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Here is a list of eight important Indian stratigraphical system.
1. The Jurassic System:
Jurassic System overlies Triassic System rocks in the Standard Stratigraphic Scale and is well represented in Indian Stratigraphy both in extra-Peninsular and Peninsular India.
In the extra-Peninsula, the Jurassic rocks of marine facies conformably overlie in the Triassic rocks at many places such as in Spiti, Kashmir and Salt Range. In Peninsular India, development of coastal marine and estuarine facies of Jurassic rocks has been studied in Rajasthan, Kutch and east coast of the Peninsula.
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We shall first describe in outline the Jurassic of extra-Peninsula.
Spiti:
Rocks of Jurassic age succeed the Upper Triassic (massive limestone series) formations in Spiti and have been distinguished into a lower group of great thickness of massive limestones—the Kioto Limestones – and an Upper Group, chiefly composed of Shales and hence known as Spiti Shales.
The Kioto limestone is classed by many as of uppermost Triassic age. The series considered to be truly Jurassic in age is the Spiti Shales. It is separated by an unconformity at the base from the so-called Sulcacutus Beds.
The Spiti Shales are further distinguished into three stages namely Belemnite beds, Chidamu beds and Lochambal beds.
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The Belemnite Beds are made up of brown shales of earthy texture containing calcareous concretions and nodules built around ammonitic nuclei. The typical fossil of the beds is Belemnopsis gerardi and hence the name.
The Chidamu Beds overlie the Belemnite beds and consist of black shales with iron oxide partings. These shales also contain calcareous concretions and nodules aggregated around ammonitic nuclei. These shales are characterized with typical fauna such as Phylloceras plicatus, Balnfordia wallichs, Perisphinctus tibetanus etc. from the Cephalopods and Rhynconella lacunose from the Brachiopods.
The Lochambal Beds are similar to Chidamu beds in their lithology being made of shinning splintery shales of black colour rich in calcareous concretions. The typical fossils of these layers include Basriasella, Blamfoldia and Acanthodiscus.
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From the fossil assemblage an Upper Jurassic Age (Portiandian) has been assigned to Spiti Shales.
Kashmir:
Jurassic rocks have been traced in Ladakh and Banihal areas in Kashmir. At Ladakh, Jurassic formations comprising chiefly of limestones and shales (similar to Spiti sequence) directly overlie the Triassic sequence in a conformable manner. In Banihal, however, they occur on the north side of Banihal pass within a synclinal sequence of Triassic rocks. The Ladakh Shales do not contain any calcareous concretions like the Spiti Shales. Their fossils yield is also very meager and not well confirmed.
Salt Range:
Ceratite beds of Triassic age are overlain by Jurassic rocks of middle to upper Jurassic age in Salt Range. Lithologically these comprise sandstones, shales and limestones, the latter being oolitic. The shales are rich in gypsum and pyrite bands. Fossils found from Upper Jurassic include Pecten, Lima (Lamellibranchs), Ammonites and some Belemnites whereas from Middle Jurassic, genra like Pleuratomaria, Pecten, Cerithium etc. have been found. Middle Jurassic formations are traversed by thin coal seams at a few places.
The Jurassic of Kutch:
Jurassic rocks are regarded as the oldest (for the greater part) and most important stratigraphical formations in Kutch (Gujarat state). The formations are about 2000 m thick and rest in most cases over the oldest Pre-Cambrian. These are broken by faults striking east-west so that the sequence is repeated alongside the faults.
The system has been divided into four series as follows:
The base of the sequence is not seen.
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Brief description of these series is as follows:
i. Patcham Series:
It is made up mostly of yellow limestones which are shelly in texture (indicating marine origin). The series has yielded fossils like Corbula lyrata and species of Trigonia belonging to Pelecypod group. Some corals and plant fossils have also been yielded.
ii. Chari Series:
It is a complex series that has been distinguished into many stages of which five prominent stages are named as Macrocephalus stage, Diadematus zone, Rehmani zone, the Ancep Beds and the Dhosa oolites. Lithologically, the Chari series is made up of Oolitic limestones, dark Shales and white Limetsones.
This series has also yielded a rich assemblage of Jurassic fossils such as Macrocephalites, Indocephalites, Rieneckeia, Perisphinctes, Petloceras, Mayaites, Epimayaites and Dhosaites and so on. The series range in age from Callovian to Lower Oxfordian in Standard Stratigraphic Scale.
iii. Katrol Series:
Lithology, it is made up of sandstones and shales (Limetsones are scant). The series has yielded fossils of Epimayaites sp, Prosophinctes sp of upper Oxfordian age. Some plant remains have also been found.
Umia Series:
This overlies the Katrol series and is made of shales, sandstones and conglomerates. The series is itself divided into three stages- Umia stage, Ukra stage and Bhuj stage. The formations have yielded fossils of Trigonia, Australiceras, Ptilophyllum, respectively, among other fossils. The last mentioned fossil suggests an estuarine origin for the Bhuj stage.
2. The Cretaceous System:
The Cretaceous System may be described as the most widespread and at the same time lithologically most complex system of Indian Stratigraphy. This system includes rock formation formed under entirely different conditions during the same general time span – the Cretaceous Period.
Thus, under Cretaceous system are grouped:
(a) The marine rocks of Northern Himalayas and Coromandal Coast;
(b) The fluviatile rocks of Madhya Pradesh and Deccan;
(c) Part of igneous rocks (volcanic and plutonic) of Peninsula (the Deccan Traps).
