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

Utpal Chakrabarti ¹

Affiliations
1 Geological Survey of India, Coal Wing, AE 623, Salt Lake City, Calcutta 700064, IN

Abstract

Spatial and temporal variations in small·scale subsidence pattern in a part of Sohagpur Coalfield have been identified and mapped using the method of graphic correlation. Subsidence was differential at the initial stage which later gave way to a tectonically stable condition of deposition. Lateral variations in thickness and continuity of mineable coal seams, however, cannot be satisfactorily explained in terms of relative rates of subsidence only. Geometric properties of the coal seams instead reveal a dominant control of depositional environment on seam morphology.

Keywords

Coal, Barakar Formation, Sobagpur Coalfield, M.P.

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Utpal Chakrabarti ¹

Affiliations
1 Geological Survey of India, Calcutta 700064, IN

Abstract

The Barakar Formation of Sohagpur Coalfield is an alluvial succession consisting principally of channel sandstones with subordinate amounts of overbank deposits comprising finer clastics and coal. The major factor controlling peat accumulation is the physical characteristic of sub-peat sediments. Coal is thin and high in ash in sand-rich areas which compacted least and produced elevated surfaces where peat was sub-aerially exposed and oxidised. Two modes of accumulation of thicker and relatively low-ash peat operated in the study area. First, relatively sand-deficient areas compacted easily and yielded morphologically low surfaces where peat could accumulate in greater quantity and the water table was sufficiently maintained to prevent peat destruction. Secondly, an optimal sand concentration in the substrate created intermediate topographic sites where a delicate balance between the water table and the depositional interface favoured concentration of thicker and better quality coals.

Keywords

Coal, Palaeotopography, Barakar Formation, Sohagpur Coalfield, Madhya Pradesh, Sedimentology.

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P. C. Bandopadhyay ¹, Utpal Chakrabarti ¹, N. R. Mohapatra ¹, Abhinaba Roy ¹

Affiliations
1 Geological Survey of India, Eastern Region, DK-6, Salt Lake City, Kolkata - 700 091, IN

Abstract

This report documents ground uplift of a part of the eastern coast of North Andaman Island during the December 2004 earthquake. Emergence of a part of fringing coral reef above the high tide level has been recorded from the Kahpur beach. The newly emergent part of the coral reef is -75m long, -25m wide and trends ESE paralleling the trend of the shore line. The study records appearance of a -600m long and 200m wide tombolo joining the Smith Island to the Ross Island off the eastern coast of North Andaman Island. Before the December 2004 earthquake it was a sand bar that emerged only during the low tides. Vertical movement of 0.4m is calculated for the Kahpur beach while that between the Smith and Ross Islands is 0.7-0.8m.

Keywords

Coral Reef, Tombolo, December 2004 Earthquake, Kahpur, Smith Island, Ross Island, North Andaman Island.

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Utpal Chakrabarti ¹, Abhinaba Roy ²

Affiliations
1 Geological Survey of India, Seminary Hills, Nagpur - 440 006, IN
2 Geological Survey of India, Eastern Region, Kolkata, IN

Abstract

Alluvial fans of Upper Pleistocene age occur as transverse depositional systems along the fault-Defined Satpura mountain front that marks the northern margin of Purna Valley Basin. Our findings indicate that the fans were built by three main sedimentary processes : Debris flow, sheetflood and stream flow. The debris flow fans are the smallest in size and comprise ill-Sorted, matrix supported conglomerates showing evidence of deposition on slopes. The sheetflood fans are larger than debris flow fans in size and are composed predominantly of laterally extensive, tabular, planar-Bedded couplets of relatively coarse and fine grained sediments. The stream flow fans are the largest in size and represent channelized flows with sheet-Like, clast supported, massive gravel beds as the fundamental depositional unit.

It emerges from this study that two essential factors controlled the sedimentary processes acting on the alluvial fans. These are : The size of the drainage basins and the tectonic geomorphology of the mountain front. The debris flow and sheetflood fans are restricted to the linear, tectonically active part of the mountain front dissected by small streams only. The stream flow fans developed adjacent to that part of the mountain front which is tectonically less active, erosionally embayed and drained by rivers with large catchments.

Keywords

Alluvial Fan, Facies, Sedimentary Processes, Mountain Front, Upper Pleistocene, Purna Valley Basin, Central India.

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Utpal Chakrabarti ¹, Abhinaba Roy ¹, Subhash Bhandari ², Rachna Raj ², D. M. Maurya ², L. S. Chamyal ²

Affiliations
1 Geological Survey of India, Central Region, Nagpur, IN
2 Department of Geology, M. S. University of Baroda, Vadodara-390002, IN

Abstract

No Abstract.

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P. C. Bandopadhyay ¹, Utpal Chakrabarti ¹, Abhinaba Roy ¹

Affiliations
1 Geological Survey of India, Eastern Region, Salt Lake City, DK-6, Kolkata - 700 091, IN

Abstract

The Namunagarh grit of Eocene age in South Andaman island is a submarine fan deposit where sedimentation occurred in deep marine environment mainly by turbidity currents. The mudstone and fine-grained sandstone of the turbidite sequence yielded three ichnogenera viz Thalassinoides, Teichichnus and Lorenzinia.

Typically shallow marine Thalassinoides burrows co-occur with deep water burrows represented by Teichichnus and Lorenzinia, indicating a dominant control of substrate, availability of food and rate of sedimentation against bathymetry on distribution of trace fossils. Trace fossils are particularly abundant in the mudstones pointing to a strong lithofacies control. A well-oxygenated muddy bottom with a low rate of sedimentation is envisaged.

An order in the succession of traces could be deciphered. Simply constructed burrows of Thalassinoides appeared initially followed by complex structures of Teichichnus and ornate Lorenzinia.

Keywords

Trace Fossils, Eocene, Namunagarh Grit, South Andaman Island.

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