bluuebunny/crossref_metadata_2025_split 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

AbstractIn 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

AbstractThe 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

AbstractA 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).
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

AbstractProminent 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

AbstractIn 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

AbstractIn 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

AbstractThe 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

AbstractTwo 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

AbstractRegional 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

AbstractA 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

AbstractChlorite–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

AbstractArchaean 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

AbstractThe 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

AbstractTime-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

AbstractThe 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

AbstractNodular 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

AbstractA 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.