Abstract The Upper Devonian to Lower Carboniferous Skrinkle Sandstones of the Pembroke Peninsula are predominantly continental deposits from the post-Caledonian syn-rift succession at the southern margin of the Late Palaeozoic Welsh Landmass. The Sandstones record deposition in the 30 km × 10 km Tenby–Angle fault block, the southernmost of a series of fault-bounded depositional basins in SW Dyfed. Activity on the bounding faults strongly influenced sedimentation through Lower Palaeozoic time. The Skrinkle Sandstones are conventionally assigned to a phase of relative fault inactivity, passive transgression of the area and southward drainage off the landmass. The Ritec Fault at the northern block boundary defined a temporary shoreline during final submergence. In contrast to this, it is argued that the lower half of the Skrinkle Sandstones represent a separate structural configuration, where SE-directed palaeocurrents and the high textural maturity of two superimposed basin-fill sequences indicate axial basin fill and potential closure to the south. The upper half records an influx of immature clastic deposits as fluvial sediments that disperse to the southwest, indicating relative uplift of either the Ritec or a more northerly fault. This phase records the true transgression, during which a thick barrier–lagoon coastline is preserved against the footwall ramp of the Ritec Fault.

Abstract Calcic pedocomplexes in the Siluro-Devonian Old Red Sandstone (ORS) of the Anglo-Welsh Basin (UK) have traditionally been interpreted as paleosols developed in dryland depositional environments. Their recognition has been used to indicate a range of controls, including climate, landscape stability, sedimentation rate, soil residence time, and proximity to alluvial channels (the pedofacies concept). A study of the Devonian Ridgeway Conglomerate Formation (RCF) in Pembrokeshire, southwest Wales, has, however challenged some of these notions, recognizing that many calcretes were not developed in soil horizons. The RCF was deposited as part of a dryland alluvial fan and axial fluvial valley complex. Regionally, structural blocks and basins were defined by a series of extensional faults, with the RCF being deposited in a half-graben as a hanging-wall alluvial fan. The RCF is heterolithic, comprising conglomerates, sandstones, and mudstones that reflect differences in processes, suggesting sheetfloods, low-relief lateral accretion, and cohesive debris flows across the alluvial fan. Pedogenic calcretes are common in all areas of the fan and axial fluvial zone. In mudstone and sandstone-grade lithofacies they comprise common horizonated nodules and subhorizontal crystallaria sheets in association with pedogenic indicators such as drab haloes, desiccation cracks, and ped textures. Also observed are both horizontal and vertical root traces, some of which have been the focus of micrite nodule growth (rhizogenic calcretes). Wedge-shaped peds are absent. Pedogenic profiles display upward-increasing percentages of nodules, and may be capped by blocky, massive calcrete and laminated micrite that developed in small ponded areas. In gravel-grade lithofacies, the pedogenic expression is different, and comprises carbonate-coated clasts with pendant and pore-occluding calcrete fabrics. Pedogenic calcretes are best developed in proximal areas of the fan, possibly on terraces adjacent to fan-channel entrenchment zones (the pedofacies concept). Proximal fan areas may also have had increased soil residence times due to reduced sedimentation rates compared to distal fan and axial fluvial valley zones. In distal fan and axial alluvial zones, thin layerbound micritic groundwater calcretes are common, typically being sharp based with upper surfaces comprising vertical and cylindrical nodules that possibly developed in the capillary-fringe zone. Inclined heterolithic bedsets, the deposits of laterally accreted ephemeral channels also commonly contain layer-bound micritic calcretes, again interpreted as having a groundwater origin. Lake-margin calcretes comprising centimeter-thick, laminated micrite, represent possible calcretized matgrounds in fan-toe, ephemeral ponds. The identification of common non-pedogenic calcretes in the RCF begs the question: how much of the ORS calcretes are similarly non-pedogenic in nature? Our analysis may act as a cautionary check for subsurface work where carbonate horizons in alluvial suites are being modelled solely in accordance with the pedofacies concept.

