Bees Sandstone Research Articles

How groundwater time series and aquifer property data explain heterogeneity in the Permo-Triassic sandstone aquifers of the Eden Valley, Cumbria, UK

A novel investigation of the impact of meteorological and geological heterogeneity within the Permo-Triassic Sandstone aquifers of the River Eden catchment, Cumbria (UK), is described. Quantifying the impact of heterogeneity on the water cycle is increasingly important to sustainably manage water resources and minimise flood risk. Traditional investigations on heterogeneity at the catchment scale require a considerable amount of data, and this has led to the analysis of available time series to interpret the impact of heterogeneity. The current research integrated groundwater-level and meteorological time series in conjunction with aquifer property data at 11 borehole locations to quantify the impact of heterogeneity and inform the hydrogeological conceptual understanding. The study visually categorised and used seasonal trend decomposition by LOESS (STL) on 11 groundwater and meteorological time series. Decomposition components of the different time series were compared using variance ratios. Though the Eden catchment exhibits highly heterogeneous rainfall distribution, comparative analysis at borehole locations showed that (1) meteorological drivers at borehole locations are broadly homogeneous and (2) the meteorological drivers are not sufficient to generate the variation observed in the groundwater-level time series. Three distinct hydrogeological regimes were identified and shown to coincide with heterogeneous features in the southern Brockram facies, which is the northern silicified region of the Penrith Sandstone and the St Bees Sandstone. The use of STL analysis in combination with detailed aquifer property data is a low-impact insightful investigative tool that helps guide the development of hydrogeological conceptual models.

Characterization of a fluvial aquifer at a range of depths and scales: the Triassic St Bees Sandstone Formation, Cumbria, UK

Fluvial sedimentary successions represent porous media that host groundwater and geothermal resources. Additionally, they overlie crystalline rocks hosting nuclear waste repositories in rift settings. The permeability characteristics of an arenaceous fluvial succession, the Triassic St Bees Sandstone Formation in England (UK), are described, from core-plug to well-test scale up to ~1 km depth. Within such lithified successions, dissolution associated with the circulation of meteoric water results in increased permeability (K~10−1–100 m/day) to depths of at least 150 m below ground level (BGL) in aquifer systems that are subject to rapid groundwater circulation. Thus, contaminant transport is likely to occur at relatively high rates. In a deeper investigation (> 150 m depth), where the aquifer has not been subjected to rapid groundwater circulation, well-test-scale hydraulic conductivity is lower, decreasing from K~10−2 m/day at 150–400 m BGL to 10−3 m/day down-dip at ~1 km BGL, where the pore fluid is hypersaline. Here, pore-scale permeability becomes progressively dominant with increasing lithostatic load. Notably, this work investigates a sandstone aquifer of fluvial origin at investigation depths consistent with highly enthalpy geothermal reservoirs (~0.7–1.1 km). At such depths, intergranular flow dominates in unfaulted areas with only minor contribution by bedding plane fractures. However, extensional faults represent preferential flow pathways, due to presence of high connective open fractures. Therefore, such faults may (1) drive nuclear waste contaminants towards the highly permeable shallow (< 150 m BGL) zone of the aquifer, and (2) influence fluid recovery in geothermal fields.

A combined pumping test and heat extraction/recirculation trial in an abandoned haematite ore mine shaft, Egremont, Cumbria, UK

A pumping test at rates of up to 50 L s−1 was carried out in the 256 m-deep Florence Shaft of the Beckermet–Winscales–Florence haematite ore mine in Cumbria, UK, between 8th January and 25th March 2015. Drawdowns in mine water level did not exceed 4 m and the entire interconnected mine complex behaved as a single reservoir. Pumping did, however, induce drawdowns of around 1 m in the St. Bees Sandstone aquifer overlying the Carboniferous Limestone host rock. During a second phase of the pumping test, a proportion of the 11.3–12 °C mine water was directed through a heat pump, which extracted up to 103 kW heat from the water and recirculated it back to the top of the shaft. Provided that an issue with elevated arsenic concentrations (20–30 µg L−1) can be resolved, the Florence mine could provide not only a valuable resource of high-quality water for industrial or even potable uses, it could also provide several hundred to several thousand kW of ground sourced heating and/or cooling, if a suitable demand can be identified. The ultimate constraint would be potential hydraulic impacts on the overlying St Bees Sandstone aquifer. The practice of recirculating thermally spent water in the Florence Shaft produced only a rather modest additional thermal benefit.

