[en] Siliceous tuff in southern Nevada occurs in a complex and locally active geological environment. Regional thrust faulting, Basin and Range faulting, and present-day seismicity complicate exploration and site characterization activities. The inherent variability of tuff and the complexity of caldera complexes also complicate siting efforts, but may serve to enhance long-term containment. Time--space trends of silicic volcanism are moderately well-established, while those of recent basaltic volcanism are not. At present, the final consequences for repository siting of the geologic complexities described in this paper are not known. Evidence from laboratory cation exchange measurements indicate that tuff and tuffaceous alluvium can serve as effective natural barriers to migration of radionuclides. This fact, coupled with multiple hydrologic barriers and long flow paths, as in the vicinity of the Nevada Test Site, might well result in tuff being a suitable medium for the safe long-term geologic disposal of nuclear wastes. Preliminary thermal modeling indicates the strong influence of varying assumptions regarding in situ fluid pressures and geothermal heat flux on acceptable initial areal power loadings