My seminar At Periyar University Salem I'm Thomas chinnappan Keeranur Pudukkottai tamilnadu India @tcoftheworld @tcorganization

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

13. 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.

14. • 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.

15. • 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.

16. 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.

17. • 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.

18. • 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 Types 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.

19. 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.

20. • 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.

21. 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.

22. • 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.

23. • 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.

24. 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.

25. 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

26. 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.

27. • 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.

28. • 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.

29. 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.

30. • 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)

31. 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.

32. • 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.

33. 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.

34. • 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.

35. • 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.

36. Thank-you