Showing papers in "Quarterly Journal of The Geological Society in 1958"

The structural and geomorphic evolution of the dead sea rift

Abstract: The two methods of projected profiles and reconstructed stream profiles are applied in a geomorphic study of the region on both sides of the Dead Sea Rift. This supplements an earlier study of the tectonics. There are three recognizable surfaces at high levels. The earliest is cut partly in Eocene marine sediments. On both the Arabia and the Sinai–Palestine blocks the third surface has been well preserved and extends far into central Arabia. Warping, tilting and local uplift which affected these early erosion surfaces are related to the first phase of horizontal movement on the wrench-faults of the Rift. Along some sectors tendency to oblique overthrusting and underthrusting caused the downwarping or upwarping of the margins of the blocks ; along others the margins separated and there was an absence of distortion. There followed a period of quiescence, and base-level changed negatively and then remained stationary on more than seven occasions. The last major still-stand was interrupted by a second phase of horizontal movement on the Rift faults, which is continuing. Except for the outpouring of the plateau basalts of the Hauran of southern Syria, volcanism in the region was confined almost entirely to the Arabia block south of Amman. It was characterized by short-lived and occasionally violent eruptions. It began shortly after the initial warping of the early surfaces.

The sedimentation and sedimentary history of the aberystwyth grits (upper llandoverian)

Abstract: The greywacke beds were deposited from turbidity currents in a region within the mud belt. These currents came from an area lying to the south-west and south-south-west, its configuration varying with time. Variation in type of deposit is correlated with distance from land, and evidence of successive rejuvenations of the source area is presented. Sedimentary injection dykes occurring in the south are held to prove that some currents were set in motion by earthquakes, but slumping from the edge of the sedimentary terrace caused by loading, together with the action of storms (in the south), probably also assisted in the transfer of material to deeper water.

Geology of the reydarfjördur area, eastern iceland

Abstract: A succession of nearly 15,000 feet of volcanic rocks, mostly plateau-basalt lavas, is described from the Reydarfjordur area. This area is in the middle of the large Tertiary outcrop of eastern Iceland, and the rocks described constitute the lower parts of the exposed succession there. They dip uniformly west-south-west at 3–7 degrees. Tholeiites, olivine-basalts, and porphyritic basalts rich in phenocrysts of basic plagioclase make up the bulk of the lava pile, and there is little sign of any systematic distribution of these types. Lavas of a particular type tend to form groups of flows, and these can be readily mapped in the field and clearly have a wide horizontal extension. Two long periods of acid volcanism are distinguished, both having given rise to a considerable bulk of acid and intermediate lavas and pyroclastic rocks. Two more acid volcanic episodes are represented in the mapped ground by acid tuffs, and a fifth is represented by the copious outpourings of acid and intermediate lavas and pyroclasts of the Thingmuli central volcano which was later built on the platform of plateau lavas described in this paper. The bulk of the lavas are regarded as the product of fissure eruption, and two examples have been found of a lava connected to its dyke feeder. The dyke swarm in the area is estimated to contain over 500 dykes, mostly basic, with a general northerly trend.

The relationship of the red sea to the east african rift system

Abstract: The Red Sea is characterized by positive gravity anomalies along its centre, whereas the East African Rift Valleys have negative anomalies. The Red Sea trough is more than 200 miles wide but the gravity maximum appears to be associated with a deep trough (about 30 to 40 miles wide), the anomalies on each side being small. A table is given of gravity data for the Red Sea area, and the values have been revised so that they are directly comparable. To assist in the interpretation of these data a brief review is given of the dimensions of the Rift Valleys, and the general geological structure and history of the area. The interpretation of negative anomalies over the East African Rift Valleys is discussed and the Red Sea anomalies are described and contrasted with those of the East African Rifts. The gravity maximum is interpreted as being due to a large basic mass intruded through the down-faulted basement rocks. It is suggested that the formation of the fault trough which the Red Sea occupies and the deep, relatively narrow axial trough is connected with the intrusion and a possible mechanism is given.

