bluuebunny/crossref_metadata_2025 huggingface dataset:
Cambrian and Ordovician stratigraphy related to structure and seismic profiles in the western part of the English Midlands
Authors: Smith N. J. P. , Rushton A. W. A.
Published: May 2009

In England the Cambrian successions include relatively continuous, dominantly argillaceous sequences (Nuneaton area, > 900 m thick) and arenaceous successions punctuated by hiatuses (Malvern, Shropshire). The Tremadoc (lower Ordovician) succeeds paraconformably; it is widespread, dominantly argillaceous and locally very thick (> 2000 m). Deposition of the lower Tremadoc is thought to have been rapid. Uplift of regional extent occurred in early Ordovician (post-Tremadoc) times and inversion of the basins, such as the Tremadoc Worcester Graben, occurred during the Variscan Orogeny. The disposition of the Cambrian–Tremadoc beneath the Upper Llandovery unconformity is thought to reflect the arrangement of basins and highs at that time. Evidence from seismic profiles indicates deposition in half-grabens. In the Welsh Borderland, evidence from outcrops and boreholes suggests that the St David's Series (Middle Cambrian) is locally thickened against syn-depositional faults. The thinning and local absence of the St David's Series across such faults is attributed to the influence of tilt-block highs. Rifting and tilting allowed thick Tremadoc to accumulate (e.g. in a precursor Worcester Graben), but evidence for the direction of thickening is masked by lack of seismic markers and by the effect of pre-landovery erosion.
Foreland basin development and tectonics on the northwest margin of eastern Avalonia
Authors: Kneller B. C. , King L. M. , Bell A. M.
Published: May 2009

The early Palaeozoic convergence of Avalonia and Laurentia created a foreland basin at the suture zone of the former lapetus Ocean. Sedimentological and stratigraphic evidence of shallowing and contemporaneous shortening suggests that the southern part of the basin (the Windermere Group) became detached from its basement in the late Ludlow, and began to invert. The detachment beneath the basin rooted into a northwest-dipping mid-crustal thrust system. Contemporaneous uplift to the north of the late Silurian basin involved shortening of the Avalonian foreland basement by thrusting. Basin inversion occurred ahead of a southeastward-advancing mountain front. We postulate a foreland (southeast) prograding sequence of thrusting through the Ludlow in the Lake District. The basin continued to migrate onto the Avalonian foreland through the early Devonian, ahead of an advancing orogenic wedge, finally coming to a stop in the Emsian.
Review of the stratigraphy of the Ordovician in the Brabant Massif, Belgium
Authors: Servais Thomas , Vanguestaine Michel , Herbosch Alain
Published: May 2009

A review of isolated data from the literature and our own observations make it possible to establish a model of the stratigraphical history of the Ordovician on the southern margin of the Brabant Massif. The succession is composed of seven informal formations, totalling a minimum of 2500 m of sediments, mainly comprising mudstones, siltstones and sandstones. This succession is compared and partly correlated with the sections observed in the adjoining areas, in the Bande de Sambre-et-Meuse, the Stavelot and Rocroi Massifs and the Remscheid–Altena and Ebbe anticlines (western Germany).
Caledonian terrane relationships in Britain: an introduction
Authors: Stone P. , Kimbell G. S.
Published: May 2009

terrane (ter-rane’) A fault-bounded body of rock of regional extent, characterized by a geological history different from that of contiguous terranes. A terrane is generally considered to be a discrete allochthonous fragment of oceanic or continental material added to a craton at an active margin by accretion. A. G. I. Glossary of Geology. 3rd edition, 1987.
Magnetic and tectonostratigraphic correlation at a terrane boundary: the Tappins Group of the Southern Uplands
Authors: Floyd J. D. , Kimbell G. S.
Published: May 2009

Prominent aeromagnetic anomalies around Ballantrae in southwest Scotland have long been associated with extensive outcrops of basic/ultrabasic igneous rocks in the Midland Valley terrane. However, recent reprocessing of data from a 1978 detailed aeromagnetic survey has demonstrated excellent correlation between discrete aeromagnetic features and certain magnetic greywacke formations within the early Caradoc Tappins Group. This heterogeneous unit forms the oldest and most northwesterly tectonostratigraphic tract in the Southern Uplands terrane and is characterized by relatively high detrital magnetite(and serpentinite) content and elevated values for Cr and Ni. This suggests original close sedimentary links with an ophiolitic terrane not unlike the presently adjacent Ballantrae Complex (or its extensive along-strike correlatives), a consideration which has important implications for terrane reconstructions in this region.
The tectonic significance of Ordovician basic igneous rocks in the Southern Uplands, southwest Scotland
Authors: Phillips E. R. , Barnes R. P. , Merriman R. J. , Floyd J. D.
Published: May 2009

