Showing papers on "Gondwana published in 1970"

Reconstruction of Pangaea: Breakup and dispersion of continents, Permian to Present

Abstract: We present a new continental drift reconstruction of the universal continent of Pangaea in the Permian plus a series of five world maps to depict the breakup and dispersion of continents with each subsequent geologic period, Triassic to Recent. Plate tectonics and sea-floor spreading are accepted as the guiding rationale. Also utilized are the morphologic fitting of continental margins and paleomagnetic pole positions. Rigor is imposed by the geometric requirements involved in presenting continental drift dispersion on maps in orderly time sequence and by following certain assumed rules of plate tectonics. The reconstructions were first made on a globe and then transferred to an Aitoff world projection. In the Permian, the Atlantic and Indian oceans were closed so that all the continents were configured into the universal landmass of Pangaea. The reconstruction is based largely on the morphologic best fit of continental margins to the 1000-fathom isobath, except for India, the east coast of which is placed against Antarctica, as dictated by plate tectonics. In the Triassic the breakup of Pangaea commenced. The southwest Indian Ocean rift was created, which split West Gondwana (South America and Africa) away from East Gondwana while a Y junction lifted India off Antarctica. An independent North Atlantic–Caribbean rift also formed, which lifted Laurasia (North America and Eurasia) off of South America and the bulge of Africa. In the Jurassic, northward and westward sea-floor spreading further opened the central North Atlantic and the Indian oceans. At the end of the period, a new rift incipiently split South America away from Africa. The Walvis mantle thermal center or ‘hot spot’ formed, which would subsequently provide an absolute geographic reference point for subsequent continental drift. In the Cretaceous, the motions already established continued. The North Atlantic rift grew northward, blocking out the Grand Banks and the western margin of Greenland. Spain rotated sinistrally, forming the Bay of Biscay. An offshoot rift split Madagascar from Africa, dropping off this subcontinent from Africa, which continued its northern flight. The northward trek of India continued, and Australia incipiently split away from Antarctica. During the Cenozoic, Antarctica rotated further westward. Australia experienced a remarkable flight northward, and New Zealand was split away from its east coast. The North and South Atlantic oceans continued to open; the rift that formerly passed west of Greenland now switched to the east and split Greenland away from northern Europe and extended through the Arctic Ocean. Africa moved slightly northward, continuing sinistral rotation. The Tethyan megashear became dextral for the first time, India collided with and underran Asia.

Tertiary Floras of India and Their Bearing on the Historical Geology of the Region

Abstract: Geologically India is divisible into three units (1) Peninsular, (2) Extra-Peninsular and (3) Indo-Gangetic Plain. The Tertiary floras of India can conveniently be divided into two groups Palaeogene and Neogene. As known today, Palaeogene floras are found only in the Peninsular India, while Neogene occur in both the Peninsular and extra-Peninsular regions. They are predominantly tropical floras, made up of genera now largely confined to the Old World. A notable feature of the Indian Palaeogene is the occurrence of a few southern hemisphere taxa which may recall the pre-Cenozoic relationships between India and the Gondwana continents to the south. The London Clay flora shows noticeable general resemblance with the Indian Palaeogene. This feature is discussed taking into account the Tertiary plant fossils known from northern Africa. Phytogeographic comparison is also made with the Malaysian region. It is envisaged that there were large scale migrations and intermingling of floras over Malaysia, India, Arabia and Eastern Africa during Neogene time. Records of Dipterocarpaceae provide significant evidence. Water seems to have been a major factor in controlling the distribution of plants at low latitudes throughout the Cenozoic era. The palaeogeography of India during the Early Eocene and Miocene epochs has been reconstructed on the joint evidence of plant and animal fossils. Much work remains to be done before we shall have a clear understanding of the sources and migrations of the plants which have survived in southern Asia throughout Tertiary times to the present.

Petrified peat from a permian coal bed in antarctica.

Abstract: Petrified plant remains that composed a Permian peat deposit occur at a coal horizon in a local area of Mount Augusta near the Beardmore Glacier in Antarctica. This discovery is the first in the entire Gondwana area that yields plant materials as exquisitely preserved as the materials of the well-known coal-ball localities of the Northern Hemisphere. A sampling of anatomical details is illustrated.

Fit between Africa and Antarctica: A Continental Drift Reconstruction.

