Richard A. Waters

A revised correlation of Carboniferous rocks in the British Isles

The report revises and expands upon the 1976 and 1978 publications for the Dinantian and Silesian, respectively, combining them into a single account of British and Irish Carboniferous stratigraphy. The need to update the two Special Reports reflects the considerable advances in Carboniferous geology over the last 30 years. The report covers developments in international chronostratigraphy and incorporates wholesale reassessments of British lithostratigraphy. A huge volume of biostratigraphical information has been published over recent decades and the report summarizes the key information. Carboniferous rocks have long been of economic importance, but it is the search for hydrocarbons, in its infancy at the time of the previous reports, which has greatly increased our understanding of Carboniferous successions offshore and at depth, particularly in southern and eastern England.

Heavy minerals as a guide to turbidite provenance in the lower Palaeozoic southern Welsh Basin; a pilot study

A pilot study has demonstrated that heavy mineral analysis is a useful guide to the provenance of Silurian turbidites in the Southern Welsh Basin. The results confirm the sedimentological evidence for two distinct source areas of coarse clastic detritus, one lying to the south and the other to the east. They also provide mineralogical criteria by which the two source areas may be distinguished. The southern area provided material with relatively low mineral diversity, and is characteristic in having low rutile/zircon ratios, whereas the eastern source provided more diverse assemblages, generally with high rutile/zircon ratios. The southern source shows variations in terms of apatite/tourmaline ratio, with the older Aberystwyth Grits Group tending to contain relatively low apatite compared with the younger Cwmystwyth Grits Group (Rhuddnant and Pysgotwr Grits formations). There is evidence for polycyclic material and volcanic detritus in both southerly and easterly derived samples; however, easterly-sourced sandstones apparently tapped a more lithologically-diverse terrain.

Structural controls on Upper Palaeozoic sedimentation in south-east Wales

Marked thickness changes in early Dinantian (Courceyan-Chadian) sediments occur across the Cardiff-Cowbridge Anticline, a major east-west Variscan fold in the Vale of Glamorgan. They resulted from differential subsidence over an active concealed basement fault zone, the Vale of Glamorgan Axis, which is coincident with the hinge of the Anticline; the latter formed in response to inversion on this basement fault. A Caledonian ancestry for the Vale of Glamorgan Axis is indicated by pronounced Upper Old Red Sandstone overstep across it. Across the Severn Estuary, between South Wales and the Bristol-Mendips region, there is a significant offset of isopachs and facies belts in both the Courceyan-Chadian and Arundian sequences. It is suggested that these result from intra-Carboniferous dextral strike-slip along a major fault zone underlying the Severn Estuary (the Severn Estuary Fault Zone), and that the Vale of Glamorgan Axis was a synthetic structure, developed in conjunction with this strike-slip movement. By inference, the Severn Estuary Fault Zone shares a common history with the Vale of Glamorgan Axis, extending back to the Devonian and possibly earlier. Late Dinantian (Holkerian) facies distribution reflects uplift and emergence on the north-south Usk Anticline and Malvern Lineament. Continued uplift is recorded in Silesian sequences in south-east Wales and the Bristol-Mendips area. The orientation of structures bordering the Severn Estuary is consistent with their development in a right-lateral strike-slip zone.

A fault-controlled depositional model for the Aberystwyth Grits turbidite system

The Aberystwyth Grits Group was the first in a series of southerly-supplied sandstone-rich turbidite systems that invaded the Welsh Basin during the Telychian Stage (late Llandovery; Silurian). Structural and stratigraphical evidence indicates that deposition of the sand-dominated parts of the system was largely restricted to the east by a zone of syndepositional faulting, now marked by a major vergence divide. Downfaulting and reverse drag along the fault hangingwall created a bathymetric low, which focused deposition of high-density, sand-carrying turbidite flows and flow components, allowing substantial thicknesses of sediment to accumulate. In contrast, low-density, mud-rich turbidites were deposited over a wider area, enhancing the topographic relief generated by uplift above the fault footwall, to form a feature which restricted the spread of the coeval easterly-supplied turbidites of the Devils Bridge Formation.

