Showing papers in "Geological Magazine in 1972"
TL;DR: In this paper, a reconstruction of the tectonic history of part of the Benue Trough is made from a study of the marine transgressions and regressions in SE Nigeria.
Abstract: A reconstruction of the tectonic history of part of the Benue Trough is made from a study of the marine transgressions and regressions in SE Nigeria. Two periods of deformation involving essentially the interaction of continental plates are suggested. The oceanic crust produced by crustal attenuation in the Benue rift is very minimal. Unstable tectonic conditions in the Benue Trough may have created conditions for the development of an evaporite sequence.
274 citations
TL;DR: In this paper, the pre-Arctic ocean concept is reviewed in the light of the Arctic Caledonides and named Iapetus, and it is shown that the ocean was in existence up to early Ordovician but had substantially closed by mid Ordovian times.
Abstract: Consideration of the arctic configuration of the Caledonides leads to a distinction between eastern and western geosynclinal belts. The western belt, comprising the East Greenland, East Svalbard and southern Barents Sea Caledonides is postulated to continue northwards into the Lomonosov Ridge, whilst the western Spitsbergen Caledonides are thought to have originated as part of the North Greenland geosyncline which is also thought to continue northwards to form the western part of the Lomonosov Ridge. The eastern Caledonian geosynclinal belt comprising the Scandinavian Caledonides appears to swing eastwards to link with the Timan Chain and possibly the Urals.The already postulated (‘Proto-Atlantic’) ocean concept is reviewed in the light of the Arctic Caledonides and named Iapetus. Faunal provincialism suggests that the ocean was in existence up to early Ordovician but had substantially closed by mid Ordovician times. Possible relics of the suture marking the closure of this ocean suggest that it lay to the west of the Arctic Scandinavian Caledonides trending NE to latitude 70° N and thence veered eastwards separating the southern Barents Sea Caledonides from those of Arctic Scandinavia, possibly connecting with the northern Uralian ocean. A previous branch of the ocean may have separated East Svalbard and East Greenland as an ocean-like trough. A further (pre-Arctic) ocean may have existed to the north of the North Greenland–Lomonosov Ridge geosynclines. This is named Pelagus.The closure of these oceanic areas and the deformation of the bordering geosynclines delineates three principal continental plates, namely, Baltic, Greenland and Barents Plates. Their relative dominantly E–W motion up to Silurian times produced compression between the Greenland and both the Baltic and Barents plates but dextral transpression and transcurrence between the latter plates. In Late Silurian to Devonian times an increasing northward component controlled late Caledonian transpression and sinistral transcurrence between the Greenland plate and the combined Baltic and Barents plates.
258 citations
TL;DR: Although the structures and textures of spilites and keratophyres are comparable to those of fresh lavas, recent work has established that they did not acquire their present mineralogy at the time of extrusion as discussed by the authors.
Abstract: Although the structures and textures of spilites and keratophyres are comparable to those of fresh lavas, recent work has established that they did not acquire their present mineralogy at the time of extrusion. Furthermore, the compositions of many analyzed samples of spilites and keratophyres are shown to lie outside a ‘spectrum’ of compositions of all comparable fresh igneous rocks. A later metamorphism has therefore been responsible for their formation and has commonly been accompanied by metasomatism. This possibility of later metasomatism must be appreciated before conclusions are drawn that are based upon analysis of rocks of a spilitic or keratophyric mineralogy. This applies to age-dating and to analytically based petrographic and petrogenetic conclusions.
226 citations
TL;DR: In this article, the U-Pb isotopic systems of zircons from two samples of hypersthene granulite from the Kylesku group have been examined and it has been shown that a major isotopic event or events took place approximately 2700 m.y.
Abstract: The overprinting of the high grade rocks of the Scourian episode, during the Inverian and Laxfordian episodes, has so far prevented a reliable estimate being made of the age of the Scourian granulite–facies metamorphism in the Central Region of the Lewisian of northwestern Scotland. In an attempt to solve this problem the U–Pb isotopic systems of zircons from two samples of hypersthene granulite from the Kylesku group have been examined. These zircons have discordant apparent ages complicating the interpretation of the results. However, by measuring a number of size fractions from each of the two zircon suites it has been possible to demonstrate that a major isotopic event or events took place approximately 2700 m.y. ago in these granulite–facies rocks. It has also been shown that the two discordance trends found for the two zircon populations can be explained in terms of episodic disturbances of the U–Pb isotopic systems at times which correspond to the known ages of ‘Laxfordian’ and ‘Caledonian’ events.
98 citations
TL;DR: In this paper, the authors presented the ages from shale horizons in the Late Pre-Cambrian tillite succession in Varangerfjord, Northern Norway, for the Tanafjord Group.
