The mid-Dinantian stratigraphy of a portion of central Pembrokeshire

Sullivan, Raymond

doi:10.1016/s0016-7878(65)80031-1

Cited by 7 publications

(2 citation statements)

References 4 publications

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“…Its presence immediately west of the VND is confirmed at Blaen Onneu Quarry, where it is developed in the BOOFm, but further west of the VND it is either not exposure or eroded. A palaeokarst occurs in the same stratigraphic position in Pembrokeshire (Spalton, 1982; Sullivan, 1965), supporting the concept of regional uplift and exposure. The karst appears as a rubbly horizon up to 6 m thick consisting of interconnected, centimetre‐sized pipes and fissures filled with green clay, with rare, larger dissolution pipes.…”

Section: Clydach Valley Subgroup Overviewmentioning

confidence: 77%

“…The Clydach Valley Subgroup (CVSGp), the subject of this paper, and its westward equivalent, the Abercriban Oolite Subgroup (AOSGp), outcrops at the eastern end of the northcrop (Figure 2). Further west the AOSGp has been removed by erosion at the sub-Holkerian unconformity, although rocks of equivalent age are preserved in the northcrop extension in Pembrokeshire (George et al, 1976;Sullivan, 1965). Figure 3 summarises the lithostratigraphy of the northcrop and southcrop/Gower areas and should be used as a guide to the abbreviated lithostratigraphic unit names used in the text.…”

Section: Clydach Valley Subgroup Stratigraphymentioning

confidence: 99%

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Carbon and oxygen isotope characteristics of the Clydach Valley Subgroup, Courceyan, South Wales‐Mendip shelf, UK

Raven

2023

The Depositional Record

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The Clydach Valley Subgroup (Courceyan) records a dip section through an Early Carboniferous shallow marine, carbonate shelf and consists of three oolitic formations separated by paludal/peritidal units with abundant evidence of subaerial exposure in proximal areas. The lower part correlates with the Kinderhookian–Osagean Boundary Excursion, with allochem data indicating a minimum δ13C value for marine carbonate of +4.5‰, with associated δ18O of −4.6‰. Marine carbonate δ13C and δ18O values of +2‰ and −2‰, respectively, were estimated for the younger part. Allochem isotopic data yield a well‐defined mixing line consistent with stabilisation in meteoric water at varied water: rock ratios, the degree of stabilisation increasing up‐dip and up‐stratigraphy. Two distinct diagenetic styles closely correlate with evidence for the presence (Diagenetic Regime 1) or absence (Diagenetic Regime 2) of subaerial exposure, non‐ferroan and ferroan calcite cement dominating respectively. Five cement zones (Zones 2–6) defined by iron content occur in Diagenetic Regime 1, irrespective of formation, indicating a similar sequence of palaeohydrological changes affected repeated depositional cycles. Zones 3–5 are considered meteoric based on their isotopic composition (δ18O: −5.8 to −11.1‰ and δ13C: −3.7 to −6.2‰) and form distinct clusters dependent on zone, age and location. Pedogenic carbonates and meteoric cements record a long term increase in meteoric δ18O values: −7.8‰ during the Kinderhookian–Osagean Boundary Excursion, −6.9‰ during diagenesis of the upper part of the subgroup and −6.3‰ associated with initial deposition of the overlying Llanelly Formation. This is consistent with global sea water trends, but an element of climate change cannot be ruled out. Increases in cement δ18O values as meteoric systems become established (Zone 3–4) indicate repeated short term variation in rainwater composition probably driven by climate change, but also suggests a link between climate and the depositional cycle.

