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Here is a list of top twelve stratigraphical systems developed in India.
1. The Archean Groups:
Broadly speaking, all those rocks that are the oldest, have absolutely no evidence of existence of life (in terms of fossils) and undoubtedly form the basic, foundation rocks for all the subsequent formations in Indian stratigraphy are grouped together as Archean.
Geologically, these lie below the Cambrian system of rocks which begins above the Eparchean unconformity. In terms of time span, they represent millions of years and are made up of igneous rocks and their metamorphosed variations, like gneisses, schists, all highly folded and altered. Rocks of definite sedimentary origin are also part of this group developed typically in many areas where they are specially referred to as Dharwar System.
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Due to their occupying the lowest or basal position in stratigraphy, the Archean rocks are commonly referred as BASEMENT COMPLEX or FUNDAMENTAL GNEISS, the later term being indicative of their predominantly metamorphic (gneissic) structure in all the areas of their development.
The Archeans form a very complex group in Indian stratigraphy. Stratigraphically, structurally and petrologically Archean group presents diversification that is yet incompletely explained in many respects. Stratigraphically, the rocks are of such extensive development, both in time and space, that these are easily accorded the title of a group.
Within this biggest group of Indian Stratigraphy, however, it has been extremely difficult to make out any subdivisions into systems, epochs and stages. Even the distinction of this group into the Archean system and the Dharwar system is purely arbitrary. In some works Dharwar system is mentioned along with Archean system without there being any chronological significance.
The Archean system and the Dharwar system described below are not ‘systems’ so well defined in the Stratigraphical or chronological order as on their penological characters. This must be understood clearly at the very outset. The Dharwar system of Karnataka, in that sense, may be taken as a subgroup of Archeans exposed in that type area.
Distribution:
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The Archean system is most widespread in southern and eastern India though its development in extra-peninsular region, especially in the sub-Himalayan ranges is equally of great significance. In fact they form the core of the inner Himalayas. The Archean rocks of southern India, especially those developed in Mysore and adjoining regions have been specially described as Dharwar system. The typically Archean Rocks are developed in vast areas of south India, Madhya Pradesh, Chhattisgarh, Orissa, Maharashtra, Rajasthan and sub-Himalayas.
Lithology:
The Archean rocks are made up predominantly of two rock types – gneisses and schists, which show a great deal of variation with respect to mineralogical composition and grade of metamorphism.
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Gneisses are the most common rocks of Archean system. They range in composition from only slightly altered Granite (Igneous) or a Gabbro (Igneous) to a banded or foliated, true Gneiss (Metamorphic). The common minerals of Archean Gneiss are – Felspar Orthoclase and Plagioclase, quartz, muscovite, biotite, hornblende, magnetite, epidote and zircon.
Gneissic rocks of the Archean system are often divided into following three groups represented typically in some areas:
(i) Bengal Gneiss:
Highly heterogeneous gneisses of Bengal, Bihar and Orissa and of many other parts of Peninsular India are included under this group. This type of rock is sometimes referred as Khondalite which is actually either sillimanite gneiss or a sillimanite-schist.
(ii) Bundhelkhand Gneiss:
This group includes massive granite-like gneisses of Bundhelkhand and some other areas like Balaghat, Arcot and Cuddapah. The most important character of these gneisses is that in hand specimens they appear like Granites – foliation being present on a very small scale. At many places of their occurrence, these are intruded by sills and dykes of coarse grained igneous rocks.
(iii) Charnockite Series:
This series comprises massive, granite-like hypersthene rich gneisses of south India, developed extensively in Madras. In their petrological character, they are medium to coarse-grained and show mineralogical variations ranging from acid to ultrabasic range though the orthoproxenye (hypersthene) is always an important constituent. The rock was so named on its having been used in the tombstone of JOB CHARNOCK, founder of the city of Calcutta.
The rock type that has been recorded in many other parts of the world presents a challenging problem on its origin. It has been suggested by some that charnockites are actually of igneous, plutonic origin that have been intruded into older rocks of Archean group. These often occur as dykes, lenses and such igneous intrusions but also show prominent banding in other cases indicating sufficient metamorphism.
