Bristol Channel Area Research Articles

New insights into the Quaternary evolution of the Bristol Channel, UK

ABSTRACTA synthesis of new publically available borehole and bathymetric data, combined with a wealth of other existing disparate data sources, reveals new insights into the Quaternary history of the Bristol Channel area. Sediment boreholes throughout the Bristol Channel confirm the area was glaciated in the Pleistocene. Till is present below marine deposits and, in some areas, is visible morphologically as submerged moraines. In the central and eastern Bristol Channel the submerged valley course of the palaeo‐Severn is very clear in new high‐resolution bathymetric surveys. This former river course and associated tributaries cross‐cut through glacial sediments in the Bristol Channel. At least three phases of glaciation are recorded in the Bristol Channel, one related to the southern limits of a Late Devensian Substage (∼Marine Isotope Stage (MIS) 2) Welsh Ice Cap which reached into Swansea Bay, an earlier Devensian (MIS 4–3) glaciation associated with Irish Sea ice, and another older glaciation that is associated with ice that filled the entire outer and central Bristol Channel. The age of the older Bristol Channel glaciation is still open, although it pre‐dates the Devensian (Late Pleistocene) and must date to the Middle Pleistocene. It is therefore evident that Pleistocene glacial and fluvial activity, combined with subsequent post‐glacial sea transgression, directly account for current morphometries of the Bristol Channel and Severn Estuary, and the current geography of the SW British Isles.

Lithospheric flexure, uplift, and landscape evolution in south-central England

The high level river gravels in Oxfordshire (the Northern Drift Group), which range in age from Early Pleistocene to around 450 ka, contain pebbles that were derived from a source area to the north near Birmingham. The pebbles could not have been transported across the Upper Triassic and Lower Jurassic clay outcrops of the Midlands unless the region was at or close to sea level. The present day Cotswold escarpment ranges from 70 to 215 m: this uplift must therefore have occurred after deposition of the Northern Drift Group, when the drainage of the Midlands appears to have shifted from towards the Thames basin to the Bristol Channel area. It is concluded that lithospheric flexure due to the removal of >500 km 3 of soft Late Triassic and Early Jurassic clays and marls from large areas of the Midlands and their re-deposition in the Celtic Deep might account for this Late Pleistocene uplift. Uplift in the Midlands is the likely explanation for both the tilted (approximately 0.3°) plateau surface in north Oxfordshire and the change in strike of the Jurassic beds SW of Northamptonshire. Although some uplift occurred in SE England during the Early Cenozoic and Miocene, we demonstrate here that there has been significant uplift of the region in the Late Pleistocene. The tilted plateau surface in north Oxfordshire may therefore be a relatively young (i.e. post-450 ka) feature related to tectonic uplift in the Midlands.

Fault slip evolution determined from crack-seal veins in pull-aparts and their implications for general slip models

Most observations of fault slip history are limited to a short period of a fault's lifetime (usually less than five major earthquake cycles) and are therefore not representative of long-term slip behaviour. A new approach to determine more complete slip histories on small inactive faults is outlined, where calcite-filled veins produced by individual slip events are measured. Two normal faults are described from the south Bristol Channel area, near Kilve, north Somerset, U.K. Fault 1 had a variable slip evolution, with variable slip magnitudes through time, but a smooth and fairly constant cumulative increase in displacement. Fault 2 also has a variable slip history but with temporary repetition of similar-sized (characteristic) slip events which were limited to a maximum of eight successive increments. Each set of temporary characteristic events was followed by a period of variable slip. On both faults there were no significant changes in slip increments immediately preceding or following the larger slip events. The average amount of cumulative slip per event at different localities was either constant, accelerated, or decelerated during the fault growth history. The two faults show a slip pattern where the total displacement along the fault varies smoothly, and magnitude of slip increment at any point is highly variable through time. The data from Fault 2 suggests that the central area, where displacement was a maximum, resulted from larger slip for each event rather than more frequent events of similar magnitude, whereas Fault 1 suggests more frequent slip events occurred in the centre of the fault to produce the displacement maximum.

