Alan T. Thomas

Late Wenlock sequence stratigraphy in central England

The late Wenlock Series (Homerian Stage) of the northern Midland Platform (central England) comprises silty mudstones and limestones of the upper part of the Coalbrookdale and overlying Much Wenlock Limestone formations. Based on outcrop studies and borehole data, the sequence stratigraphical interpretation developed for the inliers of the West Midlands is slightly revised, and extended to the stratotype sections along Wenlock Edge. A single third-order cycle of sea-level change is identified, punctuated by a regressive–transgressive episode associated with a higher-order glacioeustatic cycle, allowing the upper Wenlock Series of the area to be divided into two subsequences (A and B). Subsequence A and the early transgressive systems tract began with regression associated with the basal sequence boundary in late Cyrtograptus lundgreni Biozone times. This was followed by a period of slow transgression or stillstand, allowing shallower water carbonate environments to prograde. A minor phase of regression followed, resulting in the generation of the shallowest water deposits of both the Lower Quarried Limestone and Farley members (of the Much Wenlock Limestone and Coalbrookdale formations, respectively). The overlying Subsequence B and the late transgressive systems tract are marked by transgression and a period of rapid sea-level fluctuation and are likely contained within the Gothograptus nassa Biozone. A minor highstand is widely recognizable at this time. The rest of Subsequence B consists of an initial phase of weak progradation (highstand systems tract), followed by a marked regression (falling stage systems tract) culminating in an erosive upper sequence boundary at or close to the top of the Monograptus ludensis Biozone, but within the uppermost Much Wenlock Limestone Formation. Above Subsequence B is a marked transgression into the Lower Elton Formation and the Ludlow Series. Both late Wenlock lowstands and the succeeding flooding events have been recognized on other palaeocontinents, reflecting the eustatic nature of sea-level changes reported here.

Carbonate depositional environments in the late Wenlock of England and Wales

Based on outcrop studies and borehole data, six bedded lithofacies and two reef types are recognized within the Much Wenlock Limestone Formation of the English Midlands and Welsh Borderland. The lithofacies are interpreted to represent a series of carbonate shelf environments extending from below storm wave-base to well above fair weather wave-base. In common with many other shallow marine carbonate depositional systems, the principal controls on lithofacies development were hydrodynamic energy, the supply of fine clastic sediment, and patterns of colonization of the sea floor by organisms. Reef distribution was probably controlled by the nature of the substrate, water circulation, and rate of siliciclastic sedimentation. A depositional model is proposed which incorporates biostratigraphical evidence suggesting that the formation youngs to the west on the northern part of the shelf. Deposition of the Much Wenlock Limestone Formation there began in the West Midlands, where 12 m of microbial limestone were lain down in a mid-shelf setting during a local regression. The remainder of the shelf was dominated by low energy siliciclastic deposition at that time. The West Midlands then returned to somewhat deeper water, lower energy deposition, the resulting impure calcareous muds becoming diagenetically changed into the nodular limestone lithofacies. That lithofacies is commonly overlain successively by the interbedded limestone and silty mudstone lithofacies, and then the crinoidal grainstone lithofacies. This vertical lithofacies sequence is uniform over the entire northern part of the shelf, reflecting a gradual decrease in water depth. The crinoidal grainstone lithofacies was deposited as a wave-influenced carbonate sandbody which prograded from east to west. Lithofacies sequences on the southern shelf are laterally impersistent, probably due to greater tectonic instability and topographical variablity.