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Some other typical gneisses of Archean group of rocks exposed in various parts and mentioned by specific names are as follows:
a. Champion Gneiss:
It is exposed in Karnataka and consists of fine grained grey gneiss with good mica content. It is at places intruded into Dharwar rocks. The name is of restricted use.
b. Peninsular Gneiss:
It is a common gneissic rock of Archean age occurring at various places in southern India including Karnataka. The rock consists mostly of banded gneiss consisting of bands of quartz-felsper alternating with biotite, hornblende and other accessory minerals. These rocks, like Champion Gneiss, are also intrusive into Dharwar rocks at many places.
Schists are next in abundance in Archean rocks. They are thoroughly crystalline and occur in a number of varieties such as the mica-schists, the hornblende-schist, the epidote-schist and sillimanite-schist etc.
The question of origin of Archean Rocks is yet incompletely answered.
Different modes of origin have been suggested:
(a) They are believed to represent the first formed crust of the Earth in this part of the globe.
(b) They are regarded as earliest formed sediments, which have been subjected to extreme conditions of metamorphism.
(c) According to another view, these are originally igneous plutonic rocks that have undergone metamorphism during subsequent earth movements.
2. Dharwar System:
The Dharwar system, as understood in its latest connotation, comprises the “Oldest rocks exposed in the Karnataka State”. Their best exposures are in the Mysore region, in Shimoga and Chitaldurg area. Principally, the rocks are made up of Schists, Quartzites, Phyllites and metamorphosed conglomerates.
Although a good proportion of Dharwarian rocks are decidedly of igneous origin, it is also broadly agreed that many rocks are of sedimentary origin that has undergone metamorphism. At best, these may be described as representing a definitely sedimentary phase of the Archean System.
In their distribution, the Dharwarian rocks are very closely associated with the Archean gneisses and schists. Very extensive and important exposures are in Southern Deccan including the type areas of Dharwar, Bellary and Mysore as well as in Chhotta Nagpur and Jabalpur in Madhya Pradesh.
Equivalents of the system are also developed in extra-Peninsula where it is described under different names. Thus, in Spiti, the Dharwarian equivalents are named as Vaikrita Series; in Simla Himalayas as Jutogh Series, in Kashmir Himalayas as Salkhalas, and as the Daling Series in the Eastern Himalayas.
Lithology:
Dharwarian rocks show much complex variations in their lithological character. The earlier view of holding all the Dharwarian rocks as primarily of an igneous origin that have subsequently been metamorphosed is no longer held valid. Some rocks are decidedly of sedimentary origin that has undergone metamorphism. Majority of these rocks occur in the form of phyllites, schists, slates, quartzites and conglomerates of different varieties. Granites and gneisses are also well represented in the Dharwar.
Another character of these rocks is their having suffered complex folding. Intense metamorphism aided by extensive folding has resulted in obliterating most of the sedimentary textures and structures of those rocks which originally might have belonged to that group. The Dharwars are cut across by numerous intrusives of later ages.
Classification:
The Dharwarian Rocks in Shimoga are often divided into three rock groups:
a. The Lower Dharwars,
b. The Middle Dharwars and
c. The Upper Dharwars, mainly on their lithological characters.
a. Lower Dharwars:
These are made up mostly of schists such as mica schists, chlorite schists and greenstone schists. These also contain some lava flows, altered into hornstones due to metamorphism and also sills of quartz porphyry. The complex rock is of an igneous origin.
b. Middle Dharwars:
These are a mixture of conglomerates (at the base), quartzites, phyllites, limestones and dolomites (highly metamorphosed) topped with layers of banded haematite-quartzite series. They go by the name of HOSUR SERIES. Their sedimentary origin, at least for a great part, is hardly doubted now.
c. Upper Dharwars:
These begin with a conglomerate bed followed upwards by quartzites, calcareous and ferruginous silts and quartzites and cherty ferruginous slates.
Life:
No fossils have yet been found from Dharwarian rocks. This fact does not appear queer in circumstances to which these rocks – though mostly sedimentary in origin – have been subjected subsequent to their formation- extensive folding and faulting and intense metamorphism.
Economic Importance of Archean Group:
The Archean Group of rocks, of which Dharwar system forms a very important sub-group, are regarded as the most important economic-mineral-bearing groups of Indian stratigraphy. Baring Coal and Petroleum, most other economic minerals like those of Iron, Copper, Manganese, Lead, Zinc and Gold etc. have come from rocks which broadly belong to Archean time.
