CONSTRAINING FOLD GEOMETRY AT SHEEP MOUNTAIN ANTICLINE, WY, USING AIRBORNE LASER SWATH MAPPING DATA AND GEOLOGIC MAPPING

Folded sedimentary rocks often exhibit complex, non-cylindrical geometries. Improved methods for constraining complex fold geometry from surface data such as measurements of strike, dip and geographic coordinates of bedding contacts are necessary to advance our understanding of the complex relationship between fold geometry and geologic fracture patterns. Seismic data quality is generally poor in areas of high topographic relief while outcrop quality may be excellent, thus motivating an approach that utilizes outcrop data. This approach involves using a high resolution digital elevation model (DEM) derived from airborne laser swath mapping, in conjunction with geologic field mapping, to gather the necessary data to construct a surface model of the fold geometry of the Sheep Mountain anticline, WY. Detailed geologic mapping of bedding plane outcrops representing twelve stratigraphic contacts spanning nearly 700m of stratigraphic section identifies points on the DEM representing these contacts. These points are projected along the local bedding normal vector, which is determined either from triangulation of the outcrop surface using the DEM data or from strike and dip measurements taken in the field to the stratigraphic position of the base Sundance formation. This method provides spatially extensive constraint to interpolate a surface representing the fold. The effects of different interpolation and projection techniques on the quality and accuracy of the surface are described and evaluated. Tools of differential geometry, including maps of principal curvature magnitude and orientation and the local geometric form of the surface (planar, cylindrical, dome, basin, saddle) are used to compare the new model with an existing surface model, which was generated by the method of sequential kinematically balanced cross sections and constrained by much sparser measurements of the strike, dip, and location of exposed stratigraphic contacts.