Rates and processes of soil development on Quaternary terraces in Cajon Pass, California
Field and laboratory analyses of soils on 11 well-dated fluvial terraces spanning the past 0.5 m.y. demonstrate that a threshold governs changes in several morphological and chemical characteristics of increasingly older soils. Correlations with respect to time among iron species, soil morphology, and soil silt and clay demonstrate that the chronosequence at Cajon Pass reflects primarily an evolutionary, largely time-dependent trend and does not reflect differences in external factors such as climate. Most of the soil development on Holocene terraces of the Cajon Pass area is due to physical incorporation of eolian dust and organic material into initially very permeable gravels. This process decreases soil permeability and is conducive for an increase in the magnitude of chemical weathering. Latest Pleistocene and older Pleistocene soils have developed clay and authigenic iron oxide-rich B horizons at the expense of organic-matter-rich A horizons and color B horizons as the extent of chemical weathering has increased. This conversion of the soil from a noncolloidal system to a much more colloidal system takes place over a relatively short period of time (< 4,000 yr) and is herein defined as a type of pedologic threshold. In the Cajon Pass area, the attainment of the threshold and subsequent development of the argillic B horizon of soils on latest Pleistocene terraces occurred during the Holocene; thus, the absence of argillic horizons in soils on Holocene terraces is attributable to simply their younger age rather than to the Pleistocene-to-Holocene climatic change. The threshold is a function of several variables, including influx rate of eolian dust and initial soil permeability; therefore, the time required to attain the threshold will vary in chronose-quences characterized by geomorphic or geographic settings that are different from conditions found in Cajon Pass.