bluuebunny/crossref_metadata_2025 huggingface dataset:
Cretaceous to Eocene volcanic arc activity in the Andes of northern Ecuador
Authors: Hendenson W. G.
Published: December 2007

During the Cretaceous to Eocene the W. part of N. Ecuador received the tholeiitic and calk-alkaline magmatic products of one volcanic arc, which built up on a thin or dense basement, at least partly of oceanic origin. Accumulation of this arc was preceded by early Cretaceous volcanicity and flexuring in the E. and was brought to a close by late Senonian to Eocene orogenic movements. The partly ensimatic arc of N. Ecuador contrasts markedly with the totally ensialic arc of S. Ecuador and Peru, and suggests an important change in basement type near 2°30'S (the latitude of Guayaquil). Oceanic basement in N. Ecuador was probably trapped in the Lower Cretaceous by initiation of subduction at a trench in approximately the same relative offshore position as at present.
The Cenozoic stratigraphy and structure of the western part of the Rift Valley in southern Kenya
Authors: Crossley Robert
Published: December 2007

Mapping, K–Ar dating and palaeomagnetic polarity measurements were used to study the geological history of the predominantly volcanic sequences in the western part of the Kenya rift valley S of 1º30'S. The rift was initiated as a downwarp which was later infilled by melanephelinitic agglomerates and lavas (15–12 Ma ago). Major faulting (12–7 Ma ago) downthrew these volcanics to the E and flood trachytes (7–5 Ma ago) were erupted onto the newly-formed rift floor. Minor faulting and volcanism preceded the second period of major faulting (5–3 Ma ago). Extensive flood basalts (3–2.5 Ma ago) infilled the resulting rift structure. Later subsidiary depressions in the rift floor were infiIled by two large porphyritic trachyte flows in the N (the older one 2.1 Ma ago), by flood basalts (2.1–1.7 Ma ago) in the S and by flood trachytes (1.4–0.7 Ma ago) in the E. The palaeomagnetic polarity sequence from the rift floor lavas represents a substantial part of the Matuyama epoch. A period of major faulting (most of it post-0.6 Ma ago) created much of the present rift topography and the resulting Magadi–Natron basin was later flooded by an extensive temporary lake.
A new look at the Mona Complex (Anglesey, North Wales)
Authors: Barber A. J. , Max M. D.
Published: December 2007

Re-examination of the Mona Complex (Anglesey, North Wales) has led to a radical re-interpretation of its stratigraphy, structure and geological history. The gneisses are regarded as representatives of an earlier continental basement, as
Greenly (1919)
originally proposed. In the northern part of Anglesey the Bedded Succession can be separated into 3 structural units. The South Stack Unit of Holy Island is overlain by the more highly deformed New Harbour Unit; the contact between these two units is interpreted as a thrust plane. A third unit, the Cemlyn Unit, outcropping in the NW, comprises the Church Bay Tuffs, Skerries and Gwna Groups of Greenly's succession, as well as greywackes and slates, correlated by Greenly with the New Harbour Group. However, the Cemlyn Unit is less deformed than the New Harbour Unit and their relationships may be unconformable. It is proposed that Greenly's Fydlyn Group does not form part of the Mona Complex but may be correlated with the Caradocian volcanics of Parys Mountain. No major structural discordance occurs between the Gwna Group and the overlying Ordovician rocks on the N coast. The time gap represented by this contact has also been reduced by recent fossil finds, which indicate that some members of the Mona Complex are of Cambrian Age. The implications of these discoveries are that there was no 'Irish Sea land mass' in the Anglesey area in the Lower Palaeozoic and that major structures in the complex previously attributed to a late Precambrian orogeny are of Caledonian age. Using the new structural and stratigraphic data, a synthesis of the sedimentary, tectonic and structural evolution of the complex is proposed, in the context of plate tectonics.
Stratigraphical relationships of the early Palaeogene ash-series of NW Europe
Authors: Knox R. W. O’B. , Harland R.
Published: December 2007

Lithostratigraphical correlation and dinoflagellate cyst evidence point to the presence of a pronounced hiatus at the base of the London Clay and equivalent beds in NW Europe. Following a late Palaeocene regression, the sea became restricted to a small area in the central part of the North Sea Basin. In the subsequent 'London Clay' transgression the sea at first remained confined to the inner basin area, but during deposition of the ash-series it extended southwards over the southern North Sea area and into Germany, the Netherlands, and East Anglia. It was apparently not until after eruption of the ashes that the 'London Clay' sea became established over the S of England, Belgium, and northern France. Sedimentation was then more or less continuous in these southern areas during the lower Ypresian, but a phase of uplift and non-deposition in the central North Sea and Denmark resulted in a second hiatus above the ash-series.

The proposed presence of a stratigraphical break beneath the London Clay (and the equivalent Ypres Clay) is relevant to the problems of defining the Palaeocene/Eocene boundary and associated stage boundaries in their type areas.
Shape fabrics and superimposed simple shear strain in a Precambrian shear belt, W Greenland
Authors: Grocott John
Published: December 2007

Shape fabric type in the northern part of the Ikertôq shear belt and the adjacent area has been recorded both qualitatively and quantitatively, and its variation is described. Shape fabric variation is shown to be closely related to structural changes, and both are reconciled with a deformation model involving superimposed strain.

