bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cadomian geodynamic evolution of the northeastern Armorican Massif (Normandy and Maine)
Authors: Dupret L. , Dissler E. , Doré F , Gresselin F. , Le Gall J. 
Published: April 2008

Abstract
The oldest Precambrian rocks of the northeastern Armorican Massif are represented by the migmatites and gneisses of La Hague which make up the Lower Proterozoic Pentevrian basement. This is overlain by the low-grade Upper Proterozoic rocks of the Brioverian succession which were affected by Cadomian deformation and plutonism.
The Lower Brioverian Group consists of four tholeiitic volcanic formations overlain by terrigenous units which contain some black chert horizons. In the Coutances area this assemblage was affected by a local tectonometamorphic event which led to the formation of the Constantian arc (585 Ma). The uplift and erosion of this arc yielded the Upper Brioverian flysch which includes spilitic rocks at Vassy and sulphide deposits at Rouez.

A plate tectonic model is proposed for the Cadomian orogeny in the northeastern Armorican Massif. Major and trace element geochemical features of the Brioverian igneous rocks allow us to assign it to a ‘Pacific’ tectonic setting. An island-arc system was established on Pentevrian crust remnants and led to magmatic arcs separated by marginal basins floored by MORB. Strain analysis shows that Cadomian thrusts and folds verge northwestwards, synthetic to southeasterly-directed subduction. A second southeastward-dipping subduction zone developed and was responsible for the closure of the back-arc basin and the generation of the Mancellian Batholith. The folded Upper Brioverian rocks were then intruded by these late-tectonic granodiorites at
c.
540 Ma. The Brioverian rocks and Cadomian plutons were then uplifted, eroded and unconformably overlain by the Lower Cambrian cover.

In the northeastern Armorican Massif, this Cadomian evolution corresponds to a major period of crustal growth and represents progressive cratonisation of the Brioverian basin and arc system.
Shear zone pattern and granite emplacement within a Cadomian sinistral wrench zone at St Cast, N. Brittany
Authors: Gapais D. , Balé P. 
Published: April 2008

Abstract
The St Cast shear zone, situated in Northern Brittany, is a crustal-scale sinistral wrench zone of Cadomian age. We describe the associated deformation history on the basis of a detailed analysis of structures, microstructures and quartz fabrics of rocks outcropping at the Pointe de St Cast.
The study area shows rocks of dominantly granodioritic composition which are intruded by a leucogranite (the Roche de St Cast). Strain measurements, foliation trajectories and outcrop-scale shear zones reveal a heterogeneous pattern of plane-strain type, associated with a bulk sinistral transcurrent shearing within both intrusion and country rocks. rocks.
Local shear zone patterns are however different within the two units. Country-rocks show decimetre scale shear zones anastomosed around low strain domains. In contrast, the intrusion shows a more homogeneous structure with a single set of parallel pervasive millimetre scale shear bands (sinistral C-S structures). These differences are interpreted in terms of different local kinematics and bulk thermal history as expected between a magmatic intrusion and its surrounding country-rocks during syntectonic emplacement. The overall structural pattern emphasizes that the Roche de St Cast is a syntectonic granite emplaced within a major zone of Cadomian sinistral wrenching.
Late Precambrian M-type granitoid genesis in the Cadomian belt of NW France
Authors: Graviou Pierrick , Auvray Bernard 
Published: April 2008

Abstract
A subduction model has been proposed for the geodynamic evolution of the North Armorican Massif during Upper Proterozoic times. An early period of plutonism (600 ± 20 Ma) is characterized by M-type granitoids with largely intermediate composition. Based on petrological, geochemical and isotopic (Rb-Sr) data from the best exposed batholith (North Tregor Batholith, NTB), the following major conclusions can be proposed.

(a)
Various rock-types have resulted from fractional crystalization processes occurring in granodioritic parent magmas.

(b)
The Lower Proterozoic basement (Icartian/Pentevrian) has no major role in the genesis of these granitoids.

