On the origin of septarian structure

Richardson, W. Alfred

doi:10.1180/minmag.1919.018.86.04

Cited by 10 publications

(5 citation statements)

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“…Macrofossils often, but not invariably, comprise the nuclei of concretions (e.g. Richardson, 1919; Allison, 1988a), but are commonly present in a compacted state in host strata (e.g. Seilacher et al ., 1976 ).…”

Section: Composition Of Septarian Concretionsmentioning

confidence: 99%

“…Weeks, 1953, 1957; Berner, 1968b; Zangerl et al ., 1969 ; Sass & Kolodny, 1972; Dickson & Barber, 1976; Allison, 1988b) have suggested that concretions formed preferentially around animal carcasses because putrefaction of organic material created methane and alkalinity. However, Richardson (1919) and Raiswell (1987) emphasized that identifiable organic carcasses are rare and were probably not involved in the siting of concretions in general. Also, there is an incongruence between the normally high rate of decay of carcasses in aerobic conditions (e.g.…”

Section: Origin Of Septarian Cracksmentioning

confidence: 99%

“…It has been suggested that gel-like concretion interiors suffered shrinkage from chemical dehydration during clay mineral transformation Septarian I concretions 203 (Richardson, 1919). The normal-marine aspect of the majority of host deposits indicates that salinity changes conceivably required to induce a passive syneresis phenomenon were unlikely.…”

Section: Chemical Dehydrationmentioning

confidence: 99%

“…However, Richardson (1919) and Raiswell (1987) emphasized that identi®able organic carcasses are rare and were probably not involved in the siting of concretions in general. Also, there is an incongruence between the normally high rate of decay of carcasses in aerobic conditions (e.g.…”

Section: Gas Expansionmentioning

confidence: 99%

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Septarian concretions: internal cracking caused by synsedimentary earthquakes

Pratt¹

2001

Sedimentology

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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.

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Section: Composition Of Septarian Concretionsmentioning

confidence: 99%

Section: Origin Of Septarian Cracksmentioning

confidence: 99%

Section: Chemical Dehydrationmentioning

confidence: 99%

Section: Gas Expansionmentioning

confidence: 99%

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Septarian concretions: internal cracking caused by synsedimentary earthquakes

Pratt¹

2001

Sedimentology

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“…Richardson (1919) gave the first accurate description of the septarian structure and stated that it resulted from ‘chemical desiccation of a colloid’. At present, however, there is consensus neither on the timing nor on the mechanism of septarian crack formation (e.g.…”

Section: Discussionmentioning

confidence: 99%

Spherical and columnar, septarian, ¹⁸O‐depleted, calcite concretions from Middle–Upper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids

et al. 2007

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Calcite septarian concretions from the Permian Beaufort Group in the Maniamba Graben (NW Mozambique) allow controls on the composition and nature of diagenetic fluids to be investigated. The concretions formed in lacustrine siltstones, where they occur in spherical (1 to 70 cm in diameter) and columnar (up to 50 cm long) forms within three closely spaced, discrete beds totalling 2AE5 m in thickness. Cementation began at an early stage of diagenesis and entrapped non-compacted burrows and calcified plant roots. The cylindrical concretions overgrew calcified vertical plant roots, which experienced shrinkage cracking after entrapment. Two generations of concretionary body cement and two generations of septarian crack infill are distinguished. The early generation in both cases is a low-Mn, Mg-rich calcite, whereas the later generation is a low-Mg, Mn-rich calcite. The change in chemistry is broadly consistent with a time (burial)-related transition from oxic to sub-oxic/anoxic conditions close to the sediment-water interface. Geochemical features of all types of cement were controlled by the sulphate-poor environment and by the absence of bacterial sulphate reduction. All types of cement present have d 13 C ranging between 0& and )15& (Vienna Peedee Belemnite, V-PDB), and highly variable and highly depleted d 18 O (down to 14& Vienna Standard Mean Ocean Water, V-SMOW). The late generation of cement is most depleted in both 13 C and 18 O. The geochemical and isotopic patterns are best explained by interaction between surface oxic waters, pore waters and underground, 18 Odepleted, reducing, ice-meltwaters accumulated in the underlying coal-bearing sediments during the Permian deglaciation. The invariant d 13 C distribution across core-to-rim transects for each individual concretion is consistent with rapid lithification and involvement of a limited range of carbon sources derived via oxidation of buried plant material and from dissolved clastic carbonates. Syneresis of the cement during an advanced stage of lithification at early diagenesis is considered to be the cause of development of the septarian cracks. After cracking, the concretions retained a small volume of porosity, allowing infiltration of anoxic, Ba-bearing fluids, resulting in the formation of barite. The results obtained contribute to a better understanding of diagenetic processes at the shallow burial depths occurring in rift-bound, lacustrine depositional systems.

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Pyrite and calcite in septarian concretions from the Rupelian Clay (Oligocene) at Rumst (Antwerpen, Belgium) and their geochemical composition

Vochten¹,

Geys²

1974

Chemical Geology

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On the origin of septarian structure

Cited by 10 publications

References 0 publications

Septarian concretions: internal cracking caused by synsedimentary earthquakes

Septarian concretions: internal cracking caused by synsedimentary earthquakes

Spherical and columnar, septarian, ¹⁸O‐depleted, calcite concretions from Middle–Upper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids

Pyrite and calcite in septarian concretions from the Rupelian Clay (Oligocene) at Rumst (Antwerpen, Belgium) and their geochemical composition

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On the origin of septarian structure

Cited by 10 publications

References 0 publications

Septarian concretions: internal cracking caused by synsedimentary earthquakes

Septarian concretions: internal cracking caused by synsedimentary earthquakes

Spherical and columnar, septarian, 18O‐depleted, calcite concretions from Middle–Upper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids

Pyrite and calcite in septarian concretions from the Rupelian Clay (Oligocene) at Rumst (Antwerpen, Belgium) and their geochemical composition

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Spherical and columnar, septarian, ¹⁸O‐depleted, calcite concretions from Middle–Upper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids