SUMMARY Strata from the base of the Coal Measures up to the top of the Elland Flags were apparently all derived from a distant northern source, probably the same as that which supplied the underlying Millstone Grit. They show strongly micaceous lithologies, clean sandstones, resistate heavy minerals with monazite but no chrome spinel, and palynomorph assemblages exclusively of indigenous Langsettian miospores. A sudden change in sediment type is apparent at the base of the overlying Greenmoor Rock with the appearance of weakly micaceous lithologies of a greenish-grey colour, clayey sandstones with remnants of mudstone lithoclasts, chrome spinel among the heavy minerals (but no monazite) and reworked Devonian palynomorphs in addition to the Langsettian types. Regional palaeocurrent flow during deposition of the Greenmoor Rock was from the west but the ultimate source of this sediment is still in doubt. The northern type of sediment then reappeared in the Grenoside Sandstone, mixed locally with western-derived material. At higher stratigraphical levels, different relationships between lithology and mineral composition appear. Strongly micaceous lithologies become rare, and the presence of chrome spinel is not necessarily associated with greenish grey lithologies.

SUMMARY Spheroidal structures composed typically of radial-fibrous dolomite or calcite are described from the St Bees Sandstone and Calder Sandstone formations of the Triassic of the Sellafield area, Cumbria. Etching of the spheroids in some sandstones, probably by present-day groundwaters, has revealed intricate internal structures composed of ferruginous clay, forming near-concentric bands and radial spokes. The spheroids are interpreted as early diagenetic groundwater precipitates. They typically occur in non-compacted fabrics where cementation prior to burial compaction has preserved a generally uncompacted grain fabric. It is unclear whether they are inorganic nodules or biologically mediated precipitates.

SUMMARY A study of the petrology and diagenesis of the Permo-Triassic rocks of the Sellafield area has been undertaken as part of the Core Characterisation Programme for UK NIREX Ltd. This paper summarizes observations of the petrology and diagenesis from boreholes 1/1A, 2, 3, 7A and 7B of the Brockram, St Bees Evaporites, St Bees Shale and St Bees Sandstone and proposes a series of Diagenetic Episodes (denoted DE1 to DE10) to describe paragenetic relationships inferred from these observations. As might be expected from closely spaced boreholes, overall paragenetic characteristics are laterally similar for each stratigraphic unit, although the degree of development and preservation of authigenic minerals, cements and porosity characteristics varies between boreholes. However, there are more significant differences in diagenetic characteristics between stratigraphic units. In general, the diagenesis of the Brockram breccias is characterized by precipitation of anhydrite, calcite, ferroan calcite, dolomite, ferroan dolomite, ankerite, hematite, barite, kaolinite and illite; the St Bees Shales and Evaporites by sulphates and dolomite; and the St Bees Sandstone by non-ferroan dolomite, quartz, ferroan and non-ferroan calcite and late illite. Total porosity in the Brockram, St Bees Evaporites and St Bees Shales is negligible (<1 %), but there is significant porosity ( c. 15–25%) in the St Bees Sandstone, most of which is considered here to be secondary porosity attributed to removal of carbonates by present-day groundwaters, but ultimately due to early preservation of non-compacted fabrics by an inferred early evaporite cement, probably anhydrite, which was subsequently removed during diagenesis.

SUMMARY This paper presents a summary of the present state of knowledge of the Dinantian (Carboniferous Limestone) rocks of the Sellafield area as a result of drilling investigations for UK NIREX Ltd. An easterly thinning wedge, formed as a result of pre-Permian erosion has a maximum onshore thickness of 300 m north of Sellafield and a feather edge about 1 km east of the Sellafield site. The Dinantian rocks rest unconformably on the Ordovician Borrowdale Volcanic Group and are overlain unconformably by Permian breccias and conglomerates, known locally as ‘Brockram’. The Basal Beds comprise a thin, impersistent succession of siliciclastic beds overlying the sub-Carboniferous unconformity. A maximum thickness of 149 m of Dinantian strata was proved in Sellafield Borehole 3. A thin development of beds of late Chadian age, the Martin Limestone, is overlain by 100m of peritidal to shallow inner ramp limestones of the Frizington Limestone (new name), of Holkerian age, and 42 m of cyclic, platform carbonates of the Urswick Limestone of late Asbian age.

