Geological Formation

8/8/2019 Geological Formation

1/19

DECCAN BASALT VOLCANISM".. It all happened nearly 70 million years back.. The mighty Himalayas were just beginning to come up, the great dinosaurs that ruled theearth were disappearing and the crust of the earth in the Western India was fractured. This was a remarkable event - large quantities ofmolten rock (Lava) were poured out from these fractures filling up valleys and covering the hills. The eruption continued intermittently withperiods of quiescence, building up a huge volcanic province..." (West, W.D. 1988)The geological world identifies this volcanic province either as the "Deccan Volcanic Province" (DVP) or simply the "Deccan Traps" andrecognizes it as one of the Large Igneous Provinces (LIPs) representing high magmatic fluxes involving large amount of thermal energy inshort period of geological time. The term Deccan Trap was coined by W.H. Sykes in 1833 and it is derived from a Sanskrit word Dakshinmeaning south or southern and a Swedish word Trapp/ Trappa meaning Stair. The term was aimed to describe the step like or terrace liketopography peculiar to this terrain.

A view of Mahabaleshwar section showing typical "Trappean" topography

In Western and Central India, DVP is exposed mainly in the states of Maharashtra, Madhya Pradesh, Karnataka, Gujarat, and Andhra Pradeshand also has its nominal presence in southern parts of Uttar Pradesh and eastern parts of Rajasthan. Based on the area of occurrence, the

Deccan Lavas have been further classified into four classes viz. Malwa Trap: occurring in Malwa region of Madhya Pradesh, Mandla Traps:occurring in Mandla region of Madhya Pradesh, Saurashtra Trap: occurring in Saurashtra region of Gujarat and Main Deccan plateau:occurring in States of Maharashtra, Karnataka and Andhra Pradesh.

Page 1 of 19Geological Survey of India(GSI),Ministry of Mines,Govern...

11/17/2010mhtml:file://C:\Documents and Settings\User\Desktop\Geolo...


2/19

Map showing distribution of Deccan lavas and associated intrusives in the Western and Central India

Today, the DVP occupies an area of over 5,00,000 sq. km however, original estimates of the extent of the lava pile prior to erosion andpossible down-throw on the western side into the Arabian Sea are of the order of 1 to 1.5 million sq km. The lava pile has a maximumthickness of over 1.5 km in the western parts of India and it reduces to few tens of meters near exposed margins of the province. Kalsubai, aprominent peak in the western parts of Ahamadnagar district bordering Thane district of Maharashtra is the tallest peak of the province(>1640m).The DVP is largely made of basaltic lava flows with subordinate volumes of intrusive bodies and relatively trivial proportion of pyroclastic. Thelava flows are nearly horizontal over vast regions of the province but they assume measurable dips in the western parts of the province (nearMumbai) and in Satpura ranges. The average thickness of the individual lava flows is around 20 m and they can be traced for few to nearlyhundred kilometers.

As far as the rock sequences hidden beneath the Deccan lavas are concerned, direct evidences are provided by the localities where theDeccan lavas are seen resting over these sequences. If continuation of the older sequences is assumed beneath the Deccan Trap then, avariety of rock types including granitic rocks, sedimentary rocks as well as metamorphic rocks of Achaean to Jurassic age can be presumedto underlie the DVP. Further, crustal fragments brought up by dykes also provide a ready-made window to peep into the rocks that underliethe DVP.




3/19

Close up of a granitic xenolith occurring in a dyke exposed near Nandurbar

A granitic xenolith in a tinguite dyke near Bhiwandi

Unfortunately, such xenolith-bearing dykes are very few in number and they have so far hinted very limited variation in the crustal portion




