bluuebunny/crossref_metadata_2025_split huggingface dataset:
Do seismic reflections necessarily have chronostratigraphic significance?
Authors: Tipper John C. 
Published: May 2009

AbstractMany seismic reflections from within sedimentary successions are thought to be generated along stratal surfaces because those surfaces are laterally continuous and have marked acoustic impedance contrasts. As stratal surfaces are isochronous, those reflections are then also taken as being chronostratigraphically significant. In contrast, seismic reflections are thought not to be generated along the boundaries of lithostratigraphic units because those boundaries are discontinuous and gradational. Nevertheless, synthetic seismic analysis shows that seismic reflections should in many circumstances be expected to follow lithostratigraphic unit boundaries, not stratal surfaces. As these lithostratigraphic unit boundaries will generally be diachronous, seismic reflections from within sedimentary successions should evidently not be treated as necessarily having chronostratigraphic significance.
The stratigraphy and setting of the Precambrian rocks of the Llangynog Inlier, Dyfed, South Wales
Authors: Cope J. C. W. , Bevins R. E. 
Published: May 2009

AbstractDetailed mapping of the Precambrian rocks of the Llangynog Inlier, made possible by a series of new exposures, has allowed the establishment for the first time of the 1100 m thick succession within the Precambrian rocks of the area. All boundaries of the Precambrian rocks with younger strata in the area are either faults or unconformities. Some 350–400 m of rhyolitic lavas form the oldest rocks seen in the area; they are extensively silicified and devitrified and show little of their original texture, although flow-banding is visible in some outcrops and the presence of perlitic and snowflake textures imply that originally they were glassy in part. These lavas are succeeded by a varied succession dominated by volcaniclastic siltstones showing evidence of shallow water deposition and containing medusoids and trace fossils of Ediacaran age. Interbedded in the sediments are basaltic lavas, autobreccias and rare hyaloclastites displaying textural features characteristic of submarine eruption, and rhyolitic pumiceous and shardic ash-flow tuffs. The name Coomb Volcanic Formation is proposed for this succession, within which are recognized two members, a lower Castell Cogan Rhyolite Member succeeded by the Coed Cochion Volcaniclastic Member. In association are varied intrusions, comprising both dolerites and dacites. A palaeoenvironmental model, built on evidence of petrography, palaeontology and sedimentology is that of shallow water to emergent volcanic islands.
An oceanic volcano in an island arc setting – Seatura Volcano, Fiji
Authors: Hindle W. H. , Colley H. 
Published: May 2009

SummarySeatura is a Pliocene caldera-type, shield volcano situated in SW Vanua Levu, Fiji. Shield-building basaltic lavas are predominantly subaerial though there is a shoreline facies of fossiliferous sediment, pillow lava and hyaloclastite. Late-stage events include the intrusion of a dyke swarm in and around the caldera, and deposition of laharic conglomerates. Chemically the rocks are intermediate between tholeiite and alkali basalt and have intraoceanic levels of TiO2. Late-stage derivatives include benmoreite, trachyte, and propylitically altered rocks. The parent magmas are thought to have been partial melts of undepleted mantle at relatively deep levels. Tapping of these deep levels may have been facilitated by extensional plate tectonic conditions existing in the Fiji area during the development of Seatura Volcano.
Gravity study of Lower Benue Trough, Nigeria
Authors: Adighije C. I. 
Published: May 2009

SummaryAbout 300 gravity stations were established at 2–4 km intervals along an E-W profile across the Lower Benue Trough from Gboko-Nsukka-Auchi with the main objectives of investigating the nature and configuration of the basement complex underlying the trough and estimating the thickness of the sedimentary cover rocks. The resulting Bouguer anomaly profile shows a series of alternating positives and negatives. For a preliminary interpretation, the positive anomalies are attributed to shallow basement and the negative anomalies to sediment-fill of maximum thickness, 4250 m.
Application of backscattered electron microscopy in shale petrology
Authors: Krinsley D. H. , Pye K. , Kearsley A. T. 
Published: May 2009

