bluuebunny/crossref_metadata_2025 huggingface dataset:
Excess argon in amphiboles linked to greenschist facies alteration in the Kamila Amphibolite Belt, Kohistan island arc system, northern Pakistan: insights from 40Ar/39Ar step-heating and acid leaching experiments
Authors: Wartho Jo Anne , Rex David C. , Guise Philip G.
Published: May 2009

A mineralogical and 4OAr/39Ar study of 13 amphibole samples in the Kamila Amphibolite Belt and Kamila Shear Zone in northern Pakistan has found a correlation between the degree of greenschist facies alteration and quantity of excess 40Ar. Additionally, there is a north–south divide with amphibole samples from the northern region showing larger degrees of gree schist facies alteration, brittle deformation, and excess 40Ar incorporation compared to the predominantly plastically deformed, less altered, amphibole samples from the Kamila Shear Zone in the south. Acid leaching of two amphiboles from the Kamila Amphibolite Belt indicates that a large proportion of the excess 40Ar is correlated with later greenschist facies alteration hases, and can be easily removed by acid etching, thus revealing acceptable regional 40Ar/39Ar plateau ages.
Phytoplankton from the Lower Cambrian Læså formation on Bornholm, Denmark: biostratigraphy and palaeoenvironmental constraints
Authors: Moczydłowska Małgorzata , Vidal Gonzalo
Published: May 2009

Acritarchs from the Lower Cambrian Læsså formation on Bornholm, Denmark, are taxonomically diverse. Their state of preservation, including thermal, mechanical and chemical alteration, is discussed. Different states of thermal maturation of acritarchs in shales and phosphorites of the Broens Odde member could be explained in terms of possible irradiation from natural radioactive decay. The microfossils form two age-diagnostic assemblages that allow recognition of theSkiagia ornata–Fimbriaglomerella membranaceaandHeliosphaeridium dissimilare–Skiagia ciliosaAssemblage Zones within the Broens Odde member of the Laeså formation. Acritarch-based biostratigraphy indicates that the Lower Cambrian Balka Formation and Læså formation correspond to theSchmidliellus mickwitziZone andHolmia kjerulfiAssemblage Zone recognized in Baltoscandia and the East European Platform. Acritarch distribution within three different depositional settings indicates that comparable spectra of morphotypes occurred in different depositional environments. This suggests the absence of facies control. During early Cambrian times palaeoenvironmental barriers in shallow, epicontinental shelf basins constituted a minor obstacle for widespread distribution of acritarch taxa. Formerly proposed early Palaeozoic acritarch provincialism appears insufficiently documented in the fossil record and no evidence could be extracted from the Cambrian record. Following a rapid radiation at the onset of the Phanerozoic, Cambrian phytoplankton populations underwent dispersion following oxygenic and nutrient-rich bodies of water within epicontinental and presumably basinal environments. Lower Cambrian acritarch taxa were largely cosmopolitan and little affected by lithofacies associations. A continuous flow of data is contributing to the emergence of acritarch-based biostratigraphy. Its apparent consistency suggests great usefulness for interregional and detailed event correlation.
Climatic and oceanographic isotopic signals from the carbonate rock record and their preservation
Authors: Marshall James D.
Published: May 2009

Stable isotopic data from marine limestones and their constituent fossils and marine cements can provide quantitative evidence for changes in global climate and ocean circulation. Oxygen isotopic data can indicate changes in temperature and ocean composition whereas stratigraphic variation in carbon isotope ratios may reflect changes in the carbon cycle that can be linked to changes in oceanic productivity and atmospheric greenhouse gases. Terrestrial carbonates–meteoric cements, calcretes and speleothems–similarly offer significant potential for understanding the evolution of terrestrial climates by providing evidence for the composition of rainwater and the nature of vegetative cover.

Primary environmental isotopic signals may be obscured by the effects of post-depositional diagenetic alteration. Cementation and replacement reactions can take place in a wide range of diagenetic environments; the diagenetic history of an individual limestone is determined by a combination of its mineralogical diagenetic potential and depositional setting, together with subsequent changes in relative sea-level and burial history. Carbon isotopic values are less prone to alteration during diagenesis than oxygen values but shifts can be significant where organogenic carbon is incorporated. Linear covariation of carbon and oxygen values is not a reliable indicator of diagenetic alteration: water-rock interaction and fluid mixing may produce non-linear distributions.

Attempts to determine long-term changes in climatic and oceanographie conditions through isotope stratigraphy of shallow-water limestones must include an assessment of the diagenetic history of the materials analysed. Pétrographic examination using conventional microscopy backed up, where appropriate, by cathodoluminescence and scanning electron microscopy together with elemental and strontium isotopic analysis can help to identify the effects of diagenetic alteration. Where material with a range of different degrees of alteration is preserved in the same sediment it may be possible to compare patterns of isotopic and elemental variation and to attempt to unravel the effects of diagenesis in order to determine primary, environmental, isotopic signals. Recent research has shown that these techniques can be successfully employed in both Phanerozoic and Precambrian sediments.
A review of terrestrial and marine climates in the Cretaceous with implications for modelling the ‘Greenhouse Earth’
Authors: Spicer Robert A. , Corfield Richard M.
Published: May 2009

