Faults, fractures and fluids in mudstones during Cenozoic extension in the Cleveland Basin

Abstract

Fractures and faults may act as important fluid-flow pathways through low permeability mudstone sequences; understanding their occurrence and properties is thus important when considering the risks of leakage of petroleum from underlying reservoirs, or CO2 from storage sites. Fluid-flow is also an important in radioactive waste storage sites during construction or operation of the sites. This project studies calcite-filled fractures and faults developed within the exhumed, early-mature, Jurassic mudstone succession of the Cleveland Basin, NE England, combining structural geology with isotope geochemistry and geochronology. The abundance of well-exposed, natural fractures with different orientations and failure modes provides an opportunity to investigate the properties of these fractures and provides a basin-wide temporal and spatial framework of evolving deformation.
Calcite veins and fault fills are present in N-S to NNW-SSE trending normal faults and associated fractures in the north of the Cleveland Basin. U-Pb calcite geochronology has yielded ages in the range 45-20Ma for sampled calcite. This describes a previously unrecognised phase of Cenozoic faulting. I propose that this deformation relates salt-related deformation following regional tilting, uplift and eastwards gravity sliding related to Atlantic opening and the development of the proto-Icelandic plume to the NW of Britain.
Structural and petrographic observations suggest that N-S trending faults have complex kinematic and fluid-flow histories. Faults are characterised by damage zones with widespread calcite mineralisation, extensional jog structures and fracture reactivation. Stable isotope and clumped isotope analyses suggest that the regional fracture-controlled fluid-flow during the Cenozoic deformation involved post-exhumation mixing of cool meteoric waters (20°C) with warmer (80°C) basinal fluids. I hypothesise that fluid is driven by gravity-driven regional extension. Whilst fault-related displacements are modest, meaning that the structures are unlikely to be widely resolved in offshore seismic reflection profiles, they are widespread and therefore are inferred to represent highly effective fluid-flow pathways. Our findings place new important constraints on the poorly constrained Cenozoic tectonic history of the Cleveland Basin and northeast Britain.