In the northern part of the Southern Uplands, restricted volumes of basic igneous rocks occur at or near the base of the Ordovician sedimentary strata. These rocks have previously been interpreted as ocean-floor tholeiites representative of the subducted Iapetus oceanic plate, preserved as tectonic slivers in a fore-arc accretionary prism. The alternative, back-arc basin model proposed for the Southern Uplands on sedimentological evidence raises questions over the origin of these rocks. New geochemical data and previously published data clearly indicate that the volcanic material does not have a simple single source. The oldest (Arenig) volcanic rocks from the Moffat Shale Group associated with the Leadhills Fault include alkaline within-plate basalts and tholeiitic lavas which possibly display geochemical characteristics of midocean ridge basalts. In the northernmost occurrence, alkaline and tholeiitic basalts contained within the Caradoc Marchburn Formation are both of within-plate ocean island affinity. To the south, in the Gabsnout Burn area, the Moffat Shale Group contains lenticular bodies of dolerite and basalt which have characteristics of island-arc to transitional basalts. This complex association of basaltic volcanic rocks is, at the present time, difficult to reconcile with either a simple fore-arc or back-arc setting for the Southern Uplands. However, the increasing arc-related chemical influence on basic rock geochemistry towards the southeast may tentatively be used in support of a southern arc-terrane, and as a result, a back-arc situation for the Southern Uplands basin. An alternative is that these volcanic rocks may represent the local basement to the basin and include remnants of an arc precursor to the Southern Uplands basin.
Terrane geochemistry contrasts across the Iapetus Suture in Ireland
Authors: Ryan P. D. , Stillman C. J. , Stillman C. J. , Pow S.
Published: May 2009

In the Irish Caledonides, volcanism has been significant in terrane identification and in reconstructions of the Appalachian/Caledonian orogen. Crucial to these reconstructions is the recognition of ocean margins using obducted ocean floor relics (ophiolites) and supra-subduction zone volcanic assemblages. The volcanic rocks provide much evidence for the affinity of a terrane, however, by analogy with present day examples, the ocean floor sediments may provide the best way of tracing both ocean-floor magmatic activity, and continental source areas.
This investigation shows that the Irish Lower Palaeozoic volcanogenic terranes can be discriminated in terms of their shale geochemistry, which also gives information on their provenance and environment of deposition. South Mayo shales are dominated by volcaniclastic material derived both from both an arc and from an ophiolitic source. The Northern and Central belts of the Central Terrane show very similar lithogeochemistries, apparently derived in part from intermediate to silicic volcanic complexes. The Ordovician-Silurian inliers that straddle the Suture Zone, here termed the Southern Domain, show a chemistry close to that of the Leinster Terrane, which, coupled with a greater degree of sea-floor weathering, suggests a terrane with sediment of both volcanic and continental provenance being deposited in deeper water further from land. Across the suture the Leinster Terrane shows a mature chemistry which clearly suggests a continental provenance, together with a volcanogenic input from supra-subduction volcanism. This maturity is probably due to slower rates of sedimentation with longer residence times for volcanic detritus, plus the existence of a deeply weathered continental basement.
Constraints on the age and genesis of the Llanrwst and Llanfair–Talhaiarn orefields, North Wales from K–Ar and Rb–Sr studies
Authors: Haggerty R. , Bottrell S. H. , Cliff R. A. , Rex D. C.
Published: May 2009

The Llanrwst Pb–Zn and Llanfair–Talhaiarn Cu–Pb–Zn orefields contain vein mineralization similar to Lower Palaeozoic-hosted base metal deposits elsewhere in the British Isles, which are believed to have formed in the late Palaeozoic. K–Ar and Rb–Sr data on intensely sericitized wallrock material indicate that the main phase of mineralization at both orefields occurred between the mid-Devonian and theTournasian (386–350 Ma). The latest veins at Llanrwst formed in Visean–Westphalian times. Gangue mineral Sr isotope compositions indicate a mixture of radiogenic wallrock-derived Sr with less radiogenic Sr, with a poorly-defined trend to more radiogenic compositions with time.

The age of the main mineralization precludes dewatering of Carboniferous basins as a source of ore fluid. More generally Sr isotopes rule out unmodified basinal brines or metamorphic fluids as mineralizing agents, since their Sr content would be too radiogenic. Local sources of unradiogenic Sr are available at Llanrwst in metadolerites and calcareous sandstones and at Llanfair–Talhaiarn in thin Ludlow Limestone beds.
Graptolite ontogeny and the size of the graptolite zooid
Authors: Rigby Susan , Sudbury Margaret
Published: May 2009

Two methods of estimating the size of the graptolite zooid are described and discussed. It is possible to size a zooid by reference to cortical bandages or with reference to modem Rhabdopleura and its tubes. These two methods give very different results, with the first suggesting small zooids relative to thecal size and the second suggesting zooids that filled their thecae completely. Comparison is made with the evidence available from rare cases of preserved zooids. When all these observations are considered in the light of the ontogeny of modem pterobranchs, an ontogenetic sequence for the graptolite zooid can be inferred which helps to reconcile the two methods of estimating zooid size. This sequence postulates that most skeletal building occurred early in the life of the zooid, before it developed the capacity to feed or to reproduce, and it implies that only the first of three stages in the zooid's life is recorded in the skeleton. The later stages occurred without normally leaving any trace on the preservable remains of the colony. Finally, there is discussion of the effects which different zooid sizes would have had on some aspects of the functional morphology of a theca and on the hydrodynamic behaviour of the rhabdosome as a whole.
Interpretation of the regional gravity and magnetic surveys of Wales, using shaded relief and Euler deconvolution techniques
Authors: McDonald A. J. W. , Fletchert C. J. N. , Carruthers R. M. , Wilson D. , Evans R. B.
Published: May 2009