Following is just an overview:
i. Manganese:
Almost the whole of manganese annually produced in India is derived from Archean rocks.
The chief manganese bearing rocks are:
(a) Gondites developed in Balaghat, Bhandare, Chinddwar, Gangapur and Chhotta-Nagpur. These are typically sedimentary rocks that show prominent structure and are highly manganiferous.
(b) Kodurites, those are in fact hybrid rocks which have been formed due to intrusion of granitic magma into originally sedimentary rocks like limestones. These consist of quartz, feldspar orthoclase, sillimanite, manganese-pyroxenes and garnet. These are developed along Eastern Ghats.
(c) Khondalites that are garnetiferous schists, which contain associated ore deposits of manganese and occur in Eastern Ghats, especially in the states of Orissa and in Madhya Pradesh.
ii. Gold:
The gold-bearing quartz-veins of Mysore are Dharwarian in age. The famous Kolar gold mines of India are located in these rocks.
iii. Iron:
The iron ore series of Singhbhum are associated with the Archean rocks. These are the richest sources of good quality iron ore in India.
iv. Copper:
The Singhbhum belt is also rich in copper ores which are mined in Bihar. In Rajasthan also, the Khetri belt is another source of copper ores.
v. Lead-Zinc Ores:
The so famous Zawar deposits of lead and zinc occur in the rocks of Aravali age which are broadly equivalent of Dharwar in geological age.
vi. Mica:
India occupies a very prominent position in the world market of Mica and Dharwarian rocks are the main source of the mica obtained in India. The source rock of micas is a pegmatite which contains many minerals of industrial and other uses such as tourmaline, beryl etc. besides mica.
Among many other occurrences of economic minerals may be mentioned Nickel (Rajasthan), Uranium and Tin (Bihar), Tungsten (Bihar), Chromites, Asbestos, Kyanite and Columbite.
3. The Cuddapah System:
In Indian Stratigraphy, rocks of distinctly Proterozoic Group (that is younger to Archean Group and older than Cambrian with some evidence of most primitive life in some other parts of the world) are said to be represented by the Cuddapah System which is well developed in the Cuddapah basin of Andhra Pradesh. Though considered equivalent to Proterozoic, the system has not yielded any fossils so far. Its sedimentary origin and nature is, however, undoubted.
Three major outcrops of rocks of Cuddapah age studied in some detail are:
A. Cuddapah basin of Andhra
B. Cuddapah basin of Madhya Pradesh
C. The Delhi System.
A. Cuddapah Basin of Andhra:
These may be said as the type rocks of Cuddapah system. The system is developed in the form of a crescent (half-moon shaped) basin, with concave side facing eastwardly and the convex side facing westwardly.
Its approximate dimensions are:
i. Length:
About 340 km, extending from Singareni (north) to Nagarihills (south).
ii. Width:
Maximum 145 km in the middle part of the basin.
iii. Thickness:
Variable in all directions; it is deepest on the concave front, where it acquires thickness of more than 4000 m.
General geology of the area indicates that the sediments of the system were derived from the western side of the basin. The eastern side shows folding and faulting.
Stratigraphy:
The Cuddapah rocks rest on the Archean gneisses and schists over and above the eparchean unconformity. The system is divided into four distinct series having specific lithological characters and firm evidence of overlapping and order of superposition. The upward continuity of these four series, however, is clearly marked by a gap or unconformity. The series, their thickness and typical lithology is summarized in the Table 16.4.
B. Cuppapah Basin of Madhya Pradesh:
Rocks broadly thought to be equivalent to Cuddapah of Madhya Pradesh have been recorded in different areas of Madhya Pradesh and Chhattisgarh where they occur in isolated patches in Gwalior, Bijawar and Rajpur etc.
The Gwalior series consisting of thin bedded sandstones and siliceous to ferruginous shales is considered by some belonging to Cuddapah age. Similarly, the Bajawar series made up of quartzites and sandstones with conglomeratic beds lying above Archean rocks in isolated patches are also believed to be of Cuddapah age. Similar formations found in identical relationship with Archeans in Jabalpur are assigned the same status.
