GEOLOGY AND JOINT ANALYSIS OF THE SOUTH 1/2 OF THE CUSTER QUADRANGLE, SOUTH DAKOTA

    The objective of this study is to prepare and present the South Dakota Geological Survey (SDGS) with a digital copy of the Custer Quadrangle geologic map, to collect and analyze joint data to define dominant joint trends and morphological characteristics, and to discuss the implications of joint trends (structural anisotropy) on groundwater anisotropy. The 7.5-minute Custer Quadrangle is located in the southern Black Hills, South Dakota; the study area is the south half of the quadrangle. Unpublished field maps of Dr. Jack Redden were examined, compiled, and field checked. Additional mapping of several critical areas was performed. Joint data and additional orientation data were collected and added to the compiled map which was digitized on-screen using Corel DrawTM and presented to the SDGS for publication. The general geology of the study area is described. Structural domains were defined and joint data was collected per the selection method and analyzed using STEREONETT as was data collected from aerial photographs.

    Analysis of joint data was separated based on lithology, granite or metasediments and aerial photograph data. Although analysis of individual domains shows considerable variability in trend data, the following generalizations are recognized. Data from aerial photographs for larger granite outcrop indicate joint trends at N45E, N25W, and N55W. Data collected on smaller granite outcrop agrees closely with data from aerial photographs; joint trends are N65E, N25W, and N55W. Analysis of the metasediment data indicates joint trends at N65E, N35W, and N25E. Locally, water wells are located along discontinuities; the Custer city wells are located along the Fourmile fault and wells for Mount Rushmore National Monument are located along joint trends. Results of this study and similar studies may present helpful guides for placement of future well sites in this area.