MULTI-SCALE FRACTURE CHARACTERISTICS AND STRUCTURAL GEOMETRY OF THE GLEN CANYON GROUP; IMPLICATIONS FOR GROUND-WATER FLOW IN THE PRINCIPAL AQUIFER NEAR MOAB-SPANISH VALLEY, UTAH

The rocks of the Glen Canyon Group comprise the principal aquifer for the rapidly growing Moab-Spanish Valley area in southeastern Utah. Rock fracturing and structural geometry in the Glen Canyon Group control fluid flow and therefore water yield and availability. Fracture types including joints, joint zones, deformation bands, and faults, represent either a potential conduit or barrier to fluid flow, and have a cumulative effect on permeability across fractured reservoirs. This study examines the extent and characteristics of joints and joint zones and the structural geometry of the Glen Canyon Group along the northeast margin of Moab-Spanish Valley and their potential effect on fluid flow. Lineaments, commonly corresponding to laterally continuous joint zones, digitized from three separate imagery sets were analyzed and compared with outcrop-scale joint analysis. Lineament densities and orientations correlate well across image sets. However, no statistical correlation exists between joint density measured at outcrop scale and remote lineament density at specific sites. Disconnects in joint and lineament density peaks may result from imagery vs. outcrop-scale sampling bias or may reflect different stress states or mechanical controls on fracturing at different scales. Most lineaments and joints measured at outcrop strike to the northwest. Individual joints at outcrops are either open or infilled, most commonly by calcite and occasionally by hematite. No systematic variation in fracture infill exists. Schematic permeability tensors calculated for joints at outcrop, and lineament-domain analysis support increased regional- and well-scale permeability parallel to the valley axis due to joints and joint zones. Northwest-striking deformation bands near Moab-Spanish Valley locally offset preexisting calcite-filled joints and likely inhibit fluid flow transverse to their plane. Northwest-striking normal faults along the valley margin that cut Glen Canyon Group rocks have fault cores consisting of 1-2 m thick zones of cataclastic rock and clay gouge along defined slip surfaces. Late timing and the inherently low transverse permeability of northwest-striking faults and deformation bands may reduce the overall permeability of the fractured aquifer transverse to the valley axis.