(c)
The mantle wedge above the subducted oceanic slab constitutes one possible source region for the magmas. Fluids escaping from the downgoing slab induced melting of a garnet lherzolite mantle which was also slightly metasomatized. During the ascent of these primary magmas, fractionation of olivine and spinel produced the parent magmas of the Tregor plutonic suite thus explaining the low transition element abundances of these rocks (e.g. Ni, Co contents).
Contrasting metamorphic terranes in northwest Wales
Authors: Gibbons Wes , Horák Jana 
Published: April 2008

Abstract

The oldest rocks exposed in northwest Wales crop out as a series of fault-bounded slices. Whereas some of these fault-bounded units are clearly part of the same stratigraphic package (e.g. those belonging to the Monian Supergroup), others show little or no affinity with contiguous areas. Three suspect terranes are identified on Anglesey: (1) the Monian Supergroup; (2) the gneisses and granitic rocks of central Anglesey; (3) the Monian blueschists. The metamorphic histories of these terranes are markedly different and
PT
conditions vary from anchizonal/lower greenschist (1) to sillimanite grade (2) to epidote- or lawsonite-bearing blueschists (3). Each terrane has been juxtaposed by faults, the earliest movements along which (where not obscured by brittle reactivation) can often be shown to have been ductile and accompanied by dynamic recrystallization. The most prominent of these ductile fault boundaries occur between terranes 1 and 3 in Anglesey (Berw Shear Zone), and between terrane 1 and the Sarn Complex (terrane 4) in the Llŷn peninsula (Llŷn Shear Zone). A similar ductile mylonite zone exists between the Cullenstown Group (terrane 1?) and the Rosslare gneisses (terrane 2?) in SE Eire. All these shear zones are steep, and kinematic indicators indicate oblique sinistral shear. Radiometric, lithostratigraphic and palaeontological data constrain the ductile fault movement to
c.
550–540 Ma. This latter event is interpreted as recording an important phase of terrane dispersion and subsequent assembly that occurred along the Avalonian margin immediately prior to marine transgression over southern Britain. It is likely that this transcurrent event is recorded inboard from the Monian area by shear zones within the late Precambrian (Avalonian/Cadomian) basement in southern Britain and northwest France.
Kinematic history of the Bass River Complex, Nova Scotia: Cadomian tectonostratigraphic relations in the Avalon terrane of the Canadian Appalachians
Authors: Nance R. Damian , Murphy J. Brendan 
Published: April 2008

Abstract

The Late Proterozoic Bass River Complex of Nova Scotia, Canada, forms part of the Northern Appalachian Avalon terrane and comprises the Great Village River Gneiss, the platformal metasedimentary Gamble Brook Formation, and the mafic metavolcanic Folly River Formation. Three major periods of deformation within the complex are considered to be of late Precambrian age. The earliest (D1) appears to affect only the gneiss. The second additionally affects the Gamble Brook Formation and records polyphase ductile shear (D2a/D2b) along contacts with the gneiss attributed to sinistral transtension at
c.
630 Ma. Locally sheeted, late-kinematic mafic dykes suggest limited crustal extension and may herald the development of basinal conditions in the Folly River Formation. The third deformation (D3a/D3b) records dextral transpression and polyphase, NW-directed thrusting in the Folly River Formation. D3 is tentatively attributed to basin closure during the latest Precambrian and may have accompanied thrust emplacement of late Precambrian bimodal volcanic rocks and turbidites preserved in the nearby Jeffers Group. Regional considerations suggest D2 and D3 kinematics respectively reflect the development and destruction of a late Precambrian volcanic arc rift.
Diagenetic stability of the isotopic composition of phosphate-oxygen: palaeoenvironmental implications
Authors: McArthur J. M. , Herczeg A. 
Published: April 2008

Abstract

Meteoric diagenesis of the Lower Pliocene Varswater Formation phosphate deposit, Cape Province, South Africa, has altered the chemistry of the francolite component. The isotopic composition of the phosphate-oxygen has been changed, as have other chemical and isotopic parameters that are known to be susceptible to meteoric diagenesis. Reduced major axis regressions between δ
18
O

PO
4

and Sr/Ca,
87
Sr/
86
Sr and francolite-δ
13
C

CO
3

, and the correlation of francolite-δ
18
O

CO
3

and francolite-δ
13
C

CO
3

, predict an original isotopic composition of +30.7‰ (SMOW) for francolite-δ
18
O

