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Septarian crack formation in carbonate concretions from shales and mudstones

Abstract The textural evidence in septarian concretions shows that the septa formed as tensile fractures during burial and compaction of the host shale. Comparison of the combinations of vertical and horizontal stresses and pore-fluid pressure required for tensile fracture of cemented concretions and those likely to occur during shale burial indicates that septarian fracturing will be favoured by overpressuring and low horizontal stress, most likely to occur during times of rapid burial. Burial histories of septarian concretions from the Eocene London Clay and the Jurassic Kimmeridge Clay show that fracturing can form under as little as 50 m of sediment. By relating episodes of septarian fracturing to times of rapid burial, the timing of diagenetic cement formation can be constrained.