bluuebunny/crossref_metadata_2025_split huggingface dataset:
Conodont palaeobiology: recent progress and unsolved problems
Authors: Morris S. Conway 
Published: July 2007

ABSTRACTThe fossilized soft‐parts of conodonts are extraordinarily rare; excluding dubious and refuted examples, only 10 specimens are known. Nine come from the Lower Carboniferous of Granton, Scotland, and represent at least two genera with complex polygnathacean apparatuses. The soft‐parts include well‐defined myotomal segmentation and a probable notochord, thus strongly suggesting affinity with the chordates. The tenth specimen, from the Lower Silurian of Waukesha, Wisconsin, bears a simpler apparatus of panderodontid conodont elements. The soft‐parts are very poorly preserved, but appear to be unlike those of the Granton specimens. In both occurrences the conodont apparatus appears well adapted for grasping and biting, supporting earlier suggestions that conodont elements acted as teeth. Many conodonts were probably predators or scavengers, but other feeding modes are not necessarily excluded. Other aspects of conodont palaeobiology reviewed here include mode of life and possible migration, reproduction and genetics, and palaeopathology. All these topics rely almost exclusively on the study of conodont elements, and many problems of conodont palaeobiology await further research.
San Quintín volcanic field, Baja California, Mexico: ‘within‐plate’ magmatism following ridge subduction
Authors: Storey M. , Rogers G. , Saunders A.D. , Terrell D.J. 
Published: July 2007

ABSTRACTThe Holocene San Quintín volcanic province in northern Baja California comprises spinel‐lherzolite‐bearing alkali basalts. Trace element (La/Nb = 0.57–0.73; K/Rb = 402–479; LaN/YbN= 8.4, 9.9) and isotopic ratios (87Sr/86r = 0.70323–0.70352; 143Nd/144Nd = 0.512924–0.512996; 206Pb/204Pb = 19.108, 19.250; 207Pb/204Pb = 15.567, 15.589; 208Pb/204Pb = 38.82, 38.85) show that the lavas are compositionally indistinguishable from some ocean island, plume‐associated basalts such as Hawaii and the Azores, and testify to an asthenospheric source for the magmas. The occurrence in Baja of such lavas may be related to the nature of the cessation of plate subduction beneath the peninsula; at present, San Quintín (and volcanic provinces to the north) are underlain by a ‘no‐slab window', whereas immediately to the south, remanent oceanic lithosphere may be preserved as a relict slab. This may act as a bamer to the upward passage of diapirs or magmas from the asthenosphere.
Archimedes revisited: a structural test of eclogite emplacement models in the Austrian Alps
Authors: Behrmann J.H. , Ratschbacher L. 
Published: July 2007

ABSTRACTHigh‐pressure rocks of the eclogite zone (Tauern Window, Eastern Alps) formed at 20 kb/600°C. Uplift was fast without post‐burial heating. Extensive near‐field and far‐field deformation of the eclogite zone and its surroundings provide evidence for a tectonic uplift mechanism. Published emplacement models (buoyant rise, underplating and extension, wrench faulting) create distinct patterns of crustal deformation, and are therefore testable by structural analysis. We show that emplacement‐related deformation and its kinematics are consistent with underplating and extension. Extension is two‐phased. The first phase may be driven by changes in rate or direction of plate convergence. The second phase is due to large‐scale underplating of continental crust.
On the Chouet model of excitation of a buried magmaticpipe
Authors: Salusti E. , Travaglioni F. 
Published: July 2007

ABSTRACTRhythmic motion of the ground (tremor) is often associated with activity before and during a volcanic eruption. Volcanic tremor is characterized by dominant peaks located in the 0.1–10 Hz range of the spectrum. In the light of previous formulations concerning the vibrations of a uniform vertical magma‐filled pipe, this work presents the analysis of the motion when the section of the pipe is not constant but has constrictions and meanders. Our results show that the magma velocity modulus and pressure are inversely proportional to the pipe's cross‐section. Consequently the effect of the motion of the magma gives rise to enlargements of the pipe's constrictions.
Deep‐sea biogenic silica: new structural and analytical data from infrared analysis ‐ geological implications
Authors: Fröhlich François 
Published: July 2007