Lithostratigraphy and palaeoenvironments of the Cambrian in SW Wales

Abstract The classic Cambrian succession of SW Wales comprises a succession of clastic rocks some 1250 m thick that make up part of the Dyfed Supergroup. The existing lithostratigraphical framework owes much to the Nineteenth Century researchers, with relatively little detailed work having been published since. We here present a detailed, rationalized and revised lithostratigraphy for the Cambrian part of the Dyfed Supergroup of the area. Where possible and appropriate, we have retained original and well-known names for formations and other units in harmony with current usage. However, the recognition of four lithostratigraphical groups is overly complex and sometimes unworkable, and a two-fold subdivision is proposed instead. The Caerfai and much of the Lower and Middle Solva groups of earlier usage comprise conglomerates, pebbly sandstones and brightly coloured fine- to coarse-grained sandstones: all are here combined into a revised Caerfai Group. By excluding the silt-dominated Upper Solva division, the Caerfai Group is readily divided into lower and upper parts comprising conglomerates and/or pebbly sandstones, with sandstones and siltstones dominant in between. These rocks comprise the Ogof Golchfa (new), St Nons, Caerfai Bay, Caer Bwdy Bay, Newgale (new) and Porth Clais (new) formations. The following members (all new) are recognized within the Newgale Formation: the Trwyncynddeiriog, Pen-y-Cyfrwy and Cwm Mawr members. Ichnofossils from the upper part of the Ogof Golchfa Formation suggest that Cambrian deposition in the area began in Unnamed Cambrian Series 2 (C2) times. Body fossils and radiometric dating show that the Caerfai Bay to Porth Clais formations span the interval from Cambrian Series 2, Unnamed Cambrian Stage 3 (such stages are denoted by CS3, CS4 etc. below), probably to the Ptychagnostus gibbus Biozone (CS5). The former Upper Solva Group, Menevian Group and Lingula Flags comprise the newly erected Porth-y-rhaw Group, a 687 m-thick unit of fine sandstones, siltstones and mudstones, within which a further five formations are recognized. These are the Whitesands Bay, Menevia, Aber Llong (all new), Ogof Velvet (revised) and Treffgarne Bridge formations. The Porth-y-rhaw Group spans the Tomagnostus fissus Biozone (C3, Drumian) to the Olenus cataractes Biosubzone (Furongian, Paibian). The overall two-fold subdivision of the Cambrian part of the Dyfed Supergroup in the area accords broadly with the first lithostratigraphical scheme proposed for the area, and is comparable with the separation of the Harlech Grits and Mawddach groups in northern and central Wales. The oldest part of the Caerfai Group formed in a tectonically active context and records a transition from alluvial fan deposits, through braided stream environments, into transgressive nearshore marine sandstones. These sediments were derived from the west, and as well as locally-sourced material, include lithic clasts and grains derived from a presumed southwestwards extension of the Monian Composite Terrane. Above lie sedimentary deposits formed under a wide range of conditions, ranging from tidally-influenced to turbidites. Further evidence of active tectonism is found in the easterly-derived fan-delta sedimentary deposits of the higher part of the Caerfai Group and lower part of the Porth-y-rhaw Group. Deposition of the latter began with fine-grained turbidites deposited in a mid–outer shelf setting. These pass up first into hemipelagites, then into sedimentary rocks deposited on a storm-dominated shelf, and finally a very extensive shallow subaqueous delta platform formed in a passive margin setting.

Overview and biostratigraphy

Abstract Excellent coastal exposures of strongly folded and faulted Cambrian rocks occur around the St Davids Peninsula of SW Wales. The best sections occur in the St Davids Anticline, in the vicinity of St Davids itself, and these have been the subject of scientific study since the mid-Nineteenth Century. Based on this early work, a quadripartite lithostratigraphical scheme emerged, comprising the Caerfai, Solva and Menevian groups and the Lingula Flags Formation. However, this scheme is unnecessarily complex, and sometimes unworkable in practice. The Caerfai Group is here redefined to embrace much of the Solva Group of earlier authors, while the upper part of the Solva Group, the Menevian Group and Lingula Flags are combined in the new Porth-y-rhaw Group. Traditionally, all these Cambrian rocks have been thought of as being deposited on a stable cratonic platform, but there is substantial evidence of tectonic influences on sedimentation, particularly in the older part of the succession. The Cambrian was originally recognized and defined in North Wales. There, as in many other areas, it became common practice to divide the period into three parts, but more recent work favours a four-fold chronostratigraphical subdivision. The oldest part of the Cambrian is missing in South Wales, but rocks of Terreneuvian, C2, C3 and Furongian age are all represented. Age-diagnostic fossils are few in the older part of the local succession. Trace fossils low in the Caerfai Group indicate the avalonensis Ichnozone or Teichichnus Interval of the Terreneuvian, while a radiometric date of early C2 (CS3) age has been obtained higher up. Trilobite faunas occur more commonly in the higher parts of the Caerfai Group and in the Porth-y-rhaw Group, and these facilitate international correlation of the C3 (CS5) to Furongian (Paibian) parts of the succession. Trilobites characteristic of the oelandicus Biosuperzone and the gibbus, fissus, parvifrons, punctuosus, pisiformis and Olenus biozones all occur.

Fan-delta sedimentation in the Silurian Coralliferous Formation of SW Wales: implications for the structure of the southern margin of the Welsh Basin

The uppermost Llandovery to lower Wenlock Coralliferous ‘Group’ in SW Pembrokeshire is here redefined as a single formation with two members. The Coralliferous Formation is approximately 150 m thick and comprises a basal unit of granule- to pebble-grade rudite beds, the Renney Slip Member, overlain by interbedded mudstones and fine sandstones of the Deadman’s Bay Member. The Renney Slip Member lies unconformably above the Skomer Volcanic Group and includes 12.4 m of coarse grained, granule- and pebble-rich rudites, with beds up to 0.94 m thick. Three lithofacies are recognized within this unit: coarse, granule-rich rudite beds are interpreted as a variety of mass flow deposits, some of which have been reworked in a marine environment; thick sandstones with planar and ripple lamination are shoreface to offshore transition zone deposits; silty mudstones interbedded with very fine grained sandstones represent marine offshore deposits, formed largely below mean storm wave base. These facies associations, and abundant bioturbation, indicate an environment with a strong marine influence, and a proximal source of coarse grained sediment. The Renney Slip Member is reinterpreted in the context of a fan delta depositional model. At least seven cycles of deposition are recognized, each showing an upwards-fining pattern, representing deposition from fan delta, shoreface–transition zone to open marine environments. These patterns of deposition are attributed to localized tectonic movements causing variations in relative sea level. At the time the Renney Slip Member was deposited, the southern Welsh Basin margin is interpreted as a fault-block extensional margin, with the landmass of Pretannia to the south. Though fan-delta deposition took place southwards against the uplifted footwall of the Wenall Fault, the basin margin lay to the south of the Ritec Fault.