“This heterogeneous constitution of Cretaceous” is, in the words of Dr. Wadia, “proof of the prevalence of much diversified physical conditions in India at the time of its formation and existence of quite a different order of geographical features.”
The Marine Facies of Cretaceous System:
1. The Coromandal Coast:
The Coromandal Cretaceous rocks, which are actually of Upper Cretaceous age, are regarded as the most thoroughly studied formations of Southern India because of their geological and palaeontological importance. These rocks cover a large area starting from Coromandal coast and extending into the valley of Cauvery. The Cretaceous succession studied in Trichinopoly may be described as an illustration.
In Trichinopoly, the cretaceous rocks mostly rest over a platform of Archean age formed by granitic gneisses and charnockites. The rocks are mostly limestones, sandstones, grits and gravel beds. They have been divided into four stages as described in Table 16.13.
2. Cretaceous of Narbada Valley:
The so called Bagh Beds of Narbada Valley provide a second example of development of marine facies of Cretaceous rocks in Peninsular India. These consist of isolated outcrops of Cretaceous rocks running along the Narbada Valley, in an east-west direction, starting from Bagh in Gwalior (Madhya Pradesh) and reaching Baroda in Gujarat.
In the Bagh area, the Cretaceous sequence is distinguished into a Lower and an Upper series.
The Lower Bagh beds called the Nimar Sandstones rest over a basal conglomerate separating them from the metamorphic rocks of earlier ages. These Nimar sandstones make excellent quality building stones and millstones.
The Upper Bagh beds are made up of three layers of rocks in ascending order- Nodular clayey limestone, Deola Marls and Coralline limestones.
For most of their thickness, the Bagh Beds are unfossiliferous. But Deola Marls, only a few metres thick, have yielded rich assemblage of Cretaceous fossils of Brachiopods, Cephalopods, Lamellibranchs, Gastropods, Echinoids and Fishes indicating a Turonian age for the series.
3. Marine Cretaceous of Extra-Peninsula:
In extra-Peninsula, marine Cretaceous rocks are developed at Spiti, Northern Hazara, Sind and Baluchistan.
In Spiti, the rocks of Cretaceous age are divided into two series- Guimal Series and Chikkim Series. The Guimal series is made up of sandstones and quartzites overlying the Spiti Shales of Jurassic age. The Chikkim series is made up primarily of limestones which are, grey or white colour compared to brownish to yellow sandstones of the underlying Guimal series.
The distinction is, therefore, quite easily made from a distance. The Chikkim series is in turn overlain by a newer formation of sandstones and Shales of the Flysch facies which indicated a very shallow environment of deposition almost unsuitable for supporting fauna or flora.
As such Flysch formations are totally unfossilferous whereas both the Guimal and Chikkim series have yielded rich Cretaceous fauna, chief among which are:
Stephanoceras; Holostephanus, Belemnites, Cristellaria, Texularia, Nodosaria and the like.
The Cretaceous sequence is also developed in Kumaon and Tibet part of the Himalayas. In Kashmir, Cretaceous limestones have been recorded developed along the Leh-Yarkand Road.
4. Origin of Marine Cretaceous:
Rocks, belonging to marine Cretaceous in extra-Peninsula and also those of Bagh Beds in MP- Gujarat belt are regarded as having been deposited in the sea of Tethys (considered to be an extension of European Sea and which is believed to have suffered a retreat during this period). The lithological and fossil sequence in the Cretaceous rocks of Spiti supplies enough evidence that Tethys started shallowing and ultimately disappeared during this period.
The marine Coromandal Cretaceous rocks are, however, believed to have been deposited by a different sea, most likely a branch of southern Ocean which had transgressed temporarily over the coastal areas during Cretaceous Age.
The Fluviatile Cretaceous (The Lameta Series):
The fluviatile and estuarine facies of Cretaceous system is well represented by the Lameta Series that comprises of a number of isolated outcrops developed in Madhya Pradesh at Jabalpur, Bhopal, Indore and also in the western part of the Narmada valley. They rest over older formations and are in turn overlain by Deccan Traps for which reason they are sometimes referred as Infra-Trappean Beds.
Lithologically the Lametas consist mostly of sediments deposited and consolidated in fluviatile, estuarine and at best shallow marine environments. Lameta series is composed of sandstones, grits, clays and siliceous limestones which seldom acquire any great thickness but are comparatively of wider lateral extent.
Life:
Fossils of freshwater animals and land animals like reptiles (chiefly dinosaurian) have been found from these rocks. This series has provided with a large number of Dinosaurian reptiles which show close resemblance to the Madagascar Dinosauria of Cretaceous age.
Chief fossils of Lameta series are:
Dinosaurs:
Antarctosauras septentrionalis, Titanosauras indicus, Indosauras matleyl, Lametosauras indicus.
Fossil Molluscks like Melania, Physa and Pauldina and Fishes like Lepidostens have also been found from Lameta layers. These fossils collectively indicate a Tuoronian Age for the Lameta Series.
3. The Tertiary Group:
The Tertiary Group, also termed Kainozoic (Cainozoic) Group includes four systems- The Eocene, The Oligocene, The Miocene and The Pliocene system. In Europe and in Indian subcontinent each system has been further subdivided into numerous series, stages and sub-stages mostly with local names.