ABSTRACTThe main parameters of infrared spectra have been investigated for marine siliceous skeletons and for silica polymorphs. From infrared (IR)data, the mean values for the Si‐O bond length (1.62 Å) and for the Si‐O‐Si angle (about 142°6) are inferred for biogenic silica. A model is proposed for the molecular structure of biogenic, amorphous silica, which consists of a discontinuous, three‐dimensional framework of short chains of [SiO4] tetrahedra, bounded with apical hydroxyls. According to the relationship between the structure of the silica polymorphs and their infrared spectra, it is shown that a quantitative analysis of biogenic silica seems to be possible in quartz‐rich sediments. Some geological implications of these data are emphasized, including the processes of siliceous skeleton sedimentation and diagenetic evolution.
Viscosity ratio estimates from natural Rayleigh‐Taylor instabilities
Authors: Rönniund Peter 
Published: July 2007

ABSTRACTThe geometry of natural gravity‐driven overturns potentially contains information on viscosity ratios of the material involved. Irregularities at the interface between two superposed horizontal fluid layers tend to amplify and grow with time with a regular wavelength if the top iayer is more dense, Published formulas (Woidt), for such Rayleigh‐Taylor (R‐T) instabilities have been used to construct graphs which can constrain the effective viscosity ratio at the onset of the overturn. Only the initial thicknesses of the two layers and the final wavelength of the structures need be known. The theory is based on an infinite two‐layer system of Newtonian fluids between rigid boundaries with no slip or free slip at contacts.Geological examples included are; soft‐sediment dewatering on the scale of decimetres in northern Norway, an unusual instability between basalt and sand in Australia of hundreds of metres, salt diapirism of kilometres in Germany and the Persian Gulf, and granitic diapirism from northern Sweden of tens of kilometres. Viscosity ratios in the order of 1–10 are found for soft sediments with the top layer as the more viscous. Ratios inferred from patterns of salt diapirs are very large, with the overburden approximately 103‐−105 times more viscous than the salt. The viscosities of granites are indicated as lower than their overburden and with ratios of 103.
Evidence from Mössbauer spectroscopy for intense localization of metamorphic fluid flow during the formation of spaced cleavage
Authors: Boulter C.A. , McConchie D.M. , Clarke E.M. 
Published: July 2007

ABSTRACTArenites in the low‐grade metasedimentary rocks of the Stirling‐Barren Group, Western Australia, contain discrete spaced cleavages. These in part are pure white mica, but up to 30% of the cleavage zones consists of fine‐grained quartz resulting from crystal plastic deformation. A modified Gresens analysis shows that the cleavage domains formed by a 50% volume loss associated primarily with SiO2 and MgO concentration decreases of 58% and 37%, respectively, and a 12% decrease in Fe2O3. Mössbauer spectroscopy shows that the Fe2+: Fe3+ ratio from lithon to cleavage changed from 50:SO to 15:85. This substantial difference cannot be accounted for by loss of iron. The data are best explained by the focusing of a highly oxidizing fluid through the cleavage domains where volumetric fluid rock ratios around 150 to 1 are indicated. In this example advectic processes appear to be important in low‐grade regional metamorphism.
A psittacosaurid dinosaur from the Cretaceous of Thailand and its implications for the palaeogeographical history of Asia
Authors: Buffetaut Eric , Sattayarak Nares , Suteethorn Varavudh 
Published: July 2007

ABSTRACTPrimitive ceratopian dinosaurs of the family Psittacosauridae were hitherto known only from central and northern Asia, from northern China to Mongolia and Siberia. This paper reports the discovery of psittacosaurid jaws in Early Cretaceous rocks of the Khorat Plateau in north‐eastern Thailand. These fossils are clearly referrable to the genusPsittacosaurusOsborn and may represent a new species. They constitute a significant new element of the hitherto poorly known Cretaceous continental vertebrate fauna of South‐East Asia, suggesting that it was basically similar to that of northern and central Asia. This in turn confirms that, contrary to some recent palaeogeographical reconstructions, by Early Cretaceous times the Indochina block, on which the Thai psittacosaurids have been found, had become part of mainland Asia.
Outcrop to subsurface cycle correlation in the Milankovitch frequency band: Middle Cretaceous, central Italy
Authors: Melnyk David H. , Smith David G. 
Published: July 2007

