Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Hayat A. Qidwai ¹, Satyendra M. Casshyap ²

Affiliations
1 Atomic Minerals Division, Department of Atomic Energy, New Delhi, IN
2 Department of Geology, Aligarh Muslim University, Aligarh, IN

Abstract

Probability plots of grain size analysis of Lower Gondwana Sandstones reveal three types of grain size distribution characterised by one, two and three inflection points between sediment populations. In the given sandstones, traction load occurs in minor (< 1.0%) to small (~15%) proportion, but may be locally abundant (~71%). Saltation load generally forms the bulk of the sediment, and fine clastics as suspension load occurs in subordinate amount. Probability plots reveal overlapping particle size limits between traction and saltation loads and saltation and suspension loads. Analysis of inclination values of probability plots suggests that traction load is poor - to moderately sorted; saltation population on the average moderately - to well sorted in the case of Talchir and Barakar sandstones, but poor - to moderately sorted for sandstone of the Motur Formation. Relevant attributes of sediment size are genetically interpreted, in order to decipher differencesbetween the processes which operated during the deposition of Lower Gondwana sandstones in the Pench Valley basin.

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Zahid A. Khan ¹, Satyendra M. Casshyap ¹

Affiliations
1 Department of Geology, Aligarh Muslim University, Aligarh-202001, IN

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.

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Zahid A. Khan ¹, Satyendra M. Casshyap ²

Affiliations
1 Lucknow 226001, U. P., IN
2 Department of Geology, Aligarh Muslim Universlty, Aligarh 202001, IN

Abstract

Three scales of cross-beds were recognised and investigated for the Barakar strata of the Jharia basin, attributed to deposition largely by streams of low to moderate channel Sinuosity. Following the recent modifications in statistical methods of crossbedding azimuthal data, sample resultant instead of mean was compared between three scales of cross beds. Sample resultant so computed was tested for 16 sample pairs in 6 sectors and 3 sample, pairs at formation level by applying a modified 'P' statistics. Results are found to be non-significant at 95% level of confidence in 16 out of 19 sample pairs.

Synthesis of paleocurrent data reveals With high degree of probability that all scales of cross beds in fluviatile deposits may be regarded as reliable indicators, provided a proper interpretation is made in respect of the environment of deposition of the host bed form and associated sediments.

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Satyendra M. Casshyap ¹, Zahid A. Khan ²

Affiliations
1 Department of Geology, Aligarh Muslim University, Aligarh 202001, IN
2 Geology and Mining Directorate, 2-Way Road, Lucknow 226001, IN

Abstract

The clastic assemblage of Lower Permian Karharbari (70 m thick) and Barakar (800 m thick) Formations of the East Bokaro Basin shows a progressive upward decrease in grain size and scale of cross-bedding, illustrating a fining upward mega-cycle. A paleo-flow analysis reveals that the Gondwana sediments were deposited by streams which flowed towards north-northwest. The azimuthal variability is 2240 for Karharbari, 3304 for Lower Barakar and 4429 for Upper Barakar. Empirical relationships from modern streams are used to estimate various channel morphological and hydraulic attributes of ancient Karharbari and Barakar streams. Results indicate a progressive decrease in channel width and depth, channel slope. flow velocity and Froude number, with consequent increase in sediment load parameter, channel sinuosity for the Gondwana streams. as sedimentation continued from Karharbari through Lower to Upper Barakar. It is suggested that amelioration of climate through time possibly had a great control on river metamorphosis and sedimentation than tectonism.

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