An investigation of some sediment-filled fractures within redbed sandstones of the UK

Abstract

SUMMARY Permo-Triassic sandstones form the most important aquifer in northern England. These sandstones are susceptible to contamination from a legacy of industrial development, associated with the expansion of urban areas that include Liverpool, Manchester, Birmingham, Nottingham and Teesside. The permeability of the sandstone is derived from the intergranular pore spaces and fractures. However, unconsolidated sediment found within fractures can have a profound effect on fluid flow, depending on whether it is clay or sand. Sediment-filled fractures are not well described in the literature; consequently there are few observations or measurements of the hydraulic properties of these features that would enable an assessment to be made of their hydro-geological significance, and it is unclear how they should be included in solute transport models. A field survey was undertaken to test whether sediment-filled fractures are a common feature within redbed sandstone terrains. Exposures from most of the main UK Permo-Triassic sedimentary basins, and some from the Devonian Old Red Sandstone in south Wales, were examined. The field survey, which included surface and underground tunnel exposures, enabled the investigation of both unweathered and weathered examples of sediment-filled fractures. Detailed mineralogical and petrographical analysis of fracture fills and host rock indicate that, in some cases, fills are historic, and that there is a component to some fills that indicates a glacial origin. Where the fill is unconsolidated, sediment may be flushed from fractures; this may explain why some water wells abstracting from the Permo-Triassic sandstones of the UK pump large quantities of sand throughout their active life, and this may be a contributory factor to the observation that transmissivity around abstraction wells in sandstone can increase with time. It is likely that, in certain circumstances, sediment-filled fractures may locally alter fluid migration paths through sandstones, which has important implications for the transport of both dissolved contaminants and hydrocarbons.