Natural fractures can form by various mechanisms in mudstone (or in shales). They can be tectonic (related to folding/faulting). Bedding parallel fractures may be related to depth of burial. Also hydrocarbon generation can create a volume increase leading to fluid overpressure which can fracture the rock. Desiccation cracks are also common in mudstone.
The fractured (silty) mudstones in fig. 1 outcrop on the northern bank of the River Tay in Perthshire, Scotland (Fig. 2). These belong to the Cromlix mudstone Formation (Strathmore Group) which is Lower Devonian in age. There is a high density of fractures (joints) which give the outcrop a very blocky appearance. Desiccation cracks can also be seen on the joint surfaces. Further along the river bank (from fig 1) the mudstones are exposed along a road cutting - here the blocky joints are cut by later fractures which form conjugate sets (Fig. 3). These fractures are filled with calcite (Fig. 4). The calcite fibres run perpendicular to the fracture and often the mudstone is found as pods between the cements.
The most interesting structure related to the through going fractures has a splaying geometry (Fig 5). I wondered if these were tail cracks - formed as a result of stress concentrations around the ends of a slipping fault. They have been later filled by calcite cements. There are also regions where the through going fractures have a green stain indicating that they were pathways for reducing fluids (Fig. 6).
Figure 1. High density of joints in pink coloured (silty) mudstones on the northern bank of River Tay, Stanley, Perthshire, UK.
Figure 2. The mighty River Tay at Stanley, Perthshire, UK.
Figure 3. Blocky jointing in the Cromlix mudstone is cut by larger, more through going fractures which form conjugate sets. From a road cutting above the level of the trees in fig 1.
Figure 4. Calcite filled through going 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.
Figure 5. Through going fracture with a splaying geometry in the Cromlix mudstone.
Figure 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.
Excursion Guide
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