Development and flow structures of sand injectites: The Hind Sandstone Member injectite complex, Carboniferous, UK

Abstract

The Hind Sandstone Member of the Carboniferous Craven Basin occurs within a thick succession of marine mudstone and is here interpreted as a sandstone injectite complex. Injectites take the form of (i) sills; (ii) upwards emplaced dykes; (iii) downwards emplaced dykes; (iv) a sedimentary laccolith; and (v) sandstone bodies remobilised in-situ. The remobilised and injected sand originated from small turbidite scour fills and potentially a more significant sand body such as a submarine slope gully fill. The sedimentary laccolith appears to jack-up the units it injects, and is filled by sandstone with a chaotic fabric suggestive of brecciation of partly cemented sand. The laccolith and sill-dominated nature are suggestive of relatively shallow remobilisation. Sills are commonly laterally gradational with upwards emplaced dykes, and in some cases these feed apparently downwards emplaced dykes. A series of flow structures are preserved within some of the injectites, and these provide clues to the emplacement history. Crack propagation direction is inferred from the orientation of undulating crests developed on the upper and lower surfaces of injectites. Peak-flow is indicated by scour marks, resembling flute and tool marks, which are superimposed onto these undulating surfaces. Internal flow structures highlighted by pyrite mineralistion record the waning stage of injection. The observed flow structures suggest that the apparent crack propagation direction was generally oblique to the peak flow, and nearly perpendicular to the direction of late-stage fluidised sediment flow. Crack propagation relates to the local stress field, whilst injection flow may be related to the local pore pressure distribution. Recognition of this suite of structures may provide a useful interpretational tool for other injectites in both core and outcrop. Pore fluid overpressure which drove injection developed through the migration of petroleum and mineralising fluids from the underlying Bowland Shales and older limestones. A complex brecciated fracture network within the mudstones underlying the injectites records fluid migration pathways. Sliced blocks and thin sections from injectites reveal flow structures defined by iron pyrite mineralisation. The trigger is here related to growth of compressional structures and associated syn-sedimentary slumps which occur at the same stratigraphic level as the injectites.