Abstract Boreholes at Kirkham [SD 4324 3247] and Weeton Camp [SD 3888 3603] in Lancashire penetrate Triassic sandstones of the Sherwood Sandstone Group that occur near the eastern margins of the Manx-Furness (East Irish Sea) Basin. Petrological analysis of sandstone samples from cores was undertaken to determine diagenetic characteristics. Some very porous sandstones with porosities of the order of 25–30% have cement dissolution textures indicating that earlier extensive cements have largely been removed by aggressive pore fluids, probably during the Triassic. These early cements were calcite and non-ferroan dolomite, and possibly later evaporitic cements (anhydrite, gypsum, halite) introduced during the period of deposition of the Mercia Mudstones. Non-ferroan dolomite post-dates and locally replaces early calcite. There is minor ferroan dolomite, late calcite and pyrite. Extensive early quartz overgrowths and cements have developed in some quartz arenites. Porosity values are about 10 to 13%, but permeabilities are likely to be low. These quartzcemented quartz arenite units may correlate between the boreholes. The clay mineral assemblages of the sandstones of each borehole are significantly different. The Kirkham sequence is characterized by illite with Fe-rich chlorite and minor smectite, whereas the Weeton Camp sequence is dominated by kaolinite with minor illite and smectite. These differences are attributed to different pore fluid histories, possibly structurally controlled.

ABSTRACTGlacial gravels of Late Devensian Dimlington Stadial age (26 000–13 000 years BP) at West Tanfield, North Yorkshire, England, have been cemented by carbonate‐rich solutions to produce a strongly indurated calcrete horizon. The low‐Mg cements occur as drusy spar, needle fibres, alveolar septal structures, micrite and micropinnacles, indicative of vadose‐zone cementation. Some complex pore partition structures attributed to precipitation along meniscus films also occur. These partitions separate air‐dominated and water‐dominated microenvironments of the vadose zone. The abundance of vadose fabrics shows that the accumulation is not a groundwater calcrete. In addition, much of the carbonate appears to have been precipitated by biological mediation.Carbon and oxygen isotopic data suggest that the carbonate did not form as a result of freezing, as has been suggested for some ‘arctic’soil carbonates. The pollen history of the area since the Devensian suggests that this calcrete precipitated at low temperatures; this contrasts with widely reported occurrences of calcrete in soils of hot arid or semi‐arid regions, and suggests that palaeo‐calcretes should not be used as absolute palaeoclimatic indicators.The unusual occurrence, albeit localized, of a thick calcrete under a cool and wet climate probably reflects the well‐drained nature of the gravels, the abundance of CaCO3 as limestone clasts in the gravel and a high degree of biological activity beneath a forest cover, which created a local environment favouring carbonate precipitation.

Data from released wells indicate that the subdivision of the Sherwood Sandstone Group into the Budleigh Salterton Pebble Beds and the Otter Sandstone Formation is equally applicable in both the outcrop and subcrop of the Group in the Wessex Basin. The Sherwood Sandstone Group was deposited largely from braided streams but an inland sabkha may have occupied the depocentre during deposition of the lower parts of the Otter Sandstone Formation. The Budleigh Salterton Pebble Beds may have been removed from parts of the Wessex Basin by erosion prior to deposition of the Otter Sandstone Formation.

The Sherwood Sandstone and the Basal Permian Sands contain hot brines in the eastern deeper parts of the East Yorkshire and Lincolnshire Basin. In this eastern area, the Sherwood Sandstone is up to 500 m thick and has an average permeability of 250 mD. In the Cleethorpes Well, the upper part of the formation has been cemented by anhydrite and this is probably a widespread feature in the deeper parts of the basin. Fibrous illite forms bridging meshes across the pore throats reducing the permeability. The transmissivity of the Sherwood Sandstone in the Cleethorpes Well is believed to be about 60 D.m, although this could be a conservative value. The Basal Permian Sands are relatively thin. In the the Cleethorpes Well they are represented by fluviatile sandstones and conglomerates. The sandstones in the Cleethorpes Well are only weakly cemented, contrasting with the situation in the conglomerates. Porosity in the sandstones has been reduced by compaction and precipitation of anhydrite, quartz, ankerite and illite. The transmissivity of the Basal Permian Sands does not exceed 2 D.m in the Cleethorpes Well but in east Lincolnshire the mean value is likely to be higher than this as aeolian sands, rather than fluviatile deposits, are believed to represent the formation.

