Interpretations of magnetic anomalies at a potential repository site located in the Yucca Mountain area, Nevada Test Site

Abstract

In the Yucca Mountain area near the southwestern border of the Nevada Test Site, studies of the relation of magnetic properties to geologic features have provided structural information at and near a potential site for storage of radioactive waste. Interpreted features include a tabular mass of magnetized sedimentary rock beneath thick deposits of volcanic rock, and 11 major faults that strike generally northward and displace magnetized volcanic rock. A positive anomaly in a high-altitude aeromagnetic survey over exposures of strongly magnetized argil! ite of the Eleana Formation extends westward 20 km into the site area where interpretations indicate an argil!ite thickness of 800 m at a depth of 2.25 km. The high magnetite content of the argi!lite is not typical of the region, and was probably introduced by the heating effects of an underlying piuton. The basis for mapping traces of faults, and identifying their upthrown sides, was developed elsewhere at Yucca fault in the relatively simple volcanic terrains of Yucca Flat. In the site area, analyses of aeromagnetic anomalies from a low-altitude east-west aeromagnetic survey show the Topopah Spring Member of the Paintbrush Tuff as the primary source of anomalies from faulted sequences of volcanic rock. Faults related to belts of positive and negative anomalies surrounding the site have been identified. The possibility that an east-west pattern of anomalies is related to structure crossing the site was investigated by a recent aeromagnetic survey flown at low altitude in north-south directions. A significant reduction in amplitude of these anomalies resulted when effects of the deeply buried argil!ite were removed. The remaining anomalies over the site can be explained by a change in lateral extent, or magnetic properties, of volcanic units beneath the Topopah Spring Member.