4/19

that lie below the Deccan lavas. For areas that do not have any exposures of the older sequences, geophysical methods mainly seismic andgravity surveys are useful in exploring the possibilities, and indicate the presence of largely granitic terrain underneath most parts of theDVP. The only exceptions come from the Narmada-Tapi tectonic zone where, two nearly E-W trending sedimentary basins with more thanfew hundred metre thick sedimentary pile have been interpreted. Last but not the least, the strontium (Sr) as well as other radio-isotopeanalyses of Deccan lavas too indirectly suggest presence of granitic crust and its involvement in the Deccan volcanism.In 19th century and early part of 20th century, the lava flows were thought to be without any significant minerals of economic importance.Therefore, the province attracted only limited attention of the geologists and many of the early descriptions are restricted to thephysiographic and petrographic accounts of the basaltic rocks and cavity minerals occurring in them. In 1960s Dr. G.P.L. Walker visited theDVP and described the lava flows as compound (made of many flow units) and simple (made of single flow unit). Subsequently, owing to thesimilarities seen between the recent eruptions in Hawaii islands and the DVP, the Hawaiian terminology was also extended to describe theDeccan lava flows. Based on surface morphology, viscosity, rate of eruption and strain rate, the Hawaiian terminology divides the lava flow

into two main classes viz. aà and pahoehoe. Akin to the Hawaiian lava flows, the aà flows in Deccan too have upper and lower surfacescomposed of rubble blocks and flattened and irregular, larger but fewer vesicles in their core.

Angular rubble blocks occurring near the upper surface of a typical aà flow exposed in Diveghat area, Pune district




5/19


6/19

Lava toes exposed in Girna River bed in Nasik district, Maharashtra

In addition to the two lava flow types described above, few Deccan Lava flows share characters of a`a and pahoehoe types and thus theyhave been rather loosely described as mixed flows. Based on the distribution of predominant lava flow types, the DVP can be divided intotwo parts viz. areas with dominantly compound/ pahoehoe flows and area with simple / a`a flows.




7/19

Distribution of predominantly pahoehoe /compound and a`a/ simple flows in the Deccan Volcanic Province

The presence of vesicles in the lava flows is due to escape of gases during the process of cooling and solidification. These vesicles are eitherempty or host many varieties of secondary minerals such as zeolites, quartz, calcite etc. in them

A large vesicle hosting a variety of secondary/ cavity minerals. Location: Tamhani Ghat

Irrespective of the flow types, the lavas show various types of joints and fractures. Many of these joints have resulted during cooling of hotlava.

Columnar joints in lava flow from the DVP. Location: Near Kolhapur




8/19

A planar view of the columnar joints in a Lava flow. Location: Ajanta

The fractures are largely post-cooling in age and tectonic in origin. Many a times, these fracture zones guide the development of drainagesand ridges and are traceable over long distances. As a result of water and wind action facilitated by the jointed characters of the rockmass aswell as the process of scarp retreat, spectacular land forms such as potholes, spheroidal weathering, mesas, buttes, spinacles etc have beencarved out from the lava pile.




9/19

Fracture zones cutting across the Deccan lava pile exposed in Malshej Ghat, Thane district

Fracture zones/ master joints cutting across the Deccan Trap escarpment in Kalsubai ranges, Ahamadnagar district, Maharashtra

Spheroidal weathering in a Deccan lava flow. Location: Katraj Ghat, Pune District




10/19

A series of mesas and buttes from Trimbakeshwar hill, Nasik district.

To an unaided eye, the lava flow is usually light to dark grey, fine to medium grained rock with aphyric (without identifiable minerals) tophyric (with identifiable minerals/phenocrysts) texture. Sometimes the rock contains crystals (mainly plagioclase feldspar-an aluminosilicate)as large as 2cm or even more and is called a "megacrystic basalt" or "Giant Plagioclase Basalt (GPB)".

A giant phenocrystic/ megacrystic basalt horizon, Thalghat, Thane district, Maharashtra

Page 10 of 19Geological Survey of India(GSI),Ministry of Mines,Gover...



11/19

A giant phenocrystic/ megacrystic basalt from Dhadgaon, Nandurbar district, Maharashtra

Under microscope, the rock shows fine to medium grained, equigranular to inequigranular texture and is made of primary minerals such asplagioclase, intergranular, inter-sertal, ophitic, sub-ophitic, pophyritic etc.

