bluuebunny/crossref_metadata_2025_split huggingface dataset:
Neoproterozoic-early Cambrian geology and palaeontology of Iberia
Authors: Vidal Gonzalo , Palacios Teodoro , Gámez-Vintaned Jose Antonio , Balda Maria Antonia Díez , Grant Stephen W. F. 
Published: May 2009

AbstractNeoproterozoic-early Cambrian successions in Iberia are reexamined. A gradual transition across the Neoproterozoic-Cambrian boundary is present in Central Iberia, whereas in the Cantabrian region and the Iberian Chains Lower Cambrian arenaceous successions rest with profound angular unconformity on Neoproterozoic turbidites. In Central Iberia, the Neoproterozoic sedimentary succession is referred to the informal Domo Extremeño group, representing mostly basinal facies, and the overlying Rio Huso group consisting of slope deposits and proximal turbidites that grade into shallower marine deposits. The latter is inferred to represent distal slope to outer platform depositional conditions and contains widespread carbonate olistostromic units. The position of the Precambrian-Cambrian boundary is within the Pusa shale of the Rio Huso group and can be correlated at the regional level by the occurrence of trace fossils, acritarchs, and in particular the abundant shelly metazoan Cloudina. The succession also yielded megascopic carbonaceous fossils, such as vendotaenids and Beltanelioides? sp. ind., and Sabellidites. Contrary to former interpretations assuming transport of older platform carbonates from the Ibor region into ‘younger’ olistostromic beds of the Rio Huso group, we interpret sedimentary and fossil evidence to suggest that shallower platform deposits of the informal Ibor group were penecontemporaneously incorporated in the olistostromic lower part of the Rio Huso group. Hence, the olistostromes are not believed to mark a major erosive unconformity. Based on the ichnofossil record and recent U-Pb age determinations, we argue that a proposed disconformity between Lower and Upper ‘Alcudian’ strata is neither regional nor does it mark a significant hiatus. Neoproterozoic-early Cambrian deposition in Central Iberia can be accommodated in a model that implies a generalized stretching of the crust during an extensional event which closely followed the Cadomian phase of the Pan-African Orogeny and which eventually could have included transcurrent components. An extensional phase with transcurrent components during the deposition of the Ibor and lower Rio Huso groups is regarded as a probable cause of widespread ponding resulting in the juxtaposition of platform and basinal successions, eventually leading to anoxic conditions in Pusa shale deposition times. A possible cause for repeated collapse events developing olistostromes and intra-sequential folding could be sought in this tectonic context.
A carbon isotope reference scale for the Lower Cambrian succession in Siberia: report of IGCP Project 303
Authors: Brasier M. D. , Rozanov A. Yu. , Zhuravlev A. Yu. , Corfield R. M. , Derry L. A. 
Published: May 2009

AbstractFour sections from the mid-Atdabanian to lowest Toyonian (middle Cambrian) along the Lena River of Siberia were sampled for carbon isotope stratigraphy. These show a mainly heavy but highly oscillatory δ13C signature for the Atdabanian to mid-Botomian interval, coincident with the major phase of invertebrate innovation. A prolonged interval of negative δ13C followed until late Toyonian times, coincident with Botomian-Toyonian mass extinctions. Eleven carbon isotope cycles are identified through the lower Cambrian, which should now be tested for their utility in global correlation and relationship to bioevents in the Cambrian explosion.
The Middle and Upper Quaternary deposits at Morston, north Norfolk, U.K.
Authors: Gale S. J. , Hoare P. G. , Hunt C. O. , Pye K. 
Published: May 2009

AbstractInvestigation of a section near Morston on the central north Norfolk coast has revealed evidence of a complex sequence of beach deposits laid down under interglacial conditions in association with relative sea levels similar to those of the present. This material rests upon a highly calcareous till and is itself overlain by a deposit comparable with the Hunstanton Till of northwest Norfolk. This site therefore represents the first location at which Hunstanton Till has been found overlying older till, and the only location so far discovered in East Anglia at which interglacial deposits separate glacigenic sediments. The site also contains the only indubitable Quaternary warm stage deposits to have been recognized in central north Norfolk.
Open-system magma chamber process in the Freetown Complex of Sierra Leone: evidence from Zone 3
Authors: Chalokwu Christopher I. , Seney Pamela J. 
Published: May 2009

