ALBERT

Description: A series of boreholes in Horton Quarry, northwest Yorkshire (Horton-in-Ribblesdale Inlier) penetrated mudstones and slates belonging to the Austwick Formation (Windermere Supergroup) overlying laminated mudstones of the Ingleton Group. Illite (IC) and chlorite (ChC) crystallinity measurements indicate a metamorphic inversion between the two groups of mudrocks. The Windermere Supergroup mudrocks are mostly in the high anchizone or epizone, whereas the Ingletonian samples are lower grade in terms of IC, and are mostly deep diagenetic zone or low anchizone. Hence younger strata at higher grades rest on older strata at lower grades, creating a metamorphic inversion. Ingletonian slates exposed at Pecca Falls on the River Twiss show epizonal and anchizonal IC values, and greywacke samples from Ingleton Quarry contain pumpellyite. This suggests that grade in the Ingletonian may increase to the NW from the Horton to Ingleton inliers. K-white mica b cell dimensions show further differences between the Ingleton Group and the Windermere Supergroup. The Ingletonian samples are characterized by low b cell values (8.989–9.035, mean 9.007 Å), whereas the Windermere Supergroup has higher values in the range 9.022–9.034, mean 9.027 Å. The Windermere Supergroup values are similar to those recorded from the Windermere Supergroup of the southern Lake District, and Lower Palaeozoic rocks from the Scottish Southern Uplands, and are consistent with metamorphism in a low heat flow, convergent geotectonic setting. The Ingletonian b cell values suggest metamorphism in a higher heat flow setting, most likely an extensional basin. The metamorphic inversion at Horton and differences in K-white mica b cell dimensions suggest that the Ingleton Group and Windermere Supergroup strata evolved in different geotectonic settings and record two separate metamorphic events. The discovery of the metamorphic inversion at Horton provides further evidence in favour of an Ordovician rather than Neoproterozoic depositional age for the controversial Ingleton Group.

Description: 40Ar–39Ar dating by a combination of spot analysis and step-heating has been carried out on mica fabrics from slaty cleavage and a shear zone in the Neoproterozoic to Cambrian rocks of Charnwood Forest, Leicestershire. The results indicate that crust adjacent to the eastern margin of the Midlands Microcraton was affected by localized epizonal metamorphism and penetrative deformation between 425 and 416 Ma (Silurian to earliest Devonian), somewhat earlier than the Acadian Phase of the Caledonian orogeny recorded elsewhere in southern Britain. The Charnwood cleavage geometry suggests that the deformation arose within a dextral transpressional regime along the eastern margin of the Midlands Microcraton. This tectonism may have overlapped with pre-Acadian sinistral transtensional movements documented for the Welsh Basin and Lake District areas, to the west and north of the Midlands Microcraton.

Description: The Lòn Ostatoin stream section, Trotternish Peninsula, Isle of Skye, exposes a sequence of Middle Jurassic mudstones and limestones which have been locally metasomatized by a transgressive sill of Tertiary age. Limestones in the sequence, including some previously reported as bentonite, have been altered to an unusual assemblage of grossular garnet and saponite clay. The mudstones also contain large proportions of saponite together with pyroxene and zeolites. Saponite also occurs within the basalt intrusion. Grossular and pyroxene represent artifacts of relatively high-temperature assemblages that formed during an early phase of alteration. As the intrusion and adjacent altered country rocks cooled, lower-temperature fluids flowed through a late set of contraction (micro)fractures. Back-reacted saponite, analcime and clinoptilolite were formed, possibly as alteration products of the unstable higher-temperature minerals. The lower-temperature mineral assemblage eventually sealed the late fracture system.This paper highlights an important concept for the study of analogue sites used to investigate thermal effects on engineered liners or barrier host rocks for the landfill and radioactive-waste industries. This is that the original thermally altered mineral assemblage may be overprinted by later, lower-temperature back-reactions. A detailed understanding of both processes is necessary in order to construct a sensible model for the thermal and mineralogical evolution of the site.