C. The Delhi System:
These are outcrops of rocks forming a belt and thought to be equivalent of Cuddapah system rocks of Andhra Pradesh. These are developed along the main axis of folding of the Aravali mountains starting from Delhi (hence the name) in the north, through Ajmer and Mewar to Palanpur in the south. The rocks of Delhi system rest in an unconformable manner, over older gneisses of the Archean rocks at most places, and also are overlain, by rocks of Vindhyan system, which are decidedly younger in age to them.
Archean Gneisses & Schists:
Structurally:
The rocks of the Delhi System form a broad synclinorium about 300 km in length, which have undergone high degree of folding and faulting as a result of having been involved in the mountain building process. In this respect, these rocks differ considerably from their equivalents of Andhra Pradesh.
Lithologically:
The Delhi System comprises of huge thickness of sediments—estimated at 6000 m— which have undergone metamorphism changing them to phyllites, schists, gneisses and quartzites. Arkose grits are also abundant in the main synclinorium. The schists are of calc-schist type and inter- bedded with thin flaggy limestones that form their source.
Similarly, the gnessies are also of calc- gneiss type containing good proportion of unmetamorphosed limestones. In Alwar, the predominant rocks of the Delhi System are thick massive quartzites, some of which contain pure quartz grains, besides oxides of iron and copper in good proportion. The quartzite ridge of Delhi is a continuation of the same formation.
Other Cuddapah Equivalents in Extra-Peninsula:
The Dogra slates of Kashmir, the Chail and Simla slates of Simla Hills, Himachal Pradesh, and The Haimanta System of Kumaon (Uttaranchal) are thought to be some other exposures of rocks which are broadly equivalent in age to the Cuddapah of Andhra Pradesh and the Delhi System of Delhi- Rajasthan synclinorium.
Economic Minerals of Cuddapah System:
Quite a few important economic minerals occur in the rocks of Cuddapah System or its equivalents.
Examples are:
(i) The Barite and Asbestos of Papaghani series at Cuddapah, Anantapur and Kurnool.
(ii) The Quartzitiers of Delhi system have been used extensively as building stones and roadstones.
4. The Palaeozoic Group (Cambrian to Permian):
The Palaeozoic Group is the first post-Archean Group of rocks that has been studied in many parts of the world and its correlation established with good degree of certainty. This has been possible due to abundant fossil evidence yielded by rocks of this group.
In Indian sub-continent, the Palaeozoic rocks of marine origin are generally absent from peninsular India. Their development is most extensive and relatively perfect in many extra-Peninsular regions, especially in Spiti, Kashmir and Salt Range (in Pakistan). The group comprises six systems namely, Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian, in that order (Refer to standard stratigraphic scale). Following is only a brief account of main features of these six systems.
5. The Cambrian System:
The name of the system is derived from CAMBRIAN, the other name of Wales in the British Isle and has been applied to all the rocks of post Archean age occurring at the base of Palaeozoic group and correlated on the fossil evidence. These rocks have been recorded in North Europe, North America, China, India and Pakistan and in many other parts of the world.
In all regions of their development, they are richly fossiliferous, fossils representing mostly trilobites and brachiopods. In Indian Stratigraphy also, the name Cambrian has been used for the formations resting un-conformably over pre-Cambrian rocks. These have been studied exhaustively in the three regions in the subcontinent – Spiti in Uttar Pradesh, Kashmir and Salt Range.
Cambrian of Spiti (Haimanta System):
In Spiti, the Cambrian rocks form the basal beds of a nearly complete sequence of fossiliferous Palaeozoic and Mesozoic rocks. This system is also known by the name of Haimanta System of Spiti. The Cambrian rocks rest over the Vaikraita series (of Dharwar age) of highly metamorphosed schists and have been developed on the slopes of central Himalayas starting from Spiti and continuing to Lahaul in the north-west and Garhwal and Kumaon in north-east.
Lithology:
Shales, slates and quartzites are the chief types of Cambrian rocks in spiti. The strata are clearly of sedimentary origin and highly fossiliferous in some sections.
Classification:
The Haimanta System of Spiti has been studied and classified by HYDEN into three divisions as follows:
Cambrian (Haimanta) of Spiti:
Upper Haimanta is of 400 m Thickness – Consist of mainly grey and green quartzites, slates and fossiliferous Shales.
