Can mudstones and shales be heavily fractured ?
Yes, although they often deform in a more ductile manner than sandstones and we often see a fracture in a sandstone bed dying out in the neighouring shale. Fractures in shales can form in various ways including unloading and through tectonic processes such as folding and faulting.
Outcrop example near Perth, Scotland
The Cromlix mudstone formation (Strathmore Gp) is of Devonian age and outcrops on the northern bank of the River Tay (Fig. 1) in Perthshire, Scotland. This is a real hidden gem close to the Stanley Mills carpark which I discovered by picking up a BGS guide online
http://earthwise.bgs.ac.uk/index.php/Stanley_and_Campsie_Linn_-_an_excursion
The main types of fractures in the mudstone are joints (Fig. 2), although later shear fractures (faults) are also present (Fig. 3). Closely spaced fractures are possible decompaction - stress relief fractures (Fig. 3). The mudstone is quite silty in places which perhaps makes it more susceptible to fracturing. Dessication cracks can also be seen on the joint surfaces (Fig. 4).
Fig. 1. The mighty River Tay at Stanley, Perthshire
Fig. 2. Joints give the mudstone outcrop a blocky appearance in the Cromlix Mudstone.
Fig. 3. Background joints and later shear fractures which have conjugate geometries. Scale circled (car keys).
Fig. 4. Dessication cracks in the Cromlix mudstone. From BGS guide referenced in text.
Note that the shear fractures jut out of the outcrop due to relative resistance to erosion. This is because they are filled with calcite cements (Fig. 5). The mineral fibres run perpendicular to the fracture and often the host mudstone is found a pods between the cements. In other places, the through going fractures have a green stain indicating that they were pathways for reducing fluids (Fig. 6). Therefore, this fracture network was open and connected at the time of fluid movement.
Fig. 5. Calcite filled fractures in the jointed mudstones. Note how the calcite fibres run perpendicular to the fracture and that pods of mudstone are enclosed between the cements.
Fig. 6. Green stain along some of the through going fractures indicating presence of reducing fluids. Note the intensive, small scale blocky jointing in the background.
The most interesting structure has a splaying geometry (Fig 7). I wondered if these were tail cracks - formed as a result of stress concentrations around the ends of a slipping fault. Its more likely that they are fringe structures related to plumose structures in joints and that we a looking straight onto a joint face. The plumose pattern (Fig. 7 right) develops from local twists and tilts during propagation of the joint. The likely fringe structure in outcrop has been later filled by calcite cements.
Fig. 7. Splaying structure in the Cromlix mudstone. It is possible that these are fringe structures related to plumose structures.
Key takeaways
In sand-shale sequences we often observe fractures in sands disappearing into neighbouring shales (as are more ductile)
However, mudstones and shales can host connected fracture networks with a variety of structures as demonstrated here from the Cromlix mudstone, Perthshire, Scotland.