Description: The clay mineral assemblages and microtextures of a suite of mudrocks from the Lias Group of England and Wales indicate important regional differences in burial history.Samples from the northern Cleveland Basin are characterized by illite-smectite (I-S, 90% illite) and little carbonate whilst samples from the southern Worcester and Wessex basins contain less mature discrete smectite and are often calcite- and dolomite-rich. Lias Group rocks have been buried to 4 km in the Cleveland Basin but to

Description: The origin of the regional and stratigraphical variation in the Triassic authigenic clay assemblages of England is discussed in relation to new estimates of the palaeotemperatures experienced by their host sediments and a preliminary study by transmission electron microscopy of their microtextural features. Spore colour index measurements, based on the spore type Deltoidospora s.l. occurring in the sediments (Penarth Group) at the very top of the Triassic sequence, give estimated palaeotemperatures ranging from 60–74°C (south Devon) to 89–97°C (northeast Yorkshire). Calculated palaeotemperatures, based on a gradient of 25°C/km, for the main zone of authigenic clay minerals range from 63–77°C to 89–97°C for the top to 71–85°C to 94–104°C for the base. Irregular mixed-layer smectite-chlorite, corrensite and Mg-rich chlorite are associated with calculated palaeotemperatures of 66–86°C, 66–104°C and 75–104°C respectively. The suggestion that elsewhere in the UK corrensite and Mg-rich chlorite were formed at temperatures in excess of 100°C finds no support. Geothermal gradients would have to have been of the order of at least 100–300°C/km to obtain these temperatures within the Triassic sediments; such values are associated typically with high-level magmatic intrusions or geothermal systems of which there is no geological evidence. The balance of evidence suggests that the Triassic authigenic clay assemblages formed by neoformation during the early stages of sediment diagenesis under the influence of variation in the alkalinity of the depositional environments.

Description: Lower Palaeozoic rocks crop out extensively in Wales, the Lake District of northern England and the Southern Uplands of Scotland; they also form the subcrop concealed beneath the English Midlands and East Anglia. These mainly marine sedimentary rocks were deposited in basins created during plate tectonic assembly of the various terranes that amalgamated to form the British Isles, 400-600 Ma ago. Final amalgamation occurred during the late Lower Devonian Acadian Orogeny when the basins were uplifted and deformed, producing belts of cleaved, low-grade metasediments, so-called slate belts, with a predominantly Caledonian (NE-SW) trend. The clay mineralogy of mudrock lithologies - including mudstone, shale and slate - found in these belts is reviewed. Using X-ray diffraction data from the

