Chapter-2:- LITERATURE REVIEW

CHAPTER-II

LITERATURE REVIEW

2.1 Introduction

A detailed review presented in Chapter -II includes published and unpublished literature on general geology, stratigraphy, physical volcanology, geophysical aspects and engineering geology of DTB. The geological literature of DTB dates back to writings of British Officers from Geological Survey of India (GSI). In last fifty years, exponential growth in the literature is seen in the form of various memoirs published by Geological Society of India, Bengaluru, Gondwana Geological Society, Records of Geological Survey of India and many other academic institutes. Primary objective of present work is to understand engineering geology of basalt from parts of Western Maharashtra. The literature review presented in this chapter is subdivided into three parts. First part of chapter describes review on general geology including geomorphology, physical volcanology, petrology and stratigraphy of DTB. Second part includes geophysical aspects and third part includes engineering geology of DTB.

Earlier writings of GSI officers and Gazzatier of Govt of India working in the Bombay Province (pre-independence India) have given good insight on morphology, petrology of DTB. Foot (1876) asserted that flatness of the Deccan country is a consequence of flat lying lava flows and negated the marine planation theory. Oldham (1893) compiled information available till 1890 on Deccan country and proposed usage of name Sahyādri instead of Western Ghats. Vast literature on DTB has been published since 1980. Geological Society of India (Geol. Socy of India), Bengaluru, has published various memoirs such as proceedings on Deccan Volcanism (1981), Volcanism-Radhakrishna volume (1994), Deccan Volcanic Province (1999), and Sahyādri- the Great Escarpment of Indian Subcontinent (2001).

Agashe et al (1971), Karmarkar (1978) and Gupte et al (1980), have proposed alternate models of volcanicity, types of flows and various rock types of DTB. On the other hand Hawaii nomenclature scheme proposed by (Macdonald, 1953) and (Walker, 1971) is followed by Geological Survey of India and many others. After 2001, researchers such as Bondre et al, (2004a, b), Brown et al, (2011), Duraiswami

18 et al., (2003), (2004), (2009), (2014), Sheth et al , (2005), (2006), (2011), Sen et al (2011), Sen Bibhas (2012), (2017) have paid attention to physical volcanology of DTB to understand mode of emplacement. These researchers have also challenged earlier views of volcanological characters of `A`Ā and Pāhoehoe flows and promoted usage of rubbly pahoehoe instead of `A`Ā flows for most of Western DTB.

Second part of the chapter discusses reviews from geophysical studies. These studies have been included in the thesis to develop a correlation between site conditions and local geophysical signatures. Many shallow subsurface infrastructure projects studied for this thesis are located in the river basins of Mula-Mutha with thick alluvial cover. This makes the locations / sites vulnerable if ground motions are associated with earthquake. Role of subsurface particularly extent of weathering, velocity of P and S waves are crucial parameters in the consideration of a stable site conditions. To understand this, a review of literature encompassing, probability of seismicity and nature of seismicity in and around the region was taken e.g. Verma et al (2014). Alongwith this, literature review, to undersatnad the crustal structure beneath the DTB, Glennie, (1951), Qureshy (1981), Kaila et al(1968),(1981) were reviewed and for Koyana-Warna region, Guha et al (1981) , Talwani (1995; Talwani et al (1997), (2000) were also reviewed to understand the phenomena.,

Third part of chapter describes the contributions on engineering geology of DTB. Earlier understating of DTB as a monolith has led to the misconceptions about the engineering geological problems in DTB. Most of the literature till date is restricted to estimation of various parameters as per different IS codes in laboratory conditions. These works lack inclusion of understanding of volcanological character of basalt and its engineering properties. This need was however addressed by Gupte et al (1980) who helped Public Works Department (PWD) of Govt. of Maharashtra, India to publish a handbook on engineering geology of projects in the state of Maharashtra. This work has discussed various flow types of basalts and their lithological variation based on presence of vesicularity. Handbook also throws light on lithological varieties of basalts and their laboratory tests such as UCS in kg.cm2, porosity%, specific gravity etc. This happens to be the first attempt to put lithological and engineering aspect together. Among other workers , Parthasarathy et al (1981) provided rock mass characteristics data for DTB, Ghosh (1987) provided

engineering geological zoning of India including DTB. GSI has carried out engineering geological investigation in many projects in Maharashtra. But these reports were seldomely published in the literature. Indra Prakash et al (1993), Chakraborty et al., (1996), Gupta et al., (2011), Ansari et al., (2014), Jain et al (2014), Kainthola et al (2015), Singh et al (2017) have published literature in the recent times on engineering geology of DTB.

2.2 Geology of Deccan Trap Basalt

The Deccan Trap Basalt (DTB) occupies western and central part of India (Fig: 1.1). It is one of the remarkable Continental Flood Basalt (CFB) provinces in the world. The term ‗trap‘ was coined by Skyes in 1833 after a Swedish word- Trapp or Trappa meaning step like occurrence. These CFBs are also called as flood basalt because of the vast expanse or ―Plateau Basalts‖ as they often stand out as tableland. Eruption of DTB is also one of the most interesting events in the geological history as the lavas were erupted over the Cretaceous / Tertiary boundary (KTB), ~ 65 Ma as given by Cande et al (1995). This event is considered responsible for one of the largest mass extinctions in the geological time where about 40% of the genera extinguished (Sepkoski, 1996). The Western Ghats represent escarpment to the west, parallel to the west coast of India. Estimated extent of DTB prior to erosion, including their concealed extensions under Arabian sea may be of the order of 15, 00,000 km2 (Krishnan, 1963). Presently, it occupies more than 85% area of State of Maharashtra and parts of Gujarat, Madhya Pradesh, Karnataka, and Andhra Pradesh. At places along the western part of the province, a continuous vertical succession of basaltic flows with a thickness of more than 1, 600 m can be observed, and geophysical studies have indicated that the thickness attained by the lava pile is over 2,000 m in the western part of the province (Kaila et al., 1981).

Its maximum exposed vertical thickness is about 1700 m in the Igatpuri area (Beane et al., 1986). Near Mahabaleshwar, roughly 300 km farther south, about 1200 m of flows are exposed (e.g. Deshmukh (1988), Najafi et al (1981)); southwest of Mahabaleshwar some 40 km, on the coastal lowland near Khed (elevation + 39 msl) 500 m of basalt thickness is reported (Mahoney, 1988). To the north, in the Cambay region, drill holes have encountered more than 1000 m of largely buried basalts, in some places lying beneath 4-6 km of younger sediments (West,1981). Drilling carried out near at Rasati village near Koyana dam site confirmed the occurrence of 20

granitic basement at 350 m below msl (Roy et al., 2013). Thickness of the basalt decreases from west to east as well as in the north and south. It reaches maximum thickness along the Western Ghats.

Two different theories are proposed to explain the occurrence of this vast DTB. One is known as mantle plume hypothesis. This states that DTB formed around 65 Ma in response to the passage of the Indian plate over the Reunion hotspot. The eruption of the Deccan lavas is intimately related to the formation of the passive margin along the western coast of India, as the Seychelles micro-plate separated from the Indian plate. This view is endorsed by Beane et al., (1986), Cox et al , (1985). However, Senthil Kumar et al., (2007) have questioned apparent thermal trace of a Réunion plume or any other thermal anomaly responsible for the genesis of the Deccan basalts, in the present-day thermal regime.

Another theory suggested by Sheth et al (2005) contradicting the plume hypothesis, suggests rifting in three different zones in the DTB, leading to widespread and extensive sedimentation. The DTB is constituted dominantly of tholeiitic basaltic lava flows, nearly horizontal, stacked one above the other and exhibits a low dip of about 1° to the southeast (Courtillot et al., 1986). The basaltic flows are intruded at a number of locations by doleritic dykes.

