SIWALIKS AND OIL BEARING TERTIARY BEDS

INTRODUCTION:

The name Siwalik was originally given by Captain P.T. Cautley in the early nineteenth century (1832) to the outermost Sub – Himalayan Ranges. He discovered the Siwalik fossils in the course of digging for the Ganga canal near a Shiva Temple at Haridwar (now in Uttaranchal) in the Ganga Valley. Hence the name Siwalik was given to the sedimentary rocks entombing the fossils. .Subsequently it was applied to the mammal – bearing horizon of the Neogene continental deposits of the Himalayan foothills.

Falconer adopted the term in 1835 to designate the nearly continuous series of Tertiary formations stretching from the Indus River in the west to Irrawady River in Myanmar in the east. The succession was later sub-divided three – fold without their actual extent being defined. H.B. Medlicott substantially contributed to our understanding of the stratigraphy of the Siwalik Group, particularly between Ganga and the Ravi rivers. Besides Pilgrims (1910 – 1944), who made a very detailed study of the Siwalik vertebrate fauna, others who in the earlier years contributed to our understanding of the sediments of the Siwalik fauna, others who in the earlier years contributed to our understanding of the Siwalik were E.H. Colbert, G.E. Lewis, R. Lydekker and D.N. Wadia.

The Siwalik group occurs in Potwar basin in NW Pakistan and extends along the Himalayan foothills to Arunachal Pradesh foothills in the SE, except for a small break near Sikkim. The group is about 7km thick and ranges from Middle Miocene to Early (?) Pleistocene (16.8 to 0.22 Ma) in age.

Siwalik Hills are relatively low and rarely exceed 1000m above the mean sea level with conformable trends running parallel to the Himalaya. The strata are folded and have an outcrop pattern, more or less bounded by a major thrust in the north – the Main Himalayan Frontal Thrust (HFT). The Siwalik is separated from the Indo – Gangetic plain in the south by the Main Boundary Fault (MBF). The Siwalik outcrops are generally 10 – 12 km wide and present themselves as a series of parallel ridges with a steep scarp towards south. Between the outermost hills in the south and the one in the interior in the north are many narrow valleys calls “Duns” filled up mostly with Quaternary sediments.

The termination of the Murree period in Mid Miocene coincided with the third and perhaps the most violent episode of mountain building on the northern border of India. This must have accompanied by a considerable rising up and folding of strata laid down in the Tethys into mountain ranges and by large intrusions of igneous rocks into the cores of the folds. A long narrow depression was formed in front of rising mountains i.e. towards the side of the Peninsula. This depression called Fore Deep was the site of deposition of the Siwalik strata which commenced in Mid Miocene. The 16,000 to 18,000 feet thickness gives the evidence of continuous deposition in the shallow water whose depth kept pace with the accumulation of sediments. Based on lithology, structure and fossils the succession has been divided as follows:

Sub Group Formation Fossils Age ThicknessUpper Siwaliks

Boulder ConglomeratePinjor FormationTatrot Formation

Primates, Carnivores, rodents, Elephants, Ungulates.

Pleistocene2300m

Middle Siwaliks

Dhokpathan FormationNagri Formation

Primates, Carnivores, rodents, Elephants, Ungulates

Pliocene1400 m to 2000 m

Lower Siwaliks

Chinji FormationKamlial Formation

Primates, Carnivores, Birds, Proboscidians, Ungulates, Fishes, Reptiles.

Miocene1600 m

Upper Murrees

LITHOLOGY:

The Siwalik system is made up of ungraded Sandstones, Conglomerates, Shales, Clays and silts having characteristic of Fluviatile deposits of torrential (rapidly flowing) streams and floods in shallow fresh – water and swampy areas. The fossils in them show that the earlier beds in them were deposited in somewhat brackish environment as compared with the later ones. Some of the latest deposits may be continental i.e. left on land by temporary heavy floods. There is a temporary ferruginous matter, especially in some of the older horizons, which indicates that the sediments were derived partly from an old and well oxidised terrain. Coarser and finer sediments alternate each other. The schists show current bedding and poor stratification with feldspar and mica which are clearly derived from the breakdown of Central Himalayan Granites.

