DOI: 10.5066/p13bafsv
title: Digital input data and model output for a three-dimensional hydrogeologic framework for the Great Basin carbonate and alluvial aquifer system of Nevada, Utah, and parts of adjacent states

description: This digital dataset was created as part of a U.S. Geological Survey (USGS) regional groundwater assessment of the eastern Great Basin; the project was called the Great Basin Carbonate-Alluvial Aquifer Study (GBCAAS). The study area has a geographic extent of 110,000 square miles, predominantly in eastern Nevada and western Utah. As part of this larger study, the USGS developed digital geologic data and a three-dimensional hydrogeologic framework model (3DHFM) that defines the elevation, thickness, and extent of nine hydrogeologic units in the regional study area. The hydrogeologic framework model became the principal digital geologic input to a regional numerical groundwater flow model. A USGS report published in 2011 presented a conceptualization of the regional hydrogeology and major aquifer systems and included a description of the construction of the 3DHFM. This data release formalizes the input geologic data and model outputs as a digital dataset. Nine hydrogeologic units (HGUs) were modeled within the GBCAAS 3DHFM; six of the units describe consolidated pre-Cenozoic rocks and the other three describe Cenozoic basin-fill and volcanic rocks. The modeled HGUs include, from deepest to shallowest, a noncarbonate confining unit (NCCU) representing low-permeability Precambrian siliciclastic formations, (2) a lower carbonate aquifer unit (LCAU) representing high-permeability Cambrian through Devonian limestone and dolomite, (3) an upper siliciclastic confining unit (USCU) representing low-permeability Mississippian shale, (4) an upper carbonate aquifer unit (UCAU) representing high-permeability Pennsylvanian and Permian carbonate rocks, (5) a thrusted noncarbonate confining unit (TNCCU) representing low-permeability siliciclastic rocks incorporated in regional thrust faults, (6) a thrusted lower carbonate aquifer unit (TLCAU) representing high-permeability limestone and dolomite incorporated in regional thrust faults, (7) a volcanic unit (VU) representing outcrop areas of volcanic rocks, (8) a lower basin-fill aquifer unit (LBFAU) representing the lower one-third of the Cenozoic basin fill, and (9) an upper basin-fill aquifer unit (UBFAU) representing the upper two-thirds of the Cenozoic basin fill. The GBCAAS 3DHFM was built by extracting and combining information from digital elevation models, geologic maps, cross sections, drill hole logs, existing hydrogeologic frameworks, and geophysical data. Compared to areas with active oil and gas production, deep well data for the GBCAAS study area are generally sparse; the primary input data for the 3DHFM were map and cross section data. Input surface and subsurface data have been reduced to points that define the top elevation of each hydrogeologic unit at x,y locations; these point data sets serve as digital input to the framework models. Faults and caldera boundaries that offset or affect hydrogeologic units are provided as a separate line feature class. XYZ data from all input sources were combined and interpolated using standard grid interpolation methods. Each interpolated surface was then sampled at the centroid nodes of a polygonal cellular array of square cells at a node spacing of 1 mile in both the north-south and east-west directions. The array of polygonal cells is essentially a “flattened”, two-dimensional representation of the digital 3DHFM. The mesh of model cells have multiple attributes that describe the geometry of the regional hydrogeologic framework including x,y location, elevation, and thickness of each hydrogeologic unit. Spatial data in this digital dataset are stored in file geodatabase format and are also released as shapefiles. Data include a polygon dataset representing a hydrogeologic map of the region, a polyline dataset that contains the map traces of faults and calderas included in the 3DHFM, a second polyline dataset that contains the map traces of geologic cross sections from which HGU tops were derived, and a large point features class that contains as x,y,z data all of the geologic input data used to construct the 3DHFM. The GBCAAS 3DHFM is contained in a polygon dataset that is a polygonal array of square cells with multiple attributes. A separate folder contains raster output from the 3DHFM including elevation and thickness grids for each HGU in the 3DHFM. Also included is a raster dataset of a gravity based regional “depth-to-basement” surface compiled from previous studies that was used to represent the altitude of the pre-Cenozoic rock surface and the base of the Cenozoic sedimentary basin-fill deposits and volcanic rocks. The spatial data are accompanied by non-spatial tables that describe the sources of geologic information, a glossary of terms, a description of model units, and a Data Dictionary that duplicates the Entity and Attribute information contained in the metadata file.