2.3 Geomorphology of the Deccan Trap Basalt

In the early geomorphological studies of Deccan trap terrain, Foot (1876) suggested that flatness of scenery in the Upland Maharashtra is a consequence of structural factors such as flat lying lava and sedimentary succession beneath. Oldham (1893) inferred that the cliffs of Western Ghats are of marine origin based on parallelism to the sea cost as well as occurrence of Cremnoconchus - a genus of fresh water Mollusca in the hills of Western Ghats. The Imperial Gazetteer of India (1907) noted that greater wall of Western Ghats represents water divide of Peninsula and upheaval to the present altitudes is comparatively based on the gradients of westerly flowing river. According to Pascoe (1950) , differential erosion has played a vital role in the mountain formation as compared to earth movement as successive lava flows are practically undisturbed. He further noted that strike and dip of Archaean rocks which are concealed below the Deccan Trap have influence over the contours of Deccan

Trap country. King (1963) gave emphasis on deformation by tectonic movements which have elevated late – Cenozoic plateau leading to rejuvenation of river systems.

Radhakrishna, (1965) pointed out following important observations that Western Ghats are not a true mountain range but represent the precipitous edge of the elevated plateau which appears to be the combined effect of uplift and erosion. He added that easterly flowing rivers are graded right upto their heads indicating inheritance from a previous denudational cycle and the ghat edge coincide water divide between westerly and easterly flowing rivers. Important observation of Radhakrishna, (1965) is along the west coast, sediments older than Miocene are not recorded, hence land must have existed prior to this date and retreat of ghat is accelerated by westerly flowing rivers which are rapidly eroding headwords and capturing the well-established drainage pattern on the plateau.

Dikshit (1981) has reviewed Fault Escarpment, Erosional Escarpment, and dead cliff as three different hypotheses for formation of Western Ghats. A drop of elevation from 400 to 600 m in the escarpment face of most of the Western Ghats is the main argument behind hypothesis. But lack of evidence such as slickensides, structural lineaments as well as continuity of flows on either side of escarpment does not support the argument. In the Erosional escarpment argument, presence of east west ridges i.e. plateau offshoots in Konkan, separating large basins in the area with more than 500 m elevation msl is the main logic.

Dikshit (1981) reviewed a threefold relief of the Western Ghat as Relief of the plateau, East of Continental Divide which has a height around 800 m msl all over Western Maharashtra and as major sources of rivers such as Krishna, Bhima, Godavari and their tributaries. This relief is seen in the form of series of alternating valleys and plateaus. Valleys are relatively flat bottomed with width dependent upon their catchment. The plateau usually bounded by crenulated escarpments rise 100 to 300 m above the valley bottom. Further to east, this arrangement of alternating valley and plateau ceases to exist and giving rise to extensive plains with no divide. Second Relief of the Crest Zone, Western Ghats proper, which is 25 km wide longitudinal zone characterized by higher surfaces of planation, steep escarpment, locally with extreme dissection. Steep precipitous slope of Western Ghat, heavy rainfall and high humidity, promote intense chemical weathering and excessive erosion by fluvial processes result in the present relief. The third Relief belongs to

22 western face and projecting escarpment in which the escarpment face is divided into number of parallel steep slopes separated by near horizontal levels forming spurs projected westward in Konkan with slopes upto 20.

Radhakrishna (1993) proposed a scheme of events for DTB since Cretaceous to present as illustrated in Table 2-1.

Table 2-1:-Scheme of events for DTB as proposed by Radhakrishna (1993)

Time Event

Recent Reduction of old plateau to new plateau surface, carving present day landscape Migration and nick points and river capture, excavation of deep gorges, retreat Pleistocene of scarps, waterfalls Formation of Western Ghat scarp and retreat, Minor up warps, -rejuvenation of Pliocene rivers Miocene Stability and plantation, Lateritation of part of the surface Eocene Rifting of uplifted segment, creation of Western Ghat and its scarp facing sea, Deccan Volcanic episode, extensive trap cover over eroded sediments of an Cretaceous earlier surface.

Cox et al (1996) correlated uplift history and drainage pattern in DTB using laterite occurrence and concluded that low level laterites form Konkan region are developed during Mid Tertiary and represent a totally separate phase of laterites as compared to high level laterites developed upon newly developed lava plateau.

According to Kale et al (2007), morphological data demonstrates that Western Ghat escarpment is most likely receding at a modest rate. Kale et al (2008) carried out geomorphometric studies for thirty rivers in the Deccan trap area. In this study, he found that use of geomorphometric tools provide a very modest support to the widespread view that the western margin of India has undergone protracted uplift and tectonic deformation from Tertiary to recent times. If the DTB area has indeed undergone significant uplift till recent times, the effects of the tectonic activity have left only subtle imprints on the present-day landscape, which cannot be easily detected by the commonly used geomorphic indices of active tectonics, at least on this spatial scale. The ―Indian great escarpment‖ could be labelled as ―creeping divide‖ rather than ―leaping divide‖. He further emphasizes that the data on morphology and geochronology are inadequate to make a clear cut distinction

between escarpments created by mechanisms of parallel retreat and plateau down wearing.

This regional understating of escarpment evolution points out that slope decline, slope replacement, parallel retreat are the three theories which would together explain the evolution of slopes in Sahyādri. In this entire review, it is observed that, understanding of regional slope is very well established in the DTB but at higher resolution, local scales have not been generated so far. This has been attempted during the present studies (Chapter-VII).

2.4 Lithological Variation in Deccan Trap Basalt (DTB)

Flows in DTB show lithological variation within the flow and can be identified megascopically. These rock types show similar mineralogical constituents but differ in terms of presence or absence of as well as size, and shape of vesicles. Pioneering work in identifying the different rock types in Deccan trap is done by Gupte et al (1980) as follows:

a) Compact Aphinitic Basalt (CAB): These fine grained rocks do not contain the visible crystals of constituent minerals, hence known as Aphinitic. These rocks show typical inequigranular porphyritic, ophitic/ subophitic and glomeroporphyritic texture under the microscope. These rocks are devoid of gas vesicles. They show dark purple to, dark grey to dark black colour in hand specimens. b) Giant Porphyritic Basalt (GPB): This rock type can be considered as a variety of compact aphinitic basalt. They show wide variation in the grain size of constituent minerals. Most of the grains are plagioclase feldspar. However, olivine variety is also reported. The porphyry –larger grain of size from few mm to around 10 cm appeared to float in a glassy or fine grained groundmass. c) Amygdaloidal Basalt (AB): This variety of basalt contains various sizes and shapes of gas vesicles filled or unfilled with zeolites. These rocks show different varieties of gas cavities entrapped. The size range is from few mm to few cm. If these cavities are unfilled then, it is known as vesicular basalt. d) Volcanic Breccia (VB): These rocks are formed as agglomerated of angular fragments caught up in a lava matrix. The grains have variable composition, size and shape.

e) Tachylitic basalt: These rocks occur at an interface of two basalt flow having either red/green/brown colour. For these rocks many authors have used different terminologies such as tuff or ash beds (Kshirsagar, 1979). Commonly these rocks are referred as boles or bole beds.

Occurrence of these different varieties of basalt are a function of cooling history , viscosity, vesicle zonation, available pre-eruption topography of the lava flow is explained by many workers e.g. Macdonald (1953), Agashe et al ( 1971), Walker (1971), Gupte et al (1980), Aubele et al(1988), Bondre et al (2000), Duraiswami et al., (2009), (2014), Lockwood et al (2010) etc.