Characterizing flow pathways in a sandstone aquifer: Tectonic vs sedimentary heterogeneities

Sandstone aquifers are commonly assumed to represent porous media characterized by a permeable matrix. However, such aquifers may be heavy fractured when rock properties and timing of deformation favour brittle failure and crack opening. In many aquifer types, fractures associated with faults, bedding planes and stratabound joints represent preferential pathways for fluids and contaminants. In this paper, well test and outcrop-scale studies reveal how strongly lithified siliciclastic rocks may be entirely dominated by fracture flow at shallow depths (≤180m), similar to limestone and crystalline aquifers. However, sedimentary heterogeneities can primarily control fluid flow where fracture apertures are reduced by overburden pressures or mineral infills at greater depths.The Triassic St Bees Sandstone Formation (UK) of the East Irish Sea Basin represents an optimum example for study of the influence of both sedimentary and tectonic aquifer heterogeneities in a strongly lithified sandstone aquifer-type. This fluvial sedimentary succession accumulated in rapidly subsiding basins, which typically favours preservation of complete depositional cycles including fine grained layers (mudstone and silty sandstone) interbedded in sandstone fluvial channels. Additionally, vertical joints in the St Bees Sandstone Formation form a pervasive stratabound system whereby joints terminate at bedding discontinuities. Additionally, normal faults are present through the succession showing particular development of open-fractures. Here, the shallow aquifer (depth≤180m) was characterized using hydro-geophysics. Fluid temperature, conductivity and flow-velocity logs record inflows and outflows from normal faults, as well as from pervasive bed-parallel fractures. Quantitative flow logging analyses in boreholes that cut fault planes indicate that zones of fault-related open fractures characterize ~50% of water flow. The remaining flow component is dominated by bed-parallel fractures. However, such sub-horizontal fissures become the principal flow conduits in wells that penetrate the exterior parts of fault damage zones, as well as in non-faulted areas.The findings of this study have been compared with those of an earlier investigation of the deeper St Bees Sandstone aquifer (180 to 400m subsurface depth) undertaken as part of an investigation for a proposed nuclear waste repository. The deeper aquifer is characterized by significantly lower transmissivities. High overburden pressure and the presence of mineral infillings, have reduced the relative impact of tectonic heterogeneities on transmissivity here, thereby allowing matrix flow in the deeper part of the aquifer. The St Bees Sandstone aquifer contrasts the hydraulic behaviour of low-mechanically resistant sandstone rock-types. In fact, the UK Triassic Sandstone of the Cheshire Basin is low-mechanically resistant and flow is supported both by matrix and fracture. Additionally, faults in such weak-rocks are dominated by granulation seams representing flow-barriers which strongly compartmentalize the UK Triassic Sandstone in the Cheshire Basin.

Palaeoenvironment of braided fluvial systems in different tectonic realms of the Triassic Sherwood Sandstone Group, UK

Fluvial successions comprising the fills of sedimentary basins occur in a variety of tectonic realms related to extensional, compressional and strike-slip settings, as well as on slowly subsiding, passive basin margins. A major rifting phase affected NW Europe during the Triassic and resulted in the generation of numerous sedimentary basins. In the UK, much of the fill of these basins is represented by fluvial and aeolian successions of the Sherwood Sandstone Group. Additionally, regions that experienced slow rates of Mesozoic subsidence unrelated to Triassic rifting also acted as sites of accumulation of the Sherwood Sandstone Group, one well-exposed example being the eastern England Shelf. The fluvial depositional architecture of deposits of the Sherwood Sandstone Group of the eastern England Shelf (a shelf-edge basin) is compared with similar fluvial deposits of the St Bees Sandstone Formation, eastern Irish Sea Basin (a half-graben). The two studied successions represent the preserved deposits of braided fluvial systems that were influenced by common allogenic factors (climate, sediment source, delivery style); differences in preserved sedimentary style principally reflect their different tectonics settings. Analysis of lithofacies and architectural elements demonstrates that both studied successions are characterized by amalgamated channel-fill elements that are recorded predominantly by downstream-accreting sandy barforms. The different tectonic settings in which the two braided-fluvial systems accumulated exerted a dominant control on preserved sedimentary style and long-term preservation potential. On the eastern England Shelf, the vertical stacking of pebbly units and the general absence of fine-grained units reflect a slow rate of sediment accommodation generation (18–19.4m/Myr). In this shelf-edge basin, successive fluvial cycles repeatedly reworked the uppermost parts of earlier fluvial deposits such that only the lowermost channel lags tend to be preserved. By contrast, in the eastern Irish Sea Basin of west Cumbria, the rate of sediment accommodation generation was substantially greater (119m/Myr) such that space was available to preserve complete fluvial cycles, including silty drape units that cap the channelized deposits.

