Deccan traps in stratigraphy

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1. BY: THOMAS CHINNAPPAN .A , M.SC.APPLIED GEOLOGY, PERIYAR UNIVERSITY, SALEM. Stratigraphy DECCAN TRAPS

2. Introduction : The Deccan Traps are basaltic lava flows which now occupy some 200,000 square miles of Western and Central India and which may originally have occupied more than twice this area. Basaltic lavas are often called traps because the flows produce step like topography. On account of their tendency to form plateau-like features they are called plateau basalt. It is surmised that they issued out of several fissures in the earth's crust with a high degree of super-heat which enabled the lavas to spread out far and wide into horizontal sheets.

3. Evidences of the existence of this latter type in the Deccan Trap area has also been recorded from a few places in western India in recent years, the eruptions in such cases being accompanied by some differentiated rock type of acid, intermediate and ultra-basic composition. Such eruptions are called fissers-eruptions in contrast to central-eruptions which produce cone and crater.

4. The lower and upper parts of the Deccan Trap succession can also be differentiated on the basis of their differing chemical characters (Ghosh,1976) The lower unit exposed in the eastern southern parts of the Deccan country is composed of uniform horizontal thoelitic flows representing the quiet type of eruptions. The upper unit exposed in the northern parts of the Deccan country is characterised by an explosive acitivity.

5. The trap succession has been generally classified into three stratigraphic units. The lower about 150m thick succession of traps exposed in parts of Madhya Pradesh and eastern areas are associated with numerous intertrappean beds. The middle unit comprising about 1200 m thick lava flows and ash beds in practically devoid of inter-trappean beds. This unit is exposed in Central India and Malva region of the northern Peninsula. The upper unit of the succession consisting of about 450m thick sequence of lava flows with numerous inter-trappean beds is exposed in the northwestern parts of the Peninsula.

6. History : The Deccan Traps began forming 66.25 million years ago,at the end of the Cretaceous period. The bulk of the volcanic eruption occurred at the Western Ghats some 66 million years ago. This series of eruptions may have lasted fewer than 30,000 years.

7. The original area covered by the lava flows is estimated to have been as large as 1.5 million km2 (0.58 million sq mi), approximately half the size of modern India. The Deccan Traps region was reduced to its current size by erosion and plate tectonics; the present area of directly observable lava flows is around 500,000 km2 (200,000 sq mi).

8. The Deccan Traps are famous for the beds of fossils that have been found between layers of lava. Particularly well known species include the frog Oxyglossus pusillus of the Eocene of India and the toothed frog Indobatrachus, an early lineage of modern frogs, which is now placed in the Australian family Myobatrachidae. The Infratrappean Beds (Lameta Formation) and Intertrappean Beds also contain fossil freshwater molluscs. Fossils :

9. Distribution : The Deccan Traps occupy large areas of Bombay, Kathiawar, Central India and Madhya Pradesh, with outlying patches in Bihar, Madras and Kutch. It would appear that they extended for some distance west of the present Bombay coast, but this portion has been faulted down and is now covered by the sea. The straightness of the continental shelf of the western coast and the large thickness of the traps here, estimated at over 7,000 feet, go to support this idea.

10. The traps are divided into three groups as follows: Upper Traps (1,500 ft.) Middle Traps (4.000 ft.) Lower Traps (500 ft.) Bombay; contain numerous layers of volcanic ash and intertrappean beds. Central India; numerous Bagh-beds in the upper portion but few intertrappeans. Madhya Pradesh and further east; several inter-trappeans. but few ash-beds.

11. Classification of traps

12. Structural features : The traps have been poured out in a series of flows, the individual flows in different areas varying in thickness from a few feet to as much as 100 feet. The average of a large number of flows is probably of the order of 50 feet. The flows have a great areal extent, and a few individual flows have been traced for distances of 50 or 60 miles. Some of the flows are compact but may show variation in coarseness as between the centre and the Top and bottom surfaces.

13. In thick flows, the bottom may be coarser than the other portions. Amygdaloidal flows are common, the amygdular cavities being often filled with secondary minerals such as calcite, varieties of silica (quartz, chalcedony, agate, jasper, Carnelian, etc.), zeolites or 'green earth.' Ash beds are common in the Upper Traps. Columnar jointing is seen only in a few places, Eg: Salsette Island and Malwa. The Traps occasionally show slight undulations and folding, attributable to some earth movement Subsequent to their formation.

