Namurian, Silesian, Carboniferous, Wales
From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.
Across south Wales, Namurian strata form a persistent outcrop about the main basin and westwards into Pembrokeshire. The sequence (P916187) represents a fluviodeltaic to basinal succession which overstepped on to older rocks, for example the Ordovician strata near Haverfordwest. Towards the eastern edge of the basin, along the north crop (P662427), the sequence progressively oversteps the north–south-aligned Dinantian zones, reflecting contemporaneous activity along the Usk Axis, which was probably emergent for most of Namurian times. The southern limit of the basin is now concealed below the Bristol Channel. In north Wales, the succession accumulated at the south-western margin of the Pennine Basin; it forms a narrow outcrop between the Dee estuary and Oswestry. Chronostratigraphical classification and correlation is based on the marine bands that define the stage boundaries in the Pennine Basin (Table 5).
In south Wales, the Namurian Millstone Grit sequence, now named the Marros Group, is most complete in Gower where it is represented by some 800 m of mainly prodelta mudstone. To the north and east, the sequence thins due to non-deposition, unconformities and the proximity of the basin margin. It is thinnest in the east crop, near Pontypool (P916188), where about 20 m of fluvial quartz sandstones with palaeosols and one or two marine mudstones are present. On the north crop, the sequence is dominated by deltaic braidplain deposits that prograded southwards into the basin. Contemporary movement on faults contributed to facies and thickness variations that were determined mainly by post-rift thermal subsidence of the basin. Uplift along the Carreg Cennen Disturbance (P916176) caused emergence of the Penlan Axis, which separated the main basin from the Pembrokeshire sub-basin.
Changes in the sea floor environment from Dinantian to Namurian times resulted in progressively more muddy conditions, and bivalves and ammonoids replaced the coral-brachiopod fauna. However, the fluvial to shallow marine sandstones are largely unfossiliferous apart from drifted plant remains. In the shallow water, close to the basin margins, calcareous shales contain bivalves, gastropods, brachiopods (orthids, chonetids, spirifers, productids, common mud burrowing Lingula and Orbiculoidea cf nitida), few bryozoans and rare zaphrentoid corals. In the upper part of the sequence, shales associated with thin coal seams contain freshwater and brackish water mussels (Carbonicola lenicurvata, Anthraconaia cf. bellula), and indicate the initiation of the swampy conditions that became such a characteristic feature of later, Westphalian times. The designated marine bands refer to those beds with a noticeably enriched fauna, and the distribution of these bands demonstrates most clearly the expansion of the basin throughout the Namurian, with only the upper (Marsdenian and Yeadonian) stages present in the north and east (P916187). However, even in Gower, where the succession is most complete, the basal parts of some stages are missing. In the context of sequence stratigraphy, the marine bands represent high-stand flooding surfaces induced by sea level rise. The maximum flooding surfaces are marked by a concentration of thick-shelled ammonoids, calcareous brachiopods and limestones. On either side of these surfaces the character of the faunal assemblages is determined by salinity and water depth which are broadly influenced by distance from the shore. The determined phases with increasing depth are:
1 Planolites opthalmoides
3 Mollusc spat
4 Anthracoceras or Dimorphoceras
6 Calcareous brachiopod
On this basis, three broad biofacies belts, Lingula biofacies, shelly benthic biofacies and the ammonoid-pectinoid biofacies, have been determined (P916189). Thus many of the marine bands of the north crop, closest to the landmass, are of Lingula biofacies, those in Gower and south Pembrokeshire are of the ammonoid-pectinoid biofacies, and a zone of shelly benthic biofacies occurs in between.
