SUMMARY Three new marine bands are recognized in the exceptionally thick Lower Namurian succession of the northern part of the Central Pennine Basin, namely the Blacko (late Pendleian), Cravenoceras gressinghamense (early Arnsbergian) and Saleswheel (early Arnsbergian) marine bands. The stratotype of the Blacko Marine Band (E 1c 2) is within delta-slope siltstone and sandstone of the Surgill Shale Member at the top of the turbiditic Pendle Grit Formation of the Blacko Borehole (near Colne). It comprises marginal marine siltstone and contains the bivalve Sanguinolites. The band is correlated with a bivalve-spat phase in the Widmerpool Gulf, and a Lingula band lying within the delta-top Grassington Grit Formation on the south-eastern part of the Askrigg Block. The Cravenoceras gressinghamense Marine Band (E 2a 2α) succeeds the Eumorphoceras ferrimontanum Marine Band (E 2a 2) in the Roeburndale and Sabden Shale formations. At its type locality, along Gressingham Beck, Hornby, near Lancaster, it has yielded the new name-bearing ammonoid. It also contains the lowest appearance of Selenimyalina variabilis (Hind) and is characterized by the presence of Posidonia lamellosa (de Koninck). In the Widmerpool Gulf, the band is represented by the Posidonia phase at two stratigraphical levels. The Saleswheel Marine Band (E 2a 2β) lies between the C. gressinghamense and Eumorphoceras yatesae (E2 a 3) marine bands. Its type locality is in the lower part of the Sabden Shale Formation at Ribchester, near Blackburn, where it contains Anthracoceras. The marine band thickens dramatically towards the Lancaster district into siltstone beds of the Close Hill Siltstone Member containing a Sanguinolites -dominated faunal phase. In the Widmerpool Gulf it is represented by a Lingula band. The dramatic thickness and facies changes of early Namurian marine bands across the Central Pennine Basin and on to the Askrigg Block appear to be controlled primarily by proximity to sediment supply, availability of accommodation space, and the duration, rate and amplitude of glacio-eustatic marine flooding events. A regional non-sequence is developed at the base of the Ward’s Stone Sandstone (and equivalents), associated with marked tectonic discordance. This boundary, together with the newly resolved marine band stratigraphy, necessitates lithostratigraphical revision and re-correlation of early Namurian sandstones in the northern part of the Central Pennine Basin and the Askrigg Block. One new species, Cravenoceras gressinghamense sp. nov., is described.
Abstract We conducted a high-resolution multi-disciplinary analysis of two core sections in the borehole Ellesmere Port-1, Cheshire, UK. Biostratigraphic analysis indicates that the core sections are Kinderscoutian and late Arnsbergian–Chokierian in age, respectively. Both cores are assigned to the Bowland Shale Formation (Holywell Shale). Coupled core scan and discrete geochemical analysis enables interpretation of syngenetic processes at a high stratigraphic resolution. Both cores exhibit the classic cyclicity of limestones, calcareous to non-calcareous mudstones and siltstones, interpreted to represent sediment deposition during fourth-order sea-level fluctuation. Machine learning of the well log data coupled to the core scan data enabled prediction of the key lithofacies through the entire Bowland Shale interval in Ellesmere Port-1. The machine predictions show that the Bowland Shale is interfingered with three turbiditic leaves of the Cefn-y-Fedw Sandstone Formation and contains at least 12 complete fourth-order cycles. The Bowland Shale exhibits high radiogenic heat productivity in comparison with other sedimentary rocks, due primarily to relative U enrichment under intermittently euxinic conditions. Thermal modelling, however, shows that the radiogenic heat productivity of the Bowland Shale contributes a negligible source of additional heat at the scale of hundreds of metres.
The following discussion concentrates on the rhythmites described by Aitkenhead & Riley (1996) from Carboniferous successions in the Central Pennine Basin. These rhythmites have a high probability of being tidal in origin as noted by the authors. The available evidence, however, makes a much stronger case than that tentatively advocated in the text. The implications of a tidal origin of such laminae have a significant impact on the interpretation of the Carboniferous depositional environments in the area of study.
1. Previous research
Within a depositional basin that has been traditionally interpreted as mainly non-marine, acceptance of marine and/or tidal influences can be initially difficult. As an example, in the early 1980s, research was undertaken on laminated facies associated with coals in the Illinois Basin (Eastern Interior Coal Basin) of central USA. Based upon the existing interpretation of basin analysis and palaeogeography, coupled with a recurring association of arthropod trackways and coals, the sequence was interpreted to be non-marine, and more specifically, lacustrine (Archer & Maples 1984; Maples & Archer 1987).