The Siwaliks have been involved in the later phases of Himalayan Orogeny as indicated from their folding, faulting, over thrust, inversion and lying at steep angles against other formations.

CLIMATE:

The alternations of coarse and fine sediments suggest seasonal deposition. The coarse material during the floods or wet season (from the Northern Himalayas) and fine sediments during drier season (from Peninsula). The extraordinary similarity of deposits over long distance along the strike would show that the source rocks were similar and that the basin of deposition (Indo – Brahma or Siwalik River Basin) was practically continuous. The large thickness shows that the area of deposition was sinking keeping pace with the sedimentation. At the same time there was gradual southward shift of the basin with each fresh pulse of the

uplift. Thus the Siwaliks extend for kilometres below the alluvial cover of the Indus and Ganges Valley.

The Siwalik sediments gave the evidences of a warm and humid climate through the greater part. The characters of the sediments indicate that the basin of deposition was first brackish and then it became increasingly fresh and that there was a variation from Lacustrine to fluviatile conditions.

FOSSILS:

Bulk of Siwalik formation is unfossiliferous but certain horizons and areas are very rich in fossils. These include leaf impressions, tree trunks, mollusca, ostracoda, charopyta, spores, pollens, fishes, reptiles and mammals being most important to subdivide the formations into stages. The mammals throw great light on their origin, evolution and migration of groups like pigs, hippopotamus, ancestral elephants and horses. Many mammalian genera disclose the wealth of species. The immediate ancestors of our modern species of land mammals of not far distant age from our times, but most of them have now become extinct (only one third exists now) e.g. out of 30 species of elephants (Probescidians) only one is found living today. Several river fishes, lignite, dicotyledonous trees and few land and freshwater shells, Mastodontoids (elephants with skulls and jaws of gigantic size) are found. Thus not only showing the number and variety of species but an extraordinary assemblage of animals like mastodon elephants, giraffes, hippopotami, rhinoceroses, horses, camels, antelopes, monkeys, pigs, crocodilians and chelonian reptiles, birds, etc.

STRUCTURE:

An important difference between the recent Indo – Gangetic alluvial deposits and Siwalik deposits is that the Siwalik have been involved in the latest Himalayan System of upheaval by which they have been folded and elevated into the outermost foothills of Himalayas. Folding has imparted them high dips and indurations which is absent in recent alluvium. Some of the folds have been inverted or reversed with overturning of missal limb (which has to suffer severe tension) having reached the limit passed into high inclined fracture or thrust plane along which the disrupted part of the fold has slipped bodily for long distances. Thrusting the older Pre Siwalik rocks of the ranges of mountain over rocks of the outer ranges Main Boundary Fault extends all along the Siwalik foothills from Indus to Brahmaputra valley. The M.B> Fault is not a single fault but is one series of more or less parallel faults among the Tertiary zone of strata all of which cover the Siwalik under older Tertiaries under Puranas of Mid Himalayas. The M.B. Fault is not only a fault but also marks the original limit of depositional of Siwalik strata against the cliff of foot of the then existing mountain beyond which they could not extend. This view of MBT will be made clear by imagining that if the Indo – Gangetic alluvium at present lying against the Siwalik foot hills, were to be involved in the future Himalayan Upheaval they would exhibit much the same

relation to the Siwalik as the latter do to the older rocks of the Himalayas. These reversed faults were not contemporaneous but successional while some are subsequent to folding.

The reversed over thrust faults giving rise to sheet – like recumbent folds (nappes) are characteristic and highly significant features of the outer Himalayas (extending for hundreds of miles). Such faults have been named as Murree and Nahan followed inwards by Panjal and Krol thrust followed by zanskar, Giri, and Gharwal thrusts as the central ranges are approached. The width of foot hills zones is thus determined by outermost of these thrust.

LITHOLOGY AND SUBDIVISIONS OF SIWALIKS

The Siwaliks are subdivided into three major divisions. The various sub-divisions take their names from the localities in the Potwar Region which is the type area of Siwaliks.

