bluuebunny/crossref_metadata_2025 huggingface dataset:
The margin of the West African craton: the Voltaian Basin
Authors: Ako Justice A. , Wellman Peter
Published: December 2007

The Voltaian Basin in Ghana is interpreted as a foreland basin formed by sediments filling a flexural depression at the margin of the West African Craton. The flexure is due to northwest obduction onto the craton of younger, Pan African crust. This model is consistent with our knowledge of the Voltaian Basin from geology, seismic refraction profiles, and depths to magnetic basement. Major gravity and magnetic anomalies over the middle of the Voltaian Basin, approximately along 0
o
longitude, are interpreted as due to major structures in the West African Craton underlying the basin, the structures being older than the basin sediments. The anomalies could be due to either a combination of an elongate N–S striking change in crustal thickness or density in the craton and two large areas of mafic igneous rock or, preferably, to a major thrust within the West African Craton along a 300-km long boundary between crusts of different ages.
Intrusive metabasite belts within the Moine assemblage, west of Loch Ness, Scotland: evidence for metabasite modification by country rock interactions
Authors: Rock N. M. S. , Macdonald R. , Walker B. H. , May F. , Peacock J. D. , Scott P.
Published: December 2007

Metabasites representing an extensive differentiated sill-complex (with minor dykes) now form two
c
. 10 km-wide NE-SW ‘belts’ in western Inverness-shire: a
Western Belt,
overlapping the ‘Quoich Line’, and an
Eastern Belt,
beside the Great Glen Fault. Regional evidence and, where interpretable. Rb-Sr isotope data, imply Grenvillian emplacement. More than one intrusive episode may be represented by contrasting adjacent discordant/unfoliated versus concordant/foliated bodies. although some such contrasts reflect later tectonism. Despite variations in form, size, internal homogeneity, paragenesis, grain-size and fabric, 120 newly analysed metabasites have much more uniformly tholeiitic chemistry than metabasites in the Moine of Ross-shire or Sutherland. Discriminant analysis nevertheless reveals that the Eastern Belt is compositionally invariable from NE to SW. whereas the Western differs between three sub-areas studied in detail, and shows more numerous evolved compositions. A single andesitic rock (
c
. 57% SiO
2
) is the most evolved metabasite so far reported from the Moine. Metabasite–country rock interactions during igneous emplacement and/or regional metamorphism caused major chemical transfers. For example, host psammites and granite-gneisses gained Ca. causing hornblende to crystallize; metabasites in limestones gained Ca, Mg, Sr, and lost Fe, Ti, Rb, Rb, Y and Zr, totally suppressing garnet; metabasites in pelites lost Na, enhancing garnet relative to plagioclase; metabasites within psammites gained K and Rb, enhancing biotite; and metabasites within granite-gneisses gained Na, K, Mn, Rb and Sr, suppressing epidote. Overall chemical differences between metabasites with or without biotite, epidote and garnet may reflect the same interactions, rather than original magmatic variations. Even the ‘immobile’ element chemistry of high-grade metabasites may be an unreliable tracer of magmatic chemistry, unless country rock interactions are allowed for.
A reinterpretation of the concealed Variscan structure beneath southern England by section balancing
Authors: Williams G. D. , Brooks M.
Published: December 2007

Recently, Chadwick
et al.
1983 presented an interpretation of the concealed Variscan structure beneath southern England based on deep seismic reflection profiles. We re-evaluate the status of reflection events identified along some of the profiles and draw attention to major uncertainties in the interpretation presented. The geological section of Chadwick
et al.
is admissible in that it portrays similar structures to those seen to the west in the Mendips. However, the section is not a valid balanced section as it cannot be restored to a pre-thrusting stratigraphy. Using the published seismic reflection data and borehole information, we present a new balanced cross-section of the Variscan structures for which a total tectonic shortening of about 20% is calculated.
Placer deposits symposium
Authors: Pitty Alistair F.
Published: December 2007

Report of a whole-day joint meeting of the Institution of Mining and Metallurgy and the British Geomorphological Research Group held at Burlington House, 15 November 1984. The meeting was organized by Professor F. K. G. Hosking, Dr Ian Reid, Dr D. G. Sutherland and Professor M. F. Thomas, and convened by Drs Reid and Sutherland.

At this joint meeting, some 160 participants explored those areas where process geomorphology is instrumental in the identification of metalliferous palaeosediments. Water transport and reworking of debris are fundamental controls on placer concentration by mechanical processes, and chemical deepweathering of land surfaces is a feature of environments where residual enrichments are concentrated.

The initial session, chaired by K. F. G. Hosking, was an immediate reminder of the global span and regional diversity of the topic. D. G. Sutherland’s overview featured the main areas in the world of placer deposits. He examined the fundamental role of chemical deep weathering in releasing minerals from the host rock, noting the temperature and moisture conditions required. Climate is also important and controls the volume of runoff, which then determines the amount of debris moved. The climatic changes of the past, particularly during the last 2 Ma, were emphasized.