2.5 Classification of Different Flows based on volcanological characters Basalt flows are classified in the field based on their physical volcanological properties. Different approaches have been proposed for description of these flows in the field. Two main approaches are existing; one belongs to Gupte et al (1980) and other belongs to Hawaii terminology advocated by Macdonald (1953), Walker (1971) and GSI etc. These are described in detail in the following section.

2.5.1 Flow Morphology as per Nomenclature System as Developed by Gupte

Gupte et al (1980) described the field characters of DVP basalts based on form, dimension of the flow as well as number size and shape of gas vesicles.

(a) Compact basalt flows: These are thick and extensive flows with more or less tabular form. The top and bottom are nearly parallel, planar surface, the considerable lateral extent ~ 20 km. These flows show vertical to sub vertical cooling joints. (b) Amygdaloidal basalt Flows (A.B.): These flows show some variations in the form, generally smaller in size and irregular from. Some of Amygdaloidal basalt flows are upto 1 km in lateral extent and 10 m in thickness. These are marked by pipe vesicles at the margins of the flow. These flows are free of joints when fresh in condition. These flows are commonly known as Thin/Thick Irregular Amygdaloidal Basalt (TIAB). (c) Tachylitic basalt (T.B.): These are thin, irregular flows of basalts. These are occurring in red /black or green in colour. Usually occur in between two flows of basalt with a varying thickness of about 50 cm to 100 cm.

(d) Volcanic breccia (V.B.): Caught up fragments in a lava matrix or held due to zeolitization. Lava matrix is often tachylitic, or hydrothermally altered. Most of these occur as individual flow. Gupte et al (1980) further observed that, volcanic breccia occur to be intrusive at places.

2.5.2 Flow Morphology as per Hawaiiˋ Nomenclature System

Other approach is based on the understanding of the basaltic lava systems studied in the Hawaiiˋ Island. Basaltic lava flows have been traditionally classified according to their surface morphology, initial viscosity and style of emplacement /eruption into two categories such as Pāhoehoe and `A`Ā (Macdonald, 1953). The pāhoehoe flow is characterized by presence of a smooth surface, at times in form of coiling of ropes, whereas `A`Ā has a spinose autobreccia surface.

Pāhoehoe lava flow: C. E. Dutton introduced the Hawaiian terms Pāhoehoe and À`Ā into the scientific literature in 1884. Macdonald (1953) reviewed the literature and gave a scientific definition of Pāhoehoe, À`Ā and block lava. According to him, pāhoehoe flow is characterized by presence of a smooth surface, at times in form of coiling of ropes. MacDonald further emphasized that, along with surface features, the internal structure of the lava flow plays important role. The internal structure of the lava flow was better understood using the concept of flow unit (a term coined by Nichols in 1936). Walker (1971) used a term ―compound pāhoehoe‖ and ―simple À`Ā. According to him, many Deccan Trap flows are essentially compound pāhoehoe as they are divisible into different flow units and individual flow-units vary from 5 cm (Figure 2:1(a)) to more than 10 m thick (Figure 2:1(b)) (though mostly in the range 50 cm to 5 m) and as seen in cross-section (Figure 2:1) vary from bun-like to tabular bodies. These buns have concentration of vesicles in the uppermost third of a unit - coalescence of vesicles may produce man-sized vesicles or lava blisters there - and a zone of pipe vesicles along the base. In the upper chilled salvage, contraction cracks are much closer.

Aubele et al(1988) based on vesicle zonation subdivided these flow in three distinct zones i.e. Upper vesicular zone, which generally extends down to and below a depth equal to one-half the flow thickness; non-vesicular or dense central zone which generally contains few or no vesicles in flows > 2 m thick and only a few vesicles in thinner flows, is medium-grained to granular and may be flow layered; and A lower

26 vesicular zone, which is characteristically about 30-40 cm thick, regardless of total flow thickness.

Figure 2:1:-Sections across Part of a Compound Pahoehoe Basalt Lava Flow (Walker, 1971)

(a)Section across part of a compound pahoehoe basalt lava flow at the Mula Dam in the Central DTB, showing the distribution of vesicles and pipe vesicles (b) Section across part of a compound pahoehoe basalt lava flow, Near Ellora, Central DTB)

Wilmoth et al (1993) classified pāhoehoe lobes on the basis of pipe vesicles as P and S type. S-type lobes lack pipe vesicles and are vesicular throughout their vertical extent. P-type lobes are characterized by pipe vesicles and display a typical internal structure with a vesicular base and top, and a relatively vesicle poor core. Keszthelyi (1995) measured temperature at the bottom of a pāhoehoe flow and observed that as flow advances slowly, the insulated crust would form to enable the transport of thermally efficient system for lava flow.

Self et al.(1998) proposed a four stage growth model for propagation and subsequent development of Pāhoehoe lava flow. This includes stage one taking place at the time of eruption with slow moving lobes, followed by stage two in which inflation i.e. lifting of upper crust would take place. Stage three develops due to continued injection of the lava leading to more inflation and depressurization from the formation of new breakouts leads to pulse of vesiculation within liquid lava. Pipes are developed during this stage and in the final stage, after stagnation, the remaining primary bubbles rise to the top of the flow in a few days to weeks. Slow cooling of stationary core forms more regular joints. Cooling is enhanced around clefts. Self et al (1998) further subdivide this pāhoehoe lobe into three sections (Figure 2:2) based on the vesicle structures, jointing, and crystal texture. The upper crust makes 40-

60% of the lobe and the lower crust is 20-100 cm thick, irrespective of the total lobe thickness.

Upper Crust is vesicular, often with horizontal vesicular bands (VZs) that form during active inflation. Bubble size increases with depth. Sometimes mega-vesicles (MVs) are also found. Prismatic or irregular jointing are seen, sometimes equivalent to entablatures in thick lava flows. Petrographic texture ranges from hypohyaline to hypocrystalline (90-10% glass). Core consists of very few vesicles. Porosity is dominated by diktytaxitic voids. Vesicles are mostly in silicic residuum, which forms vesicle cylinders (VCs) and Vesicle Sheets (VSs), Holocrystalline (<10% glass). Lower Crust consists of basal vesicular zone (BVZ), which is nearly as vesicular as the upper crust; pipe vesicles (PVs) are found; few joints and 50-100% glass (Self et al., 1998).

Using this description and Figure 2:2, Pāhoehoe lobes/flows were studied for different porjects and locations to carry out work reported in the present thesis.

Figure 2:2:-Idealized cross section through an inflated Pāhoehoe lobe(Self et al, 1998)

Transition of Pāhoehoe to `A`Ā Flow: Hulme (1974) observed that lava flows are non-Newtonians liquids or a Bingham fluid and yield stress governs the dimensional aspects of the flow. This experiment provided a physical basis to suggest that there

28 exists an inverse relation between viscosity and shear rate. Further, Peterson et al, (1980) provided a model for the transition from pāhoehoe to À`Ā as seen in Figure 2:3. During the transport of the flow, stiff discrete, sticky masses are formed where shear rate is highest. These clots are fragmented and rolled as discrete glassy masses which are characterized by spinose, granulated surfaces; as flow movement continues, the masses and fragments aggregate, fracture, and grind together, completing the transition to À`Ā. In recent times, many authors (Duraiswami, 2009a; Brown et al., 2011; Sheth et al., 2011, 2017; Sen Bibhas, 2012; Duraiswami et al., 2014; Bibhas, 2017) have identified various localities from DTB where a transition between Pāhoehoe and À`Ā flow has occurred. This has given rise to variety of intermediate types of flows such as sheet flow or hummocky flow or rubbly Pāhoehoe flows. According to these authors, this transition has occurred due to complexities in paleosurfaces, strain rate etc.

