Depositional architecture and facies variability in anhydrite and polyhalite sequences: a multi-scale study of the Jurassic (Weald Basin, Brightling Mine) and Permian (Zechstein Basin, Boulby Mine) of the UK

Abstract

Quantifying the geometries of evaporite deposits at a <1 km scale is critical in our understanding of similar ancient depositional systems, but is challenging given evaporite mineral dissolution at surface conditions. Two high-resolution stratigraphic studies in mines provide insight into the geometries, dimensions, and spatial distributions of sedimentary bodies in 3-D at a <1-km scale in evaporites. A field mapping study was conducted in Brightling (Purbeck Group) and Boulby (Zechstein Group) mines, in southeast and northeast England, respectively. This is integrated with XRD, petrography, and δ13C and δ18O isotope analyses. The evolution and conditions of sedimentation during the Tithonian in the Weald Basin is also evaluated. A newly defined megasequence boundary at the base of the Purbeck Group is suggested to mark the onset of rifting of the Bay of Biscay and to the north of the Charlie-Gibbs Fracture Zone, which implies an earlier rifting phase than previously proposed. Basal Purbeck lateral facies changes are influenced by the position in the Weald Basin, normal fault systems, and relative sea level changes. In Brightling Mine, the basal Purbeck exhibits carbonate-evaporite shoaling upward cycles, likely controlled by localized high-frequency relative sea level changes and/or sabkha hydrology. The dynamic process of evaporite deposition led to subtle stratigraphic heterogeneities and changes in bed thicknesses, but largely continuous lateral bedding. Boulby Mine offers a unique opportunity to study early deformation structures in ancient polyhalite that formed in playa conditions. The controlling mechanism that formed these syndepositional polyhalite tepees is attributed to soft sediment deformation via polyhalite dewatering coupled with penecontemporaneous precipitation of halite during fluid escape. This study offers new insight into the types of heterogeneity observed in ancient evaporites formed in marginal playa and sabkha environments at a <1 km-scale, which can include a variety of compositions and morphologies at a range of scales.