ABSTRACTTime series analysis of the Cenomanian through Aptian interval, in well logs from offshore Adriatic boreholes, facilitates a cycle‐by‐cycle correlation with measured sections of equivalent age in Umbria, central Italy. The cycle‐by‐cycle match can be demonstrated between filtered spontaneous potential logs and filtered bed thickness data, but is unequivocal only at wavelengths of greater than 10 metres, which are believed to represent a 2 Myr eccentricity signal; at shorter wavelengths tool resolution becomes a limiting factor. As a result of low sedimentation rates the 100, 41, and 21 kyr Milankovitch signals all appear to be aliased to longer wavelengths in the sonic and gamma ray log records and tend to blend with the 400 kyr signal; this is especially true in the Aptian‐Albian interval. Attempts to filter and correlate on the bases of these shorter wavelengths were therefore unsuccessful.Accepting a 2 Myr duration for each cycle permits an estimation of less than 7 Myr to be made for the duration of the Cenomanian Stage.
Primary versus diagenetic origin of Blue Lias rhythms (Dorset, UK): evidence from sulphur geochemistry
Authors: Bottrell Simon , Raiswell Robert 
Published: July 2007

ABSTRACTThe question of a primary versus diagenetic origin for the limestone‐shale rhythms of the Blue Lias has been addressed through a study of pyrite abundance and isotopic composition. Pyrite is relatively abundant and isotopically light in the central portions of the bioturbated limestones as compared to adjacent, less calcareous, sediment. The abundance of pyrite shows that the limestones were a focus for prolonged sulphate reduction and pyrite formation. The isotopic data indicate that bioturbation oxidized some pyrite to produce isotopically light sulphate, part of which was subsequently reduced back to pyrite before preservation by burial. Acidity generated by pyrite oxidation was buffered in the limestones by carbonate dissolution, hence supersaturation of sulphides could be maintained. By contrast, in adjacent less calcareous sediments, carbonate dissolution was unable to buffer acidity and bioturbational oxidation of pyrite formed iron‐rich pore solutions. Continued sulphate reduction in the limestones acted as a sink for iron from the adjacent sediments and, with burial below the zone of bioturbation, the alkalinity so generated caused cementation of the limestones. Diagenetic cementation would be enhanced during an hiatus in sedimentation, an event which might be related to a Milankovitch forcing mechanism, but which would not be recorded in bioturbated, less calcareous sediment, thus leaving an imperfect record. Only cyclicity in pre‐diagenetic sedimentation patterns may be safely related to a Milankovitch forcing mechanism as proposed by Weedon.
Cyclic sedimentation patterns in Lower Lias mudstones of Yorkshire (GB)
Authors: van Buchem F.S.P. , McCave I.N. 
Published: July 2007

ABSTRACTDistinct stratification patterns in Lower Liassic mudstones are due to the regular occurrence of shell beds, silty/sandy layers and concretionary horizons. The mudstones formed in four depositional environments: (1) a storm dominated shallow marine system; (2) a hemipelagic system; (3) a shallow marine system influenced by climatic changes at astronomical frequencies; and (4) a pro‐deltaic offshore transition/shallow marine system. The tectonic setting played an important role in determining the facies sequence, whereas eustatic sea‐level changes were of lesser influence.
Milankovitch type cycles in the Lower Carboniferous of NW Ireland
Authors: Schwarzacher W. 
Published: July 2007

ABSTRACTA short description of the Lower Carboniferous near Sligo (NW Ireland) is given. Limestones and mark in four measured sections in the Glencar Limestone were numerically coded as plus and minus one, respectively. Such sequences show good and persistent cyclicity with a basic wave length of 200 cm. Filtered sections have been adjusted to give each cycle a constant length. Such records show a much longer cycle in the form of amplitude modulation of the basic cycle.The 200 cm cycle is very regular and its approximate duration is around 100 kyr. It could, therefore be the result of eccentricity variations in the earth's orbit. The longer cycle represents approximately 1–1.5 Myr and may be equivalent to the Yoredale cycles in the northern England.
The project ‘Marine‐environmental development of Danish Waters’ at the Geological Survey of Denmark
Authors: Konradi Peter B. 
Published: July 2007