Figure 2:3:Pāhoehoe–`A`Ā transition (Lockwood et al, 2010)

`A`Ā lava flow: According to Macdonald (1953), `A`Ā flow is characterized by a rough jagged or spinose surface. Most of the surface is covered with loose fragmental material known as clinker or flow breccia. Thickness of the clinker layer varies from few inches to several feet. Each of these clinker fragments are rough, irregular or spiny. This clinker crust is also known as fragmentary top breccia.

Important aspect of `A`Ā is that most of the these flows are result of transition from pāhoehoe flow due to various reasons (Figure 2:4). Noteworthy work about `A`Ā flow from various CFB provinces are summarized as below.

Solidified À`Ā flows can be recognized in the field by basal and flow-top breccia and massive cores that are commonly texturally uniform, aphinitic and contain sparse, highly-deformed vesicles (Macdonald 1953). According to Rowland et al (1990), these morphologies reflect fundamentally different emplacement conditions. It is suggested that À`Ā flow fields usually develop under high effusion rate conditions (<5–10 m3/s, based on observations on Hawai‘i,). Cashman et al (1999), (2005) noted that rapid groundmass crystallization is critical in the formation of À`Ā lava. Hon et al., (2003) suggested that transition of pāhoehoe to À`Ā occurs due to high strain rate because of change in slope. Glaze et al., (2014) experimentally proved for various eruptions that many lava flows experience multiple slope breaks of various magnitudes that can combine to disrupt the surface.

Figure 2:4:-:(a) shows moving front of AA lava flow and (b) showing the main components of a `A`Ā lava flow in a vertical section (Lockwood et al, 2010)

Flow Morphology in Deccan Trap Basalt (DTB) using Hawaiiˋ Nomenclature Deccan Trap Basalt has been a topic of research since last hundred years which has resulted in significant data on physical volcanology. Even though, the understating of DTB started in the early 19th Century, the physical aspects started gaining importance since the works of Agashe et al (1971) who emphasized the need to look into field aspects of lava flow. Phadke et al (1971) tried to develop a relationship of flow morphology and relevant slopes around various localities in Pune. Meanwhile Walker (1971) emphasized that the flow exposed around Pune and adjoining region

are essentially compound pāhoehoe (consisting of two or more flow units) and `A`Ā simple flow (generally do not contain flow units). GSI and other agencies contributed significantly in understanding of physical volcanology of DTB.

2.6 Contribution of Geological Survey of India (GSI)

GSI has carried out a systematic geological mapping of most of the DTB area at a scale of 1:50,000 using Survey of India (SOI) toposheets as a base. Most of the reports published since 1971, have started using the terminology proposed by Walker (1971).Various unpublished reports are available on their website www.portal.gsi.gov.in. Many outcrop scale features are not reported as the mapping is carried out at 1:50,000 scale in these reports. Many of these reports contain geological map of the concerned area showing group of flows. Significant contributions of GSI officers for mapping of DTB, based on occurrence of compound pāhoehoe, simple `A`Ā flow has eventually given rise to lithostratigraphic classification of DTB proposed by Godbole et al (1996). Relevant GSI reports are used in the present work and are referred at appropriate place.

2.7 Contribution by others

The importance of physical volcanology was renewed after Bondre et al (2000) , who investigated flows around Sangamner area and compared them with the Hawaiˋan and Columbia River Basalt lava flow. These flows were investigated to quantify various morphological features such as flow lobes, crust of the flow, vesicle cylinders etc. Duraiswami et al. (2001) identified few more features such as tumuli. Tumuli‘s are features generally associated with hummocky pāhoehoe flow; however, they are also associated with sheet lobes of the flow. These are characterized by lava inflation clefts with squeeze ups. Few of these clefts are associated with single flow lobe, whereas others display multiple flow lobes. Some tumuli appear to have developed along anastomosing tube systems. Duraiswami et al., (2003) recorded a transition from Pāhoehoe -`A`Ā flow at Bodshil village, Near Lonavala which was identified as a slabby pāhoehoe. Evolution of this flow is shown in Figure 2:5.

Bondre et al., (2004a) noted that formations of Kalsubai subgroup and Bushe formation of Lonavala subgroup consists of thick compound pāhoehoe flows which are mostly P-type flows with the thickness of the upper crust ranges from about a

31 third to half the thickness of the lobe, and it commonly displays vesicle layering. Well-developed vertical jointing is rarely observed in crusts of these lobes. The interface between the crust and the core in thick lobes is usually sharp, marked by a change in the style of jointing and the distribution of vesicles. The basal crust generally does not vary greatly in thickness irrespective of the thickness of the lobe and always shows an inverted ‗Y‘ geometry filled by cryptocrystalline silica. These features are indicative of multiple injections of lava and suggest endogenous growth.

Figure 2:5:Stages in the evolution of the Bodshil Flow(Duraiswami et al., 2003)

They further noted that Simple `A`Ā flows belong to Khandala Formation of the Lonavala Subgroup and all formations of the Wai Subgroup (i.e. Chemostratigraphic classification). Some flows do display preserved crusts, while many are characterized by reddened (oxidized), rubbly tops or flow-top breccia. This usually consists of irregular, highly vesicular/amygdaloidal fragments in a matrix of fine, glassy material upto a thickness 5.00 m. These flows generally exhibit well- developed and persistent jointing patterns. These are referred to as colonnade and entablature. Many times, columns fan out in a radial manner, encircling breccia. Most flows display a sharp, slightly undulating, glassy contact with the top of the underlying flow. This zone has a variable thickness and often shows close spaced

32 jointing and bluish-black sheen. It is rarely red as in the pāhoehoe flows. This is particularly well observed in Dive Ghat.

Jay Anne (2005) attempted to develop an integrated model for SW part of DTB. The volcanic architecture of lobes and flow-fields is comparable in thickness of the lava flows within the base of the Mahabaleshwar Formation. According to Sheth (2006), internal architecture of Kilauea flows and DTB is same but size and areal extent of DTB is much greater. Many compound flows of the Deccan Traps were emplaced in a gentle, effusive, Kilauea-like fashion. Duraiswami et al., (2008) observed that DTB flows do not indicate the true `A`Ā characteristics. These flows, termed as rubbly pāhoehoe show patchy flow-top breccias.

Duraiswami, (2009a) identified a flow from Morgaon (~ 80 km SE of Pune) belonging to terminal part of Bushe Formation which was transported through a complex network of interconnected lobes. Brown et al., (2011) reported occurrence of `A`Ā lava from SW of Sangamner area (Pune- Nashik Highway) from Thakurwadi Formation i.e. older part of DTB. Sheth et al., (2011) proposed the recycling of flow top breccia in the molten interior of lava flow, exposed in a quarry near Koynanagar (S of Pune) challenging the previous hypothesis of volcanic vents of Gupte et al (1980). Duraiswami et al., (2014) identified Bushe Formation with two compound flows having vertically alternating sequence of hummocky and sheet pāhoehoe, large inflation clefts, and squeeze-ups. `A`Ā flow in the Uruli Ghat section consists of a massive but jointed core portion between thick flow-top and flow bottom breccias. The author would like to remark that such isolated occurrences cannot be taken for developing model of eruption of the entire Deccan Traps.