I] Lower Siwaliks:

a) Kamlial Stage:It is composed of hard red sandstones with clay nodules, purple Shales ( 550 ft thick). The chief mammalian fossils are Carnivora, Probiscidae, and Suidae.

b) Chinji Stage:Alternating ash gray sandstones and bright red Shales contain large number of vertebrate fossils and wood. The main mammalian fossils are Primates, Carnivora, Proboscidae, Rhinoceratidae, suidae, Anthrocotheratidae, Traqulidae and girafidae, etc.

II] Middle Siwaliks:

c) Nagri Stage:It comprises of hard gray sandstones with small proportion of clays. It is the starting point of Major Siwalik Fauna. It is poorly fossiliferous. The fossils found are Primates, Carnivora and Pigs.

d) Dhok Pathan Stage:It includes brown sandstones, Shales and some beds of gravels. Its source of sediments is from NW i.e. from present Indus Basin, the drainage being transverse to the basin of deposits. This feature is the evidence against Indo – Brahm River. This stage is richly fossiliferous. Most important of these are Primates, Rhodentia, Ursidae, Mustelidae, Felidae, Praboscidae, Equidae and Rhinoceratidae, Suidae, Anthrocotheridae, Hippopotamidae, Tringulidae, Carvidae, Giraffidae and Sovidae.

At the end of the Dhok pathan time an uplift occurred and the strata were folded and eroded before deposition of Upper Siwaliks. This erosion interval is thought cover Upper Siwalik Pliocene period which is unrepresented either by sediments or by fauna.

III] Upper Siwaliks:

e) Tatrot Stage:It consists of Conglomerates, soft sandstones and brown Clays. These beds were laid down in basins which were results from the folding movements, the folds having NE – SW axis. They often lie unconformably over the planed surface, in which there are prominent upturned ridges of Nummulitic strata (benthonic). The sediments are coarser then before and indicate quick deposition by river and delta like structure. His stage is marked by heavy rainfall as the beds do not show red colour and encloses remnants of Elephant, pigs, hippopotamus, bovids, etc, indicating good moist conditions.

f) Pinjor Stage:It is composed of pebble beds and sandstones with pink silts. The pink colour indicates somewhat drier conditions and deposition under Aeolian condition. They are rich in fossils like Primates, Rhodantia, Carnivora, Probpscodae, Equids, Rhinoceratidae, suidae, Anthrocotheridaem Giraffidae and Bovidae, etc.

g) Boulder Conglomerates:They Overlie the older beds disconformably mainly as fans. The boulders are derived from newly ridden Pir Panjal Hill and are faceted, striated and poorly graded indicating deposition from glaciers. The conglomerates contain few silt horizons which may indicate Aeoline deposited during dry period. Boulder conglomerates also consist of relics of pre – historic man whose appearance coincided with the rapid disappearance of mammals.

Jammu and Kashmir

The Siwalik sediments overlie Murree in this Sector of Kashmir Himalaya. Complete development of Siwalik can be seen in the Jammu Hills, where they attain a thickness of about 6000 m with an outcrop width of about 40km as seen along the Chenab section in Raisi – Aknoor sctor and in Ramnagar – Samba sector.

Lower Siwalik occurs in Poonch – Mirpur region. Trace fossils are found in the lower siwalik rocks at Mansar & Jhajjar Kotli in Jammu region. The siltstones are bioturbated with branched burrows and resting traces along with semicircular and circular imbricate structures.

In Middle Siwalik, the Chinji member grades into the Helen member of the Middle Siwalik in Poonch region. They attain thickness of about 500 m.

The Upper Siwalik these have a very large thickness and are overlain by about thick 1000 m thick boulder conglomerate. The Upper Siwalik of Jammu region is divided into Parmandal, Nagrota and Tawi Boulder Conglomerate (Jammu Fm) Formations.

