INFLUENCE OF PARENT MATERIAL LITHOLOGY ON DUST ACCUMULATION IN ALLUVIAL FAN SOILS, MOJAVE DESERT CALIFORNIA U.S.A

Studies have shown that soil development in deserts is controlled by the accumulation of fine-grained eolian sediment (dust). Few studies, however, have shown how the lithologic composition of the soil parent material may also moderate the addition of dust during soil formation. We compare trends in dust accumulation among three lithologically different soil chronosequences composed of limestone/marble (LS), mixtures of rhyodacite, limestone, and granitic clasts (VX), and mixed-granitic lithologies (MG). The sequences are associated with alluvial fan surfaces of the Providence Mountains (Mojave Desert, California) that range in age from ~3-70 ka.

Significant variability of silt and clay inventories occurs among the three soil-chronosequences with total profile mass accumulation ranging from: 0.5 to 7 g/cm² (Qf6, ~4 ka); 3 to 20 g/cm² (Qf5, ~10 ka), 3 to 25 g/cm² (Qf4, ~30 ka), and 4 to 33 g/cm² (Qf3, ~70 ka). Mass accumulation is lowest in the LS sequence, highest in the MG sequence, and intermediate for the VX sequence. Variations in dust accumulation are also reflected in B horizon development. In the MG sequence, soil B horizons range from weak Bw horizons (Qf6), to weak Bt horizons (Qf5), to strong Bt horizons (Qf4, Qf3). By comparison, in the LS chronosequence, B horizons range from weak Bk horizons (Qf6, Qf5) to weak-moderate Bkm horizons (Qf4, Qf3). Soils developed in VX sequence represent a mid-way point among the three sequences, and exhibit weak Bwk horizons (Qf6, Qf5), weak Bwk-Bkm horizons (Qf4), and weak Bt horizons and moderate Bkm horizons.

Temporal differences in the rate of desert pavement development among the LS, VX, and MG sequences explain part of the variations dust accumulation rates. The rate of desert pavement development occurs faster on the VX and LS fan deposits relative to that on the MG surfaces. A reduction in dust-trapping efficiency would coincide with increases in the packing and smoothing of clasts during development of desert pavements. Increases in available calcium associated with limestone content may also limit downward translocation of silt and clay. Recognition of this type of intrinsic feedback between soil formation and the modification of alluvial fan surfaces is critical for interpreting the systematic changes in desert soil formation among soils formed in the different rock.