The Tertiary Group has been of great significance in the global stratigraphy in many ways.
The end of previous geological Era, the Cretaceous Era marked a number of changes, most important of which are:
(a) The great Mediterranean Sea- The Tethys that existed throughout Createous period started getting compressed due to northward directed forces (due to plate movements) and shallowed, gradually disappearing totally from the scene.
(b) The sediments accumulated in the Tethys since the earlier periods, under the influence of orogenic movements that became prominent during the Tertiary period, started rising as equatorial mountain systems that came up in the form of Great Himalayas and also the Alps, The Atlas, the Pyresea, the Caucausus and the Burma Andaman Arc. The process of rising of the Himalayas is believed to be continuing even today.
(c) The resulting formations were distinctly of two types- the Lower being TYPICALLY of marine origin, bearing all the evidence of a gradually shallowing sea, and the upper formations being typically of fresh water origin, laid down in streams and lakes.
(d) The quality of life, as evidenced by the fossils, suffered a great change. Many important and ruling genra of animal and plant life such as the great reptiles and the ammonites having disappeared and the mammals making their appearance with great vigour.
With this type of information at the background, we study the Tertiary Group in an established systematic manner.
Distribution:
The Tertiary Group rocks are developed extensively in the extra-Peninsular India and adjoining areas of Pakistan in North-West and to Assam in the North-East. In the Peninsular India, Tertiary group rocks are developed in prominent isolated patches in Kutch (Gujarat) and Chennai and Orissa in the eastern coast.
Tertiary Group of Extra-Peninsula:
Important development of rocks of Tertiary Group have been recorded in extra-Peninsula in outer Himalayas of Jammu, the Punjab, Assam and also in Sindh, Baluchistan and Salt Range areas of Pakistan. We shall give outline classification of the Tertiary Groups in these areas for a general reference.
Outer Himalayas:
Tertiary rocks compose almost the whole of the outermost Himalayan range. These have been thoroughly studied at many places in Jammu (Kashmir), the Punjab, Simla (Himachal Pradesh) and other areas. In these areas, they form two belts- the outer belt of the Upper Tertiary Group and the inner belt of Lower Tertiary Group. Towards the north of the Central Himalayas, Tertiary rocks are almost totally absent except at few places in Ladakh, where they rest un-conformably over older gneisses and schists.
Classification:
The Tertiary group of Kashmir Himalayas, Simla Himalayas, Assam Himalayas, Baluchistan and Salt Range has been classified as shown in table 16.16 (A-D). These tables are meant for a comparative study.
4. The Eocene System:
The Eocene system is the oldest of the Tertiary Group and chiefly marine in origin having been formed in the gradually dwindling extensions of the seas. It is developed in Kashmir Himalayas, Simla Himalayas, Assam Himalayas, Sind and Baluchistan and Salt Range in Pakistan.
Classification:
The Eocene system, where developed can be divided into three subdivisions- Lower, Middle and Upper. These divisions are well represented by the Laki Series, Ranikot Series and Kirthar Series respectively in Sind where they are best developed.
Lithology:
Sandstones, shales, clays and limestones are common rock types developed in Eocene system. These rocks, however, show great variation in their texture, structure and colour from place to place indicating quite unstable conditions of deposition.
The Eocene rocks are developed at many places on the southern flanks of Pir Panjal in Jammu and Kashmir. In Jammu, outcrops of Eocene age are found near Reasi in the form of inliers around Sirban Limestones. The sequence being similar to Subathu series of Simla is also named accordingly. Lithologically, they are made up of laterite zone at the base followed upward by carbonaceous shales and nummulitic limestones.
The Eocene rocks of this area are economically important as they yield coal from the pyritic shales. The pisolitic bauxite forming a part of the Subathu series is considered suitable for a refractory use. The Subathu series of Jammu rests unconformably over the Permo-carboniferous rocks and is overlain in a similar manner by Murree series.
Subathu series of Eocene age is also developed near Simla where it forms a typical example of development of Eocene rocks in outer Himalayas. Shales of red and grey colours and sandstones form the Subathu of Simla. These are thought geologically equivalent to Kirthar series (Upper Eocene) of Sind.
Other Examples of Eocene:
i. Assam:
Eocene Rocks are exposed in many parts of Assam such as:
a. Jaintia Series – in Shillong Plateau;
b. Disang Series – in Upper Assam. The Jaintia Series is made up of limestones and sandstones, which have yielded quite good number of fossils.
ii. Bengal:
Formations named as Jalangi, Sylhet and Kopili and containing variable thickness of shales indicating an estuarine environment have been found in the East Bengal region.
iii. Gujarat:
Exposures of rocks of decidedly early Eocene age have been named as Kadi series in Kutch. Some isolated outcrops have also been found between Surat and Baroch.
Life:
Among the common fossils of Eocene may be mentioned:
i. Foraminifera – Nummalites nuttali, N. obtusus, N. petforatus, Assilina granulose, Orbitolites complanata, Dictyoconoides cooki
ii. Echinoids – Phyllacanthus sindensis, Echinocyammus nummulitics, Conoclypeus rostratus, Micraster tumidus.
iii. Lamellibranchs – Ostrea multicostata, O. vesicularis, O. pharaonum, Spondylus roxanae- Vulsella legumen.
iv. Gastropods – Surcula polycesta, Conus blagrovei, Turritella halaensis, T. angulata, and also species of Corals, Cephelopods etc.