Himachal Pradesh

The Siwalik group in Himachal Pradesh forms a parallel foothill belt along the southern margin of the Palaeogene Sirmur Belt between the Ravi and the Yamuna Rivers. In the Punjab re – entrant, it has a maximum width of about 100 km between Hoshiarpur and Jogindernagar. The Siwalik sediments overlie the Dharamsala group (having the two formations Dagshai and Kssauli). A gradual transition from Kasauli to Lower Siwalik (Nahan) is seen. However, at Dharamsala, Sarkarghat and Nalagarh the Siwalik Group directly overlies the Sirmur Group. At Haritalyangar near Bilaspur, the Lower Siwalik rests on Dagshai with a profound unconformity, indicating a period of Earth movements. Detailed studies on the heavy mineral content and fossils present in the Himachal Siwalik give a clue to the nature of the sedimentary provenance.

Lower Siwalik: the heavy mineral assemblage consists of various opaques, garnet, tourmaline, epidote, staurolite, zoisite, zircon, rutile and chlorite. Epidote and Staurolite are uniformly present in the sediments. The lower Siwalik has yielded a rich mammalian fauna in the area around Nurpur along with a few reptilian remains and shells of Unio. The lower Siwalik sediments are characterised by the preponderance of angiosperm pollen over pteridophytic spores and gymnospermous pollen.

Middle Siwalik: pertologically, the lower boundary of the Middle Siwalik coincides with the influx of kyanite in the heavy mineral assemblage. Moderately rich assemblage of opaques, garnet, tourmaline, epidote, staurolite, zoisite, zircon, rutile, chlorite and kyanitewith sporadic sphene occur. The appearance of kyanite marker attended by lithological changes in the Siwalik points to a major change in depositional pattern, accompanied by earth movements resulting in the unroofing of extensive kyanite bearing crystalline thrusts sheets in the area, north of the Siwalik Basin.

The recovery of Ramapithecus, an hominoid, from the Middle Siwalik, is of great significance as this is one of the forerunners of early man. The occurrence of many advanced apes and Ramapithecus, according to some, clearly indicates that the Siwalik of India formed the radiative centre for the evolution of apes and man. Siwalik have also yielded fossils of Gigantopethicus, man-like ape, which stood about 180cm and possessed massive jaw and teeth.

Upper Siwalik: the conglomeratic facies of the Upper Siwalik is found in the Mandi re-entrant, mainly in the Lambagraon syncline and in the area immediately to the south east of the main tectonic divide between the Siwalik belt and the older Sirmur belt.

The Uper Siwalik is characterized by a very complex suite of heavy minerals including hornblende, sillimanite and andalusite indicating a derivation in part from the advancing crystalline thrust sheet from the north.

Uttaranchal

Siwalik Group here extends for over 250km and is 25km wide, but much less than those in Himachal Pradesh. Kalagarh, one of the principal areas investigated, yielded hominid and other vertebrate fossils. Total thickness is about 4500m here.

In Uttaranchal, between Laldhang and Kotdwara, the Lower Siwalik is dominantly arenaceous. Clays, however, appear in Ramganga-Palain river sections. Middle Siwalik is well exposed south of Dehra Dun in the Mohand section in the core of Mohand anticline, where it attains a thickness of about 1800 m. The Siwalik group of rocks (Sandstones and siltstones) exposed near Kalagarh basin (Paui, Garhwal) are more than 600 m thick.

West Bengal

In the Terai region of the northern part of Jalpaiguri and Darjeeling districts of West Bengal, the Sandstone is usually soft and friable but close to the thrust, it is highly indurated and sheared. In the upper pebbly horizon, pebbles mostly made up of quartzite, jasper and rarely phyllite are present. Besides these, lenses of lignite and fossil wood commonly occur in the Siwalik.

Arunachal Pradesh

The Siwalik group occurs as a linear belt all along the foothills from the border with Bhutan in the west, to Roing in the Dibang Valley in the east where it ends against the Roing fault. Its southern limit is defined by the Main Boundary Fault separating it from the pre –tertiary sequence, while in south the Brahmaputra alluvium defines its boundary which at places appears to be tectonic (foothill fault). The sediments have been divided into Kimi, Dafla, Subansiri and Kimin Formations.

