MAPPING SLIM BUTTES STRATIGRAPHIC AND STRUCTURAL FEATURES BY UAV PHOTOGRAMMETRY

Slim Buttes, in Harding Co. in NW South Dakota, is one of the few areas north of the Black Hills where the Oligocene and Miocene strata of the White River Group are well exposed, including the Chadron Formation and the overlying Brule Formation. Cliff exposures locally stretch semi-continuously for several kilometers. There is a major angular unconformity between the overlying Arikaree Group and faulted Brule Fm. strata. Both units contain evidence of nested channel complexes, with > 15 m of relief on individual channels. Building upon previous studies at Slim Buttes, drone/UAV technology was used to capture and map some of these formations in order to create 3D virtual outcrops that can be studied offsite and expand/complement previous findings. Collecting photogrammetric information in 3D and being able to model it in a quantifiable format gives the geologic community a powerful tool to add to traditional field work. This data can be ultimately shared with other geology professionals and academic entities. Initial data processing in the field provides checks and the ability to adapt image acquisition parameters. The final product is a high resolution, photorealistic 3D model from which measurable features can be extracted, including the orientations of fractures and cross beds, stratigraphic thicknesses, and xyz positions on surfaces that can then be contoured. Analysis is presently focused on identifying evidence of channel morphology and fill architecture, as well as lateral facies changes. This includes characterization of possible point bar architecture in the Arikaree Grp. strata, which could be suggestive of more channelized than braided flows, and which will inform paleocurrent analysis. In our case, using UAV photogrammetry to map these geologic features not only provided visual access to normally inaccessible areas, it also provided the ability to gather large amounts of data in a short amount of time and on a slender budget.