In the shear belt itself finite strain shape fabrics are shown to be a consequence of superimposition of 2 differently orientated simple shear strains. The earlier simple shear strain initiated the shear belt and was characterized by horizontal ENE-WSW movements in a vertical shear plane. The second deformation was produced by overthrusting in a SSE direction. In effect the
XZ
plane of the later deformation was superimposed on YZ sections through shape fabrics produced by transcurrent movements. The overthrusting deformation was superimposed on earlier structures with gradually increasing intensity, allowing the resultant deformation path to be recorded in detail.

A steep strain gradient marks the northern boundary of the shear belt, but low deformation of shear belt age can still be recognized further N. Shape fabric variation N of this limit of intense strain is described. Structural and shape fabric variation in this area is a consequence of superimposition of low magnitudes of overthrusting simple shear strain on pre-shear belt structures and fabrics.
Palaeomagnetic evidence for rotation and northward drift of Sumatra
Authors: Haile N.S.
Published: December 2007

The rocks sampled include ?Permian, Upper Triassic, Lower Cretaceous, and Lower Tertiary, with rocks at 2 sites being of uncertain age, probably Mesozoic. A total of 137 orientated samples was collected at 25 sites from 13 localities in N and central Sumatra.

2 Lower Tertiary sites (Geunteut & Breueh Island) yield magnetic directions of D=8, I = l l and D=34, 1=3 respectively, equivalent to apparent palaeomagnetic poles at 82°N 183°E, and 56°N 190°E. An Upper Triassic site gives D=220,
I=26,
equivalent to a pole at 49°N 210°E, assuming, as seems likely, that the field was reversed.

These results indicate a probable northward drift of 12° since the late Triassic, accompanied by a clockwise rotation of 40°.

The results from remaining localities are less reliable because of uncertainties
of
age, low stability, or small number of samples, but all results except one confirm clockwise rotation, and low palaeolatitudes (within 26° of present latitude) since the Permian.

The clockwise rotation of Sumatra contrasts with anti-clockwise rotation of W Borneo, the Malay Peninsula, and SW Sulawesi indicated by palaeomagnetic evidence, and suggests that Sumatra, or at least the parts sampled, was not coupled to 'Sundaland' until the mid-Tertiary.
Reinterpretation of the geology of Seram: implications for the Banda Arcs and northern Australia
Authors: Audley-Charles M.G. , Carter D.J. , Barber A.J. , Norvick M.S. , Tjokrosapoetro S.
Published: December 2007

Reconnaissance field traverses in Seram have led to major revisions in the stratigraphy, structure, tectonic history and geological maps. The island is composed of 4 principal stratigraphical-structural elements: (1) metamorphic continental of uncertain structural status and palaeogeographical affinity, (2) an entirely marine early Triassic-Miocene imbricate succession regarded as para-autochthonous, (3) an allochthon composed of several different thrust sheets, including metamorphic rocks, Triassic limestone and a late Miocene olistostrome, (4) a Plio-Pleistocene post-orogenic autochthon. The apparently over-thrust slices of metamorphic basement complex can be interpreted as derived from either the Asian or Australian craton. The Australian shelf, slope and rise sediments, possibly including some oceanic sediment, are regarded as para-autochthonous. Remarkably close correlation is demonstrated between the stratigraphical breaks reported from the Mesozoic-Cenozoic succession of the NW Australian shelf, from Misool, and the para-autochthonous rocks of Seram and Timor. This emphasizes the presence of the Australian craton underlying these 3 islands. Close correlation is also found between the allochthonous rocks of Seram and Timor. Some of these thrust sheets are interpreted as having been derived from the Asian continental margin. The ultrabasic rocks of SW Seram and Ambon seem to form the highest thrust sheets. The main period of orogenesis, involving over-thrusting, olistostrome emplacement and imbrication of the underlying Australian cover-rock sequences occurred in the late Miocene-early Pliocene (N.18). The structural position of the volcanic rocks of Ambon is uncertain. A tentative interpretation is that they are
in situ
, having been extruded from deep-seated fractures that penetrate the 'Asian' thrust sheets and the underlying Australian continental basement.
Metallogenesis at oceanic spreading centres
Authors: Cronan D. S.
Published: December 2007

Report of a combined OGM and Marine, Mineral Deposits and Volcanic Studies Group Meeting held at Burlington House, 21–22 November 1978. The meeting was organized by Dr D. S. Cronan.

Metalliferous sediments associated with active spreading centres can be divided into 3 types: (1) sulphide deposits associated with silicates and oxides, as in the Red Sea; (2) sharply fractionated silicate and oxide deposits of very localized extent, as on some mid-ocean ridges in the open ocean such as in the FAMOUS area; and (3) widely dispersed predominantly oxidic deposits of iron and manganese which comprise the bulk of metalliferous sediments on mid-ocean ridges. These different deposits could all be formed as a result of fractional precipitation of metals from hydrothermal solutions derived by seawater leaching hot newly formed oceanic crust. Sulphide deposits would represent the early formed precipitates, silicates those precipitated next, and oxides those precipitated last. The widely dispersed predominantly oxide bearing metalliferous sediments common on mid-ocean ridges have been formed as a result of iron and manganese bearing solutions representing the residual liquids of sub sea-floor hydrothermal fractionation processes discharging from hydrothermal vents, together with some particulate iron and manganese oxides, becoming well mixed in the bottom waters, and precipating their constituents relatively uniformly over a wide area. That such a process is not confined just to the present time is evinced by the widespread occurrence of metalliferous sediments directly overlying oceanic basement found in many Deep Sea Drilling Project cores on the flanks of mid-ocean ridges.
New age determinations and the geology of the Kenya Rift-Kavirondo Rift junction, W Kenya
Authors: Jones W. B. , Lippard S. J.
Published: December 2007