Regional gravity and aeromagnetic data of Wales have been processed using a variety of techniques. Image processing has greatly assisted qualitative interpretation, whilst automated procedures have provided additional quantitative information. The shaded relief images emphasize gradients in the potential fields, and are useful for displaying strong linear features. The Euler deconvolution method produces plotted solution maps, which define the position of the source of the gravity and magnetic anomalies. Euler solution maps of Wales and the adjacent continental shelf are presented for the first time. These maps are interpreted in relation to the known geology, with special emphasis on the Lower Palaeozoic Welsh Basin. It is proposed that the Euler solutions define a network of fault-bounded blocks within the Precambrian basement.
Heavy minerals as a guide to turbidite provenance in the Lower Palaeozoic Southern Welsh Basin: a pilot study
Authors: Morton A. C. , Davies J. R. , Waters R. A.
Published: May 2009

A pilot study has demonstrated that heavy mineral analysis is a useful guide to the provenance of Silurian turbidites in the Southern Welsh Basin. The results confirm the sedimentological evidence for two distinct source areas of coarse clastic detritus, one lying to the south and the other to the east. They also provide mineralogical criteria by which the two source areas may be distinguished. The southern area provided material with relatively low mineral diversity, and is characteristic in having low rutile/zircon ratios, whereas the eastern source provided more diverse assemblages, generally with high rutile/zircon ratios. The southern source shows variations in terms of apatite/tourmaline ratio, with the older Aberystwyth Grits Group tending to contain relatively low apatite compared with the younger Cwmystwyth Grits Group (Rhuddnant and Pysgotwr Grits formations). There is evidence for polycyclic material and volcanic detritus in both southerly and easterly derived samples; however, easterly-sourced sandstones apparently tapped a more lithologically-diverse terrain.
The origin and sedimentary, diagenetic and metamorphic evolution of chlorite–mica stacks in Llandovery sediments of central Wales, U.K.
Authors: Milodowski A. E. , Zalasiewicz J. A.
Published: May 2009

Chlorite–mica stacks in the Llandovery sedimentary rocks of central Wales have been studied, both within cleaved turbidite mudrocks, and within early diagenetic apatite, carbonate and monazite concretions. Stacks show a graded distribution within turbidite units, indicating an origin as detrital particles. Concretions provided an arrested diagenetic environment which restricted subsequent deformation and so revealed the nature of the stack precursor minerals. These included detrital mica, and also probable pyroxenes, amphiboles and volcanic rock particles, relic textures of which can be seen preserved within the concretions. Diagenetic alteration to chlorite, probably with some white mica intergrowth, occurred, most probably via smectite and then mixed-layer or swelling chlorite intermediates. Subsequent modification, during the Caledonian cleavage-forming episode, determined the present shape and mineralogy of the stacks. Outside concretions, chlorite pseudomorphs were rotated by tectonic compression, crushed at their margins and pulled apart along (001) cleavage and slip planes. Growth of at least two generations of secondary mica infillings has been recognized, and this was probably accompanied by the growth of secondary chlorite. This resulted in barrel-shaped stacks, elongated parallel with cleavage, and with the (001) plane rotated from the bedding towards the perpendicular to the cleavage. Within early diagenetic concretions it is evident that some stacks had formed prior to lithification. However, these have a relatively simple morphology and the original bedding-parallel grain shapes are largely preserved.
Petrology and stratigraphy of some texturally well preserved thin komatiites from Kambalda, Western Australia
Authors: Thomson B.
Published: May 2009

Archaean komatiite volcanics at Kambalda, Western Australia have been metamorphosed to upper greenschist–lower amphibolite grade and have experienced intense though heterogeneously developed polyphase deformation. Despite this, preservation of igneous textural features is often good, particularly in areas which underwent only ‘static style’ metamorphism. Thin lavas from the Tripod Hill Member of the Kambalda Komatiite Formation over the western margin of the Hunt nickel shoot display textural elements and facies variations which are virtually identical to those found in fresher thin komatiite sequences in other Archaean greenstone belts. Four principal flow profile (facies) types are defined, comprising nine subtypes. These represent stages in a facies continuum, ranging from ‘mature’ profiles which comprise thick spinifex textured tops and close packed cumulate bases through to massive, jointed ‘immature’ profiles devoid of mesoscopic spinifex texture. The causes of textural diversity within and between profiles are many and complex. However, facies variations can be attributed mainly to the effects of lava velocity at the time of major heat loss, combined with relative lateral position within any flow. The most mature textural (and geochemical) profiles developed in parts of lavas which had become ponded prior to major heat loss, whereas the least evolved profiles developed along the lateral margins (levees) of moving lavas. The study area komatiites occur as alternating stacks of flows of similar type. This stratigraphy records temporal and spatial shifts in the locus of lava ponding over the western margin of the Hunt nickel shoot. Such shifts may have been caused by irregularities in the underlying volcanic topography and/or by synvolcanic faulting and subsidence.
Earliest skeletal fossils from Precambrian–Cambrian boundary strata, Elburz Mountains, Iran
Authors: Hamdi B. , Brasier M. D. , Zhiwen Jiang
Published: May 2009