T. K. Smith’s talk featured an area of former hydraulic mining in the Sierra Nevada in the San Juan placer gold deposits. The significance of the unlikely name of his study area, the Red Dog–You Bet district, was only explained during the informal discussions, which were a conspicuous feature of
Role of settling, entrainment and dispersive equivalence and of interstice trapping in placer formation
Authors: Reid I. a. n. , Frostick Lynne E.
Published: December 2007

Grain-scale placer mineral concentrating processes are reviewed. Early explanations of localized enrichment of heavy particles in terms of sorting associated with the settling equivalence of clasts of differing density have been superseded. Entrainment equivalence is shown to be a more appropriate concentrating factor in both fluvial and littoral sedimentary environments. The vertical shuffling of particles of different density that is associated with dispersive equivalence is an ancillary process that presents appropriate particles for entrainment sorting. However, the role of entrapment of comparatively small grains in the interstices of the stream bed or the beach face is shown to be more important that hitherto envisaged. This depositional enrichment is controlled by the relative sizes of the bed sediment pores and the ingressing particles. Local vertical changes in framework particle size dictate patterns of interstice clogging and hence the quantity of placer mineral rich matrix passing into the bed.
Late Quaternary alluvial placer development in the humid tropics: the case of the Birim Diamond Placer, Ghana
Authors: Hall A. M. , Thomas M. F. , Thorp M. B.
Published: December 2007

Examination of the diamondiferous sediments of the Birim River floodplain in Ghana using radiocarbon dating allows the recognition of three chronostratigraphic units: 13000–11000, 9000–7000, and 2100 years BP to the present, while older sediments rest on bedrock benches and form terraces. The late Pleistocene sediments infill deeper, scour channels, are generally both thicker and coarser, and return higher diamond grades. One thousand nine hundred samples from 700 boreholes indicate that diamond grade is influenced by bedrock consistency, gravel thickness, gravel calibre, and stratigraphic position, but other external factors include supply of coarse quartz from bedrock, valley morphology, and the location of tributary junctions and diamond sources. The influence of palaeoenvironmental conditions on fluvial deposition and placer formation in W Africa appears widespread and relates to the known late Quaternary climatic changes established from Lake Bosumtwi, Ghana, and from the Sahara.
Indonesian fluvial cassiterite placers and their genetic environment
Authors: Aleva G. J. J.
Published: December 2007

Placer valley systems in the areas of Belitung and Singkep islands and in the Tujuh archipelago, Indonesia, are described with the use of geological and contour maps, longitudinal sections and grade distribution charts. The main cassiterite concentration (>95% of all minable cassiterite) in these deposits rests directly on weathered bedrock. Cover-layer (Miencan) cassiterite concentrations occur at various levels, indicating continued upstream erosion and downstream sedimentation of cassiterite. The valley bottoms currently reach depths below mean sea level of ~50 m; greater depths, down to –100 m, are recorded on sonic sections and can be deduced from wildcat drill-holes downstream from the mined areas. All offshore and large parts of the onshore placers are palaeoplacers, which have been protected against erosion by marine, littoral and paludal sediments. The stratigraphical succession of this cover has been established in the Tujuh area, but a definite dating has not yet been achieved. The collected data indicate a concentration process on the bottoms of fluvial valleys, which at that stage contained only a thin sediment layer that was composed mainly of coarse, angular clastics and heavy minerals. The distribution of cassiterite on the valley floors is locally determined by morphology—at the lowest parts, in local depressions and behind obstacles—whereas on a larger scale it indicates a largely autochthonous or residual provenance, with maximum fluvial transportation distances of ~500 m. In contrast, many of the W Malaysian cassiterite placers are alluvial concentrations and found at various levels within the relatively fine-grained sedimentary valley fill.
Intertidal drainage and mass-movement processes in the Severn Estuary: rills and creeks (pills)
Authors: Allen J. R. L.
Published: December 2007

Mass-movement processes acting at up to three scales influence the intertidal forms of drainage in the Severn Estuary and its bordering estuarine alluvium. The open mudflats are marked by delicate drainage rills into which some freshly deposited silt is transferred at low tide by downslope creep, as shown by widespread systems of small extensional fractures. The pills traverse the areas of reclaimed alluvium and the saltings. They retain a relatively symmetrical cross-sectional profile, even where tortuous, and are remarkably stable in both general position and detailed form. The pills are maintained by large rotational slips accompanied in many cases by channelled mudflows formed from much smaller failures. A profile of long-term sediment accumulation that favours the loading of the upper sides of the pills, irrespective of local channel curvature, appears to be a significant factor in the maintenance of these drainage lines.
Volcanogenic mineralization related to the Snowdon Volcanic Group (Ordovician), Gwynedd, North Wales
Authors: Reedman A. J. , Colman T. B. , Campbell S. D. G. , Howells M. F.
Published: December 2007