The Marros Group comprises a basal quartzitic sandstone (Twrch Sandstone Formation) overlain by a mudstone-dominated sequence (Bishopston Mudstone Formation) and an upper lithic sandstone (Telpyn Point Sandstone Formation; (P916187)). The lowest Namurian strata (Aberkenfig Formation) were encountered in the Kenfig Borehole near Bridgend and comprise dark grey mudstone and black chert with some sandstones (35 m). The base of the group is markedly diachronous. At Ragwen Point in Carmarthen Bay conodonts (including Gnathodus bilineatus, G. girty simplex, G. girty subspindit and G. commutatus) found in septarian nodules that occur about 20 m above the base suggest a Pendleian (E1) age. In the Bridgend area there is a non-sequence, which is marked by pebbly lags of chert intraclasts, phosphatic nodules, abraded bone fragments and coprolites. The Pendleian (E1) stage is absent, and the lowest recorded faunas include Posidonia corrugata, G. bilineatus and Cravenoceras nitidus, indicating a mid- to late-Arnsgergian age (E2b2–E2b3). The magnitude of the unconformity increases northwards and eastwards, the basal Namurian strata becoming younger as the underlying Dinantian strata become older. The oldest strata on the north-east crop are of late-Marsdenian (R2) age. The top of the group lies at the base of the Gastrioceras subcrenatum Marine Band, which is recognised throughout south Wales (P662428); (P662429).
At Bishopton, in Gower, basal radiolarian chert layers, with Cenosphaera, Carposphaera, Cenellipsis and Sphaerozocum, are local equivalents of the Aberkenfig Formation, as are strata formerly referred to as the Plastic Clay Beds between Drefach and Llygad Llwchwr on the north crop. The age of these beds may range from Brigantian to Arnsbergian, although the lowest proven Namurian zone is E2b in shales within the overlying Twrch Sandstone Formation near Capel hir bach in Carmarthenshire and some of the Plastic Clay Beds may be decalcified Brigantian.
The Twrch Sandstone Formation, formerly the Basal Grit, is thickest, about 190 m, in the Twrch valley and it ranges in age from Pendleian (E1) to Marsdenian (R2). It consists mainly of quartzitic sandstone with conglomerate and thin beds of mudstone and siltstone. Pebbly sandstone with clasts of Dinantian limestone are common along the north crop and are locally clean and mature; some beds of orthoquartzite contain 98 per cent detrital quartz and have been worked for refractory material. The coarsest quartz sandstones fine upwards, and are deposits of braided, fluviodeltaic distributary channels. The channels became progressively more clearly developed through Namurian times and a localised southerly source is indicated in the sediments in the Aberkenfig–Margam sector of the south crop (P916190). Mudstone beds record freshwater lacustrine or floodplain deposition, or in some cases marine incursions. Some fine-grained, well-sorted quartz sandstones coarsen upwards or, more commonly, have sharp bases with hummocky and swaley cross-stratification and parallel to low-angle cross-stratification; these beds have been interpreted as storm-dominated delta front and shoreface deposits. The quartzitic rocks form prominent escarpments about the coalfield, and especially where the Upper Old Red Sandstone thins westwards in the Carmarthenshire Fans.
The most detailed facies and environmental interpretation of the sequence is based on the well exposed coastal cliff sections, such as those at Marros Sands and Tenby, in Pembrokeshire. Close to the south end of Ragwen Point, the Twrch Sandstone Formation overlies limestones interbedded with calcareous shales (Brigantian). The contact is erosional, although conodonts from septarian nodules in the overlying sandstones indicate an early Namurian (Pendleian E1) age and a non-sequence rather than a disconformity at the base of the sequence. The occurrence of mature quartz sandstones and marine mudstones suggests a littoral setting, but the abundance of palaeosol horizons, with roots and trunks of giant club mosses, as exposed at Ragwen Point, are indicative of more continental conditions.
