GIS as an aid to visualizing and mapping geology and rock properties in regions of subtle topography

This study visualizes, quantifies, and evaluates relationships between bedrock geology and topography through the use of GIS. The study area contains weakly consolidated Permian-aged sandstone and shale of slightly differing ages that have been dissected by the regional drainage. The erosion of these rocks has produced a subtle but well-defined topography. Data for this work were obtained from a 30 m resolution digital elevation model (DEM) and the bedrock geology map of Oklahoma. Numeric values of local slope angle and relief were extracted from the DEM and associations with geologic formations were summarized and compared. The maps reveal that local topographic variations are dependent on the relative abundance of sand to shale in the underlying bedrock. This finding is significant because the topographic expression is so subtle that associations between topography and bedrock geology would not be easily recognized or quantified with conventional field techniques. To identify controls on topograpy, sandstone thicknesses were measured in outcrop and the bulk density of field samples was measured in the laboratory. We find that sandstone thickness is greater in areas of higher local relief and thinner in areas of lower relief. In contrast, bulk density, used as a proxy for susceptibility to erosion, is not significantly different between areas. These findings suggest that presence and thickness of sandstone, even if weakly consolidated, plays a role in determining topographic expression of bedrock. This GIS-based technique, when constrained by geology, can enhance the quality of multiuse mapping programs in the geosciences, agriculture, and civil engineering.