Middle Haimanta is of 300 m Thickness – Red and pink quartzites and shales; Black carbonaceous slates.
Lower Haimanta is of 600-900 m Thickness – Greenish phyllites, shales and quartzites.
Life:
In spiti, the Cambrian rocks have yielded fossils mostly from the Upper Haimanta series on the basis of which a middle to Upper Cambrian age has been assigned to these rocks. Among the fossils, remains belong to Trilobites, Brachiopod and Echinoderms.
Some species reported from Upper Haimanta are:
i. Trilobite:
Agnostus spitiensis, Anomocare conjuctiva, Conocephalitis memor, Microdiscus greisbachi, Ptychoporia spitiensis etc.
ii. Brachiopods:
Acrothela praestans, Linguleka haimantensis, Nisusia depsaensis.
iii. Echinoderms:
Eocystitis Sp.
The Cambrian of Kashmir:
The Palaeozoic sequence, often including Cambrian are developed at a number of places in the valley of Kashmir, major exposures being in Hundwara in Baramulla district, in Basmai anticline (Sind Valley) and Lidar Valley in Anantnag district. The section of Cambrian age is best seen in Hundwara where the sequence rests upon the slaty rocks of Purana age named as Dogra Slates.
Lithologically, the Cambrian rocks in this section consist mostly of:
(i) Clayey slates, greywacks and quartzites in the lower regions.
(ii) Thick-bedded clays and thin-bedded limestones in the upper regions.
Both the zones have yielded fossils belonging chiefly to Trilobites and Brachiopods.
Some prominent species are:
i. Trilobites:
Agnostus Sp; Microdiscus Sp, Anomocare hundwarensis.
ii. Brachiopods:
Acrothele aff; Botsfordia coelata, Obolus kashmiricus.
iii. Pteropod:
Hyolithes Sp.
From these fossils, a lower Cambrian age has been suggested for this sequence.
Cambrian of Salt Range:
The Salt Range Cambrian rocks are developed in the form of a thick, conformable and stratified sequence which has been divided into the following series on the basis of order of superposition.
The Saline Series is the thickest among the Cambrian sequence of Salt Range, and is best developed in Khewra (Pakistan). It is well known for its Rock Salt deposits that occur in thick layers inter-bedded with red marls, gypsum and dolomite. There is considerable difference of opinion about the age of Saline Series. The series lies at most places below the purple sandstones of Cambrian age but in some other cases evidence suggesting an Eocene age for Saline Series is also very strong. The problem is yet unresolved.
The Purple Sandstone is a sequence of sandstone of mainly purple colour (hence the name) that show ripple marks and current bedding at many places. In lower regions they are of flaggy type. The series is totally unfossiliferous.
The Neobolus Beds are so named because they have yielded fossils of Neobolus-a primitive brachiopod. The series is made up of shales and dolomitic limestones. Among other fossils found from these rocks Trilobites like Ptychoporia richteri and Conocephalus warthi, Brachiopods like Lingula warthi and Mabergia granulates and Ptenopod like Hyolithes wyeni are worth mentioning which indicate a Middle Cambrian age for the series.
The Magnesian Sandstones:
The series is made up principally of cream coloured sandstones that are massive and dolomitic. At places they also contain bands of different colours.
Salt Pseudomorph Series:
The series is so named because it contains cubic replacement type crystals (pseudomorphs) of salt in clay. The principal rock type is shale whereas laminated sandstones are also of common occurrence.
6. The Ordovician System:
The Ordovician rocks are found to overlie the Cambrian rocks in Kashmir, Spiti and Kumaon Himalayas. In all these areas these rocks are themselves overlain by more well defined outcrops of Silurian System. Often they are described together. The Ordovician rocks in Kashmir contain mostly slates and limestones.
In Spiti, the Ordovician are made up of flaggy quartzites with conglomerates at the base. The quartzites pass upwards into red quartzites, flaggy sandstones and dark limestones. In Northern Kumaon, the Ordovicians are represented by layers of Shales and marly limestones designated as Shiala Series.
Ordovician rocks of Spiti and Northern Kumaon are highly fossiliferous, especially when considered in succession with the Silurian rocks. The Ordovician rocks of Kashmir have, however, not yielded any significant fossils.