Stratigraphical and palaeoecological importance of Caradoc (Upper Ordovician) graptolites from the Cardigan area, southwest Wales

Graptolites from more than 60 horizons in the basinal Caradoc succession of southwest Wales, between Fishguard and Cardigan, allow recognition of the multidens , clingani and linearis biozones. The biostratigraphy permits recognition of major differences in the sedimentary rock-sequence north and south of structures associated with the Fishguard–Cardigan Fault Belt. The Penyraber Mudstone Formation, disconformably overlying the Fishguard Volcanic Group (Llanvirn), is partly of multidens Biozone age. It is succeeded south of the Newport Sands Fault by the Cwm yr Eglwys Mudstone Formation of clingani to linearis biozones age. North of the fault the Cwm yr Eglwys Mudstone Formation is replaced laterally by the northwards-thickening, sandstone turbidite-dominated Dinas Island Formation ( clingani and linearis biozones). Graptolite stratigraphical distribution indicates that Dicranograptus clingani occurs only rarely within the caudatus Subzone of the clingani Biozone and that Climacograptus antiquus s.l . also does not range above the lower part of the clingani Biozone. The first occurrence of Dicellograptus morrisi , within the upper clingani Biozone, confirms its value as a marker for the morrisi Subzone, and is associated with the first occurrences of Diplacanthograptus dorotheus and Normalograptus minimus . Dicellograptus flexuosus , used to indicate the morrisi Subzone elsewhere, occurs throughout the clingani Biozone in the Cardigan area. The linearis Biozone is recognized by Climacograptus tubuliferus . Oxic bottom conditions in early and early mid-Caradoc times largely precluded the influx of, or preservation of, graptolite faunas in the Penyraber Mudstone Formation. Anoxic mudstones of the Cwm yr Eglwys Mudstone and Dinas Island formations preserve graptolite assemblages of 21 and 26 species, signalling strong open marine influences which persisted in this area until late Caradoc times. This contrasts with the shelfal faunas in the Whitland area (south Pembrokeshire), where the late Caradoc is dominated by low-diversity Normalograptus- dominated assemblages.

Sequence stratigraphical analysis of late ordovician and early Silurian depositional systems in the Welsh Basin: a critical assessment

Abstract Sequence stratigraphical concepts are applied to a 55 km long transect through the uppermost Ordovician and lower Silurian rocks of the Welsh Basin and the adjoining Midland Platform. The study focuses on sedimentary rocks deposited during the Llandovery epoch (about 439–430 Ma). An early Llandovery slope apron of hemipelagite and laterally supplied mudstone turbidites shows the influence of eustatic sea-level changes. Two complete depositional sequences are recognized. Transgressions and highstands on the platform were accompanied by laminated hemipelagite deposition in the basin, recording anoxic bottom waters in a stratified watermass. The intervening regressions generated unconformities on the platform and produced bioturbated basinal sediments in a more oxic environment. By contrast, the extent and timing of sandstone-turbidite and debrite systems in the basin are strongly affected by tectonic activity. Easterly derived late Llandovery facies relate directly to faulting and submarine mass wasting along the basin margin. Southerly derived late Llandovery to mid-Wenlock sandstone-lobe systems were the products of tectonic uplift in extrabasinal source areas and were partially confined within intrabasinal tilted fault blocks. Subsidence analysis confirms a basinal stretching event in late Llandovery (Telychian) time and shows that it also affected the adjacent Midland Platform. The depositional systems of this period constitute elements of a dual-sourced depositional sequence, within which eustatic effects are masked by the strong influence of relative base-level changes attendant to tectonism. The architecture of the early Silurian Welsh Basin illustrates the complexities of applying sequence stratigraphical models where there is an interleaving of several depositional systems, each modulated by a different mix of eustatic, tectonic and input controls.

Integrated Upper Ordovician graptolite-chitinozoan biostratigraphy of the Cardigan and Whitland areas, southwest Wales

To help calibrate the emerging Upper Ordovician chitinozoan biozonation with the graptolite biozonation in the Anglo-Welsh, historical type basin, the graptolite-bearing Caradoc–Ashgill successions between Fishguard and Cardigan, and at Whitland, SW Wales, have been collected for chitinozoans. In the Cardigan district, finds of Armoricochitina reticulifera within strata referred to the clingani graptolite Biozone ( morrisi Subzone), together with accessory species, indicate the Fungochitina spinifera chitinozoan Biozone, known from several Ordovician sections in northern England that span the base of the Ashgill Series. Tanuchitina ? bergstroemi , eponymous of the succeeding chitinozoan biozone, has tentatively been recovered from strata of Pleurograptus linearis graptolite Biozone age in the Cardigan area. The T. ? bergstroemi Biozone can also be correlated with the type Ashgill Series of northern England. Chitinozoans suggest that the widespread Welsh Basin anoxic–oxic transition at the base of the Nantmel Mudstones Formation in Wales, traditionally equated with the Caradoc–Ashgill boundary, is of Cautleyan (or younger Ashgill) age in the Cardigan area. In the broadly time-equivalent, graptolite-rich Whitland section, also in SW Wales, two Baltoscandian chitinozoan biozones and a subzone have been recognized (again using accessory species), namely the Spinachitina cervicornis Biozone?, the Fungochitina spinifera Biozone and the Armoricochitina reticulifera Subzone. The new chitinozoan data provide a more precise means of correlation between the Whitland and Cardigan successions and suggest that the Normalograptus proliferation interval of the Whitland section is at least partly attributable to the Dicellograptus morrisi Subzone of the Dicranograptus clingani Biozone, rather than equating with the overlying Pleurograptus linearis Biozone.

