Subsequent to the deposition and uplift of the Vlndhyan rocks during the Pre-cambrian era, the peninsula witnessed no further deposition of sediments for a pretty long time. During the Up. carboniferous period, however there commenced a. new cycle of sedimentation in interconnected inland basins of fluviatile and lacustrine origin.
This new phase of deposition of sediments continued up to the end of the Jurassic period. These inland sediments of Up carboniferous to Jurassic age occupying a vast tract in the peninsula and together constitute the Gondwana Group or system (named after the Gond Kingdom of M.P., where they were first, studied by H.B Medlicott in 1872). The Southern continents of the present day namely Australia, Antarctica, South America and India were during the Gondwana period united together to form one continuous stretch of land, known as Gondwana land.
Important features:
1. The deposition of Gondwana sediments commenced under glacial climatic conditions.
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2. Then there prevailed a warm and humid climate during the rest of the Up, carboniferous and the whole of the Permian.
3. Throughout ‘triassic’, there prevailed a dry and arid climatic condition.
4. During Jurassic again the country appears to have witnessed a more or less warm and humid climate.
5. Each individual cycle commenced with the – deposition of coarse sand. The Gondwana rocks were subjected to tectonic disturbances during the Mid triassic, Jurassic and Post-Eocene periods leading to the development of a number of faults in them.
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They were also traversed by doleritic, lamprophyre dykes and sills which in the Damodar valley area are said to be genetically related to the Rajmahal- traps.
Classification:
On the basis of the palaeontologica!, strati-graphical and lithological criteria, two types of classifications have been proposed for the rocks of this sequence :
(a) The two-fold classification have been proposed by C S. Fox, M S. Krishnan etc., in which they have divided the Gondwana rocks on the basis of floral characteristics into Lower Gondwana rocks characterised by Glossopteris flora and the Upper-Gondwana sediments marked by the advent of the Ptilophyllum flora.
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(b) The three-fold classification has been proposed by Feist- mantel, Vredenburg, D.N. Wadia etc. on the basis of prevailing climatic conditions and faunal characteristics of the Gondwana era. The three subdivisions are as follows:
In spite of several views for and against either of the classification, the modern trend is more in favour of the two-fold classification.
Important facts:
(i) Talchir stage is made up of greenish shales known as ‘Needle shales’, due to the characteristic weathering which they exhibit.
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(ii) Damuda series contains coal seams of Permian age.
(iii) The Barakar stage is well developed in the Jharia coal field.
(iv) The Barren measures is devoid of fossils and contain nodules of clay-ironstone.
(v) Raniganj stage contains thick horizons of coal seams.
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(vi) Panchet series was deposited in an arid climatic condition, and is devoid of coal seams.
(vii)Himgiri beds in Mahanadi valley: Pali beds of South-Rewa. Kamthi beds of Satpura in Madhya Pradesh belong to Damuda series.
(viii) Overlying the Damuda series of South Rewa is the Pasa- sora stage.
(ix) Bap beds and Pokran beds in Rajasthan contain boalders of Vindhyan limestone and are assigned to the age of Talchir Ullite.
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(x) The Tiki beds of Rewa and Denwa and Bogra stages. off Satpura are equivalent to the Maleri stage.
(xi) Rajmahal stage is principally made up of lava, flows.
(xii) Dubrajpur Sandstone belongs to the Mahadeva series in the Raniganj hills.
(xiii) The upper part of the Kota stage is known as Chikiala.
(xiv) The Kota stage is well developed in Godavari valley.
(xv) Chaugan and Jabalpur stage belongs to Jabalpur series.
(xvi) The Barakar and Raniganj stages of the Damuda series constitute the most important coal-bearing horizons of the Gondwana succession.
(xvii) The rocks of the Gondwana system constitute a total thickness of 6,000 to 7,000 metres.
(xviii) The glaciated boulder bed called the Blaini Conglomerates of Simla Hills lying at the base is equivalent to the Talchirs.
(xix) The Tanaki Boulder Bed (Glacial) of Hazara is believed to be contemporaneous with that of the Talchir.