From the unique perspective of the geological record, it appears that the ‘Greenhouse Earth’ was a feature of climate for up to 80 % of the last 500 Ma, and that therefore our present glacially dominated climate is an anomaly. The Cretaceous in particular was a time of global warmth, an extreme greenhouse world apparently warmer than our current Earth. The geological record provides perspective and constraints against which the success of climate models can be evaluated. At present there are no ways of evaluating model predictions for the future of our ‘Greenhouse Earth’ until after the event. Retrodicting the past is therefore a very useful way of testing model sensitivity and robustness. The geological record tells us that the characteristics of the Cretaceous greenhouse world were a shallower equator-to-pole temperature gradient, shallow, well-stratified epicontinental seas with a tendency towards periodic dysaerobism, and a well-developed terrestrial flora extending to the high latitudes. Both marine and non-marine data show a global cooling trend throughout Late Cretaceous time, a trend that seems to correlate with declining atmospheric carbon dioxide.
Ediacaran fossils from the Innerelv Member (late Proterozoic) of the Tanafjorden area, northeastern Finnmark
Authors: Farmer J. , Vidal G. , Moczydłowska M. , Strauss H. , Ahlberg P. , Siedlecka A.
Published: May 2009

An Ediacaran assemblage dominated by an unidentified species ofCyclomedusaSprigg 1947, along with species ofEdiacaria?Sprigg 1947,BeltanellaSprigg 1947,HiemaloraFedonkin 1982, andNimbia?Fedonkin 1980, is described for the first time from the Innerelv Member of the Stappogiedde Formation exposed in coastal outcrops west of Tanafjorden on Digermul Peninsula, in northeastern Finnmark, northern Norway. The fossil assemblage is dominated by discoidal forms which share certain affinities with the cosmopolitan genera.CyclomedusaandEdiacaria. However, our specimens differ from these and other discoidal Ediacaran fossils in the absence of radial sculpture. This, along with a basically concentric organization, are characteristics shared withKullingiafrom the Dividal Group of northern Scandinavia, the White Sea, Podolia, and northwestern Canada, along with undescribed discoidal remains from the Charnian Supergroup, Charnwood Forest, Leicestershire, England, and the Conception Group, Avalon Peninsula, Newfoundland.

Our discovery of an Ediacaran-type assemblage within the Middle Innerelv Member provides support for previous suggestions of a late Vendian age for this sequence. This general conclusion is consistent with the occurrence of early Cambrian taxa, including the trace fossilPhycodes, and the problematical formsVendotaeniaandSabellidites,in basal portions of the Lower Breivik Formation, within the same stratigraphie section. The lowest formally-proposed faunal zone in northern Scandinavia is theKullingiaZone, based on the occurrence of the fossil medusoidKullingia concentricain Member III (Middle Sandstone C) of the Dividal Group, northern Scandinavia.Kullingiais a distinctly chambered form that was probably pelagic. In contrast,Cyclomedusa,and related genera of the so-calledCyclomedusaplexus, comprise an informal grouping of intergrading, probably benthic, taxa that possess radial and/or concentric organization. In light of the intergradational nature of taxa, present difficulties in taxonomic interpretation and correlation, and the abundance of cyclomedusoids in many Ediacaran assemblages, we suggest that the concept of theKullingiaZone, as originally defined for northern Scandanavia, be broadened to include the common form genera of theCyclomedusaplexus, inclusive of the occurrences in the Innerelv Member described herein. It is our hope that additional fieldwork will provide a basis for more refined taxonomic evaluations and biozonation.
A combined petrographical—geochemical provenance study of the Newland Formation, Mid-Proterozoic of Montana
Authors: Schieber Jürgen
Published: May 2009

A provenance study was conducted on the Mid-Proterozoic Newland Formation, in which petrographical features of sandstones and geochemical characteristics of shales were integrated to arrive at an internally consistent interpretation.

Sandstones of the Newland Formation are typically arkosic sands and arkoses with very-well-rounded quartz and feldspar grains and only minor amounts of extrabasinal rock fragments. The predominant feldspar types are K-spar and microcline, feldspar grains are smaller than quartz grains, and feldspars show little alteration due to weathering. Detrital modes of Newland sandstones (QFL diagrams) indicate that they were derived from a stable cratonic source. These petrographical features imply a source area dominated by granites and granitoid gneisses, semi-arid to arid climate, tectonic quiescence, and overall peneplain conditions.

Shales of the Newland Formation are dominated by illite, quartz silt, and fine crystalline dolomite. They have small La/Th rations, relatively large Hf contents, and small contents of Cr, Co, and Ni, all indicative of derivation from crust of granitic composition. Small Tio2/Al2O3ratios also suggest source rocks of granitic composition. The average chemical index of alteration (CIA) for Newland shales is 71.8, which in light of the probable granitoid source indicates modest amounts of chemical weathering. Relatively large SiO2contents and large K2O/Na2O ratios reflect derivation from stable cratonic areas and tectonic quiescence.

