bluuebunny/crossref_metadata_2025 huggingface dataset:
Lena-Tunguska Upper Proterozoic-Palaeozoic petroleum superprovince
Authors: Kontorovich A. E. , Mandel’Baum M. M. , Surkov V. S. , Trofimuk A. A. , Zolotov A. N.
Published: April 2008

The Lena-Tunguska Superprovince is the only region of the world where extensive economic Upper Proterozoic petroleum production is proved. This province extends the stratigraphical range of hydrocarbon accumulations to the deposits older than one billion years.

The Upper Proterozoic deposits are represented mainly by Riphean carbonate formations and terrigene carbonate sulphate-bearing sediments of Vendian (Eocambrian). These deposits cover an area of some two million square kilometres.

Oil and gas potential in the Vendian and Upper Cambrian sediments in the central and southern regions of the province is proven and more than thirty fields have been discovered. Sheet, roof, lithologically and tectonically screened and non-anticlinal pools dominate these discoveries. The Lower Cambrian salts are the regional seal.

Riphean petroleum potential has been proved in the western part of the province where the extensive Yurubcheno-Takhomsk zone of oil and gas accumulations occur within the Baikit antecline and Katanga Trough. Oils appear to be of one genetical type as low sulphur, naphthenic-rich oils containing
n
-alkanes, isoprenoids and are enriched in 12,13-monomethylalkanes,
12
C carbon isotope.

The main current oil-producing plays are confined to Riphean Formations.
Sequence stratigraphy and the habitat of hydrocarbons, Gippsland Basin, Australia
Authors: Rahmanian V. D. , Moore P. S. , Mudge W. J. , Spring D. E.
Published: April 2008

The Gippsland Basin is Australia’s most prolific oil and gas province, with initial reserves estimated at 3.6 billion barrels crude/condensate, 0.6 billion barrels liquid petroleum gas (LPG) and 8.3 TCF dry gas. It is located in southeastern Australia, mainly offshore. Significant hydrocarbons were first discovered in 1964, in large structures at the top of the Late Cretaceous-Eocene Latrobe Group. All of the large structures have now been drilled, with current exploration concentrating on fault-dependent and combined stratigraphic structural traps within the Latrobe Group. Hydrocarbon, source rock and maturation data suggest that there is a widespread and uniform non-marine source of Late Cretaceous age. The concentration of hydrocarbons at the top of the Latrobe Group is due to the presence of large structures, an excellent cap seal, and the relatively sandy nature of the underlying, intra-Latrobe section. The distribution of oil versus gas at this level is largely explained by maturation variations and migration pathways within the basin. Vertical migration of up to 2 km is required in most cases. The distribution of oil versus gas is also influenced by minor source rock variations, fresh water washing, biodegradation, and the variable quality of fault seal. Recent exploration studies have concentrated on locating stratigraphic and combination traps, especially within the Latrobe Group. Prediction and subsequent discovery of these traps is a challenging task that has been aided by sequence-stratigraphic and seismic-stratigraphic analysis of the rock record. This has allowed the prediction of reservoir, source and seal geometries and distributions. The techniques used to construct this chronostratigraphic framework are presented, together with examples of its application within fields and on a regional basis.
U-Pb geochronology of the Upper Proterozoic Cadomian orogeny in the northern Armorican Massif, France
Authors: Guerrot C. , Peucat J. J.
Published: April 2008

U-Pb zircon dating has clarified the timing of magmatic and metamorphic events during the Cadomian orogeny from 670–660 Ma to 530 Ma. This indicates a time-span of nearly 150 Ma for the Cadomian orogeny in the northern part of the Armorican Massif.

The ages of
c.
667 Ma and 656 Ma obtained for granodiorite boulders from a Brioverian conglomerate are the oldest known dates which may relate to the Cadomian orogeny, and imply an early cycle of uplift, erosion and sedimentation before major plutonism around 590 Ma.

Two main phases of evolution are recognized. The first one, probably related to an active margin, is characterized by the emplacement of granodiorites at 670–660 Ma, 645 Ma and 615 Ma ago followed by basic magmatism at 600 Ma in a back-arc environment. Some of these rocks underwent metamorphism or cooling a short time after emplacement, thus indicating an active tectonic setting with rapid uplift of the crust and some crustal thickening. This phase of evolution ended with diorite emplacement around 590–580 Ma ago. The second phase corresponds to an important crustal melting event recorded by migmatitic gneisses and anatectic magmas formed 540 Ma ago.

