Abstract

The geometry and distribution of joints and faults in the Jurassic Navajo Sandstone result in structural and hydrologic compartmentalization and heterogeneous, anisotropic permeability. Wells in the Navajo provide half the public water supply for southwestern Utah, a region experiencing rapid population growth, which highlights the importance of understanding the influence of fractures on ground-water conditions.

Neogene extensional faults and Sevier-age contractional folds and faults severed the Navajo into six structural compartments. Compartmentalization restricts regional groundwater circulation patterns and isolates some regions from the primary regional recharge source, the Pine Valley Mountains.

Joint zones, linear zones of high joint density bounded by less densely jointed regions, are common in the Navajo and result in heterogeneous joint density. Joint populations typically have one or two strongly defined preferred orientations. Faults consist of 1 to 3 ft- (0.31 m-) thick gouge zones bounded by 3 to 6 ft- (1-2 m-) thick, densely jointed damage zones, and are conduits parallel to the fault plane but have low transverse permeability.

The permeability of joint zones is at least 35 times greater than that of adjacent, less densely jointed rock masses. Ground water is likely channelized into joint zones and faults, resulting in complex flow paths. The orientation and anisotropy of bulk permeability tensors can be estimated from the shape of ellipses drawn around quantitative rose diagrams, and these ellipses can be used to investigate the influence of fractures on regional-scale flow of ground water. Joint zones require either relatively large representative elemental volumes or thorough mapping and hydrologic characterization to incorporate their channelizing effects into regional ground-water models.