SummaryA grey pyritic mudstone from Central Wales (Red Vein unit of the Dicellograptus anceps zone, Upper Ordovician Ashgill) has been examined in thin section by scanning electron microscopy using backscattered electrons. Using backscatter it is possible to identify individual mineral constituents of the mudstone by virtue of their atomic number (Z) contrast and differential hardness (relief). The amount of detail observable is far greater than that possible with optical microscopy. Valuable information can be obtained relating to particle form, orientation, texture and internal structure which aids in interpretation of the deformational and diagenetic history of the rock. The adoption of electron microscopical methods in the study of thin sections and polished rock chip surfaces promises to revolutionize the field of shale petrology.
Late Precambrian resedimented conglomerates from Bellsund, Spitsbergen
Authors: Waddams P. 
Published: May 2009

SummaryLate Precambrian polymict conglomerates are described from Bellsund, western Spitsbergen, where they are intimately associated with authentic glacigenic sediments. They display normal, compound and reverse grading. Spectacular flame, injection and incipient rip-up structures are developed at the base of some beds. The conglomerates were initially deposited either as foreset debris beneath a wet-based glacier, or on the subaqueous part of a glaciomarine outwash delta, and were subsequently redeposited from mass gravity flows initiated by slumping on the unstable glacial detritus slope. An informal stratigraphic nomenclature is proposed for the Kapp Martin area to the north of Bellsund and the Kapp Lyell area to the south.
Correlation of Late Proterozoic tillites in the North Atlantic region and Europe
Authors: Hambrey M. J. 
Published: May 2009

SummaryWell-established Late Proterozoic tillites are widely distributed in Svalbard, East and North Greenland, the Norwegian and Swedish Caledonides, Scotland, northwestern Ireland, western USSR, the Urals, Newfoundland and the Appalachians. Recent biostratigraphic and geochronometric data have provided the following sequence of glacial events. The earliest evidence of glaciation is represented by tillites in Newfoundland, Scotland and possibly NE Svalbard, and may have taken place in Late Riphean (sensuVidal) or Sturtian (sensuHarland) time, perhapsc.800 Ma ago. A glacial period in Vendian time, the Varangian, centred on 650 Ma was a much more severe climatic event, represented in all the above mentioned areas except Newfoundland. Two Varangian glacial epochs are recorded in Svalbard, East Greenland, northern Norway and possibly western USSR, and although most of the deposits are marine, terrestrial conditions prevailed in some areas with associated periglacial phenomena. The earlier epoch was completed before 650 Ma; the later one occurred within the rangec.650–610 Ma.A tectonic reconstruction of the region places eastern Svalbard adjacent to East Greenland, which has implications for major strike-slip motions in late and post-Caledonian times. A discrete ice-sheet is envisaged, centred on the western USSR (terrestrial tillites) with ice flowing out to the then peripheral area of the northwestern Europe continental margin and to the Urals where shallow platformal glaciomarine sediments accumulated.
Geochemistry and relationships to mineralization of granitic rocks from the Afu Younger Granite Complex, Central Nigeria
Authors: Imeokparia E. G. 
Published: May 2009

SummaryGeochemical analyses of fresh bedrock samples indicate that the Afu biotite granites are characterized by enhanced values of a suite of trace elements, including Sn, Nb, W, Zn, Rb, Li, F th, Y and depleted in Ba Sr, and Mn. These geochemical data are also examined by R-mode factor analysis with the primary objective of isolating the significant factors accounting for the sample composition as derived from mineralization, alteration and lithology.The Orthogonal varimax solution yields a five-factor model that accounts for 76% of the total data variance. Factors 1 (Nb, Rb, Li F) and 3 (Sn, W) can be attributed to the effect of mineralization and postmagmatic alteration; while Factor 2 (Mn, Sr, Ba Zr) is lithological, associated with barren rocks, unaffected by metasomatism. Factor 4 (Th, Y, Ga) and Factor 5 (Pb and Zn) are also associated with prominent mineralization. Thus the mineralized granites are characterized by high concentrations of Sn, Nb, W, Zn, Rb, Li F, Th, Y and Pb but ore formation is considered a product of postmagmatic processes.
Stratigraphy of the Mesozoic rocks above the Semail Ophiolite, Oman
Authors: Woodcock N. H. , Robertson A. H. F. 
Published: May 2009

