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 rocksequence 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.

-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 intraWenlock 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.

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 Devil's Bridge Formation.Peer reviewe

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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 producedbioturbated 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.