Deccan Trap

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

v 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.

v  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.

v 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. 

§  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.

 

v 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.

v  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.

 

 

 

 

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. Below figure 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.