Sulphur isotopes in Permian-Triassic evaporites: A tool for stratigraphy and correlation

Abstract

The latest Permian-Triassic time interval is associated with major perturbations in the global sulphur cycle, expressed as variability in the sulphur isotopic composition (δ34S) of seawater sulphate and recorded in sedimentary sulphates of marine origin. Recent efforts have applied sulphur isotope stratigraphy for the correlation of evaporitic strata in central Europe and Spain. However, despite the abundance of evaporitic material, sulphur isotope stratigraphy has yet to see widespread application to the UK late Permian-Triassic. This thesis presents evaporite-based sulphur isotope (δ34Sevap) records from multiple UK sub-basins, the Larne, East Irish Sea, Wessex, Cleveland, and UK Southern North Sea (SNS) basins. The data herein expands the geospatial coverage of the Permian-Triassic sulphur isotope age curve and provides further age-calibration for sedimentary evaporites with limited biostratigraphic constraint.
δ34Sevap data from drillcore of the Staithes S-20 borehole, Cleveland Basin, provides coverage of the uppermost Zechstein, through to the uppermost Mercia Mudstone Group (MMG) and forms the basis from which to correlate other UK latest Permian-Triassic δ34Sevap records. The Staithes S-20 profile is correlated with the global δ34Sevap curve to form a composite record, which is assimilated into a sulphur biogeochemical box model, producing the most complete record of pyrite burial rates for the latest Permian-Triassic.
An equivalent δ34Sevap profile is presented from drill cuttings of well 42/28-2 in the UK SNS and correlated with the Staithes S-20 record. The successful correlation demonstrates the potential suitability of drill cuttings for sulphur isotope stratigraphy of evaporitic lithologies, greatly increasing the number of sites available for sampling, presenting the potential to expand the geospatial coverage of the Triassic δ34Sevap age curve. In addition, δ34Sevap data from the Bunter Shale and Bunter Sandstone (Sherwood Sandstone Group – SSG) in both Staithes S-20 and 42/28-2 are within the range for late Permian seawater. Additional δ34Sevap data from nodular calcium sulphates of drillcore GT-01 Kilroot, Larne Basin, within an evaporitic siltstone interval at the base of the Ormskirk Sandstone (SSG), also fall within the range established for Permian seawater. δ34Sevap data from the uppermost SSG in another core within the Larne Basin, Cairncastle-2, compare well to the δ34S of latest Early Triassic seawater sulphate. Together, these data suggest that in the locations sampled, the position of the Permian-Triassic boundary should be placed at a currently unconfirmed position within the SSG. However, uncertainties persist regarding the precise degree of marine vs. terrestrial influence and additional sampling is necessary to confirm the reproducibility of these δ34Sevap data.
Further δ34Sevap data were generated from Middle to Late Triassic strata of 110/13-8, East Irish Sea Basin (EISB). Although the δ34Sevap profile exhibits a greater degree of scatter than the equivalent records of Staithes S-20 and 42/28-2, the broad, general declining trend established for the Middle-Late Triassic is clearly replicated, enabling correlation. These data constitute the highest resolution δ34Sevap profile generated for Triassic strata of the EISB and assist in providing tighter age constraint beyond that which can be achieved with sparse biostratigraphy alone. Further sampling of the EISB is required to better confirm the trends observed in the δ34Sevap profile of 110/13-8.
It can be difficult to confidently distinguish between marine and nonmarine evaporite systems in the geologic record, due to sedimentological and mineralogical ambiguities. The δ34Sevap profiles for the MMG of Carnduff-02 (Larne Basin) and Burton Row (Wessex Basin) are largely offset from the global composite δ34Sevap age curve, which are interpreted here as evidence for terrestrial deposition and hydrologic restriction from the seawater sulphate reservoir. When compared with the δ34Sevap profiles of 110/13-8, Staithes S-20, and 42/28-2, the data imply a spatial distinction in the degree of marine influence during of the UK Triassic, helping to reconcile apparent disagreements between sedimentological, mineralogical, and palaeontological data.
A suite of stable isotopic (δ34Sevap, δ34SCRS, δ15Ntot, δ13Corg) and organic geochemical data were generated from the late Permian Eden Shales, Vale of Eden, Cumbria. Two stages defined by distinct biogeochemistry can be defined, with Stage 1 spanning evaporite bed A and Stage 2 evaporite beds B and C. The data for Stage 1 is particularly speculative and requires further refinement. In Stage 2, isotopic and lipid biomarker data possibly indicate photic zone euxinia. In addition, extreme δ13Corg enrichments (>0 ‰), along with the detection of β-carotane, potentially reflect a combination of hypersalinity and enhanced primary productivity. The interpretations put forth here remain speculative, and the generation of additional geochemical data and further sampling of the Eden Shales should be pursued to further constrain the palaeoenvironmental conditions during deposition.