Abstract: A computerized (smallest average misfit) best fit position is obtained for the juxtaposition of Africa and Antarctica in a continental drift reconstruction. An S-shaped portion of the Weddell and Princess Martha Coast regions of western East Antarctica is fitted into a similar profile along southeastern Africa. The total amount of overlap is 36,300 square kilometers, and the underlap is 23,600 square kilometers; the total mismatch is thus of 59,900 square kilometers. The congruency along the 1000-fathom isobath is remarkably good and suggests that this reconstruction is valid within the overall framework of the Gondwana supercontinent.

Gondwana Sedimentation Around Bheemaram (Bhimaram), Pranhita-Godavari Valley, India

Abstract: The Permian and Triassic Gondwana deposits around Bheemaram belong to six mapable rock units ("formations"): Barakar, Ironstone Shale, Kamthi, Yerrapalli, Bhimaram and Maleri. Of these, the Barakar, Middle and Upper Kamthi and Bhimaram formation are composed of profusely cross-bedded, prismatic or lenticular bodies of coarse, argillaceous sandstones. The other formations are made of structureless, thin sheets of clay stone (or shale) and silty sandstones with inter-bedded lenses of calcareous sandstones and sandy limestones. The sandy and clayey formation juxtapose, often, with interfingering contacts. Lithology, sand, body geometry, primary structures and patterns of grain-size distribution of these Gondwana rocks are comparable to those of fluvial sediments. The coarser fractions of the rocks which were transported mainly by tractive currents, constituted point-bars and channel-bars of the Gondwana river and the finer fractions were deposited from suspension in the interchannel floodplain areas. Cross-bedding dip directions indicate a northerly paleocurrent. In the Lower Kamtha, the measurements show large dispersion due to arcuate directions of flow at the river meanders. With time, the direction of sediment transport shifted gradually, but statistically significantly, to the west. The flow was essentially unidirectional during the later phase of Kamthi sedimentaion. Orientation of long-axes of elongate pebbles and ripple mark lee-slopes are in conformity with the local foreset dip directions. Within the limited area studied, only slight suggestion of a northerly decrease in cross-bedding thickness, indicating a northerly paleocurrent, is noted. Contrary to these observations, pebble sizes in the Upper Kamthi decrease to the west-southwest, possibly due to a local swing in t e size trends. Variations in pebble size seem to yield useful paleocurrent clues only when studied on a regional scale with the observations restricted to the same depositional plane. Pebble roundness is found to vary only as a function of pebble size, and provide no clue to the paleocurrent direction. The study reveals an uninterrupted fluvial sedimentation during the Upper Paleozoic and Mesozoic, during which time, a number of dissimilar sedimentological units were contemporaneously deposited in the varied environs of the same fluvial system. On the other hand, several similar units were also deposited in similar environmental conditions at different times. An appropriate scheme of Gondwana classification should be developed to depict the complex relationship between the various lithological, chronological and biological units which have developed in this process.

Palaeomagnetic Results on Gondwana Dykes from the Damodar Valley Coal‐fields and their Bearing on the Sequence of Mesozoic Igneous Activity in India

Abstract: Summary Detailed palaeomagnetic measurements have been carried out on twelve dolerite dykes intruding a belt of Gondwana coalfields exposed in the Damodar valley region and stretching between the Rajmahal traps on the east and the Deccan traps on the west. The problem of age and correlation of the dolerite dykes activity with either the Rajmahal or the Deccan traps activity has been examined in the light of the palaeomagnetic data on these formations. The palaeomagnetic data suggests that the Rajmahal-Sylhet group of traps on one hand and the Deccan traps on the other do not represent independent igneous activities as was hitherto believed and that they are phases of a continuous Mesozoic igneous activity which began on the eastern side with a more or less contemporaneous eruption of lavas giving rise to the Rajmahal, Sylhet and Rajmahendri traps, and which passed through a hypabyssal phase giving rise to the coalfield dykes to the west of Rajmahal traps and then again manifested itself further west in the vast sequence of lava flows constituting the Deccan traps.

