Current Science

A. B. Roy ¹, Alokesh Chatterjee ²

Affiliations
1 Department of Geology, Presidency University, Kolkata 700 073, IN
2 Hoogli Moshin College, Hoogli 712 101, IN

Abstract

The Bengal Basin evolved as a rift-controlled extensional basin along the NNE-SSW trending Basin Margin Fault coevally with the 85° East Ridge in the Bay of Bengal during the short-lived hotspot activity south of Bhubaneswar. The basin opening post-dated the Kereguelen Plume magmatism (at ~116 Ma), but predated the phase of continental collision that triggered the rise of the Himalaya in the north. Supply of sediments in the initial stages of basin opening was from the west, mainly through the denudation and erosion of the uplifted Precambrian Shield. Following virtually similar tectonic and depositional pattern in the entire basin, an abrupt change in depositional pattern was recorded during the Oligocene with the emergence of easterly source of sediments derived from the uplifting of Indo-Myanmarese Ranges. Between the Oligocene and Late Pleistocene different parts of the Sylhet Trough (the best-studied region in the deeper part of the Bengal Basin) received huge volumes of sediments, which resulted in deposition measuring between 10 km and over 17 km in thickness. This was followed by an equally sudden drop in the sediment supply from the east due to the basin inversion concurrently with the westward advance of the Indo-Burmese mountain front during early and mid-Pleistocene. Followed by a short hiatus, the depositional scenario changed completely with the arrival of thick volumes of sediment during the late Pleistocene-Holocene, which covered the entire Bengal basin with the sediments brought by the Ganga and Brahmaputra from the Himalayan sources.

Keywords

Basin Evolution Tectonics, Extensional Rift Basin, Depositional Pattern, Palaeogeographic Setting, Sediment Supply.

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A. B. Roy ¹, N. K. Chauhan ², Alokesh Chatterjee ¹

Affiliations
1 Department of Geology, Presidency University, Kolkata 700 073, IN
2 Department of Geology, ML Sukhadia University, Udaipur 313 001, IN

Abstract

Kachchh domes are recognized by the oval to elliptical- shaped outcrop patterns marked by outlines of bedding surfaces which invariably dip in the outward directions. The occurrence of domes in rows without having corresponding basin-like features implies that these are not superposed folds resulting due to constriction- type tectonic forces. Further, in spite of the close time-space relationship, the occurrence of domes is exclusively in the Mesozoic rocks on the uplifted block of the fault. The absence of any such rock formation on the other side of the fault rules out the possibility that these are 'drape folds' developed during the adjustment of the sedimentary blanket over the faulted-up edges of the basement blocks. Hinging on the evidence of intrusive plutonic (mafic) masses in the core of some of the domes, we suggest that the structures evolved through diapiric rise of magma bodies causing dome-shaped up-warping (bending) of the pre-existing (Mesozoic) flat-lying sedimentary formations. Linear disposition of domes is explained as due to channellization of magma along the fractures that developed around large-scale crustal doming during the early phase of the Reunion Plume impingement under the Indian Lithosphere.

Keywords

Diapiric Folds, Domes, Evolutionary History, Magma Bodies.

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A. B. Roy ¹, Alokesh Chatterjee ², N. K. Chauhan ³

Affiliations
1 Niloy Apartment, Flat 3/2G, 46A, R.N. Das Road, Kolkata 700 031, IN
2 Geology Department, Hooghly Mohsin College, Chinsura 712 101, IN
3 1014 Hiran Magri, Sector 4, Udaipur 310 002, IN

Abstract

Kachchh geological province is conventionally referred to as 'Kachchh basin' or 'Kachchh rift basin' or even 'Kachchh aulacogen'. The geological records, however, recount a different history of the diverse depositional environment under diverse tectonic situations. The earliest Phanerozoic event was the deposition of Palaeozoic sediments during upper Permian and lower Triassic. Next event was a major marine incursion along rift zones coinciding with the Gondwana break-up and the initiation of northward drifting of the 'Indian land mass'. This was followed by the Reunion Plume related magmatism centring on the K- T boundary. Overlying this occur lignite deposits in the Naredi Formation correlatable with the global Eocene excursion. The Cenozoic basin closure corresponds to the major uplift in the Himalayas during the early Quaternary. Finally, the evidence of recurrent youngest block-movement-type active tectonics transformed the terrain into a zone of high seismicity.

Keywords

Cenozoic Depositories, Kachchh, Palaeocene–Eocene Thermal Maximum, Phanerozoic Events, Quaternary.

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