bluuebunny/crossref_metadata_2025_split huggingface dataset:
Transcurrent ductile shear zones and the dispersal of the Avalon superterrane
Authors: Gibbons Wes 
Published: April 2008

Abstract
A mosaic of steep, often sinistral, transcurrent shear zones cuts late Precambrian to earliest Cambrian rocks in eastern Canada, southern Britain, and northwest France. Examples include mylonite belts in Saint John (New Brunswick), Cobequid Highlands (Nova Scotia), Burin Peninsula (Newfoundland), Malverns (England), and N Brittany (France). These shear zones often separate geologically distinct areas, each broadly of Avalonian or Cadomian character, that together are grouped into an Avalon superterrane. The shear zones occur as: (1) major fault systems juxtaposing unrelated geological units, e.g. the Dover-Rosslare-Menai Strait mylonite belt, a well defined transcurrent terrane boundary that locally contains exotic slivers of blueschist; (2) shear zones separating high grade gneisses from low grade metasediments which may represent tectonized basement/cover sequences (e.g. Brookville Gneiss/Green Head Group of New Brunswick; Bass River Complex/Gamble Brook Formation of Nova Scotia; Rosslare Complex/Cullenstown Formation in SE Ireland); (3) shear zones (locally with syntectonic granites) separating migmatitic gneisses from low grade rocks of similar protolith, e.g. the Cancale shear zone affecting the Brioverian of northern Brittany; such boundaries may form due to transtensional shearing within an arc system above an obliquely subducting oceanic plate; (4) shear zones separating rocks of similar character so that no case exists for defining the ductile fault as a suspect terrane boundary (e.g. Malverns); (5) long-active steep fault systems, the earliest movements along which have been obscured by Phanerozoic reactivation (e.g. Welsh Borderlands Fault System). Constraints on the timing of the ductile fault movements remain poor, although in Britain and France an early Cambrian age is implied. The transcurrent faulting dismembered and dispersed a long-active late Precambrian arc system and provided a structural template that localized subsequent fault movements and so exerted a major controlling influence on the Phanerozoic geology of the region. Use of the term ‘Avalonian/Cadomian Orogeny’ refers to subduction and terrane dispersal events during the early history of the SE side of the Caledonian/Appalachian Orogen.
Phosphorite research: a historical overview
Authors: Cook P. J. , Shergold J. H. , Burnett W. C. , Riggs S. R. 
Published: April 2008

Abstract
The nature and origin of phosphorites have been a matter of much speculation since were they first discovered more than 150 years ago. They are also of wide scientific interest in that they provide evidence of past changes in the biology and chemistry of the world’s oceans.
In the past ten years or so, due to a considerable extent to the efforts of the many participants in IGCP Project 156, our knowledge and understanding of phosphorites has increased enormously: it is now evident that there were times in earth history when phosphorites were preferentially deposited in many parts of the world. This probably was the consequence of the interaction of a number of factors including oceanography, changes of sea-level, and plate tectonics. Perhaps one of the most intriguing features to have become apparent in recent years is the link between times of phosphogenesis and periods of major evolutionary change. Not only has our knowledge of ancient deposits increased but much information now exists on present-day deposits, such as those off Chile—Peru and the more enigmatic deposits off southern Australia. In addition, there are better data and a greater understanding of some of the details of phosphorite formation and phosphate hardgrounds, on the role of bacteria in phosphogenesis, and on the petrology, mineralogy, geochemistry and biology of phosphorites.
Whilst a great deal has been achieved in the past 10 years, there still remains much to be done: there is scope for grass-root phosphate exploration in countries presently lacking their own sources of phosphate; the nature of the relationship between phosphate and some diagenetic minerals such as zeolites has yet to be elucidated; similarly the link between phosphogenesis and the formation of petroleum source rocks needs to be better understood and much more work is required on Precambrian phosphorites.
Mineralogy of sedimentary apatites
Authors: McClellan G. H. , Van Kauwenbergh S. J. 
Published: April 2008