THE TERTIARY SYSTEMS IN INDIA

Rock formations belonging to Tertiary Period are found in both Peninsular as well as Extra-peninsular India. Those of Extra peninsular India are much more important while those of peninsular India have less significance.

Extra – peninsular India:

The Tertiary rocks of Extra – peninsular India are of much more importance and cover an enormous superficial extent of the country. These formations occur continuously from Mekharan coast of Baluchistan through Sindh, Kashmir along the foothills of Himalayas to Brahmaputra gorge in the extreme north of Assam. Then the outcrop continues southwards with an acute bend of strike up to extreme south of Myanmar. The formations of Tertiary Period are most prominently displayed in a belt running along the foot of the Himalayan Mountains. Width of these formations is large in the Eastern and Western portions i.e. in Trans Indus Hazara and Kashmir and in Eastern Assam and upper and lower Myanmar while in central portion they are of lesser aerial extent.

In all these areas Tertiary Systems exhibit dual facies of deposition. An early or lower Marine facies and a later or Upper Estuarine and Fluviatile facies.

In North – western Himalayas, the Eocene strata are marine, the succeeding Murree Series (Lower Miocene) estuarine while Siwaliks (Mid Miocene to lower Pleistocene) are distinctly fluviatile or freshwater in nature.

The sea, in which the lower tertiary sediments were deposited, gradually receded with a rise of its bottom. The retreat of the sea was southwards from the two extremities of the extra – peninsular, one towards Bay of Bengal and the other towards Sindh and Rann of Kutch.

As the seas dwindled and receded they were replaced by the broad estuaries of the rivers like Indus, Ganges, Brahmaputra and Irawaddy.

The whole tertiary history of India is recorded in the sediments filling up the two principal gulfs viz. Sindh Gulf extending through Kutch, Western Rajasthan, Punjab, Simla and Nepal and the eastern gulf which was divided into two by the ridge of Arakan Yoma into the Assam gulf and the Myanmar Gulf.

However the succession of the entire tertiary period is not continuous. There are two important breaks or discontinuity in succession, the first one is at the close of Oligocene when there was a temporary but a wide spread retreat of Sea because of which in Extra – Peninsular region passage from Oligocene to Miocene is not continuous. The second important break affected the eastern part. These local folding movement in late Miocene affected parts of Assam and Myanmar. During these break thousands of metres of Miocene deposits were eroded away from the uplifted areas,

Tertiary Systems of Peninsular India:

Rock systems of this period are less significant as compared to those of Extra – Peninsular India, in both vertical thickness and horizontal extension. Sequence of tertiary rocks is found in Kerala, Gujarat and Sourashtra on small scale. Along the Eastern coast a fairly large area is covered by the Cuddalore Sandstones, from Orissa to southern trip of peninsula. In Kutch the rocks of this period are found overlying the fringe of Deccan Traps.

TERTIARIES OF ASSAM:

INTRODUCTION:

The tertiary rocks are well developed in the North-eastern and South-western parts of Assam. Here the succession attains a great thickness (more than 15000 metres), probably exceeding that of any other part of India, exhibiting more or less a complete geological succession ranging in age from Palaeocene to lower Pleistocene. However there are several gaps or breaks in the succession, the most important being the absence of top part of Oligocene i.e. between the Barail and Surma series. Then there is another break observed at the Late Miocene. In addition there are minor unconformities also, which indicate temporary pause in sedimentation.

There is some variation in lithology of different systems developed in upper (north-eastern) Assam and central and lower (south – eastern) Assam. The fossil content of this system is very less and many horizons of great thickness are devoid of fossils. Therefore ascertaining the age of this formation and correlating with other formations is very difficult only on paleontological grounds. However, geologists of Assam Oil Company have used lithological characters and heavy mineral crops for the purpose of correlation.

Tertiary formations of Assam are very important from the economical point of view since they contain a number of workable coal horizons and also petroleum bearing horizons.

