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Showing papers on "Gondwana published in 1973"


Journal ArticleDOI
TL;DR: Palaeomagnetic data from the British Isles are reviewed and are found to be consistent with erratic polar shift from the vicinity of 10° N, 180° E in the Ordovician to 0°, 145°E in the Siluro-Devonian and to 25° n, 160° E.
Abstract: Summary Palaeozoic palaeomagnetic data from the British Isles are reviewed and are found to be consistent with erratic polar shift from the vicinity of 10° N, 180° E in the Ordovician to 0°, 145° E in the Siluro-Devonian and to 25° N, 160° E in the early Carboniferous. The Cambrian pole is less well established near 25° N, 170° E. The rates and times of shifts are discussed. World-wide palaeomagnetic data suggest the existence of three groups of continents in the Early and Middle Palaeozoic—the Gondwanan, Euramerican and Siberian groups—separated by large oceans which closed in the Late Palaeozoic along the lines of the Hercynides and Uralides. The data from Gondwana and Siberia are internally consistent and each may have been a single plate to which only peripheral slices were added subsequently. Data from Euramerica are less coherent. The most probable explanation seems to be that this group of continents was loosely-knit and consisted of the North American and the Baltic/Russian plates (which joined in the Silurian on the site of the Scandinavian Caledonides) and the British Isles sub-plate which joined in the Devonian (on previously unrecognized sutures lying in the present North Atlantic and North Sea respectively). This interpretation takes all the data at face value and assumes that time-coverage is adequate in all parts of the region. Alternatively, if time-coverage was not adequate, Britain might have been part of the North American plate, but further work is required to discriminate between these interpretations. Palaeomagnetic evidence from the British Isles also indicates that little or no closure has taken place across the British Caledonides since Early Ordovician time. If so the British Isles sub-plate may have been rotated in azimuth as well as being transported laterally before joining the Euramerican plate. Examples of rotation and remagnetization on a more local scale are cited, and are ascribed to tectonic and thermal effects of orogeny respectively. The Euramerica and Gondwana groups were already close together by early Carboniferous times, which is distinctly earlier than the Hercynian-Alleghenian orogenic belt which may mark their juncture. It may be generally true that large scale crustal drift, involving consumption of a large area of oceanic crust between two continents, significantly predates the orogenic belt which forms when the two continents have met. If this is so, large scale closure across the British Caledonides might have occurred in Precambrian or Cambrian time, but palaeomagnetic data are not yet adequate to evaluate this.

97 citations


Journal ArticleDOI
TL;DR: The stratigraphy and tectonics of the Cauvery, Palar and Godavari-Krishna basins, on the east coast of India, are considered on the basis of geologic, geophysical, and drilling data as mentioned in this paper.
Abstract: The stratigraphy and tectonics of the Cauvery, Palar, and Godavari-Krishna basins, on the east coast of India, are considered on the basis of geologic, geophysical, and drilling data. The Cauvery basin was formed during Late Jurassic time by sagging of a part of the Indian shield, mainly along the dominant northeast-southwest basement trends. The basin consists of several depressions separated from each other by subsurface basement ridges aligned parallel or subparallel with the dominant basement trends. The sedimentation in the respective depressions was controlled by movements along these trends from the time of deposition of the Upper Gondwana beds. The depocenters, which were mainly open toward the west during Upper Gondwana deposition, shifted toward the east as a result of general basinal tilt at the beginning of the Tertiary. These movements were responsible for repeated transgressions and regressions, as is evidenced by lithofacies, biofacies, and thickness variations, as well as by sedimentation breaks. Outcrops in the Palar basin include thin Permian boulder beds and shales, a thick paralic Cretaceous sequence, and a thin continental Neogene succession. In the Godavari-Krishna basin the outcropping strata are Middle Jurassic to Lower Cretaceous paralic to shallow-marine sedimentary rocks and thin Trap Volcanics (basalts) overlain by a continental Neogene succession. A synthesis of geologic and geophysical data suggests that the tectonic and sedimentation patterns in the Palar and Godavari-Krishna basins are likely to be analogous to those of the Cauvery basin.

80 citations


Journal ArticleDOI
TL;DR: In this paper, four palaeogeographical reconstructions for the southern Cape covering the period Late Permian to Late Cretaceous are presented, spanning the commencement to an advanced stage of breakup of Gondwanaland, during which the area moved from a mid-continental, high latitude, to an ocean dominated, middle latitude position.

