Showing papers on "Gondwana published in 1985"

Alleghenian reconstruction and subsequent evolution of the Gulf of Mexico, Bahamas, and Proto-Caribbean

Abstract: A detailed model for the evolution of the Gulf of Mexico, the Bahamas and the Proto-Caribbean is built within the framework provided by a detailed initial Alleghenian (western Pangean) reconstruction and an accurate subsequent relative-motion history between North America and Gondwana (northern Africa and South America). The Alleghenian reconstruction closes all pre-Jurassic oceans; accounts for Jurassic attenuation of continental crust by restoring that attenuation to original prerift continental thicknesses; incorporates an improved Equatorial Atlantic fit between northern Brazil and the Guinea margin of Africa; quantitatively removes changes in shape of northern South America due to Late Cretaceous and Cenozoic accretion and internal deformation; includes pre-Mesozoic continental crust presently underlying the western Bahamas and southern Florida; and correlates Late Paleozoic geology of Yucatan with its neighboring continental masses. Extension occurred within the Gulf of Mexico from Late Triassic to earliest Cretaceous time, but seafloor spreading was delayed until the Late Callovian. This divided a single Gulf-wide salt basin into the Louann and Campeche salt provinces. The Yucatan block progressively rotated about 43 degrees counterclockwise away from the Texas-Louisiana margin around a pole in northern Florida. The Tamaulipas-Golden Lane-Chiapas fault zone of eastern Mexico is interpreted as the remains of an initially intracontinental transform system along which Yucatan migrated. Attenuated continental crust beneath southern Florida and the western Bahamas, termed here the Florida Straits block, migrated approximately 300 km out of the eastern Gulf, approximately along Central Atlantic flow lines. These rotations are consistent with recently suggested magnetic anomaly trends in the Gulf of Mexico (Shepherd et al., 1982; S. Hall, personal communication, 1984). The Proto-Caribbean formed synchronously by a fan-like rotation of Yucatan away from Venezuela.

Migration of glacial centers across Gondwana during Paleozoic Era

Abstract: Continental glaciation, as recorded by sedimentary facies and by pavements scoured into underlying rocks, affected northern Africa at the end of the Proterozoic Era. This glaciation was followed in the Cambrian Period by a long, warmer interval without recorded ice sheets upon the Gondwana supercontinent. Strong glaciation ensued in Late Ordovician time in central northern Africa, and centers moved into then adjoining northern Brazil and on westward into southern Brazil, southern Africa, and Bolivia and into northern Argentina by the Early Silurian. From Middle Silurian time, world-wide and Gondwanan climate ameliorated until Late Devonian (Famennian) time, when glaciation again affected Brazil and perhaps parts of Africa. Glacial centers apparently waned after the late Famennian (latest Devonian) but waxed again in Andean regions and northern Brazil in Early and mid-Early Carboniferous times to begin the strong Late Paleozoic Ice Age. Many ice caps and ice sheets came and went across the wide reaches of Gondwana during the late Paleozoic, beginning on the west, culminating in southern Africa in the Late Carboniferous, continuing strongly in India and Australia in the Permian, and dying out in eastern Australia and in Antarctica in early Late Permian time. The path of ice-center migration closely follows published paleomagnetic wander paths. The record suggests that, among other terrestrial factors which cause ice ages, glaciation flourished when Gondwana lay in south polar regions and that glaciation disappeared when Gondwana glided, so that the pole lay in oceanic or coastal regions.

Preliminary Phanerozoic polar wander paths for the North and South China blocks

Abstract: Polar wander paths for the North and South China blocks suggest that (1) both were parts of Gondwana in the Palaeozoic, (2) the North China block accreted to Siberia in the late Permian and (3) the South China block accreted to the North China block in the middle Triassic to the early Jurassic. Comparison of the polar wander path for the South China block with that for northern Eurasia suggests that relative motion of over 4,000 km has occurred between them.

Disintegration of the continental margin of northwestern Gondwana: Late Devonian of the eastern Anti-Atlas (Morocco)

Abstract: The Devonian–early Carboniferous sequence in the eastern Anti-Atlas represents a complete record of the last stage of the depositional and tectonic evolution along the northwestern margin of Gondwana. As a consequence of early Variscan block faulting, a platform and basin topography was established during the Middle and Late Devonian. Platforms were covered by condensed cephalopod limestones; sedimentation in the basins was mainly argillaceous with calcareous and turbiditic intercalations. In the latest Famennian/early Tournaisian the whole area was covered by delta deposits in the south passing into turbidites and olistostromes toward a continental slope farther north. This sedimentary and structural evolution reflects the gradual foundering and disintegration of the northwestern continental margin of Gondwana prior to the collisional stage in the late Visean/late Carboniferous.

