The Deccan Trap is a significant geological formation in India, characterized by extensive flood basalt deposits created by volcanic activity approximately 65 million years ago. It features layered structures, magnetic polarity variations, and intertrappean beds containing fossils, marking its stratigraphy and geological history. The formation has implications for climate change due to the release of volcanic gases and is associated with significant mineral resources such as native copper and agricultural soil known as 'regur.'

1. The term "trap" has been used in geology since 1785–95 for rock formations. It is derived from the Swedish word
for stairs (trapp , trappa) and refers to the step-like hills forming the landscape of the region.
The plateau: also called a high plain or tableland, is an area of highland, usually consisting of relatively flat
terrain. A plateau is an elevated land. It is a flat-topped table standing above the surrounding area. A plateau may
have one or more sides with steep slopes.
LIP – Large Igneous provinces. (Province = Area / Region)
DVP - The Deccan Volcanic Province is one of the Earth’s giant continental flood basalts and has a total exposed
area of about half a million square kilometers in Maharashtra, Madhya Pradesh, Gujrat and some part of Andhra
Pradesh. Deccan trap has maximum thickness 3400m in western ghat and its thickness goes decrease toward east
side. At Amrakantat on east its thickness is just 160m. Geographical distribution is between latitudes 16° - 24° N
and longitudes 70° - 77° E.
Age – 65 Ma late cretaceous, it begins to
form at this age and it take almost 1Ma to
complete the episode of fissure type
volcanic eruption. The release of volcanic
gases, particularly sulphur dioxide, during
the formation of the traps contributed
to climate change. Data points to an average
drop in temperature of 2°C in this period.
Source – from lower mantle - outer core
boundary, Association – Reunion mantle
plume. Less viscous lava with massive,
compact and vesicular basalt. 95% -
Tholeiitic basalt, 5 % - Alkali basalt,
Nephelinite, Carbonatite and lamprophyre.
Fossil – Frog (Oxyglossus pusillus) Eocene
Deccan basalts are nearly horizontal over
vast areas, except where they are
tectonically disturbed as in the West Coast,
Western Ghats, Khambhat graben and Son-
Narmada (SONATA) rift. The lavas are
thinnest in the east But at some places it gently dip by 10°. Total 48 flows are recorded in deccan trap in which 11
flows can identified in SW of DVP on the basis of major elements and Trace elements like – Sr, Nd, Pb isotopes
❖ Structural Features – Deccan volcanic province are layered and layers are almost horizontal
everywhere, average dip (angle of inclination) is 10°. The individual single horizontal layer formed from
lava is called lava flow. Its thickness is varying from 1m to 30m or even more. Bole beds are considered as
marker horizons, which are valuable in the identification of flows. They are commonly termed as red boles and
green boles or green earth. Dykes are present in all over deccan trap primarily in Gujrat and konkan area with N-S
trende.
❖ Magnetostratigraphy: The entire lava pile including the Kalsubai and Lonavala Subgroups, as well as the
lower two formations (Ambenali and Poladpur of Wai Subgroup), is reversely magnetised with the exception of
local normal magnetisation in thin flows. This means that nearly 95% of -3000m thick Deccan lavas are reversely
magnetised. The top portion of the stratigraphy of the Western Deccan Province (Mahabaleshwar Formation and
younger formations of Wai Subgroup) shows normal magnetisation. This top-normal characteristic may be used
as a significant marker in mapping the DVP.
❖ Intertrappean beds –
The Intertrappean Beds are a Late Cretaceous geologic formation in India. Indeterminate theropod and pterosaur
remains have been recovered from the formation, as well as dinosaur eggs. Before the advent of radiometric dating,
the age of the Deccan Trap was invariably fixed based on the age of the youngest underlying fossiliferous
formations, mostly of Cretaceous age, designated as Infra-trappean beds, and ages obtained for the youngest Inter-
trappean beds. It was soon noted that there are such fossiliferous horizons in different places at different levels in
different sections of the Deccan Traps and they may be infra-, inter- or supra-trappean beds. The same horizon may
be infra-trappean in one place and intertrappean in another. At short intervals traps are separated by sedimentary
beds of small vertical as well as horizontal extent of lacustrine or fluviatile deposition on the irregular surface
During eruptive intervals, Which are formed under water. These sedimentary beds are called as Infratrappeans or
Intertrappean beds.
GL-JAN-2019 Last update Aug - 2019 S t r a t i g r a p h y
Q U A R T Z – E C I E S Nikhil V. Sherekar
Deccan Trap