Outline of Tectonic History of Bolivian Andes

Abstract: Stille's chronologic sequence of events for the "typical" tectonic cycle does not apply in the Bolivian Andes because (1) the deep-sea sedimentation phase was limited to the Ordovician through Devonian, (2) the batholiths are Triassic, and (3) the principal folding phase was late Tertiary Cambrian and older rocks are poorly known in Bolivia, but an Ordovician-Devonian cycle of marine sedimentation is well established During this cycle, up to 10,000 m of mud, silt, and sand was deposited in the Eastern Cordillera A Silurian glacial episode took place on the Altiplano massif west of the Eastern Cordillera The marine Upper Devonian is overlain by another glacial sequence--the Carboniferous Gondwana Group; as the age of the oldest Gondwana beds is not known, the beginning of this glacial cycle cannot be dated precisely The Gondwana cycle terminated in Bolivia with a marine transgression that deposited the Copacabana Group of Early Permian (Wolfcampian) age Salt stocks, presumably containing salt of Permian age, are present along the eastern margin of the Altiplano, between it and the Eastern Cordillera Triassic time is represented by a sandstone sequence in the high Andes and by a sandstone-limestone-salt sequence in the southeast Granite intrusion took place in Late Triassic time (180-199 my ago), but was not preceded by any important folding as it should have been if Stille's tectonic cycle were valid There are similarities between the granite and salt intrusions Jurassic strata are scarce in the Bolivian Andes They might be represented partly by nonfossiliferous sandstone and conglomerate The post-Jurassic section consists mainly of Cretaceous through Miocene continental deposits, although two marine incursions are known to have occurred during Cretaceous time The principal folding of the Bolivian Andes was completed by Miocene or Pliocene time, and the vertical uplift which formed the present ranges began Almost all structural evidence shows a history of compression in the area until the Pliocene uplift Holocene uplift movements are described from the La Paz and Santa Cruz areas (western and eastern edges of the Bolivian Andes respectively) Petroleum, tin, and copper prospects are discussed

Relation of floras of the southern hemisphere to continental drift

Abstract: Summary Anomalous distribution of the Southern Hemisphere floras has long been recognized, and many explanations have been proposed. The occurrences of plants, modern or fossil, rarely point to a unique mode of distribution. A synthesis of pertinent information drawn from many disciplines (geonomy) is required to determine a most probable explanation. Recent studies of crustal mechanics by geophysicists show that sea-floor spreading can serve as an adequate mechanism for continental drift and can be important in explaining the distribution of ancient plants of the Southern Hemisphere. The Permian Glossopteris flora of southern continents contrasts strikingly with the Permian flora of the Northern Hemisphere. The identity of the northern flora that was contemporary with the Glossopteris flora was controversial for many years because of the great degree of isolation existing between the two floras. The botanical relationships of the glossopterids are still imperfectly established, particularly with reference to foliage occasionally found in northern areas or in deposits younger than Permian. Fertile structures of glossopterids seem to provide the only reliable basis for determining botanical relationship. The Glossopteris flora is, in general, extremely distinctive and consists taxonomically of a relatively small array of plants which occurs over an exceptionally large area. It represents an ecologic assemblage with both climatic and edaphic implications. A tentative Permian Gondwanaland reconstruction is presented that is somewhat more condensed than other reconstructions; the area thus reconstructed is more likely to have had a reasonably consistent, seasonal temperate climate in keeping with consistent similarities of the Glossopteris flora. According to this interpretation, most of the present area of the Indian Ocean seems to have been occupied by Permian Gondwanaland. The ridges and rises as recently mapped across the Indian Ocean floor are incompatible with foundered continents or isthmian links, but they can be interpreted as reflecting an historical sequence in sea-floor spreading that can account for the present dispersal of Gondwana continents.

Permian-Triassic Beacon Group of the Shackleton Glacier Area, Queen Maud Range, Transantarctic Mountains, Antarctica

Abstract: Beacon Group sediments, consisting of approximately 1425 m of Permian-Triassic graywacke sandstones, siltstones, shales, and coals, are well exposed in the Shackleton Glacier area. The strata have a terrestrial origin and are subdivided into five formations. In ascending order, the formations are Pagoda Formation, Mackellar Formation, Mt. Butters Formation (new name), Buckley Formation, and Mt. Kenyon Formation (new name). Numerous Jurassic diabase sheets, individually as thick as 300 m, have an aggregate thickness about equal to that of the sedimentary section. The rock succession is similar to Beacon and diabase rocks in adjacent areas along 800 km of the Transantarctic Mountains and Gondwana rocks of other southern continents.