SEM petrography of samples of the Lias Group of England and Wales

This report summarises work undertaken in support of the Ground Movements: Shrink/Swell Project. It provides petrographical descriptions acquired using scanning electron microscopy (SEM) of a suite of samples (predominantly mudrocks) from the main Lias Group depositional basins across England and Wales. The main Formations sampled are the Scunthorpe Mudstone (n = 5), the Blue Lias (n = 8), the Charmouth Mudstone (n = 21 including 15 samples from weathering profiles), the Marlstone Rock (n = 4), and the Whitby Mudstone (n = 5). Small numbers of samples from the other Lias Formations are also described. The analyses largely confirm the observations made by earlier workers, and are broadly consistent with the results of XRD analyses. The Scunthorpe Mudstone Formation samples are typically laminated with mineralogies probably dominated by illite and/or smectite (although XRD analysis would be required to confirm this). Very finely crystalline gypsum/anhydrite occurs throughout. A sample from the Barnstone Limestone Member is typical, dominated by fine grained calcite, with minor dolomite and clay. A sample from the Frodingham Ironstone Member is also typical being a goethitic oolitic ironstone. The Blue Lias Formation samples are typically massive, hard, dense, highly calcareous mudrocks. Some less calcareous samples preserve lamination, and minor framboidal and euhedral pyrite is present. SEM analysis confirms that variations in surface area (Appendix Table 1.1) broadly correspond to variations in the relative proportions of carbonate and clay, with variations matching a regional trend in calcite content reported by Kemp and Mc Kervey (2001). Clay mineral assemblages in the two southernmost samples from this formation contain major illite, with minor kaolinite, chlorite and illite/smectite, whereas in the samples from further north, illite tends to be less dominant of the clay mineral assemblage and smectite and/or illitesmectite become more significant, suggesting either a difference in the primary makeup of the sediment, or that the smectite to illite transformation is more advanced in the most southernmost samples. The Charmouth Mudstone Formation is characterised by well-laminated mudrocks with only minor amounts of silt and sand-grade material. EDXA analysis suggests a predominance of Kbearing (illitic) clays, although XRD indicates that the clay mineral assemblages comprise major kaolinite, with minor illite, chlorite and possible illite-smectite, with minor smectite also present in the samples from the Worcester basin. Samples of this formation from the Dorset Coast contain well-developed calcite veining, with cone-in-cone (beef) fabrics present. Gypsum/anhydrite is locally developed along lamination surfaces and early framboidal pyrite is present. The samples from weathering profiles at Dimmer and Blockley reveal development of locally abundant gypsum/anhydrite, nodularisation, rootleting and oxidation towards the surface as seen in hand specimen, but these features are poorly defined on the scale of SEM stubs. No appreciable, systematic differences clay mineralogy is noted through the weathering profiles. The Marlstone Rock Formation is peloidal/ooidal with thick coatings of probable chloritic or glauconitic clay on well-rounded grains in a fine clay matrix. Cone-in-cone calcite cement is locally present. The Whitby Mudstone Formation is confirmed as a typically well-laminated mudrock with minor silty material. EDXA suggests a predominance of illite and/or illite-smectite (K and KCa-bearing) compositions, consistent with earlier, quantitative, XRD data on samples from the Cleveland Basin by Kemp and Mc Kervey (2001) which indicates that the smectite to illite transformation is well advanced. Qualitative XRD on these samples indicates major kaolinite with minor illite, illite-smectite and chlorite

SEM petrography of samples of the London Clay of Southern England

This report summarises work undertaken in support of the Ground Movements: Shrink/Swell Project undertaken as part of the BGS Physical hazards Programme. It provides petrographical descriptions acquired using scanning electron microscopy (SEM) of a suite of samples (n = 24, from 19 sites) collected from the London Clay of the London and Hampshire Basins in southern England. The analysis indicates that the samples from both basins display considerable variations in grain size both on a millimetric scale (mm-scale laminae of clay-rich, or silt-rich or fine-sand material), and also on a regional scale, with a broad trend to increasing grain size and decreasing clay-content from NE to SW. The clay content of a given mudstone is likely to be an important factor in governing the shrinking and swelling capacity of the London Clay at a given site – clearly if clay is volumetrically of minor significance within the sediment, then it will have proportionally less impact on sediment volume as it shrinks/swells, irrespective of it’s mineralogy. As might be anticipated, porosities, and hence permeabilities, will be higher where grain sizes are coarser, this will affect the susceptibility of the lithology at a given site to wetting/drying. It is recommended that a more quantitative assessment of sediment grain size is carried out. X-ray microchemical analyses qualitatively confirm the results of XRD analysis (Kemp and Wagner 2006), indicating that the clays are dominated by illite and smectite species, which might be expected to undergo significant shrinking and swelling in response to wetting and drying