Use of seasonal trend decomposition to understand groundwater behaviour in the Permo-Triassic Sandstone aquifer, Eden Valley, UK

The daily groundwater level (GWL) response in the Permo-Triassic Sandstone aquifers in the Eden Valley, England (UK), has been studied using the seasonal trend decomposition by LOESS (STL) technique. The hydrographs from 18 boreholes in the Permo-Triassic Sandstone were decomposed into three components: seasonality, general trend and remainder. The decomposition was analysed first visually, then using tools involving a variance ratio, time-series hierarchical clustering and correlation analysis. Differences and similarities in decomposition pattern were explained using the physical and hydrogeological information associated with each borehole. The Penrith Sandstone exhibits vertical and horizontal heterogeneity, whereas the more homogeneous St Bees Sandstone groundwater hydrographs characterize a well-identified seasonality; however, exceptions can be identified. A stronger trend component is obtained in the silicified parts of the northern Penrith Sandstone, while the southern Penrith, containing Brockram (breccias) Formation, shows a greater relative variability of the seasonal component. Other boreholes drilled as shallow/deep pairs show differences in responses, revealing the potential vertical heterogeneities within the Penrith Sandstone. The differences in bedrock characteristics between and within the Penrith and St Bees Sandstone formations appear to influence the GWL response. The de-seasonalized and de-trended GWL time series were then used to characterize the response, for example in terms of memory effect (autocorrelation analysis). By applying the STL method, it is possible to analyse GWL hydrographs leading to better conceptual understanding of the groundwater flow. Thus, variation in groundwater response can be used to gain insight into the aquifer physical properties and understand differences in groundwater behaviour.

Modelling groundwater flow changes due to thermal effects of radioactive waste disposal at a hypothetical repository site near Sellafield, UK

High-level radioactive waste (HLW) and spent fuel from nuclear power plants (SF) require very long duration disposal because of its long-lived toxicity. Specific repository design is necessary to accommodate the associated large heat generation from continual decay. Conceptual designs propose geological containment within salt, claystone, or crystalline bedrock. However, while many studies have investigated the safety of disposal in these host rocks, the interaction of near-field heat coupled to far-field groundwater flow for UK-relevant conditions has not been fully investigated. Using an excellent dataset from a repository investigation in saturated fractured rock at Sellafield, UK, with deep groundwater flow, a preliminary investigation of the effect of the addition of heat from HLW and SF on regional groundwater flow is undertaken. A coupled thermo-hydrogeological groundwater flow model dependent on fluid density and viscosity has been calibrated to field data, before heat is added to simulate waste. Substantial heating reduces fluid density and dynamic viscosity. Groundwater flows upwards through permeable damage zones of high-angle faults and increases natural velocity by 1.5×. Elevated groundwater temperatures are predicted at the surface in just hundreds of years. Travel times for conservative chemical species from the hypothetical repository to the St. Bees Sandstone aquifer are reduced by over one-third if engineered barriers fail at the point of containment, and remain reduced by hundreds of years for particles released thousands of years after containment. The results imply that in conditions similar to those modelled, the high-performance guarantee of the engineered barrier is essential to repository safety. A very good understanding of the coupled interaction of near-field heat and far-field groundwater flow will be important for safety cases, especially accuracy concerning heterogeneous fault discontinuities. It is necessary to model heat advection and faults at fractured sites.