The area of the triple-rift junction in W Kenya, between the E-W Kavirondo Rift, the N-S Northern Kenya Rift and the NW-SE Central Kenya Rift, has been the site of continuous volcanic and tectonic activity since the Middle Miocene. From about 20 to 7 Ma the area was dominated by the accumulation of a thick pile of nephelinitic and phonolitic volcanics in the Timboroa area. From 7 Ma to the present day, apart from local eruptions of basanite in the W, the volcanic and tectonic activity has been progressively concentrated towards the axial zone of the Kenya Rift with alkali basalt-trachyte sequences predominating. During this period the Kavirondo Rift was inactive. The stratigraphic sequence in the area has been dated by 19 new K-Ar age determinations. These enable the absolute and relative ages of the Tinderet, Timboroa, Kapkut, Londiani and Kilombe volcanoes to be established for the first time.

Topographical and structural considerations show that the Kavirondo Rift largely fades out 50 km W of the Kenya Rift and is barely perceptible at its edge. The W wall of the Kenya Rift adjacent to the Kavirondo Rift is a monocline and is very similar to the South Turkana region. Although the area is adjacent to major rift-bounding faults there is little faulting within it, the tectonic pattern being dominated by monoclines and volcano-tectonic structures.
Diagenetic alteration in black shales
Authors: Curtis C. D.
Published: December 2007

To most field geologists 'black shales' are dark, fissile sedimentary rocks which are rich in both carbon and sulphur. The property of fissility probably develops only in sediments deposited from anoxic bottom waters. Carbon and sulphur-rich sediments are formed wherever relatively organic-rich material accumulates at such a rate that intensive diagenetic alteration takes place within the uppermost few metres of the sediment column, but relatively little happens below. Depositional waters must be marine (or at least sulphate-rich) but not necessarily anoxic.
Unmixing a mélange: the petrology and history of a disrupted and metamorphosed ophiolite, SE Turkey
Authors: Hall R.
Published: December 2007

An ophiolitic tectonic mélange occurs to the S and structurally below pre-Permian crystalline rocks of the Bitlis Massif in SE Turkey. Basic igneous rocks occur in the mélange as tectonic blocks in a sheared serpentinite matrix and most have recrystallized during metamorphism or metasomatism. The basic rocks once formed part of a single igneous complex subsequently disrupted to form the mélange. The volcanic rocks and cherts were metamorphosed under conditions transitional between the blueschist and epidote-amphibolite facies. The mineralogy of the basic schists is dependent on composition: blueschists (crossite + magnetite ± chlorite ± phengite) differ from greenschists (hornblende + chlorite ± biotite) in having higher Fe/Fe + Mg and Fe
3+
/(Fe
3+
+ Fe
2+
). In contrast, the gabbros were metamorphosed under greenschist facies conditions and the resulting amphibolites are characterized by static fabrics and contain actinolite rather than hornblende. Later calcium metasomatism produced rodingites. It is suggested that the greenschist facies metamorphism was a sub-ocean-floor event and the higher pressure metamorphism occurred during subduction. The thrusting during the later stages of compression in the orogenic belt led to the emplacement of the mélange in its present position, but played little part in its formation.
Caledonian deformation in Norway
Authors: Roberts D. , Sturt B. A.
Published: December 2007

Geological research and mapping over the last decade has led to a reassessment of the Caledonian orogen in Norway. Two principal stages of orogenesis are now established: (1) a late Cambrian to early Ordovician
Finnmarkian
or Grampian event recognized mainly in Finnmark and probably also developed in western Norway; and (2) the widespread Middle-Upper Silurian
Scandinavian
stage of central and southern areas. Another significant finding, aided by radiometric studies, is that of incorporation of Precambrian crystallines as discrete tectonic slices within the Caledonian nappe complexes, reminiscent of the Lewisian-within-Moine relationship of Scotland. In the extensive gneiss terrain of western districts and of Lofoten the effects of Caledonian tectonometamorphic reconstitution have been minimal.

Biostratigraphic and geochronological indicators provide evidence that diachroneity is a major feature common to both the above evorogenic stages, with deformation younging progressively towards the southeastern quadrant. In S Norway, folding and local thrusting continued into the Devonian. In terms of geometry and strain, individual nappes are generally extensive, thin, sheet or westward-thinning elements in which protracted high strains in basal zones have produced complex, banded mylonitic and pseudo-psammitic lithologies. Indications of an overall, primary, progressive simple shear deformation are ubiquitous, commonly with evidence of fold rotation into the stretching lineation trend in areas of high ductile strain; this is masked by important episodic increments of flattening and extensional strain producing lensoid, mega-boudin structures and leading to excision of some nappe units. This vertical shortening and gravitational spreading is recognized in both the Cambrian and the Silurian stages of the orogenesis and affects calculations of nappe translation, estimated in hundreds of km for central and southern areas. In conclusion, some comparisons are outlined between the Scandinavian and East Greenland Caledonides; in these two segments of the orogen, nappe displacement is in opposite directions.
Rb-Sr and O isotopic relationships in 3 zoned Caledonian granitic plutons, Southern Uplands, Scotland: evidence for varied sources and hybridization of magmas
Authors: Halliday A. N. , Stephens W. E. , Harmon R. S.
Published: December 2007

The 3 largest plutons in southern Scotland yield Rb-Sr mineral-whole-rock ages of 408 ±2 Ma for Doon, 397 ±2 Ma for Criffell and 392 ±2 Ma for Fleet. This emplacement sequence is accompanied by a progression towards higher average Thornton and Tuttle Indices,
87
Sr/
86
Sr initial ratios and δ
18
O values. All 3 plutons are composite and normally zoned and display an overall increase in (
87
Sr/
86
Sr)
i
and δ
18
O inward from the margins.

