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In this article we will discuss about:- 1. Origin of Gondwana Rocks 2. Distribution of Gondwana Rocks 3. Classification of Gondwana Rocks 4. Lithology of Gondwana Rocks 5. Life of Gondwana Rocks 6. Economic Importance of Gondwana Rocks.
Origin of Gondwana Rocks:
It is widely accepted that Gondwana Rocks are of fluviatile origin, that is, these have been deposited by streams and rivers in the so called Gondwana basin. The basin itself is thought to have been of the nature of a gradually sinking broad trough where deposition of the sediments was accompanied by the subsidence of the basin (under the load) so that huge accumulations were possible in due course of time. This mode of origin is confirmed by their fossil content as well as their geotectonic (structural) relations with other rocks in the area of occurrence.
The Gondwana Group of rocks form the most important – stratigraphically, geologically and economically—groups of India.
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Stratigraphically, they form evidence of beginning of the end of a southern continent including present India, Australia, South America, South Africa, Madagascar and Antarctica that existed as a unit after Vindhyan times and has been named as Gondwanaland. Numerous streams, rivers, lakes and other shallow-water-bodies that existed during that period received sediments and continued to get depressed right through Triassic and Jurassic times.
It was sometime during Cretaceous period that this great continent cracked and the resulting parts drifted through subsequent times to the present locations. During the period it remained together as a land mass, it accumulated huge volume of sediments along with remains of great variety of life that existed during that period and migrated very freely.
Peninsular India bears great evidence of these accumulations that are spread over vast areas of Bengal, Bihar, Orissa, Madhya Pradesh and Maharashtra. The rocks form a composite group, in stratigraphical sense, rather than a single system, in as much as it contains rocks belonging to more than one true system, that is, of Triassic system, Jurassic system and Cretaceous system.
Geologically, the Gondwana group presents us with best example of subsidence along major trough faults amidst the older rocks. In fact, deposition and preservation of great accumulation of sediments could have been possible due to such sinking basins. The total thickness of Gondwana sediments is estimated to be around 6000 m or even more.
Economically, the Gondwana rocks are the biggest source of COAL deposits in India. Besides this black gold, Gondwana have yielded good quality building stones, clays and iron ores of importance.
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Distribution of Gondwana Rocks:
The Gondwana group has been named after the ancient ‘Gond’ kingdom of Madhya Pradesh, the name having been used first by H.B. Medlicott in 1872.
The Gondwana rocks are traced along three large tracts in Peninsular and Central India:
(i) In Bengal along the valley of Damodar.
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(ii) In Maharashtra along the Godavari River.
(iii) In Madhya Pradesh, parallel to Mahanadi Valley.
Isolated tracts of Gondwana rocks are also found in extra Peninsula, especially in Himalayan foot-hills in Nepal, Bhuttan, Assam and in the middle Himalayas of Kashmir.
Classification of Gondwana Rocks:
Two Classifications have been suggested for the Gondwana group of rocks- three-fold classification and two-fold classification.
Following is the outline of three-fold classification which is based primarily on stratigraphical evidence, each division being assigned to a particular system in Standard Geological Scale.
In the two-fold division, based primarily on fossil evidence, the dividing line lies between the Panchet Series and the Mahadeva Series as shown in Table 16.15B.
Lithology of Gondwana Rocks:
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1. Lower Gondwanas:
This division is made up primarily of shales and sandstones with occasional layers of grits and very wide occurrence of coal in two of the three stages.
The Talchir Series is the lowest member and has a tillite or boulder bed, at its own base, which is of great stratigraphical significance. It is 15 to 60 m thick and is considered to be of fluvioglacial origin indicating extension of glacial climate to those areas of Peninsula at the time of its deposition. The boulder bed is followed upwards by greenish, fragmentary type shales and greenish coloured sandstones in that sequence.
The Damuda System, so called because of their considerable thickness, and named after Damodar River, is made up of:
(i) Sandstones and grits, generally white in appearance and about 750 m in thickness, designated as the Barker stage. This stage contains numerous coal seams and is considered Chief Coal Bearing Stage in Lower Gondwana.
(ii) Finer varieties of sandstones, about 600 m thick, but without any coal seams, hence called Barren Series.
