SUMMARY The Eycott Volcanic Group, at the northern margin of the Lower Palaeozoic inlier in the Lake District, is the smaller of two substantial middle Ordovician (Caradoc) subduction-related volcanic successions that stratigraphically separate marine sedimentary successions of the Skiddaw Group and Windermere Supergroup. Tabular lavas and subordinate sills, in a sequence up to 2400 m thick, mainly comprise porphyritic basalt, basaltic andesite, andesite and dacite, and are locally interbedded with thin units of volcaniclastic sandstone and pyroclastic rocks; these are overlain by 800 m of acid andesitic pyroclastic rocks. The distribution and form of the lava/sill fades association are consistent with emplacement as a lava plateau sequence, remarkably similar to the Birker Fell Formation, the lower part of the Borrowdale Volcanic Group in the central Lake District. The Eycott Volcanic Group rocks are geochemically coherent with characteristics transitional between medium-K, continental-margin tholeiitic, and calc-alkaline andesite suites. Rocks within the suite can be linked by fractionation of an assemblage of plagioclase, pyroxene, Fe–Ti oxide and apatite. A prominent compositional gap between about 58 and 65% SiO 2 is attributed to the rapid precipitation and segregation of Fe–Ti oxide. Incompatible element concentrations in the mafic members suggest that magmas were derived possibly from a subcontinental lithosphere source, similar to that of the Borrowdale Volcanic Group. The geochemical differences between these two suites arose through the incorporation of different amounts of the subduction component and different fractionation histories.
The 6 km thick Ordovician Borrowdale Volcanic Group is readily divisible into a lower 2.2–2.7 km thick predominantly pre-caldera succession dominated by basalt, andesite and dacite sheets, and an upper succession of caldera-related ignimbrites and volcaniclastic sedimentary rocks of c. 3 km thickness. The lower Borrowdale Volcanic Group rocks, here included within a single lithostratigraphical unit, the Birker Fell Formation, affords a well-exposed section through a pre-caldera sequence. The Birker Fell Formation is dominated by andesites which comprise 60% of the stratigraphy. Thin sequences of reworked volcanic detritus are commonly interbedded with the andesites; locally there are thicker units of volcaniclastic sedimentary rocks. Interpretation of the stratigraphy and eruptive history of the formation has been aided by the recognition of some distinctive lithological units including: (1) a 200-600 m thick sequence of weakly parallel-bedded basaltic tuffs resting on the pre-volcanic basement, (2) a 200-600 m sequence of single flow unit aa basalts, (3) dacite flows and ignimbrites up to > 1100 m thick in the middle and upper parts of the formation and (4) aphyric to pyroxene-phyric, simple and compound flows of basalt, 50-400 m thick, locally present in the uppermost parts. Analysis of the facies comprising the Birker Fell Formation indicates that it was emplaced as a sub-aerial, plateau-andesite sequence, formed by the coalescence of products erupted from a number of centres or fissures in an extensional, subsiding volcano-tectonic rift zone. Volcano-tectonic faulting and eruption of thick ignimbrites locally influenced development of this field.
When developing a 3-D view of the subsurface, a geologist has to collect, validate and integrate a wide range of data. The resulting understanding of the concealed geology can then be brought together as a 3D model. All of this data and understanding should be accessible as the model is viewed, rather like a digital 3D geological map with descriptions, keys and links to the observational data. The term digital geoscience spatial model (DGSM) was coined by Vic Loudon for this concept, which he describes in the first section of this report. The DGSM has seen the concept turned into a working system.
In this report we summarise the objectives and achievements of the DGSM and describe the strands that have been woven together to bring the programme to completion. Our intention here is to provide an outline of the system so that BGS collegues, other environmental scientists and our clients can use it to further the understanding of the complex Earth on which we depend. In order to put the idea into practice, the BGS carried out a scoping study and won funding from the NERC to carry out a five-year project which started in 2000. We designed the programme to have two main parts: the 'framework' of information technology developments and the 'population projects' where the framework could be tested.
Summary In the Northumberland–Solway Basin and Alston Block of northern England, some aspects of the stratigraphical and sedimentological relationships between the Millstone Grit Group, the Stainmore Formation (Namurian part of the Yoredale Group) and the Westphalian Pennine Coal Measures Group are uncertain. Also, confusion has resulted from discontinuation of Millstone Grit as a formal lithostratigraphical term north of the Stainmore Basin. This paper presents the evidence for, and describes the nature of, a Kinderscoutian (early Pennsylvanian) abrupt transition from typical ‘Yoredale cyclicity’, characterized by marine limestones in a dominantly siliciclastic succession but including marked fluvial channels, to a sandstone-dominated fluvial succession recognizable as the Millstone Grit Group. Sandbodies present in this region are probably the fluvial feeder systems to many of the fluvio-deltaic successions recorded farther south in the Central Pennine Basin. However, onset of the Millstone Grit Group occurs much earlier to the south, during the Pendleian (late Mississippian), despite the entry of fluvial systems into the Central Pennines Basin from the north. In addition to explaining this counter-intuitive relationship, the paper also recognizes continuation of the fluvial regime into the lowermost part of the Pennine Coal Measures Group.
