Dolomites formed in upper Turonian–lower Coniacian Cueva Formation carbonates in the El Ribero area (Burgos province, northern Spain) have been investigated. Carbonate banks up to 30 m thick are massive or poorly structured along several kilometres, but at this location they contain clinoforms that dip away from the central area of the Rosío diapir, producing a dome-like structure. Three main diagenetic processes affected the El Ribero carbonates: silicification, dolomitization and subsequent dedolomitization. Silicification was mainly early, producing chert nodules from Thalassinoides burrows and replacing anhydrite cauliflower-type geodes, converting them into quartz geodes and silicified fossils (corals, echinoderms and oysters). Dolomite occurs immediately below the first order Coniacian Sequence Boundary (CSB) and is restricted to the diapiric area of influence. The dolomite has a flat top, whereas its base is irregular, suggesting downward movement of dolomitic fluids. Isotopic data from the dolomite (δ18O=-3.4‰PDB and δ13C= 1.1 ‰PDB) are similar to values reported from both evaporative and mixing zone dolomite. Finally, an intense and locally destructive dedolomitization produced significant dissolution. This affected the unsilicified organisms, and was followed by the development of meteoric-water poikilotopic calcite cements and radiaxial calcite geodes infilling dissolution cavities (isotopic values for later calcite are α18O=-7.4‰PDB and δ13C=-9.0‰PDB). In addition, palaeokarstic horizons formed as a result of diapiric uplifts that caused collapse structures and vugs of several centimetres in size infilled by laminated sediments. All these features suggest that the diapiric movements could have been synsedimentary and particularly active during the Cueva Formation deposition.

The South Wales Coalfield comprises Namurian, Westphalian and Stephanian sequences which developed in the external zone of the Variscan Orogenic Belt. Sedimentation is thought to have been concentrated in a paralic basin related to crustal flexure in a foreland basin setting, linked to Variscan tectonic loading to the south. It has long been recognized that coal rank varies across the Coalfield, from high-volatile bituminous rank in the east and southeast, to anthracite in the northwest. This is confirmed here on the basis of comprehensive volatile matter and vitrinite reflectance data sets, computer contoured to generate isovol and isoreflectance maps. A steady increase in coalification with depth is also recorded. Clay mineral investigations show the presence of illite and kaolinite across the entire coalfield area, while neither pyrophyllite nor mixed clays were detected. Illite crystallinity values from mudstones are in the range 1.20–0.44 Δ°2θ.

Vitrinite reflectance data indicate maturation temperatures in the range 123–290 °C; clay mineral and IC values suggest temperatures up to 230°C, while previous descriptions of pyrophyllite from the Coalfield would allow locally for even higher temperatures. Coalification appears to have occurred rapidly (maximum 5 Ma) following deposition, implying high heat flux during Westphalian times. This is also indicated by fluid inclusion evidence from quartz crystals in clay ironstone nodules.

The inference is of a high geothermal gradient (c. 50°C km-1) during Westphalian times, contrasting with an earlier gradient estimate of 20–37°C km-1. Relatively high gradients have also been determined previously for the Ruhr Coalfield, which occupies a similar tectonic setting in the external zone of the Variscan Orogenic Belt. Such a gradient is at odds with the proposed foreland basin tectonic setting for the South Wales Coalfield, in which low geothermal gradients would be anticipated.

The South Wales Coalfield comprises Namurian, Westphalian and Stephanian sequences which developed in the external zone of the Variscan Orogenic Belt. Sedimentation is thought to have been concentrated in a paralic basin related to crustal flexure in a foreland basin setting, linked to Variscan tectonic loading to the south. It has long been recognized that coal rank varies across the Coalfield, from high-volatile bituminous rank in the east and southeast, to anthracite in the northwest. This is confirmed here on the basis of comprehensive volatile matter and vitrinite reflectance data sets, computer contoured to generate isovol and isoreflectance maps. A steady increase in coalification with depth is also recorded. Clay mineral investigations show the presence of illite and kaolinite across the entire coalfield area, while neither pyrophyllite nor mixed clays were detected. Illite crystallinity values from mudstones are in the range 1.20–0.44 Δ°2θ.

Vitrinite reflectance data indicate maturation temperatures in the range 123–290 °C; clay mineral and IC values suggest temperatures up to 230°C, while previous descriptions of pyrophyllite from the Coalfield would allow locally for even higher temperatures. Coalification appears to have occurred rapidly (maximum 5 Ma) following deposition, implying high heat flux during Westphalian times. This is also indicated by fluid inclusion evidence from quartz crystals in clay ironstone nodules.

The inference is of a high geothermal gradient (c. 50 °C km−1) during Westphalian times, contrasting with an earlier gradient estimate of 20–37°C km−1. Relatively high gradients have also been determined previously for the Ruhr Coalfield, which occupies a similar tectonic setting in the external zone of the Variscan Orogenic Belt. Such a gradient is at odds with the proposed foreland basin tectonic setting for the South Wales Coalfield, in which low geothermal gradients would be anticipated.