Carboniferous and Early Permian biogeography

Abstract: During the Carboniferous, changes in the biogeographical distribution of shelf-dwelling, benthic marine invertebrates were made in response to changes in physical paleogeography and climatic variations Calcareous foraminifers and bryozoans are principal examples of the general trends during the Early Carboniferous, which show that Tournaisian and early and middle Visean faunas were broadly cosmopolitan in a circumequatorial belt and that latitudinal diversity gradients were relatively minor During the later part of the Visean and early part of the Namurian, the Hercynian orogeny, caused by the collision of Euramerica with Gondwana, disrupted these cosmopolitan equatorial faunal patterns This was also a time of progressively cooler temperatures throughout the world, of dramatic reduction in faunal diversity, and of high rates of extinction of both species and genera During middle Carboniferous time, strongly provincial faunas were common Spasmodic, but limited, dispersals gave a few widespread genera significantly different stratigraphic ranges in different provinces so that Tethyan and non-Tethyan distributions are recognizable Faunal diversity gradually increased during the middle Carboniferous and then declined at the end of the epoch with additional high levels of extinction of species and genera During late Carboniferous time, several new faunal lineages became well established Some filled vacant ecological niches; however, others took advantage of the increase in the number of niches that became available because of gradually warming climates Provinciality continued to be pronounced Diversity gradually increased and continued to do so through the Early Permian

Importance of thrust faulting in the tectonic development of northern Victoria Land, Antarctica

Abstract: The present configuration of Wilson Group, Bowers Supergroup and Robertson Bay Group in northern Victoria Land (Fig. 1) has been attributed to high-angle block-faulting1–3 or more recently4 to major strike-slip faulting along the Lanterman and Leap Year Fault zones. In this latter interpretation4, adjacent units have allochthonous relationships and constitute separate geological terranes. We offer additional support for the concept of allochthonous terranes in northern Victoria Land but suggest here an alternative model to explain terrane juxtapositions. Our model emphasizes the role of thrust tectonics and implies severe structural telescoping of entire terranes with significant loss of intervening crust. We further suggest that this occurred during the Lower Palaeozoic Ross Orogeny in response to the collision of Antarctica with a westwardly directed eastern continental block. This requires that the northern Victoria Land segment of Gondwana was a zone of major plate convergence throughout the Cambrian–early Ordovician period.

Crustal structure across the Narmada-Son lineament, Central India from deep seismic soundings

Abstract: The crustal structure along Ujjain-Mahan profile reveals a layered structure in the vertical direction and block structure in the lateral direction. This profile from north to south is divided into four crustal blocks: I (Ujjain-Sanwer), II (Indore-Dorwa), III (Dorwa-Tapti) and IV (Tapti-Mahan) which were relatively displaced up or down during different times along deep faults bounding them and extending up to the Moho discontinuity. The depth to Moho varies between 37 to 42 km along the entire length of the profile with a velocity jump of 7.3 to 7.8 km/sec across the boundary. It is concluded that during Precambrian, blocks I and II north of Dorwa were downthrown with respect to block III leading to the development of the Vindhyan basin in that region. Blocks III and IV being uplifted at that time, formed the land part and hence no Vindhyan sedimentation took place there. Subsequently during Gondwana times, reverse tectonic movement resulted in downfaulting of block III where Gondwana sedimentation took place. During this period, blocks I and II formed the land part and hence no Gondwanas were deposited there. A shallow refraction boundary at a depth of 8 to 12 km has also been observed along this profile with a velocity jump from 6.0 to 6.9 km/sec, which may represent the Conrad discontinuity in this region. In the Ujjain-Dorwa section comprising blocks I and II, under a thin cover of about 100 m of Deccan Traps, there lie Cretaceous Lameta beds (200m thick) and Vindhyan quartzites (lOa m thick) and Bijawars (200 to 300 m thick). Thus the total sedimentary thickness above the crystalline basement is hardly 600metres in this region. On the other hand, the Dora-Mahan section, consisting of blocks III and IV, has a maximum thickness of about 1.7 km Gondwana sediments underlying 400 metres of Deccan Traps. This hidden Gondwana basin is believed to be a northwestward continuation under the Deccan Trap of the exposed Gondwana-Godavari graben.

