Subaqueous shrinkage cracks and early sediment fabrics preserved in Pleistocene calcareous concretions

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    Abstract

    Septarian cracks within concretions are widely believed to have formed by dehydration of initially soft concretion interiors. Other workers believe that such cracks form as tensile fractures in response to stresses induced during burial by 50 m and more of overburden. Herein attention is drawn to the fabrics of Pleistocene, calcareous concretions which have been subjected to a maximum overburden thickness of 10–12 m. These bodies, up to 10.5 cm in maximum dimension, are common in the silty clays of the Late Devensian (18000–13500 years BP), glacimarine Errol Beds of east-central Scotland. Within the concretions occur open fractures; microscopic lenticular, macroscopic, margin-parallel lenticular and septarian-like (radial) forms. These are interpreted as a product of subaqueous shrinkage during very shallow burial. This mode of formation is supported by SEM studies of the clastic fabrics preserved in the concretions which are indicative of deposition from originally flocculated suspensions. Spherical cavities, 30–35 µm in diameter, are interpreted as former gas bubbles derived from the decay of proteinaceous organic matter. This is believed to have promoted localized Ca2+ precipitation, in the form of a soap of fatty acids, through a rise in pH. The shrinkage cracks are thought to have been induced by the density increase which accompanies the conversion of Ca soap to CaCO3. The former gas cavities exert some control on the position of microcracks. Moreover, the delicate open fabrics preserved confirm that the concretions are of early diagenetic origin and formed close to the sediment water interface.
    Original languageEnglish
    Pages (from-to)151-156
    JournalJournal of the Geological Society
    Volume152
    Issue number1
    DOIs
    Publication statusPublished - 1995

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    concretion
    crack
    Pleistocene
    sediment
    overburden
    cavity
    Devensian
    silty clay
    microcrack
    sediment-water interface
    gas
    dehydration
    fabric
    bubble
    fatty acid
    scanning electron microscopy
    organic matter

    Cite this

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    title = "Subaqueous shrinkage cracks and early sediment fabrics preserved in Pleistocene calcareous concretions",
    abstract = "Septarian cracks within concretions are widely believed to have formed by dehydration of initially soft concretion interiors. Other workers believe that such cracks form as tensile fractures in response to stresses induced during burial by 50 m and more of overburden. Herein attention is drawn to the fabrics of Pleistocene, calcareous concretions which have been subjected to a maximum overburden thickness of 10–12 m. These bodies, up to 10.5 cm in maximum dimension, are common in the silty clays of the Late Devensian (18000–13500 years BP), glacimarine Errol Beds of east-central Scotland. Within the concretions occur open fractures; microscopic lenticular, macroscopic, margin-parallel lenticular and septarian-like (radial) forms. These are interpreted as a product of subaqueous shrinkage during very shallow burial. This mode of formation is supported by SEM studies of the clastic fabrics preserved in the concretions which are indicative of deposition from originally flocculated suspensions. Spherical cavities, 30–35 µm in diameter, are interpreted as former gas bubbles derived from the decay of proteinaceous organic matter. This is believed to have promoted localized Ca2+ precipitation, in the form of a soap of fatty acids, through a rise in pH. The shrinkage cracks are thought to have been induced by the density increase which accompanies the conversion of Ca soap to CaCO3. The former gas cavities exert some control on the position of microcracks. Moreover, the delicate open fabrics preserved confirm that the concretions are of early diagenetic origin and formed close to the sediment water interface.",
    author = "Duck, {R. W.}",
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    T1 - Subaqueous shrinkage cracks and early sediment fabrics preserved in Pleistocene calcareous concretions

    AU - Duck, R. W.

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    N2 - Septarian cracks within concretions are widely believed to have formed by dehydration of initially soft concretion interiors. Other workers believe that such cracks form as tensile fractures in response to stresses induced during burial by 50 m and more of overburden. Herein attention is drawn to the fabrics of Pleistocene, calcareous concretions which have been subjected to a maximum overburden thickness of 10–12 m. These bodies, up to 10.5 cm in maximum dimension, are common in the silty clays of the Late Devensian (18000–13500 years BP), glacimarine Errol Beds of east-central Scotland. Within the concretions occur open fractures; microscopic lenticular, macroscopic, margin-parallel lenticular and septarian-like (radial) forms. These are interpreted as a product of subaqueous shrinkage during very shallow burial. This mode of formation is supported by SEM studies of the clastic fabrics preserved in the concretions which are indicative of deposition from originally flocculated suspensions. Spherical cavities, 30–35 µm in diameter, are interpreted as former gas bubbles derived from the decay of proteinaceous organic matter. This is believed to have promoted localized Ca2+ precipitation, in the form of a soap of fatty acids, through a rise in pH. The shrinkage cracks are thought to have been induced by the density increase which accompanies the conversion of Ca soap to CaCO3. The former gas cavities exert some control on the position of microcracks. Moreover, the delicate open fabrics preserved confirm that the concretions are of early diagenetic origin and formed close to the sediment water interface.

    AB - Septarian cracks within concretions are widely believed to have formed by dehydration of initially soft concretion interiors. Other workers believe that such cracks form as tensile fractures in response to stresses induced during burial by 50 m and more of overburden. Herein attention is drawn to the fabrics of Pleistocene, calcareous concretions which have been subjected to a maximum overburden thickness of 10–12 m. These bodies, up to 10.5 cm in maximum dimension, are common in the silty clays of the Late Devensian (18000–13500 years BP), glacimarine Errol Beds of east-central Scotland. Within the concretions occur open fractures; microscopic lenticular, macroscopic, margin-parallel lenticular and septarian-like (radial) forms. These are interpreted as a product of subaqueous shrinkage during very shallow burial. This mode of formation is supported by SEM studies of the clastic fabrics preserved in the concretions which are indicative of deposition from originally flocculated suspensions. Spherical cavities, 30–35 µm in diameter, are interpreted as former gas bubbles derived from the decay of proteinaceous organic matter. This is believed to have promoted localized Ca2+ precipitation, in the form of a soap of fatty acids, through a rise in pH. The shrinkage cracks are thought to have been induced by the density increase which accompanies the conversion of Ca soap to CaCO3. The former gas cavities exert some control on the position of microcracks. Moreover, the delicate open fabrics preserved confirm that the concretions are of early diagenetic origin and formed close to the sediment water interface.

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