AbstractThe ˜ 7 km thick Freetown layered complex of Sierra Leone consists of four zones each composed of a cyclically layered sequence of troctolite, gabbro, olivine gabbro, gabbronorite, and anorthosite. The complex is thought by previous workers to have solidified in situ from a single parental magma without stratigraphic changes in mineral compositions. Evidence for cryptic variation is presented based on electron microprobe analyses of mega-unit Zone 3. Two reversals in olivine forsterite content, plagioclase anorthite content, and 100 Mg/(Mg Fe2) in clinopyroxene match the variations of Ni in olivine and Cr in clinopyroxene. These changes are consistent with a magma chamber that was open to periodic influxes of new magmas, and the mixing of new and fractionated resident magmas. Expansion of the magma chamber is thought to have occurred at 2000 m, corresponding to the level of a major influx.
The petrology of the Harkerbreen Group, Ny Friesland, Svalbard: protoliths and tectonic significance
Authors: Manby G. M. 
Published: May 2009

AbstractThe late Precambrian–early Palaeozoic rocks of Ny Friesland, which have been subjected to Caledonian deformation and metamorphism, constitute part of the Eastern Province or Terrane of Svalbard. The Harkerbreen group and other divisions of the Stubendorffbreen supergroup form a high-grade and intensely deformed core complex to this terrane which is bounded to the west by the Billefjorden Fault Zone and to the east by a major north–south shear zone. The Stubendorffbreen rocks exhibit two gneissic foliations, one axial planar to a large scale, F1fold nappe closing to the east and the other axial planar to kilometre-scale upright F2folds subsidiary to the Atomfjella Arch. Metamorphism in the mid-amphibolite facies range coincided with generation of these folds, and F3.crenulation folding was accompanied by waningP–Tconditions. A significant proportion of the gneisses within the Harkerbreen group display silica–major element covariation patterns consistent with their position in the granodiorite field on the AFM plot. Incompatible, immobile element ratios Zr/Ti v. Nb/Y indicate affinities with rhyolites to rhyodacites which is also suggested by their REE profiles. Normalized multi-element plots of the gneisses are similar to those of granites from attenuated within-plate settings such as Mull and Skaergaard. The amphibolites which were intruded in the D1–D2interval appear to be derivatives of fractionated basalts. They plot across the calk-alkaline tholeiite boundary on the AFM diagram, and the calc-alkaline character of some of the amphibolites is further suggested by their Yb-normalized Ce-Ta abundances. Zr-Ti-Y and REE abundances would support their derivation from a related suite of fractionated basalt liquids. On the Zr/Y v. Zr discrimination diagram the amphibolites appear to have compositions transitional between Mid Ocean Ridge and Within-Plate basalts whilst the Zr-Nb-Ta plot indicates Volcanic Arc Basalt affinities. Th-Hf-Ta and multi-element plots, however, indicate a marginal to back-arc basin setting possibly above a mature subduction zone. The late Caledonian Chydenius granite is an adamellite with mixed within-plate and syn-orogenic characteristics typical of post-collisional granites.
Cambrian and Ordovician metabentonites and their relevance to the origins of associated mudrocks in the northern sector of the Lower Palaeozoic Welsh marginal basin
Authors: Roberts B. , Merriman R. J. 
Published: May 2009

AbstractMiddle Cambrian and Caradoc metabentonites and associated mudrocks are described from the northern part of the Welsh Lower Palaeozoic basin. They occur in areas of slight deformation, distant from thick piles of contemporaneous volcanics. The < 2 μm fraction from anchizonal Middle Cambrian metabentonites consists dominantly of 1Md micas (with up to 5% randomly interstratified smectite) with lesser 1M and 2M1 muscovite, corrensite, quartz, chlorite and kutnohorite. Interbedded mudrocks are similar but contain, additionally, minor paragonite and regular (6:4) paragonite-muscovite. Late diagenetic grade Caradoc bentonite consists of rectorite, pyrophyllite and small amounts of chlorite. Interbedded mudrocks are similar but contain, additionally, 1Md mica with up to 10% smectite interlayers, minor kaolinite and quartz.Metabentonites contain high concentrations of the trace elements Ce, La, Hf, Nb, Ta, Th, Y and Zr in comparison with associated mudrocks. Discriminant diagrams indicate derivation from trachyandesite–dacite–rhyolite magmas in a within-plate setting. Major element variations in Cambrian metabentonites compare closely with those described in Silurian examples whereas the Caradoc metabentonite is closely similar to rhyolitic tuffs in the Caradoc Llewelyn Volcanic Group.Both geochemical and field evidence indicate that whereas Middle Cambrian mudrocks were derived from bentonitic material adulterated variably by a terrigenous input. Lower Cambrian mudrocks were derived from a strongly depleted source such as basic Mona Complex rocks. Geochemical and field evidence suggest Caradoc mudrocks southeast of the Aber–Dinlle fault are largely volcanogenic; and the geological setting of Tremadoc, Arenig and Llanvirn mudrocks demands some volcanogenic input.The Caradoc volcanic succession of Snowdonia is represented by a single bentonite northwest of the Aber–Dinlle fault. The fault system was either a highly effective contemporaneous barrier or, more probably, a site of post-Caradoc transcurrent movement.
The Upper Band-Better Bed sequence (Lower Coal Measures, Westphalian A) in the central and south Pennine area of England
Authors: Chisholm J. I. 
Published: May 2009