The list of foraminifer’s fossils is very exhaustive.
5. The Oligocene-Lower Miocene Systems:
No well-defined development of purely oligocene rocks has yet been observed in Kashmir, the Punjab and Assam Himalayas. It is inferred that the regression of the sea was in great progress from this part during the oligocene. In the Sind and Baluchistan areas, however, the extensions of the sea, though in regression, continued to exist and receive sediments.
Hence it is in Sind (Pakistan) that the best exposures of Oligocene-Miocene system are recorded:
1. The Oligo-Miocene Strata of Sind:
It is divided into two series:
a. The Nari Series and
b. The Gaj Series.
a. The Nari Series:
It is made up mainly of limestones in the lower region and sandstones and conglomerates in the upper region. Both limestones and sandstones show considerable variation in colour when traced upwards. The series has yielded formanifera, corals, echinoids, lamellibranchs and gastropod fossils that indicate an Oligocene age. Its total thickness is about 1800 m.
b. The Gaj Series:
This overlies conformably the Nari series at most places and consists mostly of Limestones of various lighter shales intercalated with gypsum, clays and enriched with sands. The sequence of deposition points out to a gradual shallowing from an original marine environment. Fossils indicate an uppermost Oligocence-Lower Miocene age for the rocks of Gaj Series.
2. West Punjab-Jammu-Simla Himalayas:
The Oligocence-Lower Miocene formations with modification as dictated by environment have extensive development as indicated by deposits of brackish and fresh water origin in an almost contiguoun basin extending from N.W. Punjab through Jammu to Simla Himalayas. Although separated presently by good distances, these Oligocence- Lower Miocene deposits show remarkable similarities in their stratigraphical and fossils character.
The different important formations of this sequence are discussed briefly as below:
The Murree Series:
These are developed best in NW Punjab, Jammu Hills and are also represented eastwardly in the Simla Himalayas by the Dagshai and Kasauli Beds.
In the NW Punjab, the Murree Series begin with a basal bed of conglomerate resting un-conformably over the Chharrat Series of Eocene age. The basal zone is called Fatehjang zone.
The Murree Series is made up of shales and sandstones, generally of various shales of purple, maroon and red, indicating their having been derived from iron-rich sediments. It is divided into two stages- the Lower Murree and the Upper Murree.
The Lower Murrees are characterized with:
(i) Shales of bright purple colour;
(ii) Sandstones of grey to grayish brown colour;
(iii) Some plant and animal fossils.
The Upper Murrees are made up mostly of sandstones, which are soft in texture and pale-brown in colour. They also contain fossils of some animals and plants. Age of the Murrees is placed at Lower to Middle Miocene.
The Dagshai Beds occur only in Simla Himalayas and overlie the Subathu series of Eocene age and are made chiefly of massive quartzitic sandstones intervened by red mottled, clay seams.
Kasauli Beds:
These sandstone layers overlie conformably the Dagshai beds in the Simla Himalayas. The sandstones are grey to greenish in colour and show at some places clayey bands (but the distinctly red clays of Dagshai formations are typically absent). They have visible traces of mica and are much soft compared to Dagshai sandstones. The Dagshai and Kasauli beds may be considered extensions of Murrees.
3. Oligocence-Lower Miocene of Assam:
The sequence of this time period is represented in Assam by the well-known Barail Series and the Surma Series which are important economically as well as being the source rocks of coal and oil deposits of the north eastern India.
Barail Series:
The name is derived from a hill range. The series is made up of sandstones and shales. The shales are carbonaceous in character and the sandstones are of thin-bedded and flaggy type in Upper Assam exposures and hard and massive in their exposures in Khasi and Jaintia Hills.
Although the Marine-estuarine origin of Barail rocks is easily established from lithological evidence, these have not yielded fossils of any significance.
Surma Series:
Rocks of this series rest unconformably over the Barail series in Upper Assam in Surma Valley and Arakan region of the state. They acquire a thickness of more than 5500 m in the Arakan region. The series has been divided into a lower Bhuban stage (1200-2000 m) which is made up of sandstones and arenaceous shales and an upper Boka Bill stage, of silts, conglomerates and sandstones which at places are distinctly ferruginous in character. It is typical of Surma series that they contain no coal seams.
6. The Siwalik System (Middle Miocene to Lower Pleistocene):
Definition and Distribution:
Under Siwalik System are grouped rocks of low lying outermost hills of the Himalayas that had been deposited in shallow water conditions in times ranging from Middle Miocene to Lower Pleistocene in the geological history of this area. The name is derived from the Siwalik Hills of Hardwar in Uttaranchal.
Rocks of the same age and type had been deposited in similar conditions in many other parts of northern Indian subcontinent and are known by such names as- the Manchhar System in Sind (Pakistan), the Makran System of Baluchistan (Pakistan), the Dihing System in Assam and the Irrawady System of Burma. The Siwalik System under description extends from the Brahmaputra Valley in the east to the Bannu Plains in Pakistan on the west, local terminology not-withstanding.
