GEOLOGY ADAPTED TO HYDROLOGY—SILENT CANYON CALDERA

Some geologic features fit well into a hydrogeologic framework, others must be modified to have hydrologic meaning. A collapse-caldera is used as an example of a geologic system that is adapted for use as part of a regional ground water flow model and exemplifies the evolution of geologic thought concerning the mapping of volcanic units and its application, or lack thereof, to ground water flow models.

The Miocene Silent Canyon caldera in southern Nevada is a well defined saucer shaped depression about 15 miles in diameter and as much as 20,000 feet deep that formed by multiple collapses on steeply-dipping arcuate faults following eruption of at least two major ash-flow tuffs. The caldera filled with low- density eruptive rocks from both internal and external sources to produce one of the largest gravity lows in the western United States. Data used to construct a 3D geologic model include: (1) gravity inversion modeling to delineate the shape and dimensions of the caldera, (2) geologic mapping to outline surface structure and rock distribution and, (3) drill-hole intercepts of stratigraphic units in and around the caldera to give the distribution of the units in the subsurface. While the 3D geometric framework of the caldera is readily transferred to a hydrologic model, the ash-flow sheet stratigraphic units are not. Ash-flow sheets, the basic volcanic stratigraphic units in the Silent Canyon caldera, are not the fundamental aquifer and confining hydrologic units so conversion of geologic stratigraphy to hydrostratigraphy is necessary. This involves splitting the basic eruptive units on the basis of compaction and welding characteristics and creating new hydrologic units by lumping rocks with similar permeability characteristics that are not necessarily geologically or genetically related. Although modern geologic mapping focuses on the eruptive unit, which may form several different lithologic types, it is interesting to note that the earliest mapping of the volcanic rocks in southern Nevada tended to delineate map units on the basis of material properties which would be more amenable to direct input into ground water flow models than their modern counterparts.