Thus, in general, the petrography of sandstones and geochemistry of shales provides the same provenance clues for the Newland Formation. One notable discrepancy between the two approaches is that the sandstones indicate an arid to semi-arid climate with very minor chemical weathering, whereas the CIA of the shales indicates at least modest amounts of chemical weathering. This indicates on one hand the need to better calibrate the CIA with a large variety of muds from modern climatic settings, and on the other hand the possibility that this discrepancy is due to transport segregation.
Oxygen isotope evidence for major fluid flow along the contact zone of the Rum ultrabasic intrusion
Authors: Greenwood R. C. , Fallick A. E. , Donaldson C. H.
Published: May 2009

Recent studies indicate that the Rum Tertiary ultrabasic intrusion formed in situ, and was not emplaced as a fault-bounded plug. The suggestion that the Main Ring Fault was the primary pathway for the flow of meteoric-hydrothermal fluids on Rum is therefore seriously flawed. Oxygen isotope evidence is presented indicating that the contact zone of the intrusion was the major pathway for meteoric fluids during cooling of the pluton. δ18O depletions of over 12‰ correlate with hydrothermal alteration assemblages, indicating that the bulk of the interaction with meteoric fluids took place at low temperatures (200–450 °C).
Two new graptolites from the early Silurian (Llandovery: Aeronian) of central Wales: an origin for monoclimacid thecal morphology
Authors: Zalasiewicz J. A.
Published: May 2009

Monograptus chrysalissp. nov. from themagnusZone (Aeronian) of central Wales shows a previously unrecognized combination of thecal types. Distally, the thecae possess marked lappets and short lateral horns, and approximate to those of therevolutusgroup of monograptids and some described examples ofPribylograptus. Proximally, the thecae are elongate and slender, and possess slit-like apertures below a distinct geniculum. They resemble the proximal thecae ofMonograptus imagosp. nov., one of the earliest recognized British monoclimacid monograptids, from the succeedingleptothecaZone of central Wales and the Lake District. It is suggested thatM. chrysalisarose from a graptolite of therevolutusgroup by retraction or suppression of the hooked proximal thecae, while the distal thecae remained constant, and thatM. imagothen evolved fromM. chrysalisby a spreading of the proximal end characteristics distally. The proximal morphology and overall record of the monoclimacids, though, suggests that they may be polyphyletic; in this paper, the generic nameMonoclimacisis reserved for the late Telychian graptolites of thevomerinagroup.
Pliocene—Quaternary sedimentation in the Northern Apennine Foredeep and related denudation
Authors: Bartolini Carlo , Caputo Riccardo , Pieri Marco
Published: May 2009

The deposits of the Pliocene—Quaternary foredeep of the Northern Apennine cover at present an area of 103000 km2. The original boundaries of the basin are not known, since marginal deposits have been eroded, in particular those of the inner, southwestern border. During Pliocene times the basin area was reduced by thrust tectonics and the amount of shortening may be tentatively estimated.

The present volume of Pliocene and Quaternary sediments may be inferred with good approximation from the maps of the base of the Pliocene and of the Quaternary (base of the Hyalinea balthica Zone) successions. The Pliocene volume has been corrected adding the estimate of the underthrust sediments, while no correction has been attempted for the eroded marginal deposits. The estimates of 97000 and 95000km3. reflecting the present volume of the Pliocene and Quaternary deposits, are therefore significantly less than the volumes originally deposited.

Present volumes have been transformed in ‘net’ (0% porosity) volumes, in order to obtain the relative net supply rates: 0.021 (Pliocene) and 0.047 (Quaternary) km3/a. Other unmeasurable factors (volume variations due to the weathering of silicates, loss of leached carbonates) may induce a probably unimportant underestimate of the supply rates.

Available data allow an approximate estimate of the range of the net volume of the Holocene sediments deposited during the last 6000 a BP (221–276km3) and of the relative net supply rate (0.037–0.046km3/a), that is not significantly different from the Quaternary one. Applying a porosity correction, these supply rates may be related to the Quaternary source area (128000km2) to obtain the relative denudation rates: 0.41–0.46mm/a (Quaternary) and 0.36–0.51 mm/a (Holocene). Present supply and denudation rates, deduced from the direct measurements of the bed load and suspended load of the apenninic and alpine rivers, do not differ significantly from the Quaternary and Holocene ones.

Available data do not allow a reliable estimate of the Pliocene source area, and consequently of the Pliocene denudation rate. However, a minimum of 160000–177 000 km3 has been eroded during Pliocene and Quaternary times. Assuming, as a working hypothesis, that the Pliocene source area did not significantly differ from the present one, an average thickness of 1240–1390 m could have been eroded since the beginning of Pliocene. This estimate is in agreement with the values obtained from the measurements of coalification of vegetal organic matter in the outcrops, and suggests that post-orogenic successions and ‘higher’ thrust sheets may have been completely removed in vast areas.
Evidence for late Precambrian to early Cambrian orogeny in northwest Himalaya, Pakistan
Authors: Baig M. S. , Lawrence R. D. , Snee L. W.
Published: May 2009

An angular unconformity below Cambrian rocks is present in the northwest Himalaya in the Hazara district, Pakistan. Low-grade metamorphism and folding with axial planar cleavage present in Precambrian rocks below the unconformity, but not in those above it, confirm orogenic deformation at this time. This is the first clear evidence for such a deformation episode and it may be referred to locally as the Hazaran orogeny. Anatectic peraluminous granites of the Himalaya are of only slightly younger age and may be related to this orogenic episode.
Stratigraphy and geodynamics of the Mombasa Basin (Kenya) in relation to the genesis of the proto-Indian Ocean
Authors: Rais-Assa R.
Published: May 2009