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Section: Clydach Valley Subgroup Overviewmentioning

confidence: 77%

Section: Clydach Valley Subgroup Stratigraphymentioning

confidence: 99%

Carbon and oxygen isotope characteristics of the Clydach Valley Subgroup, Courceyan, South Wales‐Mendip shelf, UK

Raven

2023

The Depositional Record

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Upper Palaeozoic Wales—a review of studies in the past 25 years

Owen

1984

Proceedings of the Geologists' Association

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The classification of Avonian limestones

George¹

1972

JGS

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In systematised stratigraphical classification 'Avonian' is a term junior to 'Dinantian'; but in present ignorance of the precise correlative equivalents of Dinantian assises and Avonian zones the term is conveniently retained for the Lower Carboniferous rocks, most of them typical 'Carboniferous Limestone', of the British South-Western Province, where there is also ignorance of the precise horizons of the Fammennian-Dinantian junction and the Dinantian-Namurian junction. At the same time, the Avonian assemblage zones have significance only in the carbonate rocks of the northern part of the Province: they are not at all applicable to the Lower Culm of the southern part, and they are applicable only with uncertainty to other Lower Carboniferous provinces in Britain. 'Avonian' is of only temporary and local validity: it is not the name of a standard series. Avonian rocks are mainly shelf limestones of a variety of kinds. They display marked lateral and sequential changes that reflect the influence of many factors, of which contemporary earth-movement, with the consequent migration of facies belts and the occurrence of sharp lithological breaks and nonsequences, was of major importance. The fossiliferous members of the series reflect subtly variant biotopes that throw light on a multiplicity of shelf environments whose characteristics and distribution have strong analogies, in associations of organisms and in community balance, with near-shore shelf sediments of many of the warmer present-day seas; and recurrent inorganic limestones (mainly calcite muds and oolites) demonstrate the general shallowness of the shelf seas over very wide areas. Detailed palaeogeographical reconstruction is possible for many of the rock groups over much of the ground. A sedimentological classification of the rocks is not readily accommodated by a usual system of parameters inherited from the classification of detrital terrigenes. Nearly all the limestones are endogenic, their calcareous constituents almost never the product of hinterland erosion or of prolonged transport even when the 'clasts' in them are abraded fragments. Grain size in such sediments is commonly an accident of origin, not a reliable sign of the restlessness of the environment of deposition: a spirifer shell is much larger than the individual crinoid plates of the bed in which it lies; a stromatolitic algal sheet is the welded product of growing algal nodules, and conversely an algal nodule may show partial disintegration into 'amorphous' algal mud; a coral bush is the containing frame of the shells it houses and of the organic debris caught in its mesh, and at the same time is itself embedded in a bank of the same kinds of shells or in wrappings of the same kinds of debris; drewite mudstone, a faecal pellet, an oolith, reflect related factors in carbonate formation. 'Matrix' also has many ambiguities: it may be an accumulate virtually in situ; it may be detritus transported some distance, or fragmented and redistributed locally; it may be mainly of monogenetic provenance, in its shelly or its algal or its evaporitic constituents, and then be coarse-grained or fine-grained, apparently well-sorted, through immediately environmental 'accident,' or it may be, equally accidentally,' unsorted or ill-sorted in grain size as an 'internal' product of a mixed biotope; may be diagenetic, 'contemporaneous' penecontemporaneous' or 'subsequent.' In a naive petrographical system mechanically applied, classification can result in the imposition of mutually incommensurable incompatible criteria in attempts to distinguish between petrological kinds resistant to the system: rocks similar on the arbitrary criteria for instance of grain size) may have little mutual affinity; rocks very different on the criteria may be closely allied. Principles the classification of limestones, as they are illustrated in the Avonian rocks, need to self-consistent and comprehensive: they cannot be adequately of use in description unless they are also genetic.

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The mid-Dinantian stratigraphy of a portion of central Pembrokeshire

Cited by 7 publications

References 4 publications

Carbon and oxygen isotope characteristics of the Clydach Valley Subgroup, Courceyan, South Wales‐Mendip shelf, UK

Carbon and oxygen isotope characteristics of the Clydach Valley Subgroup, Courceyan, South Wales‐Mendip shelf, UK

Upper Palaeozoic Wales—a review of studies in the past 25 years

The classification of Avonian limestones

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