The nitrate time bomb: a numerical way to investigate nitrate storage and lag time in the unsaturated zone

Nitrate pollution in groundwater, which is mainly from agricultural activities, remains an international problem. It threatens the environment, economics and human health. There is a rising trend in nitrate concentrations in many UK groundwater bodies. Research has shown it can take decades for leached nitrate from the soil to discharge into groundwater and surface water due to the 'store' of nitrate and its potentially long travel time in the unsaturated and saturated zones. However, this time lag is rarely considered in current water nitrate management and policy development. The aim of this study was to develop a catchment-scale integrated numerical method to investigate the nitrate lag time in the groundwater system, and the Eden Valley, UK, was selected as a case study area. The method involves three models, namely the nitrate time bomb-a process-based model to simulate the nitrate transport in the unsaturated zone (USZ), GISGroundwater--a GISGroundwater flow model, and N-FM--a model to simulate the nitrate transport in the saturated zone. This study answers the scientific questions of when the nitrate currently in the groundwater was loaded into the unsaturated zones and eventually reached the water table; is the rising groundwater nitrate concentration in the study area caused by historic nitrate load; what caused the uneven distribution of groundwater nitrate concentration in the study area; and whether the historic peak nitrate loading has reached the water table in the area. The groundwater nitrate in the area was mainly from the 1980s to 2000s, whilst the groundwater nitrate in most of the source protection zones leached into the system during 1940s-1970s; the large and spatially variable thickness of the USZ is one of the major reasons for unevenly distributed groundwater nitrate concentrations in the study area; the peak nitrate loading around 1983 has affected most of the study area. For areas around the Bowscar, Beacon Edge, Low Plains, Nord Vue, Dale Springs, Gamblesby, Bankwood Springs, and Cliburn, the peak nitrate loading will arrive at the water table in the next 34 years; statistical analysis shows that 8.7 % of the Penrith Sandstone and 7.3 % of the St Bees Sandstone have not been affected by peak nitrate. This research can improve the scientific understanding of nitrate processes in the groundwater system and support the effective management of groundwater nitrate pollution for the study area. With a limited number of parameters, the method and models developed in this study are readily transferable to other areas.

Experimental investigation into salt precipitation during CO2 injection in saline aquifers

Carbon Capture and Storage (CCS) is recognised as one of the most effective technologies for reducing CO2 emissions in the short to medium term. From a capacity point of view, deep saline aquifers offer the greatest potential. As supercritical CO2 is injected in the aquifer, a variety of strongly coupled physical and chemical processes occur. Among these various mechanisms, vaporisation of water requires particular attention as it can cause salt precipitation, which reduces the porosity and the permeability of the reservoir in the vicinity of the wellbore. This can lead to significant reduction in injectivity. Research described in this paper aims to provide a relationship between porosity changes and resulting permeability variations representing the effect of salt precipitation due to vaporisation. Supercritical CO2 core flooding experiments were conducted on a St. Bees sandstone core with fully saturated saline water obtaining several levels of alteration due to halite scaling. Porosity reduction ranged from around 4 to 29% of the initial value. Permeability impairments ranged from 30 to 86%. Results were used to calibrate a Verma-Pruess “tube-in-series” model which can be used to obtain more accurate results from numerical simulations.

Lithostratigraphical subdivision of the Sherwood Sandstone Group (Triassic) of the northeastern part of the Carlisle Basin, Cumbria and Dumfries and Galloway, UK

Synopsis Two formations, the St Bees Sandstone and Kirklinton Sandstone, have been mapped in the past within the Triassic Sherwood Sandstone Group of the eastern part of the Carlisle Basin, Cumbria, and adjacent parts of Dumfries and Galloway, UK. However, previous workers have found considerable difficulty in consistently identifying, defining and mapping the Kirklinton Sandstone Formation. The principal lithological change within the Sherwood Sandstone Group is between mainly fine-grained sandstones that are commonly micaceous and contain numerous mudstone interbeds in the lower and middle parts of the group, and fine- to coarse-grained sandstones with rare or no mica and few mudstone partings at the top of the group. This change occurs within the Kirklinton Sandstone Formation as previously mapped. This paper considers ways of subdividing the group lithostratigraphically and the nomenclature to be adopted. All options have some associated problems, but, in view of the recently determined similarities with the contiguous sandstones offshore, the adoption of the same terminology as that in the adjacent East Irish Sea and Solway Firth basins is suggested, i.e. the St Bees Sandstone Formation below (subdivided where possible into Rottington Sandstone and Calder Sandstone members) and the Ormskirk Sandstone Formation above.