The Doon southern microdiorites are chemically distinct from the later diorites and some have (
87
Sr/
86
Sr)
i
as low as 0.7041. The main diorites in the S have (
87
Sr/
86
Sr)
i
similar to proximal granodiorites and granites (~0.705) suggesting that the chemical variations were chiefly produced by a process like fractional crystallization. Central granites further N have higher (
87
Sr/
86
Sr)
i
(up to 0.7059). δ
18
O values for the range of rock types vary from 7.83 to 10.32%o.

Magmatic (
87
Sr/
86
Sr)
i
values from Criffell lie between 0.7052 and 0.7069, δ
18
O values ranging from 8.54 to 11.84%o. Fractional crystallization was important in the centre of the pluton. There was greater interaction between the granite and granodiorite components in the S than in the NE.

The Fleet granites have high and variable (
87
Sr/
86
Sr)
i
ranging from 0.7060 to 0.7109. δ
18
O values obtained are relatively constant (11.17–11.33‰).

These data, combined with isotopic studies of the surrounding Lower Palaeozoic sediments, common Pb and U-Pb (zircon) data and geophysical constraints, indicate that the magmas were derived by melting of mantle and /or 'new' basic lower crust and metasediment, the isotopic variations being ascribable to incomplete hybridization between magmas derived from these differing sources.
TAG Hydrothermal Field: Mid-Atlantic Ridge crest at latitude 26°N
Authors: Rona P. A.
Published: December 2007

Results of ongoing interdisciplinary investigation of the TAG Hydrothermal Field since discovery in 1972 by the NOAA Trans-Atlantic Geotraverse (TAG) Project are synthesized to document the first and only known active submarine hydrothermal field on a slow-spreading oceanic ridge. The structural setting of the field is the E wall of the rift valley of the Mid-Atlantic Ridge at lat. 26°N where high-intensity hydrothermal activity is favoured by exceptionally close spacing of faults (tens of metres) that enhances permeability and facilitates voluminous circulation, and by projection of the wall directly over intrusive heat sources beneath the rift valley that increases thermal gradients which vigorously drive the upwelling limb of a sub-seafloor hydrothermal convection cell discharging through faults in the wall. A low in residual magnetic intensity coincides with the field and is attributed to hydrothermal alteration of the magnetic mineral component of basalt in the discharge zone. Hydrothermal precipitates in the form of manganese oxide crusts of extreme purity (40% Mn) and rapid deposition rate (200 mm 10
-6
yr) cover about 10% of the sea-floor within the field, distributed along faults sub-parallel to the rift valley. The composition and mineralogy of basalts exposed at the sea-floor is indicative of low-temperature zeolite metamorphic facies. Thin sediments of the field exhibit average metal-to-aluminum ratios and non-detrital metal accumulation rates (Fe, Mn, Ni, Co, Cu, Cr) that are high relative to other areas of the Mid-Atlantic Ridge. Present hydrothermal activity is evidenced in the near-bottom water by temperature anomalies and excesses of the primordial isotope
3
He, and in the surrounding water column by increases in weak-acid soluble suspended particulate matter enriched in certain metals (Fe × 10; Mn 10%). The observed distribution of hydrothermal products indicates that the special structural and thermal conditions that have concentrated hydrothermal activity in the TAG Hydrothermal Field have persisted for at least 1.4 × 10
6
yr.
On the stability of lower stage plane beds and the absence of current ripples in coarse sands
Authors: Leeder M. R.
Published: December 2007

Previous hypotheses for the absence of current ripples in coarse sands and for the stability of lower stage plane beds are critically reviewed and refuted. It is proposed that the lower stage plane bed remains stable and does not ripple because flow separation over bed defects is inhibited by the enhanced vertical mixing of boundary layer fluid over transitional to rough boundaries. This vertical mixing neutralises the increased static pressure over bed defects and prevents separation and re-attachment so that the defects are unable to amplify and propagate downstream. Eventually, as flow strength is increased, larger defects advance as bar bedforms which are regarded as kinematic waves. Further increases of flow strength give rise to dune bedforms related to effective flow re-attachment from pre-existing bar crests and whose downstream repeat distances are controlled by the magnitude of burst events occupying the whole turbulent boundary layer.
Sorting mechanisms in coarse-grained alluvial sediments: fresh evidence from a basalt plateau gravel, Kenya
Authors: Frostick L. E. , Reid I.
Published: December 2007

