CROSS JOINT GEOMETRIES IN THE UPPER ENTRADA SANDSTONES; SOUTHEASTERN ARCHES NATIONAL PARK AS SEEN IN GOOGLE EARTH

Within Arches National Park an extensive bedding plane sub-parallel exposure of the upper Entrada Fm. shows a longitudinal and cross joint pattern visible in Google Earth. The longitudinal joints form panels with interior cross joints, whose geometry as seen in Google Earth was the research focus. Results are constrained by image resolution, and how geomorphic processes make the fractures visible. Three specific questions were investigated. Spacing of intra-layer joints is known to be a function of thickness. Is cross joint spacing here similarly a function of the panel width? Does cross joint strike orientation variance increase with distance from the truncating longitudinal joint? Are the cross joint intersections on opposing sides of a longitudinal fracture dependent on each other? Answers to these questions may produce a better understanding of how these joints formed. Interior cross joint spacing was measured along traverses along panel centers of varying width, and panel width vs. average spacing plotted. Cross joints were traced with constant length line segments. Line segment orientations were calculated, and standard deviations of sliding window sub-samples as a function of the average perpendicular distance from the longitudinal joint were plotted. X, Y positions of opposing cross joint intersections along a longitudinal joint were used to calculate the percentage position of each intersection relative to the distance between nearest intersections on the either side. Histograms of percentage positions were constructed, and Chi-square statistics were used to test if the distribution was non-random. Cross joint spacing histograms were unimodal and slightly right skewed. The average spacing vs. panel width relationship is linear for the range of panel widths used. Spacing is ≈50-60% of the longitudinal panel width, a ratio similar to that documented elsewhere for layered rocks. The orientation organization as measured by standard deviation of the cross joint orientations decreases nonlinearly with distance from the adjacent longitudinal joints. In initial results, orientation variability increases significantly at 8 m or greater from an adjacent longitudinal joint. Initial results indicate that there is a relationship between opposing cross joint intersection positions.