Basic palaeogeography of gondwanaland during the late palaeozoic and mesozoic eras

Abstract: The mighty sequences of late Palaeozoic and Mesozoic continental sediments characteristic of Gondwanaland are reviewed and interpreted palaeoclimatically. The glacial and succeeding beds are not susceptible of explanation on the simple hypothesis of a “Permo-Carboniferous” refrigeration—actually the glacial episode had passed in western Argentina before it began in Australia and. climates in western Gondwana were warming up after glaciation while those in the east were cooling towards refrigeration. From the Carboniferous period until the Jurassic, facies divisions within the continental rock sequences cross the time divisions progressively from weat to east. The palaeoclimatic data are plotted on a series of restorations of Gondwanaland that had been independently derived several years before from a study of the pattern of major sub-oceanic ridges which had suggested successive positions for the reassembly on the globe. The data so plotted are found to fall into climatic girdles consonant with latitude that are precisely similar to the major girdles—frigid, cold, temperate and tropical—of the present day. This agreement of the palaeoclimatic data with modern conditions is of such an order as to verify both the accuracy of the reassembly and the positions for it deduced previously from the pattern of mid-oceanic ridges.

Magmatic differentiation and alteration in the st. austell granite

Abstract: Mineralogical and chemical evidence enables the St. Austell granite to be divided into four main varieties, of which the earliest probably constitutes a separate intrusion. The distribution of minerals and chemical components makes it clear that variation is due to magmatic differentiation. Suggestions as to the causes of albite enrichment and fine grain in the later varieties are offered. Further evidence shows that the granite has been altered by both late magmatic and post-magmatic processes. The latter are related to the joint system, which forms part of a regional pattern. Greisening, a late magmatic process, was initiated by aqueous residual solutions containing small amounts of boron and fluorine and was controlled by pressure, temperature, acidity and the concentration of various elements. Kaolinization, the important post-magmatic process, resulted from the attack on the solid rock of solutions, doubtless from unconsolidated magma below, escaping through the joints. These solutions, also aqueous, did not apparently contain any of the agents of alteration usually attributed to them. Reaction was controlled by pressure, temperature, acidity, concentration and also by ionic potential of the elements involved and the ion-exchange capacity of the newly formed clay minerals.

On the nappe structure of north-east shetland

Abstract: The main serpentine and greenstone block of Unst is traced into Fetlar and shown to be a nappe (the lower nappe) folded into a southerly-trending synform. It lies above a basement of high-grade regional metamorphic and migmatitic rocks and is separated from this basement by the lower schuppen zone. The lower nappe is overlain by an upper nappe (the Vord Hill serpentine in Fetlar and the Clibberswick serpentine in Unst) which forms the core of the synform. It is separated from this nappe by the middle schuppen zone. The upper nappe is overlain in Fetlar by the upper schuppen zone. The three schuppen zones contain slices of migmatite-gneiss and schist from the basement, serpentine and greenstone from the nappes, and Norwick Hornblendie Schist and Phyllite Group rocks which occur only in these zones. Sedimentary structures in the Phyllite Group show that they were formed, in part at least, of the erosion products of the nappes as they were being emplaced. Continued movement of the nappes resulted in the metamorphism and deformation of the rocks in the schuppen zones and of the nappes and basement beside the zones. This, the second metamorphism, was a constructive synkine-matic greenschist facies metamorphism, which completely or partially transformed both metamorphic and sedimentary rocks of the schuppen zones into phyllites. During the metamorphism these rocks were folded and elongated parallel to the fold-axes as a result of constriction in the plane normal to the fold-axes. This deformation was not directly caused by movements on the thrust-planes; it was caused by the flow of the relatively incompetent rocks of the schuppen zones towards the direction of the easiest relief, due to pressure from the relatively rigid masses of the nappes. The direction of movement of the nappes themselves cannot be determined as the roots are not exposed and the thrust-planes lack directional structures.

