After the formation of the Archaean group of rocks they were subsequently subjected to diastrophism, erosion and denudation, which led ultimately to the development of what is known as ‘Eparchaean unconformity’. The pre-cambrian rocks lie above this unconformity. In India the pre-cambrian rocks are divided into two systems known as (1) Cuddapah system, (2) Vindhyan system. The rocks of the Cuddapah system are older and structurally more complicated than the younger Vindhyan rocks.
Cuddapah System:
Structure:
The huge Cuddapah basin is more or less crescentic in shape with the concave side facing the east. Enormous thickness of the Cuddapah sediments (6000 metres) indicates that a slow and quiet submergence was in progress all through their deposition. The western side exhibits undisturbed sequence of rocks while the eastern side shows steep folding.
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The name of the Cuddapah system has been derived from the Cuddapah basin of Andhra state, where it is best developed.
Lithology:
The rocks of the system are devoid of fossils and include quartzites, indurated sandstones, slates, shales are left in a series of bedded traps and tuff-beds.
The Cuddapah system is made up of alternate layers of quartzite’s and shales and there exists an unconformity between any two successive series of the system.
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The Vempalle limestones have been intruded by dolerite and basaltic sills which range in thickness between wide limits are possibly responsible for the development of deposits of barites, asbestos, etc. Limestone has been metamorphosed to marbles and on account of its being impure has developed minerals such as serpentine and talc. This kind of marble is known as Ophicalcite’.
Other Outcrops:
1. Kaladgi series:
Cuddapah rocks occurring in south Maharashtra in the Bijapur district.
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2. Chandrapur series, Raipur series and Bijawar and Gwaliorseries are rocks of chddapah are occurring in respective localities of Madhya Pradesh.
3. Kolhan series:
Cuddapah rocks of Singhbhum.
4. Delhi system:
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Cuddapah rocks of Rajasthan and its tratigraphic succession is
5. Pakhal series:
Cuddapah rocks in Godavari valleys.
6. Pengana beds:
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Pranhita valley to the vest of Wardha coal field Kuddaph rocks.
Erinpura Granite and Malani Igneous suite belong to the Cuddapah system.
Vindhyan system:
Epeiorogenic movements visited the peninsula after the Cuddapah system. The Vindhyan system lies unconformably on the Cuddapah rocks and constitutes an unmetamorphosed column of calcareous, arenaceous and argillaceous sediments The Vindhyan rocks are developed characteristically in the Sone valley and in certain parts of Rajasthan, Madhya Pradesh and Andhra Pradesh. The Vindhyan rocks do not show any sign of metamorphism. They retain their horizontally.
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The lower part of the system is made up of calcareous and argillaceous sediments deposited under a marine environment. The upper part is made up principally of arenaceous rocks of Estuarine or Fluviatite origin. These rocks said to be devoid of fossils except a few doubtful organic remaine found in Suket shales of Lower Vindhyan rocks in Rajasthan.
Two genera of primitive brachiopods ‘Fermoria’ and “Krishnania’ have so far been described in the Vindhyan rocks.