The lithological and biotic succession across the late Precambrian to early Cambrian interval is outlined for the Dalir and Valiabad successions of the Elburz Mountains of Iran. The Lower Dolomite Member contains an assemblage of phosphatic tubes and other poorly preserved remains. The succeeding Lower Shale Member bears macroscopic chuariamorphid algae. Early skeletal fossil diversity rises through the Middle Dolomite Member, with the successive appearance ofProtohertzina anabarica, Cambrotubulus decurvatusandAnabarites trisulcatus, culminating near the top of the dolomites with the appearance ofPurellasp.,Maikhanella multa, Tiksitheca licisand circothecids. This succession is compared with lower to upper parts of the Nemakit-Daldyn Formation of Siberia. The overlying Upper Shale Member bears phosphatic beds at its base with allathecids and an uncoiled pelagiellid (?Aldanellasp.) that suggest comparison with lower Tommotian strata and the Precambrian–Cambrian boundary phosphorite event of southern and central Asia. A rich assemblage of molluscs appears high in the Upper Shale Member, including theLatouchella korobkovigroup and thePelagiella lorenzigroup. The succession is broadly homotaxial with those from the Siberian Platform and Mongolia and those platforms bordering Gondwana (India, Kazakhstan, South China).
Time-level plots and accumulation patterns of sediment sequences
Authors: Friend P. F. , Johnson N. M. , McRae L. E.
Published: May 2009

Time-level plots represent changes of level of sediment surfaces over one, or more, episodes. The durations of the episodes must be specified, because they can vary over many orders of magnitude in different studies. This study is concerned with the relationships between sedimentological episodes, resulting in deposition or erosion of beds, centimetres to metres thick, and stratigraphic episodes resulting in deposition of tens to hundreds of metres. The object of this work is to develop a method of analysing patterns of sedimentological episodes which will allow the examination of their relationship with the stratigraphic rates of accumulation that reflect tectonic and/or climatic controls.

The method plots the sedimentological episodes as a sequence of small time-level vectors, plotted head to tail. The resultant of this vector sequence is the large vector that represents the net change of level over the whole stratigraphic time episode. The gradient of this large vector is the rate of stratigraphic accumulation.

The Chinji Formation (Miocene), Siwalik Group, Pakistan, is a fluvial unit, whose major channel sandstones occur between intervals of floodplain overbank mudstones with minor sandstones. Using time-level plots, interpretation of a 36 m interval from the formation is progressively improved in the light of our growing understanding of the surface processes. Having first outlined a simple first timelevel plot, the following modifications are introduced, and their time and rate significance can be assessed from new plots: (1) thick mudstone episodes are replaced by many thinner episodes; (2) long episodes of hiatus are removed unless marked by distinct soils; (3) episodes are inserted below the sandstones.
A terrane interpretation of the Archaean Limpopo Belt
Authors: Rollinson H. R.
Published: May 2009

The Limpopo Belt is a zone of thickened Archaean crust whose origin is currently explained by a late Archaean continent-continent collision between the Kaapvaal and Zimbabwe cratons. This review shows that the two cratons have fundamentally different geological histories and that the Zimbabwe Craton was unlikely to have behaved as a stable ‘cratonic’ block at the time of the Limpopo Belt collision. The geological histories of the Zimbabwe Craton, the North Marginal, Central and South Marginal zones of the Limpopo Belt and the Kaapvaal Craton are shown to be sufficiently different from one another to warrant their consideration as discrete terranes. The boundaries between the five units outlined above are all major shear zones, further supporting a terrane model for the Limpopo Belt. The five units were all intruded by late- to syn-tectonic granites c.2.6 Ga, constraining the accretion event to c. 2.6 Ga.
Chert appearance in the Cueva-Bedón carbonate platform (upper Cretaceous, northern Spain)
Authors: Elorza J. , Garcia-Garmilla F.
Published: May 2009

Nodular chert from the Cueva-Bedón carbonate platform (late Turonian-earliest Coniacian) occurs above the type 1 sequence boundary and in association with a transgressive systems tract deposit. It formed by selective replacement of grainstones-packstones, and Thalassinoides burrow fillings. Other minor types of selective silicification include the replacement of anhydrite nodules, forming quartz geodes, and the partial or total replacement of oysters and inoceramids. The chert nodules consist predominantly of microquartz with volumetrically minor fibrous quartz (length-fast chalcedony) and megaquartz. The silica has a mainly biogenic source (siliceous sponge spicules and radiolarians). Lepisphere textures that are indicative of an intermediate opal-CT precursor are evident in chert nodules, which indicates the chert formed by a maturation process from opalA to opal-CT to quartz. The δ 18O values of microquartz suggest a precipitation from marine fluids. The fabric of quartz geodes consists of irregular bands of heterometric megaquartz, quartzine and lutecite spherulites, and a well developed band of petaloidal megaquartz that contains small anhydrite inclusions. Bivalve shells are replaced mainly by fibrousquartz (quartzine-lutecite), which forms extensive beekite rings. The silica source of the replaced fossils is also biogenic, but silicification couldhave occurred by direct quartz replacement without an opal-CT precursor. Field and petrographic evidence indicate that silicification was an early diagenetic process that affected both uncompacted and compacted sediments, andwas inhibited when the dolomitization began. There seems to exist a syncrony in age (late Turonian-earliest Coniacian) between the nodular chert in the Cueva-Bedón carbonate platform and the bedded and nodular chert ofthe Plentzia carbonate turbidites in the deeper Basque Arc.
A concentration of exceptionally well-preserved large tubular foraminifera in the Eocene Zumaya flysch, northern Spain
Authors: Crimes T. P. , Uchman A.
Published: May 2009

