boudinage

Boudinage is a structural feature in geology in which a competent, layered rock body is segmented into a chain of elongated blocks called boudins, separated by thinner zones of more strongly deformed material. The term derives from the French word for sausage, reflecting the sausage-like shape of the blocks.

Formation occurs under extensional deformation of layered rocks. When a relatively stiff bed is surrounded by more ductile material, it behaves as an isolated slab that lengthens and breaks into shorter, laterally continuous blocks. The boudins tend to align perpendicular to the direction of extension, and the spacing between blocks depends on the thickness of the competent layer, the rheology contrast with the host rock, and the amount of strain.

Geometry and patterns vary. Simple boudinage features relatively regular, cylindrical to pillowy blocks with necks of thinned surrounding material between them. The overall pattern records finite stretching of the layer and can be preserved within metamorphic or sedimentary sequences.

Occurrence and examples include abundant observations in deformed sedimentary rocks, such as sandstone or limestone beds within more ductile shales, where extension or tectonic shear leads to segmentation. Boudinage is also seen in metamorphic rocks and in salt tectonics, where contrasts in rheology and extension produce similar sausage-shaped structures.

Interpretation and significance lie in using boudinage to infer the direction and magnitude of extension, assess rheological contrasts, and reconstruct tectonic histories. The orientation, scale, and distribution of boudins provide practical clues about the stress regime and deformation processes that affected a rock sequence.