The ages obtained lead us to propose a new chronostratigraphy for the Upper Proterozoic Brioverian Supergroup with a Lower Brioverian Group deposited between 660 and 590–580 Ma in the Baie de Saint-Brieuc and before 585 Ma in Normandy. The Upper Brioverian Group in Normandy was formed between 585 and 540 Ma. There is no evidence for the existence of Pentevrian or Icartian basement in the Baie de Saint-Brieuc.
The geology of the Penthièvre crystalline massif: a reappraisal of the type-Pentevrian area, northern Brittany
Authors: Shufflebotham M. M.
Published: April 2008

Field evidence suggests a complex history for the Penthièvre crystalline massif (PCM) prior to the deposition of the local Brioverian supracrustal sequences, the Erquy Volcanic Formation and the Hillion Volcanic Formation. The PCM comprises three assemblages of igneous derivation: (a) the Yffiniac Group, consisting of amphibolites and banded orthogneisses which were deformed and metamorphosed to amphibolite facies during D
1
and D
2
; (b) the Port Morvan Group, comprising Grey and Green Gneisses of trondhjemitic to quartz dioritic composition, the precursors of which intruded the Yffiniac Group and were deformed and metamorphosed during D
3
and D
4
; and (c) the Jospinet Group of granodiorites, the St Maurice quartz diorite and, probably, the Fort de la Latte Complex (the latter two were emplaced during D
5a
). A shearing event (D
5b
) produced cataclastic fabrics in the basement, and the Hillion—Cesson Mylonite Belt, prior to the deposition of the Brioverian in the area.

The Erquy and Hillion Volcanic Formations were deposited in an ensialic basin upon a basement complex represented by the PCM. Island-arc and active continental margin magmatism occurred from before 670 Ma to as late as the Cambrian. This magmatism can be viewed as representing the effects of the Cadomian orogeny, during the early stages of which the local Brioverian sequences were deposited.
Brioverian volcanism and Cadomian tectonics, Baie de St Brieuc, Brittany: stages in the evolution of a late Precambrian ensialic basin
Authors: Roach R. A. , Lees G. J. , Shufflebotham M. M.
Published: April 2008

During late Precambrian times, the Northern Brittany and Lower Normandy region was the site of extensive submarine basaltic volcanism. On the southern side of the Baie de St Brieuc, two volcanic formations have been identified. In the east the Erquy Volcanic Formation (EVF), over 2 km thick, is a sedimentary-volcanic sequence in which the basalts occur mainly as pillowed, massive or composite massive-pillowed flows, interdigitated with intrusive sheets. Whole- and broken-pillow breccias, hyaloclastites, and peperites are locally common. Further west, the Hillion Volcanic Formation (HVF) has a similar thickness and attitude, and exhibits the same range of lithologies as the EVF, though in a more deformed state and metamorphosed to a higher grade. While assigned by some workers to the Ordovician, several lines of evidence are presented which show that the basic volcanics of the EVF form part of the same late Precambrian volcanic suite as the HVF.

The suite was erupted in a marginal basin (the Lanvollon basin) environment founded upon a slightly older crust of calc-alkaline plutonic rocks, now represented in part by the adjacent Penthièvre massif. Basic sheets (the Dahouët dykes), cutting this basement, are considered co-genetic with the volcanics of the EVF and HVF. Chemically, the metabasic lavas, sub-volcanic sills and dyke rocks are subalkaline, tholeiitic basalts and basaltic andesites. The Lanvollon basin was subsequently filled with clastic sediments and then subjected to Cadomian deformation and metamorphism, the grade of which increases westwards, varying from prehnite-pumpellyite to lower amphibolite facies.
The Brioverian (Upper Proterozoic) and the Cadomian orogeny in the Armorican Massif
Authors: Rabu D. , Chantraine J. , Chauvel J. J. , Denis E. , Balé P. , Bardy Ph.
Published: April 2008

The Brioverian in Brittany has generally been divided into the volcanogenic Lower Brioverian and the detrital Upper Brioverian, representing respectively the first and second episodes of the Cadomian orogeny, Cadomian I and Cadomian II. This scheme was subsequently incorporated into an active margin model of evolution and related to subduction during closure of the Celtic Ocean, in a geochronological framework defined by ages of 2000 Ma for the basement, 670 Ma for magmatic rocks of the first episode (Cadomian I) and 600 Ma for those of the second (Cadomian II).