Following is the general succession of Tertiaries in Assam:

Dihing Series Thick pebble beds with clays and sandstones Pliocene

----------------------------------------- Unconformity ------------------------------------------

Dupitila Series Coarse Ferruginous sandstones and clays with Upper

(3000 m) Fossil wood. Miocene

----------------------------------------- Unconformity ------------------------------------------

Tipam Series Thick coarse ferruginous Sandstone, Mottled Middle

Sandy clays, shales with fossil wood and Lignite. Miocene

Surma Series (200 m) Sandy shales, sandstones and Conglomerates Lower Miocene

----------------------------------------- Unconformity ------------------------------------------

Barail Series Sandstones and Carbonaceous Shales Lower Oligocene

(1200 m) to Upper Eocene

Jaintia Series Alternating Sandstone and Shales with Coal Middle Eocene (900 m) seems including Limestones

----------------------------------------- Unconformity ------------------------------------------

Cretaceous and Older Rocks.

The portion of Upper Assam (eastern) the succession below the Oligocene – Miocene unconformity is somewhat different. In this area very thick series of shales and sandstones have developed which is partly equivalent to Jaintia Series and partly older than that. The succession observed here is as follows:

The succession above this unconformity is same as western part.

----------------------------------------- Unconformity ------------------------------------------

Barail Series Sandstones and Carbonaceous Shales Oligocene to

Upper Eocene

Disang Series Thick series of gray shales with fine sandstones Middle to Lower

Which are partly equivalent to Jaintia Series. Eocene, partly Cretaceous.

----------------------------------------- Unconformity ------------------------------------------

Basement not seen

EOCENE OF ASSAM:

The Eocene system is well developed in Assam and occupies a very large area. The rocks of this are represented by two different facies. They are:

1. In the southern and the western part of Shilling Plateau and Jaintia Hill Ranges (Presently Manipur) the Eocene rocks show Calcareous Facies known as Jaintia Series which cover the entire Eocene Period.

2. In the Naga Hills (presently Nagaland) extending from the Brahmaputra valley southwards into Manipur, the lower Eocene formations show geosynclinal facies and are known as Disang Series. These range from Upper Cretaceous to Upper part of Eocene.

These two facies if successions have been brought into juxtaposition by the Haflong – Disang thrust fault.

JAINTIA SERIES:

The 900 metres thick succession of mainly calcareous facies of rocks covering Southern and Eastern parts of the Shillong Plateau (Meghalaya) are termed Jaintia Series, named after Jaintia Hill ranges. This series have been divided into three main divisions or stages and further into several formations based on lithology and fossil content. Exploration for coal and petroleum has helped to study these successions extensively.

The Jaintia Series is made up of chiefly limestones with subordinate shells and sandstones. Fossils are comparatively rare and also of cosmopolitan nature. However there are some fossils indicative of Lower Palaeocene to Upper Eocene age for this series.

DISANG SERIES:

This facies of the Eocene is well developed in the Eastern portion of Assam in the Naga Hills and Manipur. The sequence is geosynclinal, mainly argillaceous and is about 3000 m thick. The successions largely consist of Splintery dark Shales intercalated with fine grained well cemented Sandstones. The entire sequence is mostly devoid of fossils. In the hills of Nagaland the Shales are metamorphosed o slates. The bottom portion of this series may belong to Upper Cretaceous age but the upper portion represents whole of Eocene.

OLIGOCENE AND LOWER MIOCENE OF ASSAM:

INTRODUCTION:

One of the major and spectacular event of Cainozoic era i.e. upheaval of Himalayas continued during this span of period. Towards the end of Eocene there occurred the second major phase. As a result of this event Tethyan Himalayan zone was uplifted as a landmass. This resulted in driving out the sea from most of Himalayan region except along the southern border. The lesser Himalayan basin became shallower with partial withdrawal of marine

water. This shallow bay extended along the foothill region of freshly uplifted young Himalayas. Brackish water sediments were deposited in these basin during the rest if the Tertiary.

However on the western side of the Baluchistan Arc shallow sea continued its hold and received large thickness of Calcareous facies of rocks. The sequence consists of calcareous Sandstones and Shales of singularly uniform appearance. They form the bulk of Flysch formation resembling the Oligocene Flysch of Alpine Region.

