New Mexico Geological Society Annual Spring Meeting — Abstracts

The Alamogordo fault has displaced alluvial fans along the western base of the Sacramento Mountains. Steep and tall (up to 11 meters) fault scarps in Quaternary alluvium indicate impressive Quaternary offset by this fault system. This offset, in addition to the fault's total length (~150 km) and close proximity to Alamogordo and two military bases, substantiate the need for paleoseismic analyses. The sedimentology and geomorphology of the alluvial fans south of Alamogordo were studied in order to interpret the paleoseismicity of the Alamogordo fault.

Fault scarps and the proximal piedmont were mapped at a scale of 1:24,000 along 24 km of the mountain front. Three depositional units (Qf1-Qf3) were distinguished based on sedimentologic and pedologic properties. Qf3 was divided into three surfaces called Qs3o, Qs3m, and Qs3y based on surficial characteristics and elevational differences. Natural exposures of faulted sediments were also described and documented.

All of the map units consist of gravelly to bouldery alluvium derived from the carbonate bedrock of the Sacramento Mountains. The oldest unit, Qf1, is composed of at least four stacked allostratigraphic subunits bounded by paleosols. The paleosols typically contain a distinct reddish Bw or Bt overlying a white Bk with Stage I+ to III+ carbonate morphology. The stacked sequence of depositional units and buried soils is similar to that described for the Jornado II alluvium and Camp Rice piedmont facies of the Desert Project- thought to be at least 25 ka (Gile et aI., 1981). Additionally, soils on Qf1 resemble those on the Jornado II alluvium. Based on this comparison, have assigned Qf1 a conservative minimum age of ~15 ka.

Qf2, the next youngest unit. overlies Qf1 and has not been offset by the fault. Qf2 lacks buried paleosols and rarely contains Bt or Bw soil horizons. In addition, Qf2 is slightly coarser than Qf1. Qf2 has a soil characterized by Stage II carbonate morphology, similar to soils found on the Isaack's Ranch alluvium in the Desert Project (Gile et aI., 1981). Considering the age assigned to the Isaack's Ranch alluvium and the comparably more calcic parent material of my study area, the age of Qf2 is probably in the range of 7-12 ka. Thus, the last rupture event occurred prior to the age of Qf2.

Unit Qf3 is commonly inset into Qf1 and Qf2 near the mountain front. In the distal regions of the alluvial fans. Qf3 tends to overlie the older units. Soil, stratigraphic, and surface characteristics of Qf3 are consistent with a mid to late Holocene age.

Two rupture events can be interpreted from exposures of faulted sediment and scarp-derived colluvium at the base of a ~6-meter-high fault scarp in Qf1.