The Bishopston Mudstone Formation, formerly the Middle Shales, consists predominantly of mudstone with minor quartz sandstones. It is thickest in Gower, where at least 20 marine bands have been recorded at Barland Common, Bishopston. However, it is most clearly exposed in the cliff sections at its type locality in Marros Sands, south Pembrokeshire, which have been the subject of detailed sedimentological analysis. Three major coarsening-upwards deltaic sequences have been determined; the lowest sequence is capped by distributary channel sandstones. The cycles consist mainly of interbedded mudstone and siltstone, with Lingula and Sanguinolites, deposited in brackish to marine distributary bays. Fine-grained sandstones represent a range of mouth bar and coastal barrier environments. The marine transgression at the base of the third cycle is marked by the Gastrioceras cancellatum Marine Band, which is widespread in south Wales and indicates a major expansion of the basin. Two thin, but widespread quartz sandstones, the Twelve Feet Sandstone and the Cumbriense Quartzite, occur above and below the marine band in the north-east of the main basin. The latter has been interpreted as a storm-influenced, regressive barrier sand complex.
The Telpyn Point Sandstone Formation in Pembrokeshire is one of many sandstones in south Wales that have been named the Farewell Rock. It has also been termed the Upper Sandstone Group and it may be the equivalent of the Llanelen Sandstone in north Gower. It is well exposed at a number of coastal localities, for example west of Telpyn Point, at Settling Point, south of Broadhaven, in the core of a syncline at First Point and in the core of an anticline, at Waterwynch Bay, north of Tenby. The formation comprises a fining-upward, dominantly fluvial distributary channel sequence. The basal, massive, medium-grained sandstones display internal erosional surfaces marked by lag concentrates and casts of Calamites, and local soft-sediment deformation structures. The overlying sandstones show more typical trough cross-stratification, and are overlain, in turn, by fine-grained sandstones with well developed ripple lamination showing a wide variation in palaeocurrent orientations. Correlation of this fining-upwards pattern along strike has identified an incised north-north-west-trending valley fill with a sharply erosional base that cuts out both the Cumbriense and Anthracoceras marine bands to the west of Cwm Pedol (P916191). Up-sequence, the channels are better preserved, and evidence of decreasing stream energy has been attributed to decreasing channel slope as relative sea level rose. This trend culminated in the deposition of the upper siltstone as a channel plug. The palaeosol cap to the fluvial sequence indicates that the valley was filled prior to the flood at the start of the transgression.
To the east of Afon Twrch, the erosional unconformity at the base of the channel extends on to the interfluve, a terrace-like emergent surface beyond the incised valley. On the interfluve, the palaeosol overlies the Cumbriense Marine Band and consequently its base defines a marked basinward facies shift. The widespread fluvial incision surface and the laterally correlative interfluve surface is regarded as a sequence boundary, which can be related to a similar surface in the Namurian sequence in northern England and southern Ireland.
In Pembrokeshire, large-scale cross-stratification indicates that in the north crop the derivation was from the south whereas in the south crop the flow was dominantly towards the east.
This disparity in the direction of the fluviodeltaic systems may be the result of their separation by an active east–west tectonic zone along the line of the Ritec Fault. In response to late Namurian (Yeadonian) uplift, the deltas advanced to the south and east, and their channels deeply incised the shelf and delta slope deposits of the Bishopston Mudstone Formation. As in the main basin, the top of the sequence is marked by the widespread Gastrioceras subcrenatum Marine Band at the base of the Westphalian. However, in the main basin, sandstones of similar lithology occur above the marine band (P916191), which suggests that the locus of delta deposition migrated to the north-east.
In Namurian times, north Wales lay at the south-western edge of the Pennine Basin on the north side of the Wales–Brabant Massif. Currently, the only remnant of the Namurian sequence lies on the east side of the Dinantian outcrop, from near Prestatyn in the north to Oswestry in the south (P916192). The facies changes across this outcrop, from dominantly sandstone in the south (Cefn y Fedw Sandstone Formation) to predominantly mudstone (Bowland Shale Formation) overlain by lithic sandstone (Gwespyr Sandstone Formation) in the north. All the succession, from E1 to G1, is present, but the base is markedly diachronous, with E1 strata probably present but unproved by ammonoids in basinal areas around Holywell and Flint, E2 beds overlying Carboniferous Limestone around Hope Mountain, and G1 beds forming the base at Rhydymwyn.
