N.S. Jones

A revised correlation of Carboniferous rocks in the British Isles

The report revises and expands upon the 1976 and 1978 publications for the Dinantian and Silesian, respectively, combining them into a single account of British and Irish Carboniferous stratigraphy. The need to update the two Special Reports reflects the considerable advances in Carboniferous geology over the last 30 years. The report covers developments in international chronostratigraphy and incorporates wholesale reassessments of British lithostratigraphy. A huge volume of biostratigraphical information has been published over recent decades and the report summarizes the key information. Carboniferous rocks have long been of economic importance, but it is the search for hydrocarbons, in its infancy at the time of the previous reports, which has greatly increased our understanding of Carboniferous successions offshore and at depth, particularly in southern and eastern England.

The Exeter Group, south Devon, England: a contribution to the early post-Variscan stratigraphy of northwest Europe

The lower part of the post-Variscan succession around Exeter, south Devon, England, comprises some 800 m of breccias, with subordinate sandstones and mudstones, which rest upon Devonian and Carboniferous rocks folded during the Variscan Orogeny and are overlain, disconformably, by the Aylesbeare Mudstone Group (Early Triassic?). These deposits comprise the most westerly of the early post-Variscan successions preserved onshore in northwest Europe and lie to the south of the Variscan Deformation Front; they are assigned to the Exeter Group (new term). Geochronological and palaeontological studies, in conjunction with detailed geological mapping, show that the constituent formations comprise a lower (Late Carboniferous(?)–Early Permian) sequence separated from an upper (Late Permian) sequence by an unconformity which represents an hiatus with a duration of at least 20 m.y. The lower sequence contains volcanic rocks dated at between 291 and 282 Ma (Early Permian) and pre-dates intrusion of the nearby Dartmoor Granite (280 Ma). In the overlying, palynologically-dated, Late Permian sequence, older breccias contain clasts of the Dartmoor Granite aureole rocks, and younger ones contain clasts of that granite. The lower sequence occurs mainly within the Crediton Trough, an east–west trending, partly fault-bounded, sedimentary basin that probably formed by extensional reactivation of a Variscan thrust. Breccias in this sequence formed largely on alluvial fans; the common occurrence of debris flows and a down-fan passage from gravity flows into fluvially deposited sediments is typical of deposition on semi-arid fans. The upper (Late Permian) sequence is more widespread but includes similar deposits overlain, at the top of the Exeter Group, by aeolian dune and interdune deposits. Correlation within the laterally variable facies associations which comprise these sequences has been achieved using a combination of sedimentary facies analysis, sedimentary geochemistry, and petrographical and geochemical clast typing. The stratigraphy revealed within the Exeter Group is broadly comparable with that recognized in the early post-Variscan Rotliegend successions elsewhere in Europe. This similarity may, however, be deceptive; the upper part of the Exeter Group may be coeval with the Zechstein, and apparently correlatable major unconformities in the group and the Rotliegend may reflect different events in the Variscan fold-belt and Variscan Foreland areas, respectively.

Warwickshire Group (Pennsylvanian) red-beds of the Canonbie Coalfield, England–Scotland border, and their regional palaeogeographical implications

Late Carboniferous red-beds, < 700 m thick, at outcrop and in the subsurface of the Canonbie Coalfield can be assigned to the Warwickshire Group. They are preserved within the axial part of the Solway Syncline and are divisible into the Eskbank Wood, Canonbie Bridge Sandstone and Becklees Sandstone formations. Sedimentation largely took place on a well-drained alluvial plain, characterized mainly by early, primary oxidation of the strata. Large, northerly-flowing braided river systems were common, with overbank and floodplain fines deposited lateral to the channels; soils formed during intervals of low sediment aggradation. The Canonbie succession includes some of the youngest Carboniferous rocks preserved in the UK. Correlation of the Eskbank Wood Formation is equivocal, but using petrographical, heavy mineral, zircon age dating and palaeocurrent data, the Canonbie Bridge Sandstone Formation can be unambiguously correlated with the Halesowen Formation of Warwickshire, the Pennant Sandstone Formation of South Wales and the offshore Boulton Formation. This suggests that southerly-derived detritus travelled considerable distances from the Variscan highlands of Brittany and/or central Germany across the southern North Sea and UK areas, to a position some hundreds of kilometres north of that previously recognized. The Becklees Sandstone Formation has much in common with the Salop Formation of the English Midlands. It appears to have no preserved equivalent elsewhere in the UK or in the UK sector of the southern North Sea but resembles stratigraphically higher parts of the southern North Sea succession seen in the Dutch sector.