A hydrogeological model for the early diagenesis of Late Triassic alluvial sediments

The late Triassic Mercia Mudstone Group of southwest Britain, a problematic, complex arid playa-floodplain-aeolian deposit, contains a variety of Mg clays and carbonates. Recent interpretations have invoked marine waters as a source of Mg in clays. However, both the Mg-rich clays and other early diagenetic features seen in the late Triassic in Europe are directly analogous to those found in the inland drainage basins of Australia. A simple hydrogeological model is offered based on these basins where shallow groundwater evolution takes place within the basin producing a range of early diagenetic products including groundwater dolocretes and Mg clays. Such a model, as developed in Australia, is probably widely applicable to ancient arid alluvial systems. The Mercia Mudstone Group is a problematic sequence of predominantly red siltstones and mudstones with minor sand, dolomite and limestone intercalations ( Ruffell 1991 ). It has been extensively studied in outcrops in southwest and southern Britain, particularly in the Bristol Channel area. The siltstones and mudstones, of late Triassic age, are the distal equivalents to alluvial fan deposits and are associated, at least locally, with lake margin and spring travertine deposits ( Leslie et al. 1992 ). Some workers have favoured a marine-influenced setting for the deposition of the mudrocks or playa lakes (see review in Taylor 1983 ). Most recently they have been interpreted as the deposits of complex floodplain-playa systems with a signficant aeolian input ( Talbot et al. 1994 ). Extensive evaporites occur locally in the unit, including thick halite deposits and have been interpreted as evidence of marine

Organic facies variations in the Westbury Formation (Rhaetic, Bristol channel, SW England)

Despite their dark colour, laminated fabric, and dysoxic character, the thermally immature shales of the Rhaetic Westbury Formation of the Bristol Channel area typically have total organic carbon (TOC) values of only 1.0–1.5%, and hydrogen indices (HI) >200. Variations in TOC and HI are correlated with the amorphous organic matter (AOM) fluorescence, but not the % AOM, suggesting that redox-related variations in AOM preservation control the organic facies. The diameter of equidimensional black “wood” phytoclasts decreases in intervals with higher TOC and HI, suggesting an association with lower energy depositional conditions. Such intervals also show a relative increase in light brown “wood”, probably a reflection of more humid onshore conditions and lower phytoclast oxidation at source. PhytOC values suggest the input of terrestrial OM was relatively low and stable during Westbury deposition, indicating a distal setting removed from fluviodeltaic sediment sources.

Basement tectonics and their relationship to Mesozoic megasequences in the Celtic Seas and Bristol Channel area

Abstract A synthesis of new and pre-existing data on the deep geological structure of the Celtic Sea and Bristol Channel basins shows similarities to published models of deep structure from the onshore Wessex Basin to the east. A series of Variscan thrust planes, reactivated in the Mesozoic as normal faults, underlie the study area. Beneath the Bristol Channel, one such southward-dipping thrust may be equated with the ‘Variscan Front’. This structure is offset by NW-SE orientated transcurrent faults, structures thought to be active from the late Palaeozoic to the present-day. Mesozoic stratigraphic successions from the North Celtic Sea Basin and South Celtic Sea; Bristol Channel, and northern Wessex Basins suggest uplift and erosion of the latter areas in the late Jurassic. This is contrasted with increased subsidence at the same time in the North Celtic Sea. This uplift may have been facilitated by reactivation of the ‘Variscan Front’ thrust as a thrust in the east of the area, and by similar thrusts beneath the Pembrokeshire Ridge to the west. This reactivation and uplift during North Atlantic opening is suggested to have controlled the preservation of Upper Jurassic-Lower Cretaceous sedimentary megasequences.

Analysis of twenty microseim storms during the winter of 1987–1988 and comparison with wave hindcasts

In this paper an attempt is made to establish a quantitative relationship between the spectra of secondary microseisms recorded at Menai Bridge and the parent sea-wave spectra in the sea areas in the western approaches. Secondary microseisms have double the frequency of the parent waves. The sea-wave spectra were obtained from a hindcasting model. The problem was simplified by restricting the consideration of the microseismic energy to that associated with periods of 7 s or over and relating this to the energy spectra of sea waves of period 14 s or over. In this way only the strongest storms (usually only found in the North Atlantic) were taken into account. The hindcasting model gives a complete frequency-directional distribution of the sea-wave energy and so in coastal areas the energy moving normal to the coast can be ascertained. In the event, a workable relationship was found between the microseism spectra and those for waves in the Bristol Channel area and also the area bounded by the north coast of Ireland and the south-west coast of Scotland, including the North Channel. From the theory due to Longuet-Higgins it was possible to calculate the coastal reflection coefficients from the relationship by putting in the areas for these two regions. These were found to be between 0.15 and 0.25, which are too high for these coastal regions. It is probable therefore that these two regions act as monitors for larger sea areas. An investigation was also made into the existence of primary microseisms (these have the same frequency as the parent waves). The results confirmed a previous finding that, with the vertical component, the energy is about 100 times less than that of the secondary.

