock-derived soil contains primarily smectitic clays and Fe-oxides as secondary minerals, which extend into the underlying saprolite. At both slope positions, kaolinite is present only below 40 cm and in trace amounts (<2 wt %). The mid-slope soil is developed on colluvium, which lies above and below a buried soil. Bulk carbon from the A horizon of the paleosol gave calibrated ages of 9000 yr and 8640 yr. Most weathered samples from Betasso have bulk densities < 2.0 T m^-3 and soil horizons give values < 1.75 T m^-3. Bulk chemical changes from bedrock and colluvium to soils are relatively small, probably mediated by the relatively dry climate and the transformation of plagioclase and biotite to secondary minerals without major losses of silica, cations or minor elements. Downprofile changes in bulk chemistry are subtle, but regolith generally is enriched in constituents such as C and Zr and slightly depleted in base-metal cations such as Ca and Na with respect to bedrock. Apatite concentrations in soils and unweathered colluvial deposits range from 0.10 to 0.27 weight percent. Concentrations of apatite and plagioclase are lowest, and the smectite concentrations highest in the buried A and B horizons, which indicate that the paleosol is the most weathered material sampled. Apatite morphology does not appear to show depth-related trends, but total phosphorus concentrations increase in the buried soil profile where apatite concentrations decrease, suggesting that the high P concentrations were adsorbed on Fe-oxides during weathering of the paleosurface. Comparison to other chronosequences in the western USA suggests that the buried colluvial soil at Betasso likely accumulated clay and Fe-oxides for at least 50 to 150 kyr. The upper saprolite-derived soil represents at least 35 to 60 kyr of clay and Fe-oxide accumulation.