Bedding-parallel lenticular soft-sediment deformation structures: A type of seismite in extensional settings?

Soft-sediment deformation structures (SSDSs) with bedding-parallel lenticular morphology have been occasionally reported and called different names, such as loop bedding or pillow bed. However, there is no understanding of the formative conditions that give rise to them versus other SSDSs. Similar bedding-parallel, lenticular, dolomite bodies, tens of centimeters thick with quite variable lengths (30 cm–30 m), occur across a large area (>3000 km²), exclusively in the St. Louis Member (Late Vasean, Mississippian) of the Slade Formation in east-central Kentucky, USA. Field investigations indicate that the lenticular structures, along with other associated SSDSs, represent a persistent deformed horizon that is correlative with and occurs symmetrically around the more intensely deformed Big Sinking Bed, which was previously interpreted as earthquake-induced SSDSs. Based on multiple lines of evidence, the lenticular structures are interpreted to represent distal expressions of the same seismic event that generated the proximal Big Sinking Bed. The lenticular structures were subsequently dolomitized and their selective dolomitization is attributable to their brittle deformation during the earthquake, which resulted in preferentially developed fracture porosity that allowed more efficient circulation of Mg²⁺ fluids throughout the lenticular structures. Petrographic and stable-isotopic evidence also suggests that the selective dolomitization postdates the deformation, probably related to a brine-reflux system developed during the first marine transgression following subaerial exposure of the St. Louis carbonates. Based on comparative analysis with other global cases, the bedding-parallel, lenticular structures are interpreted to represent a distinct type of SSDS, comparable to boudinage in tectonic deformation. Their formation reflects a situation in which seismically-elevated pore-fluid pressure is lower than vertical confining stress but higher than horizontal confining stress. A prominent occurrence of lenticular-type SSDS is a potential indicator of syndepositional extensional stress regime, which is testified by tectonic settings of all known cases, including the Kentucky dolomite bodies.