Fissure fills along faults: Variscan examples from Gower, South Wales

Abstract

-The extent to which persistent, rather than transient, fissures (wide planar voids) can exist along upper crustal faults is important in assessing fault permeability to mineral and hydrocarbonbearing fluids. Variscan (late Carboniferous) faults cutting Dinantian (Lower Carboniferous) limestones on the Gower peninsula, South Wales, host clear evidence for fissures up to several metres wide. Evidence includes dendritic hematite growth and elongate calcite growth into open voids, spar ball and cockade breccia formation, laminated sediment infill and void-collapse breccias. Detailed mapping reveals cross-cutting geometries and brecciation of earlier fissure fills, showing that fissures were formed during, rather than after, active faulting. Fissures therefore probably formed by geometric mismatch between displaced fault walls, rather than by solution widening along inactive faults.

Chaotic breccia zones on the Pembroke Peninsula, south Wales: Evidence for collapse into voids along dilational faults

“…The age of the Gower fault fissures is unproven. Wright et al (2009) suggested a late Variscan age, but we favour the Triassic age suggested by Strahan (1907) on the basis of supposedly Triassic red sandstones unconformably overlying the Pembroke Group at Port Eynon. A later age cannot be ruled out.…”

“…3c). Wright et al (2009) studied strike-slip cross faults in the Pembroke Group of the Gower Peninsula, there contain void-filling veins and breccias produced during active faulting rather than by solution. One of the westernmost faults contains megabreccia identical to the Pembrokeshire examples.…”

“…Southeast Ireland is more remote from such Mesozoic basins, and the western end of the Pembroke Peninsula seems to have been just outside the zone of basin extension. The Gower Peninsula does have one megabreccia zone at Mewslade (Wright et al, 2009) and many other sediment-and vein-filled dilational faults. 3.…”

“…These faults have also been reactivated in the Pembroke Group with a dilational component, and filled mostly with haematite-calcite veins and laminated red sediment (Wright et al, 2009). One locality (Mewslade, Fig.…”

Relationship between karstification and burial dolomitization in Permian platform carbonates (Lower Khuff — Oman)

“…A pure tectonically related mechanical diminution or abrasion scenario (Jébrak, 1997;Keulen et al, 2007;Mort and Woodcock, 2008;Wright et al, 2009) Another scenario comprises the dissolution of local patches of sulphates which requires a brine density flow (Anderson and Kirkland, 1980) triggered by pressurized meteoric water exploring a present fracture system. The potential presence of some sulphate minerals is supported by the presence of celestite laths within and next to the SSB fabric.…”