New data obtained from the study of the Kenyan and Tanzanian coastal basins establish a rifting period at the end of the Karoo sedimentation (Upper Triassic/Lower Jurassic), in the northeastern margin of Gondwanaland. The rifting period was followed by an erosive phase and then transgression of a Tethyan sea during the Middle Jurassic. This leads to a modification of the stratigraphic succession of the region and gives new datas about the nature of separation and drifting of Gondwanaland, the genesis of the proto-Indian ocean and the paleoposition of Madagascar
Apatite fission track data from southeast Ireland: implications for post-Variscan burial history
Authors: Keeley M. L. , Lewis C. L. E. , Sevastopulo G. D. , Clayton G. , Blackmore R.
Published: May 2009

The results of a reconnaissance apatite fission track study from southeast Ireland are compared with modelled ages and length distributions predicted using both traditional thermal histories and the thermal history indicated by the fission track age and length data. All samples have experienced total annealing during the Variscan orogeny. There is strong evidence for significant post-Variscan, pre-Mid Jurassic heating and cooling. The modelling also recognizes a third thermal event subsequent to 100 Ma. The implications of these results are examined in terms of the timing and extent of subsidence and inversion along the northern margin of the North Celtic Sea Basin.
Conodonts from the lower Ordovician at Hunneberg, south-central Sweden
Authors: Löfgren A.
Published: May 2009

Six sections through lower Ordovician shale and limestone at Hunneberg have been sampled for conodonts. Forty-nine limestone horizons and 34 shale surface samples yielded over 40000 conodont elements. The degree of biostratigraphic resolution permitted further subdivision of the post-TremadocParoistodus proteusZone into four successive intervals. Preliminary studies of coeval rocks in other areas of Sweden show that this subdivision applies elsewhere as well. Comparison with published sections from, for example, Newfoundland, Estonia and Kazakhstan indicates that the fourfold subdivision of theP. proteusZone proposed here could significantly increase the correlation accuracy, even internationally, within this critical part of the Ordovician. Graptolites and trilobites found together with the conodonts at Hunneberg indicated that the new subdivision could help to correlate biostratigraphic units based on these fossil groups. Thus the two lower intervals of theP. proteusZone correspond to theM. (E.) armatatrilobite Zone, and the upper two are approximately equal to theM. (V.) planilimbataZone. The base of theT. phyllograptoidesgraptolite Zone at Hunneberg lies close to the boundary between the upper middle and uppermost intervals of theP. proteusZone, but the graptolitic chronozonal boundary may be lower. The overlyingPrioniodus elegansZone is mainly represented by graptolitic shale at Hunneberg, but conodont faunas have been found in it, and co-occurrences of conodonts and graptolites have been noted.
Relative sea-level changes and the development of a Cambrian transgression
Authors: McKie T.
Published: May 2009

The Lower Cambrian in northwest Scotland is one example of a Lower Palaeozoic ‘orthoquartzite-carbonate’ succession deposited on a slowly subsiding, peneplained Precambrian basement during a period of relative sea-level rise. This particular setting led to the development of a very wide, low gradient shelf which was extremely sensitive to minor sea-level changes. The basal quartz arenite section (Lower Member-Pipe Rock) is a transgressive, tide-dominated systems tract, but lacks a systematic parasequence architecture because of three factors: a fluvial sediment flux was insufficient to induce shoreline progradation, accommodation space was limited during sea-level falls (which are commonly expressed by widespread erosional surfaces), and sediment yield to the shelf by transgressive reworking was a major contributor towards the preserved stratigraphy. The storm-dominated Fucoid Beds represent a condensed section and also show the effects of rapid and widespread facies belt oscillations because of the low shelf gradient. An overlying highstand systems tract is also lacking, partly due to the absence of a large fluvial sediment yield and also due to lowstand and transgressive reworking. An erosively based tidal sandsheet at the top of the Fucoid Beds, interpreted to be a lowstand systems tract, therefore rests directly on the condensed section of the underlying sequence. This was in turn reworked into linear tidal sandbanks (Salterella Grit) during slow sea-level rise, prior to the next major transgression. The limited accommodation space therefore introduced a preservational bias towards deepening-upward trends on a parasequence and sequence scale. The oscillations in facies belts, episodic subareal exposure and the potential to remove substantial portions of systems tracts suggests that Lower Palaeozoic ‘orthoquartzite’ successions may exhibit regular and abrupt vertical shifts in depositional environment which, given their subtle lithological character, may require detailed analysis to identify. Such successions may also display incomplete development of several components of transgressive-regressive sequence architecture.
Mesozoic extensional tectonics in the southeast Iberian Chain
Authors: Roca E. , Guimerà J. , Salas R.
Published: May 2009

The Desert de les Palmes area, in the southeast Iberian Chain, belongs to a Mesozoic NE–SW high which separated the early Cretaceous basins of the Maestrat and Aliaga-Penyagolosa from the little Orpesa basin. Its structure is characterized by the development of a system of NE–SW to ENE–WSW extensional listric faults detached in a shallow upper crustal level (1.7–2.2 km), mostly affecting the pre-Upper Cretaceous rocks. These faults record two well-differentiated rifting periods: (1) a first late Triassic–early Jurassic rifting period that divided the Desert de les Palmes high in several blocks; (2) a second early Cretaceous rifting period, only developed in the eastern margin of the Desert de les Palmes high, which was related to the opening of the Maestrat, Aliaga-Penyagolosa and Orpesa basins. Based on the comparison of the main features of this Mesozoic structure with an analysis of the structural and subsidence data already known in the neighbouring Mesozoic basins (Maestrat, Aliaga-Penyagolosa and Columbrets), a geodynamic scenario for the crustal evolution of the eastern Iberian Chain is also suggested. This involves four evolutionary stages: (1) Triassic rift (late Permian–Hettangian); (2) early and middle Jurassic postrift (Sinemurian–Oxfordian); (3) late Jurassic and early Cretaceous rift (Kimmeridgian–middle Albian), which includes a short Hauterivian postrift period; and (4) late Cretaceous postrift (late Albian–Maastrichtian).
Neoproterozoic (Vendian) ichnofossils from Lower Alcudian strata in central Spain
Authors: Vidal Gonzalo , Jensen Sören , Palacios Teodoro
Published: May 2009