Abstract

Members of the apatite family are the most common phosphate minerals. Apatites in igneous and metamorphic rocks generally approach fluorapatite in composition. In sedimentary rocks, the apatite is usually the carbonate fluorapatite variety, francolite, which produces x-ray diffraction patterns similar to but distinctive from those of fluorapatite. Variations in francolite compositions and physical properties have also been identified. The substitution of CO
3
−2
for PO
4
−3
, from zero to about 25% of phosphate sites on a 1:1 basis, influences all other changes in composition and crystallography of francolites. Statistical models have reduced the francolite composition to a single parameter problem, the dominant factor being the variation in unit-cell
a
-value with increasing CO
3
−2
substitution. Within a deposit, variations in francolite composition with stratigraphical position and particle size may indicate the extent and type of post-depositional alteration. Replacements of SO
4
−2
for PO
4
−3
and Ca
+2
by Na
+2
and Mg
+2
are also discussed but these are of more limited significance.
Experimental study of the substitution of Ca by Sr and Ba in synthetic apatites
Authors: Lucas Jacques , El Faleh El Mâti , Prevot Liliane 
Published: April 2008

Abstract

Experiments into apatite synthesis through bacterial mediation were carried out using the now well-established methods of either replacement of a carbonate precursor, or direct precipitation from solution. In one set of experiments, strontium was added in varying amounts, either to the solution, or through the carbonate precursor. In another set of experiments, barium was added. The final products were analysed by X-ray diffraction and chemical methods. It is evident that both Ba and Sr can enter the lattice of the synthetized apatite. Strontium is able to replace calcium in any proportion and there is a continuous solid-solution series between purely calcitic and purely strontic end-members. Conversely, barium can replace calcium in only small amounts (less than 10% by weight), and its introduction in the lattice soon decreases the crystallinity of the apatite. The barium phosphate which forms does not belong to the apatite group, but approximates to a phosphate having the formula BaHPO
4
. This difference in behaviour between Sr and Ba results from their different ionic radii and may help constrain the genesis of natural phosphorites.
Humic substances in phosphorites: occurrence, characterization and significance
Authors: Nathan Y. 
Published: April 2008

Abstract
Humic substances form an appreciable part of the organic matter in many phosphorites of various ages, including some deposited as early as Cambrian times. In some phosphorites the humic substances are primary, having formed during sedimentation or early diagenesis, and seem to have been preserved as such. In others they appear to be secondary and were probably formed by oxidation of kerogen. While the presence of humic substances in phosphorites (or any sediment) does not necessarily indicate thermal immaturity, their characterization (aromaticity, size of particles, etc.) provides valuable information about their environment of deposition (oxic, anoxic) and their geological history (extent of diagenesis, burial depth, thermal history).
The results of the present work show clearly that the humic substances in the Permian phosphorites of the Western US Phosphate Field were derived from late degradation of kerogen, while those in the Eocene phosphorites from Tunisia are primary. The origin of humic substances in other fields is less clear-cut and it is possible that in some instances, for example, in the Late Cretaceous phosphorites from the Negev, Israel, they are polygenetic.
Condensation and phosphogenesis: example of the Helvetic mid-Cretaceous (northern Tethyan margin)
Authors: Föllmi Karl B. 
Published: April 2008

Abstract

The Aptian to Lower Cenomanian condensed phosphatic beds, deposited in the Helvetic Shelf along the northern Tethyan margin, consist of thin strata (generally <50 cm) of densely packed phosphatized particles and crudely laminated crusts embedded in glauconitic sands, marls, and pelagic micrites. The beds record very low net sediment-accumulation rates (typically 2–20 cm Ma
−1
). The presences of complex internal and laterally rapidly changing microstratigraphies, multiple phosphate generations, intimate mixtures of fossils from different biostratigraphical zones and ecological habitats, a proximal transition into bioturbated glauconitic sands, and a distal transition into allochthonous sediments deposited in channel and fan systems, indicate that the condensed phosphatic beds formed along the axis of a stable westward-flowing current system contouring the northern Tethyan margin. Repetitive cycles of deposition, phosphogenesis, re-exposure and reworking, and deposition (Baturin Cycles), caused by current-induced lateral migration of sand bodies, shaped the internal stratification and composition of the condensed phosphatic beds. Catastrophic burial of entire benthic communities was crucial to phosphogenesis. The sudden presence of large amounts of organic matter buried in siliciclasts created a reactive environment favourable for the concentration and precipitation of phosphates, probably with the help of physicochemical cycles of the Fe
3+
-Fe
2+
and Mn
4+
-Mn
2+
redox pairs.
Events leading to global phosphogenesis around the Proterozoic/Cambrian boundary
Authors: Donnelly T. H. , Shergold J. H. , Southgate P. N. , Barnes C. J. 
Published: April 2008