An analysis of the Koobi Fora Plateau Gravel, N Kenya, reveals clear-cut but contradictory patterns of sedimentary sorting. Basalt particles form the major constituent and decrease in diameter downstream at a rate of 3 mm km
-1
, while feldspar is virtually eliminated from the sediment within 30 km. But over the same ground, agate particles increase in size more than 4-fold while the sphericity of quartzite clasts decreases. These apparently conflicting sorting patterns are used to formulate a clearer understanding of the sedimentary processes that operate in widespread gravel deposits. The nature of the basalt particle size-gradient is contrasted with those published for alluvial fans and simple undivided channels to suggest diagnostic features that distinguish braided channel deposits.
Lithosphere evolution and changing tectonic regimes
Authors: Tarling D. H.
Published: December 2007

A model is presented in which the Earth is characterized by the mantle deep convection, and the thickness of the continental and oceanic lithospheres change with time as the pressure-temperature-geochemical conditions alter in the upper mantle. This model predicts that no island arc differentiation took place before 4000 Ma. During the Archaean, the shallow lithosphere allowed local, tectonic coherence and a gradual increase in the depth of subduction differentiation processes. The change to large cratonic-scale tectonic stability during the Proterozoic occurred as a result of lower lithosphere conditions allowing granulites to be stable—the consequential dehydration of the continental lithosphere creating extensive lateral and vertical stability. The change to ‘modern’ plate tectonics finally took place when the strengths of the oceanic and continental lithospheres became sufficiently close that mantle convection could produce continental splitting. It is suggested that this model accounts for the overall changing tectonic and geochemical features of the continental crust, although such generalities must be applied cautiously to specific localities.
Fibrous minerals and health
Authors: Elmes P. C.
Published: December 2007

The differences between fibrous and isometric dust particles in causing lung disease are outlined; the former cause diffuse lung scarring and 2 types of cancer, while the latter do not normally cause cancer; asbestosis and lung cancer can be caused by the retention of <1 g of asbestos, and mesothelioma with <1 mg. The factors causing asbestos dust to be so dangerous are outlined, including retention, clearance, physico-chemical properties, and smoking. Mesotheliomas are more likely where crocidolite exposure occurs. Potency of mineral fibres seems to be dependent on size and independent of the chemistry. Palygorskite is also an active agent. The epidemiology of mesothelioma in Turkey is described; in 2 areas the mesotheliomas appear to be due to fibrous erionite (a zeolite) in the dust. Urgent research and control measures are needed concerning fibres other than asbestos as a cause of cancer, involving collaboration between geologists and physicians interested in environmental health.
Correlation of late Proterozoic stratigraphies, NE Africa and Arabia: Summary of an IGCP Project 164 Report
Authors: Jackson N. J.
Published: December 2007

The most important late Precambrian stratigraphic units of NE Africa and Arabia are tabulated within a crude radiometric framework, and inferred correlations are suggested. Two geographical groupings of stratigraphies can be recognized. Continental clastic sedimentary and anorogenic volcanic successions are found around the Tanzanian craton, while orogenic volcanic-volcanoclastic successions typify areas surrounding the the Red Sea and Gulf of Aden.
The rare earth element geochemistry of glauconites and celadonites
Authors: Fleet A. J. , Buckley H. A. , Johnson L. R.
Published: December 2007

The few available rare earth element (REE) data on glauconites (Balashov & Kazakov 1968) show that, like authigenic phases and sediments in general, they are enriched in the light relative to the heavy REE and suggest that glauconites preferentially incorporate Ce from seawater and have distinct positive Ce anomalies. In order to add to these data the REE contents of 15 glauconites and, for comparison, 4 celadonites were determined by instrumental neutron activation analysis. These analyses indicate that the REE abundances in glauconites are similar to those in other sedimentary materials and that glauconites rarely exhibit significant Ce anomalies. Sea-floor samples have no or only slight negative Ce anomalies, indicating no significant preferential uptake of Ce. Glauconites from Cambrian and Cretaceous terrestrial outcrops, however, generally exhibit small positive Ce anomalies, possibly because they formed in extensive shallow seas rather than under truly oceanic conditions. Celadonites contain lower REE abundances than glauconites. Marine celadonites, unlike ones from terrestrial outcrops, exhibit distinct negative Ce anomalies. Most of the REE contents of glauconites and celadonites seems to come from lithogenous material; little is apparently contributed by
Petrology, mineralogy and geochemistry of the late Precambrian phosphate deposits of Upper Volta (W Africa)
Authors: Lucas J. , Prévôt L. , Trompette R.
Published: December 2007

Samples from different occurrences (trenches or borings) of late Precambrian phosphate deposits in Upper Volta were studied. At Kodjari, the phosphorites are fine-grained and clayey; in cores they are calcareous in the lower part but siliceous above. At Arly the phosphorites are coarse and the grains are frequently rimmed by fibrous apatite and recrystallized. The phosphate mineral is a fluorapatite with a low CO
2
content, generally <1%, but with an excess of Ca and F. 3 trace elements are present in notable quantity: Sr, Ba and Mn. The primary deposit was formed in a carbonaceous environment with abundant biological activity. Recrystallization is probably due to deep diagenesis. At the surface, weathering produces aluminium phosphates.
Sedimentology and stratigraphy of theTownsend Tuff Bed (Lower Old Red Sandstone) in South Wales and the Welsh Borders
Authors: Allen J. R.L. , Williams B.P.J.
Published: December 2007

