Circular 196—Late Holocene displacement along the Organ Mountains fault in southern New Mexico

By L. H. Gile, 1987, 42 pp., 8 tables, 24 figs., glossary.

A comprehensive study of soils and geomorphology of an extensive fault system in southern New Mexico. Alluvial fans of several ages have been displaced by the fault system, indicating long-term faulting at different times in the same place. These episodes of climatically controlled erosion and deposition provide both discrete deposits and, where the deposits can be dated, a chronology that is independent of faulting and the sedimentation associated with it. The faulting, which was determined to be relatively recent, has particular significance because of potential effects on current urban development and storage of hazardous wastes.

An extensive fault system occurs along the east side of the Organ and San Andres Mountains in southern New Mexico. Alluvial fans of several ages have been displaced by the fault, indicating long-term faulting at different times in the same place. In many arid regions there is widespread evidence of erosion caused by past changes in climate, particularly by long, severe droughts after cooler periods with more effective moisture. Thus, episodes of climatically controlled erosion and deposition provide both discrete deposits and, where the deposits can be dated, a chronology that is independent of faulting and the sedimentation associated with it. Charcoal in the lower parts of such deposits has been dated by 14C methods at the Desert Soil-Geomorphology Project west and north of the Organ Mountains. These dates give chronological control on important episodes of sedimentation in the middle and late Holocene for the area. Soil morphology can be used to distinguish deposits of different ages if the morphological range has been determined for soils that have formed deposits. Both the faulting event and the deposits caused by faulting can then be placed in this chronological framework. Soil features having chronological significance in the study area are accumulations of organic carbon, silicate clay, and carbonate, as well as soil color, consistence, and structure. Soil morphology, coordinated with the geomorphic and stratigraphic arrangement of the deposits, was used to extrapolate the chronology from the dated sites to the fault area. The evidence indicates that a major displacement took place about 1,000 yrs B. P. Maximum displacement is estimated to have been about 15 ft.