Lithology:
The Siwaliks are a huge thickness (5000-6000 m) of sandstones, shales, clays conglomerates, and the last named rock occurring as a cap over the Siwaliks. The sandstones are hard, compact and red coloured in the lower Siwaliks becoming grayish and softer in the middle levels and virtually becoming masses of loosely packed coarse sands with grits in the upper region. Similarly, shales show significant lithological variation as they pass upwards in the stratigraphical column, changing from bright purple shales to light orange and drab colours in the younger formations.
Classification:
The Siwalik System has been divided into three sub-divisions, primarily on the basis of their fossil contents. Each subdivision has been further divided into stages named after places of typical exposures. Rich assemblage of fossils has been collected from many stages. The classification is summarized in Table 16.17.
Life:
A rich assemblage of fossils has been recovered from the Siwalik rocks which have given this system a very important status in Indian stratigraphy. Most of the stages in Siwalik system are typically rich in mammalian fossils.
Some important fossils genra from Siwalik are mentioned below:
i. Carnivora:
Amphicyon; Hyaeneluros; Dissopsalis; Sivanasua; Crocuta; Canis; Mellivora, Lutra; Enhydriodon.
ii. Primates:
Dryopithecus, Brahmapithecus, Macacus, Sivapithecus, Papio; Simia.
iii. Proboscidea:
Dinotherium; Trilophodon; Stegolophodon.
iv. Equidae:
Hipparion; Equus.
v. Rhinoceratid:
Gaindatherium; Aeratherium.
vi. Suidae:
Palaeochoerus; Listriodon; Conohyus; Sanitherium; Tetraconodon.
vii. Giraffidae:
Hydaspitherium, Vishnutherium; Giraffai brahmatherium.
viii. Bovidae:
Perimia; Cobus; Leptobos; Bos; Bison.
Structure:
Rocks of Siwalik System show a great deal of folding and faulting indicating many orogenic movements during and after their formation. The system rocks have a thrust fault relationship with the older Murree rocks, which have come to rest over them for a great part of their development. This faulted contact between the Siwaliks and other Tertiary rocks is called the Main Boundary Fault in Indian Stratigraphy.
It is now well established that a number of other smaller thrusts have affected the sequence of Siwaliks. Similarly, Siwaliks have been thrown into folds – anticlinal and over-folds – at number of places. The Siwalik formations show at most places steep inclinations implying influence of tectonic uplift.
Origin:
The origin of Siwalik System is closely related to the orogenic activity that is believed to have taken place towards close of Middle Miocene age. With the uplift of sediments deposited in the Tethys due to northward compression, a depression, fore-deep type of topography, is believed to have come into being which started receiving sedimentation from the mountains of north (the Himalayas of that time) and the highlands of south.
The rise of the mountains in the north is also believed to have caused beginning of monsoon cycles so that periods of high abrupt rains were intercepted by relatively dry spells. The high monsoon rains might have caused floods in numerous streams contributing sediments to the fore- deep.
This type of environment contributed:
(i) Towards great thickness of the sediments brought to the fore-deep-about 6000 m.
(ii) Towards alternating coarse and fine grade texture of the sediments; the coarse grained being deposited during high-water times.
The enormous thickness of Siwaliks is also indicative of gradual sinking of the basin under the load of sediments. The sinking might have been even aided by the compression under tectonic forces directed from south.
The great thickness of fore-deep was ultimately compressed and raised to the present heights in the subsequent phases of mountain building towards the close of Pleistocene.
E.H. Pascoe and G.E. Pilgrim have earlier advanced a view that the Siwaliks represent fluviatile deposits along the flood plains of a single major river, (named SIWALIK by them) which were subsequently compressed and raised into the outer Himalayas. The ‘fore-deep’ hypothesis, has, however better structural evidence in support of the origin of the Siwalik System.
7. The Pleistocene System:
The deposits belonging to Pleistocene and recent ages cover an enormous extent of the country, estimated around 80,00,000 sq. km. and are spread over a great part of plains of northern and central India, lying between the southern flanks of Himalayan foot hills and northern borders of Peninsular India.
The alluvium formed between Indus, Ganges and Brahmaputra are all of Pleistocene and recent origin. Pleistocene and recent deposits are also laid in many other parts of the country such as Karewas in Kashmir, the Deserts of Rajasthan and Laterites of Madhya Pradesh and Deccan.
Indo-Gangetic Alluvium:
The Indo-Gangetic alluvial plains are spread over whole of Uttar Pradesh, greater part of the Punjab (both East and West Punjab), Sindh and northern parts of Bihar, Bengal and Rajasthan. It has not been possible to determine their actual thickness; it is variable from place to place but runs over many hundred, rather thousands of metres. Figures for the base of the Indo-Gangetic alluvium ranging between 500 to 5000 m at different places have been accepted without much objection for different places.
Lithological Characters:
The rocks of Indo-Gangetic alluvium, or the material making this vast expanse of consolidated, poorly compacted and even loose sediments is of fluviatile origin, made mostly by flood waters of the great and small rivers. These consist mostly of clays, gravels and compact sands.
These deposits generally form a continuous as well as uniformly homogeneous sequence (which may be said being formed even at present), not distinguishable at most places into series and stages as has been possible in the deposits of earlier ages. It is, however, customary to classify them into Newer Alluvium including the Khadar of Punjab and the Older Alluvium, comprising the Bhangar of the Ganges Valley.
The Bhangar or Older Alluvium covers areas of Uttar Pradesh and northern parts of Bengal where they generally form highlands. The Khadar or newer alluvium is lowlying in the southern parts; it merges with the deltas of the Ganges and the Brahmaputra. The delta of Indus is also, in general, an extension of Khadar.