The Townsend Tuff Bed has a highly distinctive internal stratigraphy which allows it to be recognized at a large number of localities scattered over the Welsh Borders and South Wales. It is a group of 3 almost directly superimposed, normally graded air-fall tuffs that originated in distant but powerful Plinian eruptions (centre unknown) and became spread over extensive, but practically featureless, coastal mud flats. The bed occurs in a barren interval within the early Lower Old Red Sandstone, between a Silurian invertebrate-vertebrate fauna below and an already Devonian vertebrate assemblage above. The base of the Devonian System in South Wales and the Welsh Borders is accordingly defined as occurring at a regionally uniform lithological contact within the Townsend Tuff Bed.
Glaucophanic amphibole in a Monian shear zone on the mainland of N Wales
Authors: Gibbons W.
Published: December 2007

Glaucophanic amphibole occurs within recrystallized basic volcanic rocks derived from the Gwna Group at Penrhyn Nefyn in the Lleyn Peninsula. The glaucophanic rocks are restricted to the ‘Penmynydd Zone of Metamorphism’ which at Nefyn is interpreted as a narrow blastomylonite belt produced by the shearing of the Gwna Group against a body of tonalite. The blue amphibole coexists with blue-green amphibole and grades into actinolite greenschist away from the focus of shearing. Fragments of Gwna Group (dated as Lower Cambrian) and blastomylonitic ‘Penmynydd’ lithologies lie in the Cambro–Ordovician sediments of the Welsh Basin. Therefore, the metamorphism of parts of the Gwna Group during the ‘Penmynydd’ (subduction zone?) shearing is interpreted as having occurred in Cambrian times.
Caesium from former Dartmoor volcanism: its incorporation in New Red sediments of SW England
Authors: Merefield J. R. , Brice C. J. , Palmer A. J.
Published: December 2007

High concentrations of caesium occur in the New Red Sandstone of Devon, England. Values are most abundant in breccias at the base of the Permian and decrease upwards through the sandstones and marls. Caesium resides in the clay fraction (<2 μm) where it reaches 468 ppm in Watcombe breccia, 146 ppm in Exmouth mudstone and 35 ppm in Triassic Otter sandstone. Credition Trough values attain 108 ppm in the Cadbury breccia.

Extensive volcanic activity on the south eastern edge of the Cornubian Massif provided a supply of Cs. Due to its low ionic potential, this alkali was concentrated at source under extreme weathering in the hydrolysate fraction of red bed sediments. The source area was largely exhausted by late Permian, reflected by a reduction in Cs content of New Red Sandstone detritus.
The climate of the Eocene ocean
Authors: Shackleton N. , Boersma A.
Published: December 2007

Oxygen isotope palaeotemperature estimates are presented for a number of deep sea sediment samples of Eocene age, covering a wide geographical area. These are plotted on a geographical reconstruction for 50 Ma. The latitudinal temperature gradient at that time was less than half its present value: surface temperature at high latitudes was about 10°C, and at low latitudes little over 20°C. It is concluded that Ocean currents must have contributed a greater proportion of the poleward transport of heat than they do at present.
Major marine cycles in the Mesozoic
Authors: Ager Derek V.
Published: December 2007

The cycles postulated by
Fischer (1979)
are considered in the light of the Mesozoic stratigraphical and palaeontological record. It is suggested that the most important events were:
(i)
the late Permian regression;
(ii)
the late mid-Triassic transgression;
(iii)
the late late-Transgression;
(iv)
the late early Jurassic transgression; (
v
) the late mid-Jurassic transgression;
(vi)
the end-Jurassicre gression;
(vii)
the late early Cretaceous transgression;
(viii)
the end-Cretaceous regression. These are related to episodicity in ocean-floor spreading, climatic changes and major happenings in the history of the shallow marine benthos.
Periodicity in the early Palaeozoic marine realm
Authors: Leggett J. K. , McKerrow W. S. , Cocks L. R. M. , Rickards R. B.
Published: December 2007

Evidence from the British Isles suggests that during 3 intervals in the early Palaeozoic conditions approximating to those of a non-uniformitarian ‘greenhouse (G)’ state Earth were realized: middle–early Upper Cambrian (St. Davids–Merioneth ages, ~30 Ma), mid-Ordovician (Llandeilo–Caradoc ages, ~32 Ma) and early Silurian (Llandovery–early Wenlock ages, ~12 Ma). During these times sea levels were high and ocean waters below the zone of surface overturning, poorly aerated and, at times, euxinic; pelagic biotas (graptoloids) were diverse, and in aerated portions of the highly productive shelf seas, benthic biotas (brachiopods) flourished. Opposing conditions, marking a well oxygenated (‘O’ state) Earth, occurred to varying degrees in the intervening intervals: early Cambrian (Comley age, ~20 Ma), late Cambrian–mid-Ordovician (Tremadoc–late Llandeilo ages, ~46 Ma), late Ordovician (Ashgill age, ~4 Ma) and mid to late Silurian (post early Wenlock age, >15 Ma). Within the constraints of the present data, perturbations in the early Palaeozoic marine realm do not appear to follow the same regular periodicity as in the Mesozoic and Caenozoic. Among a number of likely causes for this, the greater tendency for the early Palaeozoic seas to become anoxic seems the most important.
Sedimentary features of the Nunarssuit and Klokken syenites, S Greenland
Authors: Parsons I. , Butterfield A. W.
Published: December 2007

