Folding in intertidal sediments on the west Lancashire Coast, England

Folded mud layers are observed to be spatially associated with dune bedforms in the runnels of a wide multi-barred foreshore. An hypothesis is presented linking the formation of the folds, by porewater and sediment movements, to the growth of the dune bedforms. The hypothesis is supported by published reports of laboratory experiments by other authors.     

Variscan very low-grade metamorphism in southwest England: a diastathermal and thrust-related origin

Abstract Regional metamorphism in the external Variscides of southwest England varied from diagenetic level to greenschist facies. There is a fundamental difference in the metamorphic character between the northern and southern regions of the area. In the north, M1 metamorphism is of a sedimentary burial character associated with high heat flow, whilst to the south it is related to tectonic burial during thrust thickening processes, with lower geothermal gradients. This pattern appears to be related to the character of basin development and its subsequent tectonic evolution. The northern region has features that accord with a diastathermal (extensional) origin for the very low-grade metamorphism whilst in the southern region the very low-grade metamorphism is linked to thrusting as a consequence of Variscan compression. The Tintagel High-Strain Zone presents an anomaly in this regional pattern where an M2 metamorphic phase is attributed to localized D2 thrust stacking along the southern margin of the Culm Basin.
There is no extensive overprint of the regional metamorphic pattern by the contact aureoles surrounding the granite plutons of the region. However, there is a noticeable coincidence between the areas of regional epizone grade and the extent of the geophysically defined subsurface limit of the granite batholith (excluding the North Devon area). This link is attributed largely to the late-stage structural up-doming of the higher grade areas over the roof of the batholith.     

Seasonality of modern pollen and sediment deposition in an estuarine context: the Severn Estuary Levels,southwest England

Recent sedimentological and palynological research on subfossil Holocene banded sediments from the Severn Estuary Levels suggested seasonality of deposition, registered by variations in mineral grain‐size and pollen assemblages between different parts of the bands. Here we provide data that strengthen this interpretation from sampling of modern sediments and pollen deposition on an active mudflat and saltmarsh on the margin of the Severn Estuary, and comparison with a vegetation survey and contemporary records of climate, river and tidal regimes. The results of grain‐size analysis indicate deposition of comparatively coarse‐grained silts during the relatively cool and windy conditions of winter and comparatively fine‐grained sediments during relatively warm and calm summer months. Pollen analysis demonstrates the significance of long‐term storage of pollen grains and fern spores in the estuarine waterbody, superimposed on which seasonal variations in pollen inputs from local and regional vegetation remain detectable. Copyright © 2007 John Wiley & Sons, Ltd.     

Petrography,mineral chemistry and origin of Type I enstatite chondrites

Optical and cathodoluminescence petrography were coupled with electron microprobe analysis to relate the textures and chemical compositions of minerals in the chondrules and matrix of the Indarch, Kota-Kota, Adhi-Kot and Abee Type I enstatite chondrites. Clinoenstatites fall into two distinct chemical groups with characteristic red or blue luminescence; red crystals are higher in Ti, Al, Cr, Mn and Ca, and lower in Na, than blue ones. Rare forsterites in Indarch and Kota-Kota show distinct compositions associated with orange or blue luminescence. The chemical ranges are indistinguishable for each color type in chondrules of all textural types, and the presence of both color types in a single chondrule or a metal fragment requires mechanical aggregation of both crystals and liquids of both color types. Porphyritic chondrules are ascribed mainly to aggregation of existing crystals because both types of pyroxene and olivine occur in the same chondrule. Large crystals of one color type are surrounded by fine-grained crystals of another type in some barred and radiating chondrules. All types of chondrules are surrounded by fine-grained rims rich in sulfide. The matrix contains many broken chondrules and individual silicate grains but is rich in sulfide and metal. Analyses are given of albite (minor elements and luminescence color vary between chondrites), kamacite, schreibersite, oldhamite and niningerite.Although the mineral assemblages do not fit theoretical condensation sequences in detail, the red pyroxene and orange olivine might result ultimately from near-equilibrium crystallization in which early reduced condensates reacted with a gas, while the blue crystals might result from fractional condensation in which early condensates were removed mechanically from a gas. Subsequent episodes involving mixing, melting, crystallization, condensation, fracturing, and mechanical aggregation would be needed to produce the complex textures.     