Recent work has led to substantial modification of this model. The two parts of the Brioverian are now considered not to have been superposed one upon the other but to be juxtaposed along a suture, and the superimposition of Cadomian structures not to have been due to successive events but to southward migration of deformation from the ocean towards the continent.

In the northern, orogenic domain the volcanogenic Brioverian represents a volcanic arc-back-arc basin system located on a rifted continental margin. At about 600 Ma the closure of this system resulted in the overthrusting of the orogenic belt onto the continental margin, with the development of anatectic magmatism up to 540 Ma.

In the southern, continental domain the detrital Brioverian includes black chert in the lower part (probably contemporaneous with the volcanogenic deposits of the orogenic zone) and reworked black chert in the upper part. In Normandy, the deformation of these rocks was related to the rise of the Mancellian granites about 540 Ma. In central Brittany, the main deformation is Variscan in age, and the age of the Brioverian there is questionable as the lower part of the Palaeozoic cover is of Ordovician age.
Tectonic evolution of the Cadomian belt in north Brittany
Authors: Strachan R. A. , Roach R. A.
Published: April 2008

A discrete geotectonic unit, here termed the St Brieuc Terrane, has been identified within the late Precambrian Cadomian belt of the Armorican Massif. This terrane contains syn-orogenic Brioverian supracrustal rocks which accumulated in a marginal basin and were intruded by subduction-related calc-alkaline plutonic complexes during the period
c.
615–580 Ma. The Brioverian unconformably overlies orthogneisses and granitoids of the Penthièvre complex which formed during an early phase of Cadomian calc-alkaline activity. Regional transpressive deformation resulted in the internal imbrication of the St Brieuc Terrane into five tectonic blocks, mainly bounded by ductile shear zones which have both strike-slip and dip-slip senses of movement. Amalgamation of the St Brieuc Terrane with the St Malo Terrane to the SE occurred at
c.
540 Ma, during regional sinistral transpression. Major subvertical faulting localised along Cadomian shear zones was probably initiated during post-Cadomian extension and continued through Variscan events.
Post-Cadomian erosion, deposition and basin development in the Channel Islands and northern Brittany
Authors: Went David , Andrews Michael
Published: April 2008

Facies analysis, provenance, and palaeocurrent studies of the Rozel Conglomerate Formation (Jersey), Alderney Sandstone Formation and the Erquy—Fréhel Group (northern Brittany), have established patterns of ‘molasse-like’ sedimentation in a series of basins related to a phase of post-Cadomian tectonism.

Although shallow marine sediments form minor constituents of the Alderney Sandstone Formation and Erquy-Fréhel Group, fluviatile sediments deposited by braided streams axially draining eastwards predominate. Palaeocurrents within alluvial fan deposits in Jersey, and similar tributary systems in Alderney and Brittany, indicate drainage orientated either transverse to the basin axes, or at a high angle to the main trunk streams. Gross sediment dispersal was towards the Cambrian shoreline that lay to the east.

These basins and their equivalents in Normandy are elongate parallel to the east-west structural grain of the Cadomian Orogen. The basins opened, and the sediments were shed, in response to a phase of post-Cadomian extension. The geometries of these basins were possibly controlled by the re-activation of Cadomian thrust and strike-slip faults. The area to the west of Normandy, though probably mountainous in aspect, was one that was, for the most part, open to a marine connection. In Normandy itself, Cambrian sediments were deposited sometimes in continental environments, but more commonly in shallow seas.
Sedimentology, structural evolution and tectonic setting of the late Precambrian Longmyndian Supergroup of the Welsh Borderland, UK
Authors: Pauley J. C.
Published: April 2008