Among the chief fossils of Ordovician of Spiti may be mentioned:
i. Trilobites:
Asaphus emodi, Illaenus brachioniscus; Calymene nivalis.
ii. Brachiopods:
Orthis thakil, Leptaena rhomboidalis, Strophomena.
iii. Lamellibranchs:
Pternia thanamensis, Loposphira himalensis.
iv. Cephalopods:
Orthoceras kernas; Cyrtocaras centrifugum and Bryozoa, Aclinozoa and Pteropods.
7. The Silurian System:
The Ordovician and Silurian System rocks occur in a continuous conformable manner in Kashmir, Spiti and Kumaon.
In Kashmir, these rocks are exposed in the famous Lidar Valley Articline (near Pahalgam). The Silurian rocks here comprise mostly impure limestones and arenaceous shales. These are highly fossiliferous and have yielded corals, crinoids and strophomenids of typical Silurian age.
In Spiti, the Ordovician rocks pass upwards without any distinction into Silurian rocks consisting of hard dolomitic limestones, shaly limestones, coral limestones and grey limestones which become prominently siliceous at the top. These rocks have also yielded a good assemblage of Trilobites, Brachiopods, Lamellibrabchs, Gastropods, Cephalopods and Actinozoa of distinctly Silurian age. The important character of Silurian is their richness in corals, a feature in which they are often compared with Silurian fauna of North America.
In Northern Kumaon, the Silurian rocks are developed at Shiala Pass in the form of a thick (200-300 m) formation of various coloured Shales. The series is often referred as Variegated Series and also contains limestones and shales making another 500-600 m thickness.
8. Devonian System:
Both in Kashmir and Spiti and to some extent in Northern Kumaon also, the fossiliferous Silurian rocks are succeeded upwards by a totally unfossiliferous sequence of massive sedimentary rocks, the Quartzites. These are commonly known as Muth Quartzites, after the pass of that name in Spiti in which they are prominently displayed.
In Kashmir, the Muth Quartzites are developed both in the Lidar Valley anticline and the Shamus Abari syncline. The quartzites are quite pure, white coloured and very hard. In Spiti, the rocks are similar to Kashmir quartzites and are about- only 100 m thick compared to a thickness of more than 500 m in Kashmir.
9. The Carboniferous System:
In India, the Carboniferous System is developed prominently only in Spiti and Kashmir in the Himalayas region. A very characteristic feature of development of these rocks is that while the Lower and Middle Carboniferous rocks are conformable with respect to each other, there is often a clear break between them and the Upper Carboniferous formations which go more with the overlying Permian System. The break or hiatus, where developed is often referred as Mid Palaeozoic Unconformity.
We shall discuss the Carboniferous System as a whole with respect to the two type areas: Spiti and Kashmir.
Carboniferous of Spiti:
This system is made up of massive thick succession of limestones, shales and quartzites resting over the Devonian rocks- the Muth Quartzites. The rocks of this system are clearly divisible on the basis of their fossil content and lithology, into two series, the Lipak series and the Po series.
The Lipak Series, named after a river, consist mainly of limestones; Quartzites are next abundant rocks whereas shales are subordinate to them. Total thickness of the series is about 600 m.
Typical fossils of this series are Syringothyris cuspidate. It has been assigned a Lower Carboniferous age.
The Po Series- This is composed mainly of shales and quartzites which rest conformably over the Lipak series. This series has yielded fossils of Rhacopteris group of plants from the Thabo stage formation.
The Po series pass upward, with a clear stratigraphic break, into grits, quartzites and conglomerates that have been assigned an Upper Carboniferous Age.
Carboniferous of Kashmir:
Carboniferous rocks overlie conformably the Muth Quartzites of Devonian age in Lidar Valley. These rocks are composed of a huge thickness of grey coloured limestones, unfossiliferous quartzites and shales.
The continuous Carboniferous System of Kashmir is often distinguished into following series:
The Syringothyris Limestones form the basal series of Carboniferous in Lidar Valley at Eishmukam and are also developed in Banihal area where they rest unconformably over much older pre Cambrian rocks. They are composed mainly of grey coloured flaggy limestones which have yielded a good assemblage of Brachiopod fossils, chief among which is Syringothyris (and hence the name). Conetes, Products, Athyrus and Derbya are other chief genra of Brachiopods found in Syringothyris limestones.