Description: The Devonian sedimentary rocks of the UK are made up of a continental red bed facies, the Old Red Sandstone (ORS), and sediments of a marine origin. The latter are confined to southwest England whereas the ORS occurs much more extensively, particularly in South Wales, the West Midlands, Northern England, the Midland Valley of Scotland and the Orcadian basin. The ORS also occurs extensively offshore where it contains important hydrocarbon reservoirs. Highly variable suites of clay mineral assemblages are characteristic of the ORS. In the West Midlands and Monmouthshire, the Downton Group is characterized by illitic, smectitic, and mixed-layer illite-smectite minerals. A tuff bed (Townsend Tuff) also contains expansible minerals but when the bed is traced westwards it is found that the clay mineralogy changes progressively to an illite-chlorite assemblage, suggesting the influence of diagenetic or metamorphic change. It is not known, however, whether such a transformation is typical of the Downton Group as a whole. The overlying Ditton Group in its eastern outcrops contains a high-spacing mineral identified as tosudite, together with regularly interstratified illite-smectite and well crystallized kaolinite. Further west this assemblage gives way to illite and chlorite, with the latter being trioctahedral or dioctahedral, while in Dyfed the Ditton Group may contain smectite and poorly crystallized kaolinite in addition to illite and chlorite. The geographical distribution of clay minerals in the Ditton Group may also be accounted for by progressive diagenetic to low-grade metamorphic reactions, although it is necessary to postulate retrogressive diagenesis to account for the smectite and kaolinite that occur in the Dyfed samples. The clay mineralogy of the overlying Brecon Group and the Upper ORS also consists of mixtures of illite and chlorite in the west and central parts of the region. No data are available for the Brecon Group in the eastern parts of the outcrop but the Upper ORS from around Monmouth and Portishead contains assemblages rich in kaolinite and mixed-layer illite-smectite with only minor amounts of chlorite. The distribution of clay minerals in the Upper ORS is again suggestive of a progressive westerly increase in the influence of diagenetic alteration, although the influence of provenance cannot be discounted.In Scotland the clay mineralogy of the Lower ORS in the Midland valley is characterized by a variety of interstratified minerals, including regularly interstratified trioctahedral chlorite-vermiculite, a tosudite mineral similar to that described from South Wales and illite-smectite, as well as occasional illite, chlorite and smectite. The oldest Stonehaven Group is kaolinitic but in the younger groups kaolinite is either completely absent or present in only minor amounts. It is clear that detrital inputs, particularly from associated volcanic rocks, have contributed to the clay minerals found in these rocks, although the contribution could be indirect with diagenetic clay minerals forming from volcanic detritus after deposition. Diagenetic alteration may also be important, particularly with respect to the origin of the tosudite mineral. The Middle ORS lacustrine sediments of the Orcadian Basin of Scotland are characterized by mixtures of trioctahedral chlorite and dioctahedral illite, with interstratified chlorite-smectite, illite-smectite, kaolinite and occasional reports of minor montmorillonite. The most recent interpretations of the origins of the clay minerals in these rocks emphasize the role of progressive diagenetic and low-grade metamorphic reactions, based on correlations of clay mineral assemblages and parameters such as illite crystallinity with organic maturation data. This interpretation argues that the illite-smectite in the shales is derived largely from a precursor detrital smectite. However, the finding of two populations of morphologically and structurally distinct illite-smectite particles in the Middle and Upper ORS sandstones suggests a more complex picture involving different diagenetic episodes. In addition, the likelihood of a smectitic-rich detrital input to the ORS may also be a point of debate. The clay mineralogy of the North Sea offshore is also described briefly, in addition to the marine Devonian in southwest England. The latter is characterized by chlorite and illite assemblages of low-grade metamorphic origin, although smectite and kaolinite are also found occasionally in these rocks. The offshore ORS, however, contains a variety of clay minerals, including an assemblage similar to that found in the Lower ORS south of the Highland Boundary Fault and notably contains a tosudite-like mineral.

Description: The publication of The Clay Mineralogy of British Sediments by Perrin in 1971 collated several decades of clay mineral research in the British Isles and for the first time presented all the data in a stratigraphical framework. While it quickly became a useful source of information for geologists, engineers and soil scientists, it also revealed many gaps in clay mineral data through the geological succession, stimulating further research. Within ten years of publication, a successor to Perrin's book was under discussion by the Clay Minerals Group. Inevitably, the enthusiasm for the concept of the project gave way to the patience of a long gestation. A successor to Perrin (1971) became a standing item on the agenda of Clay Minerals Group Committee meetings, and the bane of many a Chairman's three years in office. By the mid-1990s the project began to show real progress, gathering momentum from the success of an international series of 'Cambridge clay mineral diagenesis conferences' (1981, 1984, 1986, 1989, 1993, 1998) that were supported by the oil industry. A timely injection of financial support from the Joint Association for Petroleum Exploration Courses (JAPEC) ensured a successful conclusion for the project.The cost of publication has been borne by three sponsors: the Clay Minerals Group, JAPEC (UK: training), and the Mineralogical Society. Consequently, the financing of this Special Volume of Clay Minerals is entirely independent of the usual costs of publishing the journal. We owe our particular thanks to Kevin Murphy, Editorial Manager, for his care and humour in guiding Clay minerals in onshore and offshore strata of the British Isles through publication.