A concentration of exceptionally well-preserved large tubular foraminifera occurs on a specimen from the sole of a sandstone bed from the Eocene Zumaya flysch. The foraminifera were probably washed out from the sediment by a weak bottom current, orientated and concentrated in a pre-existing depression on the ocean floor. Fortunately, subsequently deposited turbidite sand, which forms the host bed, only disturbed the fossils to an infinitesimal degree.
Do seismic reflections necessarily have chronostratigraphic significance?
Authors: Tipper John C.
Published: May 2009

Many seismic reflections from within sedimentary successions are thought to be generated along stratal surfaces because those surfaces are laterally continuous and have marked acoustic impedance contrasts. As stratal surfaces are isochronous, those reflections are then also taken as being chronostratigraphically significant. In contrast, seismic reflections are thought not to be generated along the boundaries of lithostratigraphic units because those boundaries are discontinuous and gradational. Nevertheless, synthetic seismic analysis shows that seismic reflections should in many circumstances be expected to follow lithostratigraphic unit boundaries, not stratal surfaces. As these lithostratigraphic unit boundaries will generally be diachronous, seismic reflections from within sedimentary successions should evidently not be treated as necessarily having chronostratigraphic significance.
The stratigraphy and setting of the Precambrian rocks of the Llangynog Inlier, Dyfed, South Wales
Authors: Cope J. C. W. , Bevins R. E.
Published: May 2009

Detailed mapping of the Precambrian rocks of the Llangynog Inlier, made possible by a series of new exposures, has allowed the establishment for the first time of the 1100 m thick succession within the Precambrian rocks of the area. All boundaries of the Precambrian rocks with younger strata in the area are either faults or unconformities. Some 350–400 m of rhyolitic lavas form the oldest rocks seen in the area; they are extensively silicified and devitrified and show little of their original texture, although flow-banding is visible in some outcrops and the presence of perlitic and snowflake textures imply that originally they were glassy in part. These lavas are succeeded by a varied succession dominated by volcaniclastic siltstones showing evidence of shallow water deposition and containing medusoids and trace fossils of Ediacaran age. Interbedded in the sediments are basaltic lavas, autobreccias and rare hyaloclastites displaying textural features characteristic of submarine eruption, and rhyolitic pumiceous and shardic ash-flow tuffs. The name Coomb Volcanic Formation is proposed for this succession, within which are recognized two members, a lower Castell Cogan Rhyolite Member succeeded by the Coed Cochion Volcaniclastic Member. In association are varied intrusions, comprising both dolerites and dacites. A palaeoenvironmental model, built on evidence of petrography, palaeontology and sedimentology is that of shallow water to emergent volcanic islands.
An oceanic volcano in an island arc setting – Seatura Volcano, Fiji
Authors: Hindle W. H. , Colley H.
Published: May 2009

Seatura is a Pliocene caldera-type, shield volcano situated in SW Vanua Levu, Fiji. Shield-building basaltic lavas are predominantly subaerial though there is a shoreline facies of fossiliferous sediment, pillow lava and hyaloclastite. Late-stage events include the intrusion of a dyke swarm in and around the caldera, and deposition of laharic conglomerates. Chemically the rocks are intermediate between tholeiite and alkali basalt and have intraoceanic levels of TiO2. Late-stage derivatives include benmoreite, trachyte, and propylitically altered rocks. The parent magmas are thought to have been partial melts of undepleted mantle at relatively deep levels. Tapping of these deep levels may have been facilitated by extensional plate tectonic conditions existing in the Fiji area during the development of Seatura Volcano.
Gravity study of Lower Benue Trough, Nigeria
Authors: Adighije C. I.
Published: May 2009

About 300 gravity stations were established at 2–4 km intervals along an E-W profile across the Lower Benue Trough from Gboko-Nsukka-Auchi with the main objectives of investigating the nature and configuration of the basement complex underlying the trough and estimating the thickness of the sedimentary cover rocks. The resulting Bouguer anomaly profile shows a series of alternating positives and negatives. For a preliminary interpretation, the positive anomalies are attributed to shallow basement and the negative anomalies to sediment-fill of maximum thickness, 4250 m.
Application of backscattered electron microscopy in shale petrology
Authors: Krinsley D. H. , Pye K. , Kearsley A. T.
Published: May 2009

A grey pyritic mudstone from Central Wales (Red Vein unit of the Dicellograptus anceps zone, Upper Ordovician Ashgill) has been examined in thin section by scanning electron microscopy using backscattered electrons. Using backscatter it is possible to identify individual mineral constituents of the mudstone by virtue of their atomic number (Z) contrast and differential hardness (relief). The amount of detail observable is far greater than that possible with optical microscopy. Valuable information can be obtained relating to particle form, orientation, texture and internal structure which aids in interpretation of the deformational and diagenetic history of the rock. The adoption of electron microscopical methods in the study of thin sections and polished rock chip surfaces promises to revolutionize the field of shale petrology.
Late Precambrian resedimented conglomerates from Bellsund, Spitsbergen
Authors: Waddams P.
Published: May 2009