The Khadar consists of calcareous concretions occurring as bands or dispersed nodules in the alluvium. It is a characteristic formation of alluvial deposits, especially of those with a marked clayey component. Kankar is formed, as explained elsewhere, due to capillary action in which groundwater rises up during periods of drought depositing the carbonate content in the upper soil in the form of nodules or bands.
The Bhangar deposits have yielded some fossils of elephants (Elephas antiquus) and horses, (Equus namadicus) from which these have been assigned a Middle Pleistocene age. The Khadar deposits are much newer in age.
Origin:
There is much controversy regarding the exact mode of origin of the basin or depression in which the huge thickness of Indo-Gangetic alluvium has been deposited.
Some common views expressed in this regard are as follows:
(i) Depression – a Synclinorium:
This view, forwarded by Eduard, regards the depression into which these deposits accumulated during the youngest periods of the history of the Earth, as a synclinorium, a fore-deep type of structure formed in front of the Himalayas.
(ii) Depression – due to Sedimentation:
This view which is favoured by many Indian geologists, regard the depression as one having formed due to excessive deposition leading to excessive loading. This loading is believed to be a cause sufficient to induce gradual sinking of the basin (a fact already accepted for origin of many sedimentary deposits in earlier times), concurrently with the deposition. Much importance is attached to almost contemporaneous (last phase) upheaval of the Himalaya, which made the Himalayan Rivers more vigorously eroding and hence bringing greater volumes of sediments into the sinking basin.
(iii) Depression – a Rift Valley:
According to S. Burard, the depression is actually of the nature of a rift valley, that is, a large central part having been thrown down between parallel faults of great extent.
(iv) Depression – a Sag in the Crust:
This is the most recent view developed on the basis of Plate Tectonics. According to this view, the Indian Plate is gradually moving northwards. As a result of this, sag or downward buckling is caused at the front of central plateau and soft sediments of the outer Himalayas. This is assumed to have happened during the last phase of the Himalayan Orogeny.
The Karewas of Kashmir:
The Karewas of Kashmir are most conspicuous formations spread in the form of level lands, dissected plateaus and vast fields starting from Shopian in South Kashmir and extending to Baramulla for a stretch of about 90 km. Their width is comparatively less, between 13-15 km. For a greater part these are almost horizontal but the strips bordering the Panjal slopes show inclinations ranging between 5°-20°. At some heights over the southern flanks, some stretches of Karewas have shown very steep inclinations – up to 40°, indicating their having been deposited prior to elevation of that part of the Himalayas.
Lithology:
The Karewas are for a greater part purely lacustrine (lake deposits) having been made up of silts and clays of various hues, texture and structure. At places they contain lignite seams. Clays are typically varved at many places and also contain fossils of mollusks and plants. Besides clays, Karewas also contain sands and shales in the lower regions where the Karewas are more extensive. It is believed by many now that the lower part of Karewas is fluviatile and fluvio-glacial in origin rather than of pure lacustrine nature.
Classification:
Karewas of Kashmir are commonly classified into a Lower and an Upper Karewas division, there being a marked unconformity or discontinuity between the two divisions. At least two glacial periods are believed to have intervened during the deposition of Karewas.
An outline classification of Karewas is given below:
Origin:
Karewas are for greater part, decidedly lacustarine in origin which were deposited in a great lake during the Pleistocene period. The lake extended upto Baramulla where river Jhelum finally made a cut into the barrier and the lake was drained out.
Life:
Karewas have yielded animal and plant fossils from many places.
Some important examples are:
i. Plants- Acer, Berberis, Indigofera, Juglans, Juniperus, Pruntus, Quercus, Viburnum.
ii. Mammalia- Bos, Cervus, Equus, Elephas, Felis, Rhinoceras, Sivatherium.
The Desert (Thar of Rajasthan):
Distribution:
The Rajasthan deserts form an exceptionally wide tract – 650 km in length and 160 km in width covering a great part of southwestern Rajasthan known as Thar Desert. The deserts are clearly wind-blown or Aeolian in character and have covered practically the entire earlier geology under them.
Composition:
The Thar deserts are practically made up of sand particles spread in the form of a vast expanse of undulating dunes. At places the underlying rocks protrude out as local ridges. Both the longitudinal and crescent (transverse) type dunes are seen in the Thar. All of them are in persistent processes of migration.
Quartz is most common of the minerals present in the dune sands. Felspar, hornblende and calcareous grains are other important constituents.
Origin:
It is believed that Deserts of Rajputana (other name of Rajasthan) are geologically of very recent age- 3000-5000 years only. The vast area south of the Punjab, which is now a sandy region was a quite fertile area even so late as the time of invasion by Alexender the Great (323 B.C.) – in the words of Dr. Wadia, an authority on Indian Geology.
The process of transformation of this fertile region into a desert is assumed to have been favoured by the following factors:
(i) Arid climates of extreme type that prevailed in the region over considerable time.
(ii) Monsoon winds from south- west direction that caused spreading of the sands and silt.
(iii) Availability of good quantity of loose or semi-loose sands along the sea coast especially along the Rann of Kutch.
