6 research outputs found

    Welsh Basin

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    The Cambrian rocks of Wales mostly lie within the Avalon composite terrane, apart from a small area of Cambrian rocks of the Monian composite terrane that is discussed in Chapter 9. The Cambrian rocks of the Welsh basin form the greater part of the Dyfed Supergroup of Woodcock (1990), the base of which overlies a widespread early Cambrian unconformity and the top of which extends to the late Tremadocian (Ordovician). The Dyfed Supergroup extends onto the Midland microcraton in attenuated form, with substantial gaps in the successions locally. The correlation of the basal parts of the Dyfed Supergroup is uncertain because of the lack of suitable evidence, but Woodcock was able to interpret the supergroup as a megasequence composed of five sequences, labelled in ascending order Ia to Ie, each separated by eustatic, tectonic or volcanic events, or a combination of these (Woodcock 1990, fig. 6). Sequence Ib includes strata assigned to the later Terreneuvian and all of Series 2; the strata of Sequence Ic are those of Stage 5 and the Drumian. The bases of sequences Id and Ie are particularly strongly marked, mainly by eustatic regressions, the former in the Guzhangian and the latter near the top of the Furongian, so that Ie is essentially composed of Tremadocian strata. Based principally on his extensive work in the eastern North American sector of Avalonia, Landing (1996) divided the Cambrian to Tremadocian successions in Avalonia into ten epeirogenic sequences (Landing 1996, figs. 2, 5). He recognized equivalents of some of these sequences in selected Welsh and English successions (Landing 1996, p. 51, fig. 7), for example the base of his Sequence 6 (which corresponds to Woodcock’s Ic) and Sequence 9 (part of Woodcock’s Id). Although some correlations are doubtful, the presence in Britain of a hiatus at the level of his Sequence 5, as suggested by Landing’s analysis (1996, fig. 7), is a possibility that merits further investigation. On the other hand, the downward extension of the Arvon ‘Slate Belt’ succession to the base of the Terreneuvian appears speculative

    A review of Cambrian lingulate brachiopods of England and Wales

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    The taxonomy of Cambrian lingulate (phosphatic shelled) brachiopods from England and Wales are reviewed in detail for the first time in nearly a century. 37 linguliformean brachiopod species, assigned to 23 genera, are described; 19 taxa are recorded in open taxonomy. Giving provisional names, three new genera, Svenjaella, Alexellus and Kayleigha are erected and nine new species are described (Chapter 4). Detailed cladistic analysis supports previous reports that Acrotretidae is a paraphyletic grouping of brachiopods which gave rise to other families within the Acrotretoidea (Chapter 6). Curticiidae is identified as lying outside the Acrotretoidea. The remaining families are supported as valid taxonomic units, albeit with some potential minor revisions. A new Scaphelasmatidae-Ceratretidae clade is consistently recovered. Support for the previously proposed subfamilies Neotretinae and Linnarssoniinae is partially recovered. The preservation of polygonal imprints of epithelial cells in lingulate brachiopods is reviewed and supplemented by new data (Chapter 7). The imprints are confirmed as representing moulds of epithelial cells as they are best preserved in areas where the shell has been thickened, and are similar in size to cells recorded in Lingula, the closest living relative to the extinct acrotretoids. Analysis of the morphology and sizes of cell-moulds demonstrates that there is no consistent relationship between cell width and valve size, and that they are not a useful taxonomic character within this group. The distribution of lingulate brachiopods across the Iapetus region is analysed using a number of statistical measures (Chapter 8). Although lingulate brachiopods have traditionally been considered to be of little utility in assessing palaeobiogeography, this analysis shows a clear palaeobiogeographic signal where lingulate faunas are sufficiently diverse, reflecting the history of the Iapetus ocean and the relative separation of Laurentia and Baltica through the Cambrian and Ordovician and provides confirmatory evidence that Palaeozoic lingulates had long-lived planktotrophic larvae.Open Acces

    Sedimentology and palaeontology of the Withycombe Farm Borehole, Oxfordshire, England

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    The pre-trilobitic lower Cambrian of the Withycombe Formation is a 194 m thick siliciclastic succession dominated by interbedded offshore red to purple and green pyritic mudstone with minor sandstone. The mudstone contains a hyolith-dominated small shelly fauna including: orthothecid hyoliths, hyolithid hyoliths, the rostroconch Watsonella crosbyi, early brachiopods, the foraminiferan Platysolenites antiquissimus, the coiled gastropod-like Aldanella attleborensis, halkieriids, gastropods and a low diversity ichnofauna including evidence of predation by a vagile infaunal predator. The assemblage contains a number of important index fossils (Watsonella, Platysolenites, Aldanella and the trace fossil Teichichnus) that enable correlation of strata around the base of Cambrian Stage 2 from Avalonia to Baltica, as well as the assessment of the stratigraphy within the context of the lower Cambrian stratigraphic standards of southeastern Newfoundland. The pyritized nature of the assemblage has enabled the study of some of the biota using micro-CT, augmented with petrographic studies, revealing pyritized microbial filaments of probable giant sulfur bacteria. We aim to produce the first complete description of the core and the abundant small pyritized fossils preserved in it, and develop a taphonomic model for the pyritization of the “small” shelly fossils