2. ❖ Stratigraphy of Deccan trap in western ghat
Group Sub Group Formation
Thickness Magnetic
polarity
Sr87 / Sr86
ratio
DeccanBasalt
Wai
Desur ~100M N 0.7072-0.7080
Panhala >175M N 0.7046-0.7055
Mahabaleshwar 280M N 0.7040-0.7055
Ambenali 500m R 0.7035-0.7044
Poladpur 375m R 0.7053-0.7110
Lonavala Bushe 352m R 0.7078-0.7200
Khandala 140m R 0.7071-0.7124
Kalsubai
Bhimashankar 140m R 0.7067-0.7076
Thakurwadi 650m R 0.7067-0.7112
Neral 100m R 0.7062-0.7104
Jawahar - Igatpuri >700m R 0.7085-0.7128
Created by – Nikhil V. Sherekar
N – Normal magnetic polarity R – Reverse magnetic polarity
▪ Wai - Simple lava flow with small phenocryst of plagioclase + pyroxene + olivine
▪ Lonavala – Bushe compound lava flow
Khandala Simple lava flow
▪ Kalsubai - Amygdaloidal compound flow with high MgO content, Picritic basalt and picrite with
phenocryst of Olivine + CPx
The inter-university team (Subbarao et al.2005) divided the Deccan Basalt Group into 12 formations, which are
combined into 3 subgroups.
▪ Kalsubai Subgroup consists of five formations represented by amygdaloidal compound flows. Picrites
(>18% MgO) and picritic basalts (>10% MgO) containing olivine and/or clinopyroxene phenocrysts are
relatively common. GPBs (Giant plagioclase basalt) with plagioclase, 2 to 5 cm long, separate each of the
five formations of Kalsubai Subgroup.
▪ Lonavala Subgroup consists of the lower Khandala Formation which marks a distinctive change from the
Kalsubai Subgroup by the presence of simple flows having clear-cut differences in petrology and
geochemistry. The upper Bushe Formation consists of coarse grained, aphyric, amygdaloidal compound
flows having a narrow chemical range. Lonavala Subgroup is characterised by high Ba/Ti and Ba/Sr
values and higher MgO for given TiO2 values.
▪ Wai Subgroup forms the upper part of stratigraphic succession with five formations consisting mainly of
simple flows having well developed flow tops. The junction between Lonavala and Wai Subgroups is
clearly identified in the field, which is also reflected in sharp elemental and isotopic change. The flow
compositions of Wai Subgroup are more evolved than that of the older flows, but some rare picritic flows
are locally encountered.
❖ Classification of Deccan trap
Sr no Division Distribution Character
1 Upper Trap (500m) NW Peninsular Lava flow with ash bed +
intertrappean
2 Middle Trap (1300m) Central India + Malwa region Lava flow with ash bed >
Intertrappean
3 Lower Trap (160m) MP and Eastern region Lava flow with intertrappean <
ash beds
▪ The lower trap is exposed in Madhya Pradesh and toward the east of deccan trap with 160m thickness and
they contain no of intertrappean beds. In the lower trap the ash beds are rare.
▪ The middle trap occurs in central India and malwa region with 1300m thick lavaflows and some ash beds.
In this part, the intertrappean beds are almost absent.
▪ The upper trap is exposed in west part of the peninsula, particularly in Mumbai and Kathiawar with
500m thickness and are associated with intertrappean and ash beds.
❖ ECONOMIC MINERALS
Flood basalts are known to host important deposits of native copper and platinoids. Epithermal gold and gold-
silver deposits are also known elsewhere. Reconnaissance studies on Deccan Traps have not found any indicators
of the mineralisation of this type. Bauxite capping over Deccan Trap as in Belgaum are useful as aluminium ore.
Natural zeolites filling the cavities in volcanic are useful as gemstones and have industrial and agricultural
applications. They are also sources of good agricultural soil formed by the decomposition of basalt containing Ca
& Mg carbonates, potash and phosphates. This soil is known as ʻRegurʻ & is used for cultivation of cotton.

3. What is Flood basalts ?
Flood basalts, also known as large igneous provinces (LIP), are the product of massive outpourings of
low viscosity basaltic lava that envelop hundreds of thousands of square kilometres. Flood basalts form above
mantle hotspots in the marine setting as oceanic flood plateaus or on land as continental flood plateaus.
Oceanic flood basalts include the Ontong Plateau deposits in the western Pacific Ocean basin and the Kerguelen
Plateau in the Indian Ocean. Spectacular examples of continental flood basalts include India’s Deccan Traps, the
Siberian flood basalts, East Africa’s Karoo flood basalts and North America’s Columbia River/Snake River Plain
flood basalts. The largest flood basalt of all – the Central Atlantic Magmatic Province (CAMP) – includes both
marine and continental deposits distributed throughout the Atlantic Ocean margins recording the break-up of the
Pangea supercontinent. We will discuss these examples in greater detail in Chapter 10 .
Flood basalts are somewhat unique among volcanic features in that no modern examples are known,
which prevents the direct observation of eruptive style and their impact upon Earth’s climate and inhabitants. The
Columbia River/Snake River Plain flood basalts are the youngest flood basalt deposits on Earth, having erupted
within the past 17Ma.
Flood basalts typically release lava through multiple fissures. Multiple, long fissures and low lava
viscosity together with large volumes of magma promote widespread flooding as opposed to localized
accumulation around central vents. Where fluid lavas flow into basins, deep lava lakes may form to produce
unusually thick accumulations. Flood basalts may erupt repeatedly over millions of years to produce stacked
sequences of flood deposits, hundreds to thousands of meters thick. The tops of each individual flow may show
vesiculation as gas bubbles migrate toward the top of fl ows. Thick flows display columnar jointing, formed as the
lava cools and contracts. The joint sets create polygonal columns less than 1m in diameter but meters to hundreds
of meters in height where lava lakes once existed. The relatively flat surfaces produced as fluid lavas fill in low
areas generate immense lava plateaus. Figure 9.6 illustrates columnar joints exposed at Giant’s Causeway in
Ireland, an ancient lava lake that formed from a fissure eruption in the Irish Sea 60 million years ago.
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