The caledonian sequence in ny friesland, spitsbergen

Abstract: Arising mainly out of observations in the last decade the structure of the Ny Friesland sector of Spitsbergen is outlined for the first time and summarized in a map and sections. It is interpreted as reflecting the formation of a geosyncline of Hecla Hoek rocks (late Pre-Cambrian to mid-Ordovician), and their deformation in the Ny Friesland orogeny, with the formation of gneisses and with the emplacement of “post-tectonic” batholiths, followed by Old Red Sandstone deposition and deformation in the Svalbard folding. Thus a mobile history (the Caledonian sequence) stretches from late Pre-Cambrian to Upper Devonian times and reaches its climax in the Ny Friesland orogeny (between mid-Ordovician and late Silurian) and the structures arising therefrom form the core of this paper as they do of the region itself. The sequence is compared with that in other areas, particularly East Greenland.

The main donegal granite

Abstract: The Main Donegal Granite is described in its regional setting and details are given of the envelope of Dalradian metasediments and the relation to the earlier members of the granite complex. A separation is made into the Main Granite Proper and the structureless Trawenagh Bay Granite, covering areas of 140 and 20 square miles respectively. The Main Granite Proper is a biotite-granite showing a medium-grained, grey, apparently marginal type in the south-east of its outcrop, and a coarser, lighter-coloured variety in the north and west. Pegmatite forms a roof facies in the south-west. Flow structures are shown by mineral orientation and a perfect banding ; the latter is produced by granites of varying grain and biotite content, the finer-grained biotite-richer type being of earlier consolidation and possibly constituting a roof phase. Banding is everywhere steep and along the NE.–SW. length of the Main Granite Proper. Within the Granite, perfectly aligned trains of free-swimming rafts of country rocks, seen to be derived from roofs or walls, extend for some dozen or more miles in the Granite and agree perfectly with its flow pattern. The contact-zones of the Main Granite Proper show marked sheeting by granite and, in these zones, high-grade contact schists are produced with staurolite and garnet—deformation, and heating have acted together. The marginal parts of the Granite and the adjacent contact-zones show an intense shoar structure, lineation and mullioning, the planar structures running vertically NE.–SW. and the linear elements plunging gently north-eastwards in the north-eastern part of the outcrop and south-westwards in the south-western. The production of these structures was contemporaneous with that of the contact schists and is related to the emplacement of the Main Granito Proper. The phenomena shown by the Main Granite Proper and by its roof and walls are consistent with the almost horizontal flow, from north-east to south-west, of viscous magma in a long narrow body beneath a pronged roof and between steep walls. A broad divergence of raft-trains as they are followed into the granite shows that it cannot be of replacement origin, nor can the room problem be solved by major stoping. The authors propose that lateral magmatic wedging is the mechanism of emplacement this great granite body.

The tectonics of the schichallion complex

Abstract: The results of the present investigation of the Schichallion complex can be summarized as follows :– (i) The succession established by Bailey & McCallien (1937) is confirmed. (ii) The imprints of four separate movements, F 1 to F 4 , have been found in the area. (iii) F 1 and F 2 are somewhat closely connected and are responsible for the two major systems of folding, which have produced the “Schichallion swing”. (iv) No indications of “twisting” or “torsion” have been found.

The geology of the usk inlier (monmouthshire)

Abstract: Calcareous and shelly Upper Silurian beds, about 2000 feet in thickness, outcropping over approximately 28 square miles in the Usk inlier (Monmouthshire) have been investigated. The lithological and faunal succession has been established and found to be constant, with minor exceptions, throughout the inlier. The Wenlock Shale is represented by at least 800 feet of mainly argillaceous beds which become sandy and micaceous in their transition upwards into a thin (40 feet) development of Wenlock Limestone. The Ludlow Series is between 1050 and 1300 feet thick and has been subdivided, mainly on the basis of total fossil assemblages, into seven stratigraphical groups. With the possible exception of a thin conglomerate band in the south of the inlier, there is no sign of any important break in the succession. This contradicts the claim by Gardiner (1917 and 1927) that the Lower Ludlow and Aymestry beds are missing. The presence of the Ludlow Bone-bed has been established and the succeeding Speckled Grit Beds are considered to be Downtonian in age. Structurally, the Usk inlier is a complex pericline whose major axis trends NNE.-SSW. A “Sudetic” phase of embryonic arching, and a possible “Malvernian” phase, preceded the main movements of Armorican age.

