SUMMARY The Stockdale Rhyolite Member of the Coniston Limestone Formation is exposed discontinuously for 13 km from Kentmere to Shap in the eastern Lake District. It occurs within marine sedimentary rocks of Cautleyan age (Ashgill Series). The member consists of a maximum of 185 m of pink to pale grey, massive to intensely fractured, platy-jointed, flow-banded and flow-folded rocks of rhyolitic composition, locally underlain and overlain by impersistent beds of clastic rocks composed largely of pumice and felsic fragments. The Stockdale ‘Rhyolite’ has previously been interpreted as a line of coeval lava flows, but outcrop length to thickness ratios, textural characteristics and associated sedimentary rocks are more diagnostic of it being a rheomorphic ignimbrite.
The Visean stage of the Mississippian was a time of rapid tetrapod diversification which marks the earliest appearance of temnospondyls, microsaurs and the limbless a€ ıstopods.Tetrapod finds from this stage are very rare and only a dozen sites are known worldwide.Here we announce the discovery of a new Visean site in Fife, Scotland, of Asbian age, and from it describe a new species of the baphetoid Spathicephalus.These specimens represent the oldest known baphetoid by three million years, yet belong to the most specialized members of the clade.Unlike typical baphetoids with large marginal teeth and palatal fangs characteristic of early tetrapods, spathicephalids had very broad flattened heads with a dentition consisting of a large number of small, uniform teeth.Spathicephalids were probably one of the earliest tetrapod groups to use suction feeding on small, aquatic prey.Palynological and sedimentological analysis indicates that the new fossil bed was deposited in a large, stratified, freshwater lake that became increasingly saline.
The southern margin of the Askrigg Block around Cracoe, North Yorkshire, shows a transition from carbonate ramp to reef-rimmed shelf margin, which, based on new foraminiferal/algal data, is now constrained to have initiated during the late Asbian. A late Holkerian to early Asbian ramp facies that included small mudmounds developed in comparatively deeper waters, in a transition zone between the proximal ramp, mudmound-free carbonates of the Scaleber Quarry Limestone Member (Kilnsey Formation) and the distal Hodderense Limestone and lower Pendleside Limestone formations of the adjacent Craven Basin. The ramp is envisaged as structurally fragmented, associated with sudden thickness and facies changes. The late Asbian to early Brigantian apron reefs and isolated reef knolls of the Cracoe Limestone Formation include massive reef core and marginal reef flank facies, the latter also including development of small mudmounds on the deeper water toes of back-reef flanks. The position of the apron/knoll reefs is constrained to the south (hangingwall) of the North Craven Fault, but it is syn-depositional displacement on the Middle Craven Fault that accounts for the thick reefal development. Subsequent inversion of this structure during the early Brigantian caused uplift and abandonment of the reefs and subsequent burial by the Bowland Shale Formation.
Abstract Spatially and temporally variable Tournaisian to Namurian Carboniferous fluvial, fluvio-deltaic, platform carbonate and shale-dominated basin sedimentary successions up to 3.5 km thick are preserved in a complex series of basins from the Outer Moray Firth (Quadrant 14) to the Silverpit Basin (Quadrant 44). Differences in stratigraphic nomenclature in the areas surrounding the Mid North Sea High and onshore, combined with sparse biostratigraphic data, have hindered the systematic regional understanding of the timing and controls on stacked source and reservoir rock intervals. Over 125 well reinterpretations, tied to seismic interpretations, provide evidence of the inception and extent of a delta system. Regional time slices highlight a long-lived laterally equivalent basinal, mud-rich succession across Quadrants 41–44. They also show that the area from the Outer Moray Firth to the Silverpit Basin was part of the same sedimentary system up to at least Namurian times. All of this is placed within a simplified stratigraphic framework.
The only two known graphite vein-deposits hosted by volcanic rocks (Borrowdale, United Kingdom, and Huelma, Southern Spain) show remarkable similarities and differences. The lithology, age of the magmatism and geodynamic contexts are distinct, but the mineralized bodies are controlled by fractures. Evidence of assimilation of metasedimentary rocks by the magmas and hydrothermal alteration are also common features to both occurrences. Graphite morphologies at the Borrowdale deposit vary from flakes (predominant) to spherulites and cryptocrystalline aggregates, whereas at Huelma, flaky graphite is the only morphology observed. The structural characterization of graphite indicates a high degree of ordering along both the c axis and the basal plane. Stable carbon isotope ratios of graphite point to a biogenic origin of carbon, most probably related to the assimilation of metasedimentary rocks. Bulk į13C values are quite homogeneous in both occurrences, probably related to precipitation in short time periods. Fluid inclusion data reveal that graphite precipitated from C-O-H fluids at moderate temperature (500 oC) in Borrowdale and crystallized at high temperature from magma in Huelma, In addition, graphite mineralization occurred under contrasting fO2 conditions. All these features can be used as potential exploration tools for volcanic-hosted graphite deposits.
Abstract Extensive evaporites in Lower Mississippian successions from palaeoequatorial regions are commonly used as evidence for an arid to semi‐arid palaeoclimate. However, in this article, detailed studies of evaporites and their context refute this interpretation. Detailed sedimentological and petrographical analysis of the Lower Mississippian of northern Britain, is combined with archived log data from more than 40 boreholes across southern Scotland, northern England and Northern Ireland, and published literature from Canada. Two key cores from the Tweed Basin and the northern margin of the Northumberland – Solway Basin contain 178 evaporite intervals and reveal twelve distinct forms of gypsum and anhydrite across seven facies that are associated with planar laminated siltstone and intercalated thin beds of ferroan dolostone. Nodular gypsum and anhydrite, typically in intervals <1 to 2 m thick, are integral components of the succession. Nodular evaporite occurs within about 1 m of a palaeosurface, but most evaporite deposits represent ephemeral brine pans to semi‐permanent hypersaline lakes or salinas on a floodplain that was subjected periodically to storm surges introducing marine waters. Formation of evaporites under a strongly seasonal climate in a coastal wetland is supported by palaeosol types and geochemical proxies, and from palaeobotanical evidence published previously. Although 65% of modern equatorial areas experience a strongly seasonal climatic regime, salinas and sabkhas are a minor component today in comparison with the evidence from these Lower Mississippian successions. This implies that the earliest terrestrial environments were complex and dynamic, providing a diverse range of habitats in which the early tetrapods became terrestrialized and represent a setting that is rarely preserved in the geological record.
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