Dissolved aluminium in the Tamar Estuary,southwest England

Distributions of dissolved Al in the Tamar Estuary have been recorded through spring-neap tidal cycles in winter and summer. Dissolved Al is a highly reactive constituent in this estuary, undergoing net removal in the very low salinity region and net input to the mid-estuary. The extent of both the depletion and the augmentation varied systematically with tidal energy input, indicating that tidal sediment disturbance was the principal controlling agency.These field data, supported by the results of laboratory simulations, show that the removal is regulated by sorption onto resuspended sediment particles, which dominates kinetically over authigenic aluminosilicate formation. Removal of dissolved Al by flocculation of riverborne colloids is not significant. The mid-estuarine input is consistent with dissolution of authigenic aluminosilicate following net remobilization of estuarine sediment. The internal cycling of Al in the estuary, generated by these processes, alters both the time-course and the soluble-particulate balance of the riverine discharge of Al to the adjacent coastal water.     

Petrography and origin of deposits at the Bentheim bitumen mine, north western Germany

The fracture-bound deposit at the Sieringhoek (Bentheim) bitumen mine in NW Germany has yielded a substantial quantity of bitumen ore. Emplacement of the bitumen into the fracture system occurred under high pressure, causing brecciation and impregnation of the host Gildehaus Sandstone. Subsequently the bitumen was brecciated and cemented by calcite and calcite-entrained sand. Bitumen injection followed fracturing into a hydro-carbon reservoir at depth, generated from a Jurassic/Cretaceous source rock. Diagenesis of the host rock was largely completed by the time of bitumen injection, reflecting the long time gap during which source rock maturation occurred. Impurities in the bitumen include large quantities of rock fragments, abraded during injection, and crystals of metal oxides and sulphides.     

Petrography and depositional environment of the liddell seam, upper hunter valley, New South Wales

The Liddell Seam occurs in the lower part of the Permian Foybrook Formation of the Sydney Basin, eastern Australia. The seam profile is dominated by banded lithotypes (clarain). Maceral and microlithotype analyses of lithotypes indicate a limited degree of uniformity in their composition. A comparison of whole-seam petrographic analyses with compositions calculated on the basis of the seam lithotype profile and lithotype petrography suggests that the latter method may provide for a rapid assessment of petrographic variation in operating mines.Petrographic analyses form the basis of an interpretation of Liddell peat-swamp facies. The seam contains few clastic layers which are associated with relatively dull coals whose petrography reflects elevated water levels during deposition in the limno-telmatic, reed-moor and open-moor facies. Between these zones the seam profile is dominated by relatively bright coals deposited between periods of inundation, in the telmatic forest-moor and reed-moor zones. The lithotype profile and the high vitrite and clarite content indicate the significant influence of arborescent vegetation, a feature typical of seams deposited in tectonically unstable areas.     

Petrography, chemistry and origin of early diagenetic concretions in the Lower Carboniferous of the Isle of Man

Three silicified limestone horizons of D1 age from the Visean of the Isle of Man contain calcitic concretions with peripheral silica crusts, occasionally surrounded by a further calcitic layer. Components of the original sediment include carbonate skeletons, carbonaceous grains, sponge spicules and muscovite. Diagenetic products include calcite, dolomite, pyrite, sphalerite, clays, feldspar and quartz. The concretions are composed of neomorphic calcite. The time of recrystallization and the identity of the neomorphic precurosor are both unknown. Displacive, fibrous calcite is chemically similar to neomorphic calcite and both are of early diagenetic age. Granular and rhombic ferroan calcites are of late diagenetic age and were precipitated from pore-waters with Sr/Ca, Mg/Ca and Fe/Ca ratios unlike those of seawater. The difference between early silicification which produced silica crusts and later diffuse silicification of the host sediment is related to a change in sediment transmissivity between the two silicification periods. A four-fold scheme of concretionary growth is proposed. The supply of silica is from sponge spicules and that of carbonate from seawater via porewater. The distribution of organic matter, either as sporadic large carcasses or as small carcasses concentrated in particular horizons, is believed to be vital for carbonate precipitation and controls the distribution of concretions. Awareness of the multiplicity of diagenetic changes is essential in interpretation of early porewater systems and in the origin of products which are often metastable and destined to subsequent changes. No single model is an explanation for all types of concretionary growth.     