CO
3

and + 22.7‰ (SMOW) for francolite-δ
18
O

PO
4

. These data fix the temperature of phosphogenesis at 16°C and the δ
18
O of the phosphatizing fluid as +0.3‰ (SMOW).
Episodes of phosphogenesis and phosphorite concretion formation in the North Sea Tertiary
Authors: Balson Peter S. 
Published: April 2008

Abstract
It is well established that certain periods through geological time have been more favourable to the formation of phosphorite deposits than others. These ‘phosphogenic episodes’ have been correlated with periods of eustatic high sea-levels, global warm temperatures and increased marine productivity, amongst other factors.
The development of phosphorite concretions in marine sediments of the southern part of the North Sea Tertiary Basin reflects episodicity corresponding to times of sea-level high-stands. In particular, concretions in Early Eocene marine mudstones make their appearance just above levels which indicate initiation of a more open connection between the Atlantic Ocean and the relatively enclosed North Sea Basin. Similarly, during the Middle Miocene, phosphorite concretions occur associated with indicators of oceanic influence. Elsewhere in the world, remote from the North Sea, phosphate-rich sediments also formed during these stratigraphically brief episodes. Occurrences of relatively sparse diagenetic phosphorite concretions in the North Sea Basin therefore appear to be linked to large scale, possibly global, cycles and not solely to the local diagenetic environment.
Role of microbial processes in the genesis of Jordanian Upper Cretaceous phosphorites
Authors: Abed Abdulkader M. , Fakhouri Khalid 
Published: April 2008

Abstract
Samples representing granular and laminated phosphorites from the main Upper Cretaceous deposits of Jordan, have been examined by SEM for microbial structures. Globular clusters and sheaths, postulated to have formed by bacterial activity or possibly as inorganic precipitates, have been found only within cavities and in the cementing phosphatic material around phosphate grains of the granular phosphorites. These structures are interpreted to have originated in association with percolation of ground-water late in the history of the sediments, rather than as a consequence of primary grain-forming processes. Organic-rich phosphate grains are believed to have acted as a food source for the bacteria and are, therefore, the cause of the microbial structures and not the result of them. On the other hand, empty cylindrical and/or spherical sheaths, interpreted to be of primary, cyanobacterial (blue-green algae) origin, were readily observed in the laminated phosphorites. These phosphorites are interpreted to have been deposited as algal mats in shallow, subtidal to intertidal environments.
The influence of magnesium ions during the formation of stromatolitic phosphorites of Udaipur, Rajasthan, India
Authors: Sisodia M. S. , Chauhan D. S. 
Published: April 2008

Abstract
The stromatolites of the Precambrian Aravalli Supergroup outcropping around Udaipur City are composed of carbonate (dominantly dolomite) and/or phosphate (carbonate-fluorapatite). The phosphate present in these stromatolites occurs in varying proportions, from traces levels to a major constituent. Compositional variation is attributed to the differences in the original magnesium contents stromatolites which reflects their different of the depositional environments, Intertidal stromatolites had primary magnesium calcite mineralogies, while those deposited in subtidal areas were composed of aragonite and/or lower magnesium calcite. The Mg/Ca ratio in the stromatolites appears to have been controlled by microorganisms which, it is argued, also played an important role in the process of phosphate fixation. It is inferred that the high Mg content of intertidal stromatolites prevented extensive phosphatisation in this facies.
On the description and modelling of glacimarine sediments and sedimentation
Authors: Dowdeswell Julian A. , Scourse James D. 
Published: April 2008

