DEFINITION OF BOUDINAGE

A process where a relatively stiff or rigid layer

in a layered rock is ductily extended parallel to layering into

a series of connected or disconnected segments. Softer or

more ductile layers of rock that separate the boudinaged

layer flow into the spaces created by the extending stiff layer.

Under some deformation conditions fibrous minerals or other

minerals, typically quartz or calcite, may fill the spaces

between the boudins. The degree of contrast in the competence

(strength) of the stiff and soft layers determines the

shapes of the boudins. Large competence contrasts between

the stiff and soft layers produce boudins with sharp edges,

whereas small competence contrasts produce rounded

boudins. The term boudinage is from the French boudin, for

sausage links, which these structures resemble.

Simple boudins often form during folding of layered

rock sequences, and in these cases the long axes of the

boudins (and the boudin necks) are elongate parallel to the

fold axes. Under other conditions, such as flattening perpendicular

to layers, with extension in all directions parallel to

layers, structures called chocolate block boudins may form.

These have roughly equidimensional shapes, with boudin

necks in two perpendicular directions.

See also DEFORMATION OF ROCKS; STRUCTURAL GEOLOGY.

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