The Deccan lavas are intruded by numerous dykes and sills that show preferred occurrence along the Narmada-Tapi tectonic zone and theWestern Coast. While most of the dykes of the Narmada-Tapi region trend in ENE-WSW direction, the coastal dykes are almost N-S in trend.

False Colour Composite (FCC) showing linear ENE-WSW trending linear ridges formed by dykes from Tapi region

In the other parts of the province, especially the plateau region, the dykes are fewer in number and they lack any pronounced preferredorientation. The dykes usually make linear ridges that can be traced for a long distance, even upto 50 km or more. Occasionally, dykes aremore eroded than the lava flows they intrude and thus give rise to linear depressions along their trend. Individual outcrops are often wellointed or bouldery. Compositionally as well as mineralogically the dyke rocks are similar to the Deccan lavas. However, they usually have

relatively coarser grain-size than the lava flows.




12/19

A negative impression of a dyke formed due to its preferred/ higher erosion than the lava flows. Location: Sakri, Dhule district,Maharashtra

The pyroclastic component of the Deccan Province is represented by bright red to green to khaki horizons that occur between successive lavaflows at many places.

A red pyroclastic "interflow horizon" from Panhala road section, Kolhapur district, Maharashtra




13/19

Photomicrograph of a typical mineral assemblage and textural features in a red pyroclastic "interflow horizon"

These inter-flow horizons are also known in the geological literature as bole beds. They are few cm to a couple of metre in thickness andcan be traced for few metres to many kilometres. They generally have a sharp contact with the upper flow and a gradational contact with thelower flow. The association of these horizons is more common with the aa/simple flows than the pahoehoe/compound flows. These horizonshave glass shards as well as fresh phenocrysts in them and their texture is typically pyroclastics.




14/19

An Intertrappean bed (White coloured) topped and bottomed by Deccan lava flows. Location: Rajahmundri, Andhra Pradesh

Besides the three important components described above, a number of sedimentary horizons are intimately associated with the DVP in thestates of Andhra Pradesh, Maharshtra, Madhya Pradesh, Rajasthan, Gujarat and Karnataka. Based on whether they occur immediately belowor are within the successive lava flows, these horizons are known as infra- or inter-trappeans respectively. Although over 150 localities of

infra- and inter-trappean beds are known, the restricted occurrence of these horizons near the fringes of the Deccan Province is noteworthy.These sediments consist of sandstone, siltstone, shale, limestone and chert and contain numerous plant and animal fossils. The dinosaurianremains are restricted to the infra-trappeans whereas the inter-trappeans have first reliable record of Angiosperms in them. The fossils yieldMaastrichtian to Paleocene age.




15/19


16/19

A sinus lava channel at Gunjale, Ahamadnagar district, Maharashtra

Although the DVP does not host any conventional mineral deposits, the lava flows have been proved to be ideal sites for Buddhist and Jainmonuments that date from 2000 BC to 500 AD. The strategic importance of the hillocks was well understood and exploited by the Yadava aswell as Maratha kings to build more than 350 forts in different parts of their kingdoms. Besides, in a number of temple complexesconstructed in different parts of Maharashtra large blocks of basalt have been used as building stones. It is interesting to note that thecompound/pahoehoe flow-types were preferred by the sculptors and architects over the a`a flows. In recent past, the blocks of Deccan

basalts and associated rocks have also been used by the European rulers to build majestic buildings like Gateway of India, Victoria Terminusetc.

A panoramic view of Ajanta caves, carved out of a compound flow.




17/19

Kailash temple, Ellora Caves, a marvelous rock cut architecture in a compound flow.Now-a-days, the cavity minerals hosted by the lava flows have attracted the attention of mineral collectors and many of these minerals havebeen displayed in museums world over.

Apophylite from Rahuri, Ahamadnagar




18/19

Blue cavenzite and heulandite, Pune




19/19

Green apophylite and pink stilbite

Source: Central Region, GSI


Geological Formation

Documents

Transcript of Geological Formation