Dating faulting and fluid circulation using the U-Pb carbonate geochronometer reveals Cenozoic reactivation in Gower, Wales

With renewed interest in the Bristol Channel and Celtic Sea Basins for geoenergy, South Wales offers the opportunity to study exposed basement rocks at the basin margins. Field relationships show NW-SE and NE-SW trending faults, joints, and fissure fills in South Wales underwent multiple episodes of deformation. Previous studies assume deformation initiated during the Late Variscan, with extensional reactivation during the Mesozoic (e.g. Wright et al., 2009). Recent studies demonstrate the importance of Cenozoic reactivation of Mesozoic structures in the Wessex Basin (Parrish et al., 2018), Bristol Channel Basin (Connolly et al., 2024), and Ireland (Monchal et al., 2023). Due to challenges dating fault rocks associated with low-temperature deformation, the timing of reactivation of Variscan structures is poorly constrained - only one previous study in Gower dates Cretaceous hematite (Ault et al., 2016). New data using the U-Pb carbonate geochronometer at Limeslade Bay, Gower, yields multiple U-Pb ages spanning the Mesozoic to the Cenozoic. A N-S oriented, blocky calcite vein yields a Triassic ²⁰⁶Pb/²³⁸U intercept age of 245 ± 11 Ma (MSWD = 2.3, n = 31), and alteration-seam recrystallised host rock yields a Jurassic ²⁰⁶Pb/²³⁸U intercept age of 186 ± 19 Ma (MSWD = 2.3, n = 37). Six samples associated with strike-slip deformation yield Eocene-Oligocene (39 - 24 Ma) ages. NW-SE dextral and NE-SW sinistral fault systems displace calcite veins of 27.0 ± 3.4 Ma (MSWD = 1, n = 62) and 24.53 ± 1.28 Ma (MSWD = 1.7, n = 37), respectively. Additionally, an Oligocene (28.6 ± 2.5 Ma, MSWD = 1.2, n = 55) vein is disrupted by Miocene deformation, where a ²⁰⁶Pb/²³⁸U intercept age of 13.6 ± 5 Ma (MSWD = 1.4, n = 41) was obtained for recrystallised calcite cross-cutting earlier Oligocene vein fabrics. Cenozoic fault reactivation occurred during N-S compression, resulting from far-field stress during the late Oligocene to Miocene Pyrenean-Alpine orogenies. Fluid circulation is significant in reactivating basin margin structures during basin inversion, which poses a hazard to many geoenergy applications.

 

References:

AULT, A. K., FRENZEL, M., REINERS, P. W., WOODCOCK, N. H. & THOMSON, S. N. 2016. Record of paleofluid circulation in faults revealed by hematite (U-Th)/He and apatite fission-track dating: An example from Gower Peninsula fault fissures, Wales. Lithosphere, 8, 379-385.

CONNOLLY, J., ANDERSON, M., MOTTRAM, C., PRICE, G. & SANDERSON, D. 2024. Using U-Pb carbonate dating to constrain the timing of structural development and reactivation within the Bristol Channel Basin, SW England. Journal of the Geological Society.

MONCHAL, V., DROST, K. & CHEW, D. 2023. Precise U-Pb dating of incremental calcite slickenfiber growth: Evidence for far-field Eocene fold reactivation in Ireland. Geology, 51, 611-615.

PARRISH, R. R., PARRISH, C. M. & LASALLE, S. 2018. Vein calcite dating reveals Pyrenean orogen as cause of Paleogene deformation in southern England. Journal of the Geological Society, 175, 425-442.

WRIGHT, V., WOODCOCK, N. H. & DICKSON, J. A. D. 2009. Fissure fills along faults: Variscan examples from Gower, South Wales. Geological Magazine, 146, 890-902.