8 Deformation Band Sets
Following the kinematic classification of single deformation bands, this section discusses sets of compaction bands. Sets of shear bands are discussed in Section 10 and sets of dilation bands, which are rather rare and difficult to identify both in the field as well as in cores, are not discussed.
The map in Figure 25 shows multiple sets of compaction bands first recognized by Hill (1989) in the Cottonwood Wash area of the Valley of Fire State Park, Nevada. It includes three sets marked as 1, 2 and 3 on the map. A fourth set along the low-angle cross-beds is not visible on the map. Aydin and Ahmadov (2009) subsequently documented these features and introduced the concept of bed-parallel compaction bands. Two of the sets, labeled 1 and 2, are broadly spaced and are at right angles to each other. The third set, labeled 3 by Hill (1989), is confined between sets 1 and 2 and is characterized by narrower spacings, shorter lengths and a zigzag geometry. This set is analogous to the anti-cracks of Mollema and Antonellini (1996) and shear-enhanced compaction bands of Eichhubl and others (2010). Compaction bands form in porous rocks normal to the maximum compressional stress direction, and their hydraulic behavior is like that of shear deformation bands. The maximum permeability reduction is in the direction normal to the compaction band. These structures, therefore, introduce a strong anisotropy of hydraulic conductivity in the rock masses and should be considered when assessing groundwater flow pathways.
Figure 25 – Multiple sets of compaction bands in Aztec Sandstone cropping out at Cottonwood Wash in Valley of Fire State Park, Nevada, USA, were originally mapped and described by Hill (1989). Two of the sets, labeled 1 and 2, are broadly spaced and at right angles to each other. The third set, labeled 3, is confined between sets 1 and 2 and has narrower spacings and shorter lengths. Later, this set was interpreted as a shear-enhanced compaction structure by Eichhubl and others (2010).
Figure 26 illustrates the relationship between the presence and orientation of compaction bands and cross-beds in the Aztec Sandstone. The outcrop reported in this figure is not far from the locations where Hill (1989) and Aydin and Ahmadov (2009) mapped high-angle-to-bedding and bed-parallel compaction bands. The annotated photograph shows two adjacent dune packages with cross-beds at different orientations and associated compaction bands (Figure 26). One domain of high-angle-to-cross-beds compaction bands occurs at the upper dune, and the other domain of compaction bands occurs parallel to the cross-bed at the lower dune. It then follows that as the cross-bed orientations vary from one dune package to the next, so do the compaction band orientations as shown in the schematic block diagram in Figure 27. This interdependence is attributed to the anisotropy introduced by the depositional structures, primarily cross-beds (Deng and Aydin, 2012; Deng et al., 2015a).
Figure 26 – Distribution of compaction bands (CBs) in adjacent dunes with different cross-bed architectures in the Valley of Fire State Park, Nevada, USA. One set of high-angle-to-bedding compaction bands is pointed out by horizontal arrows in the upper dune with a distinct cross-bed orientation (light and red diagenetic colors of beds). Another set of compaction bands occurs parallel to cross-beds in the lower dune with noticeably different cross-beds. From Deng and Aydin (2012).
Figure 27 – Schematic diagram illustrating compaction band (CB- bed-parallel Bp and high-angle Ha) patterns adopted from dunes (numbered D1, D2 and D6) and cross-beds therein. From Deng and Aydin (2012) and Deng and others (2017).
The schematic diagram in Figure 27 depicts the variation of compaction band orientation controlled by a geometry of multiple dune packages to form the following sets: high-angle set, bed-parallel set, two intersecting high-angle sets, and both high-angle and bed-parallel sets. These are idealized from an exposed section in the field that passes through dunes D1, D2 and D6 as labeled by Deng and Aydin (2012).