SUMMARY: The sediment-like features of two layered syenites are described and reviewed in the light of recent suggestions that layering in gabbros maybe produced by
in situ
crystal growth in a near-rigid medium. In the Nunarssuit Syenite igneous layering is normally graded (sharp mafic bases becoming more feldspathic upwards) and channel structures, some with breccias and extreme concentrations of mafic phases, are evidence of magmatic currents. Contorted bedding provides evidence of
c.
15 m of crystal mush. In the chemically similar Klokken Laminated Syenite the layers are inversely graded with well-sorted horizons of hedenbergite or fayalite at the top of graded units. Current bedding and rare channels are superimposed o n this inverse grading, and striking load structures occur at junctions with interbedded horizons of Granular Syenite, demonstrating the presence of >3 m of crystal mush. The Klokken Laminated Syenites are slightly more fractionated than Nunarssuit and approach the water-rich termination of fractionation of olivine, clinopyroxene and ternary feldspar. Cumulus phases in Nunarssuit layers are chemically less fractionated than those in ‘normal’ rock and occur with cumulus chevkinite and zircon, consistent with separation of suspended crystals from melt, but cumulus minerals at Klokken do not show this type of chemical contrast. Calculated magma densities and viscosities show that crystal settling of all phases was feasible in both complexes if the magmas were hydrous. Druses at Klokken suggest that the intercumulus liquid was water saturated during load structure formation. Sorting of locally derived crystals by currents accounts for the Nunarssuit layering, but at Klokken the crystal supply was controlled by order-of-nucleation and growth rate effects, although crystal settling occurred.
The chronology of magma influxes to the eastern compartment of the Bushveld Complex as exemplified by its marginal border groups
Authors: Sharpe M. R.
Published: December 2007

The floor contact of the eastern compartment of the Bushveld Complex has been mapped in detail for 130 km. Field relations, petrographical and geochemical data have enabled 3 important marginal border groups and a possible parent to the lower zone to be demarcated. These represent the 4 main magma influxes which formed the malic phase of the Bushveld Complex and are, in chronological order: 1, a contaminated magnesian basalt (B1) which gave rise to a saucer-shaped laminated marginal zone at the base of the Complex with diverse related sills; 2, a harrisitic peridotite representing quenched lower zone magma; 3, a border group to the more evolved, probably basaltic, critical zone magma which was intruded in two phases—an early quenched norite facies (N) being followed by a chilled bulk critical zone facies (B2) which forms a skin to the whole critical zone. Finally, 4, a border group to the main and upper zone (B3) which represents an early magnesian pulse of this last immense volume of tholeiitic liquid. There is also evidence for minor influxes of magma within the critical zone and main zone.
Horizontal and vertical zoning in the Peruvian Coastal Batholith
Authors: Atherton M. P.
Published: December 2007

Plutons of the Coastal Batholith of Peru show both horizontal and vertical zoning in mineralogy and chemistry. The horizontal variation is well seen in the areally extensive early ‘tonalite’ superunits, in which the sequence: marginal quartz diorite, tonalite adamellite, granite is often seen. This is the result of inward crystallization from the periphery of a relatively slowly cooling magma. Boundary layer convection combined with internal cell convection was effective in producing a markedly differentiated ‘rest’ liquid. The mineral assemblages are the result of incomplete separation of the evolved liquidus precipitate and the ‘rest’ liquid. In the smaller ring complex intrusions vertical variation is common and precipitation on the floor of the magma chamber resulted in the final crystallization of water rich aplites at the top. In these later intrusions magma mixing and hybridization may also be important. In both types of magma chamber stratification by boundary layer convection and diffusion seems likely to have complemented the conventional fractionation model of liquid evolution.
Convection and crystallization in the Criffell-Dalbeattie pluton
Authors: Phillips W. J. , Fuge R. , Phillips N.
Published: December 2007

It is argued that the asymmetric Dalbeattie granodiorite-granite pluton developed as a result of convectional circulation in the magma chamber during the consolidation of the outer xenolith-bearing granodiorite shell. The shell was deformed by the intrusion of aplite veins, the development of a penetrative foliation, and the emplacement of dykes with chilled margins due to the intermittent uprise of the magma body which eventually consolidated as granite. A porphyrite-porphyry series of dykes provides information concerning the degree of crystallization of the magmas that existed in the pluton the final stages of its emplacement and consolidation. Oscillatory zoning in plagioclases can be explained by constitutional supercooling or by changes occurring during convection. The effects of crystal fractionation and diffusion within the pluton are discussed.
Online searching of bibliographic geological databases and their use at the Science Reference Library
Authors: Leigh B.
Published: December 2007

Bibliographic databases consist of records of publications and many are now available for online computer searching as well as being used in the production of abstracting and indexing journals. The three major online databases of interest to geologists are GeoRef (produced by the American Geological Institute), GeoArchive (produced by Geosystems, London), and the earth sciences subfile of PASCAL (produced by the Centre de Documentation Scientifique et Technique, CNRS, Paris).

Online searching is very much quicker than manual searching of abstracting journals and allows more complex and complete searches to be carried out. The costs involved in online searching include database access fees, telecommunications charges, and the cost of equipment and staff time.

