Septarian concretions: internal cracking caused by synsedimentary earthquakes

Abstract

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.

Depositional environments of the lower Permian Dwyka diamictite and Prince Albert shale inferred from the geochemistry of early diagenetic concretions, southwest Karoo Basin, South Africa

“…b, The REE patterns of the carbonate-fluorapatite cement of the phosphatic concretions and beds in the Prince Albert Formation. (Pratt, 2001). However, because of the massive nature of the host diamictite, sedimentary structures due to differential compaction were not observed.…”

“…For example, sand-sized CFA concretions are observed in the Prince Albert Formation phosphatic chert concretions. The septarian cracks in the spherical calcite concretions from the upper Dwyka are interpreted to have formed while the interior of the concretions was still soft and not yet fully cemented (Pratt, 2001). Equally plausible mechanisms of septarian crack formation include chemical dehydration, rapid changes in overburden pressures (Seilacher, 2001) or syndepositional seismicity (Morad and Eshete, 1990;Pratt, 2001).…”

Microbially-driven formation of Cenozoic siderite and calcite concretions from eastern Austria

“…And finally, the formation of the septarian cracks in the Steinbrunn concretions required a concretion body, which had reached its final volume at the time of crack formation, but was still in a plastic state (cf. Hounslow, 1997;Pratt, 2001). To summarize, the growth mode of the Steinbrunn calcite concretions cannot be reconstructed with certainty, but pervasive growth seems more likely than concentric growth.…”

“…Most common parageneses are carbonate concretions in clayey or sandy horizons, quartz or chert in limestones, and pyrite in black shales (Coleman et al, 1985;Sellés-Martínez, 1996), and the most common concretion-forming minerals are carbonates such as calcite, Mg-calcite, Fe-calcite, siderite, and dolomite (Siegel et al, 1987;Pye et al, 1990;Mozley, 1996). Concretion size varies from millimeters to meters, and estimated growth rates vary from tens to hundreds up to thousands of years for decimeter-sized concretions (Duck, 1995;Pratt, 2001;Thomka and Lewis, 2013). The growth of concretions of meter size is assumed to take millions of years (Sellés-Martínez, 1996).…”

Bajocian ammonoids from Pumani River area (Ayacucho, Peru): Palaeobiogeographical and palaeoenvironmental implications for the Arequipa Basin

“…4B). Grey car bonate concretions, locally compressed by gravitational diagenetic compaction with deformed millimetric laminae and septarian cracks of calcite cementation indicative of syndepositional earthquake-induced shaking (Pratt, 2001), are common in the lower part of this stratigraphic interval (Fig. 4G).…”