Saline ground-waters in the Carboniferous Rocks of the English East Midlands in relation to the geology

Abstract

Summary The Carboniferous system in the East Midlands is comprised of a limestone-shale series or a massive limestone (the Carboniferous Limestone) overlain by a cyclic sequence of sandstones, mudstones, seat-earths and coals (the Millstone Grit and Coal Measures). The geology is described briefly and modifications are made to correlations of the Millstone Grit and Lower Coal Measures in the light of recent studies of information obtained by deep drilling for oil. Most of the Carboniferous ground-waters discussed are saline with total concentrations ranging up to 246 000 mg/1. The concentration in all formations increases from the Eakring-Kelham Hills area towards Gainsborough, that is from SW to NE, and the variation in ionic content associated with this increase is discussed. The ground-waters are believed to have originated from the diagenesis of Carboniferous marine and brackish waters and possibly some Permian marine waters. They have probably been concentrated by argillaceous beds acting as semi-permeable membranes and it is suggested that the increase in the Ca/Cl and Mg/Cl ratios as the total ionic concentration increases is due to the selective concentration of calcium and magnesium ions by argillaceous beds. In the south-west and south of the East Midlands the saline water has been diluted, probably by meteoric water moving through the aquifers from the outcrop areas and discharging into Permo-Triassic rocks, where these overlie Carboniferous aquifers at depth in the south-east of the region.