Tournaisian, Dinantian, Carboniferous, Wales

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From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.

Dinantian of south Wales, schematic cross-section (adapted from Wilson et al. 1990). P916183.
Vertical sections illustrating the Dinantian of the east crop, south Wales (adapted from Barclay, 1989 and Wright 1996). P916184.
Green Bridge of Wales: well bedded limestone with chert (Arundian) overlain by thick-bedded pale grey limestone (Holkerian) (MFH P662424).
Dinantian palaeogeography (adapted from Freshney and Taylor 1980). P916180.

Tournaisian strata are restricted to south Wales, where predominantly continental Upper Old Red Sandstone strata are overlain by largely marine strata of the Avon Group (previously the Lower Limestone Shale Group, and, in Gower, the Cefn Bryn Shales) at the base of the Carboniferous sequence. These shallow marine shelf and shoreline carbonates record a northward-directed marine transgression on to the landmass, and they form an important transition into the succeeding carbonate sequence. The strata reach a maximum thickness of about 145 m in the south crop, thinning to a few metres in the north; the base of the group becomes younger in the same direction.

The Avon Group contains a variety of carbonate and terrigenous lithologies that have been ascribed to three depositional environments: barrier shoal, lagoon and shelf embayment. The barrier shoal association comprises ooidal and skeletal grainstones deposited in a turbulent tidal to near-shore setting. Skeletal fossil debris is abundant, but trace fossils are rare. The lagoonal lithologies include thinly bedded ooidal-skeletal and peloidal packstones with abundant algae and a rich, but low-diversity fauna dominated by Modiolus; evaporite pseudomorphs and desiccation cracks are common. The facies reflects deposition in a shallow-water low-energy environment, probably high intertidal to shallow subtidal. In contrast, the shelf embayment association of grey mudstone and argillaceous limestone is characteristic of a low-energy, but deeper offshore environment. The calcarenites of the barrier shoal association sheltered the lagoon on the landward side from the deep water shelf embayment association on the seaward side.

Between the Vale of Glamorgan and Newport, the lowest part of the group comprises ooidal, peloidal, skeletal and sandy limestones, and interbedded mudstones with some calcretes and ironstones (Tongwynlais Formation). The accumulation was dominated by northward-directed palaeocurrents, in contrast to the southward-directed systems of the underlying fluvial sandstones of the Upper Old Red Sandstone. The areal and sequential patterns of the lithologies reflect four shoaling-upward sedimentary cycles, in environments ranging from coastal plain, peritidal, lagoon, embayment and barrier to open marine (P916183). Broadly, the cycles represent progressive deepening and the northward retreat of a barrier shoreline.

To the west, into south Pembrokeshire, the Avon Group has not yet been subdivided, but it is represented in the upper part of the clastic facies of the Skrinkle Sandstone Formation. However, towards the east crop, the Tongwynlais Formation is progressively and completely onlapped by the Castell Coch Limestone Formation south of Pontypool. The thick- and commonly cross-bedded, skeletal and ooidal grainstone is well exposed in the Taff Gorge. The skeletal debris is mainly brachiopod fragments, crinoid ossicles and bryozoan fragments. Red staining is locally intense and is generally associated with late-stage epigenetic dolomitisation. North of Pontypool, where the limestone rests directly on the Upper Old Red Sandstone, a basal pebble conglomerate (a littoral lag deposit) is overlain by transgressive littoral ooidal and skeletal calcarenites, which are correlatives of the middle and upper Tongwynlais Formation to the south, and part of the same transgression. Overall, the Castell Coch Limestone represents an extensive belt of subtidal to intertidal shoals, up to 10 km wide and over 90 km long. A strong tidal influence prevailed across its width and, particularly in the south crop, there was a distinct eastwards to north-eastwards longshore component. At the same time, to the east of the coalfield, along the Wye valley, an ooidal barrier reformed and advanced south-eastwards.

At the top of the Avon Group, dark grey to black, silty and micaceous mudstone, with subordinate thin, bioclastic graded limestones and calcareous siltstones (Cwmyniscoy Mudstone Formation) form a distinct depression east of the Vale of Glamorgan. The sequence thins from about 40 m to some 20 m, and marks a return to deeper water on the shelf, below fair weather wave base; graded bioclastic limestone beds reflect periodic storm events. The formation closely resembles the upper part of the Tongwynlais Formation, and where the Castell Coch Limestone is poorly developed they are difficult to separate.

The uppermost Tournaisian to lower Visean limestones (Black Rock Limestone Subgroup) at the base of the Pembroke Limestone Group, form a prominent feature in Gower and the Vale of Glamorgan, where they comprise two thick bioclastic units separated by an oolite. The group thins from about 500 m to some 120 m across the Vale of Glamorgan. At the base, thin bedded, coarse- to fine-grained, graded skeletal packstones (Barry Harbour Limestone Formation) indicate the continuation of the storm-influenced shelf conditions. Higher in the sequence, the mudstone content diminishes, suggesting a higher energy environment. To the north of the Vale of Glamorgan axis, the lithofacies is dolomitised and overlain by fine-grained, grey, laminated and cross-laminated secondary dolomite, suggesting a more restricted environment. These lithofacies patterns, indicating shallowing into more proximal settings, can be traced into eastern Gower (Shipway Limestone).