Abstract: Rb–Sr whole-rock isochron ages from shale horizons in the Late Pre-Cambrian tillite succession in Varangerfjord, Northern Norway, are presented. Although data from the Stappogiedde Formation, which overlies the tillite horizons, give a Caledonian, metamorphic age of 530±50 m.y., ages obtained from shales occurring between and below the tillites are believed to represent the approximate times of deposition of the sediments. An age of 810±90 m.y. for the Tanafjord Group enables this group to be correlated with the Upper Torridonian and the age of 668±23 m.y. for the Nyborg Formation, which is taken as the age of the Varanger Ice Age, allows a tentative correlation to be made with the Egan and Marinoan Glaciation in Australia.
86 citations
TL;DR: In this paper, the authors measured en-echelon veins cutting small chevron folds in Carboniferous greywackes, near Hartland in Devon, and found that they may be initiated between 10° and 46° to the shear zone in which they occur.
Abstract: Measurements of en-echelon veins cutting small chevron folds in Carboniferous greywackes, near Hartland in Devon, allow the following new conclusions about naturally occurring second-order fractures to be stated. 1. Veins may be initiated between 10° and 46° to the shear zone in which they occur. Previous studies of en-echelon fractures have generally classified them as either tension gashes formed at 45° to an array, or Riedel shears at 15° to an array. 2. Veins at about 20° to 40° to an array probably occupy surfaces which are transitional between shear and extension fractures. 3. Proof of a component of shear, in addition to dilation, on many of the veins at less than 40° to an array is provided by crystal fibres at acute angles to vein margins, and third-order en-echelon veins in arrays which are parallel to and part of a set of second-order veins.
67 citations
TL;DR: Many deep water marine muds, including lower Palaeozoic mudstones from Britain, have thin graded beds in which mud and silt laminae alternate, with the silt becoming finer and less abundant upwards.
Abstract: Many deep water marine muds, including lower Palaeozoic mudstones from Britain, have thin graded beds in which mud and silt laminae alternate, with the silt becoming finer and less abundant upwards. Of the known deep-sea depositional processes, turbidity currents are the most likely cause of such graded laminated beds. The lamination may be produced by alternating cohesive and granular bed conditions. Much more careful examination of laminated fine grained terrigenous sediment is needed.
63 citations
TL;DR: In this article, two modes of occurrence are used for observation in Banffshire, northeast Scotland: interpretation, simultaneous exsolution and replacement, and simultaneous ex-solution.
Abstract: Observations, two modes of occurrence, Banffshire, northeast Scotland; interpretation, simultaneous exsolution and replacement
51 citations
TL;DR: In this paper, the results of three XRF chemical analyses and five K/Ar radiometric determinations were presented, and the radiometric dating and field evidence both suggest that the glaucophane schists are a result not of the Tertiary orogeny which affected this part of Spitsbergen, but of the earlier Caledonian orogenys.
Abstract: Glaucophane schists from Oscar II Land in western Spitsbergen are described for the first time. Although lawsonite, aragonite and pumpellyite have not been found, the mineral assemblages correspond generally with glaucophane schist assemblages known from elsewhere. The results of three XRF chemical analyses are presented, and five K/Ar radiometric determinations. The radiometric dating and field evidence both suggest that the glaucophane schists are a result not of the Tertiary orogeny which affected this part of Spitsbergen, but of the earlier Caledonian orogeny.
51 citations
TL;DR: The age of the British late Caledonian granites is known to lie in the range 390 m.y. to 370 m.m.y as mentioned in this paper, which is similar to that of the Cheviot granite.
Abstract: Potassium–argon ages of a variety of rocks and minerals from the Cheviot Hills, northern England, lie in the range 390 m.y. to 370 m.y. The Cheviot granite, dated at 380 m.y., has an age similar to that of many of the younger members of the British late Caledonian granites.
41 citations
TL;DR: In this paper, the authors described curved and other variably oriented minor F1 folds with a common axial-planar cleavage from the Dalradian of the South-West Scottish Highlands.
Abstract: Curved and other variably oriented minor F1 folds with a common axial–planar cleavage are described from the Dalradian of the South-West Scottish Highlands. An explanation for the varying orientations is offered in terms of progressive deformation following the arguments of Flinn (1962) and Ramsay (1967).
TL;DR: In this article, a modified terminology for erosional surfaces on rocky coasts is proposed, which is best developed on promontories and terminates in a low-tide cliff, and the wave ramp, often exposed in bays, which slopes gradually down from about high water level to merge with the submarine slope near-shore.
Abstract: Examination of rocky coastal profiles in Australia and Europe suggests that two major and genetically distinct types of erosional surface should be recognized, one of these being the shore platform, whether sloping or horizontal, smooth or rough, which is best developed on promontories and terminates in a low-tide cliff, the other being the wave ramp, often exposed in bays, which slopes gradually down from about high water level to merge with the submarine slope near-shore. A modified terminology for erosional surfaces on rocky coasts is proposed.