AbstractAn analysis of outcrop and borehole information relating to the strata between the Upper Band and Better Bed coals of the central and south Pennines shows that two lithological facies can be recognized within a cyclic sequence of mudstones, siltstones and sandstones. In one facies, sandstones and siltstones contain abundant mica, and argillaceous beds are neutral grey in colour. Petrographically the sandstones are feldspathic but contain little lithic material other than multigrain quartz. In the other facies, mica is much less common, argillaceous beds are greenish grey, and sandstones and siltstones contain a notable proportion of chloritic lithoclasts. The sediments of both facies were deposited in lower delta plain/shallow-water delta environments, and palaeocurrent measurements show that the micaceous facies was supplied from the north or east while the green facies came in from the west.Three named divisions of the sequence are based on recognition of the two facies types. They correspond roughly with three upward-coarsening transgressive–regressive sedimentary cycles of presumed eustatic origin. In the lowest part of the succession (Shibden division) the main clastic input was of the micaceous type. Sandstones are found only in the north of the basin, where they form a series of superimposed delta-front bodies. Mudstones were deposited elsewhere, in a large body of open water which, although linked to the sea, was generally of lowered salinity. The restriction of the delta-front sandstones to the same geographical area over a long period of time is attributed to differential subsidence of the basin across deep-seated structures.The sediments of the overlying Brighouse division belong to the green facies, and the source of the elastics lay to the west. After an initial period of mudstone deposition in an extensive, apparently non-marine, body of open water, fluviodeltaic sand and silt spread across the entire area. There is no obvious control of sandstone distribution by basement structures.The succeeding Bradley Wood division includes sediments of both micaceous and green facies. A lobe of medium-grained micaceous sandstone around Sheffield (Thurlstone Channel) represents a fluvial incursion from the east and a linear body of fine-grained green-facies sandstone south of Chesterfield (Bole Hill Channel) represents a similar incursion from the west. Elsewhere, small units of finer-grained elastics are probably deltaic deposits linked to the same sources. The location of the channel sand-bodies suggests control by deep-seated fractures.
Combined Sm-Nd and Rb-Sr isotope systematics in the Donegal granitoids and their petrogenetic implications
Authors: Dempsey C. S. , Halliday A. N. , Meighan I. G. 
Published: May 2009

AbstractThe metaluminous to peraluminous granitoids of the Donegal batholith, northwest Ireland, were emplaced at c. 400 Ma into greenschist-amphibolite facies metasediments of the Dalradian Supergroup. Sm-Nd and Rb-Sr isotopic data are provided for eleven granitoid samples from six of the plutons and one specimen from the northeast granodiorite pluton of the Newry complex, Co. Down; the Donegal results reveal essentially similar initial Sr isotope ratios (0.7051–0.7068) but highly variable initial eNd values, −1.2 to −8.3 (and −0.5 for Newry). Certain granitoids have distinctive Nd isotopic compositions characteristic of the involvement of old, LREE-enriched continental crust in some cases or young crust and/or mantle-derived magmas in others. The Nd and Sr isotopic variations can be explained by a variety of mixing hypotheses.
SPOT image data as an aid to structural mapping in the southern Aravalli Hills of Rajasthan, India
Authors: Drury S. A. 
Published: May 2009

