New Mexico Geological Society Annual Spring Meeting — Abstracts

A catena study of soils associated with a prominent (8.5 m high) fault scarp was completed on the Deadman Canyon alluvial fan. This alluvial fan is located 17 kilometers southeast of Alamogordo, New Mexico, at the east margin of the Tularosa Basin. A laser theodolite (EDM) was used to measure the profile of the fault scarp, and soil and sedimentologic descriptions were made at four selected sites along the trace of this profile (Figure 1). Age estimates for the scarp were calculated on the basis of Profile Development Indexes (PDI) and slope diffusion analysis. These two analyses are complicated by the coarse-grained, carbonate-rich nature of the parent material. The PDI is of limited value as a dating method in this study, but it proved to be an insightful interpretative tool and was useful for correlation. Calcic horizon development is by far the dominant property reflected in the PDI calculations. Similarly, local conditions partly violate the assumptions underlying slope diffusion
analysis; consequently, this analysis yields only a very general age estimate (34 to 294 ka) but may be useful for correlation. Soils were best developed at the summit of the scarp and on the lower fan surface, less developed at the foots lope, and least developed on the midslope of the scarp (Figure 1). The degree of soil development immediately upslope and downslope of the scarp is similar, and both have stage IV calcic horizon development, indicating that the surfaces at these positions are correlative. The sedimentology and soil characteristics at the midslope and foots lope indicate at least two rupture events along this portion of the scarp. The upper 48 cm of the footslope soil profile consists of scarp-derived slopewash that overlies coarser alluvial fan deposits. Stage II+ carbonate morphology occurs at depth in the footslope, but this calcic horizon is not best developed at its upper boundary, possibly because the depth of soil carbonate precipitation has risen through time. We interpret that the upward rise in precipitation depth is a response to being buried by 48 cm of slope wash. The midslope profile contains weakly-developed soil horizons that may include either a buried soil or a weakly developed soil currently fonning at depth because of slope-related hydrologic conditions. Through soil profile descriptions, PDI calculations, and age estimates from slope diffusion analysis, the offset alluvial fan surface along the studied profile can be correlated to the Jornada II--Picacho geomorphic surface of the Desert Soils Project, indicating that the fault in this study area offsets a late Pleistocene surface. This preliminary study is a component of ongoing research regarding fault segmentation, paleoseismicity, and the nature of translation along the central Alamogordo fault.