Syringothyris limestones have been assigned a Lower Carboniferous age, thereby corresponding to Lipak Series of Spiti.
The Fenestella Shales form the second series overlying conformably over the Syringothyris limestones. The shales series is made up chiefly of calcareous shales and quartzites, which are practically unfossiliferous. The shale beds have yielded in the upper region rich assemblage of Polyzoa group of which Fenestella is the most abundant genus (and hence the name). Protoretopora is another common genus. Other groups like Brachiopods and Lamellibranchs are also adequately represented.
The Fenestella Shales have been provisionally assigned a Middle Carboniferous age because of difficulty experienced in correlating fossils of these formations with any other formations of India or abroad.
The Panjal Series:
Large scale eruptions of lava were witnessed by this part of the country semi-contemporaneously with the deposition of Fenestella shales. This resulted in deposition of volcanic agglomerates, commonly referred as the Panjal Series. This series is divided into Lower Agglomeratic Slates with angular quartz grains and gneissic fragments as inclusions in them; and, an upper Panjal Trap which consists chiefly of basaltic lava, mainly andesitic in nature.
The agglomeratic slate series have yielded fossils at few localities. These include genra like Products, Chonetes, Spirifera, Eurydesme, Pleuratomaria and Fenestella. The age and exact mode of origin of the Panjal Series is yet a disputed problem.
10. The Permian System:
Kashmir:
In Kashmir, rocks of Permian age are developed at a few places over the so-called Gangamopteris beds and go by the name of Zewan Series. The series is composed of highly fossiliferous limestones and shales. Fossils yielded from this series include varieties of brachiopods, ammonites, lamellibranchs and polyzoa, all of which suggest a Permian age for the series.
Permian rocks are also developed in the form of a chain of inliers extending from Riasi to Poonch in Jammu Province. Typical rocks are limestones which are unfossiliferous in character. These limestones are quite frequently mineralized, the ores being polymetallic sulphides of lead, copper, zinc and iron.
Salt Range:
Rocks of Permo-Carboniferous age are widely developed in Salt Range (Pakistan), where they overlie directly and unconformably on the Salt Pseudomorph Shales of Cambrian age. The Permo- Carboniferous sequence of Salt Range is a vast thickness of sandstone in the lower parts and limestones in the upper parts. The limestones are highly fossiliferous.
The Upper Carboniferous rocks are represented by Speckled Sandstone Series which overlie the Salt Pseudomorph Shales and are overlain by the Productus Limestones of Permian age. The Productus Limestone Series is itself a great thickness of limestones of more than 250 m and has been differentiated into three distinct subdivisions- the Lower, Middle and Upper Productus Limestones as given in Table 16.10.
A brief description of various stages is given below:
i. Boulder Beds:
These form the lowermost layer resting over the denuded Cambrian rocks and indicating a well-defined uniformity. Their glacial origin is well established.
ii. Speckled Sandstone:
This series is composed of shales, sandstones and clays. The lower part of the series is fossiliferous and has yielded conularia (hence the name). The sandstone resting above is of bluish shades and spotted and speckled in appearance.
iii. Productus Limestone:
This group of Permian rocks is composed of calcareous sandstone in the lower part which differ a great deal in fossil content from the Speckled sandstone over which it lies. Typical fossils of Lower Productus are Productus cora and P.spiralis.
In the Middle Productus limestones series, the rocks are mostly crinoidal and cherty limestones with typical fossils of Productus lineatus.
The Upper Productus comprises a small thickness of arenaceous rocks, chiefly sandstones, with some beds of limetsones and carboniferous shales. Productus indicus is the characteristic fossil of this series.
11. The Mesozoic Group:
At the end of Palaeozoic Era, that is, sometimes in Upper Carboniferous, mountain building activity of a semi-global scale is thought to have acted on the planet. This orogeny, known as Hercynian, was responsible for a lot of changes in the distribution of land and sea in different parts of the Earth including India, Australia, South America, South Africa, Madagascar and Antarctica.
