What is Mechanical Stratigraphy ?

Different rock types will deform differently when subject to the same amount of strain. Sandstones in sand-shale sequences are usually more fractured (open fractures) than the shales. This is because shale is usually more ductile than sandstone and when subjected to the same amount of strain, they will take longer to break. Sometimes, fractures enter the shales at a lower angle which is consistent with propagation through a rock of lower frictional strength.

Another example is given in Fig.1 where limestone and dolomites are side by side - the dolomite is fractured as it tends to be more brittle than the limestone which has little/no fractures.

Mechanical stratigraphy also controls fracture intensity through bed thickness - thicker sedimentary beds often have less fractures than thinner beds. This was recognised by McQuillan [1] in the Zagros in the early 1970s.

Note how the thick sandstone bed in the upper part of the outcrop in Fig. 2 has little/no fractures whereas the thinner sandstone beds are fractured. In this case these fractures are likely to be joints are they are near vertical and don’t tend to offset the bedding. The geomechanics reason often given is that tall fractures in thick beds tend to cast stress shadows that prevent new fractures from initiating close by. A simpler explanation is that thinner materials will in general break more easily than thicker materials.

The shales in this outcrop are less fractured than the sandstones as the ductile shales are more difficult to fracture. Therefore, mechanical stratigraphy (lithology and bed thickness) is controlling occurrence of fractures in this outcrop.

Near vertical sandstone beds are shown in Fig. 3; again the sandstones are more highly fractured than the surrounding shales with the deformation occurring pre-folding.