Showing papers on "Gondwana published in 1979"

A model for the evolution of the Indian Ocean and the breakup of Gondwanaland

Abstract: Magnetic anomaly and fracture zone information is used to develop a self-consistent tectonic history of the Indian and South Atlantic oceans. Working backward in time we have made three reasonably well constrained (39, 53, and 65 Ma) and two speculative (80 and 115 Ma) reconstructions of the positions of the Gondwana continents (Ma is m.y.B.P.). Our final fit, which is constrained by the recognition of Mesozoic anomalies off Antarctica and in the Mozambique Basin, places Dronning Maud Land against southern Mozambique and Madagascar in the northern position against Kenya. We suggest that after the initial rifting, Antarctica moved away from Africa in a southerly direction relative to present-day Africa. This started the formation of the Southwest Indian Ridge. Most of the present length and geometry of the ridge result from migration of triple junctions so do not reflect predrift continental outlines. India and Madagascar moved with Antarctica until India separated from first Antarctica then Madagascar, when it started moving north toward Asia. In our reconstructions we find no necessity for significant relative motion between the Antarctic Peninsula and South America from the early Cretaceous to the Oligocene. From the breakup of Gondwanaland to the present we identify seven significant events. These are (1) first break in the late Triassic/early Jurassic between East and West Gondwanaland with initial motion along long transform faults parallel to the present African east coast, (2) early Cretaceous separation of Africa and South America and possibly simultaneous separation between India and Australia-Antarctica, (3) cessation of motion between Africa and Madagascar, (4) break between India and Madagascar in the late Cretaceous, (5) Paleocene reorganization in the northwest Indian Ocean when the Seychelles left India, (6) Eocene separation between Australia and Antarctica with Australia joining the Indian plate, and (7) India's collision with Asia and subsequent commencement of spreading on the Central Indian Ridge, and later opening of Drake Passage.

A Lower Carboniferous Lepidodendropsis flora in Kashmir

Abstract: The concept of a more or less uniform world-wide flora (the ‘Lepidodendropsis flora’) of early Carboniferous age, was first expounded by Jongmans1. He cited the fossil plant genera Lepidodendropsis, Triphyllopteris and Rhacopteris as characterising this palaeofloristic unit, of which he claimed representation from a number of Northern and Southern Hemisphere localities. Subsequent work has generally confirmed his conclusion that in the early Carboniferous a floristically rather uniform lycopod-rich association extended into all the major continents, preceding the onset of Carbo–Permian glaciation. In its relative uniformity this flora contrasts sharply with the pronounced regional character of the floras (especially the Gondwana Glossopteris flora) that followed the glaciation. Although representation of Jongman's Lepidodendropsis flora was claimed for the major southern land masses (South America, Africa, Australia) only a very limited Carboniferous flora, of a few genera of fern/pteridosperm leaves, has been reported2,3 from India. This flora, from the Thabo stage of Spiti, in the Western Himalayas, was the only Indian Carboniferous flora known hitherto, and it differed significantly from its counterparts in other parts of Gondwanaland in the absence of Lepidodendropsis, and indeed any other lycopods. A newly discovered flora of Lower Carboniferous age from the Gund Formation of Kashmir, some 400 km west-northwest of Spiti is reported here.

The sea floor morphology seaward of Terre Adélie (Antarctica)

Abstract: All available sounding lines were used to construct general (Fig. 2) and local (Figs. 3, 4 and 5) bathymetric charts of the Antarctic sea floor between 136° E and 147° E (Terre Adelie, Southern Indian Ocean). Major coastal and submarine features of the area are delineated and described (Figs. 6 and 7). The sea bed morphology is related clearly to the tectonic and climatic influences. Complex faulting has ruptured the Gondwana crust into a series of deep basins separated by irregular plateaus. The Pleistocene maximal extension of inland ice produced the following morphological effects: (1) the overdeepening of valleys, (2) the construction of outer banks (terminal moraines), and (3) the unusual depth of the shelf edge by incomplete post-glacial isostatic readjustment.

Paleocurrent Pattern in Gondwana Coal Measures Brahmini Coalfield (Santhal Pargana), Bihar, and Possible Regional Implications

Abstract: Among the lower Gondwana outcrops in the Brahmini coalfield, sandstone is widely developed; it is thin-bedded, brownish white, coarse to medium grained and occurs in lenticular and multi-lateral bodies for the most part. Associated rock types include thin beds of siltstone, carbonaceous shale and coal. The coal measures have been correlated with the Barakar Formation. Analysis of directional data based on cross-bedding foresets reveals a unimodal, southeasterly distribution at most exposure and sector levels. Computed value of grand mean is 167 ± 53°. Ripple crests orient more or less normal to current direction, but fossil plants are obliquely to the inferred current direction. Combined evidence from cross-bedding variability and overall lithology suggests that the inferred strearns possibly had a moderately high channel sinuosity in the northern sector but low sinuosity in the southern sector, in the down current direction. Possible implications of the southerly transport in the Brahmini coalfield are discussed in relation to other Gondwana basins of Rajmahal and Eastern India.

Paleocurrent pattern in Gondwana coal measures of Brahmini coalfield (Santhal Pargana), Bihar and possible regional implications

Abstract: Among the lower Gondwana outcrops in the Brahmini coalfield, sandstone is widely developed; it is thin-bedded, brownish white, coarse to medium grained and occurs in lenticular and multi-lateral bodies for the most part. Associated rock types include thin beds of siltstone, carbonaceous shale and coal. The coal measures have been correlated with the Barakar Formation. Analysis of directional data based on cross-bedding foresets reveals a unimodal, southeasterly distribution at most exposore and sector levels. Computed value of grand mean is 167+/-53 degree. Ripple crests orient more or less normal to current direction, but fossil plants are obliquely to the inferred current direction. Combined evidence from cross-bedding variability and overall lithology suggests that the inferred streams possibly had a moderately high channel sinuosity in the northern sector but low sinuosity in the southern sector, in the down current direction. Possible implications of the southerly transport in the Brahmini coalfield are discussed in relation to other Gondwana basins of Rajmahal and Eastern India.