Sequence stratigraphy, tectonostratigraphy and basin analysis

Abstract The Dyfed Supergroup of SW Wales ranges from Terreneuvian to Furongian in age and represents a prolonged and nearly continuous phase of siliciclastic sedimentation on the southern margin of the Cambrian Welsh Basin. The lower 564 m of the Supergroup are included in an extended Caerfai Group (six formations), embracing strata previously assigned to the Solva Group. Predominantly arenaceous, the Caerfai Group has important units of conglomerate and pebbly sandstone at its base, middle and top. The Caerfai Group is overlain by 687 m of mainly argillaceous sedimentary deposits of the Porth-y-rhaw Group (five formations), a newly erected unit that includes strata previously assigned to the ‘Menevian Group’ and ‘Lingula Flags’. This two-fold division of the Dyfed Supergroup is comparable with the distinction of the laterally equivalent Harlech Grits and Mawddach groups exposed around the Harlech Dome in North Wales. High resolution sequence-stratigraphical techniques, constrained by biostratigraphical data wherever possible, are applied to the Dyfed Supergroup across southern Britain: the revised lithostratigraphy is thereby integrated with a slightly modified version of the Avalonian chronostratigraphy initially developed for the western Avalonian successions of maritime Canada. Sequences 1 and 2 are not recognized in the Dyfed supergroup of SW Wales, reflecting the inner platform setting of the area. Those sequences are represented in North Wales however, which was situated on the outer platform. The relative ease with which the Cambrian successions of southern Britain can be incorporated into the general Avalonian framework reflects the shared epeirogenic history, sediment sources and accumulation history of a microcontinent unified by early Cambrian times. The gross lithological similarities that exist between Cambrian successions across Avalonia, and between SW Wales and North Wales in particular, are better understood when basin chronostratigraphy is placed within a sequence stratigraphic framework defined by systems tracts. This is particularly evident during Unnamed Cambrian Series 3 (C3) and Furongian times, when clear systems links are demonstrated between paralic depositional environments in SW Wales and deeper basin turbidites in North Wales. Sequences 3–7 (Terreneuvian–C3) in the Welsh Basin and its hinterland were dominated by siliciclastic deposition on an epeirogenically active platform. An extensional rift-like tectonic regime is proposed, where elongate basins dominated by mudstone deposition have time equivalents formed on areas of apparent uplift that were probably tilt-block highs. Local accumulation history, the development of regional unconformities, the extent of marine onlap, and the secular succession of lithofacies were controlled principally by phases of fault-accommodated subsidence along the NE-trending lineaments that bounded these basins and their intervening horsts or platforms. In southern Britain, movements first along the Menai Straits Fault System then the Welsh Borderlands Fault System, both of which are terrane boundaries, were responsible for the dominantly coarse-grained arenaceous sedimentation of pre-Drumian times. In the Welsh Basin, this is highlighted by a change in sediment source from the Monian Superterrane to the Wrekin Terrane. A rapid switch from marginal to inner platform source areas accompanied a major phase of tectonic reorganization of the Avalonian Superterrane during the development of the boundary between sequences 3 and 4. Although the role of secular variations in eustatic sea level as a control on sequence development and architecture has been dismissed previously, prominent lowstands such as those recorded during the basal Ptychagnostus gibbus Biozone, ‘Solenopleura’ brachymetopa Biozone (mid-Paradoxides forchhammeri Biosuperzone), and the upper part of the forchhammeri Biosuperzone also influenced the depositional sequence stratigraphy. The broad subdivision of Cambrian Avalonian stratigraphy into tectonically active and passive phases of sedimentation allows two megasequences to be distinguished. Megasequence 1 (sequences 1–7) represents the transform termination of Avalonian subduction following oblique convergence, and the accretion of island arcs onto the northern periphery of Gondwana. Under transpressional regimes, late Precambrian arc-related basins were inverted and recycled into pull-apart successor basins. Initially, the margins of these were dominated by alluvial fans and coarse-grained fan-deltas built by flood-generated sediment dispersal processes. In the later transform stage of Cadomian–Avalonian orogenesis, there was a switch to sediment supplied from highlands to the east. Sequence 8, represented in Pembrokeshire by the Aber Llong and Ogof Velvet formations, lies at the base of Megasequence 2 (late C3–early Ordovician). Excellent sections occur through these formations and their equivalents in North Wales, and their interpretation significantly improves understanding of younger Cambrian sedimentary basins in Avalonia. These successions reflect passive margin sedimentation and the culmination of the Avalonian–Cadomian orogenic cycle. Deposition occurred in part of a subaqueous delta platform at the mouth of a huge river system that drained a substantial part of West Gondwana, with Avalonia acting as a sink for vast quantities of fine-grained sediment. Secular variations in eustatic sea level and/or sediment supply, rather than active tectonism, were the main mechanisms controlling sequence architecture and depositional environments at this time.