This ancient continent, believed to have continued from early Mesozoic to late Mesozoic, and known as Gondwanaland formed one type of environment of deposition for the Mesozoic that were mostly of shallow water and lacustarine type. These land masses were broadly connected with each other and almost unrestricted floral and faunal migration was possible.
In the extra-Peninsular India, however, a marine phase had set in during the same period indicted by the Tethys (presently represented partly by Himalayas). The Tethys Sea continued to receive sediments for most of the part of Triassic and Jurassic eras and preserved them faithfully. However, towards the end of Cretaceous period, waters from the Tethys Sea started migrating towards the Indian sea, thereby gradually creating shallow water conditions.
As a result of these tectonic changes, the records of Mesozoic Group in Indian Geology are conveniently studied under two main groups- The marine deposits of extra-Peninsular (Tethyian Himalayas) and the shallow water deposits of Peninsular India – The Gondwana Group. The distribution is not totally exclusive. We come across marine Mesozoic rocks in Peninsular India at places and shallow-water deposits of the same age in extra-Peninsula.
The Mesozoic of Tethys Himalayas:
Distribution:
An almost complete succession of Mesozoic rocks are seen developed in different parts of the Himalayas such as Ladakh, Kashmir, Spiti, Lahaul, Kumaom and also in Chamba (Himachal Pradesh). We shall describe below typical exposures of the main systems, that is, Triassic, Jurassic and Cretaceous systems, with respect to their type areas where these have been studied best.
12. The Triassic System:
Spiti:
Definition:
This is the basal system of the Mesozoic Group and succeeds the Permian rocks in Salt Range, Spiti and Kashmir. It is of marine origin. The most perfect development of the system rocks in Spiti is in Lilang and hence the Triassic of Spiti is sometimes also referred as Lilang System.
Lithology:
The Triassic rocks of Spiti consist chiefly of limestones and shales. Quartzites are found in the upper zones of the system. The limestones and shales are of variable colour and character as also of variable fossil content.
Classification:
Triassic of Spiti has been divided into three main groups- the Lower, the Middle and the Upper Triassic – corresponding exactly to Bunter, Muschelkalk and Keuper division of the European Triassic system.
These three groups are further sub-divided into zones as given below (Table 16.11):
Life:
The Triassic rocks of Spiti are known to the palaeontologists of the world because of their richness in typical Triassic fauna.
Among the most important fossils, following are mentioned only for example sake:
Otoceras woodwardi, Ophiceras sakuntala, Meekoceras varah, Aspidites sp, Hedenstroemia sp, Rhynconella greisbachi; Spirifera stracheyt; Daonella lommeli, Spirigera hunica, Celites trigonalis, Joannites thanamensis, Halobia comata, Trachyceras sp. and so on.
Kashmir:
Triassic rocks are extensively developed in Kashmir. They can be traced conveniently all along the flanks of Pir Panjal, in Lidar Valley, Wardwan Valley and Gurais Valley and also north of Jhelum River. The system has been divided like that in Spiti, into Lower, Middle and Upper Triassic. Each of the divisions is further subdivided into different zones mainly on the basis of typical fossils found from them.
The classification is abstracted in Table 16.12.
Lithology:
The System is chiefly composed of homogeneous, compact light blue coloured limestones with some beds of shales in the Lower Triassic. In the Middle Triassic, shales and sandstones are intercalated with Limestones whereas in the Upper Triassic massive limestones are the predominant rock types.
Life:
Like Triassic of Spiti, Kashmir Triassic has also yielded a rich assemblage of Ammonite fauna with subordinate number of lamellibranchs and brachiopod fossils. In fact the zonal classification as given in the table 16.12 is based on the important generic type of Ammonite fossils.
Among the Lamellibranchs, Pseudomonotis, Modiola, Anodontophora and Anomia may be mentioned. The Brachiopods are represented by species of genra Spiriferina, Rhynconella and Dielesma. Gastropods are also represented by a few genra.
Salt Range:
In the Salt Range proper, only Lower Triassic is well developed, though on the Trans Indus side, complete Triassic sequence has been studied. The Triassic formations of Slat Range are also known as Ceratite Beds because of occurrence of fossils of that genus in good abundance.
The lithology of Triassic of Salt Range is also broadly similar to that of Kashmir and Spiti-flaggy limestones predominate with intercalations of shales and sandstones.