Permo–Triassic rocks in boreholes in the Annan-Canonbie area, Dumfries and Galloway, southern Scotland

Synopsis Recent boreholes provide new data on the sequence, age, lithology and geophysical log stratigraphy of Late Permian and Early Triassic strata on the northern edge of the Carlisle Basin. Late Permian evaporites (gypsum-anhydrite) and microfloras are recorded and described for the first time in onshore Scotland. The lower, argillaceous and evaporite-bearing strata are assigned to the Eden Shales Formation and are divided into three informal lithostratigraphical units, in ascending order ES1, ES2 and ES3. The Eden Shales grade upwards into the St Bees Sandstone Formation. The strata closely resemble those in neigh-bouring Cumbria, and help to refine some details of regional palaeogeography and correlation. Regional log correlation suggests that a significant evaporite-bearing Late Permian section is missing, probably cut out by faulting, in the Silloth-1A Borehole, the main deep borehole penetrating the Carlisle Basin.

Late Devensian glaciotectonic deformation at St Bees, Cumbria: a critical wedge model

Glaciotectonic deformation of the Late Devensian sediments exposed at St Bees, Cumbria is represented by minor extensional faults resulting from ice-sheet loading of the unconsolidated sediments accompanied by thrusting and folding. The highly deformed northern part of the section has numerous thrust faults associated with fold structures that verge predominantly to the SE, but with some NW-vergent (backthrust) structures. A high-resolution seismic reflection survey confirms that the thrust structures form a linked fault system that detaches at, or slightly above bed rock, which is here composed of Triassic, St Bees sandstones. In the less deformed southern part of the section, the seismic survey has imaged bedding with a southerly component of dip in the St Bees sandstone cut by steep north-dipping extensional faults which, to some extent, control rockhead topography. An integrated approach combining structural geology and high-resolution seismic reflection surveying has enabled the construction of a balanced cross-section which estimates a minimum of 115 m (22%) cumulative shortening due to glaciotectonic processes. Minor structures observed in the St Bees cliffs and larger structures interpreted from the seismic profile are compatible with a critical wedge model for deformation caused by an overlying thick ice wedge with a SE-dipping surface slope.

Interpretation of structural domains in discontinuity data from Nirex deep boreholes at Sellafield

SUMMARY Knowledge of the fracture network is fundamental to the fluid transport and geotechnical modelling necessary to assess the suitability of a rock mass to host a potential underground repository for the disposal of radioactive waste. However, in the early stages of a site investigation programme, site-specific fracture data are frequently available only from boreholes. This was the case at Sellafield, where characterization of borehole fracture data presented a number of inter-related problems including the volume of data, two data sets for each borehole and borehole sampling effects. A solution to the presentation and interpretation of such data was found in simple graphical display which allows rapid appraisal of a range of fracture attributes in relation to depth, lithostratigraphy and structure or in any other context. The succession sampled by the boreholes in the immediate vicinity of the proposed repository at Sellafield comprises three major units. The Borrowdale Volcanic Group, of Ordovician age, is unconformably overlain by a Permo-Triassic cover sequence comprising up to 100 m of sedimentary breccia (Brockram) succeeded by c. 450 m of the fluvial St Bees Sandstone to rockhead. Fracture data from these are available from logging of the borehole core and from interpretation of resistivity and acoustic images of the borehole wall, providing accurate frequency and orientation information respectively. These show that the discontinuity character of each of the major units is distinctive, indicating overall lithological control on the fracture network. Fracture frequency in the sedimentary cover rocks is relatively low, especially in the Brockram, and orientation patterns are dominated by bedding. The fracture frequency in the volcanic rocks is generally high and orientation typically very variable in the volumetrically dominant welded ignimbrites. However, interbedded units of non-welded lapilli tuff and breccia have lower fracture frequency and simpler orientation patterns. In all units, variations in fracture frequency and orientation character on a scale of a few metres to 100 m can be correlated with fault intersections indicating a structural overprint. Borehole sampling effects are illustrated by comparison of data from differently orientated boreholes and the underlying causes reviewed. Orientation bias is a well known problem usually addressed by weighting according to the probability of the borehole intersecting a planar feature in a particular orientation. However, data from borehole wall imagery are subject to additional sampling effects which reduce the effectiveness of such weighting. A different approach is suggested, using the probability density function of the population from non-censored parts of the sample.

