Septarian structures are former cracks, often filled with cement and are most commonly found in concretions hosted in mudrocks, although rare occurrences are known from siltstones and sandstones. Septarian structures occur in concretions or concretionary sheets, which may be chemically and mineralogically the same as non-septarian concretions in the same mudrock. The host concretions are most commonly calcite, dolomite or siderite dominated, although rare occurrences have been interpreted from silica concretions. Septarian concretions are predominantly a pre-Pleistocene phenomenon, although a potential “proto” version has been described from the late Pleistocene (Duck, 1995).

Crack morphology
Septarian structures were initially formed as open fractures, and are most often concentrated in the central regions of concretions and reduce in width and frequency toward the outer parts of the concretion, which may or may not be cracked. The fractures take a variety of forms, from lenticular...