SummaryA revised stratigraphy is presented for the rocks which lie between the Upper Cretaceous Semail ophiolite and its post-emplacement sediment cover of uppermost Cretaceous to Tertiary age. Above the pelagic sediments of the ophiolite (Suhaylah Formation, Cenomanian–Turonian) occur ophiolite-derived clastic sediments (Zabyat Formation, ?Turonian–?Campanian) related to early stages of ophiolite emplacement. These formations are structurally overlain by the Batinah Complex, comprising Triassic to Cretaceous lithologies, all previously equated with the Hawasina rocks that now lie tectonically beneath the ophiolite. We recognize a lower melange unit to the complex, which demonstrably includes Hawasina lithologies. The melange formed by superficial rather than deep processes, but the mechanism by which Hawasina material was supplied to the ophiolite surface is unproven. The upper unit of the Batinah Complex shows a coherent stratigraphy which contrasts with that of the Hawasina. It comprises three main allochthonous tectonic sheets, within which we recognize three formations and one other informal unit. The upper Batinah Complex may have been tectonically derived not from the now subjacent Arabian margin but from another margin with a passive Mesozoic history.
Benue Trough and Benue Chain
Authors: Benkhelil J. 
Published: May 2009

SummaryIt is suggested that the Benue Trough may have been initiated by the extension of the equatorial oceanic fracture zones onto the continent during the early stages of the separation of Africa and South America. A transcurrent movement along deep-seated faults of the basement is responsible for the structure of the trough. After a period of extension characterized by the sedimentary infilling of the trough and igneous activity, the trough is turned into a chain through a short phase of compression. A discrete pattern of ‘en echelon’ folds and the strain pattern deduced from the analysis of large scale and microfracturing correspond to a N–S to N160° E direction of compression coupled to a sinistral transcurrent movement along a NE–SW direction. The difference in age of the folding phase in the northern and southern parts of the chain indicates a diachronous nature for this tectonic event in relation to the wrench regime. The Tertiary evolution of the trough marked by a tensional regime resulting in an E–W extension is to be related to the Cameroon Volcanic Line.
An early Permian cool-water fauna from pebbly mudstones in south Thailand
Authors: Waterhouse J. B. 
Published: May 2009

SummaryA small brachiopod fauna of early Permian age is recorded from pebbly mudstones and sandstones of the Phuket (Kaeng Krachan) Group at the Thai islands of Ko Muk, and Ko Phi Phi in the Andaman Sea. Most species belong to genera found in temperate to high Permian palaeolatitudes, whereas other Permian faunas from Thailand are strong in tropical affinities, suggesting that the pebbly mudstones are cool water deposits, contemporaneous with the late Asselian glacial deposits of Gondwana. New taxa are Arctitreta percostata sp.nov., Komukia solita gen.nov., sp.nov. (Rugosochonetidae), Cancrinelloides monticulus sp.nov., Rhynchopora culta sp.nov., Arionthia sapa sp.nov. and Elasmata retusus gen.nov., sp.nov. (Heterelasminidae).
Ultrastructural studies of graptolites – a review
Authors: Rickards R. B. , Crowther P. R. , Chapman A. J. 
Published: May 2009

SummaryA reinterpretation is attempted of the 25 years of graptolite ultrastructural work based upon the present state of knowledge. It is emphasized that studies need to embrace many more species of graptolites, particularly in the dichograptid groups, and with different growth stages, before the gross evolution of ultrastructural elements can be satisfactorily appraised. The factual (morphological, ultrastructural) data are themselves reappraised, summarized, and very tentatively related to Bulman's anagenetic grades of graptolite development.
Preservation of shell microstructure in silicified brachiopods from the Upper Cretaceous Wilmington Sands of Devon
Authors: Holdaway H. K. , Clayton C. J. 
Published: May 2009

SummaryFrom a study of silicified fossils, and in particular the microstructure of brachiopods, from the Wilmington Sands (Upper Cretaceous) of Devon, a model of skeletal silicification is proposed. Three distinct morphologies of silica were formed, controlled by the relative rates of silica supply and carbonate dissolution: (a) a fine-scale replacement of the original shell microstructure where silica was abundant; (b) a concentric ring morphology called ‘beekite’ where silica supply was limited, and (c) a granular white crust formed where carbonate dissolution was restricted. Silicification occurred during early diagenesis as a result of bacterial decay of organic matter intimately associated with skeletal fragments, within a sediment of restricted permeability. A build-up of CO2probably caused dissolution of skeletal carbonate, and bicarbonate released from this caused local precipitation of silica. The proposed mechanism is belived to be of general applicability to micrite-rich carbonate sediments.
The Devonian–Carboniferous transition and its structural setting at Chillaton, West Devon, England
Authors: Selwood E. B. , Stewart I. J. , Turner P. J. , Whiteley M. J. 
Published: May 2009