Later, these same rocks were reinterpreted as having been deposited under conditions of tidal influence (Kvale et al. 1989). Subsequently, much of the strata overlying coals have been interpreted in a similar manner (Kvale & Archer 1990). Acceptance of this was greatly aided by documentation of modern analogs in the Bay of Fundy, Nova Scotia, Canada (Dalrymple & Makino 1989; Dalrymple et al. 1991) and in the Bay of Mont Saint Michel in France (Tessier et al. 1995).
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SUMMARY The Worston Shale Group of Earp et al. (1961) is made up of four formations: the Clitheroe Limestone, Hodder Mudstone, Hodderense Limestone and Pendleside Limestone. Within these formations fifteen members are defined: the Thornton Limestone, Coplow Limestone, Peach Quarry Limestone, Bellman Limestone, Limekiln Wood Limestone. Phynis Mudstone, Whitemore Limestone, Hetton Beck Limestone, Leagram Mudstone, Buckbanks Sandstone, Embsay Limestone, Rain Gill Limestone, Chaigley Limestone, Twiston Sandstone and Rad Brook Mudstone. A particular bed within the Hodder Mudstone Formation, the Dunbarella Bed, is also recognised. Four unconformities are recognised. The most widespread is at the base of the Hodder Mudstone Formation; the others, which are more restricted, occur at the base of the Embsay Limestone Member (and equivalents), at the base of the Twiston Sandstone Member and at the base of the Pendleside Limestone Formation. Diagnostic Viséan foraminifera and conodonts first enter within the basal part of the Hodder Mudstone Formation, suggesting that the Tournaisian-Viséan boundary lies much higher than previously supposed. All the Dinantian stage boundaries in the Arundian to Asbian interval have been re-correlated. The Craven Basin is thought to have been underlain by an asymmetric graben within the pre-Carboniferous basement and the depositional history of the basin to have resulted from a combination of both syndepositional tectonics and eustatic processes. Consequently, the basinal sediments portray a transition from carbonate shelf to slope (Gawthorpe 1987), involving a change during deposition of the Clitheroe Limestone and Hodder Mudstone formations from shallow-water carbonates, generated within the basin, to deep-water hemi-pelagic sediments with limestone turbidites fed from the surrounding carbonate shelves and platforms. Intermittent supply into the basin of terrigenous sand and reworked faunas is related to the emergence and erosion of the surrounding areas.
New data from three shale gas exploration wells in the Bowland Basin of NW England contribute to the understanding of the stratigraphy, tectonic history and unconventional hydrocarbon resource potential of Lower Carboniferous strata. Three main prospective shales dominate the identified unconventional reservoirs: the Upper Bowland and Lower Bowland shales and the Hodder Mudstone, which are recognized by their distinctive lithology, corresponding log signatures and key zonal ammonoids. With a combined thickness of over 5000 ft ( c. 1500 m), this sequence of shales is one of thickest known potential self-sourced, unconventional hydrocarbon resources. The strata are organic rich with total organic carbon (TOC) values of between 1 and 7%, with an average of 2.65%, and organic maturity that ranges from the upper oil window (pyrolysis T max c . 450°C) in the higher part of the section to dry gas (R o = 2.4%; pyrolysis T max >470°C) in the Lower Bowland Shale. The sequence is strongly heterolithic, and up to 60% free gas is stored in thinly bedded carbonate and clastic silty turbidites. Adsorbed gas is concentrated in more organic-rich, hemipelagic shales which are distributed throughout the sequence. Near maximum burial temperatures of c. 130°C are inferred from vitrinite reflectance (R o ) and are consistent with fluid-inclusion microthermometry of carbonate-filled fractures. This indicates oil generation in the Late Carboniferous, prior to Variscan uplift. Renewed subsidence through the early Mesozoic resulted in increased maturity and gas generation. In the Bowland Shale the gas per unit volume of rock ranges from about 0.6 to 1.5 Bcf (billion cubic ft) per metre per square mile. The thick interval of gas-charged strata provides the opportunity to exploit these major hydrocarbon resources by using stacked multilateral wells from a common, strategically located and environmentally optimized surface pad.