Models of dolomitisation : a literature review

From an economic perspective (e.g. in the hydrocarbon and groundwater industries) one of the main questions relating to dolomitisation is the influence the dolomitisation process has on the nature of the dolomitised limestone’s pore system. To this end, one of the key objectives of the “Development of Integrated Methods for Characterising Faults and Fractures in Reservoirs and Aquifers” project is to understand the nature and distribution of pore-systems within dolomitised limestones and in particular to understand the influence of fractures on dolomitisation and/or porosity distribution, and subsequent fluid-flow pathways. This document provides a literature review on the current state of knowledge regarding the dolomitisation process. The “dolomite problem” is introduced, and the kinetic inhibitions on dolomitisation from seawater, which ultimately form the crux of the dolomite problem are summarised. Over the years, various models have been suggested in order to explain the dolomitisation process. Systematic summaries of these models are presented. In essence, each model attempts to provide a mechanism by which the kinetic inhibitions to dolomite formation are overcome (typically through evaporation or dilution of seawater), and a means of pumping large porevolumes of fluid through the body undergoing dolomitisation. Following the summaries of the dolomitisation models, the next section contains an introduction into the classification of dolomite fabrics and pore systems. The final sections outline the regional geology of the Lower Carboniferous in the Midlands of the United Kingdom, and describes potential analogue material in Derbyshire and the Bowland Basin

Application of mineralogical, petrological and geochemical tools for evaluating the palaeohdrogeological evolution of the PADAMOT study sites

The role of Work Package (WP) 2 of the PADAMOT project – ‘Palaeohydrogeological Data Measurements’ - has been to study late-stage fracture mineral and water samples from groundwater systems in Spain, Sweden, United Kingdom and the Czech Republic, with the aim of understanding the recent palaeohydrogeological evolution of these groundwater systems. In particular, the project sought to develop and evaluate methods for obtaining information about past groundwater evolution during the Quaternary (about the last 2 million years) by examining how the late-stage mineralization might record mineralogical, petrographical and geochemical evidence of how the groundwater system may have responded to past geological and climatological changes. Fracture-flow groundwater systems at six European sites were studied: • Melechov Hill, in the Bohemian Massif of the Czech Republic: a shallow (0-100 m) dilute groundwater flow system within the near-surface weathering zone in fractured granitic rocks; • Cloud Hill, in the English Midlands: a (~100 m) shallow dilute groundwater flow system in fractured and dolomitized Carboniferous limestone; • Los Ratones, in southwest Spain: an intermediate depth (0-500 m) dilute groundwater flow system in fractured granitic rocks; • Laxemar, in southeast Sweden: a deep (0-1000 m) groundwater flow system in fractured granitic rocks. This is a complex groundwater system with potential recharge and flushing by glacial, marine, lacustrine and freshwater during the Quaternary; • Sellafield, northwest England: a deep (0-2000 m) groundwater flow system in fractured Ordovician low-grade metamorphosed volcaniclastic rocks and discontinuous Carboniferous Limestone, overlain by a Permo-Triassic sedimentary sequence with fracture and matrix porosity. This is a complex coastal groundwater system with deep hypersaline sedimentary basinal brines, and deep saline groundwaters in crystalline basement rocks, overlain by a shallow freshwater aquifer system. The site was glaciated several times during the Quaternary and may have been affected by recharge from glacial meltwater; • Dounreay, northeast Scotland: a deep (0-1400 m) groundwater flow system in fractured Precambrian crystalline basement overlain by fractured Devonian sedimentary rocks. This is within the coastal discharge area of a complex groundwater system, comprising deep saline groundwater hosted in crystalline basement, overlain by a fracture-controlled freshwater sedimentary aquifer system. Like Sellafield, this area experienced glaciation and may potentially record the impact of glacial meltwater recharge. In addition, a study has been made of two Quaternary sedimentary sequences in Andalusia in southeastern Spain to provide a basis of estimating the palaeoclimatic history of the region that could be used in any reconstruction of the palaeoclimatic history at the Los Ratones site: • The Cúllar-Baza lacustrine sequence records information about precipitation and palaeotemperature regimes, derived largely from the analysis of the stable isotope (δ18O and δ13C) signatures from biogenic calcite (ostracod shells). • The Padul Peat Bog sequence provided information on past vegetation cover and palaeogroundwater inputs based on the study of fossil pollen and biomarkers as proxies for past climate change. Following on from the earlier EC 4th Framework EQUIP project, the focus of the PADAMOT studies has been on calcite mineralization. Calcite has been identified as a late stage mineral, closely associated with hydraulically-conductive fractures in the present-day groundwater systems at the Äspö-Laxemar, Sellafield, Dounreay and Cloud Hill sites. At Los Ratones and Melechov sites late-stage mineralization is either absent or extremely scarce, and both the quantity and fine crystal size of any late-stage fracture mineralization relevant to Quaternary palaeohydrogeological investigations is difficult to work with. The results from the material investigated during the PADAMOT studies indicate that the fracture fillings at these sites are related to hydrothermal activity, and so do not have direct relevance as Quaternary indicators. Neoformed calcite has not been found at these two sites at the present depth of the investigations. Furthermore, the HCO3 - concentration in all the Los Ratones groundwaters is mainly controlled by complex carbonate dissolution. The carbonate mineral saturation indices do not indicate precipitation conditions, and this is consistent with the fact that neoformed calcite, ankerite or dolomite have not been observed petrographically

