The Jurassic system was marked by extensive marine transgression and humid tropical climatic conditions. This system shows a more widely divergent facies of deposits in different parts of the country. In the Spiti area the deposits are marine geosynclinal; in Kutch they are marine transgressional deposits, within the peninsula the deposits are freshwater, fluviatile deposits.
In the type area of Spiti in the Kumaon Himalayas the Triassic system is conformably overlain by the rocks of the Jurassic system. The junction between the two being so transitional that but for the change in their fossil contents, it would have been well nigh impossible to mark off one from the other.
The upper portion of this system is composed of shales indicating a shallowing of the sea floor towards the later part of the period, whereas the lower portion is dominantly composed of limestone.
In the peninsula, mesozoic rocks ranging in age from Mid. Jurassic to Low. Cretaceous is particularly well developed in Kutch, where they exhibit a complete succession and a total thickness of about 2,000 metres. But it is to be borne in mind that here as elsewhere such rocks are the result of what is known as marine- transgression or the invasion of sea on land possibly due to the subsidence of the peninsular block as a whole.
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In Kutch, the Jurassic rocks constitute the oldest formation and at some places underlain by a few patches of rocks of pre- cambrian age. The Jurassic rocks in Kutch outcrop along three more or less parallel anticlinal ridges running approximately east to west. In Kutch, the Jurassic rocks are divisible into four series, viz., Patcham series, Chari or Habo series, Katrol series and Umia series.
Umia series 1000 metres thick
Katrol series 400 metres thick
Chari or Habo series 450 metres thick
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Patcham series 400 metres thick
In kutch, the underlying archaeans gneisses and schists are overlain by 2000 metres of sandstones, shales, and limestones. The geological successions of the Jurassic system in Kutch are as follows;
The Jurassic succession in Kutch commences with the Patcham series which is made up principally of limestones, some sandstones and shales. The lowest beds, seen near Khera are known as ‘Knar Bet Beds’ constituted of clastic sandstones and yellow limestones.
The Patcham basal limestones contain the characteristic fossils ‘Megateuthis’; the overlying shell limestones contain pelecypods (Trigonia and Corbula) and ammonites. The Patcham coral beds are characterised by the presence of remains of corals (stylina) and ammonites.
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Above the Patcham series lays the Chari series, which takes its name from the Chari village. According to the fresh fossil collections made from the Habo dome, and since the rocks belonging to this formation are nowhere found in the vicinity of the village ‘Chari’ but are well exposed around the village ‘Haboae’ this series has been recently named as ‘Habo series’.
The oldest stage of Habo series is made up of shales and limestones and is known as ‘Macrocephalus Beds’ characterised by the presence of ‘Macrocephalites macrocephalus’. The upper part of the middle division of this stage contains a few layers of golden oolites constituting the Diadematus zone, which is a calcareous oolite and its grains are coated with thin films of Ferric oxide giving them a golden colour. This zone is named after the common ammonite ‘Indocephalites diadematus’.
The overlying Rehmani beds contain remains of a number of ammonites-‘Reineckeia rehmanni’, Sivajiceras, Phyiloceras and Lytoceras etc. The Anceps bed is characterised by the presence of the fossils ‘Perisphinctes anceps’. The marls and gypseous shales overlying the anceps beds are called ‘Atheleta Beds’, containing fossils like ‘Peltoceras atheleta’. The topmost beds of the Chari series are the Dhosa oolites, composed of green and brown oolitic limestones. The important fossils of these beds are Dhosaites, Mayaites maya, Epimayaites polyphemus etc.
The Katrol series lies above the Chari series is composed of different types of sandstones and shales. The Kantkote sandstones is the oldest stage containing fossils like-Epimayatis transience, Torquatisphinctes torquatus. The lower Katrols contain fossils Waagenia, Streblites etc. The red sandstones from the middle katrol horizon exhibit fossils like Virgato-sphinctes, Katrol. craps, Waagenia etc. The upper Katrol rocks contain no fossils. The Gajansar beds are characterised by the presence of fossils like Belemnopsis geradi, phyiloceras etc.
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The Umia series lies above the Katrol series. The bottoms most horizon ‘Zamia shale’s’ contain cycads and other plants. It is followed upwards by ammonite beds, which in turn are overlain by sandstones and conglomerates. It is subsequently overlain by Trigonia beds characterised by fossils like Trigonia ventricosa. Above it lies the unfossiliferous Ukra beds, but some plant remains are found occasionally. The Umia bed contains plant remains to a major extent like-Brachyphyllum, Williamsonia etc. The Bhuj beds are probably estuarine and contain ptilophyllum and other plants closely related to the Jabalpur series, of Gondwana system. The marine sandstones which occur above this stage is characterised by the presence of the fossils ‘acanthoceras’.
Other Distribution:
1. In the Spiti valley, above the Kito Limestone, the upper Jurassic rocks are found to lie unconformably and are represented by the ‘Sulcacutus beds’. Above the Sulcacutus beds lies unconformably the ‘Spiti-shales’ which is made up of three stages, viz., the lower is called ‘Belemnite geradi beds’, the middle is the ‘Chidamu beds and the upper is the ‘Lochambal beds’.