On the Eastern side of India i.e. Assam area there was a ridge which divided the sedimentary basin into two portions. Eastern side of this ridge formed the Myanmar basin while the area on the western side of the ridge formed the Assam basin. The sediments in this basin show lateral variation in facies, Northern part is fluviatile and towards south gradually brackish (i.e. deposited at the head of the bay) and then marine further south in the direction of the open ocean.

In these basins sedimentation continued until the mid Miocene, when a third mountain building upheaval took place along the Himalayan region. Therefore Oligocene and lower to middle Miocene rocks form one of the stratigraphic units in Indian geology.

In India Oligocene is poorly represented because it was a period of marine regression. Possible during the part of this period a fair amount of Tertiary succession was undergoing erosion.

During Miocene epoch once again a marine transgression took place in these areas and also along parts of the east coast of Peninsula.

In Assam, the sequence of rocks of Oligocene and lower Miocene are well developed in Shillong Plateau region. They are called

Surma Series (lower Miocene)

Barail Series (Oligocene)

BARAIL SERIES:

This name is derived from the Barail hill ranges which form water divide between Brahmaputra and Surma rivers. In Assam the Barail series is divided in to three stages. The rocks making up this series are predominantly Sandstones and Shales. The Shales contain numerous coal seams. Earlier these coal bearing horizons of Barail series were called ‘Coal measure series’ of Assam. This series also contain number of horizons of Petroleum.

Extensive drilling has taken place in these areas and detailed geology has been worked out. From the economic point of view the Barail series is very important because of the presence of coal seams and oil. In the north-western region of Assam Sandstones of this series are coarser because the region was closer to the landmass or the source area of these sediments but towards N-E Assam the carbonaceous Shales become predominant. The fossils present in

the Barail series are several species of foraminifers. The Barail series is separated from overlying Surma series by a marked unconformity.

SURMA SERIES:

The succession of conglomerates, Sandstones, silts and Shales lying above the Barials with unconformity at the base are called the Surma Series which is names after the Surma River. They are generally comparatively thin in N-E Assam even at places missing where younger Tipam series is directly overlying the Barails.

The Surmas are mainly arenaceous and very poor in carbonaceous matter. In this regard they differ from lower Barails and Upper Tipams. Generally the surma series is poor in fossil content but some thin members have yielded lamellibranches and gastropods and also fishes, reptiles and mammals. These fossils are suggestive of Lower Miocene age. At several places this series has shown indication of oil. Coal seams are few in this series.

MIDDLE MIOCENE TO LOWER PLEISTOCENE OF ASSAM:

In Assam, the rock succession of this time span is divided into three series viz. Tipam Series, Dupitila Series and Dihing series. By and large these rocks are unfossiliferous except some lignite beds, fossil wood and leaf impressions. These rocks are considered as equivalents of Siwaliks.

TIPAM SERIES:

It is found as a long belt from Upper Assam to Southern coast of Burma but show different facies. In southern part, the lower part of this series consists of coarser ferruginous Sandstones and Shales with fossils of marine characters while in the north-eastern part the entire series is non-marine n nature. Here it is made up of ferruginous Sandstones and mottled Shales. The lower portion contains petroleum bearing horizons. The oldest oil-field of India i.e. the Digboi Oilfield taps oil from Lower Tipam Sandstones.

DUPITILA SERIES:

In Surma valley the Tipam series is overlain by Dupitila seires separated by an unconformity. The series is composed of about 3000m thick coarse Sandstones with subordinate mottled clays. Sandstones contain fossil wood. Otherwise the entire series is unfossiliferous.

DIHING SERIES:

This consists of great thickness f pebble beds with subordinate Sandstones and clay beds. These pebble beds are poorly consolidated and pebbles are derived from Barails and older gneisses. They are nearly unfossiliferous except carbonised wood and badly preserved leaf impressions. This series is considered to be equivalent of Upper Siwaliks.

TERTIARIES OF GUJARAT

INTRODUCTION:

Although the Tertiaries are not so well developed in the Peninsular India as compared to those in Extra-peninsular India, the outcrops of Tertiaries of Gujarat are one of the better developed successions. Few outcrops of small lateral as well as vertical extent are found in Kutch, Sourashtra and Coastal Gujarat.

