Septarian concretion formation using cavity expansion phenomenology
Description
These data are observations of septarian concretions acquired from outcrop of the Cenomanian Tropic Shale near Septarianville, Utah. Here is the abstract for the article: Septarian concretions are mudstone nodules found worldwide, including in Upper Cretaceous strata deposited across central North America within the Western Interior Seaway. They display a central vacated core with radial fractures that extend outward and taper in width from the centre to the edges. Septarian concretions are named for the septa of strata that separate the radial fractures. Slices of Cenomanian Tropic Shale septarian concretions and a computed tomography scan of a complete septarian concretion from Utah are examined. The phenomenology of septarian concretion formation is presented here as a combination of physical processes. Passing earthquake energy initially inundates a spherical volume of strata defined by its impedance contrast with surrounding strata. Spherically focused energy concentrates into a small area, initiating cavitation and accelerated densification of the strata with nonlinear rheology. A swift transition from ductile to brittle behaviour occurs, allowing radial fractures to develop from a contained explosive expansion originating in a small area. Focused energy expands outward into the now-densified strata. The physical process of cavity expansion predicts the development of radially distributed stress and deformation zones as a function of the cavity-expansion velocity. A central void, a zone of crushed and fragmented material, and a radially fractured zone, possibly with pressure-relief vent holes along the concretion’s perimeter, are observed in septarian concretions. The vent holes could facilitate subsequent fluid flow and mineral precipitation, particularly aragonite and calcite, depending on fracture openness. Forward modelling by others supports this model of septarian concretion formation. Lay summary Septarian concretions are small, spherical accumulations of densified strata that can be found worldwide. The concretions exhibit radially distributed deformation patterns throughout the nodule. The patterns can be described as the result of an explosion of energy into a material that resists the expansion. The resistance creates crushed zones and zones with radial fractures outward from a centrally expanding cavity. The energy for the expansion is thought to result from seismic energy passing through and concentrating within the initial sphere. A cavity-expansion model can describe most of the deformations observed in the analyzed septarian concretions. Forward models by others support this model of septarian concretion formation. Keywords: septarian concretion, calcite, cavity expansion, shear-thickening rheology