Joint zones, also known as swarms, clusters, or corridors, are a series of closely spaced joints separated by relatively wider unfractured or less fractured slabs of rocks. They show a wide variability in geometry and dimension as well as genesis. Perhaps one of the well known examples of joint zones occurs in the Jurassic sandstones exposed at the flanks of the Salt Valley anticline at Arches National Park. Figure 1 is an aerial photograph of the northeastern flank of the anticline showing a set of highly systematic joint zones in the Moab member of the Entrada Sandstone, which was described by Dyer (1979; 1983).

Figure 1. A set of well defined joint zones trending from left to right, sub-parallel to the strike of bedding along the axis of the Salt Valley anticline in its northeastern flank. The lighter colored formation is the Moab Tongue member of the Entrada Sandstone and has a thickness of 20-40 m. Note that the diagonal set confined between the neighboring zones is a secondary set of splay joints. From Dyer (1979).

There, the joint zones, each of which includes only a small number of joints (Figure 2), extend many kilometers sub-parallel to the strike of the bedding or from left to right in the view.

Figure 2. Details of individual joint zones in the Entrada Sandstone exposed at Arches National Park, Utah. The joint zones in this locality are quite straight and and narrow and contain fewer individual joints. From Dyer (1983).

Another example is shown in Figures 3 and 4 which are maps of joint traces at different scales in the Pictured Cliffs Sandstone exposed near Durango, Colorado (Laubach, 1991). These joint zones (called 'swarms' by the author) have less regular geometry than those in the Entrada Sandstone in Figures 1 and 2 and include many individual joints some of which have slightly different trends with implications for the role of shearing in the evolution of the joint zones in hierarchical scales at this locality.

Figure 3. Map showing a series of fracture zones in the Pictured Cliffs Sandstone exposed near Durango, Colorado. From Laubach (1991).