Mesozoic reactivation of Variscan thrusting in the Bristol Channel area, UK

New seismic reflection data from the Bristol Channel area provide direct evidence of a major Variscan thrust in the Palaeozoic basement beneath the Mesozoic Bristol Channel syncline. The thrust has an average southerly dip of about 25° and it rises into the major fault (the Central Bristol Channel fault zone, CBCFZ) at the northern margin of the Bristol Channel syncline. The CBCFZ is attributed to reactivation in extension of the Variscan thrust, probably during the early Cretaceous. There is no evidence for major synsedimentary movement on the CBCFZ, and the Bristol Channel syncline does not represent a rollover structure. Section restoration suggests that the CBCFZ has cut up through a previously tilted sequence of Triassic and Jurassic strata. Pervasive NNW–SSE cross faults are interpreted as original Variscan transfer faults that have been reactivated in the Mesozoic and Tertiary.

Sediment sources and Palaeozoic history of the Bristol Channel area

Lithological and palaeocurrent data across South Wales from sediments of late Precambrian to late Silurian age, inclusive, indicate persistent derivation from a land source of Precambrian rocks in what is now the Bristol Channel area and southwards over Cornubia. In contrast to southern Wales, there is no evidence for Lower Palaeozoic deposition across most of the region to the south of the Bristol Channel; this land area is here named Pretannia. Early Devonian southward progradation of Old Red Sandstone facies terminated the long period of non- deposition across Pretannia. Later Palaeozoic tectonic movements in the Bristol Channel region led to the intermittent exhumation of the northern fringe of the landmass.

Geophysical investigations of deep structure in the Bristol Channel area

Geophysical investigations of deep structure in the Bristol Channel area

Mid-Devonian tectonics and sedimentation in the Bristol Channel area

The Hangman Sandstone Group (Eifelian) of North Devon contains two continental 'Old Red Sandstone' formations. The Trentishoe Formation is some 1000 m thick and consists of lithic arenites which petrographically match closely with those in the higher parts of the Lower Old Red Sandstone in South Wales. This formation was derived mainly from erosion of the highest Lower Old Red Sandstone in early mid-Devonian times as a result of end-Caledonian uplift and erosion in S Wales. The Rawns Formation which follows contains 120 m of conglomerates and pebbly sandstones with angular clasts of porphyry, tuff, quartzite and lithic arenite which do not match with rocks in possible source areas in Wales, but could have been derived from the Lower Palaeozoic and Precambrian rocks believed to lie at shallow depth to the north of the Bristol Channel Fault Zone. Fault movements along this zone are believed to have generated relatively short-lived coarse clastic supply at various times and places around the Bristol Channel in Lower to early mid-Devonian times, in response to end-Caledonian movements. The later production of late Namurian to Westphalian sands from the Bristol Channel area was unrelated to these Devonian events, and represents uplift in response to Variscan events in SW England.

A seismic study of deep geological structure in the Bristol Channel area, SW Britain

Summary. Results from eight seismic refraction lines, 35–90 km long, in the Bristol Channel area are presented. The data, mostly land recordings of marine shots, have been interpreted mainly by ray-tracing and time-term modelling. Upper layer velocities through Palaeozoic rocks usually fall within the range 4.8–5.2 km s−1. Below the Carboniferous Limestone with a normal velocity of 5.1–5.2 kms−1, the Old Red Sandstone with a velocity of 4.7–4.8 kms−1 acts as a low velocity layer, as do parts of the underlying Lower Palaeozoic succession. In the central South Wales/Bristol Channel area and the Mendips, a 5.4–5.5 km s−1 refractor is correlated with a horizon at or near the top of the Lower Palaeozoic succession. Under the whole area, except for north Devon, a 6.0–6.2 km s−1 basal refractor has been located and is correlated with Precambrian crystalline basement rocks. In general, this refractor deepens southwards from a series of basement highs, which existed before the major movements of the Variscan orogeny in South Wales, resulting in a southerly thickening of the pre Upper Carboniferous supra-basement sequence. In north Devon, a 6.2 km s−1 refractor at shallow depth, interpreted as a horizon in the Devonian or Lower Palaeozoic succession, overlies a deep reflector that may represent the Precambrian crystalline basement.