Tertiary rocks are exposed in Gujarat all along the coastal region of Surat, Bharoach Cambay and also in the Southern Kathiawar and in Kutch. After the sub aerial eruption of Deccan traps, a trough fault was formed in the early Eocene times in vicinity of Cambay region. The fault trough runs North – south through Cambay for nearly 400kms which extends beyond 24 N latitude but becomes shallower towards the northern extension. It has a width of about 80 – 100kms. In this trough the full tertiary succession is deposited. The succession is 1500 to 2000 m thick. These rocks are the important oil bearing horizons of this area. For all tertiary sequences of this region the basement is Deccan traps. The Eocene is generally well developed followed by regressive Oligocene and again transgressive Miocene and younger beds. The regression during Oligocene may be due to strong earth movements in he Mediterranean Region. The marine transgression during Miocene is the result of Himalayan Orogeny and the final withdrawal of water from the Tethys to the Indian Ocean. After the deposition of Miocene and Pliocene sequence, there was a final faulting along the west coast of Indian Peninsula. The down faulting in the Arabian sea may have been accompanied by upward tilt of the Peninsular block producing a steep westward slope (Western Ghats) and a gentle Eastward slope.

Cambay Trough:

The tertiary succession as revealed by bore hole drilled for exploration of Petroleum and Natural Gas in the Cambay trough is over 2000 m thick. The thickness of the succession is maximum towards the south and gradually goes on thinning towards north. Hence the shape of the sediments in this trough is like a wedge. Beyond north 24o latitude the rocks become estuarine in character and possibility of occurrence of oil is nil. While the middle and south portions are good yielding regions. The following is the succession of tertiary rocks in the Cambay trough:

Age Thickness Lithology

Pleistocene 100 metres Sands and Clays

Pliocene 700 metres Sandy Clays, Sands and gravels

Miocene 600 – 850 metres Silt, Calcareous claystones with bands of

Pyrite and carbonaceous clays.

----------------------------------------- Unconformity ------------------------------------------

Oligocene 60 – 150 metres Shales with Limestone bands

Middle to Upper 300 metres Carbonaceous and pyritic Shales with

Eocene thin Limestone layers

Lower Eocene 200 metres Carbonaceous Shales with Sandstones

Palaeocene 40 metres Lateritic with carbonaceous clays and

Trap – wash

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Deccan Traps (more than 1200 m thick bade not encountered)

The Palaeocene rocks were deposited on the eroded surface of Deccan Traps. The intervening unconformity, lateritic clays, inclusion of carbonaceous matter and pebbles of Basalts, Agate, and chalcedony at the base are indications of shallow water deposition. The great thicknesses of Tertiaries in the trough indicate subsidence of floor of the faulted block along with sedimentation.

SAURASHTRA (KATHIAWAR) AREA:

By far the largest surface area of Sourashtra is occupied by the Deccan traps. The other rocks include Jurrasic inliers, cretaceous outcrops. Tertiaries are exposed in the South-west, south and east of Kathiawar along the coast. The formations show a gentle dip towards the sea.

In the Gulf of Cambay, a detached outcrop of Tertiary rocks occurs in the Perim (Piram) island. The ‘Ossiferous’ Conglomerates are full of fossils of many extinct mammals such as goats, pigs, rhinoceros, mastodon, etc. Belonging to Middle or Upper Tertiaries (Miocene – Pliocene). The place is very important for the collection of fossil bones among the coastal gravels.

The general succession of the Tertiary Sequence in Sourashtra region is as follows:

Age Formation Lithology

Pliestocene Porbandar Limestones Oolitic Miliolite Limestones

Pliocene Dwaraka Beds Clay, Silt and Cherty Limestones

Pliocene Piram Beds ‘Ossiliferous’ Conglomerates, grits and

Clays (exposed on Perim Island)

Mio – Pliocene Gogha Beds Thin bedded grits and Sandstones

Lower Miocene Gaj Beds Clays, Marls and impure Limestones

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Deccan Traps