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 t he genesis of septarian cracks. Initial concretion cement formation wi ll lower concretion permeability through the filling of pre-existing p ore space. During progressive burial, this may lead to increased exces s pore pressure, localized within the concretion body causing a reduct ion of the effective stress. Analysis of the stress conditions and cra ck morphology suggests that cracks in septarian concretions result fro m tensional failure (sub-critical crack growth), as a consequence of t his localized excess pore pressure. Conditions suitable for crack form ation will depend upon the magnitude of the excess pore pressure and t he stress corrosion limit of the concretion body. A review of the like ly strength of such concretions indicates that cracking could be initi ated at depths less than 10 m. A variety of observed crack morphologie s can be explained with this model, depending upon the spatial distrib ution of strength and effective stress in the concretion. Crack orient ations mostly reflect stress anisotropy, but are also influenced by di rectional 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 t o the local anisotropic pore-pressure contours, which reflect the perm eability anisotropy of the concretion and surrounding mudrock.