ArcGIS Maps Depicting Topography of the Basement-Cover Contact (the Great Unconformity), and the Traces of Faults and Folds, in the Cratonic Platform of the United States

In the cratonic platform of the United States, the crust consists of Precambrian igneous and metamorphic rock ("basement") overlain by a relatively thin veneer of Phanerozoic sedimentary strata ("cover"). Regions underlain by cratonic-platform crust include the Midcontinent (the Interior Plains, the Appalachian Plateau, and the Ozark Plateau), the Rocky Mountains, and the Colorado Plateau. In the Midcontinent, land-surface topography does not provide insight into the location and character of tectonic structures, because sedimentary basins of the region have filled—surface relief primarily reflects the consequences of fluvial dissection of flat-lying strata, or landforms resulting from Quaternary and Holocene deposition. Tectonic structures are, however, delineated by variations in the elevation of the basement-cover contact (the "Great Unconformity"). We have produced a new ArcGIS digital "basement-topography map" representing regional-scale elevation variations of the Great Unconformity, both above and below ground, across the cratonic platform in the United States. This map, which can be presented either with or without shaded relief, covers the region between the Appalachian Mountains on the east, and the Basin and Range on the west. It reveals the 3-D shape of the Great Unconformity surface as it would appear if Phanerozoic strata were removed, and assists in visualization of epeirogenic domes, basins, and arches, as well as of major basement-penetrating faults. Since this contact lies at depth in much of the region studied, the map effectively provides an impression of variations in depth to basement, much as a structure-contour map would. Because the map is digital, it can be easily combined with other maps to provide a basis for interpreting regional geophysical and geological data. To help delineate the relationship between geologic structures and features on our basement topography maps, we also compiled a map showing the traces of Midcontinent folds and faults in the cratonic platform.