Stratigraphy and origin of New Forest brickearths,England

The thin, loamy brickearth deposits overlying the flinty terrace gravels of the New Forest are divided into older and younger members. The Lower (older) Brickearth includes sediments thought to be mainly loess, with some aeolian sand and possible river floodloam (overbank sediment). These share the common feature of palaeo-argillic soil horizons in their upper layers. Two separate phases of pre-Holocene temperate pedogenesis often can be distinguished in the palaeo-argillic horizons. The Lower Brickearth is the most extensive pre-Devensian loess in Britain. The Upper (younger) Brickearth consists mainly of Late Devensian (Oxygen Isotope Stage 2) loess, but its lower layers also contain fine sand derived mainly from local Tertiary strata. Both brickearths occur on all the terrace surfaces of the New Forest and indicate that the terraces date from Oxygen Isotope Stage 6 or earlier.     

Alluvial records of medieval and prehistoric tin mining on Dartmoor,southwest England

The role of tin mining in the society of prehistoric Dartmoor and its impact on the local landscape have long been discussed despite equivocal evidence for prehistoric mine sites. A fluvial geomorphological approach, using floodplain stratigraphy, combined with sediment geochemistry and mineralogy, was employed to identify prehistoric tin mining at the catchment scale. Waste sediment, released during hydraulic mining of alluvial tin deposits, caused downstream floodplain aggradation of sands with a diagnostic signature of elevated Sn concentration within the silt fraction. At a palaeochannel site in the Erme Valley, sediment aggradation buried datable peat deposits. A period of aggradation postdating cal. A.D. 1288–1389 is consistent with the 13th century peak in tin production identified in the documentary record. An earlier phase of aggradation, however, occurred between the 4th and 7th centuries A.D., providing evidence of late Roman or early Post Roman tin mining activity on Dartmoor. © 2004 Wiley Periodicals, Inc.     

Composition and origin of coalbed gases in the Lower Silesian basin,southwest Poland

Coalbed gases in the Lower Silesian Coal Basin (LSCB) of Poland are highly variable in both their molecular and stable isotope compositions. Geochemical indices and stable isotope ratios vary within the following ranges: hydrocarbon (C_HC) index C_HC=CH₄/(C₂H₆₊ C₃H₈) from 1.1 to 5825, wet gas (C₂₊) index C₂₊=(C₂H₆₊ C₃H₈₊ C₄H₁₀₊ C₅H₁₂) / (CH₄₊ C₂H₆₊ C₃H₈₊ C₄H₁₀₊ C₅H₁₂) 100 (%) from 0.0 to 48.3%, CO₂–CH₄ (CDMI) index CDMI=CO₂/(CO₂₊ CH₄) 100 (%) from 0.1 to 99.9%, δ¹³C(CH₄) from −66.1 to −24.6‰, δD(CH₄) from −266 to −117‰, δ¹³C(C₂H₆) from −27.8 to −22.8‰, and δ¹³C(CO₂) from −26.6 to 16.8‰. Isotopic studies reveal the presence of 3 genetic types of natural gases: thermogenic (CH₄, higher gaseous hydrocarbons, and CO₂), endogenic CO₂, and microbial CH₄ and CO₂. Thermogenic gases resulted from coalification processes, which were probably completed by Late Carboniferous and Early Permian time. Endogenic CO₂ migrated along the deep-seated faults from upper mantle and/or magma chambers. Minor volumes of microbial CH₄ and CO₂ occur at shallow depths close to the abandoned mine workings. “Late-stage” microbial processes have commenced in the Upper Cretaceous and are probably active at present. However, depth-related isotopic fractionation which has resulted from physical and physicochemical (e.g. diffusion and adsorption/desorption) processes during gas migration cannot be neglected. The strongest rock and gas outbursts occur only in those parts of coal deposits of the LSCB which are dominated by large amounts of endogenic CO₂.     