The Longmyndian Supergroup is an approximately 6500 m thick sedimentary sequence of late Precambrian age. Major unconformities within this sequence are lacking. The Longmyndian Supergroup can be divided into four facies associations: (1) turbidite, (2) subaqueous delta, (3) alluvial floodplain, and (4) braided alluvial. These are organized into a single, broadly upwards-coarsening, progradational sequence, with the turbidite association at the base and the braided alluvial association towards the top. The sediments are interpreted to have been deposited in a forearc, or possibly interarc, basin which lay to the north of a contemporaneous magmatic arc, of which the (now adjacent) Uriconian Volcanic Complex is considered to have been a part. The Longmyndian Supergroup has been deformed into a tight NNE-SSW trending syncline. Fault patterns and strain markers within the Longmyndian indicate that it was deformed during sinistral transpression. The adjacent Pontesford-Linley and Church Stretton fault systems are considered to be major strike-slip fault systems which were initiated at about the same time as the deformation of the Longmyndian Supergroup. Strike-slip movements along these fault systems resulted in the juxtaposition of the Uriconian Volcanic Complex with the Longmyndian Supergroup. Late Precambrian/early Cambrian deformation is indicated by some stratigraphic and radiometric age-dating evidence. Since there is evidence that the fault systems have suffered strike-slip displacements during the Palaeozoic it is difficult to be certain of the magnitude and style of the late Precambrian/early Cambrian deformation. However, the recognition of late Precambrian/early Cambrian sinistral transpressional deformation in northwest Wales suggests that the Precambrian of the Welsh Borderland was similarly deformed during the late Precambrian/early Cambrian.
Late Precambrian-Early Palaeozoic evolution of part of the Avalon terrane in southern New Brunswick, Canada
Authors: Nance R. Damian
Published: April 2008

An important Proterozoic to early Ordovician record of the Avalon terrane is exposed between the Wheaton Brook Fault and the Bay of Fundy in southern New Brunswick, Canada. Ductilely mobilized tonalitic ?basement of possibly Helikian age (Brookville gneiss) a platform carbonate-quartzite cover (Green Head Group) containing ?mid-Riphaean stromatolites. ?Unconformably overlying carbonate slump breccias and turbidites (Martinon Formation) are interpreted to record the collapse of this platform and may be broadly contemporary with minor mafic magmatism and a mid-Hadrynian (
c.
800 Ma) tectonothermal event apparently recorded in the Brookville gneiss. However, the association may equally reflect the onset of late Hadrynian (
c.
625–550 Ma) subduction recorded in calc-alkaline granitoid plutonism (Golden Grove suite) and associated arc-related volcanism (Coldbrook Group). The broadly coeval? emplacement of bimodal sheeted dykes (Kingston complex) appears to have occurred under conditions of sinistral transtension suggesting subduction was oblique. Mafic dykes are mainly continental tholeiites and the complex is interpreted to represent the floor of a volcanic arc rift. Major ductile shear zones of dextral sense, which border the dyke complex, may record late Precambrian rift closure but were active in the mid-Palaeozoic. Post-tectonic ‘Eocambrian’ volcanogenic redbeds and continental rift volcanics pass disconformably into Cambro-Ordovician sandstones and shales of the Saint John Group which contain shallow-marine Acado-Baltic fauna of the Iapetus cycle and represent an overstep sequence to Avalon’s Precambrian evolution. Termination of subduction and the tectonic transition to Cambro-Ordovician platformal conditions lacks evidence of a major collisional event. Instead, they coincide with the break-up of a late Precambrian supercontinent that was responsible for the inception of the Iapetus cycle and are likely to reflect the major plate reorganizations that would accompany such an event.
The geochemistry and petrology of the Late Precambrian Georgeville Group: a volcanic arc-rift succession in the Avalon terrane of Nova Scotia
Authors: Murphy J. B. , Keppie J. D. , Dostal J. , Hynes A. J.
Published: April 2008

The Antigonish Highlands of northern mainland Nova Scotia lie within the Avalon terrane of the Canadian Appalachians. They are predominantly underlain by late Precambrian interbedded metasedimentary and metavolcanic rocks of the Georgeville Group. In the southern highlands, subaerial volcanic rocks of the Keppoch Formation consist of bimodal, porphyritic and aphyric mafic and felsic flows. Pophyritic mafic flows display volcanic arc calc-alkalic affinities and are extensively fractionated. The aphyric mafic flows are characterized by strong Fe and Ti enrichment and are probably continental tholeiites derived from a heterogeneous metasomatized upper mantle source. Felsic lavas display volcanic arc affinities and were probably derived by anatexis of the crust.