At the base of the sequence, the Pentre Chert Formation comprises banded, glassy to impure granular chert with siliceous mudstone and siltstone. A few thin beds of variably silicified, white to light brown, fine- to medium-grained, locally pebbly quartzose sandstone and crinoidal chert also occur. In the north, the formation rests with minor unconformity on the Brigantian Teila Formation, and thickens southwards to rest unconformably on the Brigantian Cefn Mawr Limestone. Within cycles, beds thicken upwards, and the amount of terrigenous material, both as mudstone beds and as a contaminant within the cherts, decreases. A sparse macrofauna includes crinoid, productid and rhynchonellid debris and thin-shelled ammonoids. The chert and mudstone accumulated from suspension in moderately deep water below storm wave-base; the thin sandstone and crinoidal beds are probably storm turbidites. The age of the formation is constrained by the presence of Sudeticeras sp. (splendens/stolbergi group), suggesting of the late-Brigantian P2b Subzone, in the top of the underlying Cefn Mawr Limestone, and the Cravenoceras malhamense Marine Band (E1c1) in the overlying Bowland Shale Formation. This suggests an age no younger than the E1b Subzone.
The Cefn y Fedw Sandstone Formation comprises quartz sandstone, pebbly in places with thin beds of quartz conglomerate, interbedded with subordinate mudstone, siltstone, siliceous mudstone and chert, arranged in coarsening upwards cycles. The grey sandstone is well sorted with low-angle hummocky and cross-stratification; Zoophycus and Rhizocorallium are locally abundant on bedding planes. The formation is thickest north of the Bala Fault, which probably influenced the distribution of the fluvial systems, and here it is represented by three sandstones within the Bowland Shale Formation, The lower sandstone is mainly Pendleian, the middle mainly Chokierian to Alportian, and the upper, early Kinderscoutian to late Mardsenian. To the south, the sandstones merge into a single unit. The formation reflects a fluviodeltaic system, sourced in the Wales–Brabant Massif, which prograded northwards into the Pennine Basin. The Bowland Shales Formation (formerly the Holywell Shales) consists of mudstone with thin beds of fine- to coarse-grained sandstone, ganister (siliceous seatearth) and some thin coals; most of the ammonoid zones have been distinguished. The dark grey mudstone is finely micaceous, fissile to massive and weathers distinctively to red brown. Black, highly fossiliferous marine beds, up to several metres thick, are separated by beds that are largely unfossiliferous except for fish and plant remains. Sideritic ironstone occurs locally as layers and nodules as well as small phosphatic nodules. Palaeosols with sideritic rootlets underlie rare thin coals. The thin, fine- to medium-grained, quartzose and micaceous sandstones display flaser bedding and bioturbation in places. Carbonaceous debris is both disseminated and concentrated in laminae. The lowest marine band determined is the Cravenoceras malhamense (E1c) Marine Band and all the succeeding stages are present. In the Flint area, the top of the formation is marked by the Gastrioceras cumbriense Marine Band. The formation represents deltaic to prodelta deposition with a marine signature becoming more evident to the north.
The southerly derived influx of sands diminished progressively through Namurian times, and in the late Namurian there was the first influx of sand (Gwespyr Sandstone Formation and the Aqueduct Grit of Ruabon Mountain) derived from the north-east. The brown, fine-grained, variably feldspathic and micaceous sandstones are characterised by large-scale tabular cross-bedding, but low-angle and hummocky cross-stratification also occur. Thinly interbedded sandstone and mudstone with thin coals and seatearths were deposited in delta top lagoons and interdistributary bays. Deltaic sandstones interdigitate with prodelta mudstones of the Bowland Shales Formation.
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