The Ordovician (Caradoc) volcanic rocks of NE Snowdonia constitute two major groups, the Llewelyn Volcanic Group and the Snowdon Volcanic Group, which accumulated predominantly in shallow-water marine conditions. The younger Snowdon Volcanic Group comprised a bimodal, basalt-rhyolite suite and included a major caldera-forming eruption of acidic ash-flow tuffs superseded by both Surtseyan and Strombolian basaltic volcanism. Rhyolite domes were intruded into the volcanic sequence in the vicinity of the caldera. The Snowdon Cu-Pb-Zn vein mineralization comprises five paragenetic mineral assemblages. The veins cut rocks deposited within and over the caldera and it is proposed that the dominant controls of mineralization were volcanogenic. Circulation in hydrothermal cells, involving both juvenile fluids and seawater, deposited the minerals at a late stage in the evolution of the caldera.
The geochemistry of early Proterozoic metavolcanic rocks hosting massive sulphide deposits in the Skellefte district, northern Sweden
Authors: Claesson L. Å.
Published: December 2007

The Skellefte district, an early Proterozoic massive sulphide ore province in Northern Sweden, contains a well-preserved volcano-sedimentary rock association, referred to as the Skellefte Group, metamorphosed predominantly under low-grade conditions. The volcanic rocks in the lower part of the succession host the massive sulphide deposits and comprise a bimodal sequence, composed of a calc-alkaline group of felsic rocks and a tholeiitic group of mafic rocks displaying a basaltic to andesitic composition. Derivation of the felsic rocks by fractional crystallization from the mafic rocks is not supported by the geochemical data. Application of tectonic environment discriminant diagrams involving the elements Ti, Zr, Y and Cr suggests that these rocks were deposited in a volcanic arc setting.

Stratigraphically above the volcanic rocks hosting the massive sulphide deposits, there follows a sequence of metamorphosed greywackes and pelitic sediments with intercalation of primitive basaltic to ultrabasic volcanic and high-level intrusive rocks. The volcanic rocks show geochemical similarities to both Archaean basaltic komatiites and Phanerozoic boninites. The primitive nature of these rocks present difficulties in assessing their palaeotectonic setting. A volcanic arc or transition to an inter- or back-arc rift situation is suggested.
Thermal enhancement of magnetic fabric in Cretaceous sandstones
Authors: Perarnau A. , Tarling D. H.
Published: December 2007

Heating of samples of the San Juan Cretaceous sandstones (NE Venezuela) results in large increases in their low field susceptibility, mostly above 650°C, which allows measurement of their low field susceptibility ellipsoids. Where measurements are available before and after heating, the same fabric is almost always enhanced. Samples, previously too weak for measurement, showed thermally induced fabrics which are consistent within each site, but vary between different localities. The susceptibility changes are all largely attributed to magnetite formation, probably from vermiculite at 450°C in the proximal sediments, and from illite at ≥850°C. In all areas, the induced magnetic fabrics can be interpreted in terms of either sedimentary structures or low grade metamorphism related to cleavage parallel to the fold axes. It is considered that this thermal enhancement technique offers a method for measuring low field magnetic fabrics in a wider range of geological conditions and lithologies than reviously, although the processes involved need considerable study.
Some buried Tertiary dykes of Britain and surrounding waters deduced by magnetic modelling and seismic reflection methods
Authors: Kirton S. R. , Donato J. A.
Published: December 2007

Dykes of Tertiary age have been identified by modelling offshore marine and aeromagnetic data and onshore aeromagnetic data. Dykes have been found in the North Sea off NE England, and in the Irish Sea. Published aeromagnetic maps have also been used to locate non-outcropping Tertiary dykes in the Scottish Borders. Many of the offshore anomalies are caused by multiple dykes that cannot be separately resolved. All dykes retain the dominant NW-SE trend of the known Tertiary dykes although this trend approaches E-W near the east coast of England. Under the North Sea the trend swings back and becomes increasingly southerly trending. The dykes may be the result of NW-SE compressional or NE-SW tensional stress with deep crustal weaknesses causing changes in trend.
A gravity and magnetic interpretation of the structure of the Irish Midlands and its relation to ore genesis
Authors: Brown C. , Williams B.
Published: December 2007

Regional gravity and magnetic data from the Irish Midlands have been studied in an attempt to map the structures within and beneath the Lower Carboniferous Limestones. The magnetic data suggest that the Precambrian crystalline basement is at a depth of 3–4 km and is overlain by Ordovician volcanics of varying magnetization. A Fourier analysis of the gravity field has been used to construct a residual gravity map which represents principally the gravity effect of shallow sources. These sources are probably the topographic variations of dense Ordovician volcanics beneath a less dense succession of Ordovician to Carboniferous sediments.