SummaryBoreholes penetrating the inverted limb of a major fold in an allochthonous thrust sheet at Chillaton, West Devon, revealed highly fossiliferous strata spanning the Devonian–Carboniferous boundary in continuous succession. Details of the fauna, which includes conodonts, trilobites and ostracods, are given. Abundant siphonodellid conodonts allow the boundary to be determined according to the recommendations of the working group on the Devonian–Carboniferous boundary of the IUGS Commission on Stratigraphy. The nature of the imbricate zone beneath the Main Thrust, in which the succession is situated, is described. In addition, the fabrics associated with thrusts which cut the olistostromic succession lower in the tectonic unit are discussed.
Authigenic albite in the Pendleside Formation (Dinantian) of northern England
Authors: Turner P. , Ashworth J. R. , Iskenderian F. A. 
Published: May 2009

SummaryAuthigenic albite occurring as euhedral overgrowths up to 400 μm in size is a characteristic feature of the Pendleside Limestone in the Craven Basin of northern England. The albite is very pure, low-albite in keeping with other authigenic albites in carbonate rocks. The detrital cores of the albites are angular plagioclase grains of approximately An20 composition which are sometimes altered to kaolinite. The albite crystals with their detrital cores are frequently concentrated in discrete laminae, suggesting that the plagioclase cores may have been supplied by a basic volcanogenic source. This view is supported by geochemical studies of limestones which reveal significant correlation between K–Na, Na–Al and K–Al.Interstitial alteration of detrital components, in particular the reaction intermediate plagioclase → kaolinite + albite, was primarily responsible for albite authigenesis. The initial products of this reaction probably included smectite which fixed Mg2+ in the decomposing volcanic grains. Subsequent alteration of the smectite resulted in the formation of small microdolomite inclusions which are commonly seen in the authigenic albites. An equilibrium model in terms of Na+/H4 ratios and H4SiO4 activity is proposed for the silicate assemblages in the Pendleside Formation.
Structural evolution in the Moine of northwest Scotland: a Caledonian linked thrust system?
Authors: Butler Robert W. H. 
Published: May 2009

AbstractA model is proposed whereby the Caledonian metamorphic basement-cover complex of northwest Scotland (the Moine) is considered as a linked thrust system. This system lies between the Moine thrust at its base and the Naver–Sgurr Beag slide at its top. Ductile fold and thrust zones, which developed at mid crustal levels at metamorphic grades from greenschist to amphibolite facies, are interpreted as decoupling from a detachment presently situated at relatively shallow depths. This model is illustrated by two preliminary balanced cross-sections. These imply shortening across the northwest Scottish Caledonides in excess of 130 km and probably over 200 km. When these structures are restored onto a crustal template a considerable quantity of lower crust is found to be required at depth. The most likely location for the lower crustal wedge is beneath the Grampian Highlands.
A Hoxnian interglacial doline infilling at Slade Oak Lane, Denham, Buckinghamshire, England
Authors: Gibbard P. L. , Bryant I. D. , Hall A. R. 
Published: May 2009

AbstractSite investigations for the M25 motorway at Denham proved a maximum depth of 37.5 m of deposits filling an enclosed hollow up to 40 m across beneath a small seasonally dry valley near Higher Denham. This previously unknown very steepsided hollow is infilled with gravel and sand, partially laminated clayey silt and pebbly clay. Within these deposits a bed of organic clay mud up to 4.5 m in thickness is present. Resistivity survey of the hollow proves that it is completely enclosed.Palaeobotanical investigation shows that the organic sediment contains fossil pollen, spores and macroscopic remains. The pollen spectra indicate that the sediments accumulated during the later half of the Hoxnian interglacial Stage (Ho III/IV). Macroscopic remains include ‘relict’ species Dulichium arundinaceum, and Brasenia schreberi, the latter being new to the Hoxnian Stage. The evidence for waterlevel fluctuation during the interglacial at the site is discussed.The origin of the hollow is considered and the conclusion reached is that it is a doline formed by solution of the underlying Chalk bedrock. The infilling of the basin is attributed to collapse, solifluction and slope wash of local material under cold climates before and after the interglacial. The interglacial sediment is a shallow water pool accumulation. The hollow is thought to have been formed originally as a solution pipe beneath the Thames' Gerrards Cross Gravel, rapid drainage of a neighbouring glacial lake causing the major collapse in the late Anglian Stage. Subsequent infill took place during the later part of the Anglian, Hoxnian and Wolstonian Stages and possibly even later.Heavy mineral analyses of the silt-rich sediment above and beneath the interglacial deposits are appended and indicate that it is almost all of local Reading Beds material.
Composite ichnofabrics and tiering of burrows
Authors: Bromley R. G. , Ekdale A. A. 
Published: May 2009

