Hydrogeological History Research Articles

Evolution of the gypsum karst of Sorbas (SE Spain)

The gypsum karst of Sorbas is developed within Messinian gypsum characterized by an interbedded sequence of selenitic gypsum beds and marls. This geological configuration has led to the development of an interstratal karst in which galleries have developed in the marl levels, but not in the gypsum ones. Up to six cave passage levels have been detected in the gypsum caves following stratification planes between the impervious intercalated marls and the pervious gypsum strata. The explanation for this lies in the hydrogeological history of the area. Initially, the gypsum karst evolved as a multi-layer, semi-confined aquifer under phreatic conditions, enabling the formation of small proto-conduits in the individual gypsum beds, while the intervening marls and clays acted as impervious barriers. During a second stage, after lowering of the piezometric level, vadose conditions were established in which mechanical erosion processes in the intercalated marls and clays became predominant. This genetic duality means that the gypsum karst at Sorbas can be taken as an example of interstratal karstification where the contemporary underground erosion processes and those of karstic evolution should be considered products of the hydrogeological development. The total lowering of the piezometric level during the Quaternary is at least of 120 m in the centre of the basin. Interstratal vadose caves more than 120 m below the surface possess phreatic proto-channels in their upper levels which identify the past phreatic conditions in the genesis of the caves.

The petrology and paragenesis of fracture mineralization in the Sellafield area, west Cumbria

SUMMARY The petrology and paragenesis of fracture mineralization hosted in the Ordovician, Dinantian (Carboniferous Limestone) and Permo-Triassic rocks of the Sellafield area were studied as part of the United Kingdom Nirex Limited programme of site investigations around Sellafield in west Cumbria for a deep repository for radioactive waste. This paper summarizes the petrological and mineralogical observations from 23 of the deep (up to 2 km) boreholes drilled by Nirex. A paragenetic sequence of nine Mineralization Episodes (ME1 to ME9) was recognized, and has been interpreted in the context of the geological and hydrogeological history of the Lake District massif and East Irish Sea Basin margin areas. ME1 to ME3 produced silicate and sulphide-dominated Palaeozoic epithermal to mesothermal mineralization associated with pre-Acadian hydrothermal circulation and late-Caledonian (mid-Devonian) intrusions. ME4 to ME7 represent carbonate-sulphate-fluorite-hematite-dominated mineralization associated with warm, complex, Na-Cl-Ca-SO 4 brines expelled from the East Irish Sea Basin area during progressive burial of the thick Carboniferous and Permo-Triassic sedimentary sequence. Sulphate-carrying brines expelled to the basin margin in the Sellafield area mixed locally with cooler, more dilute, sulphate-poor groundwaters carrying barium, resulting in the precipitation of barite. ME8 and ME9, by contrast, are closely related to the development of the present-day groundwater system. ME8 involved telodiagenetic, supergene alteration and weathering by near surface oxidizing groundwaters. Isotopic evidence indicates that ME8 was probably initiated following Late Tertiary uplift and may still be ongoing in some areas. ME9 may be coeval with ME8, but represents mineralization in the deeper, more-reducing groundwater environment. This mineralization episode is dominated by calcite (sometimes associated with pyrite, marcasite, barite, anhydrite or gypsum) precipitated from groundwaters of similar composition to those found in the Sellafield area at the present day. Observations suggest that ME9 mineralization occurred during the Quaternary and is ongoing in the present groundwaters.

Fluid inclusions in granites and their relationships with present-day groundwater chemistry

Some granitic deep groundwaters have high chloride contents of which the origin is still unclear. Among the different possibilities, leakage of fluid inclusions trapped in primary minerals was proposed. This hypothesis is examined by studying four granitic sites with various hydrogeological features: drill-cores from Aspo (Sweden); Boettstein and Leuggern (northern Switzerland); outcropping granites and drill-core samples from Cauterets and Luchon in the Pyrenees (France); In Tounine granitic massif (Central Ahaggar, Algeria). With the exception of In Tounine, a part of the granite samples selected in each study site displays salt-rich fluid inclusions with salinities ranging from 22 to 26 eq. wt.% NaCl, calcium contents and minimal trapping temperatures typical of deep sedimentary brines. Most salt-rich fluid inclusions occur in secondary fluid inclusion planes in magmatic quartz, but were also found within fissure-in-filling calcite at Boettstein and Aspo. The wide ranges of Cl/Br ratios in single same rock samples, due to heterogeneities in fluid inclusion distribution in rock-forming minerals, and the variability of Cl/Br ratios in fluid reservoirs render the use of Cl/Br ratios for fluid-source tracing inaccurate. Only salt-rich inclusions in the Pyrenees samples show a relatively narrow range of Cl/Br ratios, similar to those of associated groundwaters. Mass-balance calculations based on salinity estimates from microthermometry, crush-leach analyses and inclusion counting, coupled with the study of possible mechanisms for leakage, demonstrate that the relatively high salinity of deep groundwaters (Boettstein and Aspo) cannot only be accounted for by the leakage of fluid inclusions trapped in quartz. It cannot be totally excluded that fluid inclusions may be responsible for the major part of chloride in groundwaters with relatively low salinity (Leuggern 1647 m, Cauterets, Luchon, Ahaggar). The leakage of secondary fluid inclusions trapped in primary minerals cannot be considered as the main process responsible for the high salinity of present-day deep groundwaters in the studied sites. However, the presence of secondary salt-rich inclusions indicates that sedimentary brines have already percolated the granites, a datum which is important for the evaluation of the hydrogeological history and for the understanding of the modalities of chlorine introduction in the crystalline basement.

Illite dates record deep fluid movements in petroleum basins

K-Ar age dates of authigenic illite from sandstones in the UK south Central Graben have a bimodal distribution. In contrast to established hypotheses of thermal triggers, this illite growth is explained by changes in hydrogeological history. Fluid motion during burial can hence be dated. Illite growth at 84-58 Ma on the graben margins was coincident with rapid subsidence of the Graben axis and consequent expulsion of pore fluids onto the margins. This event pre-dated both overpressure, and the secondary migration and accumulation of hydrocarbon in the region. Illite growth was probably caused by increased solute transport rates during pore fluid motion. Illite growth on an axial high (33-30 Ma) occurred during overpressured conditions. This was triggered either by: (1) downward fluid migration as a consequence of overpressure release. These fluids carried carboxylic acids or hydrocarbons from the highly overpressured Kimmeridge Clay Formation into the less pressured Fulmar Sandstone Formation; or (2) a decrease in water-rock ratios as fluid flow declined within the sandstones, as a consequence of overpressure build-up. Published experiments show that illite growth is favoured by low water-rock ratios.