Abstract

In the Late Proterozoic the world’s oceans changed from being relatively oxic and well-mixed, into to a less mixed and more stagnant system. This resulted in the accumulation of massive amounts of organic matter and pyrite in anoxic sediments on the sea floor, and the enrichment of P in the anoxic deep ocean waters. Except for relatively short periods of increased turnover and better ventilation during times of Late Proterozoic glaciation, the stagnant ocean system appeared to be the ‘normal’ condition. A Cambrian seawater
87
Sr/
86
Sr curve is presented, and it is concluded that continental rather than magmatic inputs were the major influence on the Sr isotopic composition of the latest Proterozoic and Cambrian oceans. Significant rises in atmospheric O
2
levels must have accompanied the periods of greatly enhanced organic matter burial. A return to a stable more oxic ocean system occurred around the Proterozoic/Cambrian boundary, at which time rifting of the supercontinent(s) created a large number of epicontinental seas at low latitudes, enabling deep P-rich ocean waters to be moved into shallow-water environments. While increasing
p
O
2
levels during the latest Proterozoic may have been largely responsible for the Cambrian ‘radiation’ event, the increase in
p
CO
2
levels at the beginning of the Cambrian, as the oceans became more oxic, may have been responsible for the acquisition of mineralized skeletal structures by soft-bodied organisms.
Rafting in glacimarine environments
Authors: Gilbert Robert 
Published: April 2008

Abstract
Sources of rafted sediment include glacial, colluvial, fluvial, aeolian and littoral settings. Processes of incorporation and rafting include active incorporation by freezing and passive loading of icebergs and sea ice, and rafting by vegetation, principally marine algae. Passively loaded sediments, being loose on the ice surface, are more easily lost from the raft and are more likely to form deposits of more than one particle at a time. The effectiveness of passive loading depends on sea ice being in contact with the shore in the spring, when colluvial and fluvial processes are most active, and during winter when aeolian processes occur. Actively incorporated sediment becomes concentrated on the surface of the raft by melting. Other factors that control distribution of rafted sediments include the strength and stability of the raft, its rate of deterioration, its ability to leave the source area, and its movement at sea.
Sediment released from the ice is deposited as single particles (referred to as a ‘drop’), agglomerations of more than one particle (‘dump’), frozen aggregates, and deposits from the melting of sediment-laden ice in contact with the sea floor. Characteristics that have been used to distinguish the mode of rafting include grain size, sorting, shape, surface features (especially striations), fabric, fauna associated with the environment of origin, distance of transport, and water depth. The most reliable in assessing origin from an iceberg is probably the presence of striations, although none is conclusive, and interpretations must be made cautiously.
Development of slope valleys in the glacimarine environment of a complex subduction zone, Northern Gulf of Alaska
Authors: Carlson Paul R. , Bruns Terry R. , Fisher Michael A. 
Published: April 2008

Abstract
Morphological, seismic-reflection, and sedimentological evidence indicates that glacial ice tongues cut large sea valleys into the Gulf of Alaska continental shelf during the Pleistocene. During the Holocene, glacially-derived sediments from the Copper River and other meltwater streams have been prograding seaward across the shelf, covering the glacial and glacimarine upper Yakataga diamicts that blanketed the shelf and accumulated on the upper slope seaward of the sea valleys during the Pleistocene. GLORIA imagery near Middleton Island provides a new perspective on the glacimarine depositional environment on the continental slope in a collision zone between the Pacific and North American plates. Southwest of Middleton Island, along the subduction margin, sinuous valleys funnel sediment around shelf-edge-parallel, subduction-created, anticlinal ridges that have deflected and locally trapped glacimarine sediment. The slope south and southeast of Middleton Island where oblique convergence occurs, is incised by dendritic, erosional gulley systems, contains no compressional ridges, and thus, the apparently active sediment pathways to the trench are unrestricted. However, below the sea valley mouths, apparently both glacial and glacimarine sediments blanket the upper slope, covering any dendritic gulley systems that may have formed during or since the Pleistocene low stands of sea level.
The flux and preservation of organic carbon in Baffin Island fjords
Authors: Syvitski James P. M. , LeBlanc K. William G. , Cranston R. E. 
Published: April 2008