The residual gravity clearly shows the presence of a series of linear volcanic blocks and sedimentary troughs trending NE–SW. The major mineralized zones in the area all occur close to the margins of these troughs. This investigation indicates that the initial position of the Midland Plain was controlled by Caledonide factors, some of which modified the later subsidence which we attribute to Hercynian plate-margin processes. The genesis of the major ore-bodies is probably associated with de-watering of the shale-filled troughs, the subsequent concentration of metal-bearing fluids along the trough margins and their deposition in relatively unloaded carbonate host rocks on the up-standing blocks.
Stratigraphy and palaeogeography of the Siham group:direct evidence for a late Proterozoic continental microplate and active continental margin in the Saudi Arabian Shield
Authors: Agar R. A.
Published: December 2007

The presence of ancient continental crust in the Afif terrane of the Arabian Shield has been indicated by radiometric data. Detailed mapping in the central Shield reveals a volcano-sedimentary succession older than one of the final cratonizing events of the Shield and unconformable upon high grade paragneisses, providing direct evidence for a western continental margin to the Afif terrane in late Proterozoic times. The basement gneisses were metamorphosed to almandine-sillimanite amphibolite facies and preserve evidence of a deformation not seen in any other rocks of the area. The Siham group, unconformable upon this basement, was metamorphosed to upper greenschist facies. It has been divided into six formations which provide an E–W section through the group. The lower members are volcanic and are absent in the easternmost formation. Their clastic content increases eastward. A carbonaceous shale with exhalative chert and marble, tuffs and associated volcanogenic massive sulphides is associated with the volcanics. It is absent in the east where shallow marine marbles and quartzites lie unconformably upon basement. Rhyolite and conglomerate complete the succession. In the west, the base of the group is not seen but the shale is associated with numerous sheet-like bodies of mafic and ultramafic rock emplaced in an extensional tectonic setting. Other plutonic rocks are calc-alkaline and are emplaced mainly in the west. Deep water oceanic conditions prevailed in the west and a shallow marine continental shelf environment existed in the east, with deposition of the group across this continental margin. The stratigraphy of the Siham group is particularly akin to the Mesozoic of Peru. Magmatic arc and marginal basin were coincident in space and time, and the marginal basin was probably extensional. There is some limited evldence that it was ensialic. The Siham group is bounded to the west by a N-S suture zone formed in one of the final collisional events in the cratonization of the Shield. This event caused the deformation of the Siham group and marked the termination of the magmatic are.
Subsidence of the North Aegean trough: an alternative view
Authors: BROOKS M. , KIRIAKIDIS L.
Published: December 2007

The North Aegean trough is a zone of deep water trending from northeast to southwest across the N Aegean Sea. The western part of the trough, the Sporadhes basin, contains a thick sequence of late Cenozoic sediments. Le Pichon
et al.
1984 suggested that the Sporadhes basin coincides with a zone of extreme lithospheric stretching and that the crustal thickness is locally reduced to only 15 km. We argue that the geophysical evidence cited by Le Pichon
et al.
can be equally well explained by a crustal model involving a much smaller amount of local crustal thinning.
Structural evolution of the Irish Variscides
Authors: COOPER M. A. , COLLINS D. A. , FORD M. , MURPHY F. X. , TRAYNER P. M. , O’SULLIVAN M.
Published: December 2007

The effects of the Variscan orogeny on the Upper Palaeozoic succession are recognizable throughout central and southern Ireland. The deformation can be zoned using structural styles and trends which illustrate a northward decrease in deformation intensity coupled with increasing control by pre-Variscan structures in the basement. The most intense deformation occurs to the south of the Dingle–Dungarvan line, where shortening approaches 50% as a result of layer-parallel shortening, followed by buckling and finally thrusting. The deformation is considered to have developed above a shallow-dipping sole thrust which has been located offshore by the SWAT seismic reflection profiles. Cross-faults, which are common throughout the orogen, resulted from N–S shear couples; this is consistent with their origin as accommodation structures due to differential shortening along strike. The original shape of the Munster Basin has exerted a control on the distribution and overall facing of the major structures. The sole thrust extends to the north of the Dingle–Dungarvan line and the structural zonation of the orogen is due to variations in the stratigraphic level of this thrust. The Leinster Massif is considered to be essentially autochthonous; slip on the sole thrust decays towards the Massif which acted as a rigid, passive block and was partially responsible for the arcuate trend of the orogen.
The modification of chevron folds by simple shear: examples from north Cornwall and Devon
Authors: LLOYD G. E. , WHALLEY J. S.
Published: December 2007

This contribution considers the effect of a horizontal simple shear deformation on an initially upright chevron fold pair using geometrical models. The models utilize an initial fold geometry based on structures observed in the coastal sections of north Cornwall and Devon, and consider various mechanisms by which the simple shear could be accommodated. Due to their different initial orientations the limbs of the chevron folds rotate at different rates during simple shear, and this is accommodated by the modification of the fold structure. Possible modification structures include: normal faulting in the opposite direction to the sense of shear on either the axial planes or the limbs of the folds; and folding on the limb which dips in the same general direction as the sense of shear. The results of the modelling can be used to interpret the structures observed in the studied sections. It is shown that a southerly-directed simple shear deformation acted in discrete zones resulting in the local modification of certain chevron folds. This deformation is in the opposite direction to the main Variscan movements in SW England. Nevertheless, it is argued that the simple shear is an integral component of the main deformation and represents an analogous process to the mechanism of backthrusting present in most thrust belts.
Mid-Devonian tectonics and sedimentation in the Bristol Channel area
Authors: TUNBRIDGE I. P.
Published: December 2007