ABSTRACTResearch in marine geology at the Geologcal Survey is primarily related to environmental problems. At present several projects are concerned with sedimentation in the inner Danish waters during the Holocene. A primary objective is a subdivision of the sea‐floor into areas of erosion, reworking and accumulation. In the accumulation area the exact sedimentation processes and sedimentary environments are elucidated. Here, also, the organo‐chemical processes are investigated, especially with regard to the biological transformations and diagenesis of par‐ticulate organic matter, by utilizing well‐known techniques from industry.The aim is to explain the natural development of the environment at the sea‐floor and to get a better understanding of the elements of the oxygen consumption at the waterisediment interface. A primary objective is to produce a new measuring‐technique for organic matter diagenesis in the sea bed that will provide fast and reproducible results in the monitoring of the environmental parameters of the sea‐floor. Special attention will be given to the various potentials of different types of organic matter in relation to oxygen consumption during bacterial decomposition in the sediment.
On the quantitative interpretation of magnetic anomalies by pseudo‐gravimetric integration.
Authors: Fedi Maurizio 
Published: July 2007

ABSTRACTPoisson's theorem relates linearly the derivative of the gravity potential (taken along the total magnetization vector direction) and the magnetic potential due to a common, isolated source with constant density and magnetization distributions.From that theorem, two very useful functional transformations for magnetic anomalies were formulated by Baranov, that is the ‘pseudo‐gravimetric integration’ and the ‘reduction to the pole'.In this paper, it is shown that the first transformation is, in some sense, more useful than the reduction to the pole and, further, that its utilization presents noticeable advantages as regards the interpretation of magnetic anomalies. In fact, the pseudo‐gravimetric integration, in contrast to the reduction to the pole, also provides a smoothing of the anomaly map and, in particular, reduces the degree of non‐linearity existing in the mathematical relationship between the data and the source depth.In order to show this last property of the transformation, spectral expansion inversions by 21/2D structures were performed on both magnetic and pseudo‐gravimetric synthetic anomalies and the respective speed and precision were compared. The tests clearly indicate that very accurate estimates of the geometric parameters of the known source can be more easily and rapidly calculated by the transformed anomalies. Furthermore, due to its smoothing property, the method seems particularly useful when data are noisy. Tests on real anomalies confirm these results.
Do carbonate reefs form due to fluid seepage?
Authors: Hovland Martin 
Published: July 2007

ABSTRACTBuried carbonate reefs are favoured hydrocarbon prospecting targets, mainly due to their high porosity and potential for containing large quantities of petroleum. The question of the true relationship between reef structure and the internally trapped fluids (hydrocarbons) is here raised as one of cause ‐ and effect. In other words, which came first, the hydrocarbons or the carbonate reef itself?Modern bioherms and seabed carbonate reefs in, amongst other locations, the North Sea and the Gulf of Mexico, are shown to form in close association with active hydrocarbon seepages. Mainly based on results from ecological studies at deep‐ocean vent communities, a new model for carbonate reef formation is promoted: that such reefs form at locations containing high concentrations of bacteria and other microorganisms suspended in the water column as a result of seeping fluids (solutions and gases) that provide some of the energy basis and carbon source for ecosystems independently of photosynthesis. Therefore, on burial and effective sealing (‘capping’), these carbonate reefs become hydrocarbon reservoirs, trapping and accumulating the very minerals on which they ‐ in the first place ‐ were dependent.
Gold‐bearing listwaenites in the Araç Massif, Kastamonu, Turkey
Authors: Aydal Doǧan 
Published: July 2007

