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

Subhendu Kumar Baksi ¹, Tapan Kumar Ray ¹, Chirananda De ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta-700032, IN

Abstract

The present study includes the nature of burrowing and grazing activities of different types of Crabs on the beach floor of a part of shore line delta complex of Western Sundarbans, West Bengal, India (Fig. 1) along with their geological significance. Two species of the crab Ocypoda are seen forming contrasting burrows, pellets and grazing marks. One crab species with its products of burrowing activities significantly reflects microenvironment in its spatial extent with respect to the shoreline region. Such features might enable us to determine the orientation of shoreline and the basin marginal configuration of the past. Two species of the hermit crab Diogenes are located on the wave washed part of the foreshore and they form contrasting trails of grazing marks from which the orientation of the shoreline, foreshore slope and outer extremities of foreshore may be confidently indicated.

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Chirananda De ¹

Affiliations
1 C-36/1, Ramgarh, P.O. Naktala, Calcutta 700047, IN

Abstract

A five metre thick silty-sandstone unit, below the topmost coal seam of the Barakar Formation, has been given a status of 'Burrrow Zone' owing to the presence of trace fossils in large numbers and variety. Six geometric forms have been described. The first two represent horizontal intrastratal burrow system (branching and nonbranching), and were produced by fresh water crickets. The fecal pellets (type 3) and the associated vertical tubular burrows (type 6) are correlated with one or both of beetles and crustaceans. Selective search for food from organic matter-rich sediments led to the formation of these fecal pellets. Type 4 and 5 structures represent trail marks of horsehair worms on the sediment-water interface.

Late Barakar basin experienced a very low energy condition of deposition which accelerated the luxuriant growth of plants and organic activities in all possible interfaces. Sudden change over to high energy condition prior to the start of Barren Measure sedimentation ceased all biogenic activities. This resulted in the formation of a burrow zone below the Barren Measure Formation.

Keywords

Trace Fossils, Barakar, Hazaribagh District, Bihar, Gondwana.

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Chirananda De ¹

Affiliations
1 Geological Survey of India, 15, Kyd Street, Kolkata - 700 016, IN

Abstract

Ichnological findings from the Indian Gondwana basins provide a new insight into the role played by the ancient sea in shaping Upper Palaeozoic-Mesozoic geohistory. The ichnocoenoses with high density and diversity define typical shallow marine ichnozones and ichnofacies (Skolithos and Cruziana) recurring as thin tongues within continental deposits and, at times, clearly separating coal-bearing cyclic deposits. In the same way, as documented in other Gondwana fragments, these ichnofacies tongues in conjunction with complementary marine invertebrate body fossils, unquestionably suggest strong infaunal activities and short-lived shallow marine incursions, the most conspicuous of which are noted within the Talchir and Barakar Formations. They also split the coal seams into transgressive-regressive couplets, a very common feature of shallow marine lower deltaic (paralic) coal deposits.

Keywords

Marine Geology, Ichnology, Gondwana Basins.

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