TERRESTRIAL LIDAR MAPPING OF FRACTURE SETS; DETECTING SUBTLE VARIATIONS IN FRACTURE PATTERNS ASSOCIATED WITH FOLDING

The application of terrestrial lidar mapping (TLM) technology for creating digital outcrops for detailed evaluation has become an important advancement in field-based analysis of lithofacies, sedimentary depositional cycling, and fracture characteristics. Although limited to cm-scale analysis, the TLM approach is ideal for capturing entire escarpments or vertical exposures and using this data to generate high-resolution digital outcrops for laboratory evaluation. Combined with geologic analysis, TLM data can be the basis for a wide range of modeling experiments and can provide new insight on outcrop scale geologic variability. Most TLM-based studies have focused on lithologic character. Far fewer studies have been completed that use lidar-based imagery to evaluate structural fabrics found within natural and anthropogenic outcrops. In this study, TLM data was collected over a several outcrops of major sandstone strata within the Mesa Verde Group of the Black Mesa Basin of NE Arizona. These sandstone strata are exposed in stream parallel outcrops over several kilometers that span across multiple, open folds (θ > 176°) that are interpreted to have formed during the early stages of Laramide-age deformation of the Colorado Plateau. TLM data acquisition was coded to capture cm-scale point spacing at ranges of ~125 m from the scanning device. Because of beam divergence and system design, points collected beyond 125 m are more widely spaced, which reduces the sampling resolution as a function of range. Multiple stations were collected along a 25km transect. The processed data reveal two important trends: 1) that fracture sets interpreted to be of tectonic origin become well organized into bimodal sets along anticlinal fold axes with no more than 3° of flexure; and 2) that there is no significant difference in fracture populations as a function of instrument measurement range. Both observations have implications for modeling fracture networks where it can be assumed that fracture sets are scale-independent and that tectonic fractures are relatively uniform in rocks with common mechanical properties. The latter complicates the concept of “mechanical stratigraphy” because fracture connectivity and distribution within rocks of uniform character have unique characteristics that are dependent on structural position.