ABSTRACTThe mineralogy and chemistry of gold‐bearing listwaenites of Gemiköy in the Araç massif (southern part of the Daday‐Devrekâni massif) were investigated in detail. The process of listwaenitization and the reasons for gold enrichment in the area are discussed.The listwaenites have no direct contact with any magmatic intrusion or extrusion. Detailed chemical analysis showed that the listwaenitized rocks contain silica‐, silica and carbonate‐, and carbonate‐rich parts. Each part is different chemically from the others, even in terms of accessory mineral content. In general, the listwaenites differed chemically and mineralogically from the surrounding ultramafic rocks. In particular, SiO2 and CaO contents of listwaenites were higher and MgO and Al2O3 contents were lower than in the surrounding ultramafic rocks. Furthermore, Ni, As, Mn, Zn, Pb and Co enrichment occurred during listwaenitization, while K was absent and Ni enrichment was detected especially in silica and carbonate‐, and carbonate‐rich parts.Finally, Gemiköy listwaenites are interpreted as autometamorphic, even though they show similarities to allometamorphic listwaenites according to Ploshko's (1963) classification.
Hydrothermal mobility of Ti, Zr and REE: examples from the Bergell and Adamello contact aureoles (Italy)
Authors: Gieré Reto 
Published: July 2007

ABSTRACTContact metamorphic marbles, affected by metasomatic fluids at the contact to the Bergell and Adamello Intrusives, contain various accessory minerals (zirconolite, allanite, titanite, rutile, geikielite, hoegbomite), which provide new evidence for the hydrothermal mobility of Ti and Zr. In both examples, Ti and Zr migrated along with U, Th, Y and REE in a metasomatic fluid rich in potassium. The composition of the main minerals (fluorine‐rich phlogopite, pargasite and titanian clinohumite) and the abundance of fluor‐apatite demonstrate that fluorine and phosphorus were important components of the fluid. The textural relationships indicate that the formation of the accessory phases is linked to the crystallization of the hydrous minerals.
Upper Devonian tetrapod palaeoecology in the light of new discoveries in East Greenland
Authors: Bendix‐Almgreen Svend Erik , Clack Jennifer Alice , Olsen Henrik 
Published: July 2007

ABSTRACTFor the first time, new material of one of the world's oldest known tetrapods,Acanthostega, has been studied in its sedimentological context. The reported evidence suggests thatAcanthostegawas primarily aquatic and inhabited active fluvial channels in fluvial‐dominated environments.
Growth of ferruginous oncoliths in the Bajocian (Middle Jurassic) of Europe
Authors: Palmer Timothy J. , Wilson Mark A. 
Published: July 2007

ABSTRACTLimonitic oncoliths from the Bajocian of Europe, widely known as ‘snuff‐boxes’, grew by accretion of bundles of laminae on one side, followed by overturning and accretion of laminae on the other side. A diverse hard‐substrate encrusting fauna grew interleaved with the ferruginous laminations. The palaeoecological preferences of this fauna indicate that its growth, and hence the accumulation of the laminations, took place on the gloomy undersides of the oncoliths. This upsidedown growth is supported by characteristics of the lamination bundles themselves, and by the predominant present‐day orientations of the snuff‐boxes. The abundance of the associated fauna and the lack of carbonate‐dissolving macroborers cutting the laminae suggest an original ferric composition. Precipitation was probably under the control of non‐photosynthetic microorganisms that inhabited the microenvironments represented by the oncoliths' lower surfaces.
The gold mineralization of Baccu Locci (Sardinia, Italy): origin, evolution and concentration processes
Authors: Bakos F. , Carcangiut G. , Faddat S. , Mazzella A. , Valera R. 
Published: July 2007

ABSTRACTAs‐Fe‐Cu‐Pb‐Zn‐Ag‐Sb sulphides and sulphosalts, with native gold as a frequent accessory (locally concentrated with high values) are the ore minerals of a deposit occurring in a Silurian volcanosedimentary sequence, either as concordant bedded orebodies, or as discordant veins and split fillings at the contact between shales and lamprophyres of the late Hercynian magmatism. Scheelite, mainly bound to acid metavolcanics, is occasionally scattered as isolated grains in the sulphides. The local geological setting together with correlations with other deposits occurring along the same regionally distributed Lower Paleozoic sequence, permit the proposal of the following scheme for the origin and evolution of the Baccu Locci Au‐bearing mineral deposit:(1) Base‐metal sulphide deposition, with gold, in the Silurian volcanosedimentary sequence: protore.(2) Hercynian tectonometamorphic activity, accompanied by multiphase ore mobilization and concentration processes, stratabound.(3) Hercynian magmatism, locally represented by lamprophyre dikes and by the effects of multiphase hydrothermal activity: ore mobilization and redeposition.