PETROGRAPHIC EVIDENCE OF PAST SEISMICITY FROM SECONDARY CALCITE AND OPAL IN THE UNSATURATED ZONE AT YUCCA MOUNTAIN, NEVADA

Performance objectives for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada, require waste containment for a minimum of 10,000 years. The long-term stability of emplacement drifts is central to assessments of site performance. The regulatory requirements of the probabilistic seismic hazard analyses used in models of emplacement drift stability, however, result in predictions of drift-wall degradation and tuff fragmentation that may be inconsistent with the preservation of delicate textures in secondary calcite and opal in the host tuffs.

Emplacement drifts would be constructed in the 500- to 700-m-thick unsaturated zone at Yucca Mountain, which consists largely of welded and fractured Miocene-age ash-flow tuffs. Deposits of secondary calcite and silica, found on some fracture footwalls and lithophysal cavity floors, began forming at least 10 million years ago. They consist of coatings and crusts from <1 mm to as much as 5 cm thick, some with tall, thin, delicate calcite blades. Petrographic studies have shown that the deposits consist of an early stage (ES) of calcite (±fluorite) locally capped by chalcedony±quartz followed by an intermediate stage (IS) of calcite (commonly bladed) and a late stage (LS) of calcite (mainly as overgrowths); opal locally accompanies both IS and LS calcite. U-Pb dating of associated silica indicates that the stage boundaries occurred about 6 to 8 Ma (ES-IS) and 2 to 4 Ma (IS-LS). Incorporation of sand-to-gravel-sized tuff fragments is common in the ES but rare in the IS and generally absent in the LS. Furthermore, even the most delicate calcite blades are intact and some loosely attached fracture coatings (that can be removed by hand) remain in place. These observations are inconsistent with large peak ground velocities from earthquake activity during at least the past 2 to 4 million years. Whether or not seismicity-induced tuff fragmentation occurring at centimeter-to-decimeter scales in the fracture and cavity openings relates directly to degradation of the 5.5-m-diameter waste emplacement drifts, the deposits do provide a potential record of the spatial and temporal distribution of tuff fragmentation and may provide an estimate of the upper limit for recent ground motion at Yucca Mountain.