Significance of localized pore pressures to the genesis of septarian concretions

The burial-stress and hydrologic conditions existing during concretion formation in mudrocks are evaluated and integrated into a model for the genesis of septarian cracks. Initial concretion cement formation will lower concretion permeability through the filling of pre-existing pore space. During progressive burial, this may lead to increased excess pore pressure, localized within the concretion body causing a reduction of the effective stress. Analysis of the stress conditions and crack morphology suggests that cracks in septarian concretions result from tensional failure (sub-critical crack growth), as a consequence of this localized excess pore pressure. Conditions suitable for crack formation will depend upon the magnitude of the excess pore pressure and the stress corrosion limit of the concretion body. A review of the likely strength of such concretions indicates that cracking could be initiated at depths less than 10 m. A variety of observed crack morphologies can be explained with this model, depending upon the spatial distribution of strength and effective stress in the concretion. Crack orientations mostly reflect stress anisotropy, but are also influenced by directional anisotropy in the crack growth rates. Locally increased pore pressure also likely occurs in non-septarian concretions, but is not sufficient to cause cracking. This enhanced local pressure may assist the crystal surface growth reactions of the carbonate cement. Through this enhancement process, the shape of concretions may be a response to the local anisotropic pore-pressure contours, which reflect the permeability anisotropy of the concretion and surrounding mudrock.