BGS karst report series : P2. Karst in the southern outcrop of Permian limestones (and associated gypsum)

Maurice, L.D.; Cooper, A.H.; Farrant, A.R.; Mathewson, E.; Murphy, P.J.. 2024 BGS karst report series : P2. Karst in the southern outcrop of Permian limestones (and associated gypsum). Nottingham, UK, British Geological Survey, 124pp. (OR/23/057) (Unpublished)

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Abstract/Summary

This report documents the evidence for karst and rapid groundwater flow in the southern outcrop of the Permian limestones in Northern England, together with the associated gypsum karst. It is part of the BGS karst report series on those karst aquifers in England in which cave development is limited – principally the Upper Cretaceous Chalk and the Jurassic and Permian limestones. The term “karst” applies to rocks that are soluble. In classic karst there are extensive caves and large-scale surface karst landforms such as dolines, shafts, stream/river sinks, and springs. In the past, the Chalk and the Jurassic and Permian limestones of England were not considered karstic because they have limited cave development, and because karst features are usually small and have not been well documented. These reports provide data and information on karst in each area. The Permian dolomitic limestones comprise the Cadeby Formation and the Brotherton Formation. These formations are separated by the Edlington Formation and overlain by the Roxby Formation, both of which contain extensive evaporite karst (in gypsum and anhydrite) interspersed with low permeability mudstones penetrated by karstic collapse features. This report is primarily focused on the dolomitic limestone karst, but the variable geology results in high complexity with interactions between the limestone and evaporite karst. Karst in the Permian gypsum associated with the limestones has not been well-recognised in hydrogeological studies. Evaporite karst in the area is well developed and documented with caves, dolines, and dissolution pipes. Collapse features extend into the limestones, especially in the Brotherton Formation, and there is evidence of groundwater connectivity between the evaporite and limestone karst, for example with sulphate-rich groundwater in the limestones and overlying strata. The gypsum sequences in contact with the limestones result in locally very high transmissivities and mixing of waters from the two types of karstified rocks. There is also clear evidence for karstification within the dolomitic limestones. Quite large, although short and dry, dolomitic limestone caves occur in the Cadeby Formation, with 21 karst caves recorded, ranging from 2.5 to 290 m in length. There is also evidence that smaller solutional conduits and fissures occur in the limestones which are likely to be an important component of groundwater flow. There are many other caves, some of which are formed by mass movement (slip rift caves) and some for which it is unclear whether they are mass movement caves or karst caves. In some instances, slip rift caves form a focus for recharge to the limestones, but many may be largely dry. There are significant karst stream sinks into the Cadeby Formation at Wadworth Wood near Doncaster and near Darrington, and a major karst river sink into the Cadeby Formation on the River Skell. Some limestone dolines and dissolution pipes have been recorded in the Cadeby and Brotherton formations. There are large numbers of springs in the Permian limestones, and although their flows and characteristics are generally not well documented, it appears that many are quite small, and a few may have substantial flows. Tracer tests have been carried out at one location, and these demonstrated connectivity between a groundwater abstraction and both a leaking sewer and a surface water course, over distances of 10s to 100s metres. At this locality high transmissivity, and high yields (> 80 l/s) also indicate solutional development of permeability. There are other abstractions in the P2 area with high yields and transmissivities of > 1000 m2/day suggesting they may be supplied by karstic solutional networks. Detailed borehole investigations using slug tests and water level monitoring have revealed rapid flow velocities of 13 to 242 m/day in the Cadeby Formation at the Leeds University study site, and modelling work over a wider area suggested very rapid flows of up to 9000 m/day (Medici et al 2019a,b). These studies also showed that very low effective porosities (2.8 x 10-4) are needed to represent the karstic development of permeability in the Permian limestones and the potential large scale of contaminant transport. The data collated in this report demonstrate that a component of unsaturated zone flow in the Permian limestones is rapid, but the proportion of rapid flow, and the frequency of rapid flowpaths that extend through the entire unsaturated zone, is uncertain. Evidence of cave, conduit and solutional fissure development; and some high transmissivities and high borehole yields, especially where gypsum is present, suggest that there are saturated zone networks of solutional fissures and conduits which might enable pollutant transport over long distances and in unexpected directions, but further work is needed to determine how frequently these networks occur and how extensive they are. Such networks can form along stream sink to spring flowpaths or through mixing corrosion. Overall, this report highlights the importance of karst in the Permian limestones, the complexities of the interactions with the evaporite karst, and the need for further development of karst datasets and conceptualisation of the karst hydrogeology to assist with groundwater studies and management in this area.

Item Type:	Publication - Report
Funders/Sponsors:	British Geological Survey
Additional Information. Not used in RCUK Gateway to Research.:	This item has been internally reviewed, but not externally peer-reviewed.
Date made live:	01 Jul 2024 13:15 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/537658

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