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.
Fossils and tectonics along the Highland Boundary Fault in Scotland
Authors: CURRY G. B.
Published: December 2007

The tectonic significance of fossil discoveries from the Highland Border Complex of Scotland is described. These sparse but diverse macro- and microfossils have been recovered from a wide range of lithologies across Scotland, and range in age from early to late Ordovician, thereby refuting any attempt to incorporate the HBC within the Dalradian Supergroup. The Lower Cambrian trilobite fauna from the Leny Limestone could represent an isolated remnant of an early development of the Ordovician HBC basin(s), or alternatively may provide an upper age limit for Dalradian sedimentation. Further palaeontological prospecting in the zone, especially in relatively undeformed late Dalradian sediments, could resolve this and other problems which complicate the regional tectonic interpretations.
Geochemistry of bimodal basalt-subalkaline/peralkaline rhyolite provinces within the Southern British Caledonides
Authors: LEAT P. T. , JACKSON S. E. , THORPE R. S. , STILLMAN C. J.
Published: December 2007

Bimodal associations of basalt and rhyolite of Upper Ordovician age which were erupted in a submarine environment occur within the Caledonian orogenic belt of South Britain at Parys Mountain (Anglesey), in Snowdonia (North Wales) and at Avoca (SE Ireland). The volcanic rocks have experienced hydrothermal alteration and low-grade metamorphism, and therefore immobile elements (e.g. Ti, Zr, Nb, Y) have been used to identify the original geochemical characteristics. The basalts have characters transitional between volcanic ‘arc’ and ‘within plate’ types consistent with eruption on an extensional part of an active continental margin. Two groups of rhyolites have been identified. A low-Zr group (Zr<500ppm), represented at all three locations, is interpreted as originally of high-K, subalkaline type. A high-Zr group (Zr>500ppm), represented at Snowdonia and Avoca, is interpreted as originally being peralkaline in composition; their high Zr/Nb ratios (>10) are typical of peralkaline rhyolites erupted above subduction zones. The bimodal nature of the associations and the peralkaline character of some rhyolites indicates magma production in a complex tectonic setting, transitional between an active continental margin/island arc and an extensional environment. Associated sulphide mineralization is volcanogenic and probably syn-sedimentary. High-level, rhyolitic magma chambers are thought to have driven convection of the hydrothermal fluids from which the sulphides precipitated.
Revision of lithostratigraphic nomenclature in the Koobi Fora region, Kenya
Authors: BROWN F. H. , FEIBEL C. S.
Published: December 2007

Recent advances in our understanding of the Plio-Pleistocene sedimentary sequence northeast of Lake Turkana in northern Kenya has allowed a revision of the lithostratigraphic nomenclature and a more complete description of the deposits. As redefined, the Koobi Fora Formation encompasses the entire Plio-Pleistocene sedimentary sequence, and is subdivided into eight members. These members are delineated using volcanic ash horizons, which can be identified uniquely on the basis of chemical composition. The revised stratigraphy resolves all earlier conflicts with the biostratigraphic zonations based on vertebrate and molluscan studies. The sedimentary sequence of the Koobi Fora Formation attains an aggregate thickness of 560 m and spans the period from about 4.3 to 0.6 Ma. Detailed correlations have now been established to relate this sequence to other deposits within the Turkana Basin and to those at Hadar and in the Gulf of Aden.
Seismicity and tectonics in Wales
Authors: BLENKINSOP T. G. , LONG R. E. , KUSZNIR N. J. , SMITH M. J.
Published: December 2007

The distribution of instrumental and historical seismicity in Wales is described, including two events in 1984 for which good hypocentral locations were determined. Most of central Wales is aseismic throughout the historic record, suggesting that this is not a feature of reporting bias or population distribution alone. There appears to be a relation between the epicentres and some of the major fault structures of Wales, including the Bala Fault and the Swansea Valley Disturbance. The relationship between faults and seismicity is examined in detail for the two recent events, and more generally for the rest of the data, which have much lower locational accuracy. Stress measurements for NW Europe are shown to give a consistent NW-SE direction for the maximum horizontal stress. This is appropriate for sinistral strike-slip on N-S vertical faults. First motion data for both 1984 events also give a nodal plane in this orientation. In the first case, the event can be clearly attributed to mapped N-S vertical faults SE of Newtown, and in the second, a deep N-S normal fault under the Lleyn Peninsula is inferred as the most likely fault plane orientation from regional geological considerations. The proposed activity of the Bala Fault and the Swansea Valley Disturbance can also be explained within this stress field by their thrust reactivation as low angle structures. The general difficulty of relating the remaining seismicity to geological structures is demonstrated by the fact that the second 1984 event could not be directly connected with any surface structure, or even with known structures at depth.
Geomorphological approaches to the study of neotectonics
Authors: DOORNKAMP J. C.
Published: December 2007

The study of morphotectonics is concerned with the analysis of landforms whose form or origins have been affected by neotectonic activity. Traditional morphotectonic studies have been used as a basis for more refined (e.g. statistical) analyses. After the 1960s, however, there emerged new techniques and new approaches to the study of morphotectonics. These have made more precise not only the recognition of morphotectonic features, but have also improved their dating.