Abstract

Except for shell hash lags found along minor portions of fjord walls and on current swept sills, organic carbon dominates over inorganic carbon, accounting for between 95 and 1000f the total carbon composition within the fjords. The deposition of terrestrial organic carbon is of minor significance except proximal to river mouths; i.e. the atomic carbon/nitrogen ratio, both within the water column and within the fjord sediments, fluctuates around 6.4. Organic carbon is an important constituent of the suspended particulate matter throughout most of the year when the suspended sediment concentration is < 2 g m
−3
. During the summer meltwater season, the organic fraction of the suspended sediment is overwhelmed by inorganic mineral matter. In the autumn season, the organic carbon content of the suspended particulate matter can exceed 30%, especially in the seasonal layer where productivity is highest. Bacterial biomass accounts for more than 500f the total biomass found at the sediment-water interface.

Microbial respiration of organic carbon is highly effective, consuming > 900f the annual deposition of carbon to the sea floor. The remaining and mostly refractory carbon preserved within the sediment column is < 20y weight of the sediment and, in high sedimentation environments, such as proximal to tidewater glaciers, organic carbon is < 0.1%. The organic carbon content of sea floor samples is found inversely proportional to the modern suspended load delivered to these fjords. Anoxic degradation of this buried carbon appears to be a slow and ineffective process over the time period of 10 000 years. The abundance of organic carbon buried within Arctic fjord sediments is therefore considered as an inverse indicator of deglacial and post-glacial sedimentation rates. A consistent organic carbon stratigraphy relating to the deglacial and Hypsithermal period, the Neoglacial period culminating in the Little Ice Age, and the present-day warm conditions has been identified in all ten Baffin Island fjords investigated. A simple technique is presented to convert the content of organic carbon observed down core to relative or absolute sedimentation rates, depending on the availability of
14
C dates. Instantaneous deposition of sediment from slides or gravity flows remains problematic.
Carbonate minerals in glacial sediments: geochemical clues to palaeoenvironment
Authors: Fairchild Ian J. , Spiro Baruch 
Published: April 2008

Abstract
Various subglacial, proglacial, periglacial and glacimarine parageneses of carbonates are known, but have not been systematically compared. Precipitation-triggering mechanisms include subglacial regelation, lacustrine photosynthesis and evaporation, freezing in permafrost, skeletal biomineralization, and bacterial oxidation of organic matter in organic-rich sediments. New data from late Proterozoic carbonates suggest another, potentially important mechanism: recrystallization of glacially transported (and hence chemically unstable) carbonate.
Unlike the bulk of warm-water carbonate sediments, most glacial carbonates have been precipitated as stable phases which therefore preserve a good memory of environmental parameters. Carbon stable isotopic data have been used to constrain precipitation mechanisms whilst oxygen isotopes together with minor element chemistry can characterize salinity and distinguish the effects of evaporative from freezing processes. Elevated Fe and Mn contents point to anoxic precipitational conditions. Petrographic data on crystal size, shape and internal zonation are vital to avoid misinterpretations of diagenetically-altered material.
Carbonate geochemistry has proved valuable in interpretation of Quaternary and older palaeoenvironments, and could even yield information to aid Proterozoic palaeogeographic reconstructions.
Nordic Seas surface ice drift reconstructions: evidence from ice rafted coal fragments during oxygen isotope stage 6
Authors: Bischof Jens , Koch Joachim , Kubisch Michaela , Spielhagen Robert F. , Thiede Jörn 
Published: April 2008