Silurian and Devonian base maps

Abstract: During the Silurian and Devonian, the sequence of continental collisions that were ultimately to result in the formation of the supercontinent of Pangaea had begun. By the Early to Middle Devonian North America (Laurentia), Acadia, Great Britain, and Northern Europe (Baltica) had collided to form the `Old Red Sandstone' continent (Laurussia). Palaeomagnetic data, however, indicate that the configuration of the continents that made up Laurussia did not resemble the pre-breakup, Mesozoic reassembly. Rather, Britain, Baltica, and Acadia were displaced 10-20 $^\circ$ to the south with respect to Laurentia. New palaeomagnetic data for Laurentia and Gondwana, suggests that the ocean separating the northern and southern continents was relatively narrow during the early Devonian, and may have been nearly closed by the late Devonian.

Intercontinental correlation by sea-level events in the Early Silurian of North America and China (Yangtze Platform)

Abstract: Potential for eustatic sea-level changes on continental platforms during Early Silurian (Llandovery) time was highly propitious due to the double effects of lingering glaciation in Gondwana and the rapid rate of sea-floor spreading which assisted the general dispersion of many era tonic realms through middle Paleozoic oceans. Following the initial mass melting of the Late Ordovician/Early Silurian glaciers in North Africa and Arabia, four smaller cycles of sea-level fluctuation were recorded by recurrent, shelly communities in a predominantly carbonate setting on the North American Platform. Peaks in sea-level rise occurred in late Rhuddanian or early Idwian (A 4 –B 1 positions), early Fronian (C 1 position), early Telychian (C 4 position), and late Telychian (C 6 position) times. Although absolute time control is poor, the periodicity of these events may have been every 2.5 m.y. In the more submerged regions of the North American Platform, such as Iowa and Anticosti Island, diverse faunal associations distinguished by the presence of stricklandiid brachiopods were especially productive at peak times of transgression. Co-occurrence of the calcareous green alga Cyclocrinites with certain of these faunas indicates that maximum water depth was still within the photic zone. At the other extreme, low stands in sea level are identified by diminished faunal diversity, as well as a combination of typical sedimentary structures and evaporite indicators. Located on a well-defined Precambrian craton, the Yangtze Platform is now a large region in central and southwestern China. During successive stages of the Early Silurian, a thick sequence of graptolitic shales, shelly carbonates, and red beds were deposited on the platform. Of the four sea-level peaks defined in North America, three are recognized in the Yangtze Region. There is very good evidence for a latest Llandovery or earliest Wenlock transgression, based on the extensive distribution of the Salopinella - Coronocephalus - Sichuanoceras fauna. Another clear and well-dated sea-level peak involves the occurrence of faunas with characteristic stricklandiid and pentamerid brachiopods at a level equivalent to the lower Telychian (C 4 position). The initial Silurian transgression appears to have reached its peak on the Yangtze Platform primarily during early Idwian time (B 1 position), but this is based less surely on changing patterns in graptolite diversity. At present, there is no compelling evidence for a widespread, early Fronian (C 1 position) transgression in China. Successful matching of the other sea-level events, however, substantially refines intercontinental correlation for Lower Silurian strata in the Yangtze Region of China and the central regions of North America.

Oolitic ironstones and contrasting Ordovician and Jurassic paleogeography

Abstract: Distribution of abundant Ordovician and Jurassic oolitic ironstones provides a test for current ideas about factors controlling their origin. These include warm climate and deep weathering, dispersed continents, and highstand of global sea level. Most of the Jurassic ironstones developed in middle northern latitudes on the unstable European part of assembled Laurasia at a time of low global sea level, mild moist climate, and abundant vegetation. Most of the Ordovician ironstones accumulated around the northwestern margin of assembled Gondwana and Armorica during highstand of sea level and an absence of land plants, and in high southern latitudes that supported an ice cap by the end of the period. These contrasts demonstrate that neither dispersed continents nor major highstand of sea level was a necessary factor. Moreover, requisite weathering in Ordovician time may have been induced by adequate soil-air CO 2 pressure maintained by elevated atmospheric P co 2 in tne absence of land plants.