In the presence of these factors, the process of development of Rajasthan Deserts took roughly the following shape:
(a) The temperature difference between the day time and night time and also during different seasons of the year being very high resulted in the intense type of mechanical disintegration (physical weathering) of the rocks into smaller and still smaller fragments. These fragments accumulated mostly in situ (because there was very little rainfall, less than an inch a month) and suffered further mechanical breakdown through the ages getting converted to coarse sands.
(b) The south-westerly monsoons are characterized with sand-lifting and eroding velocities before actually precipitating. These lifted and transported huge quantities of the sand and dust from Rann of Kutch and other areas accumulated during dry spells and spread them over as they advanced eastwardly and northwardly. This process must have continued for hundreds of years. The Aravallis being aligned parallel to the winds could not check the advancing deserts.
There is sufficient evidence to indicate that spread of Deserts in Rajasthan in pre-historic and even during historic times has been the cause of desolation of numerous well-established human settlements in these areas.
The Laterite Deposits:
Definition:
In the Peninsular India, Laterite deposits are the most important Pleistocene formations. The deposits consist chiefly of the rock known as Laterile, which is a hydrated oxide of aluminium and iron in composition. It is characterized with a typical structure- soft and vesicular when fresh but becomes hard and compact on exposure to air.
The laterite rocks generally occur as capping the hills and plateaus of Madhya Pradesh, Chhattisgarh and Deccan at heights ranging from a few metres to 2000 metres and with a thickness ranging from 20-60 metres. It may be found at height as low as 600 m above sea level (low-lying laterite) or above 600 m (high level laterite), the former being washed down accumulation of high level laterite.
Composition and Texture:
Laterite is broadly described as ferruginous rock with good content of aluminium oxide, thereby resembling bauxite in more than one way. Other hydroxides which are present in most laterites are of: manganese, titanium and zirconium. Accordingly, laterites may be classified as ferruginous (iron oxide dominating; brown colour); aluminiferous (aluminium hydroxide dominating; grey colour); manganiferous (manganese dioxide present – dark colour). In texture, the dugout rock may be hard, compact and massive. The fresh rock, however, is porous, often pisolitic, and full of vermicular cavities.
Origin:
Latrerites are of common occurrence in tropical countries such as Tropical America, Malaya, West Indies and East Indies besides India. As such laterization, the process of formation of laterites from ‘parent’ rocks has been studied extensively during last 60 years or so.
It is now broadly agreed that laterite is formed due to chemical weathering of certain preexisting rocks under favourable tropical type climates.
Such climates necessarily have:
(a) Warm and humid atmospheric conditions;
(b) Abundant and regular rainy seasons, with well-defined dry and wet seasons;
(c) Good drainage, so that leached out elements migrates quickly to new places instead of recirculating within the effected rocks and getting reprecipitated into them.
The source rocks that are most suitable for weathering into laterite include such diverse types as:
i. Feldspathic rocks like granites and gneisses;
ii. Alkaline rocks like syenites and nepheline syenites;
iii. Intermediate to basic rocks like diorites and basalts;
iv. Sedimentary rocks like limestones and shales with impurities.
It is believed that bauxite (aluminum hydroxide) and laterite (hydrous aluminium-iron oxide) are closely related in their origin in this process. A free circulating and highly alkaline environment will cause quick removal of dissolved clay ions from the decomposing rock leaving behind bauxite (rich in iron ions), whereas an acidic environment and interrupted drainage will favour accumulation of clay ions along with some iron ions so that the resulting rock is a double hydroxide of iron as well as aluminium.
Uses:
Laterites find various uses depending upon their colour, composition and availability. The ferruginous varieties are used for construction whereas clay-rich varieties approaching bauxite find use in chemical industry.
8. The Vindhyan System:
The Vindhyan System of Indian Stratigraphy derives its name from the Vindhyan Mountains of Madhya Pradesh and consists of an extensively developed sequence of “stratified formations” which cover more than one lakh square km of area extending from Sasaram and Rohtas in Bihar to Chittorgarh in Aravalis.
These formations represent the Proterozoic Group of the world stratigraphy lying between the lifeless Archean Group and full-of-life Palaeozoic Group. This is despite the fact that fossil evidence of unmistakable identity is yet to be discovered from the Vindhyan formations.
The Vindhyan system rocks rest unconformably over the older Cuddapah rocks wherever the latter are developed. Further, within the Vindhyan system itself, two sub-divisions are easily recognized on the basis of an unconformity- the Lower Vindhyan and the Upper Vindhyan.
These are the rocks of the Upper Vindhyan System that are exposed extensively; those of Lower Vindhyan are best developed in the Sone Valley only.
In the Peninsular India, Vindhyan rocks are developed overlying the Cuddapahs in Andhra Pradesh where they are named as KURNOOL SERIES.
Lithology:
The Vindhyan System sequence is made up essentially of sedimentary rocks – sandstones, shales and limestones – attaining huge thickness, at places estimated at 4200 m.
The sedimentary rocks of Vindhyan System show two distinct characters:
i. Firstly, the lower part (Lower Vindhyan) is composed mainly of rocks of marine origin (that is, rocks deposited in seas), such as limestones and shales, whereas the upper part (Upper Vindhyan) is made up mainly of rocks of fluviatile or estuarine origin (that is, deposited in shallow waters and bays) and consist of arenaceous sedimentary rocks, chiefly sandstones.
ii. Secondly, these rocks are least disturbed and unaltered structurally as well as chemically. There is no evidence of metamorphism or folding and faulting of any considerable magnitude in the rocks of upper Vindhyan. Some folding etc. seen in the lower Vindhyan has not been able to disturb it much as the original sedimentary textures, structures and composition are all well preserved.
