METHODS OF DOCUMENTING VARIABILITY IN LITHOSTRATIGRAPHIC FEATURES, FRACTURES, AND PROPERTIES IN THE TOPOPAH SPRING TUFF AT YUCCA MOUNTAIN, NEVADA

Geologic field and laboratory methods are used to obtain geotechnical information about the volcanic host rocks for a potential nuclear-waste repository at Yucca Mountain, Nevada. Hydrogeologic, thermal, and mechanical properties of the 12.8-m.y.-old Topopah Spring Tuff are constrained within the lithostratigraphy defined by detailed geologic mapping. Current studies emphasize the upper lithophysal (UL), middle nonlithophysal (MN), lower lithophysal (LL), and lower nonlithophysal (LN) zones that would host the potential repository. Data are collected from surface exposures, tunnels, and surface- and tunnel-based boreholes. Lithostratigraphic contacts, fractures with trace lengths >1 m, and faults are shown on full-periphery, 1:125-scale maps. Detailed line surveys (DLS) of fractures with trace lengths >1 m (locally >30 cm) were conducted underground and in the MN on surface exposures. Fractures with shorter trace lengths were captured by DLS of the MN, LL, and LN. One-by-three-meter panel maps at 1:10 scale are overlain on low-angle illumination photographs to document abundance, size, shape, and spacing of lithophysal cavities, rims, and spots in the LL. Linear traverses are used to map the abundance of lithostratigraphic features in the LL. Video logs and maps of boreholes are used to document abundance and location of lithostratigraphic features and fractures. Petrographic studies of thin sections for the UL, MN, and LL reveal lithostratigraphic and fracture characteristics at the centimeter and finer scale.

Collectively, these studies indicate lateral continuity of the zones (and even some subzones) across the potential repository area and document the differences between and within the zones and subzones. For example, fracture sets are consistent in the potential repository area and differ only slightly from sets outside the area. The MN has more fractures with >1-m trace lengths than other zones, but the LL has proportionally more fractures with shorter trace lengths. The LL locally has 5 to 30 percent lithophysal cavities and 2 to 30 percent combined rims and spots. Thin sections from UL, MN, and LL indicate similar lithostratigraphic and fracture characteristics in borehole, surface, and tunnel exposures.