AbstractThe 10 to 20 m resolution of SPOT image data, together with their potential for stereoscopic viewing, provides an excellent base for geological mapping inremote and rugged terrain that is akin to high-level aerial photographs. Their large format (60 × 60 km) also gives the advantage of synoptic coverage that ranks with images from the Landsat series of satellites. Use of stereo pairs of single-band SPOT images has enabled some revision of existing geological maps of the southern Aravalli Hills in Rajasthan at a scale of 1:100000, and has added significantly to knowledge of their complex mid-Proterozoic structure. In particular, many possibly early low-angled faults have been discovered, together with the tectonic nature of a major terrain boundary and much detail of intricate structures has been added in the more remote areas. The potentialfor lithological discrimination of multispectral SPOT data is severely limited by its restricted coverage of geologically important spectral features, and it is far surpassed by that of Landsat Thematic Mapper data, which would have been capable of more comprehensive lithofacies reconnaissance, had they been available.
A new Cambrian sphinctozoan sponge from North America, its relationship to archaeocyaths and the nature of early sphinctozoans
Authors: Debrenne Françoise , Wood Rachel 
Published: May 2009

AbstractA new early Cambrian sponge of sphinctozoan organization named Polythalamia americana nov. gen. et sp. is described from Nevada and Alaska, U.S.A. P. americana nov. gen. et sp. shows little resemblance to other Cambrian reported sphinctozoans, but is similar to late Palaeozoic to early Mesozoic forms from Europe and Asia, e.g. Stylothalamia and Amblysiphonella. The polyphyletic origin of sphinctozoans based upon spicule criteria from younger examples is therefore corroborated by the varied morphology and clearly widespread palaeogeographic distribution of Cambrian forms, which were hitherto recognized only from Australia. P. americana nov. gen. et sp. possesses a relatively small, globular multi-chambered calcareous skeleton, with thin but densely perforate walls and a central retrosiphonate, perforate spongocoel. The skeleton is composed of an irregular microstructure of unknown, but probable calcitic, original mineralogy. There are no spicules or primary internal structures, but secondary vesicular filling tissue is present. The pore organization, however, it directly comparable to archaeocyaths, e.g. coscinocyathines, and this character is taken to be the only synapomorphy available for taxonomic and phylogenetic purposes. Indeed, P. americana nov. gen. et sp. is closely associated with a diverse archaeocyath assemblage and draws attention to morphological similarities between some archaeocyaths and sphinctozoans, thus questioning the criteria at present used to distinguish between these groups. Accordingly, a list of nomenclatural synonymies is given to aid comparison. Coscinocyathine archaeocyaths possess chambered juvenile stages and P. americana nov. gen. et sp. is suggested to represent an aspiculate lineage of sphinctozoan grade which may be derived from forms such as Clathricoscinus by neoteny. It appears that at least three clades of sphinctozoan-grade calcified sponges were present by end of early Cambrian time and their occurrence in association with island arcs of the palaeopacific rim is confirmed.
A precise worldwide correlation of early Ordovician graptolite sequences
Authors: Cooper R. A. , Lindholm K. 
Published: May 2009

AbstractFourteen early Ordovician (Tremadoc-Llandeilo) graptolite sequences from around the world are precisely (infrazonally) correlated, based on the stratigraphic ranges of 130 species and species groups. The composite standard sequence (CSS) of graptolites has been determined from the six best regional sequences by a nonparametric graphic correlation. Two data sets were selected: one comprised first appearance events of 103 taxa, the other, first and last appearance events of 45 taxa. The results of the two runs accord well and reveal respectively 66 and 73 successive bioevents in early Ordovician time. Event spacing averages 0.7–0.8 Ma and enables fine subdivision, correlation and homotaxial testing for diachroneity. The strong correlation between each of the six regional sequences and the CSS indicates the high level of accordance among graptolite successions around the world.
Stratigraphical and structural relationships of the Dingle Group (Silurian), County Kerry, Ireland
Authors: Holland C. H. 
Published: May 2009

AbstractPrevious views on the relationship between the thick continental deposits of the Dingle Group and the older fossiliferous Silurian rocks of the Dunquin Group are briefly reviewed. Detailed field evidence is presented showing that in the coastal sections of Coosgorrib, Coosglass, and Coosshaun there is faulting between the two groups. The same situation obtains at the northern end of the Great Blasket island. Fortunately inland exposures are sufficient to show that there is stratigraphical continuity from the lower Ludfordian (Ludlow) beds of the Croaghmarhin Formation, at the top of the Dunquin Group, into the purple siltstones of the Dingle Group. The Dingle Group is followed uncomformably by Upper Old Red Sandstone (essentially Upper Devonian).
Possible cellular tissue in an Ordovician graptoblast
Authors: Crowther P. R. , Rickards R. B. , Urbanek A. 
Published: May 2009