Structures in the caledonides between loch duich and glenelg, north-west highlands

Abstract: At least three periods of folding affected the Moine and Lewisian rocks south of Loch Duich in the North-West Highlands before the formation of the Moine thrust. Two major folds are recognized and shown to belong to a set recognized elsewhere. The folding is frequently markedly disharmonic and is accompanied by sliding. The small-scale structures are described and figured. It is shown that the intensity of the regional metamorphism and related phenomena diminished during the folding. The changes in structural style resulting from this are described. A system of quartz-filled gashes in the Torridonian west of the Moine thrust is shown to be related to the Loch Alsh fold. Conjugate folds and other structures formed with the Moine thrust, are shown to be later than any of the large folds affecting the Moines.

The ludlovian and downtonian rocks of the knighton district, radnorshire

Abstract: The Knighton district displays a thick “ basin facies ” succession of over 6000 feet of Ludlovian rocks, which has been divided into five groups. The Old Red Sandstone, above the basal Platyschisma helicites Beds, is divided lithologically into Yellow, Green, and Red Downtonian. Correlation is made with successions in other Welsh Borderland areas. Conditions of deposition of the Ludlovian rocks are considered. The Ludlovian succession shows two distinct faunas separated by a group of very poorly fossiliferous strata, apparently representing conditions unfavourable to the development of benthonic forms. Structurally the most notable feature is a disturbed belt some 3½ miles wide, which occupies the western part of the area. It comprises a set of NE.–SW. folds associated with strike-faulting and much fracturing. The belt can be traced north-eastwards and south-westwards considerably beyond the limits of the Knighton district.

Autometasomatism in the lower spilites of the builth volcanic series

Abstract: The Lower Spilite formation of the Builth Volcanic Series is made up of numerous thin flows of strongly porphyritic and amygdaloidal lava. Less metasomatized specimens are composed of albite-oligoclase and chlorite with smaller amounts of pyroxene and sphene. Strongly metasomatized examples show conversion of the aibite-oligoclase to albite and replacement of chlorite, including that of the amygdales, by albite plus or minus quartz. Chemical data show that these rocks have gained Na and Si and lost Al, Ca, Mg, Fe and H 2 O. Rarely, individual sacs can be extracted from the rock. Their margins are richer in Mg, Fe and H 2 O than their cores and an example is described in which a Ca–Mg–Fe metasomatism has caused replacement of plagioclase by pumpellyite. In explanation of the observations, immiscibility of two liquid phases is suggested: one the silicate residual magmatic liquid, the other a liquid enriched in Mg, Fe, H 2 O and probably Ca and CO 2 . The amygdales arise by the separation and subsequent crystallization of this second liquid. Segregation of this phase within the magma can account for most of the variation observed in these lavas, the margins of the sacs being relatively enriched in the second phase and the strongly meta-somatized lavas correspondingly impoverished.

The carboniferous limestone of the monyash-wirksworth area, derbyshire

Abstract: The area described comprises the south-eastern portion of the Derbyshire limestone massif. A local stratigraphical succession, of Via Gellia Limestones (D 1 ), Lathkill Limestones (D 2 ) and Cawdor Limestones (P 2 ), is established. The subdivisions of the Carboniferous Limestone have unconformable relations with one another. The representatives of the D 1 zone are overlain by the Lathkill Limestones of Upper D 2 age. There are no representatives of the Station Quarry and Priestcliffe Limestones of the Miller’s Dale area. The upper division of the Lathkill Limestones overlaps the lower in the neighbourhood of Matlock. The Cawdor Limestones, of proved P 2 age at Cawdor Quarry, lie unconformably on the lower formations, cutting out the whole of the Lathkill Limestones and resting on Via Gellia Limestones at Wirksworth. Contemporaneous basalts and tuffs occur at several horizons although there is a concentration at certain levels. Dolomitization has affected much of the limestone. The lead-zinc ores are generally concentrated in the anticlinal structures, with the notable and still unexplained exception of Millclose Mine in the Stanton syncline. The main anticlines and synclines were already in existence during D 2 times. They were accentuated by post-D 2 movements causing rapid facies changes in the Cawdor Limestones and persisted at least to the base of the Millstone Grit. Late faults which are not mineralized have affected the area and of these the Bonsall fault separates areas of different tectonic pattern.

