OR/17/042 Executive summary

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Lee, J R, and Hough, E. 2017. A conceptual geological model for investigating shallow sub‐surface geology, Cheshire energy research field site. British Geological Survey Internal Report, OR/17/042.

This report provides a conceptual overview of the shallow sub-surface geology around the Cheshire Energy Research Field Site. The report assumes that the reader has a basic level of geological understanding and therefore it employs geological terminology throughout although terms are explained where appropriate. The Executive Summary provides an overview of the report for the non-geologist.

The report describes the natural geological processes that have occurred since the deposition of the youngest bedrock units (Sherwood Sandstone Group, SSG), how these have shaped the landscape and sub-surface geology and the likelihood of unpredictable geological features that may influence ground conditions. The report does not consider the impact of human activity, emplacement of fill or land development in the area. Since the deposition of the SSG about 200 million years ago, the area has undergone marked geological change. It has resulted in the deep burial of the SSG to depths of at least several kilometres followed by progressive uplift (and erosion of overlying rocks) to the land surface. The SSG has been exposed at the land surface for much of the last five million years (probably much longer), resulting in prolonged periods of weathering under a range of different climatic (temperate and arctic) regimes forming an intensely weathered zone (called a ‘saprolite’) of variable thickness (up to about 40 metres) that mantles the bedrock. An additional influence on the SSG is the likely cyclical incursion and flushing of saline groundwater during fluctuating sea level over the past two and a half million years. The flushing of fresh glacial meltwaters during glacial intervals is also a process that will have influenced the pore-water chemistry to depths of a few hundreds of metres from surface. Collectively, these weathering processes have acted to alter the physical and chemical properties of the SSG. This can make distinguishing between weathered and unweathered SSG and natural superficial deposits (some of which are largely derived from the SSG) very challenging without detailed analyses.

Over the past two million years, the bedrock surface has also been sculpted by glaciers, rivers and coastal processes forming a highly-irregular surface which is dissected by buried channels that have no surface expression within the modern landscape. Due to the sparse distribution of data points proving rockhead, the geometry and extent of these hidden channels is generally poorly-constrained but where present they typically generate windows of enhanced hydraulic conductivity within the shallow sub-surface (locally up to depths of 40–50 m). The sandstone and weathered bedrock is believed to have been largely buried beneath a veneer (metre to tens- of-metres thick) of sediment laid-down during glaciation and to a lesser extent by coastal, river and anthropogenic processes. Where the substrate has been overridden by glaciers, these sediments may have been folded, fractured or displaced. These sediments are likely to be highly-variable in terms of their composition, geometry, structure and properties creating highly-localised zones of preferred groundwater flow and complex ground conditions.

In summary, the properties of the shallow geological strata near the Cheshire Energy Research Field Site are predicted to be highly-complex. Our understanding of the geological processes that have affected the region since the deposition of the SSG provide important clues as to what these complexities may be but not where they may occur. This can only be resolved by further detailed geological investigations.