AbstractAn encysted portion of a crustoid stolotheca of Graptoblastoides sp. has revealed possible cellular tissue, but no gross zooidal morphology. Two explanations are suggested: one regarding the cellular complex as remnants of entrapped soft parts; the other, less likely, indicating a local disturbance in the secretion of the blastocrypt material. The cellular tissue is figured and described, and the opportunity is taken to review the extent of knowledge with respect to graptolite soft parts, and to compare the structures described with features of extant Rhabdopleura species.
The genesis of uranium in manganese and phosphorite assemblages, Timna Basin, Israel
Authors: Bar-Matthews Miryam 
Published: May 2009

AbstractUranium enrichments (up to 4000 ppm) occur in the manganese and phosphorite assemblages of the Lower Cambrian clastic marine sedimentary sequence, Timna Basin, Israel. Two types of mineralization assemblages can be defined. Sedimentary stratabound assemblages consist of uranium-enriched stratiform manganese and phosphatic laminae, diagenetic (type A) manganese nodules composed of pyrolusite and hollandite laminae and phosphorite lenses. Fission-track maps show that the uranium is homogeneously distributed within host manganese and phosphatic minerals of these assemblages. Epigenetic assemblages are mainly composed of manganese- and phosphorite-bearing veins and secondary (type B) manganese nodules with a coronadite dominant mineralogy. Uranium is depleted in these assemblages, relative to the sedimentary stratabound assemblages.The distribution of manganese and phosphorite assemblages has a marked bimodal character. Alternation between manganese and phosphatic laminae in the stratiform deposits reflects cycles of oxidizing and reducing conditions brought about by mixing and stratification of the waters in the Timna semi-closed depositional basins. Compaction of wet sediments led to remobilization and the formation of uranium-enriched manganese nodules at the aerated sediment–water interface, and uranium-enriched phosphorite lenses below the interface in reducing conditions. Epigenesis occurred through the passage of solution fronts which recrystallized the manganese and phosphatic minerals and remobilized metallic elements, particularly uranium which was leached away and is still being remobilized today.The mechanism of uranium uptake in manganese phases is shown most probably to involve adsorbtion of [(UO2)3. (OH)5]+ complexes on precipitating minerals. Uranium is enriched in both the pyrolusite and hollandite laminae of type A nodules, but is particularly concentrated in the former (4000–10000 ppm). Thermodynamic calculations of the relative stabilities of pyrolusite and hollandite suggest that the pH conditions of hollandite formation were close enough to the pH limit of efficient uranium adsorption to inhibit its uptake relative to pyrolusite.
Microfossils and Precambrian–Cambrian boundary stratigraphy at Maldeota, Lesser Himalaya
Authors: Brasier M. D. , Singh P. 
Published: May 2009

AbstractAn assemblage of problematical microfossils of Precambrian–Cambrian boundary age is redescribed from the Chert–Phosphorite Member, at the base of the Lower Tal Formation of Maldeota in the Lesser Himalaya of India. This assemblage has previously been ascribed to various ages, from Precambrian to Cretaceous, but is held by us to contain:Maldeotaia bandalica, Protohertzina anabaricagroup, trumpet-shaped elements, acicular elements A & B, ?Conothecasp.,Ovalithecacf.multicostata, allathecid sp. A,Barbitositheca ansata, Hexangulaconulariacf.formosa, Coleoloidesaff.typicalis, Hyolithellusaff.insolitus, H.cf.isiticus, H. vladimirovae, Spirellus shankariandOlivooides multisulcatus. These compare closely with assemblages found above the base of the first,Anabarites trisulcatus–Protohertzina anabaricaZone in China and in the second,Pseudorthotheca costataZone of southern Kazakhstan. The stratigraphic setting of the Krol–Tal succession is reviewed and several similarities are noted between the Precambrian–Cambrian boundary successions of Lesser Himalaya in India and of Yunnan and Sichuan in Southwest China, indicating that correlation between them is possible at several levels.