14. Some faults have also affected them as in the Chhindwara district, Madhya Pradesh. Dykes and sills of Trap have been noted in restricted parts of the Trap area, especially in bombay and Madhya Pradesh. The dykes may be regarded as occupying the fissures through which the lava issued out. Sills are to be seen in the Upper gondwana strata of Madhya Pradesh and in the Jurassic strata of Kutch.

15. Petrology and petrograpghy : The Deccan Traps are surprisingly uniform in mineralogical characters. They are of the nature of Dolerite or basalt with a specific gravity around 2.9. In colour they are dark grey, dark greenish grey and sometimes purplish. In a few places in Western India the Traps are been to be associated with a variety of differentiated Types--rhyolite, obsidian, granophyre, trachyte, Porphyrite, andesite, nepheline-syenite, monzonite,olivine-gabbro, lamprophyre, etc.

16. Such types are seen in the Girnar Hills of Kathiawar, in the neighbourhood of Bombay, and in the Pavagad hill in Gujaral The common type of the Trap is composed of labradorite and enstatite-augite with some interstritial -glass and granules of titaniferous magnetite. The enstatite-augite (pigeonite) is richer in iron and Magnesia and much poorer in lime than common Augite, and is characterised by a low optic axial angle. In fine-grained types, glassy matter is present but it is liable to alteration to palagonite or other material.

17. The coarser types show good ophitic texture, the feldspar being mostly the earlier mineral to form. Granophyric or micrographic texture is fairly common. Quartz and biotite are rare. Some T ypes are of the composition of olivine-dolerite. In chemical characters also the Traps tends to great uniformity of composition. The silica percentage is around 50; ferrous iron is high and lime low in comparison with ordinary basalts.

18. Alteration of the Traps : The weathering of Traps gives rise to a deep brown soil or to 'black cotton soil' (regur) . The conditions of formation of the black soil are not well Understood, but it may be said that rainfall and climate have a definite role in its formation. The black soil has the property of swelling when wetted, and drying up with numerous large cracks on drying.

19. A peculiar product of tropical weathering of a traps in a monsoon climate is laterite in which the oxides of alumina, iron and manganese are concentrated while the alkalies. Alkaline earths and silica are leached away. Laterite may be ferruginous, aluminous or manganiferous, depending on the concentration of the particular constituent.

20. Inter - trappean beds : At short intervals the lava flows are separated by sedimentary beds of small vertical as well as horizontal extent, of lacustrine or fluviatite deposition. Formed on the irregularities of the surface during the erupitive intervals. These sedimentary beds, known as inter-trappean beds, are fossiliferous, and are valuable as furnishing the history of periods of eruptive quiescence that intervened between the successive outbursts, and of the animals and plants again and again migrated to the quite centres.

21. The most common shell, which is also the most characteristic fossil of the intertrappean beds, wherever they have been discovered, is Physa(bulinus), Prinsepii--a species of fresh water Gastropod : other fossil are lymnaea, unio, Natica etc. Inter trappean beds are exposed in good sections at Bombay (Malabar hills and Worli), where about 100 feet of well bedded shales are Senn between two lava flows, containing numerous carbonised plants, many frogs, a tortoise and cypris shells.

22. A profile area for fossiliferous inter-trappeans in chhindwara in Madhya pradesh, where beautifully silicified leaves, flowers, fruits,seeds and wood of many species of plants are preserved in abundance. A type-section through a portion of the basalts will show the relations of the traps to these sedimentary interrealtions as well as to infra-trappean Lametae.

23. Infra - trappean beds : The Lameta Formation, also known as the Infratrappean Beds, is a sedimentary rock formation found in Madhya Pradesh, Gujarat, and Maharashtra, India. It is of Maastrichtian age (Late Cretaceous), and is notable for its dinosaur fossils. Many dubious names have been created for isolated bones, but several genera of dinosaurs from these rocks are well-supported, including the titanosaur sauropod Isisaurus and the abelisaurs Indosaurus, Indosuchus, Laevisuchus, and Rajasaurus. As well as mammals, snakes and other fossils.