Middle Cambrian paleomagnetism of the Avalon Terrane in Cape Breton Island, Nova Scotia

Abstract: Interbedded volcanics and sediments of the Bourinot Group (Mid-Cambrian) in central Cape Breton Island were studied as part of a paleomagnetic investigation to quantify the geologically plausible displacements of the Avalon terrane with respect to the North American craton. Both volcanics and sediments reveal prefolding magnetizations with steep to intermediate inclinations. Dual-polarity magnetizations in different volcanic flows are probably primary and they yield a mean direction of D=293°, I=−66° (N=9, k=41.8, α95=8.1°). The sediments reveal, upon detailed thermal demagnetization, complex multivectorial remanences. However, a few samples show characteristic components and a large number of samples (38) have intersecting great-circle remagnetization trajectories; when combined, this yields a mean direction of D=267°, I=−65° (k=16, mean radius of error ellipse is 8.5°). The mean directions for the Bourinot rocks correspond to a paleolatitude of 49°S±11°, which is different from the range of possible values for the predicted Mid-Cambrian paleolatitudes for Nova Scotia (29°S to 18°N), inferred from the North American apparent polar wander path. This discrepancy between predicted paleolatitudes and the observed paleolatitude implies that Nova Scotia was not part of North America during the Mid-Cambrian, but rather, they were separated by an ocean with a minimum width of 1000 km (20°±10°). This interpretation of the relative positions of Nova Scotia and North America based on paleomagnetism is consistent with the geological record in the northern Appalachians and supports previous interpretations of the dissimilar Early Paleozoic faunas in Avalon and North America. It is paleomagnetically quite plausible that the Avalon terrane formed part of a larger continent, comprising Africa (Gondwana) and Armorica during the Cambrian.

A reassessment of Taeniopteris daintreei from the Victorian Early Cretaceous: a member of the Pentoxylales and a significant Gondwanaland plant

Abstract: Plant remains from the Early Cretaceous of Victoria are described. These are leaves referred to Taeniopteris daintreei, female fructifications referred to Carnoconites cranwellii and microsporangiate structures herein described as Sahnia laxiphora sp. nov. These are considered to belong to the extinct group, the Pentoxylales, originally described from localities variously dated as Jurassic or Early Cretaceous in the Rajmahal Hills of India. It is suggested that the Pentoxylales, long regarded as an obscure group, are in fact a major and important part of the mid Mesozoic Gondwana flora in Victoria.

The Triassic and Jurassic paleogeography and evolution of the Qinghai–Xizang (Tibet) Plateau

Abstract: The Kunlun, Qilian, and Qinling mountains already existed in embryonic form as coastal ranges in Triassic times. Marine conditions predominated to the south. The southern coastline is demarcated by the northern margin of the Tertiary Siwalik sediments. The paleo-landmass of Longmenshan–Sichuan–Yunnan separated this sea from that of southwest China. By Jurassic times, the coastline of the Eurasian continent within what is now China had already moved southwards to a line along the Kunlun and Hengduan mountains.Mainly on the basis of differences between continental and oceanic crust, several different seas can be distinguished: the Himalayan, Lhasa, Qiangtang, Hengduan, and Triangle seas, together with the Qaidam Peninsula and Xining Bay. Triassic and Jurassic transgressions and regressions of these seas are discussed in detail.During Carboniferous and Permian times the Lhasa and Himalayan seas were joined together with the main Gondwana Plate to the south. In Triassic times, when the India–Pakistan Subconti...

Late Precambrian and early Paleozoic tectonism and associated sedimentation in northern Victoria Land, Antarctica

Abstract: Northern Victoria Land, part of the Trans-antarctic Mountains, occupies a key position in the reconstructed Pacific margin of Gondwana, lying near the Oligocene rift between Antarctica, Tasmania, and Australia. Sedimentology, sedimentary petrology, and structural history of the Robertson Bay Group and the Bowers Supergroup are here used to develop a regional interpretation of these rocks in plate-tectonic terms. The Robertson Bay Group, a distal shale-turbidite sequence, is interpreted to have originated in a continent-continent collision environment and was com-pressionally deformed after the Cambrian-Ordovician boundary but before Devonian time. The Sledgers Group of the Bowers Supergroup, a proximal shale-turbidite and volcanic sequence, was derived from an undissected arc environment and was deformed during the same time interval as was the Robertson Bay Group and possibly also during the Silurian-Devonian. The Cambrian Mariner and overlying Leap Year Groups represent shallow-marine and terrestrial deposition. Reworking and selective removal of unstable clasts obscure provenance, but, although these units may be derived from dissected arc sources, they are quartz-rich. They appear to have been deformed by Silurian-Devonian tectonism. The Robertson Bay Group and the Bowers Supergroup occur in large, fault-bounded terranes. The plate-tectonic interpretations developed for these units in this paper place the arc-related Bowers Supergroup between the Antarctic craton to the west and the recycled orogen-related Robertson Bay Group to the east. This geometry is not readily related to in situ tectonic environments; therefore, it is concluded that the possibility exists that these terranes are “suspect,” that is, they may be entirely allochthonous to the Antarctic craton and have been added by later accretion.