The time has come to integrate morphotectonic studies more fully both with the approaches used by other disciplines and with modern geomorphological theory.
A late Cenozoic regional mesofracture system in southern England and northern France
Authors: BEVAN T. G. , HANCOCK P. L.
Published: December 2007

The Upper Cretaceous and Palaeogene rocks of S England and N France are cut by a NW-trending system of fractures, characterized by swarms of vertical extension joints accompanied by conjugate sets of hybrid and shear joints, and normal mesofaults. Many major photolineaments exhibit the same NW trend as the fractures, and are particularly conspicuous in N France where they are coincident with rectilinear drainage channels, suggesting a relationship between fracture orientation and physiography. In S England, this relationship is absent; possibly because of the influence of additional fracture systems related to major flexures trending obliquely to the NW-SE direction. These major E-W trending flexures and associated fracture systems were formed during the Oligocene to Early Miocene 'Helvetic' phase of deformation, and are cross-cut obliquely by fractures in the NW-trending system. The NW-trending fractures and lineaments are parallel to neotectonic normal faults in the Lower Rhine embayment, and become less well developed to the west with increasing distance from these major structures. The Lower Rhine faults were reactivated and propagated into the Quaternary cover as a consequence of NE-SW regional tension generated during the late Neogene to Recent 'Jura' phase of NW-SE Alpine convergence. The NW-trending extensional structures in S England and N France are considered to be of about the same age and related to the same stress regime as the neotectonic normal faults of the Lower Rhine embayment. However, because fractures of the NW-system cutting Palaeogene sediments in SE England are truncated by the erosion surface beneath the Plio-Pleistocene Red Crag deposits, it is possible that in England fracture initiation did not continue into Quaternary times.
Towards precision in stratigraphy: geophysical log correlation of Upper Jurassic (including Callovian) strata of the Eastern England Shelf
Authors: PENN I. E. , COX B. M. , GALLOIS R. W.
Published: December 2007

Geophysical log correlation, when calibrated against cored borehole sequences, enables the relatively inexperienced geologist to subdivide the 340m of Callovian, Oxfordian and Kimmeridgian strata of the Eastern England Shelf into stratigraphical units of substantially less than zonal scale. The correspondingly detailed knowledge of shelf evolution shows the entire area to have subsided in distinct pulses which increased in strength with time, measured radiometrically, reaching a maximum in early to mid-Kimmeridgian times. The strength of the pulses varied geographically so that the northern shelf margin, the Market Weighton Block, was subject to more rapid changes in subsidence than the southern margin, the London Platform. Comparable pulsatory subsidence occurred in the Weald and Dorset Jurassic basins and adumbrates widespread tectonic events in NW Europe. An understanding of these subsidence patterns is a prerequisite to any detailed study of the sedimentation history or hydrocarbon potential of English Upper Jurassic strata.
A new gravity survey of the Lake District and three-dimensional model of the granite batholith
Authors: LEE M. K.
Published: December 2007

A new regional gravity survey of the Lake District has defined the principal anomalies in considerably more detail than previous surveys and has formed the basis for a three-dimensional interpretation of the sub-surface shape of the Lower Palaeozoic 'granite' batholith. The gravitational effects of the surrounding low density Permo-Triassic, Carboniferous and Silurian sedimentary successions have been calculated and removed from the observed field. The remaining residual gravity field due to the batholith has been interpreted on a 1 km square grid in terms of a generalized two-density model extending to a depth of 9 km. The interpretation shows that the Skiddaw Granite is a steep-sided, relatively flat-topped intrusion. The roof zone encompasses the Grainsgill, Caldew and Sinen Gill outcrops and corresponds well with the mapped thermal aureole. The southern part of the roof region of the Shap Granite coincides with the metamorphic aureole but a broad shoulder extends at about 1 km depth to the northwest beneath the Haweswater Igneous Complex. The Eskdale Granite is linked at shallow depth beneath the Borrowdale Volcanic Group to the Wasdale Granite. The central part of the Lake District is underlain by granite at relatively shallow depth (less than 2 km) which appears to be a subsurface extension of the outcrops at Eskdale and Wasdale.
Tectonic setting, mineralogy and chemistry of a metamorphosed stratiform base metal deposit within the Himalayas of Pakistan
Authors: FLETCHER C. J. N. , LEAKE R. C. , HASLAM H. W.
Published: December 2007

