Compilation of Stratigraphic Thicknesses for Caldera-Related Tertiary Volcanic Rocks, East-Central Nevada and West-Central Utah

Published isopach maps for most of the major, regionally distributed ash-flow tuffs of east-central Nevada and west-central Utah were obtained. Paper maps at a variety of scales were scanned and geo-referenced in a Geographic Information System (GIS) and the location and thickness values as measured at outcrops of individual tuffs were digitized from these published maps. In order to preserve the original author's interpreted contour patterns, additional thickness data were created by digitizing a regular series of points along each contour line of the isopach maps. For each tuff, these thickness data were gridded as 2500-m square cells within the GIS to produce a digital thickness grid as raster data sets. Thickness was gridded using either inverse distance or simple kriging algorithms; the gridding methodology was chosen on the basis of how closely the digital grid resembled the original published contour map. Grids were locally hand-edited in order to recreate abrupt thickness changes at known or inferred caldera boundaries. Using this approach, digital thickness grids were created for the following ash-flow tuffs; the geologic unit name is followed by the feature data set name in the geodatabase: the Kalamazoo Tuff (kalam_ipach), the Windous Butte Formation (windous_ipach), the Monotony Tuff (monot_ipach), the Shingle Pass Tuff (shingle_ipach), the Cottonwood Wash Tuff (cotton_ipach), the Wah Wah Springs Formation (wahwah_ipach), the Lund Formation (lund_ipach), the Isom Formation (isom_ipach), the Leach Canyon Formation (leach_ipach), the Condor Canyon Formation (condor_ipach), and the Harmony Hills Tuff (harmony_ipach). The thickness values from these individual isopach maps were summed using grid addition to produce a composite isopach that combines the gridded thickness for all of the above units (ipach_volcs).