SEM petrography of samples of the Wealden Group of southern England

This report summarises work undertaken in support of the Ground Movements: Shrink/Swell Project undertaken as part of the BGS Physical hazards Programme. It provides petrographical descriptions acquired using scanning electron microscopy (SEM) of a suite of samples (n =24, from 9 sites) from the Wealden Group (Wessex Clay, Weald Clay and Wadhurst Formations) Clay from the Weald and Wessex Basins of southern England

Development of capability in the SEM-CL of carbonates

This report describes investigations into methodologies that can be applied to overcome imaging problems associated with carbonate minerals on scanning electron microscope-based cathodoluminescence (SEM-CL) systems. The problem arises due to the persistent nature of luminescence from carbonate minerals, which causes ghosting or streaking across SEM-CL images. Two methodologies were tested: • The first methodology (Lee 2000) applied very long image acquisition times that, in certain situations proved capable of producing excellent images at higher resolution than is possible using optical-based CL systems. However, the image acquisition times are too slow (c. 40 minutes per image) to be useful in most day-to-day situations. • The second methodology (Reed and Milliken 2003) uses an optical filter to remove the portion of the CL spectrum responsible for the persistent luminescence (in this case in the yellow to red portion of the visible light spectrum). This enabled capture of SEM-CL images at far faster acquisition times (c. 5 minutes per image) than was possible without the filter. However, the resulting ‘filtered’ images suffer from relatively poor contrast and zoning apparent in these images did not always match zoning observed in optical Cl or unfiltered SEM-CL images. Poor image contrast was observed in the filtered images because the luminescence in the studied carbonates is predominantly due to activation by substitution of Mn, which predominantly occurs in the orange to red portion of the visible spectrum. Therefore, this type of luminescence was effectively excluded by the filter. Consequently the measured signals from the detector reflect the much less intense intrinsic luminescence of the carbonate or luminescence activated by other substituted cations (e.g. rare earth elements) or thermally activated luminescence. Although filtered SEM-CL carbonate imaging met with variable levels of success, the filtered imaging approach will prove useful in the SEM-CL analysis of quartz in mixed quartz-carbonate-bearing lithologies such as carbonate-cemented sandstones, which have previously been hindered by the persistence of the luminescence from the carbonates

Gold mineralization associated with low temperature basinal brines in Connemara, western Ireland

Fluids inclusion studies suggest that the gold mineralization occurring in a silica-rich fault zone in Silurian rocks at Bohaun in Connemara, western Ireland is associated with low temperature, moderate–high salinity fluids more consistent with a basinal brine than an orogenic gold lineage. This contrasts with other gold deposits in western Ireland that are typically orogenic in mineralization style. Remobilisation of pre-existing gold mineralization by low-temperature, highsalinity brines is recognised in a number of gold deposits worldwide. However, at Bohaun there is no evidence for earlier mineralization suggesting that low-temperature fluids can transport gold and potentially form gold deposits independent of other fluids