Thursday, 21 September 2017

Geology

Dayal and Duara (1963) outlined the classification of rocks in Manipur which is more or less in line with that of Oldham (1883) with some modifications after the views of Pascoe (1929) and Evans (1932). The geologic succession according to them is

Age Rock Type
Recent to sub-recent Alluvium
Oligocene Barails
Intrusive rocks Disang Series

Intrusive rocks
(Cretaceous to early Eocene)

Surpentinities
Cretaceous Axials

 

Tectonically, the whole of Manipur forms a part of the great geosynclines that apparently had original basin topography of ridges and furrows. Sediments started depositing in the geosynclines, argillaceous sediments deposited in the furrows and arenaceous and calcarious sediments in the ridges. The structural zones within the broad eugeosyncline as classified by Aubuoin (1965), from West to East are:

  • Assam-Arakan trough (Eugeosynclinal furrow)
  • Arakan Yoma Eugeosynclinal ridge)
  • West Burmese trough (Eugeosynclinal furrow)
  • Pegu Yoma (Eugeosynclinal ridge) and
  • East Burmese trough (Eugeosynclinal furrow)

The zones are broadly N-S trending and nearly eastern half of Manipur falls in Arakan Yoma (ridge) that extends from Nagaland through Manipur and sinks into sea at cape Negrais in Myanmar. They appear again in the Andaman and Nicobar Islands and again further in the outer islands of the Malayan sea.

In the Imphal valley, lenses of the argillaceous sediments were deposited in the Assam-Arakan trough (furrow).

Thus, Manipur constitutes a part of the Burmese Arc, which extends northward into the eastern syntaxial bend of the Himalayas and southwards as extension of the Patkai and Kohima synclinoria trending NNE-SSW with a high dip. It occupies rock formations, which are geologically quite young being on out, come of the Tertiary Orogeny of the Himalayas from the shallow bed of the Tethys Sea. The oldest formations are the Disangs (Eocene Age) which is about 58 Million years old. The Disangs occupy almost the entire eastern half of the state. Geological Map It is represented by a sequence of splintery shale with minor mudstone, silstone, sandstone and limestone. The limestone has yielded typical Maestrichtian microfossils such as Globogerina and Globotruncana (foraminifers) thus indicating upper Cretaceous age. Certain foraminifers like Assilina sp. indicate middle Eocene age. Presence of some Tertiary Mollusca and Coraline Algae has also been recorded.

The Disangs are overlained by the Barails which are Oligocene in age (about 36 Million years). It occupies central and western parts of Manipur. It is characterised by abundance of carbonaceous matters.

The Barails are again succeeded on top by the Surma and the Tipam which occur in the western margin characterised respectively by argillaceous sequences. An ultrabasic belt is found on the eastern fringe of Manipur which is an intrusion into the Disang group and reflects the Ophiolite Zone. The belt is considered to extend towards Indonesia on the other.

 

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