Late Precambrian polymict conglomerates are described from Bellsund, western Spitsbergen, where they are intimately associated with authentic glacigenic sediments. They display normal, compound and reverse grading. Spectacular flame, injection and incipient rip-up structures are developed at the base of some beds. The conglomerates were initially deposited either as foreset debris beneath a wet-based glacier, or on the subaqueous part of a glaciomarine outwash delta, and were subsequently redeposited from mass gravity flows initiated by slumping on the unstable glacial detritus slope. An informal stratigraphic nomenclature is proposed for the Kapp Martin area to the north of Bellsund and the Kapp Lyell area to the south.
Correlation of Late Proterozoic tillites in the North Atlantic region and Europe
Authors: Hambrey M. J.
Published: May 2009

Well-established Late Proterozoic tillites are widely distributed in Svalbard, East and North Greenland, the Norwegian and Swedish Caledonides, Scotland, northwestern Ireland, western USSR, the Urals, Newfoundland and the Appalachians. Recent biostratigraphic and geochronometric data have provided the following sequence of glacial events. The earliest evidence of glaciation is represented by tillites in Newfoundland, Scotland and possibly NE Svalbard, and may have taken place in Late Riphean (sensuVidal) or Sturtian (sensuHarland) time, perhapsc.800 Ma ago. A glacial period in Vendian time, the Varangian, centred on 650 Ma was a much more severe climatic event, represented in all the above mentioned areas except Newfoundland. Two Varangian glacial epochs are recorded in Svalbard, East Greenland, northern Norway and possibly western USSR, and although most of the deposits are marine, terrestrial conditions prevailed in some areas with associated periglacial phenomena. The earlier epoch was completed before 650 Ma; the later one occurred within the rangec.650–610 Ma.

A tectonic reconstruction of the region places eastern Svalbard adjacent to East Greenland, which has implications for major strike-slip motions in late and post-Caledonian times. A discrete ice-sheet is envisaged, centred on the western USSR (terrestrial tillites) with ice flowing out to the then peripheral area of the northwestern Europe continental margin and to the Urals where shallow platformal glaciomarine sediments accumulated.
Geochemistry and relationships to mineralization of granitic rocks from the Afu Younger Granite Complex, Central Nigeria
Authors: Imeokparia E. G.
Published: May 2009

Geochemical analyses of fresh bedrock samples indicate that the Afu biotite granites are characterized by enhanced values of a suite of trace elements, including Sn, Nb, W, Zn, Rb, Li, F th, Y and depleted in Ba Sr, and Mn. These geochemical data are also examined by R-mode factor analysis with the primary objective of isolating the significant factors accounting for the sample composition as derived from mineralization, alteration and lithology.

The Orthogonal varimax solution yields a five-factor model that accounts for 76% of the total data variance. Factors 1 (Nb, Rb, Li F) and 3 (Sn, W) can be attributed to the effect of mineralization and postmagmatic alteration; while Factor 2 (Mn, Sr, Ba Zr) is lithological, associated with barren rocks, unaffected by metasomatism. Factor 4 (Th, Y, Ga) and Factor 5 (Pb and Zn) are also associated with prominent mineralization. Thus the mineralized granites are characterized by high concentrations of Sn, Nb, W, Zn, Rb, Li F, Th, Y and Pb but ore formation is considered a product of postmagmatic processes.
Stratigraphy of the Mesozoic rocks above the Semail Ophiolite, Oman
Authors: Woodcock N. H. , Robertson A. H. F.
Published: May 2009

A revised stratigraphy is presented for the rocks which lie between the Upper Cretaceous Semail ophiolite and its post-emplacement sediment cover of uppermost Cretaceous to Tertiary age. Above the pelagic sediments of the ophiolite (Suhaylah Formation, Cenomanian–Turonian) occur ophiolite-derived clastic sediments (Zabyat Formation, ?Turonian–?Campanian) related to early stages of ophiolite emplacement. These formations are structurally overlain by the Batinah Complex, comprising Triassic to Cretaceous lithologies, all previously equated with the Hawasina rocks that now lie tectonically beneath the ophiolite. We recognize a lower melange unit to the complex, which demonstrably includes Hawasina lithologies. The melange formed by superficial rather than deep processes, but the mechanism by which Hawasina material was supplied to the ophiolite surface is unproven. The upper unit of the Batinah Complex shows a coherent stratigraphy which contrasts with that of the Hawasina. It comprises three main allochthonous tectonic sheets, within which we recognize three formations and one other informal unit. The upper Batinah Complex may have been tectonically derived not from the now subjacent Arabian margin but from another margin with a passive Mesozoic history.
Benue Trough and Benue Chain
Authors: Benkhelil J.
Published: May 2009