Fine grained interflow horizons, which are showing red / green / brown or black colour, are generally present in between two successive flows of basalt. Oldham (op. cited Krishnan, 1963) developed a threefold stratigraphy of DTB based on occurrence of these horizons. These horizons were earlier thought to be essentially of sedimentary origin, indicating a period of quiescence between lava flows. These zones were regarded as red bole or bole bed, baked soils etc. Gupte et al (1980) contradicted earlier ideas such as bole bed, baked soil etc. and advocated the term tachylitic basalt with red / green or black variety (RTB/GTB/BTB). He termed these as chilled basaltic glass i.e. tachylite with a composition of basalt but lower degree of

33 crystallization and develop close jointing while occupying interflow space with a varying thickness of about 50 cm or even greater. He treated these as showing intrusive relation with the flows. Main evidence cited was the quick succession of lava flows leaving less time for accumulation of lakes or any fresh water bodies to accumulate sediments. Colour variation from red to green is due to hydrothermal alteration of earlier formed tachylite. These tachylitic basalts (TB) are showing discontinuous irregular, pinching patches, and lenses with sometimes as vertical or steeply dipping veins.

Kshirsagar (1982) took a different view and proved that these are tuff or volcanic ash layers. He further observed that contact of these zones is gradational with lower flow and sharp undulatory with upper flow. Near the top of a lava flow, amygdularity has increased and top of flow has become highly zeolitised basalt. In this zone, the red material makes its appearance, lining the basalt fragments. Basalt fragments are dark brownish red in colour and extremely fine grained with high % of glass (~ 60%). The amount of red material goes on increasing and a distinct red horizon is observed. Such material is however restricted to the upper part of the flow and very rarely discordant veins are observed. These zones show even upto five sets of joints which are described as follows:

(a) Parallel to upper and lower surface, mostly horizontal, predominant set causing splitting along them, (b) Perpendicular to surface, vertical or highly inclined, (c) Curviplanar joints splitting along these give rise to ovoidal or semi spherical fragments, (d) Polygonal pencil joints, (e) Regional joints associated with basalt flows.

However in the recent times, interflow horizons are studied from paleoweathering and palaeoenvironmental perspective which are described as follows. Srivastava et al (2012) carried out mineral magnetic studies and revealed detrital processes involved in the deposition of interflow horizons within DTB. Sayyed et al (2013) stated that red boles have retained original weathering characteristics. Sayyed et al (2014) argued that interflow horizons are paloesols and can prove to be the important tool to develop the palaeoenvironmental reconstructions.

2.8 Summary of various flows in DTB

Table: 2.2 highlights types of basalt flow reported by various authors till date from DTB. This table includes typical characters of these flows observed in the field.

Table 2-2:-Various Types of Flows in DTB Area, Modified from Sen (2017)

Does not Should Lava type Should Have Generally has Examples from DTB show not have

Smooth, billowed top and base Lobes are inflated with pipe amygdales, few cm- (tumuli), core is Dive ghat (Ghodake et al Pāhoehoe Stretched and Fragmented thick glassy selvage; vesicular aphinitic, segregation 1973), Pune(Walker, angular, mega spinose (classical) upper crust (upto 60% features such as 1971; Bondre et al., 2000, vesicles material Flow Lobes vesicles); and lower vesicular vesicle cylinders / 2004b), crust, (10%-50% vesicles) blabes

Flow top is showing Slabby Thin Fragmented (Duraiswami et al., 2003, autobrecciation with broken Pāhoehoe bottom 2014) fragments

Fragmented top, and bottom Pune-Mumbai Express having a lobe of Pāhoehoe in Aphinitic interior; Highway(Duraiswami et Rubbly Fragmented between, crust is vesicular, rounded vesicles; al., 2014), Elephanta Pāhoehoe bottom coherent vesicular crust below smooth base caves(Sheth et al., 2017), breccia; lower vesicular crust Manmad(Bibhas, 2017)

Sinhagad ghat, Dive ghat, Rounded Top, and bottom with Core may show clasts Ropy tops, Sangamner vesicle, subangular fragments, entrained in it, pipe (Krishnamurthy et al , `A`Ā tumuli, aphinitic core, vesicles are aphinitic with amygdales 1971; Ghodake et al segregation angular curviplanar jointing at base 1973; Kshirsagar features 1982;Brown et al., 2011),

2.9 Stratigraphy of Deccan Trap Basalt (DTB)

The DTB have erupted through the Precambrian crust of Dharwar, Bastar, Aravalli, Bundelkhand cratons as well as Saputara mobile belt. At few places such as Lameta, Bagh, Dhrangdhar and Wadhavan, DTBs are underlain by Cretaceous sediments, called as Infra-trappeans. At many places these lava piles show interflow horizons with variable thickness which are either ash or tuff beds. These flows are thinnest in

35 the east. Their thickness increases towards west with a total of 2000 meter. This variation is attributed to pre- Deccan topography.

Krishnan (1963) has classified DTB into three stratigraphic groups: the Upper, Middle and Lower traps. This division was based on the distribution and relative proportions of intertrappean sedimentary beds and ash layers in relation to their elevation. The Lower Traps comprised the eastern part of the province, the Middle Traps occupying central Deccan and Malwa Plateau, whereas Upper Traps consisting of the western areas around Mumbai and Saurashtra. However, works of Athavale, (1970) based on paleomagnetic investigation carried out at Amarkantak (Madhya Pradesh), posed doubts on such simplistic stratigraphic division of the Deccan basalts and argued that the younging of basaltic sequences in DTB are from west to east rather than east to west as already believed.

In Nineteen nineties, stratigraphic classification scheme consisting of subgroup and formations was proposed by different agencies. Chemostratigraphic scheme was proposed by various authors (Section 2.7.3) and subsequently, Geological Survey of India (GSI) proposed lithostratigraphic scheme in 1996 (Godbole et al (1996). Various formations have been delineated using petrography, gross chemical and isotopic composition. Formation boundaries defined by recognizable breaks, presence of marker horizons, large chemical shifts over a wide area. The formations are clubbed together based on types of flows such as simple or compound flow, marker horizons such as presence of GPB and geochemical variation. These studies have confirmed that flows are older in west and towards east they become young. Different schemes of classifications proposed for DTB are narrated here.

2.9.1 Lithostratigraphy

Choubey (1973), Godbole et al (1996) have used criteria such as the occurrence of giant plagioclase basalts (GPBs), general hand specimen petrography, degree and appearance of weathering, `flow' thickness , presence and extent of red /green/ brown boles, jointing and the occurrence of fossiliferous beds. In the present thesis, lithostratigraphic scheme of Godbole et al (1996) is followed and hence discussed in detail which is mainly developed for the Western part of Main Deccan Plateau (Figure 2:6), (Table 2-3). It is based upon the occurrence of GPBs and compound and simple flows as defined by Walker (1971). Godbole et al (1996) further noted

many complexities are arising due to erosion, regional dip, topography and the lateral pinching out of flows to establish a volcanological stratigraphy. They gave description of subgroups and formations as follows:

Kalsubai subgroup comprises older formations i.e. Salher and Ratangad Formations, essentially consisting of compound lava flows. It is overlain by Lonavala Subgroup having simple flows of Indrayani Formations and compound flows of Karla Formation. Dive Ghat subgroup overlying Lonavala subgroup, consists of simple flows with minor occurrence of compound flows from Dive Ghat and Purandargarh Formations. Mahabaleshwar subgroup is youngest and consists of simple flows of Mahabaleshwar Formation.

1) Salher Formation: It consists of simple pāhoehoe flow of phyric to feldspar phyric type in northwest part of DTB. 2) Ratangad Formation: It consists of compound pāhoehoe flows of phyric to feldspar phyric type and are separated by a giant porphyritic basalt (GPB) also known as Megacryst horizon. This formation is divided into Lower and Upper and is exposed in north and northwest. 3) Indrayani Formation: Separated by Upper Ratangad by a GPB horizon. It consists of simple flows of columnar jointed and aphyric type exposed in the central part of the province. 4) Karla Formation: It consists of essentially compound pāhoehoe flows exposed in the central part of the province. Lower part of this formation is exposed in Elephanta Caves and Alibaug area, hence sometimes described as Elephanta Formation in the area. 5) Dive Ghat Formation: It consists of aphyric À`Ā flows with a rare pāhoehoe marker horizon. This formation demarcates transition from Pāhoehoe to À`Ā flow in DTB. 6) Purandargad Formation: It consists of simple À`Ā flows. 7) Mahabaleshwar Formation: It consists of simple À`Ā flows and separated by lower formation by a GPB horizon.

