Soil catena development on fault scarps of different ages, eastern escarpment of the Sierra Nevada, California

Abstract

Abstract In Quaternary tectonic studies, soils are useful for estimating the ages of both faulted surficial deposits and colluvial wedges on the fault-scarp slopes. Soil development on sloped landforms such as fault scarps varies with position on the slope, thus an understanding of catenary relations is needed to interpret the fault-scarp soils. Discussed here are some aspects of soil-catena development on fault scarps formed by late Quaternary normal faulting along the eastern escarpment of the Sierra Nevada, between Bishop and Lake Crowley, California. The fault scarps are multiple-event scarps formed in Pleistocene glacial and fluvial deposits composed mostly of granitic clasts and vegetated primarily with the Great Basin sagebrush community. Eolian additions to the soils are minor. Fault scarps formed since the Tioga glaciation (about 10–25 ka) are high (6 –26 m of vertical surface offset) and steep (26–38°);these fault scarps have probably been ruptured by faulting at least once during the Holocene. The upper parts of the post-Tioga fault scarps are bouldery and at the lower-slope positions a finer-grained colluvial wedge has accumulated. In contrast to the post-Tioga fault scarps, an old fault scarp (mostly formed 100 ka ago, and not ruptured by faulting in at least 15–22 ka), has been degraded to a more gentle slope (17°), and few boulders are exposed on the surface of the fault scarp. On both age groups of fault scarps, some soil properties and vegetation patterns suggest that there is more available moisture at the lower-slope positions than elsewhere in the fault-scarp catenas. Soils associated with the post-Tioga fault scarps are generally weakly developed A/Bw/Cox profiles that become cumulative at the lower-slope positions. In most places, the difference in soil development between the original surface and the fault scarp is small, and thus is a poor indicator of the age spread between the two surfaces. In contrast to the post-Tioga soils, soils on the old fault scarp are moderately developed A/Bt/Cox profiles that are better developed on the fault-scarp slope than on the original surface. Increased soil development on the slope may reflect higher available moisture on the slope and erosion of the original surface through time. On both the post-Tioga and the older fault scarps, buried soils within the colluvium were rarely encountered. The lack of buried soils may be due to a combination of low regional rates of soil development, short interseismic time intervals for events forming the scarps, and on the old fault scarp, overprinting of soils by subsequent weathering and soil development. A comparison of soil-catena development on the two age groups of fault scarps suggests that after faulting ceases, pedogenesis becomes the dominant process of the fault scarp. Although slope processes may continue to modify the fault-scarp slope, they proceed at rates that are lower than the rate of soil development.