Simple trace fossils are reported from three localities in central Spain within a monotonous succession of shale and greywacke and classical turbidites attributed to the Neoproterozoic (Riphean) Lower Alcudian megaunit. The Lower Alcudian strata are believed to have a complex tectonic history including deformation during the Pan-African Orogeny. Chronostratigraphic control is not available for this structurally complex succession and fossil evidence is sparse. Here we report on a low-diversity ichnofossil association including the ichnospecies (isp.)Gordia marinaEmmons, 1844,Gordiaisp.,G.aff.arcuataKsiążkiewicz, 1977. The stratigraphically inferred Proterozoic age of the rocks, added to the present ichnofossil association, suggests that colonization of deeper waters started before the early Cambrian. We further consider that the plausible oxygen requirements of marine invertebrate(s) which produced the present ichnofossil taxa may well have exceeded the oxygen levels in a largely dysaerobic environment. A circulation model proposed by one of us (Palacios, 1989), implying the presence of extensive upwelling and descending oxygen-rich waters, helps to explain the continuous colonization of deep-water settings in Lower and Upper Alcudian strata. We argue that the ichnofaunal record of strata older than 650 Ma is both rare and inconclusive. Moreover, the present ichnofauna, being similar to ichnofaunal associations in environmentally comparable Upper Alcudian strata, is largely documented from Neoproterozoic (Vendian) and early Palaeozoic associations elsewhere. We conclude that the new ichnofossil evidence is consistent with a Vendian or younger age for the Lower Alcudian megaunit. This view is also consistent with a recently published maximum deposition age of 565 Ma inferred from U–Pb datings of detrital zircons from the supposedly much older (Middle Riphean) Tentudia Group or ‘Serie Negra’, that appears roughly time-equivalent with Lower Alcudian strata. It thus may appear that basin formation and flysch deposition in central Iberia encompass a minor segment of Neoproterozoic time, being probably limited to the Vendian. By default, formerly postulated Pan-African deformation of Neoproterozoic Lower Alcudian strata appears implausible. We further conclude that the complex pre-Phanerozoic structural history of Alcudian strata is probably restricted to Cadomian and younger deformation. Former structural models need substantial revision.
St Audrie's Bay, Somerset, England: a candidate Global Stratotype Section and Point for the base of the Jurassic System
Authors: Warrington G. , Cope J. C. W. , Ivimey-Cook H. C.
Published: May 2009

In 1967 the Somerset coastline near Watchet was proposed as the type area of the basal (Planorbis) chronozone of the Hettangian Stage and thus of the Jurassic System. Neither at that time nor subsequently, however, has a type locality and section been nominated from those available in the area. There is urgent need to select a Global Stratotype Section and Point (GSSP) for the base of the Hettangian, and of the Jurassic System. The cliff forming the headland at the west side of St Audrie's Bay, three kilometres east of Watchet, Somerset, is here proposed as the type locality and section, with the base of the Hettangian Stage, at the base of the Planorbis Chronozone, being placed at the horizon currently recognized as that at which ammonites of the genusPsilocerasappear. In this section the base of the Planorbis Chronozone corresponds with the base of thePsiloceras planorbisBiozone. The proposal of this section is conditioned by the availability of comprehensive litho- and biostratigraphic information, and the ability of the section to fulfil International Commission on Stratigraphy (ICS) requirements for a candidate GSSP.
Diagenetic controls on multiphase pyritization of graptolites
Authors: Underwood Charlie J. , Bottrell Simon H.
Published: May 2009

Observations on pyritized graptolites have revealed a variety of pyrite morphotypes, several of which often coexist within the same fossil steinkern. The commonest forms of pyrite are those consisting of size-sorted microcrysts, which show degrees of ordering ranging from well-ordered framboids to a more homogeneous, unordered groundmass. Larger, euhedral pyrite crystals may be scattered throughout the microcrystalline groundmass, or be the dominant pyrite form in themselves. ‘Floriform’ pyrite frequently overgrows earlier framboids and may act to mould the inner periderm microstructure of the graptolite. Overpyrite occurs either as a primary ‘overspill’ of internal pyrite, or as a separate, later phase.