Carbonate spheroids in Permo-Triassic sandstones of the Sellafield area, Cumbria

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.

The interpretation of gravity and aeromagnetic data for the geological investigation of the Sellafield area, west Cumbria

SUMMARY Potential field modelling has been used to test structural models of the Permo-Triassic cover sequence in the Sellafield area. An analysis of rock density data indicated that the main density interfaces occur at the base of the Calder Sandstone and the base of the St Bees Sandstone, and these horizons were employed in 3D gravity stripping. In addition, 2.5D gravity and magnetic modelling was conducted along selected profiles, with structures constrained by boreholes and good quality seismic reflection data, but subject to modification elsewhere. These studies highlighted shortcomings in earlier models for the margins of the Ravenglass Sub-basin and have led to new versions which agree better with available data. Subsequent drilling has confirmed the validity of the re-interpretation. Gravity data provide strong evidence of a north-west-trending fault zone which bounds the north-eastern corner of the Ravenglass Sub-basin. The sharp change in the strike of the basin margin represented by this fault zone probably arose because of the influence of pre-existing basement structures (granites and lines of weakness) on basin evolution. Relatively magnetic rocks within the Borrowdale Volcanic Group can be traced beneath Permo-Triassic cover using aeromagnetic data; the magnetic signature relating to the displacement of these rocks at the margins of the Ravenglass Sub-basin provides corroboration of models based on gravity data.

Triassic sandy braidplain and aeolian sedimentation in the Sherwood Sandstone Group of the Sellafield area, west Cumbria

SUMMARY During the Permo-Triassic the Sellafield area formed part of the eastern margin to the East Irish Sea Basin. Basin-margin alluvial fan, shallow marine and coastal mudflat sedimentation created a locally thick Permian succession. Triassic sediments are represented by the Sherwood Sandstone Group, which comprises the St Bees, Calder and Ormskirk sandstone formations. The basal few metres of the St Bees Sandstone are characterized by sheetflood sandstones, interbedded with playa-mudflat deposits. These pass upwards into thick, multistorey fluvial sandbodies, formed on sandy braidplain. Rare aeolian units are interbedded with the channel facies. The top of the St Bees Sandstone is sharply defined and represents the regional abandonment of the fluvial system. The overlying Calder Sandstone Formation is coarser grained and dominated by aeolian sediments, with dune and interdune facies recognized. These sands were blown into the area from the east. Minor episodes of fluvial reworking punctuated aeolian deposition and the top of the Calder Sandstone is represented by a fluvially deposited unit. The lower part of the overlying Ormskirk Sandstone is of aeolian facies and can be distinguished from the Calder Sandstone by its finer grain size. The two formations can also be differentiated using geophysical logs, as aeolian and fluvial sandstones have diagnostic geophysical log characteristics allowing recognition of different sedimentary facies associations.

The petrology and diagenesis of Permo-Triassic rocks of the Sellafield area, Cumbria

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 (&lt;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.

Lithostratigraphical subdivision of the Triassic Sherwood Sandstone Group in west Cumbria

SUMMARY The subdivisions of the Sherwood Sandstone Group, previously recognized on geophysical and lithological criteria in boreholes offshore in the East Irish Sea Basin, can be identified in the onshore exposures and boreholes of west Cumbria. The term St Bees Sandstone Formation is here restricted to the lower, fluvial, fine-grained sandstone, with claystone and siltstone partings, exposed around St Bees. This formation is sharply overlain by softer and coarser grained, aeolian and subordinate fluvial sandstone, which contains abundant, well-rounded aeolian sand grains, referred to the newly defined Calder Sandstone Formation. Higher sandstone has been separated from the Calder Sandstone Formation by its finer grain size and by geophysical log signatures. This is referred to the Ormskirk Sandstone and is dominantly aeolian in origin. Similar divisions of the Sherwood Sandstone Group can be recognized in the Carlisle Basin, and may also be present in other onshore areas, but await formal definition.