Abstract
Sixteen long sediment cores from the eastern Arctic Ocean, the Fram Strait and the Norwegian-Greenland Sea, documenting 200 000 years of sedimentation, were studied for their qualitative dropstone composition (>500 µm-fraction). In sediments from oxygen isotope stages 1–5, coal particles are usually subordinate components of the coarse fraction. In contrast to younger deposits, coal content in oxygen isotope stage 6 (186-128 ka) varies between 20% and 65 0n the eastern Arctic Ocean and the Fram Strait and between 5% and 20 0n the Norwegian Sea. Southward decreasing coal content and similarities in maturity and petrography of the coals indicate that the coal was transported by iceberg or sea ice rafting more than 1000 km to the south. It is suggested that during intervals of oxygen-isotope stage 6 drifting ice carried abundant coal fragments from the eastern Arctic Ocean southward through the Fram Strait into the eastern Norwegian Sea. Thus, surface circulation was then opposite to that of today.
Early Holocene morainal bank sedimentology and marine ecology, Skjoldungebrae gorge, North Scoresby Land, East Greenland
Authors: Huddart D. , Peacock J. D. 
Published: April 2008

Abstract
Skjoldungebrae gorge sections have allowed a reconstruction of changing sedimentological and palaeoecological environments during the early Holocene in the Kap Petersen area of North Scoresby Land. There is evidence for a former morainal bank and associated glacimarine environments at the grounding line of the early Holocene tidewater Skjoldungebrae. The sequence has been subdivided into: bergstone/suspension, laminite, proximal morainal bank avalanche, distal submarine fan and proximal submarine fan glacimarine facies. There are three molluscan and three foraminiferal assemblages present which indicate likely palaeotemperatures, salinities and water depths. Evidence suggests that sea level fell from 105 m to 77.5 m and as it did so modified the tidewater glacimarine sequence. A readvance of Kong Oscar Fjord ice reworked part of the glacimarine sequence and deposited a basal till over the lower slopes of the Kap Petersen area and in lower Skeldal. Criteria used to distinguish between till and various diamictons are discussed.
Proximal and distal glacimarine deposits in southwestern Sweden: contrasts in sedimentation
Authors: Stevens Rodney L. 
Published: April 2008

Abstract
The balance between the meltwater supply of suspended sediment from an increasingly distal ice front and the local sources of clastic sediments through turbulent reworking is reflected in the stratigraphy of Weichselian glacimarine deposits near Göteborg. Descriptions from cored and exposed sections and process interpretations are reviewed in order to make generalized comparisons between the proximal to distal trends and the contrasting lithofacies in very distal settings. Since nearly all sediment was initially supplied by the meltwater overflow the distance to ice-front standstills, the seasonal or non-seasonal variability of the discharge and the local drainage were major influences on the textural and geochemical characteristics. This is best illustrated by the rhythmites interpreted to be annual varves. In very distal settings the fine sediments that the meltwater could supply are interrupted in the stratigraphy by silty and sand laminated clays interpreted to be the result of the increased importance of near-shore erosion. Granular sediments could be transported from local sources and concentrated in the stratigraphy during long periods of low meltwater discharge, such as with glacial stadial conditions. During high-discharge phases the fine-grained sedimentation predominated at localities along regional drainage paths for the meltwater and the coarser input from local sources was diluted.
The evolution of cataclastic fault rocks from a pre-existing mylonite
Authors: Hippler Susan J. , Knipe Robert J. 
Published: April 2008

Abstract
A microstructural investigation of cataclastic fault rock evolution from a quartzite with an original mylonitic microstructure is reported. The fault rocks produced range from clast dominated microbreccias to matrix dominated ultracataclasites. The recrystallized grain size and the sub-grain size in the original mylonite appear to control the development of the fine-grained matrix in the microbreccias and cataclasites by focusing fracture along sub-grain and grain boundaries. The ultracataclasite generation involves further grain size reduction which is dominated by transgranular fracturing. The host rock clasts present in the fault zones show a significant increase in dislocation density indicating that a component of low temperature crystal plasticity is associated with the faulting. In addition the fault rocks show evidence of partial cementation by the growth of quartz and carbonate cements. The evolution of the fault rocks studied in terms of the clast size and the clast/fine-grained matrix ratios are not a simple function of the displacement magnitude.