A GIS GEOLOGIC MAP COMPILATION: THE MT. RUSHMORE QUADRANGLE, BLACK HILLS, SOUTH DAKOTA (1:24,000 SCALE)

On-going hydrologic studies in the central Black Hills (funded by the West Dakota Water Development District) are examining individual 7.5 minute quadrangles in order to create an atlas of aquifer characteristics for sedimentary and crystalline rocks. The first step in this effort is the creation of a geologic map.

A published 7.5 minute geologic map for Mt. Rushmore quadrangle currently does not exist. Using ArcGIS, existing field geologic maps—prepared over a 40 year period by J. Norton and J. Redden—and structural data collected by S. Allard and students were digitized for publication by the South Dakota Geological Society (SDGS). Initially, the data were amalgamated and compiled onto a paper topographic map, verified and edited. The compilation was then drafted manually onto a Mylar topographic map. This Mylar map was scanned and the image was georeferenced in ArcMap based on current SDGS quadrangle boundary data. A series of shapefiles were created and organized within a personal geodatabase and digitization started. Each layer (contacts, geology, structures, foliations, etc.) was attributed appropriately based on SDGS standards.

The compilation of the geologic map helped identify areas needing additional field studies (underway) and allowed new interpretations of the complicated geology, e.g., recognition of rarely preserved early F₁ structures. NW-trending units of phyllite, metagraywacke, metagabbro, metabasalt, granite and laterally discontinuous iron-formations are paralleled by the main existing faults. The major structures—primarily refolded folds and domes—also trend NW-SE, most likely reflecting the last major period of regional deformation. The southwestern portion of the mapping area is dominated by the massive Harney Peak granite and surrounding pegmatites, sills and dikes, e.g., that used for sculpting the presidents at Mt. Rushmore. Approximately 5% of the exposed geology is alluvium located along major perennial streams and several small outcrops of Tertiary gravel. Thus, most groundwater in this area is hosted by joints in crystalline rock or faults.