Anthraconaia pulchella sp. nov. and a study of palaeoecology in the coal measures of the oldham area of lancashire

Abstract: A new species of non-marine lamellibranch, Anthraconaia pulchella, is proposed in place of A. pulchra (Wright non Hind) from the Lower similis-pulchra Zone of the Coal Measures. A detailed study is made of the relationship between the shells of A. pulchella sp. nov. and the enclosing sediments at Oak Victoria Colliery, Oldham. Shell size is shown to be directly related to the organic carbon and detrital quartz contents of the sediment and inversely related to the macroscopic pyrite content. The conditions under which the sediments were laid down are deduced, partly from study of the sediments themselves and partly from study of the disarticulation/ articulation ratios of the shells and the orientation of the shells relative to the sediments. It is concluded that A. pulchella sp. nov. thrived under conditions of relatively turbulent water but was dwarfed under conditions of relatively stagnant water. The cause of dwarfing is ascribed either to failure in food supply (possibly allochthonous organic debris in the main), or to the establishment of foul bottom conditions, or to a combination of both factors. In addition to Anthraconaia, the pulchella-bearing bed also carries occasional specimens of Anthracosia, which become more common towards the base, although they never dominate the faunal assemblage. Immediately beneath this bed, however, there is a band (10 inches thick) in which Anthracosia becomes dominant. Possible causes of the change from an Anthracosia-rich to an Anthraconaia-rich fauna are considered.

The lower margin of the shiant isles sill

Abstract: At one locality near or at the lower margin of the main Shiant Isles sill there are contacts with Jurassic marl. Immediately adjacent to the highest of these contacts there is clear evidence of an endogenous reaction—Mg-olivine→ titanaugite. Zeolites tend to be concentrated near the contacts with marl and there is some evidence that teschenitic rock can result from exogenous reactions within the marl. An unusual degree of variability in the textural relations of olivine and augite with plagioclase is described and discussed. It is suggested that where there has been assimilation of marl, augite may have been precipitated instead, or as a replacement, of early olivine and that rocks not readily distinguishable from alkaline dolerites and teschenites were produced on a limited scale by interaction with the marl of water-and soda-rich picritic material intruded slightly in advance of the main picritic facies of this sill. Interbanding of picritic and teschenitic rocks at another locality can be interpreted as due to flowage and intrusion of these products of mixed origin. Analogous phenomena in some other sills are briefly discussed.

The ostracod fauna of the shale facies of the cawdor limestones, north end of cawdor quarry

Abstract: A series of 45 samples has been collected systematically from the shale overburden of Cawdor Quarry, Matlock, and examined for their microfauna. Of the several fossil groups represented, only the ostracods, by reason of their abundance and diversity of form, are considered in this account. A total of 16 genera and 20 species, including one new genus and six new species, have been identified and their distribution in the varied lithologies of the shale succession recorded. The faunal assemblage has been given a precise stratigraphic significance by the discovery in the same beds of the goniatite G. granoaus, indicating an horizon in the lower P 2 subzone.

Some aspects of the progress in geology in the last fifty years. ii anniversary address delivered at the annual general meeting of the society on 30 april, 1958

Abstract: The advances in our knowledge of the rocks in the hidden depths are reviewed. Through seismology three discoveries were made : the crust of the earth (J. Milne), its base (A. Mohorovieiae) and the core of the earth (R. D. Oldham). The composition of the uppermost part of the mantle and the absence of the Mohorovieiae discontinuity beneath certain oceanic areas are discussed. The lavas common to continental and oceanic areas, basalts and tholeiites are considered to come from the mantle, those igneous rock types restricted to the continents from the crust or from mantle liquids modified by the crust. The significance of the fold mountain belts is discussed in the light of the evidence for gravitational sliding, and the hypothesis of a contracting globe in geological time is examined. The bearing of geomagnetic rock studies on the hypotheses of polar wandering and continental drift, and the pre-geological history of the earth’s crust during which the continent–ocean separation took place are briefly considered.