Effects of Holocene changes of sea-level on the hydrological regime of NW Europe (Les conséquences des variations du niveau de la mer sur le régime hydrologique du nord-ouest de l'Europe)

The rapid global rise of sea level that occurred c. 8400 to 5500 B.P. affected in different degree the hydrological environments of southern and northern areas of north-western Europe, although almost everywhere it produced a rise both in the base levels of rivers and in the levels of the water tables along the seaboards. In the Bristol Channel area, pre-existing river valleys were flooded progressively by marine waters rising to approximately the position of present sea level. In the Netherlands, a wide area of the seaboard was inundated and a backswamp area was formed. At the boundary of the backswamp area and the inland ground surface, numerous springs were generated which initiated a local drainage system that led indirectly to peat formation. In the Forth Valley (SE Scotland) net drops of relative sea level during the early part of the Holocene epoch produced a series of terraces related to the falling base level of the River Forth and its tributaries. Between 8400 and 5500 B.P. , rising sea level raised both fluvial base levels and the local ground water table, leading to burial of the early Holocene terraces Since 5,500 B P , fluvial incision has occurred.

Differential two-way sediment transport in the Bristol Channel and Severn Estuary, U.K.

The main sediment transport paths in the Bristol Channel and Severn Estuary are briefly described; coarse sediment is transported in a generally westward direction and fine sediment in a generally eastward direction. The concept of differential two-way sediment transport is introduced and applied to the modern Bristol Channel area. This phenomenon is considered to be of significance in interpreting ancient sediment transport patterns and shelf configurations.

THE GEOLOGY AND HYDROCARBON HABITAT OF THE BRISTOL CHANNEL BASIN

The Bristol Channel Basin forms an early Mesozoic ((?Permo‐) Triassic‐Jurassic) basin development with a relatively thin cover of Cretaceous and Tertiary sediments. On the basis of structural trends and stratigraphy, the Basin can be divided into two sub‐basins: the ENE‐WSW trending Main Bristol Channel Basin and the E‐W trending East Bristol Channel Basin. In between them there is an Intermediate Area which incorporates features of the sub‐basins on either side. This subdivision appears to be the result of the presence of major NW‐SE basement faults which intersect the Bristol Channel area in several places.As a result of intermittent periods of tectonic activity three main stages of basin development can be recognised: (1) a (?Permo‐) Triassic‐ Middle Jurassic stage terminated by mid‐Kimmerian epeirogenetic movements, (2) an Upper Jurassic—Lower Cretaceous stage terminated by a Lower Cretaceous (late Berriasian‐pre‐Aptian) period of deformation, (3) an Upper Cretaceous—Tertiary stage.The bulk of the preserved sedimentary fill in the main Bristol Channel Basin consists of up to 11,000 ft of (?Permo‐) Triassic‐Jurassic sediments. The section is severely truncated by the overlying late Lower Cretaceoussediments or, in theirabsence, by Upper Cretaceous strata. Below the unconformity, the Upper Jurassic and, depending on their structural position, parts or the whole of the Middle and Lower Jurassic are missing.The East Bristol Channel Basin hasapreserved fill of some 7,500 ft of Triassic‐Jurassic sediments. The Middle and Upper Jurassic are completely preserved in the centre of this Eastern Basin, contrasting with the situation found in the main Basin.The basin fill of both sub‐basins consists largely of Triassic continental red‐beds, mainly silty claystones and evaporites and Jurassic marine calcareous siltstonesand claystones.Structures are essentiully the result of the Lower Cretaceous tectonic phase(s) and are therefore mainly confined to the Triassic—Jurassic—early Lower Cretaceous sections. A number of structural traps of interest to exploration have been delineated and (unsuccessfully) tested in the Main Bristol Channel and Intermediate Area.Evaluation of the currently available data indicates that the absence of significant hydrocarbon indications in these wells is probably due to insufficient and untimely hydrocarbon generation. The possible generation is thought to have occurred prior to the main phases(s) of structural deformation, during a period of temporarily increased heatflow (Middle Jurassic—Lower Cretaceous). An additional unfavourable aspect for hydrocarbon prospects is the lack of good quality reservoir developments.From a megateetonic point of view the Bristol Channel Basin formspart of a regional rift basin development which also includes the Celtic Sea and Western Approaches Basins. The different basins and blocks of this area can, according to their behaviour during the Mesozoic, be grouped as follows: (a) the East Bristol Channel, E. of the zone of majorNWSE faults (e.g. Sticklepath fault), (b) a Central Fault Block, comprising the Cornubian Platform, the Main Bristol Channel and Haig Fras Basins, (c) Flanking Basins, such as the Intermediate Area, the Celtic Sea and Western Approaches Basins.