Figure 2:6:-Map showing Lithostratigraphic Succession of Deccan Trap Supergroup(Godbole et al, 1996)

According to this scheme of classification, the older flows comprising of pāhoehoe flows are exposed in the north, northwest and to some extent in central part of the province. The younger formations are comprised of `A`Ā and simple flows in the east and south of the province. This indicates that, the flows became young to south and southeast. Similar distribution of types of flows was given by Gupte(1980), however, he did not attempt any classical stratigraphy.

2.9.2 Magnetostratigraphy

Sahasrabudhe (1965), McElhinny, (1968) both concluded that there was a twofold magnetic stratigraphy consisting of lower section with reversed polarity (Chron 29R) and an upper section with normal polarity (29N). This reversal was observed to occur at approximately -600 m. Wensink et al (1971) confirmed that, elevation of the paleomagnetic reversal horizon varied between Pune and Amboli (SW of MDP). Courtillot et al., (1986) deduced that the reverse chron which contains the majority of the DTB lavas was 29R (833,000 yrs. long), with 30R is only 90,000 years long and 31 R is too old to fit with the geochronology. Thus the sequence of chrons in the DTB becomes 30N-29R-29N i.e. Late Maastrichtian to Early Danian (66 m.a.±1 m. a.), and the duration of the main pulse of basaltic volcanism. i. e., the Jawhar to lower Mahabaleshwar Formations probably did not exceed 1 Ma. The entire lava pile including the Kalsubai, Lonavala subgroup and two formations of Wai subgroup (Table: 2.3) forming lower part of Wai i.e. Ambenali and Poladpur Formations is reverse magnetized. This means that nearly 95 % of ~3000 m thick Deccan lavas are reversely magnetized.

2.9.3 Chemostratigraphy

Much of the stratigraphic work carried out on the DTB has evolved using extensive geochemical studies in order to form a robust geochemical stratigraphy proposed by Cox and Hawkeshworth, (1985), Beane et al., (1986), Devey et al (1986), Bodas et al (1988). The DTB geochemical stratigraphy of the Deccan Group, (i.e. the entire volcanic succession) is divided into three sub-groups (Table 2-3) and these subgroups into subsequent formations. Formations are defined as the `smallest mappable or traceable unit' and the formation must have internal lithological consistency and mappability. Generally, in sedimentary successions a change in lithological characteristics would define a formation boundary. However, in the DTB, this definition is not appropriate as it is predominantly tholeiitic lava units, and therefore basalt petrography, mineralogy and, most importantly, the geochemical characteristics are used to define formations. The DTB volcanic pile is divided into formations based upon geochemical criteria, most reliably 87Sr/86Sr and these flows have been shown to be traceable around the Mahabaleshwar Plateau area where they were originally defined, and also to the far south and east of the Western Ghats. Kalsubai subgroup is the oldest followed by Lonavala and Wai being the youngest of

39 the subgroups defined in the basis of geochemical parameters. Following is a brief account of different subgroups defined in the Deccan Basalt Group is as follows:

1)Kalsubai Subgroup: Consists of five formations, represented by compound pāhoehoe flows. Picrite (>18% MgO) and picritic basalt (>10% MgO) containing olivine and or clinopyroxenes phenocrysts are relatively common. GPB‘s with plagioclase as phenocrysts 2 to 5 cm long separate each of five formations of Kalsubai subgroup. This subgroup constitutes a total thickness of about 2000 m. 2)Lonavala Subgroup: It consists of 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. Upper Bushe formation consists of coarse grained, aphyric, amygdaloidal compound flows having a narrow chemical range. Lonavala subgroup is characterized by high Ba /Ti and Ba /Sr values and higher MgO for given TiO2 values. 3)Wai Subgroup: This forms the upper part of stratigraphic succession with five formations consisting mainly of simple flows having well developed flow tops. Junction between Lonavala subgroup and Wai subgroup is clearly identified in the field. It is also reflected in sharp elemental and isotopic change. Flow compositions of Wai subgroup are more evolved than older flows, but some rare picritic flows are locally encountered.

Devey et al (1986) carried out geochemical mapping for correlation of various flows and have developed cross sections in the NS and EW alignment. Figure 2:7(a) shows a cross section along EW alignment while Figure 2:7(b) shows along NS alignment.

Figure 2:7:-Geological Cross Section along N-S and E-W alignment of DTB (Devey et al, 1986)

2.9.4 Comparison between different stratigraphic schemes

Lithostratigraphic Scheme of Classification proposed by Godbole et al (1996) and Chemostratigraphy scheme of Cox and Hawkeshworth, (1985), Beane et al., (1986), Devey et al (1986), Bodas et al (1988) have achieved a broad correspondence with each other (Table 2-3). The type area for lithostratigraphy and chemostratigraphy is Western Maharashtra. In the present of thesis, lithostratigraphy scheme by Godbole et al (1996) is used.

Table 2-3:-Compiled Lithostratigraphy and Chemostratigraphy of Deccan Basalt as given by Courtillot et al., (1986), (Godbole, 1996), Cox and Hawkeshworth, (1985), Beane et al, (1986)

Geophysical Aspects of Deccan Trap Basalt (DTB)

Objective of geophysical studies is to provide necessary understanding for seismic microzonation for shallow subsurface investigations. Seismic microzonation (Finn et al., 2004) is mapping of seismic hazards at local scales to incorporate the effects of local site conditions. To understand site conditions from study area, it is necessary to review DTB region as a whole. Following part is devoted to understanding of regional subsurface image of DTB. This is followed by review on seismic studies carried out in various basaltic provinces of world and various formations of DTB. Seismic microzonation principles and their applications to Pune city are also reviewed. The outcome of this review is seismic microzonation carried out for specific projects (Table:-1.1) at higher resolution i.e. 1:1000 to 1:5000.

Geophysical studies of DTB gained importance after a fateful Koyana earthquake encountered in the region in 1967 and Latur-Killari earthquake of 1993. Hari Narain et al (1968) based on offshore drilling and geophysical surveys concluded that volcanic piles in the western offshore are either subsided or down faulted to the west in the Arabian Sea. Bose, (1972) quoted that the thickness of the Deccan peninsular crust consists of a granite layer of 22.5 km followed by a basaltic layer of 18.5 km and Moho at a depth of 41 km. Kaila et al. (1981) carried out deep-seismic-sounding (refraction) surveys, running W-E near the coast, midway across the Deccan at the latitude of Mahabaleshwar, indicating gradual decrease in thickness of DTB from coast to east. Two principle controls i.e. pre-trap topography and N-S trending Western Ghat escarpment have been inferred by Kaila et al. (1981) . Rao et al , (1984) have inferred that earthquakes at Koyana have altered common belief that Peninsula is aseismic but seismic activity is still continuing on a moderate scale. He further infers that, Frequency – Magnitude relations are indicating moderately active zones and this region has a low intracontinental seismicity, while the lineaments and rift valleys are zones of moderate seismicity.