SUMMARY Conodont Colour Alteration Indices (CAI) values in the Craven area show a general range of 2.5–3.5, the majority being a value of 3. The higher values generally occur in the south and south-west and low values are found on the Ashnott High (Ashnott Anticline and eastern closure of the Whitewell Anticline) and in Waulsortian limestones. These values are consistent with the limited vitrinite reflectance data and are a result of the Carboniferous sedimentary and tectonic history of the region. There is no evidence of significant modification by subsequent burial, igneous events, reheating or mineralization. Low values of CAI 2–2.5 on the Ashnott High are interpreted to be due to the thinner sedimentary cover in that area and possible insulation derived from underlying Waulsortian limestones. Relatively high values of CAI 4 for Dinantian strata in the Holme Chapel Borehole and 3.5 in the Silesian sediments to the south of the Craven Basin probably reflect a thicker Westphalian cover than further north.
Abstract The late Chadian Foel Formation, previously thought to be confined to the Dyserth area of North Wales, forms a poorly exposed but persistent basal unit to much of the Dinantian crop east of the Clwydian Range, necessitating a revision of the local lithostratigraphy. The formation comprises a peritidal heterolith which, together with the lowest few metres of the overlying Llanarmon Limestone, yields microfossil assemblages diagnostic of the Eoparastaffella Cf4 α Subzone. Succeeding strata, containing the lowest archaediscid foraminifera, provide the first record of Cf4β assemblages from North Wales and establish an early Arundian age for these beds. The Foel Formation was deposited as an aggradational sequence on the northern flank of St. George's Land during a pulsed transgression which began in late Chadian times. The widely recognized basal Arundian transgression is represented by the contact between the Foel Formation and overlying platform carbonates. The latter overlap the Foel Formation in the southernmost part of the Clwydian crop demonstrating, for the first time, southwards onlap on the northern side of the Bala–Bryneglwys Fault System.
SUMMARY The Bradup and Hag Farm boreholes were drilled by the British Geological Survey (BGS) into strata of the Upper Carboniferous Millstone Grit Group near Ilkley. They prove (beneath glacial till) a rock succession of Namurian (late Kinderscoutian to earliest Marsdenian) age including five faunal marker bands, two of which are the widespread Reticuloceras coreticulatum (R 1c 4) and Bilinguites gracilis (R 2a 1) marine bands. The other three yielded mainly benthonic assemblages and lacked diagnostic ammonoid taxa, but are tentatively assigned to the R 1c 2, R 1c 3, and Butterly marine bands. Four late Kinderscoutian cycles are identified, and a basal fifth cycle is incomplete. Each comprises a coarsening-upwards sequence from marine mudstones to prodelta-slope siltstones and sandstones, then to fluvial sandstones and other delta top deposits. Three of the fluvial sandstones have conglomeratic bases and overlie erosion surfaces which, in the third and fifth cycles, are immediately underlain by laminites comprising alternating grey sandstone and dark grey clay-rich sandstone laminae, the former varying rhythmically in lamina thickness, and the latter invariably having a thickness of 1–2 mm. Preliminary measurements suggest a possible tidal influence on the deposition of the laminites, an influence previously unrecorded in the Namurian of this part of the Central Pennine Basin.
Abstract Vaughan’s (1905) zonation of the Carboniferous Limestone in the Bristol district was a pioneer biostratigraphical study, which because of its meticulous execution can be reinterpreted in a modern context. The replacement of his scheme with a chronostratigraphical one by George et al. (1976) had a similar revolutionary affect on British Dinantian stratigraphy. However, it is now time to revise the British Dinantian stages so that they more closely correspond to biostratigraphical events. Dinantian biostratigraphy still requires considerable refinement, but it has now achieved a diversity of techniques and resolution far beyond that which was available at the time of these earlier proposals. It is the most pragmatic and closest approximation to widespread chronostratigraphical correlation available. This paper discusses these and related issues and presents a review and correlation of current biozonations.
SUMMARY This paper presents a summary of the present state of knowledge of the Dinantian (Carboniferous Limestone) rocks of the Sellafield area as a result of drilling investigations for UK NIREX Ltd. An easterly thinning wedge, formed as a result of pre-Permian erosion has a maximum onshore thickness of 300 m north of Sellafield and a feather edge about 1 km east of the Sellafield site. The Dinantian rocks rest unconformably on the Ordovician Borrowdale Volcanic Group and are overlain unconformably by Permian breccias and conglomerates, known locally as ‘Brockram’. The Basal Beds comprise a thin, impersistent succession of siliciclastic beds overlying the sub-Carboniferous unconformity. A maximum thickness of 149 m of Dinantian strata was proved in Sellafield Borehole 3. A thin development of beds of late Chadian age, the Martin Limestone, is overlain by 100m of peritidal to shallow inner ramp limestones of the Frizington Limestone (new name), of Holkerian age, and 42 m of cyclic, platform carbonates of the Urswick Limestone of late Asbian age.
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