Life:
The Vindhyan formations may be said to be, on the whole, totally unfossiliferous- no definitely identifiable fossils have yet been found. Existence of very primitive type of life is indicated by certain obscure traces like fucoid markings (plant impressions) and some fossils from the Suket Shales. The impressions assigned to genus Fermoria and Krishnania are the only (highly disputed) names mentioned with reference to the Vindhyan System though coal-like matter has also been reported from lower Vindhyan locations.
Stratigraphic Classification:
The Vindhyan System has been divided into two distinct divisions on the basis of a major unconformity that separates them: lower Vindhyan and upper Vindhyan. The two subdivisions are further divided into stages on the basis of lithological differences. The broad classification is given in Table 16.5.
The important characters of two subdivisions are dealt with briefly in the following paragraphs:
A. Lower Vindhyan:
The Lower Vindhyans are exposed only a few places, the best exposure being in the Sone Valley of Bihar where it is specifically termed as the Semri Series. The series starts with a basal conglomeratic layer, aptly called the basal stage. It is about 600 m in thickness.
The series shows three further stages when followed upwards:
Porcellanite stage, Kheinjua stage and the Rohtas stage. The Porcellanite stage is characterized by development of Porcellanite from the tuffs; otherwise it contains mostly sandstones and shales. Similarly, the Kheinjua stage is marked by development of ripple marks in glauconitic sandstones- indicating shallowing of deposition environment. The Rohtas stage is marked by development of best quality (high grade) limestones used extensively for cement manufacture.
Igneous intrusions of doleritic and basaltic composition have been recorded in Semri Series in the Sone Valley.
B. Upper Vindhyan:
These are the most widely developed rocks of the system and consist mostly of sandstones and shales that show very little evidence of any disturbance like folding and faulting.
Each of the three series of Upper Vindhyan (Table 16.7) has been sub-divided into a number of stages, generally on lithological ground.
The following table gives an outline of succession in the Upper Vindhyans:
Salient features of different series of the Upper Vindhyan may be summarized as follows:
i. Kaimur Series:
The series as developed in the Sone Valley section shows a succession of quartzites, shales, breccia, scarp sandstones and quartzites divisible into a lower Kaimur series and an upper Kaimur series. Ripple marks and mud-cracks are typical features of flagstones and shales besides current-bedding and gritty breccia. The Bijagarh shale stage consists of bleached yellow to black coloured shales that are micaeous, carbonaceous and contain iron pyrites at different places which are mined for their sulphur content.
ii. Rewah Series:
These rest upon the Kaimur series being separated by a diamond bearing conglomerate layer. The series is made primarily of shales and sandstones. The latter group of rock is coarse grained and often loosely bedded approaching a flagstone.
iii. Bhander Series:
These represent the uppermost series of the Upper Vindhyans and are separated from the underlying Rewah series by a diamond bearing conglomerate layer. The sandstones of this series are generally of red colour and speckled or spotted, fine grained in texture and possessing useful building stone qualities. Similarly the Bhander limestone is also of variable quality, varying between compact, calcium rich limestone to almost calcareous shales. Gypsum veins and beds occur in the Rewah series.
Kurnool System:
In South India, rocks equivalent to Lower Vindhyan System in age occur unconformably over the rocks of Cuddapah System in Andhra Pradesh. These form the Kurnool System, and are about 400 m in aggregate thickness. The system rocks consist mostly of Limestones. Sandstones, shales and quartzites also occur in subordinate scale. The limestones show great variation in their quality, ranging from high quality (calcium carbonate content higher than 90 percent) to impure limestones merging into calcareous shales.
Economic Minerals of Vindhyan System:
In order of relative importance, the mineral wealth of Vindhyan System may be discussed as follows:
(a) Diamond:
The Upper Vindhyans are commercially very important for their enclosing diamond bearing strata- the conglomerates – from which diamonds are being extracted at present. The diamonds are obtained from the base and top of Rewah series and also from the Jhiri shales at Panna area in Madhya Pradesh. However, diamond production from this source is dwindling year after year.
(b) Building Stones:
Vindhyan System is a storehouse of India with respect to good quality building stones, especially sandstones, and also, to a great extent, limestones. Numerous buildings of historical fame and importance in North India are made from Vindhyan Sandstones. This is because of excellent qualities of these stones in terms of their appearance, hardness, strength, texture and workability.
In colour, these are available in pleasing shades of red, cream and grey. In structure, they are massive as well as thick bedded and thin bedded, so that these can be used in any place in the building. The Red Fort at Delhi and Fatehpur Sikri, Agra, the administrative building in New Delhi, numerous mosques, stupas and forts are made from these stones.
The limestones of Vindhyan System are also worked in great quantity for use in building construction, especially for decorative and monumental purposes.
(c) Cement Raw Material:
The limestones of Vindhyan System form excellent quality source material for cement manufacture. These support numerous cement factories in Bihar, Uttar Pradesh, Madhya Pradesh and also in Andhra Pradesh.
Besides above three materials, some pyrite for manufacture of sulphur and glass sands is also obtained from the Vindhyan rocks.