Seismic studies and drilling for oil along the western Indian continental shelf have shown a block-faulted and rifted crust with some N–S-trending basement ridges (Biswas, 1988). Mahadevan et al (1994) have provided an insight on division of DTB in major crustal provinces such as Belt of Active Rift (BARS), Saurashtra Continental Block, Narmada-Tapti-Son (SONATA), SE platform block and NE platform block. He concluded that, SONATA and BARS are still in the process of tectonic adjustment. Sheth, (2005) developed a model based on geological and geophysical data to propose rifting mechanism for DTB as shown in Figure 2:8.

Since 1967, Koyana region has suffered many earthquakes which originated at about a depth of about 5 to 35 km from surface, with 3.5 to 6.0 as dominant magnitude on Richter scale. Koynanagar region (17.00° N, 73.40° E) has suffered a series of earthquake with >5M from Sept. 1967. On 10 Dec. 1967, an earthquake with 6.5M with source located at a depth of 27 km was felt, killing 200 people and destruction of many houses in the region. Talwani (1997), (2000) , Talwani et al (2010) have inferred Koyana Warna Fault (KWF) zone has alignment N10°-N190° and provided a compressive view of nature and possible causes of seismicity taking into considerations of variation in pore pressure.

Figure 2:8:-Map showing the Deccan Volcanic Province, Precambrian cratons, and structural trends in the Indian shield region. (Sheth, 2005)

(Dots with numbers are age of Volcanics. Breakup of Seychelles from India is shown in the inset )

Another zone with continuous seismicity is located along Latur-Killari region (18.09° N/76.47° E).This region has suffered seismic shock on Sept, 29, 1993, with source located at a depth of 5 km and 6.1 M. This region has suffered huge loss of property and more than 8000 people lost their life. It is considered as one of the deadliest intraplate earthquake known. Apparent reasons for losses in these two regions are poor quality of construction, soil liquefaction (Talwani et al, 1995) and lack of hazard preparedness. The seismic history of the region prompted author to look into various aspects of microzonation.

2.10.1 Engineering Geology of Deccan Basalts using Seismic Studies

Apart from number of laboratory tests, trend is evolved in last couple of decades to evaluate the engineering property of rocks using petrophysical tests such as ultrasonic testing, resistivity tests etc. in the laboratory and in the insitu condition so as to develop the correlation amongst them. It is observed that, mineral assemblage, texture and weathering index, are the governing factors in case of igneous rocks such as basalts (Barton, 1986, 2008). Basalts are low porosity, low water absorption rocks. These rocks even though comes under different lithologies, as long as they are fresh in condition, they are impervious, and show varying compressive strengths.

Many authors, have attempted a correlation between, various engineering properties such as porosity and UCS, porosity and Vp or Vs, bulk and Young‘s moduli etc. (Barton, 2008). Christensen (1970) calculated different velocities for basalt under different pressure conditions from 03.2 kb to 10.0 kb. Christensen et al (1980) cited velocity of shear wave from 2500 to 2900 m/s for volcanic breccia, vesicular basalt from 1800 to 2200m /s and for flow basalt 2400 to 3400 m/s for various samples tested from Deep Sea Drilling Project Leg 59.

Christensen et al (2003) quoted that from granite to gabbro or from rhyolite to basalt, quartz content decreases, feldspar becomes more calcic, and relatively fast minerals such as amphibole and pyroxene become increasingly abundant leading to rise in velocity. He could establish that the volcanic rocks have lower velocities and densities of chemically equivalent plutonic rocks and for rhyolite and basalts, a range of velocity is observed, which is attributed to amount of alteration amongst these rocks. Brocher (2005) provided correlation of Vp , Vs and density using regression analysis e.g. for Vp between 1.5 and 4.11 km/s, Vs can be established by using Brocher‘s regression fit i.e. Vs (km/s) = 0.7858 – 1.2344Vp + 0.7949Vp2 – 0.1238Vp3 + 0.0064Vp4 , and for Vp greater than 4.11 km/s, Vs (km/s) = 2.88 + 0.52 (Vp-5.25). Christensen et al (2008) for DSDP project leg 27, fresh basalts show

VP=6500 m/s and Vs= 3500 m/s, as the depth has increased, weathering effects have become less pronounced a rise in seismic velocities is observed.

Rame Gowda et al (1997) for the Koyna Hydroelectric Project Stage IV water inlet tunnels (SW of DTB), seismic refraction and reflection studies were carried out to decide the alignment. Study revealed that the top layer found on the banks is silty

44 sandy whereas a saturated sand and gravel bed in the river portion whereas rock on the eastern side is at deep i.e. between 646 to 602 m and in the western side, it is shallow i.e. between 650 m and 612.

Prasanna Lakshmi et al (2014) have plotted VP and Vs wave as a function of density and porosity as a function of depth for Bushe, Poladpur, Ambenali and Mahabaleshwar Formations from exposed in Koyana-Warna region (Figure 2:9). In this study, density, VP, Vs, porosity and Poisson‘s ratio are measured from Bushe, Poladpur, Ambenali and Mahabaleshwar Formations. Vesicular porosity and aspect ratio of the amygdales affects the Poisson‘s ratio. The Bushe Formation samples show low density, VP, and Vs as compare to other Formations.

Figure 2:9:- Variation of petrophysical properties of basalt samples from Koyana region

(Prasanna Lakshmi et al., 2014)

Vedanti et al (2015) measured VP for samples collected from Killari and found out that these samples have a high density but low VP, but show good correlation between porosity and density values (Figure 2:10).

Figure 2:10:-Variation in Vp, Vs, r and f with depth for cores from Killari (Vedanti et al., 2015)

Malkoti et al (2015) investigated viscoelastic formulation for Deccan basalt using synthetic seismograms, and opined that events are attenuated in different layers of basalts resulting in reduction in amplitude for sub basalt events. Vedanti et al (2015) carried out analysis of petrophysical properties and ultrasonic P-and S- wave attenuation measurements for basalt samples from Killari (SE of DTB) and found that the rock properties of massive and vesicular varieties from DTB are different and attenuation depends upon porosity and density of DTB with the dominant attenuation mechanism as ―squirt flow‖.

2.10.2 Seismic Microzonation studies in DTB

Methodology for zoning for ground motions, slope instability and soil liquefaction is given by ISSMGE (1999) which are serving as a base guidelines for microzonation. Finn et al (2004) proposed estimation of losses for a selected probability of exceedance of ground shaking level. Oliveira (2006), proposed results of microzonation should be incorporated in seismic regulations where dense population is present in a given urban centre. Mihalic et al (2011) Sitharam, (2008), (2011) reviewed various procedures and principles available at that time to caary out microzonation. Based on these guidelines, Verma et al (2014) carried out Seismic Microzonation for Pune and adjoining area and prepared Level-I and Level-II maps

46 for various parameters such as soil liquefaction using N value of SPT, average shear wave velocity map, Soil Vulnerability Index contour map, Peak soil amplification contour maps at 1:50000 scale. This lead to development of Level-II hazard map and classify hazard levels for Pune city.