Throughout the south-east outcrop, the early sedimentation culminated in the first extensive southward-prograding oolite shoal complex (Brofiscin Oolite Formation) comprising grey, well-sorted ooid grainstones, locally with skeletal grainstones and well developed cross-stratification. The oolite thins from the north side of the Vale of Glamorgan into the Gower where it is represented by a 3 m-thick, cross-bedded bioclastic grainstone with thin ooidal laminae. The overlying limestones (Friars Point Limestone and Tears Point Limestone) are thickly bedded, dark grey, foetid, argillaceous, richly fossiliferous skeletal packstones with shaly partings. Intense bioturbation imparts an irregular, nodular appearance to many of the beds. They range from over 400 m on the coast about Barry to 85 m on the north limb of the Cowbridge anticline, and these limestones are thought to have accumulated below storm wave base. The top of the sequence is extensively dolomitised and was previously referred to as the ‘Laminosa Dolomite’. Several phases of dolomite formation have been determined and the earliest was probably related to penecontemporaneous emergence. In the east crop and in the Monmouth and Chepstow area, the entire thickness is dolomitised, probably the result of uplift in the hinterland causing continental and marine waters to mix in the subsurface.

Across the north crop of the coalfield there is a major break within, or an overall thinning of, the uppermost Tournaisian sequence. About Merthyr Tydfil a variable group of predominantly massive ooidal grainstones and thinly bedded dolomite (Abercriban Oolite Formation) overlies the Avon Group. To the east, as in the Clydach valley, dolomitised and non-dolomitised rocks interfinger, and farther along the east crop, the sequence is totally dolomitised. Here, the pervasive dolomitisation postdates both Variscan deformation and stylolite formation, and it has been ascribed to movement of phreatic water down a hydrological gradient in proximity to the Usk Axis. Along the north-east crop, the sequence has been referred to the Clydach Valley Subgroup with many depositional breaks, some of which are marked by a palaeokarstic surface with an overlying palaeosol and massive and nodular calcrete horizon (P916184). In the Heads of the Valleys road section in the Clydach valley, the lowermost Sychnant Dolomite Formation is capped by a palaeokarstic surface and is overlain by an ooidal sequence with four bioclastic units. The lowermost Pwll y Cwm Oolite Formation is a pale grey, cross-stratified ooidal grainstone, rich in shelly debris, and the Blaen Onnen Oolite Formation is a massive grey grainstone. The oolites are separated by thin- to medium-bedded, cyclic limestone packages (Pant y darren Formation), which display five major lithofacies reflecting typical shallowing-upwards peritidal cycles. The Coed Ffyddlwn Formation, overlying the oolites, comprises fine-grained, thinly bedded dolomite with concentrations of shell and crinoid debris; it is separated from the overlying Gilwern Oolite Formation by an erosion surface. The abundance of palaeokarstic surfaces across south Wales suggests fairly broad regional uplift, which was possibly controlled by movement within the Severn Valley Fault Zone. The Tournaisian–Visean boundary probably lies close to the top of the Blaen Onnen Oolite Formation.

The broad lithological divisions of the Black Rock Limestone Subgroup can be traced into the Gower (formerly the Penmaen Burrows Limestone Group that consisted, in upward succession, of the Shipway Limestone, Tears Point Limestone and Langland Dolomite). Farther west, in Pembrokeshire (P662424), the sequence is considerably thicker and, as yet, the subdivisions have not been determined. The dominant lithofacies, as exposed in the vicinity of Bosherton and Linney Head on the south limb of the Pembroke Syncline, comprises up to 450 m of bioturbated, well-bedded bioclastic packstone and wackestone. The richly fossiliferous limestones, with crinoid, bryozoan and brachiopod debris, are typical of outer shelf environments. They are overlain by dolomitised build-ups of carbonate mud (formerly referred to as Waulsortian reef mound limestones) of the Berry Slade Formation, up to 150 m thick, which is not included in the Black Rock Limestone Subgroup. The carbonate mud formed a belt, roughly parallel to the depositional strike, which extended into west Somerset and farther into Belgium (P916180). The carbonate mud build-up consists of bioclastic calcareous mudstone, packstone and vuggy limestone; it is comparable with the downslope biohermal and biostromal constructions in distal, deep-water ramp settings. The succession accumulated in water deep enough to be unaffected by any of the relative changes in sea level that influenced sedimentation farther east. On the north limb of the syncline in south Pembrokeshire, the limestones thin markedly into a mixed neritic, nearshore sequence and display the first indications of the successive overstepping of strata, which so dominated the subsequent Lower Carboniferous sedimentation.

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