In the northern highlands, mafic flows of the Chisholm Brook Formation are dominated by calc-alkalic basalts and andesites and minor interlayered continental tholeiites. In the central highlands, submarine volcanic rocks of the Clydesdale Formation consist of continental tholeiites and are broadly comparable with those of the Keppoch Formation. The close spatial and temporal association of calc-alkalic and tholeiitic mafic rocks, the felsic rocks with volcanic arc chemistry and the stratigraphy which documents the infilling of a narrow sedimentary basin suggest a rift setting within an ensialic volcanic arc environment. The style of volcanism exhibited in the Antigonish Highlands compares favourably with other late Precambrian successions in Nova Scotia. In general late Precambrian volcanism in northern Nova Scotia is consistent with an ensialic volcanic arc environment with local development of volcanic arc rifts and provides constraints on the evolution of the Avalon terrane as a whole.
Transcurrent ductile shear zones and the dispersal of the Avalon superterrane
Authors: Gibbons Wes
Published: April 2008

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.
Brittle deformation and graphitic cataclasites in the pilot research well KTB-VB (Oberpfalz, FRG)
Authors: Zulauf G. , Kleinschmidt G. , Oncken O.
Published: April 2008

Within the research well KTB-VB, late- to post-Variscan brittle deformation is widespread, indicating that the well is situated in a broad fault zone. Graphite-enriched foliated cataclasites within paragneisses are probably connected with late-Variscan NW trending reverse faults. These faults were succeeded by further, graphite-free, faults in post-Variscan time. During the formation of the graphitic cataclasites water-consuming retrograde metamorphism was active producing large amounts of phyllosilicates, especially at the expense of feldspar. The increase in phyllosilicates and graphite was associated with a switch of the dominant deformation mechanisms from brittle fracture to frictional sliding, crystal plasticity and diffusion-controlled processes. The cataclastic fabric suggests that the main part of the movement within the graphitic cataclasites was likely to have been aseismic rather than seismic.
Instability, softening and localization of deformation
Authors: Hobbs B. E. , Mühlhaus H.-B. , Ord A.
Published: April 2008

Material strain softening is commonly taken as a necessary and sufficient condition for localization in deforming rocks. However, there is a wide range of experimental and theoretical information which shows that localization can occur in sands, brittle rocks and ductile metals under strain-hardening conditions. This paper aims to bring these two contrasted views together. Three separate criteria are necessary in order to understand localization behaviour. The first involves the stability of the deforming system. The second determines whether a deforming system will undergo bifurcation so as to cease deforming in a homogeneous mode and instead deform in an inhomogeneous mode such as barrelling or localization. The stability and bifurcation criteria are independent of each other since barrelling is a stable mode whereas localization is unstable. The third criterion establishes if the unstable bifurcation mode is one of localization or of some other kind. Localization may arise from the presence of vertices on the yield surface (as in the case of pressure insensitive, rate dependent metals and in brittle rocks due to the development of preferred microfractures for slip) or from the constitutive relation being such that the plastic strain-rate vector is not normal to the yield surface (as in the cases of pressure sensitive, dilatant rocks, of materials deforming by crystal-plastic processes involving dislocation cross-slip and/or climb, and of visco-plastic materials in which voids are forming due to diffusive processes). It is important to distinguish between
material
and
system
softening (or hardening) behaviour. The theory for a kinematically unconstrained shortening experiment (that is, rigid, frictionless platens) indicates that localization can occur in strain-hardening materials but the system must strain-soften from then on; that is, localization occurs at peak stress for the system even though the material may continue to harden (or soften). However, the addition of kinematic constraints (such as friction at elastic platens, a constraint to deform in plane strain or at constant volume) means that localization may occur in a system that is monotonically strain hardening. Shear zones in naturally deformed rocks show ample evidence of dilatant behaviour in that evidence for the passage of large volumes of fluid during localization is common as is the development of dilatant vein systems. As such, since shear zones are strongly constrained by the elastic and (limited) plastic response of the relatively undeformed rocks surrounding the shear zones, strain-hardening behaviour of the system is to be expected as the norm, even if the rocks within the shear zones are undergoing material strain-softening.