The Science Reference Library runs a publicly available online search service with access to over 50 scientific and technical databases. 1000 searches were carried out in the last financial year.
Marine geological databases: their nature, implementation and problems
Authors: Botbol J. M.
Published: December 2007

An overview of the marine geological database activity at the U.S. Geological Survey, Woods Hole, Massachusetts, is presented. Spatially-dependent, time-dependent, and topical data are collected. Major categories of data include navigation, seismicity, grab-samples and cores, photo and scanner, gravity, magnetics, physical oceanography and cruise management. Research projects in all categories generate large volumes of data that must be stored and retrieved. Seismic projects, for example, generate 700 tapes of raw seismic data each year. In order to locate data for further analysis and reference, a database index is being devized. Because of the large size of the overall database, data sets have been assigned to either active or archival status. Retrieval programs have proved to be more useful than a Data Base Management System (DBMS) in marine geologic database activity.

A major concern is the design of interactive application programs that treat retrieved data. The user interfaces are intentionally as uncomplicated as possible in order to facilitate user-computer dialogue. Data is regularly exchanged without side groups, and commercial bibliographic services employed.

Major database problems relate to the role of databases
vis-a-vis
project support and computer involvement, the relevance of data to other than original projects, generation of data on hybrid devices, effect of delays in data reduction, database user education, file maintenance and financial support.

Database activity requires support because, given modern computing advances, databases are the basis for major advances in geology.
Review of North American geological information systems
Authors: Bie S. W. , Gabert G.
Published: December 2007

A short review is given of North American geological information systems. Data systems for energy (notably petroleum), minerals, geophysics, commodity estimates of essential energy and mineral resources, physical planning (e.g. for erosion prevention, earthquake forecasting and land planning), computer-aided cartography, and ‘data-about-data’ are mentioned.
Conference Report
: Metamorphic studies: research in progress
Authors: Brown Michael
Published: December 2007

Report of the inaugural meeting of the Metamorphic Studies Group held at Burlington House, 5 March 1981. The meeting was organized by Dr M. Brown.

A Metamorphic Studies Group, jointly sponsored by the Geological Society of London and the Mineralogical Society of Great Britain, was formally established at an inaugural meeting on ‘Metamorphic studies: research in progress’ held at Burlington House on 5 March 1981. The meeting was attended by over 150 participants, ranging from professors to research students, who were rewarded with a programme of 18 papers of high quality which covered a wide range within the field of metamorphism.

A programme of topics arranged broadly in order of increasing temperature and pressure conditions was opened by Arny Sveinbjörnsdottir, one of two research students who contributed papers to the meeting. Her paper, o n water-rock interaction in Krafla (fluid derived from meteoric water) and Reykjanes (fluid derived from seawater), Iceland, both of which are high temperature hydrothermal systems, used log activity diagrams to relate alteration mineralogy to fluid composition and concluded that the present mineralogy in both systems appears to be in equilibrium with the fluid composition.

In the next 4 papers, all concerned with low-grade (or low-grade to medium-grade) metamorphism, the important role of the fluid phase in metamorphism was emphasized. Aguirre, Levi & Offler, in their contribution, demonstrated that fluid chemistry and temperature were the main variables which controlled the development of sequences of mineral assemblages characteristic of the zeolite facies, prehnitepumpellyite facies, prehnite-pumpellyite/greenschist facies transition and
Basic magmatism in Connemara, Ireland: evidence for a volcanic arc?
Authors: Yardley B. W. D. , Senior A.
Published: December 2007

It is found by the use of Pearce & Cann diagrams that basic magmatism during Dalradian sedimentation in Connemara was of a fundamentally different type to that which led to later synorogenic basic intrusions. The Dalradian magmatism is of a transitional within-plate to divergent plate margin type whereas the synorogenic intrusions are of volcanic arc type. It is suggested that the synorogenic basic intrusions of Connemara, Tyrone and NE Scotland represent the roots of a volcanic arc that developed by Arenig times over a northerly directed subduction zone.
The geological history of Maio, Cape Verde Islands
Authors: Stillman C. J. , Furnes H. , LeBas M. J. , Robertson A. H. F. , Zielonka J.
Published: December 2007

The oldest igneous rocks on Maio are pillow lavas of Mid-Ocean Ridge pillow basalts character which have been tilted and uplifted about 4 km from the ocean floor to outcrop as a partial ring, dipping steeply away from a central plutonic complex made up of pyroxenites, essexites, syenites and carbonatites. The ocean floor volcanic rocks are overlain conformably by a stratigraphically continuous pelagic carbonate succession which demonstrates a shallowing depositional environment from the Upper Jurassic to Upper Cretaceous times, when tuffaceous beds indicate renewed volcanism. The tuffs are associated with rudites demonstrating the emergence of the island and amongst the clasts are plutonics indicating Upper Cretaceous magmatism and the unroofing of the volcano to a substantial depth. Deformation under compressive stress resulted in the folding and local repetition by thrusting of this sedimentary cover, which, together with the plutonic core, had been intensively injected by major sills.

The Mesozoic succession has been planed off and overlain with marked unconformity by a largely Neogene sequence of volcanic and terrestrial sedimentary rocks. There is a hiatus throughout the Palaeogene, and constructional activity appears to recommence with ankaramitic hyaloclastite and lava deltas and subaerial ankaramitic flows. These are overlain by fluvial sediments and tuffs.

Stratigraphically above these is an extensive plateau of silica-undersaturated lavas, olivine-melilitites and nephelinites, which rest on a planed and locally lateritized surface. At topographically higher levels in the eastern part of the island there are thick ankaramitic lavas and pyroclasts which evidently flowed eastward through valleys cut down into the Mesozoic strata, and appear to be of Pliocene age.

The subsequent history of the island appears to be non-volcanic.