The use of dipmeter logs in the structural interpretation and palaeocurrent analysis of Morecambe Fields, East Irish Sea Basin

Dipmeter logs have been processed in a variety of ways as an aid to understanding the geology of the North and South Morecambe Gas Fields. They have yielded high quality structural information which has contributed to understanding the structural configuration and tectonic history of these fields. Several faults have been identified, cutting well paths, and dipmeter analysis has allowed the correct orientation and direction of throw to be determined. In one well, the deviated well path has intersected each of two listric faults in two separate places, allowing unusually precise definition of these fault planes. Throughout the Ormskirk Sandstone Formation, the predominant palaeocurrent direction has been found to be towards the west or southwest. The most consistent palaeocurrent orientations were measured in intervals of planar cross-stratified channel-fill facies sandstones, but similar results, with greater scatter were observed in ephemeral channel sandstones facies. These consistent palaeocurrent results suggest that the East Irish Sea Basin formed a distinct depo-centre during Ormskirk sandstone times. The implication is that during the deposition of the Ormskirk Sandstone, palaeocurrents were controlled by a regional dip towards the Keys Fault. There are changes in palaeoflow from the base of the Ormskirk Sandstone Formation to the top (a vertical thickness of 800 ft), also changes from the southern to the northern ends of the field. In the underlying St Bees Sandstone Formation, consistent palaeocurrent directions proved more difficult to obtain, since major channel facies sandstones are rarer. No angular unconformity was observed between the St Bees and Ormskirk Sandstone formations. Within the Ormskirk Sandstone interval, however, very consistent, 1° to 2° changes in structural dip occur at the same stratigraphic levels in several wells and indicate small-scale tectonic rotations during the deposition of these sandstones. These are probably related to tectonic reactivation which has been invoked in previous literature to explain periodic rejuvenation of the depositional system during deposition of the reservoir sandstones. The fact that the observation of such small-scale adjustments can be repeated in several wells is an indication of the degree of precision of the results obtained from the dipmeter logs in these wells.

Relationship between magnetic components and diagenetic features in reddened Triassic alluvium (St Bees Sandstone, Cumbria, UK)

Summary. The St Bees Sandstone is a reddened lower Triassic sandy alluvial sequence which has suffered a complex history of iron oxide diagenesis. Detrital clay-oxides have been post-depositionally reddened and particulate grain coatings of haematite precipitated. Syntaxial overgrowths of haematite and titaniferous haematite have been precipitated on coarse detrital oxides (specularites). In situ alteration of specularite (principally the martitization of detrital titanomagnetite and titanomaghaemite) has taken place. Also ferromagnesian phyllosilicates (principally biotite) have suffered partial to complete pseudomorphism by tine grained haematite. These diagenetic changes took place during contemporary geomagnetic field variations and each diagenetic phase carried a magnetic component recording the direction (and hence date) of its formation. In general magnetizations can be described as: pigment-dominant, specularite-dominant or mixed pigment-specularite. Using a combination of chemical and thermal demagnetization the individual magnetic components can be isolated using Zijderveld vector diagrams. Four examples are illustrated: (a) A specularite-dominant magnetization in which a reversed component carried by coarse detrital grains had a superimposed normal component carried by authigenic overgrowths of haematite. Both components are considered to be Triassic in age. (b) A pigment-dominant magnetization carried primarily by clay-oxide pellicles formed during intrastratal alteration. The main isolated component has an ‘anomalous’ direction and its precise time of formation cannot be determined. (c) A mixed specularite-pigment magnetization in which a ‘normal’ specularite component has a superimposed ‘normal’ pigment component. These normal components are considered to be of different Triassic ages. (d) A mixed specularite-pigment magnetization in which a near-horizontal specularite component, possibly due to inclination error, has a superimposed