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 μ m in diameter, are interpreted as former gas bubbles derived from the decay of proteinaceous organic matter. This is believed to have promoted localized Ca 2+ 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 CaCO 3 . 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.