12 Sandstone-Shale Sequences
Siliciclastic sequences commonly include alternating sandstone, siltstone and shale lithologies. Even small amounts of fine-grained materials, either silt or shale, have drastic consequences on deformation mechanisms due to the anisotropy they introduce and their greater ductility. This section provides examples of the role played by fine-grained lithologies on fracture occurrences, fracture terminations, fracture infill and shale entrainment.
Fine-grained units commonly occur at bedding interfaces and interdune boundaries. These have three important consequences.
- They bound sandstone beds and dune packages as separate groundwater storage units because siltstones and shales are less permeable than sandstones. The fine-grained units act as aquitards (Aydin, 2014; Cilona at al., 2016).
- They truncate joints and small faults (Helgeson and Aydin, 1991; Aydin and de Joussineau, 2014) and the fine-grained units compartmentalize aquifers.
- Due to low shear strength, these units are prone to layer-parallel slip (Cooke et al., 2000) and they form fracture corridors in the adjacent sandstone units.
Sandstone packages or, morphologically speaking, tiers can be separated by fine-grained deposits at the flats, which are instrumental for demarcation of fracture systems in sandstone formations (Figure 37).
Figure 37 – A panoramic picture showing a cliff face about 200 m high exposing a series of Mesozoic sandstone formations with multiple generations of fractures including low-angle normal faults. Sandstone packages or, morphologically speaking, tiers are separated by fine-grained deposits, which are instrumental for demarcation of fracture systems in sandstone formations. The fracture systems are color-coded for their sequence of formation. Thick white lines show low-angle normal faults with small offsets. Slightly modified from de Joussineau and Aydin (2007b).
These units normally would be aquifers. Figure 38 and Figure 39 are isolated cases that better illustrate the role of fine-grained deposits in bounding fractured sandstone domains. Figure 38 shows an interdune deposit truncating a well-developed system of joints in sandstone units above the fine-grained deposits. Figure 39 shows slip across a fine-grained unit along a dune boundary (diagonal from lower right to upper left) with an associated joint cluster or corridor in the sandstone package (upper right).
Figure 38 – Photograph showing a fine-grained interdune deposit (indicated by blue lines) truncating a well-developed system of joints in the overlying Aztec Sandstone.
Figure 39 – Photograph showing slip across a fine-grained unit along an interdune boundary. The associated joint cluster in the sandstone package in the upper right with respect to interdune deposits indicates the presence of slip along the dune boundary.