Terra Nova, 24, 16–26, 2012 Abstract A shell of Gigantoproductus okensis shows twenty growth lines with marked changes of fabric, indicating periodical reduction of growth rates caused by environmental perturbations. The number of growth lines suggests a lifespan of 20 years in agreement with the survival rates of extant brachiopods, and with spiral deviation analysis. Geochemical analyses across the growth profile show a heterogeneous distribution of stable isotopes and trace elements. It is possible to distinguish primary from altered carbonate, and to interpret the isotopic data. The oxygen isotope signal in the unaltered parts is periodical and annual, with oscillation of ∼1.1‰. The higher values are at the growth lines (winter), and therefore most likely related to monsoon circulation during the Visean. The annual periodicity seems also present in the altered part of the shell, suggesting that diagenesis could have reset the primary values, but preserved their cyclicity.
Abstract Mafic dykes occur in close association with, and both cut and are cut by, the Eskdale granite in the south-western Lake District. The dykes range compositionally from magnesian basalt to andesite and are divided into two groups: (1) high-Fe-Ti rocks of tholeiitic affinity forming most of the dykes and (2) a lower-Fe-Ti group, comparable in composition to the lavas of the Borrowdale Volcanic Group. The dykes extend the range of tholeiitic magmatism in the Lakes into late Ordovician, and possibly Silurian times, and indicate that published plate tectonic models partly based on the distribution of magma types are perhaps over-simplified. The Eskdale dykes form one end of a spectrum of Lake District compositions from tholeiitic to calc-alkaline. All the magma types may have shared a common mantle source, their final composition reflecting residence times in the crust or LIL-enriched mantle.
Within a largely concealed, caldera-related volcaniclastic succession of the Ordovician Borrowdale Volcanic Group in the western Lake District, two thick (100–350 m) ignimbrites within the Fleming Hall Formation exhibit a number of features that in combination make them unusual deposits. They are both homogeneous with comparatively low-SiO 2 (63%) bulk composition, contain only a moderate crystal content, are generally poor in lithic clasts, show uniformly very dense welding (yielding parataxitic to massive vitrophyric texture) throughout and lack associated fall-out or surge deposits. Ignimbrites of comparable bulk composition in this geological setting are usually part of zoned sheets and/or frequently very crystal-rich. Large-scale, unzoned densely welded ignimbrites are usually rhyodacitic to rhyolitic. By contrast, ignimbrites of intermediate composition that display dense welding are relatively small deposits that form by agglutination of hot, plastic spatter. It is postulated that the Fleming Hall ignimbrites were derived from low column height, low explosivity eruptions that conserved heat and minimized entrainment of accidental lithic clasts and the formation of fine ash. The very dense welding and lack of bubble-wall shard vitroclastic textures indicate that pyroclasts were hot and relatively dry, probably occurring as mildly vesicular (scoriaceous) fragments which welded or fused together during aggradational deposition rather than by post-depositional compactional loading. There is little variation in the degree of matrix or melt crystallization throughout the two ignimbrites, despite the fact that high temperatures must have been maintained for many years following deposition. Both display virtually ubiquitous development of micropoikilitic glass devitrification texture, which suggests that the viscosity of the supercooled dacitic melt was sufficiently high, probably due to initial degassing, to inhibit significant melt crystallization after deposition. The eruption of the Fleming Hall magmas was probably initiated by the rise or injection of hotter, more basic, magma, and not by overpressurization due to volatile exsolution resulting from cooling and crystallization. Foundering of the chamber roof caused forcible and rapid eruption of the magma, probably along a series of volcanotectonic faults rather than a central vent, and probably flooded the resultant caldera depression. It is predicted that this type of eruption will not have produced a widely dispersed deposit, the bulk of which may have been largely contained within its own caldera.
ABSTRACT The first half of the Mississippian or Early Carboniferous (Tournaisian to mid- Viséan), an interval of about 20 million years, has become known as “Romer's Gap” because of its poor tetrapod record. Recent discoveries emphasise the differences between pre-“Gap” Devonian tetrapods, unambiguous stem-group members retaining numerous “fish” characters indicative of an at least partially aquatic lifestyle, and post-“Gap” Carboniferous tetrapods, which are far more diverse and include fully terrestrial representatives of the main crown-group lineages. It seems that “Romer's Gap” coincided with the cladogenetic events leading to the origin of the tetrapod crown group. Here, we describe a partial right lower jaw ramus of a tetrapod from the late Tournaisian or early Viséan of Scotland. The large and robust jaw displays a distinctive character combination, including a significant mesial lamina of the strongly sculptured angular, an open sulcus for the mandibular lateral line, a non-ossified narrow Meckelian exposure, a well-defined dorsal longitudinal denticle ridge on the prearticular, and a mesially open adductor fossa. A phylogenetic analysis places this specimen in a trichotomy with Crassigyrinus and baphetids + higher tetrapods in the upper part of the tetrapod stem group, above Whatcheeria, Pederpes , Ossinodus , Sigournea and Greererpeton . It represents a small but significant step in the gradual closure of “Romer's Gap”.
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