Abstract
Models of glacimarine environments fall into two categories: (a) qualitative summaries of the depositional record; and (b) quantitative models of the physical processes of glacimarine sedimentation and their rates of operation. Qualitative approaches include descriptive vertical lithofacies profiles and models derived from observations of two- or three-dimensional sedimentary or seismic facies changes. Qualitative models are based on observations in modern glacimarine environments where the processes and settings are relatively well constrained, or on sedimentary or seismic investigations of Cenozoic and pre-Cenozoic successions. Some represent hypothetical sedimentary facies associations based on intuitive reasoning in the absence of observations (e.g. beneath ice shelves). Quantitative process and rate models can be based on theory or empirical data. The nature and rate of sedimentation beneath ice shelves has been modelled quantitatively. Scaled-down laboratory models, for example of iceberg melting, have also been used experimentally. Process models linking different parts of the glacimarine system are an important future research area.
Glacimarine processes at grounding-line fans and their growth to ice-contact deltas
Authors: Powell Ross D. 
Published: April 2008

Abstract

Grounding-line fans originate from subglacial and basal stream tunnels at grounding lines of glaciers terminating in a marine environment. During melt seasons discharge forms a turbulent jet beyond the efflux. Over the initial zone of flow establishment in the jet, it may remain in contact with the sea floor for up to 13.4
D
(
D
-tunnel diameter) along a runout distance, if velocity is constant. Plug flow in this zone deposits marine outwash sediment commonly having chaotic texture and graded and welded contacts. Texture may vary rapidly because of discharge pulses, and the jet may move laterally to produce sheet or cut-and-fill geometries. Imbricate gravel is deposited near the efflux, then farther out are sheet or weakly channelized finer-grained gravels and sands. In some instances a migrating barchanoid bar forms at the detachment zone and forms large-scale trough cross-beds. Beyond the detachment zone the turbulent jet becomes vertical and sediment cascades from it in a ‘veil’. This sediment is added to by flows from continuous failures of bed load sediment in the detachment zone. Resulting deposits are interstratified sediment gravity flows and thick, coarse end-members of cyclopsams. The jet changes to a plume when inertial forces become less than buoyancy forces. Turbulent vorticies within the jet can re-entrain particles into a buoyant plume. The plume remains vertical, although continuously spreading, until reaching neutral buoyancy. Fan depocentres have been observed to accumulate at over 10
6
m
3
a
−1
in temperate glacial areas. Eventually, at quasi-stable grounding lines, fans may aggrade to sea level to form ice-contact deltas. At incipient deltas when the delta plain is intertidal, coarse sediment is redistributed to the prodelta with each tidal cycle. Intense prodelta rhythmites mark the transition of a fan into a delta.
Suspended sediment transport and deposition of cyclically interlaminated sediment in a temperate glacial fjord, Alaska, U.S.A.
Authors: Cowan Ellen A. , Powell Ross D. 
Published: April 2008

Abstract
Cyclically interlaminated sediment is a distinctive lithofacies within the sediment package of temperate glacial fjords. The tidewater terminus of McBride Glacier is at the head of a small fjord that receives abundant suspended sediment and is a site of rapid accumulation of cyclically interlaminated sediment. Buoyant sediment-laden meltwater rises from a subglacial stream at the base of the glacier and mixes with fjord water to produce a thick, brackish overflow. Peak suspended sediment concentrations occur beneath the surface of the overflow at 3 to 10 m depth. Particle release from the overflow is controlled by semi-diurnal tidal fluctuations. The major vertical flux of suspended sediment is initiated at low tide because of low horizontal current velocities and reduced vertical eddy velocities. Sorting occurs as the particles settle; sand and coarse silt settle as single grains and finer flocculated particles settle as turbid layers at a rate between 2.5 m/h and 10.9 m/h. Each low water produces a couplet of a coarser grained lamina that is sorted as coarser single grains settle, and a finer grained lamina of flocculated particles in turbid layers. Individual couplet thicknesses and particle size decrease with distance from the discharge source. Semi-diurnal tides produce two couplets each day. Interstratified with these tidal rhythmites are coarser and poorly sorted laminae deposited by sediment gravity flows and coarse laminae contributed by peak daily discharge.
Laminated terrigenous sediments from the Antarctic Peninsula: the role of subglacial and marine processes
Authors: Domack Eugene W. 
Published: April 2008