TL;DR: Tuffs, trachybasalts, aegirine-augite trachytes and phonolitic trachyte occur on the continental shelf at the southern tip of Africa and have been radiometrically dated at 58(±2.4) million years as discussed by the authors.
Abstract: Tuffs, trachybasalts, aegirine–augite trachytes and aegirine–augite phonolitic trachytes occur on the continental shelf at the southern tip of Africa. They have been radiometrically dated at 58(±2.4) million years (Palaeocene), and together with other alkali-rich basic intrusions onshore form the Alphard Tertiary Igneous Province. Both onshore and offshore sites of intrusion appear to be strongly tectonically controlled.
TL;DR: In this article, the authors used the potassium-argon method to date 34 clay-mineral concentrates from 34 samples and concluded that repeated hydrothermal alteration of the clay minerals, reflecting at least two episodes of mineralization, one about 270 m.y.
Abstract: Episodic hydrothermal mineralization has previously been recognized in the northern section of the Pennine orefield. Igneous rocks from the southern section (the Derbyshire orefield) have yielded isotopic ages, some of which are thought to represent ages of hydrothermal metasomatism (deuteric or subsequent). In order to ascertain whether epicyclic hydrothermal events gave rise to the Derbyshire mineral deposits, samples of highly altered doleritic lava and pumice tuff were collected adjacent to areas of mineralization. Clay-mineral concentrates from 34 samples were dated by the potassium–argon method. The conclusions drawn from these analyses support a hypothesis of repeated hydrothermal alteration of the clay minerals, reflecting at least two episodes of mineralization, one about 270 m.y., the other about 235 m.y. The geo-chronological significance of these and other results is considered.
TL;DR: Biostratigraphic correlation of two boreholes drilled into sediments; foraminifera and palynomorphs, England as discussed by the authors, was found to be the case for both boreholes.
Abstract: Biostratigraphic correlation of two boreholes drilled into sediments; foraminifera and palynomorphs, England
TL;DR: Local helical flow, moderately powerful current, mechanism possibly flood tidal current, Wales.
Abstract: Local helical flow, moderately powerful current, mechanism possibly flood tidal current, Wales
TL;DR: In this paper, the authors calculated values of quartile deviations (QDa) and medians (Mdmm) from over 400 grain-size distributions of modern and ancient turbidites.
Abstract: Values of quartile deviations (QDa) and medians (Mdmm) have been calculated from over 400 grain-size distributions of modern and ancient turbidites. Each QDa–Md pair is plotted on double-log paper to establish any distinctive trends. The QDa–Md analysis of modern turbidites reveals a steep gradient trend derived from the grain-size distributions of submarine canyon and delta fan sediments, and a shallow gradient trend derived from the grain-size distributions of samples from oceanic bottoms and nepheloid water layers. The QDa–Md analysis of ancient turbidites reveals two trends, but these do not coincide with their modern counterparts. The steeper gradient trend is related to distal and proximal turbidites, while the shallow trend is related to fluxoturbidites. The disparity between the ancient and modern QDa–Md analyses is caused by textural modifications of turbidites by diagenetic disintegration of unstable minerals. The alteration products are incorporated subsequently in the matrix. The diagenetic effects on the positions of the QDa–Md plots are demonstrated theoretically by computing the textural alteration of six modern turbidites by assuming that they will lose 20%, and then 40%, of their sand-sized particles to the matrix. By working in reverse the QDa–Md plots of ancient turbidites can be ‘restored’ to their diagenetically unaltered positions. Their restored positions correspond to the QDa–Md trends of modern turbidites.
TL;DR: The lacinia in Paraglossograptus Mu is shown to consist of four ladder-like networks which are independent of each other and have no direct connection with spinose thecal apertural processes.
Abstract: The lacinia in Paraglossograptus Mu is shown to consist of four ladder-like networks which are independent of each other and have no direct connection with spinose thecal apertural processes. The lacinial networks are positioned at the corners of the quadrangular rhabdosome as in Paraglossograptus ? sp. of Whittington & Rickards, which is described here as Paraglossograptus proteus (Harris & Thomas). Paraglossograptus etheridgei (Harris, 1924), the important Ordovician zone fossil, together with P. typicalis Mu and P. latus Hsu, are considered junior synonyms of Reteograptus tentaculatus J. Hall, 1865. P. tentaculatus (J. Hall, 1865), is regarded here as the type species of Paraglossograptus Mu. Detailed comparisons are made between P. tentaculatus and the stratigraphically younger P. proteus (Harris & Thomas), and with Glossograptus holmi Bulman, a species in some ways intermediate between Paraglossograptus and Glossograptus Emmons.