Early and Middle Pleistocene river systems in eastern England: evidence from Leet Hill,southern Norfolk,England

Pleistocene sediments at Leet Hill, southern Norfolk are examined in terms of their sedimentary structures, palaeocurrent indicators, clast and heavy mineral lithology and litho- and morphostratigraphic position. Colour of the quartzite and vein-quartz clasts is used to differentiate the Bytham and the Kesgrave sands and gravels, with the Bytham sands and gravels having a significantly higher proportion of coloured material. The Kirby Cane sands and gravels are the lower sedimentary unit and were deposited by the Bytham river, which drained a catchment extending into central England. At Leet Hill, erosion of the Kesgrave Sands and Gravels by the Bytham river has given the Kirby Cane sands and gravels a distinctive lithological assemblage. Trace clast lithologies suggest that the Kesgrave Sands and Gravels in the region of Leet Hill were deposited in a coastal location with an input from northern sources as well as southern and Welsh sources diagnostic of the Thames catchment. The glaciofluvial Leet Hill Sands and Gravels were deposited by outwash from the Anglian Scandinavian ice sheet. Initially the flow direction of the outwash was determined by the Bytham river valley, but this changed to a southerly direction once the valley had been infilled. This paper provides the first indication of the location of the boundary (Early Pleistocene coastline) between the fluvial Kesgrave Sands and Gravels and the marine equivalent reworked by coastal processes, and demonstrates the way the pre-glacial relief initially controlled patterns of glaciofluvial sedimentation during the early part of the Anglian glaciation. Copyright © 1999 John Wiley & Sons, Ltd.     

Conflicting imaginations: Archaeology, anthropology and geomorphology on Leskernick Hill, Bodmin Moor, southwest Britain

In this paper we view the different practices of archaeology, anthropology, environmental reconstruction and geomorphology through the lens of fieldwork on the clitter fields and Bronze Age settlement patterns on Leskernick Hill, Bodmin Moor, southwest Britain. The moor forms one of the best preserved fossil prehistoric landscapes of Europe and has undergone repeated periglacial episodes during the Quaternary. We show that the characterisation of patterned ground by archaeologists and anthropologists can be very different from that generated by geomorphology, particularly with respect to the spatial scales at which culture/nature questions are posed. We argue that the research project at Leskernick is a good example of how multi-disciplinary work is often more fruitful than mono-disciplinary and provides an example of how conversations across the divides of disciplinary practice can be held.     

Early lithification and hardgrounds in upper Albian and Cenomanian calcarenites,southwest England

Early diagenetic lithification of calcarenites on the sea floor led to the development of a variety of hardgrounds, intraformational conglomerates, breccias and boulder beds in southwest England during late Albian and Cenomanian time. Cemented nodules commonly developed below the sea floor, and these were avoided by burrowing infauna. In some instances the nodules were exhumed and reworked on the sea floor to form distinctive intraformational conglomerates; exposed nodules frequently became bored and encrusted by organisms and mineralised by glauconite and phosphate. In other cases, sea floor cementation produced true hardgrounds whose upper surfaces were affected by the same processes, but the hardened layers were only a few tens of centimetres thick and were underlain by soft sediment. Fracturing and brecciation of some hardground layers occurred through differential compaction and through undermining and collapse as the result of burrowing and erosion beneath the hardened layer. Reworking of clasts produced in this manner yielded intraformational breccias. Petrographic analysis reveals multiple generations of carbonate cement, commonly beginning with syntaxial overgrowths on echinoderm fragments and “dog tooth” spar on polycrystalline carbonate grains. A progression exists in the Albian-Cenomanian succession of southwest England from relatively simple hardgrounds and intraformational conglomerates low in the sequence up into complex hardgrounds that may record many stages of sediment accretion, cementation, mineralisation and erosion. This progression appears to record increasing water depths and increased sea floor diagenesis.     