Stratiform Zn-Pb mineralization of probable Proterozoic age occurs in a structurally complex area near the northern margin of the Indian plate around Besham in the North West Frontier Province of Pakistan. The sphalerite and galena mineralization is associated with a well-defined stratigraphy of carbonate, layered calc-silicate quartzite, garnet-bearing quartzo-feldspathic gneiss, feldspathic granulite and albite gneiss. Associated with the ore are varying proportions of pyrrhotite, pyrite, baryte, clinopyroxene, quartz, garnet, actinolite, pyroxmangite, feldspar, carbonate and magnetite. The Mn-rich composition of the silicate minerals and the widespread enrichment of Ba in the rocks associated with the ore form a distinctive signature of the mineralization recognizable in reconnaissance exploration. This signature is distinct from that of skarn minerals formed later in carbonate rocks as a result of the intrusion of pegmatites. The recognition of the stratiform nature of the deposits within a well-defined stratigraphy enhances the exploration potential of the mineralization over a much wider area.
Transient electromagnetic studies over a Cu-Zn-Au mineral prospect Gairloch, Scotland
Authors: OGILVY R. D.
Published: December 2007

Comparative studies of different transient electromagnetic loop configurations have been made in a controlled mineral environment. The operational and interpretational advantages of the common, separated inline, and fixed transmitter loop profiling techniques are analyzed with respect to their response characteristics and detection capabilities. Laboratory and computer-aided theoretical simulations show that peak amplitude, response minima, and cross-over points are all important interpretative clues for estimating the location, attitude, and physical extent of the target mineralization. However such features are non-unique and profiling with more than one target-loop geometry is advisable to minimize interpretational ambiguities. It is shown that improved signal-to-noise ratios and detection limits are achievable with the fixed central transmitter loop system but in resistive terrains large loops and earlier channel information are needed for the detection of poor conductors at depth.
Geochemical aspects of acid rain
Authors: EDMUNDS W. M. , COE K.
Published: December 2007

A meeting on the Geochemistry of Acid Rain was held in the University of Strathclyde on 30 August 1985. It was convened by W. M. Edmunds (Institute of Hydrology, Wallingford, Oxon.) and K. Coe (University of Exeter).

The phenomenon of acid rain has been known for at least a century; one of the first records of acid rain was from Britain (Smith 1852.

Memoirs and Proceedings of the Manchester Literary and Philosophical Society
,
2
, 207–17.) Smith described acidic rainfall (with up to 38 mg 1
−
SO
2−
4
) from the industrial areas of Manchester and noted amongst other things the effects on stone buildings. General awareness of the long range international consequences as well as the widespread, as opposed to local environmental impact, of acidic deposition has grown during the last 15–20 years, mainly as a result of research in Scandinavia and North America.

There is now an extensive literature on the subject of acidic deposition and its effects, particularly in relation to the origin and fate of acidic substances in the atmosphere and their possible impact on surface waters and biota. The two most serious questions to emerge are (i) whether current levels of acidic deposition are responsible for the increase in the number of fishless streams and lakes in Scandinavia and elsewhere, and (ii) whether the presence of anthropogenic acidic substances in the atmosphere is a threat, directly or indirectly, to the productivity of European and North American forests.

Relativety little research has so far been carried out on
Influence of mineral weathering and catchment hydrology on drainage water chemistry in three upland sites in England and Wales
Authors: HORNUNG M. , ADAMSON J. K. , REYNOLDS B. , STEVENS P. A.
Published: December 2007

Solute budgets and within system variations in water chemistry have been studied at three catchments in western Britain. The solute budgets showed a net export of calcium and silica from all three sites and of aluminium and magnesium from two. Hydrogen ion budgets, based on bulk precipitation inputs and streamwater outputs, underestimated inputs of acidity and fail to take account of within system sources. At two sites the aluminium and silica were derived from the soils and the calcium and magnesium from the bedrock compartment. There was little, if any, weathering of the mudstone and slate bedrocks at these two sites; calcium and magnesium were derived from carbonate veins and/or dolerite intrusions. At the third site, aluminium and silica were released from the upper soil, and calcium and magnesium from the lower soil or underlying, calcareous till. Catchment hydrology was, a major factor influencing the pattern and rate of weathering.
Soil-acid deposition interactions and their possible effects on geochemical weathering rates in British uplands
Authors: CRESSER M. S. , EDWARDS A. C. , INGRAM S. , SKIBA U. , PEIRSON-SMITH T.
Published: December 2007

An attempt is made to quantify the effects that acidic pollutant depositions of anthropogenic origins may have on long-term geochemical weathering and soil acidification rates at upland locations. The factors which naturally regulate geochemical weathering kinetics in such areas are summarized, and the possible ways in which deposition acidity may interact with these regulatory factors are examined. Aspects considered include modifications to soil chemical, microbiological and physical characteristics. The ways in which such changes may influence the extent and pattern of mineral weathering are critically discussed.
Nutrient and proton budgets in four soil-vegetation systems underlain by Pleistocene alluvial deposits
Authors: VAN BREEMEN N. , DE VISSER P. H. B. , VAN GRINSVEN J. J. M.
Published: December 2007