Engineering Geology of Deccan Trap Basalt (DTB)

This part discusses the engineering geology of Deccan Basalts for the areas under study. Though considerable information on various case studies on basalts is available but literature on engineering behaviour of DTB is lacking. The so called monolith approach applied to understand the geology was probably the cause of lack of work and literature on DTB. However, Gupte et al (1980) contributed towards understanding of engineering behaviour (Table: ). GSI have carried out investigation of many engineering projects but their reports are not available in the literature. This felt the need to work on engineering geological mapping of various flows in DTB. Further, it is observed that, many systems of rock mass characterization are in place but application of these methods have some difficulty in the field. All these challenges are addressed in the Chapter-IV. In the following section, early understanding of engineering behaviour is addressed followed by reviews in rock mass characterization. Balakrishna (1964) assessed longitudinal velocity, UCS for basalt samples from SE of DVP. Gupte (1980) gave the laboratory details of various rock types observed in DVP (Table 4-1). Ghosh(1987) has given engineering geological map of India, in which the he quoted medium compressive strength (50 to 200 kg/cm2 ) and low to moderate permeability values (2 to 8 Lugeon) for red bole, clinker and tuff of the Deccan Trap which easily disintegrate on saturation and are compressible on sustained loading and high compressive strength (500 to ≥ 2000 kg/cm2) and low permeability (less than 2 lugeon in general and occasionally upto 5 lugeon recorded at places) for Hard Deccan Trap flows. Abdullah, (2010) correlated weathering index and water content with UCS (kg/cm2),

VP and Vs (km/sec). According to Schultz (1995) strength properties of basaltic rocks presented here differ significantly depending on the scale of observation. Ansari et al (2014) investigated slopes along the Mahabaleshwar ghat sections of DVP. Prasanna Lakshmi et al, (2014) gave petrophysical properties. Jain et al (2014) discussed possible application of TBM for construction of Maroshi- Ruparel

Tunnel in Mumbai. Ansari et al (2014) investigated slopes using Rockfall Hazard Rating System at Mahabaleshwar ghat sections of DVP. These attempts throw some light on the engineering behaviour of basalts, but lack in understanding of engineering behaviour on the basis of flow morphology of `A`Ā and Pāhoehoe flows.

Rock Mass Characterization of DTB

Rock mass characterization is generally discussed using few parameters to be measured in laboratory or in-situ conditions. However, an integrated approach is required to develop the rock mass characterization. The objective of rock mass characterization (Bieniawski, 1989) is to develop a reasonable scheme of rock mass classification, which should identify the most significant behaviour of a rock mass, should be able to derive quantitative data and guidelines for engineering design and should provide a tool of communication between engineers and geologists. Considering these objectives, this part of the chapter is prepared. In the following section, literature review citing various methods such as RMR, Q etc. and their applications have been discussed. Few references from DTB, employing these methods have been discussed at the end.

Many rock mass characterization schemes are in vogue, however, few of them are widely used and are discussed in the following section. Terzaghi (1946) developed a scheme of rock mass classification to help in knowing the type and intensity of defects in rock. Lauffer (1958) introduced the concept of the stand-up time of the active span in a tunnel, which is appropriate in determining the type and amount of tunnel support. Deere et al (1969) introduced the rock quality designation (RQD) index, which expresses number of joints per meter. Wickham et al (1972) developed a concept of rock structure rating. The Rock Mass Rating was proposed by Bieniawski (1973) to derive support systems in tunnels, later on, it was extended to rock slopes (Romana 1995), foundations, ground rippability assessment, and mining problems. Q system was proposed by Barton et al (1974) has provided a support system with precision using RQD, joint number, roughness etc. as criteria. In the present work, a recent version of Q system (NGI, 2015) is used (Chapter-VI).

Palmstrom (1995) introduced Rock Mass index (RMi) to characterise the strength of the rock mass and its suitability in the application in various types of construction

activity. Marinos and Hoek (2000) proposed rock mass index known as Geological Strength Index (GSI) based on geological observations in the field including lithology, structure, and condition of discontinuity surfaces. Most of these systems have taken into consideration different types of lithologies and structures associated with them etc. while preparing charts for the end users. Few of these systems have been included in IS Code e.g. Rock Mass Rating (RMR) is adopted in IS:13365 (Part-1), (1998), Q system in IS:13365(Part-2),(1996) and Slope Mass Rating in IS:13365(Part-3),(1997). In the present work, Rock Mass Rating (RMR), Q systems have been extensively used to evaluate, various parameters for an underground excavation (Chapter-VI).

Rock Mass Rating (RMR) system was devised by Bieniawski (1973) which utilizes parameters, such as uniaxial compressive strength of intact rock material, Rock Quality Designation (RQD), Spacing of joints / fractures, Condition of discontinuities (using orientation, persistence, Separation, roughness, Infilling, Alteration / weathering and groundwater conditions). All of these parameters are measured either on the exposure or obtained from borehole data using a particular rating i.e. a number assigned to each parameter. Once the classification parameters are determined, the ratings are assigned to each parameter according to tables and charts provided by Bieniawski (1973). The ratings of these parameters are summarized to give a value of RMR and this value is used to design supports in the underground excavation or to understand the stability of slope or suitability of foundation. Romana (1984) has modified RMR to evaluate the slope stability known as SMR. This system is further modified by Tomás et al (2007) and Lee (2008).

Barton et al (1974) proposed a Tunnelling Quality Index (Q) to determine rock mass and tunnel support systems. This involves the numerical value of the index (varying on the logarithmic scale from 0.001 to a maximum of 1,000) which can be calculated by using equation:

Q = (RQD/Jn) × (Jr/Ja) × (Jw/SRF) ……(Barton et al, 1974) Equation 1

This formula is expressed using three quotients as follows:

1) Block size: Determined by RQD/joint number (a number assigned to joint sets), 2) Inter-block shear strength: which is evaluated by joint roughness and alteration of joints

3) Active stress: Estimated using an amount of water per meter of excavation and amount of stress present in the rock due to the presence of incompetent zones, depth of excavation etc.

The value of Q obtained is plotted on the chart provided by Barton et al., (1974) to estimate support system. This Q-system is updated by Norwegian Geotechnical Institute (2015) to suit the present day needs. Palmstrom (2005) has discussed the problems associated with RQD to evaluate the block size as for higher spacing, RQD will be too low and block size would be too high. Along with this, RQD cannot be evaluated accurately for the volumetric joint count which is a deciding factor in block size. Palmstrom et al (2006) have pointed out that the Q-system does not take into account joint orientations, joint continuity, joint aperture and rock strength. RMR and Q can be used as good checklists for collecting rock mass data. Geological Strength Index (GSI) was developed by Marinos et al (2005) to fulfil the need of reliable rock mass input data required to design tunnels, slopes or foundations. In this system, visual assessment of rock has been given the prime importance. This assessment generates data which would include prediction of the strength of the rock mass involved and its ability to undergo changes under varying stress condition. It is a field method to characterize the rock considering depth at which investigation is taking pace, the presence of ground water, the aperture and the infilling of joints /fractures etc. along with the description of weathering in the in situ condition.

In DTB, few case histories are available in which these systems (mainly Q and RMR) have been applied. e.g. Gupta et al (2011) have investigated a case history from northern part of DTB; followed by Ansari et al (2014), Jain et al, (2014), (2015, (2016). Rock mass classification systems of have been applied in these works, based on lithology. Most of the work deals with performance characteristics of Tunnel Boring Machine (TBM) and response from a rock.

In our view, while applying rock mass classification systems to characterize the rocks from DTB, redefining or renaming of few input parameters is required. This need is addressed in Chapter-IV using evaluation of input parameters for rock mass classification systems in DTB.

Summary

Detail review of literature on account of theories for origin of Deccan basalt volcanism followed by various approaches of describing morphology of basaltic flows is discussed. This is followed by various schemes of stratigraphy including lithostratigraphy, magnetostratigraphy and chemostratigraphy. A regional geophysical aspect of Deccan Trap area with emphasis of seismic studies is also reviewed followed by seismic microzonation studies. The prevailing trends in engineering geology of DTB along with application of rock mass characterisation methods in various civil engineering projects are also explained. Overall review of literature on geology, geophysics and engineering geological aspects of Deccan trap basalts has been taken in this chapter. The purpose of the review is to get acquainted with the literature available on different aspects of Deccan Traps Basalt flows relevant to site suitability of a given project and establish the need and importance of literature is reviewed in this chapter.