(iii) Fine-grained sandstones, shales and Coal Seams, designated as Raniganj Series, from the type area where their thickness approaches about 1000 m.
2. Middle Gondwana:
Rocks of this subdivision are also made up primarily of Sandstones and Shales which show some variation in texture, structure and colour in different series at different places.
The Panchet Series is made up of sandstones of buff, brown and green appearance associated with shales in the upper part. They are occasionally felspathic.
The Mahadeva Series is about 1500 m thick, and is made up of typically red coloured sandstones having layers of red coloured clays at the base and top of the series. The red colour is clearly due to haematitic (iron oxide) content. The series is distinguished into a number of stages such as Pachmari stage, Maler stage, Denwa Bagre stage and Kota stage. The last named stage contains a few layers of limestone. The Kota stage is also known for having yielded dinosaurian fossil bones.
3. Upper Gondwana:
This group indicates a petrological variation in that its lower series, the Rajmahal series, is made up mostly of basaltic lava flows which contain inter-trappean clays and shales rich in Carbonaceous matter and important fossils. In many aspects, the Rajmahal Traps resemble Deccan Traps.
The Jabalpur series is lithologically a thick formation of massive sandstones and white clays in the type area.
The Umia series is developed in Kutch and is made of sandstones in the lower region (Umia stage) and calcareous shales with some typical marine fossils in the middle, Ukra stage. The upper, Bhuj stage has yielded remains of conifer plants.
Life of Gondwana Rocks:
Gondwanas have yielded a rich assemblage of animal and plant fossils that establish close relationship of Peninsular India with Australia, South America, South Africa and Madagascar etc. believed to have been a single continental unit during that span of time.
The list of Gondwana fossils is exhaustive; only a very few species are mentioned below:
i. Lower Gondwana:
Plant Fossils:
Glossopteris indica; Gangamopteris cyclopteroides; Vertebraia indica; Schizoneura sp.; Noegerathiopsis hislopi; Buriadia sewardi; Callipteridium sp., Schizoneure gondwanasis, Barakaria dichotoma; Dictyopteridum sporiferum; Phyllotheca indica; Taeniopteris feddeni.
ii. Middle Gondwana:
a. Labyrinthodonts:
Gonioglyptus longirostris, Glyptognathus fragilis, Pachygonia incurvata.
b. Reptiles:
Dicynodon orientalis, Epicampodon indicus, Rhyncosaurian sp, Phytosaurian sp, Enythrosuchoid sp.
c. Crustacea:
Estheria mangliensis.
d. Plants:
Ptilophyllum acutifolium, Elatocladus jabelpurensis, Retinosporites indica.
iii. Upper Gondwana:
Plant Fossils:
Ptilophyllum acutifolum, Nitsonnia princeps, Williamsonia indica, W. blanfordi, Taeniopteris vittata, Eletocladus conferata, Plegiophyllum peregrium, Elatocledus jabalpurensis, Brachyphyllum expansum, Araucarites macropteris; Lycopodites gracilis, Cladophlebis whitbyensis.
Economic Importance of Gondwana Rocks:
1. Coal:
Gondwana rocks, particularly the Lower Gondwanas are considered the storehouse of coal for India. These rocks contain extensive reserves spread in numerous seams distributed vertically and laterally at many places in Bihar, Jharkhand, Bengal, Orissa and Madhya Pradesh. Coking and steam coals are abundant in Barkar Stage. Bituminous coals of non-coking type, however, form the bulk of Gondwana coals.
There are different estimates for the coal reserves of Gondwana; according to one estimate, these reserves available only in thick seams (thickness more than 1.2 m) are of the order of 35000 million tonnes.
2. Iron Ore:
Gondwana rocks have been the source of carbonate iron ore (Siderite) in Raniganj coalfields, Auranga and Hutar coalfields. Their reserves are estimated at about 2000 million tonnes.
3. Clays:
Refractory clays also called fire-clays have been extracted from many places in Barkar stage coal seams. Similarly clays useful for variety of other purposes have also been found associated with coal seams in Gondwana and support a well-developed ceramic industry.
4. Building Stones:
Most Gondwana formations are made up of sandstones and clays only. Of these, there are some good quality sandstones found at Pachmari, Raniganj and Barker that are quarried for use as a dressed stone in many places.