All of the diagenetic pyrite has light sulphur isotope compositions, indicating formation during early diagenesis; isotopic evidence of the relative timing of pyrite generations matches the morphological paragenesis. Under most conditions, available iron appears not to be a limiting factor in pyrite formation, with reactive organic matter only being limiting at the lowest concentrations encountered in these sediments. The availability of organic matter does, however, exert a control on the timing of fossil pyritization. At one locality, a change in diagenetic conditions has allowed for the formation of a morphologically late generation of pyrite with isotopically light and therefore apparently early diagenetic signatures.
Conodont thermal alteration patterns in Palaeozoic rocks from Belgium, northern France and western Germany
Authors: Helsen S. , Kønigshof P.
Published: May 2009

In this paper thermal patterns based on conodont Colour Alteration Indices (CAI) have been studied in the Devonian and Carboniferous rocks from Belgium, northern France and western Germany. Four maps (with the exception of the southeastern Rheinisches Schiefergebirge) have been compiled for the investigated area: one for the Eifelian and Givetian, a second for the Frasnian, another for the Famennian and one for the Tournaisian and Viséan. Conodonts have not been found in Cambrian rocks from Belgium and northern France and as they are scarce in the Ordovician to Lower Devonian formations of this area, it was impossible to integrate these CAI data into an isograd map. In Germany (in the eastern Rheinisches Schiefergebirge) only a few CAI values for the Lower Devonian strata are available. In contrast to the Ardennes where in general conodonts show CAI values of 3.0–5.0, indices in areas adjacent to the Anglo-Brabant Massif, e.g. in the western Namur Synclinorium and in the Eifel Hills, range between 1.5 and 2.0. Slightly higher values occur in the Campine Basin north of the Anglo-Brabant Massif. By comparison with CAI data from Belgium, conodont colour alteration indices from the same stratigraphic horizons in some areas of the northern and eastern Rheinisches Schiefergebirge are up to one index higher. In most areas of the Rheinisches Schiefergebirge the level of metamorphism corresponds to the anchizone. Regional variations in CAI data appear to be related to buried intrusions and associated hydrothermal activity. In the southeastern Rheinisches Schiefergebirge CAI data are not completely consistent but it is possible to discern an increase in coalification from the north to the south. High index values in the southern Rheinisches Schiefergebirge are believed to result from regional metamorphism and heating related to numerous intrusive bodies.
The Portsoy–Duchray Hill Lineament: a review of the evidence
Authors: Goodman S.
Published: May 2009

The Portsoy–Duchray Hill Lineament has been widely quoted in the literature as a major zone of discontinuities, running from Portsoy on the Banff coast to the Duchray Hill area in Perthshire. A review of the evidence indicates that there is no single structural entity running from Portsoy to Duchray Hill; the term ‘Portsoy-Duchray Hill lineament’ should be avoided. There are, however, significant along- and across-strike variations in geological history across a zone which extends from Portsoy to the Cabrach (the ‘Portsoy Line’). Certain elements can be identified also in areas further south, e.g. the Coyles of Muick shear zone and Glen Doll fault.

The Portsoy Line was initiated as a zone of syn-depositional faulting, with resulting facies variations similar to those seen across the Glen Doll fault. The Portsoy Line was reactivated as a ductile shear zone during Caledonian orogenesis, while the Glen Doll fault is a later, more brittle, structure. The shear zone from Portsoy to the Cabrach forms the western branch of a system of shears which acted to focus syn-orogenic basic magmatism, and differential movement during post- metamorphic uplift. The Coyles of Muick shear zone is the most southerly example of the system of shears, and there is probably a transfer zone along Deeside between it and the Portsoy-Cabrach shear zone, now obscured in part by the Ballater Granite. The focused shear zones serve to delineate the Buchan area, with its low pressure metamorphism and distinctive structural style, from areas to the south and west where the structural level was deeper, and deformation more pervasive.
Fluid supersaturation and crystallization in porous media
Authors: Putnis Andrew , Prieto Manuel , Fernandez-Diaz Lurdes
Published: May 2009

The relationship between the supersaturation at the point of crystallization and the rate at which supersaturation increases has been studied from nucleation experiments on barite BaSO4, strontianite SrCO3, witherite BaCO3 and gypsum CaSO4.2H2O. The crystallization experiments have been carried out by the counter-diifusion of cations and anions through a column of porous silica gel transport medium. Nucleation is suppressed in a finely-porous medium resulting in very high values of supersaturation before crystallization from the solution begins. This threshold supersaturation for nucleation depends on the solubility of the salt, the porosity of the medium and the supersaturation rate. Nucleation inhibitors were used to extend the range of supersaturation attainable. In all cases the experimental data fits the general expression: rate of change of supersaturation ∝ (threshold supersaturation)m. These results are compared to previous work from the field of chemical engineering on the relationship between supersaturation, volume and cooling rate in aqueous salt solutions. These experiments have important implications to supersaturation in natural fluids and subsequent crystallization in relation to geological problems including crystallization in low temperature sedimentary environments and fluid-rock ratios in hydrothermal mineral deposits.
A revision of Ordovician series and stages from the historical type area
Authors: Fortey R. A. , Harper D. A. T. , Ingham J. K. , Owen A. W. , Rushton A. W. A.
Published: May 2009

This paper is a revision of the standard chronostratigraphy of the Ordovician of the historical type area in England and Wales. The revision is a response to the need for more precise definitions of series and stages, especially for practical international correlation. In some cases this has entailed moving horizons for Series bases away from those in the classical sections. The scheme will be used as the standard for a forthcoming revision of the Correlation Chart of the British Ordovician System. The Ordovician is divided into five series: Tremadoc, Arenig, Llanvirn, Caradoc and Ashgill. The Llanvirn is extended to include part of the classical Llandeilo Series, which is included within it as a stage. Where stage subdivisions of standard series have not yet been defined, as in the Tremadoc, they are proposed in this paper. However, Bancroft’s fine stadial divisions of the Caradoc, which have been criticized for their brevity and local utility, are combined into four new stages, which will have wider application. The former Caradoc subdivisions are retained as substages for the purposes of regional correlation.
Pterosaur fossils from the Cretaceous of Chile: evidence for a pterosaur colony on an inland desert plain
Authors: Bell C. M. , Padian K.
Published: May 2009