It is suggested that the Benue Trough may have been initiated by the extension of the equatorial oceanic fracture zones onto the continent during the early stages of the separation of Africa and South America. A transcurrent movement along deep-seated faults of the basement is responsible for the structure of the trough. After a period of extension characterized by the sedimentary infilling of the trough and igneous activity, the trough is turned into a chain through a short phase of compression. A discrete pattern of ‘en echelon’ folds and the strain pattern deduced from the analysis of large scale and microfracturing correspond to a N–S to N160° E direction of compression coupled to a sinistral transcurrent movement along a NE–SW direction. The difference in age of the folding phase in the northern and southern parts of the chain indicates a diachronous nature for this tectonic event in relation to the wrench regime. The Tertiary evolution of the trough marked by a tensional regime resulting in an E–W extension is to be related to the Cameroon Volcanic Line.
An early Permian cool-water fauna from pebbly mudstones in south Thailand
Authors: Waterhouse J. B.
Published: May 2009

A small brachiopod fauna of early Permian age is recorded from pebbly mudstones and sandstones of the Phuket (Kaeng Krachan) Group at the Thai islands of Ko Muk, and Ko Phi Phi in the Andaman Sea. Most species belong to genera found in temperate to high Permian palaeolatitudes, whereas other Permian faunas from Thailand are strong in tropical affinities, suggesting that the pebbly mudstones are cool water deposits, contemporaneous with the late Asselian glacial deposits of Gondwana. New taxa are Arctitreta percostata sp.nov., Komukia solita gen.nov., sp.nov. (Rugosochonetidae), Cancrinelloides monticulus sp.nov., Rhynchopora culta sp.nov., Arionthia sapa sp.nov. and Elasmata retusus gen.nov., sp.nov. (Heterelasminidae).
Ultrastructural studies of graptolites – a review
Authors: Rickards R. B. , Crowther P. R. , Chapman A. J.
Published: May 2009

A reinterpretation is attempted of the 25 years of graptolite ultrastructural work based upon the present state of knowledge. It is emphasized that studies need to embrace many more species of graptolites, particularly in the dichograptid groups, and with different growth stages, before the gross evolution of ultrastructural elements can be satisfactorily appraised. The factual (morphological, ultrastructural) data are themselves reappraised, summarized, and very tentatively related to Bulman's anagenetic grades of graptolite development.
Preservation of shell microstructure in silicified brachiopods from the Upper Cretaceous Wilmington Sands of Devon
Authors: Holdaway H. K. , Clayton C. J.
Published: May 2009

From a study of silicified fossils, and in particular the microstructure of brachiopods, from the Wilmington Sands (Upper Cretaceous) of Devon, a model of skeletal silicification is proposed. Three distinct morphologies of silica were formed, controlled by the relative rates of silica supply and carbonate dissolution: (a) a fine-scale replacement of the original shell microstructure where silica was abundant; (b) a concentric ring morphology called ‘beekite’ where silica supply was limited, and (c) a granular white crust formed where carbonate dissolution was restricted. Silicification occurred during early diagenesis as a result of bacterial decay of organic matter intimately associated with skeletal fragments, within a sediment of restricted permeability. A build-up of CO2probably caused dissolution of skeletal carbonate, and bicarbonate released from this caused local precipitation of silica. The proposed mechanism is belived to be of general applicability to micrite-rich carbonate sediments.
The Devonian–Carboniferous transition and its structural setting at Chillaton, West Devon, England
Authors: Selwood E. B. , Stewart I. J. , Turner P. J. , Whiteley M. J.
Published: May 2009

Boreholes penetrating the inverted limb of a major fold in an allochthonous thrust sheet at Chillaton, West Devon, revealed highly fossiliferous strata spanning the Devonian–Carboniferous boundary in continuous succession. Details of the fauna, which includes conodonts, trilobites and ostracods, are given. Abundant siphonodellid conodonts allow the boundary to be determined according to the recommendations of the working group on the Devonian–Carboniferous boundary of the IUGS Commission on Stratigraphy. The nature of the imbricate zone beneath the Main Thrust, in which the succession is situated, is described. In addition, the fabrics associated with thrusts which cut the olistostromic succession lower in the tectonic unit are discussed.
Authigenic albite in the Pendleside Formation (Dinantian) of northern England
Authors: Turner P. , Ashworth J. R. , Iskenderian F. A.
Published: May 2009

Authigenic albite occurring as euhedral overgrowths up to 400 μm in size is a characteristic feature of the Pendleside Limestone in the Craven Basin of northern England. The albite is very pure, low-albite in keeping with other authigenic albites in carbonate rocks. The detrital cores of the albites are angular plagioclase grains of approximately An20 composition which are sometimes altered to kaolinite. The albite crystals with their detrital cores are frequently concentrated in discrete laminae, suggesting that the plagioclase cores may have been supplied by a basic volcanogenic source. This view is supported by geochemical studies of limestones which reveal significant correlation between K–Na, Na–Al and K–Al.

