The Influence of Colluvial Deposition on Rates of Soil Development

in the Transverse Ranges, Southern California

Harrison, J.B.J., McFadden, L.D., and Weldon II, R.J., 1992

Isr. J. Earth Sci. 41: 139-154.

Soil properties shown to vary in a systematic manner over time are used to correlate and sometimes to provide relative numeric ages for otherwise undatable surfaces. Colluvial deposition from sideslopes can affect the rate of soil development and such soils cannot be used as a geochronological tool. In this study we describe the variation in soil properties due to progradation of two small colluvial aprons from different lithologies onto the same terrace treads. The relations between the cumulic soils and those on the 'stable' terrace tread were then used as a basis for determining an age estimate for another terrace tread that is completely covered by colluvium. A sequence of soil profiles was described down the long profile of each colluvial apron and compared with soils developed on the terrace tread. Where comparison was possible, the soils on the colluvium are more strongly developed than soils of the same age or slightly older developed on the terrace tread. The degree of soil development was greatest at the apex and decreased systematically with increasing distance down the colluvial apron.

Episodic deposition was recorded in the soils on one colluvial apron resulting in compound and complex soil profiles on the colluvial apron. Deposition onto the other two colluvial aprons appeared to be more continuous, resulting in cumulic soil profiles. The different processes of deposition are possibly related to the different lithologies.

Because it is difficult to clearly define the morphologic distal boundaries of the colluvial aprons, it is difficult to determine the areal extent of the influence of depositional processes on soil development. The only method may be to identify the systematic variation of soil properties down a colluvial apron and determine where the soils on the colluvial apron are indistinguishable from soils developed on the terrace tread. Thus a number of soil profiles have to be described on each surface of a chronosequence to define the natural variability of soils before any soil profile can be determined to be representative of an isochronous surface.