Petrography,geochemistry and geodynamic environment of potassic alkaline rocks in Eslamy peninsula,northwest of Iran

Eslamy peninsula, 360 km² in area, is located in the eastern coast of Urmieh lake in the northwest of Iran. This peninsula is a complex stratovolcano with a collapsed center, which is elevated due to later intrusions of sub-volcanic masses with trachytic to microsyenitic composition. The composite cone consists of a sequence of leucite tephrite, tephrite, leucite basanite, basanite and related pyroclastic rocks. Magmatic activities in the Eslamy peninsula begin with potassic alkaline to ultrapotassic and basic, silica-undersaturated shoshonitic rocks and they are followed by intrusions of lamprophyric dykes and end with acidic magmatism including trachytic, microsyenitic, syenitic and phonolitic domes. The original magma of the Eslamy peninsula rocks has a potassic alkaline nature (Roman type) rich in LREE and LILE and depleted of HREE. These characteristics suggest that the origin of magma can be from deep mantle with a garnet lherzolite composition, a low partial melting rate which has been contaminated by crustal materials in its way up. Fractional crystallization of olivine, diopsidic clinopyroxene and leucite played an important role in the evolution of magmas. Scrutinizing the geodynamic environment of Eslamy peninsula rocks in discrimination diagrams indicates that these rocks must have been formed in a post-collision magmatic arc setting.     

Fabrics,facies control and diagenesis of lacustrine ooids and associated grains from the Upper Triassic,southwest England

Petrographic analysis of ooids from the Upper Triassic (Mercia Mudstone Group) of southwest England provides an opportunity to assess in detail the origins, transport pathways and diagenesis of an ancient oolite. The Clevedon Oolite is dolomitized and contains a variety of dissolved ooids (oomoulds) and associated grains. The oomoulds occur in well‐sorted, planar and cross‐stratified grainstones, packstones, sandstones and conglomerates associated with shoreface, intershoal, foreshore, beachrock and littoral strandplain deposits. The ooids grew in suspension in the shoreface zone and developed a radial aragonite microstructure. The ooids grew to 0.80 mm in diameter, after which they fractured or ceased growing. Broken grains deposited on or near mobile shoals were rapidly recoated, while other grains, deposited in less agitated, intershoal and lower foreshore areas, were micritized or microbially bound into grapestone aggregates. Locally peloids, intraclasts, quartz grains and micritized grains from intershoal areas supplied nuclei for ooids on nearby shoals. Grains deposited in foreshore areas were rapidly cemented into beachrock and reworked into conglomerates. Soon after deposition, the ooids were subjected to widespread aragonite dissolution followed by dolomitization. The lack of pre‐dolomitization calcite, together with the abundance of early (pre‐compaction) dolospar cements and fabric‐selective dolomitization of micritic fabrics, suggest aragonite dissolution by dolomitizing fluids. Copyright © 2002 John Wiley & Sons, Ltd.     

Wave and tidal current sorting of shelf sediments southwest of England

The relative roles of waves and tidal currents in transporting bottom sediment on the continental shelf off Lands End, southwest England, are evaluated by study of (a) sediment grain size in relation to boundary layer measurements in tidal currents, (b) regional variation in sediment parameters in relation to peak tidal and wave-induced currents, and (c) visual observation of bedforms. (a) The sediments are mainly zoogenic sands. The average hydraulic equivalent median diameter is Mdφ=1.40φ (medium grade sand), and two-thirds of the median grain sizes fall between 0.97φ and 1.83φ. The linear bottom current which will just move this range of sizes is exceeded only slightly by the highest tidal drag velocities ū* measured in the area. Thus, sediment movement by tidal currents alone is restricted to areas of high bed roughness and strong peak tidal flows. In contrast, wave-induced oscillatory currents at 100 m depth (typical of the area) attain sufficient speed to disturb the same particle sizes over 3% of the time. This includes storm periods when much greater velocities occur. (b) The average Mdφ of the sediment decreases southwest and northeast from south of the Lizard. This correlates well with the pattern of maximum tidal current speeds, suggesting that tidal currents control the areal distribution of sediment median grain size. Most sediments are well sorted (mean σ_i=0.48φ). Sorting improves at shallower depths but does not improve in areas of faster tidal currents, suggesting that wave-induced currents exert the major control on sorting. Silt and clay proportions increase west of the Scilly Isles and are influenced by both wave and tidal currents. (c) Photographs and television pictures show that symmetrical bedforms due to wave action are dominant north and west of the western Channel. Asymmetric bedforms are more common in the western Channel itself, where tidal currents and bed roughness are both high. Results are used to construct a sediment transport model for the study area. Since medium grade well sorted sands occur in depths of over 100 m, many ancient, extensive, well sorted sand sheets may have been deposited at depths greater than previously suspected.