Abstract

Although laminites from the continental slope and rise are known from a number of regions in Antarctica, laminated muds on the continental shelf have not been widely recognized. During USAP-88 cruise III of the RV
Polar Duke
, a number of piston cores were collected from bays and fjords along the Danco Coast of the Antarctic Peninsula. A fine-grained sedimentary sequence of thin-bedded to randomly laminated mud and sand with dropstones is underlain by a rhythmically laminated sequence of silt and clay. The deposits were collected within a submarine valley (450 m deep) which appears to extend beneath the terminus of a tidewater glacier (Cayley Glacier). The glacial regime of the area today is polar to sub-polar with restricted ice surface melting. Fine-scale horizontal layers are observed in the water column and these transport fine-grained sediment at mid-water depths. Such features are not related to surface meltwater processes, but may be caused by tidal pumping of the basal cavity. The random bedding thickness of the sediments is related to the present depositional environment which includes iceberg rafting, bioturbation, pelagic settling of biogenic detritus, and sediment gravity flows. The rhythmic units likely represent subglacial marine deposition within the valley; possibly under the influence of tidal processes.
Glacial geomorphic features in the northern Barents Sea: direct evidence for grounded ice and implications for the pattern of deglaciation and late glacial sedimentation
Authors: Solheim Anders , Russwurm Lars , Elverhøi Anders , Nyland Berg Mona 
Published: April 2008

Abstract

Glacial landforms, in particular flutes and associated transverse ridges, have been identified in an area of roughly 4000 km
2
and between approximately 150 and 340 m water depth in the north-central Barents Sea. The transverse ridges are interpreted to be analogous to De Geer moraines and are observed only in the southern part of the fluted area. The flutes, trending roughly north-south, have a relief of less than one metre, widths on the order of 4–8 m and lengths usually 100–500 m. Although the present data do not allow firm conclusions on the exact genesis of these features, they are compared to analogous terrestrial features.

The existence of these glacigenic morphological features has major implications concerning the late glacial history of the Barents Sea. They give the first direct evidence of a grounded ice sheet in the northern Barents Sea. Ice flow directions during the latest phase before final recession were mainly southwards, but a partly topographically influenced flow pattern indicates that the main banks may have acted as local ice centres. The ice sheet was relatively thin and rested on a deformable bed. It retreated rapidly from the fluted area, as evidenced by the lack of sediment masking the low-relief flutes. The general retreat from the region took place in a stepwise manner, apparently without surging and with deposition of locally thicker glacier-proximal deposits during halts in the recession.
Geometry and genesis of the glacigenic sediments in the southern Barents Sea
Authors: Vorren Tore O. , Lebesbye Erland , Larsen Kjell B. 
Published: April 2008

Abstract
The glacigenic sequence on the continental shelf and shelf break in the Barents Sea has been mapped seismostratigraphically. Four and six major units have been distinguished on the outer and inner shelf, respectively. The outer shelf units increase in thickness towards the shelf break, except for the youngest one. The geometry of these units is determined partly by the morphology of the basement (infill), but to a larger extent by postdepositional erosion. The older, inner shelf units are located in a coast parallel trough. The two youngest units also have a blanketing character. The geometry of the younger units gives indications of source areas, as well as of ice movement direction of subsequent eroding ice sheets. The palaeoenvironment and palaeogeography during the mid-late Weichselian in the Barents Sea is reconstructed.
Sedimentology and micropalaeontology of glacimarine sediments from the Central and Southwestern Celtic Sea
Authors: Scourse J. D. , Austin W. E. N. , Bateman R. M. , Catt J. A. , Evans C. D. R. , Robinson J. E. , Young J. R. 
Published: April 2008

Abstract
Thin discontinuous glacigenic sediments occur at or close to the sea bed as far south as 49°N in the Celtic Sea. The northern samples (facies A) are clast-rich, overconsolidated lodgement tills or proximal glacimarine sediments containing sparse reworked microfaunas. The southern samples (facies B) are distal glacimarine plastic silty clays containing abundant cold water microfaunas. Both these facies are correlated with the Late Devensian Scilly Till, enabling a quantitative reconstruction of ice thicknesses, grounding line, sea-level and shoreline elevations in the Celtic Sea at 19 000 years BP. The ice advance terminated in marine waters towards the shelf edge break, and is likely to have constituted a thin lobate surge over deformable marine sediments.