    Terrane history of the Iapetus Ocean as preserved in the northern Appalachians and western Caledonides

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    The Iapetus Ocean was the first ancient ocean to be identified following the development of plate tectonics; its history has been fundamental in relating orogenesis and plate motion. The ocean probably formed following 3-way rifting between Laurentia, Baltica, and Amazonia – West Africa (a block that became incorporated in Gondwana). Closure of the ocean trapped numerous terranes during the development of the Appalachian–Caledonide Orogen. Subsequent deformation, including late Paleozoic strike slip, transpression, and transtension, and Mesozoic stretching during Pangea breakup, must be taken into account in models for orogen development. Traditional analyses of Iapetan terranes have focussed on Cambrian sedimentary successions, and on isotopic criteria, to classify terranes into larger domains: Ganderia, Avalonia and Megumia. Detrital zircon data show that these domains did not cross the Iapetus as single entities, while paleomagnetic data reveal significant vertical-axis terrane rotations. We here review and interpret 17 paleomagnetic poles and >350 published detrital zircon data sets from the northern Appalachians and western Caledonides, using consistent and rigorous criteria for the selection and presentation of data. We place these data on an integrated stratigraphic chart to show timing relations and to seek constraints on the provenance and travel of terranes in the Iapetus Ocean. We distinguish groups of terranes that likely travelled together as terrane assemblages. In the Taconian/Grampian Orogeny, Furongian to Katian continent–arc collision involved off-margin blocks along the hyperextended Laurentian margin. In New England, early Taconian collision by 475 Ma involved the Gondwana-derived Moretown assemblage. An assemblage of the Bronson and Popelogan arc terranes probably arrived at the main Laurentian margin 25-30 Myr later. Subduction polarity reversal then led to the progressive accretion of additional terrane assemblages (Salinian Orogeny). The Miramichi–Victoria assemblage arrived close to the Ordovician–Silurian boundary. The Miramichi terrane underwent partial subduction in the QuĂ©bec re-entrant, whereas the Victoria terrane was juxtaposed with the Newfoundland promontory without major metamorphism. In mid-Silurian time, an assemblage including the Gander terrane of Newfoundland and related portions of Britain and Ireland was accreted to Laurentia, along with Baltica (Scandian Orogeny). The St. Croix – La Poile assemblage may have been accreted slightly later, but is distinguished by the development of a Silurian arc–backarc system (coastal igneous belt) above a northwest-dipping subduction zone. The Avalon–Brookville assemblage encountered this system in PƙídolĂ­ to Middle Devonian time (Acadian Orogeny), leading to the collapse of the backarc basin and northwest-vergent thrust emplacement onto Laurentia during sinistral transpression in the Appalachian Orogen. Acadian deformation involved mainly sinistral strike slip in Britain and Ireland. Several of the terranes that were accreted to the Laurentian margin carried internal records of earlier deformation that took place near Amazonia – West Africa in Early Ordovician time and earlier (Monian/Penobscottian Orogeny). The Iapetus Ocean thus contained a complex array of terranes, small ocean basins, arcs, and previously emplaced ophiolites analogous to modern southeast Asia. It closed to form a complex array of sutures in an orogen within which no single Iapetus suture can be clearly identified

    Sedimentology and depositional environment of the middle Cambrian Manuels River Formation in the type locality at Conception Bay South, Newfoundland, Canada

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    A detailed bed-by-bed study of fossiliferous, marine, grey to black shales of the middle Cambrian (Drumian, Cambrian Series 3) Manuels River Formation along its type locality in Newfoundland, Canada, was carried out and provides insight into the sedimentology, depositional environment, palaeogeography and climatology of the area. The formation was deposited on the microcontinent Avalonia, the largest terrane of the peri-Gondwanan realm situated in temperate latitudes. The partly organic-rich (up to 3.36 % TOC) shales of the succession were deposited mainly conformably under dysoxic conditions. Deposition was controlled by two shallowing trends and occurred below, near and above mean storm- wave base. The contact of the top of the rst shallowing-upward trend in the succession is conformably, whereas the second shallowing-upward trend is truncated by an erosional unconformity, forming the top of the formation. An average sedimentation rate of c. 3.8 m/ Ma is estimated. The clay-mineral composition and illite crystallinity suggest a subsidence of approximately 5.7–6.3 km and a burial temperature of c. 280 °C. The low chlorite/kaolinite ratios indicate a mainly warm semi-humid climate setting at 35°–65° S, with sudden changes to a semi-arid setting. The subsidence, sedimentation rate and depositional environment suggest an Early Ordovician Avalonia-Gondwana separation

    Fossils in the Olenellus Sandstone of Nuneaton

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