Interstitial alteration of detrital components, in particular the reaction intermediate plagioclase → kaolinite + albite, was primarily responsible for albite authigenesis. The initial products of this reaction probably included smectite which fixed Mg2+ in the decomposing volcanic grains. Subsequent alteration of the smectite resulted in the formation of small microdolomite inclusions which are commonly seen in the authigenic albites. An equilibrium model in terms of Na+/H4 ratios and H4SiO4 activity is proposed for the silicate assemblages in the Pendleside Formation.
Structural evolution in the Moine of northwest Scotland: a Caledonian linked thrust system?
Authors: Butler Robert W. H.
Published: May 2009

A model is proposed whereby the Caledonian metamorphic basement-cover complex of northwest Scotland (the Moine) is considered as a linked thrust system. This system lies between the Moine thrust at its base and the Naver–Sgurr Beag slide at its top. Ductile fold and thrust zones, which developed at mid crustal levels at metamorphic grades from greenschist to amphibolite facies, are interpreted as decoupling from a detachment presently situated at relatively shallow depths. This model is illustrated by two preliminary balanced cross-sections. These imply shortening across the northwest Scottish Caledonides in excess of 130 km and probably over 200 km. When these structures are restored onto a crustal template a considerable quantity of lower crust is found to be required at depth. The most likely location for the lower crustal wedge is beneath the Grampian Highlands.
A Hoxnian interglacial doline infilling at Slade Oak Lane, Denham, Buckinghamshire, England
Authors: Gibbard P. L. , Bryant I. D. , Hall A. R.
Published: May 2009

Site investigations for the M25 motorway at Denham proved a maximum depth of 37.5 m of deposits filling an enclosed hollow up to 40 m across beneath a small seasonally dry valley near Higher Denham. This previously unknown very steepsided hollow is infilled with gravel and sand, partially laminated clayey silt and pebbly clay. Within these deposits a bed of organic clay mud up to 4.5 m in thickness is present. Resistivity survey of the hollow proves that it is completely enclosed.

Palaeobotanical investigation shows that the organic sediment contains fossil pollen, spores and macroscopic remains. The pollen spectra indicate that the sediments accumulated during the later half of the Hoxnian interglacial Stage (Ho III/IV). Macroscopic remains include ‘relict’ species Dulichium arundinaceum, and Brasenia schreberi, the latter being new to the Hoxnian Stage. The evidence for waterlevel fluctuation during the interglacial at the site is discussed.

The origin of the hollow is considered and the conclusion reached is that it is a doline formed by solution of the underlying Chalk bedrock. The infilling of the basin is attributed to collapse, solifluction and slope wash of local material under cold climates before and after the interglacial. The interglacial sediment is a shallow water pool accumulation. The hollow is thought to have been formed originally as a solution pipe beneath the Thames' Gerrards Cross Gravel, rapid drainage of a neighbouring glacial lake causing the major collapse in the late Anglian Stage. Subsequent infill took place during the later part of the Anglian, Hoxnian and Wolstonian Stages and possibly even later.

Heavy mineral analyses of the silt-rich sediment above and beneath the interglacial deposits are appended and indicate that it is almost all of local Reading Beds material.
Composite ichnofabrics and tiering of burrows
Authors: Bromley R. G. , Ekdale A. A.
Published: May 2009

Infaunal communities in marine environments typically are tiered; that is, different taxa live at different depths within the sediment. Tiered suites of biogenic structures yield complex biogenic sedimentary fabrics (ichnofabrics), with the traces of deep-burrowing organisms overprinted on those of shallow-burrowing organisms. Careful analysis of crosscutting relationships of burrows in such composite ichnofabrics allows reconstruction of the tiered nature of fossil endobenthic communities. It is important to recognize that the best preserved and most prominently displayed trace fossils in most assemblages usually represent the deepest tier. Thus, they were farther removed from the sea floor and therefore less indicative of actual sea floor conditions than the more poorly preserved traces of the shallower tiers, on which the deeper traces are juxtaposed.
Geochemical and glacio-geomorphic implications of basalt weathering in the Queen Maud Mountains, Antarctica
Authors: Talkington R. W. , Mayewski P. A. , Gaudette H. E.
Published: May 2009

The weathering of a suite of basalt clasts, that have been transported by mass wasting downslope in the Cumulus Hills region of the Queen Maud Mountains, Antarctica, is examined from both geochemical and glacio-geomorphic viewpoints. Chemical weathering, predominantly oxidation and hydration, increases in severity from clast core to rim for the suite. These weathering processes and concomitant formation of a weathering rind are suggested to be an accumulative process, culminating in the disaggregation of the rock due in part both to the chemical breakdown of the interstitial and intersertal basaltic glass and to physical weathering processes.

Mass wasting rates in the range of 1.4 × 10−3−1.13 × 10−1cm. y−1are estimated, assuming that the style of transport is dry creep. These transport rates suggest that the rock-surface/air-temperature differences in the study area may have been as little as 0.5 °C and heating and cooling cycles as few as 1 cy. y−1for the last 4.2 Ma.
Late Precambrian geochronology of the European U.S.S.R.
Authors: Keller B. M. , Krasnobaev A. A.
Published: May 2009

Summary. The available data on radiometric dating of Riphean and Vendian deposits in the European part of the U.S.S.R. enabled us to establish the age of geochronological boundaries. The Riphean lower boundary was adopted as 1650 ± 50 Ma, the Vendian lower boundary as 650 ± 10 Ma, and the Cambrian lower boundary as 590 ± 10 Ma, making the total length of the Riphean about 1000 Ma and that of the Vendian about 60 Ma. This paper suggests a draft of the geochronological scale of the Late Precambrian.