A 2-m-thick bed of sandstone and conglomerate in early Cretaceous desert sediments in the Andes of northern Chile contains thousands of scattered pterosaur bones. The bed displays sedimentary features which suggest that it was the product of a single major flood event. It seems possible that a large pterosaur colony was overwhelmed by the waters of an exceptional flood sweeping across a flat alluvial plain.
Palaeomagnetism of Late Ordovician igneous intrusions from the northern Welsh Borderlands: implications to motion of Eastern Avalonia and regional rotations
Authors: Piper J. D. A.
Published: May 2009

The palaeomagnetism of a Late Ordovician dolerite suite widely distributed in the northern Welsh Borderlands has been investigated to evaluate (i) regional rotations associated with later deformations and (ii) Lower Palaeozoic latitudes of Eastern Avalonia. Only local effects of Acadian and Variscan overprinting are observed in this region and most bodies appear to preserve a primary cooling-related magnetization. Magnetic properties in the Breidden Dolerite are related to primary igneous differentiation and a dual polarity characteristic remanence (D/I = 333.1/−71.6°, 6 sites, α95 = 10.4°) predates Late Ordovician folding. Dolerites elsewhere in the Breidden Hills Inlier record the same reversal of remanence and the combined mean (D/I = 314.3/−72.8°) is rotated clockwise from the mean remanence (D/I = 291.5/−59.5°, 21 sites, α95 = 6.0°) in a similar dolerite suite within the adjoining Shelve Inlier which is also of dual polarity and predates Late Ordovician folding. The difference in magnetic declinations is similar to the rotation of Late Ordovician fold axes and fracture systems between the two inliers and is probably the resultant effect of local rotation, possibly about a constraining bend in the Severn Valley Fault System. The Moel-y-Golfa Andesite of Caradocian (Soudleyan) age has a remanence (D/I = 294.3/−50.4°, 13 samples, α95 = 2.8°) which study of an adjoining volcanic conglomerate shows to be of primary cooling-related origin. The equivalent palaeolatitude is ˜ 31° S in early Caradocian times. A similar estimate is derived from a probable diagenetic overprint in the contemporaneous Castle Hill Conglomerate at Montgomery. The Mynydd-Bryn Andesite in the Berwyn Inlier, of probable similar origin and age to Moel-y-Golfa, has a magnetization of opposite polarity but similar inclination (D/I − 239.1/47.4°, 14 samples, α86 = 5.5°). Declination differences between these volcanic outcrops are a possible signature of later block rotations between the Lower Palaeozoic inliers and identify the importance of terrane definition in analysis of Lower Palaeozoic palaeomagnetic results from the Caledonides. A dolerite sheet complex at Hendre in the Berwyn Hills has a younger remanence (D/I − 182.3/ − 3.0°, 20 samples, α95 = 7.8°), probably of mid-Carboniferous age.

The collective Upper Ordovician results define a rapid movement of the Avalonian Plate, probably spanning no more than 10 Ma, from ˜ 30° S in early Caradocian times to ˜ 57° S prior to folding of the dolerites in late Caradocian to early Ashgill times. This motion into higher southerly palaeolatitudes marks a reversal of the motion of Avalonia into more shallow southerly palaeolatitudes between Arenig and Llanvirn times and shows that Ordovician closure of the Iapetus Ocean was more complex than recognized hitherto.
The Antrim–Galway Line: a resolution of the Highland Border Fault enigma of the Caledonides of Britain and Ireland
Authors: Ryan P. D. , Soper N. J. , Snyder D. B. , England R. W. , Hutton D. H. W.
Published: May 2009

The westward continuation of the Highland Border fault of Scotland (HBFZ) into Ireland is problematic. It is widely thought to follow a pronounced magnetic and gravity lineament, the Fair Head-Clew Bay Line (FCL). The advantage of this interpretation is that it places all the Ordovician ophiolitic complexes and associated sedimentary basins to the south of the FCL, which would represent the contact between Laurentia and the outboard terranes. Its main shortcomings are that both the deep structure and timing of strike-slip are different on the HBFZ and FCL. In Ireland the FCL is a north-dipping feature that can be traced to the Moho on BIRPS profiles, while the HBFZ has no such signature. Terrane amalgamation in western Ireland was completed by the late Ordovician, while in Scotland the Midland Valley terrane did not finally dock until the early Devonian. These considerations suggest that in western Ireland a branch of the HBFZ exists, which was active in post-Ordovician time and must lie south of Connemara. An examination of Irish geological, geophysical and image-processed magnetic data shows that a profound lineament can be traced from Antrim to Galway Bay (the Antrim–Galway Line). Stitching plutons date movement on it as pre-405 Ma. We propose that the Antrim–Galway Line represents the continuation of the Scottish HBFZ, while the FCL is a preserved Ordovician splay of the HBFZ system whose northdipping geometry is a product of Ordovician collapse of the orogen in western Ireland.
The stratigraphy, correlation, provenance and palaeogeography of the Skiddaw Group (Ordovician) in the English Lake District
Authors: Cooper A. H. , Rushton A. W. A. , Molyneux S. G. , Hughes R. A. , Moore R. M. , Webb B. C.
Published: May 2009

