10 Shear Band Sets and Their Conjugate Patterns‌

The map in Figure 30 shows the distribution and pattern of shear bands in the Entrada Sandstone that is exposed on the southwestern flank of Salt Valley Anticline in the Garden area at Arches National Park, Utah, USA (Cruikshank et al., 1991; Zhao and Johnson, 1992). This is a classic conjugate strike-slip fault pattern also described in sandstones from other regions; see for example Davis (1999) and Roznovsky and Aydin (2001). The inset is a photograph of a simple outcrop-scale conjugate fault pattern from Aydin and others (2006). The angle between the conjugate sets varies, as analyzed by Olsson and others (2004) and Haimson and Rudnicki (2010); the latter attribute the variation to the intermediate principal stress. This is a good example of how shear deformation bands can compartmentalize the top high-porosity section of the Entrada Sandstone. In this type of setting (i.e., shear deformation bands with small strike-slip offset), hydraulic conductivity is high in the vertical direction, whereas it is low in the horizontal direction. In the case shown in Figure 30, the maximum hydraulic conductivity is in the vertical direction, the minimum horizontal hydraulic conductivity is in the NW-SE direction (perpendicular to the densest set of bands) and the maximum horizontal hydraulic conductivity is in the NE-SW direction. Knowledge of the type of structures in a porous aquifer greatly helps in characterizing its anisotropic hydraulic conductivity.

Figure 30 – Shear bands pattern in Mesozoic sandstones, primarily Entrada Sandstone in the Garden area of Arches National Park, Utah, USA. This pattern is known as “conjugate.” From Cruikshank and others (1991) and Zhao and Johnson (1992).