AbstractInfaunal communities in marine environments typically are tiered; that is, different taxa live at different depths within the sediment. Tiered suites of biogenic structures yield complex biogenic sedimentary fabrics (ichnofabrics), with the traces of deep-burrowing organisms overprinted on those of shallow-burrowing organisms. Careful analysis of crosscutting relationships of burrows in such composite ichnofabrics allows reconstruction of the tiered nature of fossil endobenthic communities. It is important to recognize that the best preserved and most prominently displayed trace fossils in most assemblages usually represent the deepest tier. Thus, they were farther removed from the sea floor and therefore less indicative of actual sea floor conditions than the more poorly preserved traces of the shallower tiers, on which the deeper traces are juxtaposed.
Geochemical and glacio-geomorphic implications of basalt weathering in the Queen Maud Mountains, Antarctica
Authors: Talkington R. W. , Mayewski P. A. , Gaudette H. E. 
Published: May 2009

SummaryThe weathering of a suite of basalt clasts, that have been transported by mass wasting downslope in the Cumulus Hills region of the Queen Maud Mountains, Antarctica, is examined from both geochemical and glacio-geomorphic viewpoints. Chemical weathering, predominantly oxidation and hydration, increases in severity from clast core to rim for the suite. These weathering processes and concomitant formation of a weathering rind are suggested to be an accumulative process, culminating in the disaggregation of the rock due in part both to the chemical breakdown of the interstitial and intersertal basaltic glass and to physical weathering processes.Mass wasting rates in the range of 1.4 × 10−3−1.13 × 10−1cm. y−1are estimated, assuming that the style of transport is dry creep. These transport rates suggest that the rock-surface/air-temperature differences in the study area may have been as little as 0.5 °C and heating and cooling cycles as few as 1 cy. y−1for the last 4.2 Ma.
Submarine ‘crystal tuffs’: their origin using a Lower Devonian example from southeastern Australia
Authors: Cas R. A. F. 
Published: May 2009

Summary. The submarine crystal-rich volcaniclastics of the Lower Devonian Merrions Tuff contain concentrations of angular to euhedral volcanic quartz, plagioclase and orthoclase. Lithic fragments, largely altered vitriclasts, are minor components and the average matrix content is 37.6%. Associated dacite/andesite and rhyodacitic lavas have average groundmass contents of 64.2%. Rare shards in the graded, pelitic tops of the thick volcaniclastic sedimentation units suggest that the mode of fragmentation originally was by explosive magmatic eruptions. The sedimentology of the volcaniclastics suggests subsequent redeposition by cold-state mass-flow processes. The volcaniclastics form an isotopically coherent suite and so redeposition must have occurred essentially contemporaneously with eruption.The high crystal fragment concentration in these volcaniclastics is higher than lavas and ignimbrites and suggests some process whereby the groundmass fraction of the erupting magma is selectively removed, so concentrating the crystal fraction. The crystal-rich character of the crystal-tuffs is not simply due to explosive eruption. Several primary and secondary factors/processes could have interacted to ultimately produce the crystal-rich character, these being: (i) eruption of highly crystallized magmas (≤ 65% phenocrysts), (ii) concentration of crystals in primary eruption columns, (iii) concentration of crystals in any resulting pyroclastic flows, fine vitric ashes being elutriated out and being carried away in accompanying, trailing ash clouds, (iv) concentration of crystals in secondary eruption columns generated by the flow of hot pyroclastic flows into the ocean, and (v) concentration of crystals by the elutriation of fines into the trailing fine sediment cloud accompanying submarine mass flows resulting from the slumping of volcaniclastic aggregates from shallow marine/subaerial settings.