Abstract In general, Devonian rocks exposed north of a line from Bristol to London are of continental ‘Old Red Sandstone’ facies, while to the south a variety of marine facies predominate ( Goldring et al. 1967 ; Selwood & Durrance 1982 ; Bluck et al. 1988 ) (Fig. 1 ). Thus, Devonian ostracods are mainly restricted to Devon and Cornwall, although there are records from marine horizons in South Wales. Little taxonomic work has been carried out on these faunas and only very general palaeoecological information is available. This reflects both the complex tectonic setting of the region ( Selwood 1990 ; Selwood & Thomas 1987, 1988 ) and the often poor preservation of most of the ostracod material. Only with the establishment of a Late Devonian ostracod chronology in the Rheinish Slate Mountains of Germany ( Rabien 1954 ; Krebs & Rabien 1964 ; Buggisch et al. 1978, 1986 ) have ostracods been used to resolve geological problems in Devon and Cornwall.

Abstract Carboniferous rocks in this region occur in a broadly east-west trending syncline, the core of which includes the South Wales and Pembrokeshire coalfields (Fig. 16). Tournaisian and Visean strata (Avon and Pembroke Limestone groups) represent deposition on a southward prograding carbonate ramp evolving into a carbonate shelf (Wright 1987), in a succession which shows similarities to that of the Bristol and Mendips areas (Chapter 6). The main outcrops, in south Pembrokeshire, Gower and the Vale of Glamorgan, occur along the southern periphery of the coalfields and are commonly affected by Variscan thrusting and folding. Thinner successions occur along what are termed the East Crop and North Crop of the South Wales Coalfield, where much of the Visean succession is absent due to sub-Namurian and intra-Visean unconformities. Namurian fluvio-deltaic deposits (Marros Group) flank the South Wales and Pembrokeshire coalflelds. Much of the lower and middle Namurian succession is absent across the region, except in the west of the South Wales Coalfleld where only small parts are absent beneath an intra-Namurian unconformity. Westphalian fluvio-lacustrine deposits (South Wales Coal Measures Group) form the South Wales and Pembrokeshire coalfields, located to the east and west of Carmarthen Bay, respectively. Westphalian to Stephanian Pennant alluvial facies (Warwickshire Group) occur in the core of the South Wales Coalfield syncline.

Abstract Classically, the Old Red Sandstone (ORS) embraces the continental, predominantly siliciclastic deposits of Devonian age, being in part the terrestrial correlatives of the marine Devonian of SW England. Subsequent stratigraphical revision (e.g. House 1977 ) demonstrated that the base of the ORS is actually of Silurian age in many places. The ORS has long been of interest due to the presence of early vascular plants and vertebrate faunas. Studies of the ORS have spawned significant sedi-mentological advances, for example the now classic analysis of high-sinuosity fluvial channels ( Allen 1965a , 1970 ). Today, the ORS is a term applied to tectono-stratigraphic units of Upper Silurian–Carboniferous age bordering the North Atlantic Ocean ( Friend 1969 , Friend et al. 2000 ). It has long been seen as representing the syn- to post-orogenic depositional response (molasse) to the Caledonian Orogeny, being modified by synchronous tectonism and volcanicity. The influence of Variscan tectonics on basin formation and subsequent deformation has recently being highlighted ( Friend et al. 2000 ). Lithologically, it embraces a wide range of textural grades from mudrocks to conglomerates. Fluvial, lacustrine, aeolian, pedogenic and marginal marine deposits have been recognized. In England and Wales, the ORS crops out in four main areas ( Fig. 8.1 ): the Anglo-Welsh Basin, Anglesey, Edenside (Cumbria) and North Devon.

Abstract The Old Red Sandstone basins of the North Atlantic borderlands provide a record of diverse dynamics in very different settings, related to the Variscan, Caledonian and Ellesmerian orogenies. This paper is a first attempt to review much new information on the basins, including information presented, for the first time, in this book. Five basin groupings are distinguished: (1) Scandinavian basins of, syn- to post-Scandian (Caledonian) age, formed on greatly thickened crust by extension or transtension (Western Norway, East Greenland, Spitsbergen); (2) NE Scotland, Orcadian Basin, mid Caledonian setting, formed by extension; (3) Scotland (Midland Valley) and related Irish basins, north of the Caledonian Iapetus Suture Zone, formed by extension; (4) southern Britain and Ireland, basins south of the Iapetus Suture Zone, related to collision of Eastern Avalonia with Laurentia, and Maritime Canada and the Catskills related to collision of Western Avalonia; these are load-induced flexural basins; (5) Southern margin of Eastern Avalonia, (Munster, South Wales, SW England), of Late Devonian age, extensional basins of various (Early to Late) Devonian ages.