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.

Variscan sourcing of Westphalian (Pennsylvanian) sandstones in the Canonbie Coalfield, UK

The zircon age spectrum in a sample from the Canonbie Bridge Sandstone Formation (Asturian) of southern Scotland contains two main peaks. One is Early Carboniferous in age (348–318 Ma), and corresponds to the age of igneous activity during the Variscan Orogeny. The other is of late Neoproterozoic to early Cambrian age (693–523 Ma), corresponding to the Cadomian. Together, these two groups comprise 70 % of the zircon population. The presence of these two peaks shows unequivocally that a significant proportion of the sediment was derived from the Variscides of western or central Europe. The zircon population also contains a range of older Proterozoic zircons and a small Devonian component. These could have been derived from the Variscides, but it is possible that some were locally derived through recycling of northerly derived sandstones of Devonian–Carboniferous age. The zircon age data confirm previous suggestions of Variscide sourcing to the Canonbie area, made on the basis of petrographical, heavy mineral and palaeocurrent evidence, and extend the known northward distribution of Variscan-derived Westphalian sediment in the UK.

Fluvial and aeolian deposition in the Siluro-Devonian Swanshaw Sandstone Formation, SW Scotland

Synopsis The recognition of fluvial and aeolian facies within the Siluro-Devonian Swanshaw Sandstone Formation of SW Scotland allows a more detailed depositional history of the Lanark basin in the southern part of the Midland Valley of Scotland to be inferred. The formation is well exposed in Ayrshire and has four main sedimentary facies associations: Channel, Aeolian, Floodplain and Mudflat. Each combines several sedimentary facies: eight facies are recognized in the channel association, three in the aeolian, four in the floodplain and two in the mudflat. These facies associations co-existed in seven different depositional settings in the western Lanark basin. Deposition occurred on a semi-arid proximal alluvial plain with an aeolian component described for the first time in the Lanark basin. Sedimentation was pulsed, largely driven by periods of source area uplift that resulted in the deposition of coarse-grained conglomeratic and sandstone-dominated facies. The principal transport was via river channels, commonly in shallow braided systems. Periods of low sediment flux are marked by fewer channels and by the preservation of floodplain successions including aeolian deposits. Abandonment of the fluvial system towards the top of the formation in Ayrshire is marked by a progression through sandflats and mudflats to the overlying volcanic formation. The relationship of the fluvial and aeolian deposits is analysed. The palaeo-wind direction is compatible with that recorded in sandstones of similar age in the Orcadian basin.

Can UK coal resources contribute to a gas renaissance

The predicted shortage of indigenous UK natural gas coincides with the continuing decline of the UK deep coal mining industry. Significant capacity has been lost through mine closures over the last few years as a result of competition from cheaper sources of coal from overseas and changes to the UK energy market. It is unlikely that any new large underground mines will be developed in the UK in the short or medium term. There are, however, still vast untouched coal resources in the UK that represent a significant energy resource. Increased awareness of the potential for clean energy from coal seams, advances in technology and recognition of the environmental benefits has seen a revival of interest in the potential to release some of the energy value of UK coal via alternative, non-mining technologies such as coalbed methane production and underground coal gasification. If successful, these could supplement declining conventional UK natural gas production. The utilization of clean energy from coal seams is supported by the UK Government through its Cleaner Fossil Fuels Programme. There is some activity in this field already. Methane is being drained from most of the remaining deep mines and utilized as fuel for electricity generation or on-site boilers. Drained gas that is not utilized is conventionally vented, but at one mine this gas is now flared to reduce greenhouse gas emissions. Methane is also being produced from abandoned mines, but the present low electricity and gas prices have adversely affected its economics. For the longer-term future, there is potential for virgin coalbed methane production and underground coal gasification. The UK has coalbed methane resources estimated to be in the order of 2.9 × 10 12 m 3 CH 4 . However, the limited exploratory drilling for coalbed methane in unmined areas undertaken to date has not led to commercial production – the major barriers are perceived to be low seam permeability, drilling and completion costs, together with planning and access issues. Initial research suggests there is very large potential for underground coal gasification providing the environmental and perceived safety issues surrounding the application of this technology can be overcome.