The geology of the western region of the NNSS consists primarily of thick sequences of ash-flow tuff, lava, and volcaniclastic rocks deposited during episodic volcanic cycles associated with the formation of as many as six coalesced caldera centers from 15 million to 9 million years ago (the South­west Nevada Volcanic Field; Byers et al., 1976; Sawyer et al., 1994). These caldera centers are localized in the north-northeast trending Amargosa Desert rift zone (Wright, 1989; Carr, 1990; Fridrich, 1998), a major north — northeast trending structural trough identifiable using gravity and seismic reflection data (Healey et al., 1980; Brocher et al., 1998). The thick volcanic section in the Amargosa Desert rift zone locally replaces the carbonate aquifer as the primary pathway for regional groundwater flow. The carbon­ate aquifer is either missing, too deep in the stratigraphic section and/or impermeable from contact metamorphism associated with caldera pluton­ism to transmit significant quantities of groundwater. Regional groundwater flow in the western volcanic sequence is topographically controlled and driven by the increased recharge at higher elevations, primarily from eastern Pahute Mesa (Blankennagel and Weir, 1973 ; Laczniak et al., 1996; SNJV, 2009a; Fenelon et al., 2010).

The Miocene volcanic rocks form high elevation plateaus of welded and nonwelded ash flow sheets concentrically flanking their source calderas. This plateau topography remains preserved where basin-range deformation has locally faulted but has not significantly extended and disrupted the mesas (Pahute and Rainier mesas and Yucca Mountain) (Fig. 26.2). This layered sequence of outer caldera ash-flow sheets is replaced locally by thick sequences of densely welded ash-flow tuff and intrusive rocks within caldera depressions. This pattern of extra — and intra-caldera rock sequences is complicated in the northern part of the Amargosa Desert rift zone by multiple stages of caldera formation. Younger calderas disrupt and bury the structure and volcanic rock assemblages of older calderas.

Spatial changes in lithology and thickness of the volcanic rocks of the western volcanic highland are significant (laterally and vertically heteroge­neous), and they are locally affected by secondary alteration (zeolitization), burial diagenesis and/or hydrothermal activity. These lithologic and altera­tion features strongly affect the hydrologic properties of the rocks (tend to reduce conductivity) and form complex inter-layered aquifers and confining units locally offset or truncated by caldera structures and/or extensional faults. Groundwater flow can be rapid (tens of meters per year) within zones of higher density cooling joints within welded tuff and rhyolite lavas, both augmented by flow along faults; groundwater flow is much slower (one meter per year or less) through altered volcanic rocks and/or zones of matrix-dominated permeability.