Proton budgets of four soil-vegetation systems in a 3 ha woodland with acidic and calcareous soils were derived from three years of hydrochemical monitoring of rain- and canopy throughfall water, soil solutions at various depths and measurements on litterfall and tree growth. Soil water fluxes were obtained from dynamic simulation of water transport, calibrated with measured water potentials and water contents in the soil. Atmospheric input is predominantly ammonium sulphate. Only 5–20% of the annual uptake of nutrient elements from the soil is stored in wood, bark and roots of trees. Free H
+
inputs from the atmosphere are almost completely neutralized in the tree canopy by exchange with cations from the vegetation. The forest floor and the 0– 10 cm mineral surface layer at the acidic soil sites, however, produce free H
+
, mainly by nitrification, partly by organic acid dissociation. Most of this H
+
is neutralized at greater depth by nitrate uptake or denitrification, and by dissolution of cations from soil minerals, mainly aluminium in the acid soils. The dominant role of N transformations in the H
+
budget of these soil vegetation systems is clearly due the high anthropogenic inputs of ammonium sulphate. As a result, the rate of weathering of acidic soils has probably increased severalfold.
Towards more detailed methods for quantifying the acid susceptibility of rocks and soils
Authors: KINNIBURGH D. G.
Published: December 2007

The sensitivity of rocks and soils to acidification is best described by their acid-base titration curves. These curves tend to be relatively featureless but can be usefully summarized in terms of their initial pH, the buffer capacity at this pH, and the total acidity to pH 8.3. Unfortunately, the shapes of soil titration curves depend on the type and concentration of background electrolyte and the rate of titration. Therefore, while a set of standard titration conditions can be defined, the resulting titration curves will not simulate those applicable to
in situ
field conditions (as required for modelling purposes). To progress, it is necessary to gain a better understanding of the sources of, and sinks for, acidity in rocks and soils. For soils, partitioning of the acidity into soil solution, neutral salt extractable, and solid phase (but not neutral salt extractable) components provides a promising approach. This is demonstrated using a series of soil samples derived from an extremely acid humus-iron podzol overlying Eocene sand near Wokingham, Berks.
Stable isotopes in sedimentary geology
Authors: ANDREWS J. E. , FALLICK A. E.
Published: December 2007

In the opening session of this meeting John Hudson suggested that the measurement of
13
C/
12
C and
18
O
/
16
O isotopic ratios in a Jurassic belemnite from the Isle of Skye had been ‘the most elegant geological experiment ever performed’. Nowadays isotope geology is a major branch of earth science studies and this pioneering work of Urey’s Chicago group in the early 1950s has blossomed, with stable isotope analysis particularly useful to the sedimentary geologist. The above-titled meeting was convened to provide a forum for discussion at a time when stable isotope techniques are becoming increasingly common in contributing to the solution of palaeoclimatic, sedimentological and palaeontological problems. One day was devoted to the presentation of papers and one day to a local field excursion. The meeting was attended by 55 British delegates, of whom 12 presented papers. Presentations included three reviews dealing with carbon and oxygen isotopes in rocks and fossils; carbon and sulphur isotopes associated with the burial of organic matter and stable isotopes as tracers in phosphogenesis.

In the first review talk John Hudson (Leicester) discussed the history of carbon and oxygen isotopes in rocks and fossils. Hudson reminded delegates that the Quaternary ‘foraminiferal record’ of oceanic oxygen isotope fluctuations, caused by the storage effects of northern hemisphere ice sheets, had probably been the most successful employment of the oxygen isotope technique in sedimentary geology. No other fossil group equalled the resolving power of the foraminifers as a palaeoclimatic tool. The major problem in studies of older rocks had been
Magnetic and calcareous-nannofossil stratigraphy of the lower Palaeogene formations of the Hampshire and London basins
Authors: AUBRY M.-P. , HAILWOOD E. A. , TOWNSEND H. A.
Published: December 2007

A magnetostratigraphic study of the lower Palaeogene formations exposed in the Hampshire and London basins, southern UK, has led to the identification of eight stratigraphically distinct normal polarity magnetozones within an otherwise reverse polarity sedimentary sequence which spans the Late Palaeocene to Middle Eocene. A complementary study of the calcareous nannofossil biostratigraphy of these sections, and related sections elsewhere in NW Europe, allows a correlation of this succession of normal polarity magnetozones with the biostratigraphically-calibrated geomagnetic polarity timescale, based on marine magnetic anomalies. It is concluded that the normal polarity chrons corresponding to anomalies 20, 21 and 23 are recorded within the Huntingbridge, Earnley and Wittering Divisions of the Bracklesham Group respectively, that anomalies 24A and 24B are represented within Divisions D to B2 of the London Clay, and anomaly 26 is represented in the lower part of the Thanet beds. An additional normal polarity magnetozone within the Oldhaven beds is believed to represent a short normal polarity event between anomalies 24B and 25. Anomalies 22 and 25 appear not to be represented in the UK lower Palaeogene succession because of major unconformities at the Early/Middle Eocene boundary (within the Wittering Division of the Bracklesham Group) and within the Late Palaeocene respectively. The former appears to correlate with a widespread erosional event in the oceans identified from seismic records (

Vail
et al.
1979

) and interpreted as reflecting a major eustatic sea level fall.