The Devonian Orcadian Basin in northern Scotland belongs to a regionally linked system of post-Caledonian continental basins extending northwards to western Norway and eastern Greenland. Extensional fault systems that cut the Orcadian Basin sequences are commonly assumed to be Devonian, with some limited inversion and reactivation proposed during the Carboniferous and later times. We present a detailed structural study of the regionally recognized fault systems exposed in the Dounreay area of Caithness, which host significant amounts of authigenic mineralization (carbonate, base metal sulphides, bitumen). Structural and microstructural analyses combined with Re–Os geochronology have been used to date syndeformational fault infills (pyrite) suggesting that faulting, brecciation and fluid flow events are likely to have occurred during the Permian (267.5 ± 3.4 [3.5] Ma). Stress inversion of fault slickenline data associated with mineralization suggest NW–SE regional rifting, an episode also recognized farther west in Sutherland. Thus a dominant set of Permian age brittle faults is now recognized along the entire north coast of Scotland, forming part of the regional-scale North Coast Transfer Zone located on the southern margin of the offshore West Orkney Basin. Supplementary material: Onshore and offshore fault and fracture lineament data are available at https://doi.org/10.6084/m9.figshare.c.2182433 .
Synopsis A confluence zone existed within the northern sector of the last British Ice Sheet (BIS) where ice flowing from the Northern Highlands met ice streaming out of the Moray Firth and across the plain of Caithness. At Wester Clett, a complex 35m thick sequence of glacigenic sediments provides important evidence of dynamic interactions between these ice lobes that are consistent with recent computer simulations of the behaviour the last BIS. Sedimentological evidence suggests that early advance of Northern Highlands ice, probably between 33.5 and 30.5 ka, was followed by a first incursion of Moray Firth ice. Ice flowing out of Strath Halladale then deposited sands and gravels along its margin. Later, around the time of the Last Glacial Maximum, a powerful flow of Moray Firth ice diverted Northern Highlands ice to flow to the NW across the present coastline, depositing tills of mixed provenance. Cosmogenic exposure dates of c . 18 ka for summits in southern Caithness indicate that the ice sheet thinned subsequently, but flow to the NW continued. The youngest till and its associated moraine systems record a final movement of Northern Highlands ice. Subsequent ice retreat was accompanied by uncoupling of ice lobes at the Caithness–Sutherland border soon after 16 ka.
The Sellafield area lies over the transitional zone between the western margin of the Lake District Massif, of Lower Palaeozoic metamorphic and igneous rocks, and the East Irish Sea Basin of younger sedimentary rocks. This paper sets out the general geological framework, and includes aspects such as the structural evolution, sedimentary history, rock properties and fault and fracture characteristics. From this framework, hydrogeological units are identified and linked to hydrogeological parameters as the basis for general hydrogeological models of the Sellafield area. Site specific models of groundwater flow through and around the potential location of an underground repository for radioactive waste are being developed on the basis of detailed characterization of the rock mass. In particular, geological and geophysical methods are applied to improve understanding of groundwater flow and to develop hydrogeological models integrated with the local geological framework.
For a number of years, United Kingdom Nirex Limited investigated the suitability of the area around Sellafield in NW England to assess its suitability to host a deep repository for intermediate-level and some low-level radioactive wastes resulting from Britain's nuclear programme. The investigation programme collected a large amount of lithological, hydrogeological and hydrochemical data from 18 boreholes up to 2000 m deep. A potential route by which radionuclides may leave the repository and return to surface is the groundwater pathway. It was recognized that the groundwater regime could be disturbed significantly by major climate changes, particularly glacial–interglacial cycles, over the very long time scales considered in the assessment of the radiological safety of a repository. Available geological and hydrogeological data for the Sellafield area are reviewed in the context of current thinking on glacial hydrogeology, showing that disturbance of the hydrogeological regime is possible during glaciation, and that there are significant uncertainties in determining the timing and magnitude of the effects caused by glaciation. However, the evidence available, both from Quaternary fracture-filling minerals and from the chemistry of groundwater now present, suggests that any disturbance to the deeper hydrogeological regime was relatively minor and that any subglacial recharge has been removed by the present flow regime. There is a possibility that high hydraulic heads at depth in the low-permeability basement rock considered as a host for the repository, the Borrowdale Volcanic Group of Ordovician age beneath a cover of Triassic sandstones, may be residual.
SUMMARY Geological investigations of the Sellafield area, west Cumbria by UK NIREX Ltd are part of a site characterization process to assess its suitability for the construction of an underground radioactive waste repository. The geological characterization is an input to the NIREX post-Closure Safety Assessment. This will address all aspects of post-closure radiological safety of the repository, particularly the potential for radionuclide migration in groundwater back to the surface (the biosphere) through the various geological units (the geosphere). This characterization forms the largest geotechnical investigation in Britain and encompasses scales of study from ‘large scale, regional’, covering north-west England and the East Irish Sea Basin to ‘microscopic’, requiring detailed examination of drill cores and individual rock fractures.
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