26 citations



01 Jan 1973
TL;DR: For example, the Australian Early Triassic plant microfossil assemblages differ notably from those of the Late Permian, they contain few new forms, apart from the lycopsid genus Aratrisporites as mentioned in this paper.
Abstract: Australian Late Permian plant macrofossil associations are of the familiar Gondwana type, in which Glossopteris is a ubiquitous and predominant element. Glossopteris has never been found in Australian strata for which a Triassic age is unequivocally indicated by other fossil evidence. Spore and pollen assemblages from Australian Upper Permian sediments are among the most diverse encountered anywhere in the geological column. They are interpreted as representing part of a mature floral ecosystem, stabilized at least throughout the area of the present continent. In Western Australia virtually none of the distinctive plant microfossil taxa persists into the Triassic. Here early Scythian microfloras are characterized by lack of diversity and enormous numbers of small, spinose acritarchs. The microfloral break is less sharp at the base of the Triassic in the Sydney Basin, although the trend towards lower diversity above the boundary is still apparent. Although Australian Early Triassic plant microfossil assemblages differ notably from those of the Late Permian, they contain few new forms, apart from the lycopsid genus Aratrisporites. Almost all the well-characterized Scythian elements are known, usually as minor components, from Late Permian microfloras in Australia or other parts of the world. By late Scythian times the Dicroidium-flora had become established throughout the continent and persisted until the Early Jurassic. Floral modifications at the Permian-Triassic boundary are spectacular; if not as complete and abrupt as those among certain invertebrate groups. Like other regional and local upheavals in the vegetation of the past they are concomitant with a period of shallow marine transgression, following extensive continental emergence and erosion. Destruction of the mature and delicately balanced Late Permian continental ecosystem is related to rapid transgression of the early Scythian sea, over extensive areas of marginal lowland. This collapse initiated a new floral succession marked in its early stages by lack of diversity and high proportions of lycopods. Plant microfossil assemblages indicate gradually increasing maturity of the flora throughout the Scythian, and the attainment of essential stability by the beginning of the Middle Triassic.

18 citations


Journal ArticleDOI
31 Dec 1973
TL;DR: One of the most controverted aspects in the stratigraphy of tge Gondwana of the Parana Basin has always been the limit between the Permian and the Triassic rocks.
Abstract: One of the most controverted aspects in the stratigraphy of tge Gondwana of the Parana Basin has always been the limit between the Permian and the Triassic rocks. The greatest problem in Rio Grande do Sul is concerned to the identification of the beds situated between the Estrada Nova and the Botucatu Formations. These beds have been interpreted as different units and consequentely receiving different names.

14 citations


01 Jan 1973
TL;DR: A preliminary correlation scheme of the upper Paleozoic sequences and their relationships with lower Mesozoic strata is proposed as a basis for interpreting the paleogeography and paleoclimates of the continent as discussed by the authors.
Abstract: A preliminary correlation scheme of the upper Paleozoic sequences and their relationships with lower Mesozoic strata is proposed as a basis for interpreting the paleogeography and paleoclimates of the continent. Stratigraphic, biochronologic and radiometric data indicate that the upper Paleozoic and lower Mesozoic sequences are separated by an unconformity in the Andean region. Chronological uncertainties and/or conformable stratigraphic relationships make a separation of upper Paleozoic and Triassic sediments more difficult to trace in the southern subandean part of Bolivia, Precordillera of Argentina, in the Parnaiba Basin and parts of the Brazilian portion of Parana Basin. Uppermost Permian and lowermost Triassic sediments are virtually absent from South American sedimentary basins and so the boundary can be discussed in only the broadest of terms. The paleogeographical and paleoclimatological development of South America are compared with the history of reassembled Gondwana continents during the upper Paleozoic and lower Mesozoic, and although a general similarity is indicated, a few points deserve further study. The upper Paleozoic glaciation that affected South America started in the Early Carboniferous and ended rather abruptly in the Andean region, after Late Carboniferous time, but a glacial climate still prevailed locally during the Early Permian in parts of the Parana Basin. Starting in the Early Permian the climate became warmer and more arid conditions prevailed into the Late Permian and Triassic.

11 citations



Journal Article
TL;DR: In this article, a marine fauna bearing pebble-slate horizon is developed at the base of this sequence, and the stratigraphy and lithologic characters of this horizon are described in detail, and considered significant in the context of the palaeogeography and tectonic evolution of this part of Himalaya.
Abstract: To the south of the Central crystalline zone of the Darjeeling Himalayas, in the interior parts of the frontal zone of the nappes, a sequence of the Lower Gondwana rocks occurs in a window, overlain by the tectonic cover of the Daling-Darjeeling group of metamorphites. A prominent marine fauna bearing pebble-slate horizon is developed at the base of this sequence. The stratigraphy and lithologic characters of this horizon are described in detail, and are considered significant in the context of the palaeogeography and tectonic evolution of this part of Himalaya.

8 citations