Antarctic geology and mineral resources

Abstract: The geology of Antarctica is similar in many respects to that of the other southern continents that once formed the larger continent of Gondwana. Because of this former union with other land masses, mineral resources in Antarctica are probable, but none of any significance have been found, perhaps because of the widespread cover (97%) of ice over the continent. Offshore oil and gas are presently unknown, but would seem to hold the best prospect for any development in the near future, but perhaps not until questions of sovereignty and ownership of potential resources are resolved.

Continental Terranes in Southeast Asia: Pieces of Which Puzzle?

Abstract: Continental crust underlies most of the large pre-Tertiary core of Southeast Asia, comprising Sumatra, west Borneo, mainland Southeast Asia, and adjacent shallow sea areas. This core, joined to South China along the Song Ma-Song Da suture belt (Permo-Triassic) , is composite, consisting of two to four separate terranes. The western portion (Mitchell's "Western Southeast Asia block") represents a Paleozoic continental margin subsequently rifted on its (present) west side. Its east edge is marked by suture belts apparently formed by collision of the western continent with eastward-dipping subduction zones flanking Indochina and eastern Malaya. Late Paleozoic glacial marine deposits in western Southeast Asia indicate attachment to Gondwana, with rifting after Permian, possibly in Jurassic. Floral and paleomagnetic data are generally consistent with this and suggest land connections, at least briefly during earlier Mesozoic, from China through Southeast Asia to the main part of Gondwana, possibly as part of Pangaea. The more easterly terranes in Southeast Asia's continental core may have dispersed from a Paciflca continent, thus marking a boundary between Pacifica¬ derived and Gondwana-derived terranes. Geologic evidence, however, links at least eastern Malaya to western Southeast Asia, suggesting these eastern terranes were simply more outboard portions of a complex Gondwana margin.

Lower Carboniferous Clastic Sequence of Central Ohio

Abstract: On cover: "Field Excursion 4&9, Guide Book," "Sixth Gondwana Symposium, 19-23 August, 1985."

Gravity Field and Tectonics of Gondwana Basins of Peninsular India

Abstract: The Godavari, Mahanadi, Damodar, Narmada and Son are the major rivers along which Gondwana sediments were deposited during Carboniferous to Cretaceous times along well formed valleys, basins or rift valley structures. The present-day Gondwana occurrences in Peninsular India are shown in Figure 4.1. These include, the Godavari Valley in the south-east, the Narmada-Son-Mahanadi basin in Central India, Damodar valley basins including the Raniganj, Jharia, Bokaro, Karanpura and Ramgarh coalfields in eastern India. Besides these, Gondwanas are found in Palamau and Deoghar districts in Bihar, the Rajmahal Hills, the foot-hills of Himalaya and along the east coast of India.

Rifting and early tectonic evolution of the equatorial Atlantic

Abstract: The Equatorial Atlantic forms a giant transform connecting the Central and the South Atlantic between northwest Africa and Brazil. Improved onshore mapping and offshore seismic and drilling by Petrobras permitted the differentiation of the following phases in the tectonic evolution of the Equatorial Atlantic: (1) Basaltic volcanism along the eastern and southern margins of the Guyanan Shield during Triassic-Jurassic time as North America separated from Gondwana. (2) During Early Cretaceous time, clockwise rotation of South America caused rifting east of Fortaleza and compression and arching of the basement further west. Subsequently, the rift propagated westward along the whole length of the Equatorial Atlantic. (3) From Albian time on, there has been right-lateral divergent movement between northwest Africa and South America. (4) This movement was interrupted by intense compression in the Late Cretaceous resulting from convergence between southwest Europe and Africa, causing shortening and southward thrusting of the rift sequence. This compression was alleviated by left-lateral movement along the intracontinental Sobral transform.