Palaeoenvironments and palynofacies of a pulsed transgression: the late Devonian and early Dinantian (Lower Carboniferous) rocks of southeast Wales

Two boreholes in the Vale of Glamorgan have provided new data on the nature of the early Dinantian (Courceyan) transgression in South Wales. This transgression is manifested by the transition from the largely fluviatile, late Devonian, Upper Old Red Sandstone (Quartz Conglomerate Group) to the predominantly marine, early Dinantian, Lower Limestone Shale Group. The marine sequence comprises five shoaling upwards cycles, constructed from a suite of sedimentary lithofacies which record deposition in environments ranging from coastal plain, peritidal, lagoon, barrier and embayment to subtidal, open marine shelf. Each cycle represents a pulse of the transgression, and each successive pulse appears to have been larger than the preceding one, introducing progressively less restricted and more distal marine environments. Thirty-seven samples were processed for palynological analysis. Miospore biozonation supports the cycle correlations between the two boreholes, suggested by the sedimentary event stratigraphy. Detrital kerogens from the samples comprise both terrestrially derived and marine types in varying proportions. Each kerogen type is described as well as the size, sorting and preservation of each assemblage. A palynofacies profile is presented for eachof the depositional environments recognized.

A revised sedimentary and biostratigraphical architecture for the Type Llandovery area, Central Wales

The global standard for the Llandovery Series (early Silurian) in central Wales is re-assessed in the light of detailed geological surveying, biostratigraphical sampling and a rigorous examination of published datasets. A new sedimentary and biostratigraphical architecture is presented. Key graptolite, brachiopod, acritarch and, for the first time, chitinozoan assemblages are critically assessed. Upper Hirnantian to Aeronian strata record events that followed the Late Ordovician glacial maximum and comprise a series of progradational sequences bounded by flooding surfaces, but inferred still to be glacioeustatic in origin. Significant faunal renewals associated with many of the flooding levels underpin their potential for international recognition. Compound non-sequences are a feature of proximal parts of the system where erosion associated with fault footwall uplift was an important process. Extensive slump sheets contribute to further stratal loss and displacement in distal facies. A re-assessment of the Aeronian Stage GSSP reveals shortcomings with the biostratigraphical criteria used in its selection. Telychian portions of the succession display the disrupting effects of intra-Wenlock synsedimentary sliding; hence the relevance of key published fossil assemblages and the criteria used to erect the stage GSSP are undermined. However, the Llandovery area remains one of the best studied early Silurian successions in the world. This, together with regional considerations, supports the retention of the series standard in mid Wales where the contiguous deep-water basinal succession affords internationally cited exposure of richly graptolitic facies for the whole series and, significantly, for the post- sedgwickii Biozone interval.

A new Early Silurian turbidite system in Central Wales: insights into eustatic and tectonic controls on deposition in the southern Welsh Basin

The newly recognized Nant Brianne turbidite system was a focus of laterally supplied coarse-grade sediment deposition that, along with the Caban–Ystrad Meurig system, punctuated late Hirnantian to early Telychian, mudstone-dominated slope apron deposition along the SE margin of the southern Welsh Basin. Geological mapping coupled with detailed biostratigraphy enable the depositional influence of sea-floor topography, an active Llyn Brianne Fault and eustatic sea-level changes to be tracked. The latter may represent ‘far field’ effects relating to the retreat and advance of contemporary Gondwanan ice sheets. Slope apron mudstone facies reveal a strong response to high order eustatic events; however, the response of contemporary, easterly sourced, coarse-grade turbidite systems was more complex, with some periods of increased sand and gravel input coinciding with times of rising global sea level. These anomalous relationships are explained by invoking a series of smaller-scale movements in marine base level. It was the interaction of these lower order events with the main eustatic cycles that appears to have been the primary control on sand and gravel input to the Welsh Basin. An early Telychian expansion of the Nant Brianne turbidite system records a marked increase in tectonically generated sediment at a time of palaeo-plate collision between Baltica/Avalonia and Laurentia before intra- and peri-basinal faulting led to its abandonment as a supply path to the basin centre.