We've Moved!
Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu
Description
Soil development is initiated with the cessation of active deposition on a geomorphic surface. In the Coyote Mountains, Imperial County, California, continued tectonic activity by the right-lateral Elsinore fault has removed a number of channel bar, alluvial fan, and stream channel deposits from their depositional sites at the mouths of Alverson and Fossil Canyons. A chronosequence of thirty-three soils, forming on seven distinct geomorphic surfaces (Q1-Q7, with Q7 being the oldest), indicates that the major factor influencing local soil development is the addition, accumulation, and translocation of aeolian dust over time. Aeolian dust in this region is dominantly composed of silt and clay, carbonates, gypsum and other soluble salts. Volume of total silt and clay increase with time. However, there is no net increase in total carbonate volume with time, even though the older members (Q5-Q7) have stage II to III carbonate morphology. I infer from this data that carbonates, gypsum, and soluble salts have been leached from the soil system at some point in the past. Q1 (modern)-Q4 deposits have low volumes of silt, clay, carbonate, gypsum, and soluble salts, have similar soil development indices, and are statistically indistinguishable based on these criteria. However, they can be distinguished from one another by the use of several geomorphic relative age indicators. This indicators include: desert varnish development, reddened clay accumulations on the undersides of surficially exposed rocks, microtopography, and geomorphic position. The Q5-Q7 deposits are distinguishable from Q1-Q4 deposits based on all of the above criteria. Q5 deposits have significantly higher volumes of secondary gypsum and salts than the Q6 or Q7 deposits. This increased accumulation appears to be a function of the increased surface roughness of the Q5 deposits vis a vis the surface roughness of the Q6 and Q7 deposits. This increased surface roughness has trapped more Holocene aeolian dust than the older surfaces with their better developed desert pavements and more planar surfaces. Based on the data presented above the following inferences are made: 1) the Q1-Q4 deposits are Holocene whereas the Q5-Q7 deposits are Pleistocene in age; 2) surface roughness has a major effect on the trapping and retention of aeolian dust; and 3) there was a significant loss of calcium carbonate from the Q5-Q7 deposits, probably as a result of higher precipitation during the Pleistocene. These data suggest that the rate of aeolian dust accumulation has been non-linear in time and can be attributable to surface roughness rather than changes in aeolian flux rates.