Seismic refraction studies of geological structure in the inner part of the Bristol Channel

Nine E–W seismic refraction lines about 12 km long provide information on the thickness and structure of the Mesozoic succession in the inner Bristol Channel area. About 2000 m of mainly Jurassic strata occupy the Bristol Channel syncline north of Foreland Point but this structure attenuates eastwards and is replaced by the Glastonbury syncline which contains nearly 800 m of Mesozoic strata under Bridgwater Bay. The Central Bristol Channel fault zone has been traced eastwards towards Bridgwater Bay, and has a southerly downthrow of several hundred metres in mid-Channel north of Porlock. There is some evidence for an early phase of movement along the fault zone, bringing Lower Palaeozoic against Upper Palaeozoic rocks.

A problematic reptile from the British Upper Trias

The adaptive significance of the structural changes that led to the differentiation, during the Triassic, of two important stocks of reptiles, the Lacertilia (lizards) and the sphenodontid Rhynchocephalia, is discussed. This provides a context for the consideration of the peculiar combination of characters found in a reptile, Glevosaurus hudsoni, from the Upper Trias of the Bristol Channel area, that has some structural affinities with each of these stocks.

Depositional History of Carboniferous (Middle Visean) Limestones from Bristol and Parts of South Wales

The mid-Visean Clifton Down Limestone and parts of adjacent formations on the Gower Peninsula of southern Wales can be subdivided into informal lithologic units with dominant microfacies that reflect both minor cyclic deposition and the effect of terrigenous influx toward the type locality at Bristol, England. These units also contain distinctive biotic and geochemical constituents that support paleoenvironmental interpretations. Carbonate sediments were deposited on a shallow subsiding shelf under poorly oxygenated conditions, in a warm humid environment subject to variable physical energy, as indicated by the presence of abundant algae, relatively dark color, a relatively thick carbonate sequence without evaporites, and microfacies ranging from low-energy fine pelmicrites to high-energy coarse oointrasparudites. The seemingly complex sequence of lithologic units can be explained by postulating concurrent local regression, including seaward oolitic sand barrier migration and regional transgression, including slow basin subsidence. The oolitic barrier is thought to have been formed by waves and related longshore currents on a slope break above a structural hinge line between a shallow shelf and a slightly deeper geosynclinal basin in the present Bristol Channel area. Restriction of the shallow shelf between the barrier and St. George's Land, about 30 mi north, created a back-barrier, tidal-flat environment. Subtidal, intertidal, and questionable minor supratidal subenvironments are indicated by tidal-channel ooliths, angular intraclasts, and algal-mat fragments possibly derived from mudbanks, s me of which may have been subaerially exposed. Distal prodeltaic facies, with varying terrestrial influx through time, are thought to have resulted from the lateral swinging or intermittent progradation in a known delta complex north and east of Bristol, beyond the area of the present study.

Factors Affecting the Distribution of the Intertidal Isopods Eurydice pulchra Leach and E. affinis Hansen in Britain

Recent studies on the distribution of Eurydice pulchra Leach and E. affinis Hansen around the British coasts (Jones & Naylor 1967) have revealed that the latter species is restricted to the Bristol Channel area and, where these species occur together, they are seldom found in comparable densities. Salvat (1966, 1967) has shown some differences in the distribution of these species at Arcachon, south-west France, noting that E. affinis appeared to prefer semi-sheltered beaches, whilst E. pulchra occurred over a wide range of beaches, as is to some extent seen in Britain (Jones & Naylor 1967). Salvat also showed that the two species have different patterns of vertical zonation at Arcachon, but such differences have not been observed in British populations. In view of the reported differences in the relative distribution of these species, both from beach to beach and on the same beach, detailed investigations have been carried out in Britain with a view to elucidating the factors controlling their distribution. Two beaches in South Wales, at Rotherslade and Oxwich near Swansea, were chosen for these studies, as preliminary sampling revealed wide and consistent differences in population densities of the two species on these beaches. The physical conditions of each beach have been measured, including factors such as sand grain size, salinity, oxygen content and exposure, and these have been related to the distribution and densities of the animals collected in quantitative samples over a period of 2 years. Experimental work on the behaviour of these isopods is also considered in conjunction with the physical factors listed above.