TL;DR: A new genus and species of multiramous dichograptid from the Bendigonian and a new species of Tetragraptus from the Lancefieldian are described.
Abstract: The paper comprises descriptions of a new genus and species of multiramous dichograptid from the Bendigonian (Be 4) and a new species of Tetragraptus from the Lancefieldian (La 2).
TL;DR: The reference of Isograptus manubriatus to the genus Maeandrograptu Moberg by Skevington (1968) is argued to be invalid and a new genus, PseudisograPTus has been introduced to which the manubRIate isograptonids I. manubrianatus, I. hastatus, and I. dumosus are referred.
Abstract: Isograptus manubriatus, I. hastatus, and I. dumosus referred to new graptolite genus Pseudisograptus, Paleozoic, Australia
TL;DR: The junction in Newlands Beck between the steeply dipping Skiddaw Slates striking N-S, and Borrowdale Volcanic rocks dipping gently to the SE is interpreted as an angular unconformity, and indicates the existence of pre-Borrowdale VOLCANIC folding as mentioned in this paper.
Abstract: The junction in Newlands Beck between the steeply dipping Skiddaw Slates striking N–S, and Borrowdale Volcanic rocks dipping gently to the SE is interpreted as an angular unconformity, and indicates the existence of pre-Borrowdale Volcanic folding There is no cleavage associated with this folding, however, the ENE–WSW Caledonoid cleavage being the only one present, and this is common to both Skiddaw Slates and Borrowdale Volcanics
TL;DR: Cynodonts from two localities in Lesotho have been assigned by earlier workers to the Triassic Molteno beds of the upper part of the Karroo (Gondwana) System.
Abstract: Cynodonts from two localities in Lesotho have been assigned by earlier workers to the Triassic Molteno beds of the upper part of the Karroo (Gondwana) System. These are important inasmuch as they were thought to represent the only evidence of reptilian remains in an otherwise barren sequence and may help to bridge the palaeontological gap in this part of the Karroo succession. Critical examination of both localities during the course of a regional analysis of the Molteno basin showed discrepancies in stratigraphic interpretation. These discrepancies are pointed out and the stratigraphy revised as a result of which the cynodonts can no longer be assigned to the Molteno beds; one is in the underlying Cynognathus zone (confirmed by further finds of typical Cynognathus zone reptiles) and the other is in the Red beds above. At the Tsoaning River locality a pebble conglomerate resting on an eroded surface provides a good local marker for the base of the Molteno. The implications of this with respect to the fossil flora and fauna at these localities are briefly discussed.
TL;DR: The 18 curve from the 1390 m long ice core from Camp Century, Greenland, shows climatic changes that are easily correlated with known glacial and non-glacial events of North America and north Europe and are thus indirectly dated as mentioned in this paper.
Abstract: The 18 curve from the 1390 m long ice core from Camp Century, Greenland, shows climatic changes that are easily correlated with known glacial and non-glacial events of North America and north Europe and are thus indirectly dated. With a known chronology, the glacial dynamic changes of the Greenland Ice Sheet can be calculated for the last 125,000 years. It is concluded that the dynamics of the Greenland Ice Sheet have changed drastically during this period and that these changes are directly related to major changes of climate and extension of the Wisconsin and Weichselian glaciations. Logarithmic time scales earlier applied to this curve must therefore be incorrect.
TL;DR: The authors pointed out that metamorphic rock compositions can be significantly modified by the stable mineralogy that itself may provoke metasomatic movement, as distinct from externally actuated metasomatism.
Abstract: It is pointed out, with a few examples, that metamorphic rock compositions can be significantly modified by the stable mineralogy that itself may provoke metasomatic movement, as distinct from externally actuated metasomatism.
TL;DR: Caledonian orogeny, "way-up" criteria in lavas used to determine geometry of large-scale Stavfjord Anticline, regional extension, and axial plane trace.
Abstract: Caledonian orogeny, "way-up" criteria in lavas used to determine geometry of large-scale Stavfjord Anticline, regional extension, and axial plane trace
TL;DR: In this paper, an agreed short text in English, French and German is presented to avoid philosophical hazards and misunderstandings between the three language communities; it also harmonizes differences of three national traditions as far as possible.
Abstract: An agreed short text in English, French and German, presents a unified statement on those aspects of stratigraphy that require formal international acceptance. The statement is designed to avoid philosophical hazards and misunderstandings between the three language communities; it also harmonizes differences of three national traditions as far as is yet possible.Stratigraphy is presented as an indivisible science dependent for its successful pursuit and synthesis on the application of numerous widely different methods.The argument is presented in successive numbered steps to facilitate its critical discussion.