A new lithostratigraphy is presented for the Skiddaw Group (lower Ordovician) of the English Lake District. Two stratigraphical belts are described. Five formations are defined in the Northern Fells Belt, ranging in age from Tremadoc to early Llanvirn. They are all mudstone or sandstone dominated, of turbidite origin; in ascending order they are named the Bitter Beck, Watch Hill, Hope Beck, Loweswater and Kirk Stile formations. Two formations are defined in the Central Fells Belt, ranging in age from late Arenig to Llanvirn. These are the Buttermere Formation – a major olistostrome deposit – overlain by the Tarn Moor Formation, consisting of turbidite mudstones with volcaniclastic turbidite sandstone beds. A revised graptolite and new acritarch biostratigraphy for the Skiddaw Group is presented with eight graptolite biozones and thirteen acritarch assemblages and sub-assemblages. The provenance of the group is assessed from detailed petrographical and geochemical work. This suggests derivation, in the early Ordovician, largely from an old inactive continental arc terrane lying to the southeast, with the appearance of juvenile volcanic material in the Llanvirn. Comparisons and correlations of the Skiddaw Group are made with the Isle of Man and eastern Ireland.
Biostratigraphy of the supra-ophiolite sediments of the Troodos Massif, Cyprus: the Cretaceous Perapedhi, Kannaviou, Moni and Kathikas formations
Authors: Urquhart E. , Banner F. T.
Published: May 2009

The Troodos Massif of Cyprus exposes a classic and much-studied ophiolite sequence representing oceanic crust of Late Cretaceous age. K-Ar dating of the sheeted dykes and of the overlying pillow lavas gives a range of 83±3 Ma (earliest Campanian) and 75±5 Ma (late Campanian) respectively for the formation of the upper levels of the ophiolite. An autochthonous sequence of Late Cretaceous to Recent age sediments is exposed resting on the ophiolite, the oldest part of which reflects sedimentation in an apparently deep marine, oceanic setting. Little biostratigraphical information is available to constrain the chronostratigraphy of these sediments in relation to the complex geological history of the island, including the uplift and unroofing of the ophiolite, despite their richness in microfauna and flora. This paper provides an integrated biostratigraphical study based on radiolaria, planktonic foraminifera and calcareous nannofossils of the oldest part of the supra-ophiolite succession, of the Cretaceous Perapedhi, Kannaviou, Moni and Kathikas formations. For the first time, well-defined micropalaeontological evidence establishes the relative ages of these formations. The umbers of the Perapedhi formation are no younger than Campanian in age while the volcaniclastic sediments of the Kannaviou formation were also deposited during Campanian times. The matrix of the Moni formation contains a microfauna consistent with the hypothesis that it is derived from the Kannaviou. In contrast, the Kathikas formation is composed of sediment derived mainly from the allochthonous Mamonia complex, but autochthonous pelagic interbed horizons demonstrate that it was deposited probably within a very short time interval during the late Maastrichtian. Key index species are figured, including the first published micrographs of Cretaceous planktonic foraminifera from Cyprus.
The geometry and structural evolution of a crustal-scale Caledonian fold complex: the Ballybofey Nappe, northwest Ireland
Authors: Alsop G. I.
Published: May 2009

The gross geometries exhibited by crustal-scale fold nappes are considered a consequence of both original stratigraphic relationships associated with sub-basin configuration, coupled with the nature of the structural regime and tectonic processes involved in the generation of the nappe pile. The Neo-Proterozoic Dalradian metasediments of northwestern Ireland provide a well-constrained and correlatable stratigraphy which defines a sequence of sub-reclined, tight-isoclinal Caledonian (c. 460 Ma) fold nappes. Within this fold complex, the dominant structure is the crustal-scale Ballybofey Nappe, which may be traced for 40 km along strike and is responsible for a regional (500 km2) stratigraphie inversion. The gentle, NE-plunging attitude of this fold results in a complete spectrum of tectonic levels and deformation gradients being exposed. Relatively low strains in the upper fold limb gradually increase down through the nappe, resulting in the generation of composite foliations and lineations and the development of a 10 km thick shear zone which culminates in a high strain basal detachment with underlying pre-Caledonian basement. The Ballybofey Nappe nucleated and propagated along a major zone of lateral sedimentary facies variation, coincident with the margin of a major Dalradian sub-basin. The large amplitude of the nappe is strongly influenced by the lateral heterogeneity within the metasedimentary sequence, and is associated with a minimum of 25–30 km ESE-directed translation concentrated within the overturned limb. Additional significant displacement is also focused along the basal décollement. Generation of the nappe complex resulted in significant crustal thickening and amphibolite facies metamorphism consistent with 15–18 km of burial, induced by a sequence of nappes propagating in the direction of overshear. The ESE-directed translation of the major fold nappes is away from the Caledonian foreland and a gravity-driven mechanism of nappe emplacement is suggested. Rigorous structural analysis within the cohesive stratigraphie framework enables relationships between the tectonic evolution and stratigraphic patterns to be distinguished, thus allowing models of fold nappe generation and mid-crustal deformation to be evaluated.
Structural data from drill core
Authors: Lopez-Diaz Fernando , Bastida Fernando
Published: May 2009

A graphical method for the analysis of the orientation of planar and linear structural elements in drill core is presented. Simple computation and projection operations applied to data taken from elements traced on the cylindrical surface of the core are required. To know a structural element(s) of reference, the method requires detailed structural analysis of the surface geology around the drill hole.