Septarian concretions: internal cracking caused by synsedimentary earthquakes

Septarian concretions are abundant in many Phanerozoic marine and marginal‐marine shales and mudstones. They range from a few centimetres to several metres in size and are spherical or ellipsoidal in morphology. In general, formation by localized calcite or siderite cementation in argillaceous sediments began under less than a few metres of burial. Septarian cracks vary widely in shape and configuration: included are networks of wide, vertically, radially and sometimes concentrically oriented, lenticular shrinkage cracks; and narrower, parallel‐sided, straight to irregular tension cracks locally accompanied by brecciation, and plumose and en échelon sigmoidal cracks indicative of shear stresses. Crack types are intergradational; many concretions exhibit multiple cracking events. Enclosed macrofossils and isopachous fibrous calcite cement that lines earlier formed cracks are commonly broken and displaced. In some cases, cracks contain injected lime mud and silt. These features, taken together, testify that cracking involved a spectrum of responses in concretion interiors, from loss of shear strength, dewatering and shrinkage to brittle failure, demonstrating variations in, and contrasts between, the rheological properties of the matrix and enclosed objects. Localization to interiors and outward tapering of lenticular cracks make a case for fracture partitioning and indicate that interiors were softer than exteriors at the time of shrinkage. Parallel‐sided cracks point to greater stiffness, and evolving crack shape in multiply cracked concretions shows that rigidity increased with time. Crack orientations indicate highly variable tensile and shear stress directions within individual concretions. Rupture, brecciation, displacement of fragments, loss of shear strength, liquefaction and injection of unconsolidated granular sediment suggest that deformation events were rapid, if not virtually instantaneous. Previous explanations for the internal cracking, such as gas generation, spontaneous chemical dehydration or localized overpressuring due to compaction, seem either untenable or fail to account for the spectrum of observed features. However, syndepositional earthquake‐induced shaking of cementing bodies of varying rigidity at shallow burial depths seems to be a plausible source for the requisite short‐lived, variable to anisotropic, high‐stress regime inside the concretions. Septarian concretions may thus preserve a signature of basin seismicity as it relates to their cementation history.