Crenulation

In a geological context, crenulation or crenulation cleavage is a fabric formed in metamorphic rocks such as phyllite, schist and some gneiss by two or more stress directions causing the formation of the superimposed foliations.

Micaceous minerals form planar surfaces known as foliations perpendicular to the principal stress fields.

This process occurs at different rates in rocks and beds of different lithology and chemical composition so that it is usually valuable to look at a variety of outcrops to gain a better appreciation of the effect of crenulation or discover the orientation or presence of earlier foliations.

This may occur only in certain compositions of the rocks which favor growth of minerals under the P-T conditions at that time.

In more brittle conditions, especially in highly micaceous rocks, a crenulation may appear as 'kink bands, where S1 foliations are kinked by the S2 foliation so that the original minerals are broken or deformed.

Example of a large-scale crenulation forming kink-folds, Nathans Mine, Glengarry Basin, W.A., showing the superposition of F5 foliation on earlier F3-F4 foliation. A mechanical pencil is seen for scale comparison.