24. Lameta Formation Stratigraphic range: Maastrichtian ~70–66 Ma ↓ Underlies Deccan Traps deposits Overlies Jabalpur Group or Precambrian Area 5,000 km2 (1,900 sq mi) Thickness Variable, typically 18–45 m Lithology Claystone, sandstone limestone Other Conglomerate

25. Age of Deccan traps : The Infra-Trappeans at Dudukuru near Rajahmundry are, as noted above, to be regarded as being Cretaceous in age. The Inter-Trappeans have, in recent years, yielded some foraminifera, algae and palms which are said to point to an Eocene age for the associated Traps. There is a slight unconformity between the Bagh beds and the overlying Traps. But the magnitude of the unconformity cannot be judged precisely in terms of geological time.

26. In Surat and Broach the basal Eocene is said to be distinctly unconformable to the Traps. ln Kutch also a similar unconformity is said to be present between the Traps and the overlying Numulitic beds. In the Bor Hill near Ranikot in Sind, flows of contemporaneous basalt have been described as Occurring below and above the Cardita beaumonti bed. This bed also contains a Nautilus, some corals, echinoids and gastropods which appear to indicate an Upper-Cretaceous age.

27. There is thus some conflict of opinion regarding the age deduced from stratigraphical and palaeozoological evidence onthe one hand and from palaeobotanical evidence on the other. The upper limit of the age of the Traps may extend well into the Eocene or even later; for we know that the Traps have a large thickness in Bombay and that some phases of volcanic activity were later than the pouring out of the main mass of the Deccan Traps.

28. Mode of eruption of deccan traps : The lowermost trappean beds rest upon an uneven floor of older rocks, showing that the eruptions were subaerial and not subsequous. ln the latter case i.e. if the eruptions had taken place on the floor of the sea or lakes, the junction-plane Between the two would have been quite even, from the depositing action of water.

29. As already alluded to the actual mode of the eruptions was discharge through linear fissures, from which a highly liquid magma welled out and spread itself out in wide horizontal sheets. This view is abundantly borne out by the monotonous horizontally of the trape everywhere and the absence of any come or creater of the usual type as the foci of the eruptions, whether within the trap region or on its periphery. i.e. eruptions from a chain of craters situated along fissure-lines. (Cf. the Laki eruption of 1783)

30. Fissure-dykes in the traps : For any proof of the existence of the original fissures which served as the channels of these eruptions we should look to the peripheral tracts of the Deccan Traps, it is not easy to detect dykes and intrusions, however large, in the main mass of the lavas unless the former differ in Petrological characters from the latter, which is rarely the case actually. Looked at in this way, some evidence is forthcoming as to the original direction and distributing if the fissures.

31. The most notable of these is the Rajpipla hill tract near Broach. In Cutch likewise there are numerous large dykes and complex ramifications of intrusive masses visible, along the edge of the trap country, among the Jurassic rocks. The total area of Kathiawar is traversed by a large number of dykes intruded into the main mass of the lavas. They are of all sizes from thin veins to masses hundreds of yards wide and some miles in length, and follow different directions. Smilar fissure dykes occur in the Narmada valley and Satpura area among the Gondwana rocks; they are likewise seen in the Konkan.

32. Economic Geology of the traps : The basalts are largely employed as road-metal, in public works, and also to a certain extent as a building stone in private dwellings. From their prevailing dark colour and their generally aspect, however the rocks are not a favourite building material, except some light coloured varieties e.g. the buff trachytes of Malad, near Bombay.

33. large kernels of chalcedony often yield beautiful agates, carnelians, etc., Worked into various ornamental articles by the lapidaries for which there was once a large Market at Cambay. These are obtained from a Tertiary conglomerate in which pebbles of chalcedony, derived from the weathering of the traps, were sealed up. The sands of some of the rivers and some parts of the sea-coast are magnetite, and when sufficiently concentrated are smelted for iron.

34. Conditions of underground water storage and supply in the Deccan Trap areas are of interest. Traps contain large and important deposits of bauxite in Bombay, Madhya Pradesh and Bihar. This has been used in petroleum refining and very recently as an aluminium ore. The ferruginous laterite is a good and cheap building stone and may possibly find use in future as a source of iron.

35. Thank-you