Geological characterisation and cross-section study of the Fell Sandstone Formation in the Berwick-upon-Tweed area, Northumberland

The Carboniferous age Fell Sandstone Formation (henceforth referred to as the Fell Sandstone) forms a principal aquifer in Northern England. It comprises interbedded sandstone, siltstone and mudstone layers. Water abstracted from the sandstone layers provides drinking water for Berwick- upon-Tweed and the surrounding area. The Environment Agency (EA) believes that the licensed abstractions from the Fell Sandstone operated by Northumbrian Water Limited (NWL) may not be sustainable. Assessing the sustainability of the groundwater system requires a robust geological understanding. Existing geological interpretations of the Fell Sandstone show contrasting numbers and geometries of sandstone layers. This study, commissioned by the EA, presents a revised geological interpretation of the Fell Sandstone and the overlying superficial deposits in Berwick-upon-Tweed and the surrounding area. An integrated and iterative approach has been used, combining borehole analysis, remote sensing and targeted geological fieldwork. The results of this study depict the sandstone layers within the Fell Sandstone as a series of schematic cross-sections and a revised 1:50,000-scale bedrock map. A locally-revised superficial map is also provided. These outputs are intended for use by the EA and NWL to support their hydrogeological investigations. The revised interpretation of the Fell Sandstone shows a 225 to 350 m thick interbedded succession of mudstone-dominated and sandstone-dominated layers. Seven main (locally subdivided), and several lesser sandstone-dominated layers have been recognised in the cross-sections and inferred at the ground surface. Where possible, these layers have been traced across the study area and are depicted on the revised bedrock map. Numerous thin sandstone units occur within the mudstone- dominated layers. Similarly, the sandstone-dominated layers contain thin mudstone and siltstone units. For the purpose of this study, siltstones have generally been included in the mudstone- dominated layers. Overall, the proportion of mudstone- and sandstone-dominated layers is roughly even The Fell Sandstone dips towards the southeast across the south and central parts of the study area by about 10 degrees. In the north, the beds undergo a distinct steepening and assume a more easterly dip of about 15 to 25 degrees. The revised interpretation shows 2 conjectural geological faults in the south of the study area. It is likely that faulting of the succession is more widespread than the revised interpretation shows. Such faulting may result in the juxtaposition of different sand-dominated layers or introduce breaks in the continuity of the succession that could affect groundwater flow. Further evidence is required to confirm if additional structures are present. This study presents a revised interpretation that is consistent with, as far as practicable, a range of data including borehole records, digital elevation models and recent & historical geological map data. Modern techniques have been used to interpret and integrate these datasets and ensure consistency between the cross-sections and geological map linework. However, it should be noted that the position of geological boundaries and the correlation between boreholes and across the study area remains uncertain. The Fell Sandstone is complex (in terms of the exact number, thickness, lateral extent and connectivity of sandstone layers, and faulting). The density and distribution of borehole data, and the limited extent of recent field mapping is insufficient to resolve this complexity with a high level of confidence. To help reduce the uncertainty in the revised geological interpretation a series of recommendations are made: targeted drilling including the acquisition and detailed analysis of drill-core (sandstones and mudstones) to aid correlation and an improved understanding of sandstone connectivity across the study area; targeted geological mapping to validate the revised interpretation and gather additional geomorphological evidence to better-constrain the surface and subsurface expressions and connections of the sandstones between boreholes; geological mapping in the northern part of the study area to test and improve the existing superficial geological mapping and support a refined understanding of recharge potential in the study area.