Summary The Sherwood Sandstone Group sandstones of the Wessex Basin are important potential reservoirs for both hydrocarbons and geothermal brines. Analyses of conglomerate, sandstone and siltstone samples from outcrop and deep boreholes indicate a complex diagenetic history that has had important positive and negative effects on the reservoir properties. Over most of the basin, early diagenesis (eodiagenesis) is dominated by calcrete development but, towards the basin centre, non-ferroan dolomite associated with early evaporitic sulphate cements that appear to have been deposited in an inland sabkha or playa environment are important. Later diagenesis (mesodiagenesis) is characterized by selective framework grain dissolution of feldspars, and anhydrite cementation and subsequent dissolution—processes that have locally yielded significant secondary porosity. However, in some cases this secondary porosity has been destroyed by the precipitation of late manganiferous ferroan calcite and ferroan dolomite. Quartz cements are locally important. Rocks near the present-day outcrop have been affected by weathering processes (telodiagenesis) that have resulted in decalcification, and the precipitation of kaolinite, illite and iron hydroxides. The key factors determining the permeability of the sandstones are the original grain size and degree of sorting, the precipitation and subsequent removal of anhydrite, and the extent of early and late carbonate cementation, framework grain dissolution and overgrowth.

In the eastern parts of the East Yorkshire and Lincolnshire Basin, the Sherwood Sandstone is up to 500 m thick and has an average permeability of 250 mD; it contains brines at temperatures of about 50/sup 0/C. The porosity and permeability of the sandstone can be related to the detrital clay content and the grain size, factors that are controlled by the depositional environment of the sandstone. In the Cleethorpes Well, the upper part of the formation has been cemented by anhydrite. Fibrous illite forms bridging meshes across the pore throats reducing the permeability. At the present time anhydrite is being dissolved by meteoric groundwater to the west of Cleethorpes. As a consequence of this study, the transmissivity of the Sherwood Sandstone in the Cleethorpes Well is now believed to be about 60 D.m, although this could be a conservative value. In the Cleethorpes Well the Basal Permaian Sands are represented by fluviatile sandstones and conglomerates. Their transmissivity does not exceed 2 D.m but in east Lincolnshire the mean value is likely to be higher than this as aeolian sands, rather than fluviatile deposits, are believed to represent the formation.

Summary The flow system, hydrochemistry and isotope hydrology of the freshwater aquifer in the E Midlands Triassic sandstone are reviewed. Isotopic data suggest a mean flow velocity of around 0.6 m.y. over the last 30 k.y., in contrast with the present-day velocity of around 0.2 m.y. The difference is attributed to changes in hydraulic gradients over this period; fluctuations in recharge and gradients would have influenced meteoric water influx and diagenesis over the last 10 7 years or more. The diagenetic modifications to the arkosic sandstone are described and are divided into those taking place before and during compaction, probably with saline pore water, and those occurring as a result of meteoric water influx following uplift and denudation. The relation between the kinetics of diagenetic reactions and the groundwater flow velocity is discussed. Relevant experimental data for reaction kinetics are summarized. The hydrochemistry and flow velocity of the present regime are used to deduce mass transfers within and outside the aquifer. These are equivalent to uniform porosity increases of only about 0.17fb3e69cer million years for each of the dolomite and K-feldspar dissolution reactions. Both these dissolution reactions are incongruent, and precipitate calcite and kaolinite respectively. The spatial arrangement of reactant and product minerals in incongruent reactions is expected to reflect interplay between dissolution/precipitation kinetics and water flow rate. Scanning electron micrographs show the proximity of K-feldspar and kaolinite. Constraints can be placed on the flow rates responsible by considering chemical balances for aluminium and silicon and their maximum production rates by feldspar dissolution. This approach involves many simplifications and assumptions, but with further experimental kinetic data it offers the possibility of deducing information on past flow systems from detailed measurements of diagenetic mineralogy and geochemistry in clastic sediments.

In the Wessex Basin, the Sherwood Sandstone Group is a potential reservoir for geothermal energy. The sandstone is characterized by thin coarse-grained layers with permeabilities of up to 5 or 6 darcys and porosities of about 25 per cent. The occurrence of these zones is a consequence of the initial grain-size of the sandstone and its diagenetic history. Early cementation of significant parts of the sandstone sequence, by carbonates and anhydrite, preserved the rock fabric when it was buried subsequently to greater depths by younger sediments. Of the two cements, the anhydrite was the most significant from the point of view of reservoir properties, for its subsequent dissolution regenerated the original primary porosity and permeability. This was enhanced by the solution of potassium feldspars. The most favourable areas for developing geothermal resources, are south of the Mere Fault and, in particular, in the Dorset Basin between the Cranborne Fault and the Purbeck-Isle of Wight faults.