The Nod Glas décollement of the Welsh Basin

Pratt, Warren

doi:10.1017/s0016756800022123

Cited by 6 publications

(3 citation statements)

References 8 publications

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“…Woodcock (1990) combined this with local evidence of early Ashgill tectonic activity in Wales to postulate a basin-wide unconformity in Wales marking the base of a Powys Supergroup. However, Pratt (1991) noted that sedimentation may have been continuous but reduced, at least in the central parts of the basin, and that the Nod Glas was a level of end-Caledonian d6collement, thus making the continuity or otherwise of upper Caradoc to lower Ashgill successions difficult to establish. This study suggests that the palaeontological gap represents marked biofacies variation rather than absence of strata, and that sedimentation may have been more widely continuous across the Caradoc-Ashgill boundary in Wales than has been previously thought.…”

Section: Discussionmentioning

confidence: 99%

Late Caradoc graptolitic faunal gradients across the Iapetus Ocean

1995

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Late Caradoc graptolite assemblages across the Iapetus Ocean in Wales and Scotland became progressively more disparate despite the narrowing of the ocean. We compare faunal distributions in continuous sections from opposite sides of Iapetus, at Whitland in South Wales and Hartfell Score in the Southern Uplands of Scotland. A comprehensive graptolite range-chart is given for each. The graptolite assemblages from the clingani Biozone are subdivided into a lower Ensigraptus caudatus Subzone and upper Dicellograptus morrisi Subzone at both localities, though the faunas differ in detail. Higher in the sequence, the distinctive Scottish linearis Biozone fauna is not recognizable at Whitland, its presumed equivalent being a fauna dominated by variable morphotypes of the genus Normalograptus. This suggests that environmental gradients (depth and/or temperature) were changing more rapidly than the geography. Significantly, the subsequent inception of limestone deposition at Whitland was approximately contemporaneous with widespread replacement of black mudstones by oxic, bioturbated sediments elsewhere in the Welsh Basin and in Scotland. This was possibly a response to an early phase of cooling prior to the end-Ordovician glaciation. In Wales the claimed hiatus at the Caradoc-Ashgill boundary may rather reflect biofacies variation.

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Section: Discussionmentioning

confidence: 99%

Late Caradoc graptolitic faunal gradients across the Iapetus Ocean

1995

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“…Superimposed on this gradual increase, however, is a drop between the Ceiswyn and Nod Glas formations that is confirmed by a Student r-test. It has been noted that the pyritous and carbonaceous Nod Glas Formation acted as an horizon of mild decollement and Pratt (1991) has shown that maximum accommodation took place in the overlying Broad Vein and Narrow Vein formations, especially in the Corris Anticline, resulting in high anchizonal to low epizonal slates. Grade, however, is also seen to fall along strike to both southwest and northeast within these formations, with values (see Fig.…”

Section: B the Influence Of Strain On Crystallinitymentioning

confidence: 99%

“…Observations by Jones & Pugh (1935), however, suggested that intense shearing had occurred in the Nod Glas Formation underlying the succession of the Corris Slate Belt. Decollement within the Nod Glas Formation has recently been described by Pratt (1991). Decollement within the Nod Glas Formation has recently been described by Pratt (1991).…”

Section: Introductionmentioning

confidence: 99%

The influence of strain, lithology and stratigraphical depth on white mica (illite) crystallinity in mudrocks from the vicinity of the Corris Slate Belt, Wales: implications for the timing of metamorphism in the Welsh Basin

1991

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Low and very low grade metamorphism in a mudrock-dominated succession, ranging from Upper Cambrian to Wenlock, is characterized by the white mica (illite) crystallinity technique. Kubler indices indicate that grade ranges from late diagenetic to low epizonal. The prograde reaction most easily observed by XRD study is the progressive ordering of K/Na micas; diagnostic 2M l polytypic reflections become recognizable at 0.4 A 0 26 and strengthen as grade advances. Grade generally correlates with thickness of overburden in the > 9 km thick succession. However, strain exercises additionally a profound influence on grade, and the distribution of strain was related to decollement resulting from inhomogeneities and ductility contrasts within the succession. Strain energy is seen as an additional and important variable in the development of white mica crystallinity in slates. The distribution of metamorphic grade implies that the Lower Palaeozoic succession was augmented by several kilometres of Lower Devonian (Old Red Sandstone) strata at the metamorphic culmination. The culmination was syntectonic and represents an Acadian rather than an early diastathermal event. Subsequent dextral movement on the Bala Fault displaced the regional isocryst pattern by c. 4 km. Retrogressive smectite appears to be localized to the vicinity of the Bala Fault and is probably of Variscan age.Downloaded

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Basement controls on Acadian thrusting and fault reactivation along the southern margin of the Welsh Basin

et al. 2009

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Inversion of the Lower Palaeozoic Welsh Basin during the Early to Mid‐Devonian is generally thought to have been achieved by a combination of approximately co‐axial shortening and transcurrent movement along major faults to produce a strongly partitioned transpressional strain. However, new field observations from Rhydwilym in southwest Wales reveal superimposed deformations which indicate that thrust tectonics operated within the Welsh Borderland Fault System (WBFS) along this segment of the basin margin. An increasing regional magnetic response towards the south suggests that contrasting depth to magnetic basement across the WBFS may have buttressed basin shortening and provided the focus for thrusting and late‐Caledonian or proto‐Variscan reactivation. British Geological Survey © Nerc 2009. All rights reserved.

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The Nod Glas décollement of the Welsh Basin

Cited by 6 publications

References 8 publications

Late Caradoc graptolitic faunal gradients across the Iapetus Ocean

Late Caradoc graptolitic faunal gradients across the Iapetus Ocean

The influence of strain, lithology and stratigraphical depth on white mica (illite) crystallinity in mudrocks from the vicinity of the Corris Slate Belt, Wales: implications for the timing of metamorphism in the Welsh Basin

Basement controls on Acadian thrusting and fault reactivation along the southern margin of the Welsh Basin

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