Geology of the Area □ Detailed Geology □ Intrusives □ Flow Structures □ Quaternary Deposits □ Tufas □ Petrography □ Chemistry □ X-Ray Diffraction
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□ Introduction □ Geology of the area □ Detailed geology □ Intrusives □ Flow structures □ Quaternary deposits □ Tufas □ Petrography □ Chemistry □ X-ray diffraction Geology Chapter 2 CHAPTER II GEOLOGY Introduction A major part o f the Maharashtra state is covered with plateau basalts - the Deccan flood basalts (DFB). These flood basalts are also known commonly as the Deccan Traps. The Deccan traps are an eruptive sequence of flows which formed during the Cretaceous Tertiary period. They occur as a blanket of sub-horizontal flows over the older rocks and cover an area of over 500,000 sq. kms. These flood basalts consist dominantly of low K-tholeitic lavas. Though the Deccan Volcanic Province (DVP) is not the largest flood basalt province in the world, it has evinced keen interest. This may be because they were erupted close to or possibly at the Cretaceous Tertiary boundary and also due to the possibilities of mass extinction and meteoric impact around the time of its eruption (Cox, 1988). 16 The total thickness of the Deccan basalts varies from few metres along the eastern fringe areas to over thousands of metres in the west. Krishnan (1968) has recorded 2120 - 3030 mts thick traps in Western Maharashtra, while in northern part of Thane district. Raja Rao (1984) has reported a 2380m thick trap. Based on the chemical fingerprinting, Cox and Hawkesworth (1985), Beane et al. (1986), Devey and Lightfoot( 1986), Hooper etal. (1988) and others have established a stratigr^hic sequence of the Deccan Basalts of Western ghats. They have divided the basalt flow sequence into ten different formations, which fall in three subgroups; the Kalsubai subgroup at the base with five formations, the Lonavala subgroup in the middle with two formations and the topmost Wai subgroup with three formations. They have recognised six Giant phenocryst basalts (GPB’s) in the lower part of the sequence and have used these GPB's as markers. The simplified stratigr^hic sequence after Hooper is given in Table 2.1. Subbarao and Hooper (1988) have prepared a map showing the distribution of the various formations in western Maharashtra. Ghodke et al. (1985) had proposed a preliminary lithostratigraphy of parts of western Maharashtra. Subsequently, Godbole et al. (1996) have modified the lava stratigraphy for sequence of the Deccan basalt flows of entire western Maharashtra. They have recognised four megacryst marker horizons (Ml - M4) occurring at different stratigraphic levels for and used these for correlation of the flows. The basalt pile consisting of an older sequence of compound flows and a younger sequence of simple flows have been divided into eight formations, which have been included in the North Sahyadri Group. Godbole et al. (1996) have given a map showing the distribution of various formations. The generalised stratigraphic sequence is given in Table 2.2. 17 TABLE 2.1 Generalised Stratigraphic column of the Deccan Volcanic province in Western Ghats based on chemical characteristics ( after Hooper, 1988) GROUP SUB-GROUP FORMATION MEMBER Deccan basalts Wai Desur Panhala Mahabaleshwar Ambenali Poladpur Lonavala Bushe Khandala Monkey Hill GPB GiravaUi GPB Kalsubai Bhimashankar Manchar GPB Thakurwadi Tunnel -5 GPB Neral KasheleGPB Igatpuri Talghat GPB Jawahar TABLE 2.2 Generalised Litbostratigraphic succession of the Deccan Volcanic Province in Western Maharashtra ( after Godbole et al, 1996) SUPERGROUP GROUP SUB-GROUP FORMATION Deccan Traps North Sahyadri Mahableshwar Mahabal^war —M4 ( Marker) Dive Ghat Purandargad Dive Ghat Karla Lonavala Indrayani —M3 (Marker) Kalsubai Upper Ratangarh —M2 (Marker) Lower Ratangarh —Ml (Marker) Salher 18 The lava flows are not horizontal, but exhibit gentle gradients in varied directions. The disposition of the lava flows is important to understand the overall structure. In Nasik, Dhulia & Jalgaon districts the lava flows exhibit north easterly & easterly gradients. In parts of Pune, Ahmednagar, Satara and Kolhapur districts, the flows exhibit southerly and south easterly gradients. In parts of Ratnagiri and Kolhapur district the flows dip towards SW. In the coastal districts of Raigad and Thane and in Bombay, the flows dip steeply to the west. The basalt lava pile is intruded by dykes at numerous places. Dykes occur as clusters and swarms along two tectonic belts, the West coast and Narmada Tapi lineaments (Deshmukh and Sehgal, 1988). The dyke swarms in the Satpura range and adjoining Dhulia district, trend ENE - WSW, while the dykes in the coastal areas & the Westem districts of Maharashtra trend N - S, NNE - SSW & NNW - SSW. Numerous multiple intrusives of Nepheline-syenite, lamprophyre etc. are reported from the coastal areas near Bombay, Ahbag and Murud-Janjira (Dessai, 1987; Godbole, 1987). Acid and intermediate differentiates like rhyolites, trachytes, dacites, pitchstone and syenites have been reported from the westem pari: of Maharashtra and Saurashtra. Pyroclastics, welded tuffs and agglomerates have also been reported from the area around Bombay. Olivine rich flows occur near Bhiwandi and Igatpuri. Intertrappean beds are seen only in the fringe areas. In the eastern part of Deccan Traps a few patches of intertrappean beds are also seen in Malabar hill and Worli hills of Bombay. These beds which contain plant and animal fossils are upto 35 m thick. ' i - 19 V. ; Faults have been reported in Deccan basalts from the northern districts (Dhulia, Jalgaon) and along the coast in Thane district & near Bombay. The Kondaibari fault and Gawilgarh fault in the northern districts exhibit maximum lateral displacement of 200mts and throw to the order of 160mts. In the western part, an arcuate fault runs from south of S^hula upto Bombay, separating a sequence of pahoehoe flows to the east of fault from the volcanic and plutonic rocks exposed to the west. This arcuate fault with a subsidence of 600mts has been recognised as cauldron subsidence (Godbole, 1987). Besides this N - S and NW - SE trending faults have also been inferred along the course of Surya and Vaitama rivers. The Deccan Basalts are capped by laterite and bauxite in a number of places, especially in the western and southern districts. Alluvial and coUuvial sediments which constitute the Quaternary deposits are found as discontinuous patches mostly restricted to river valleys. Geology of the Area The area taken for study falls partly in Pune & Ahmednagar districts. The area is predominantly covered by Deccan basalt flows in a 480-500 meter thick lava pile. A very systematic study of the lava flows covering the entire area on a 1:50,000 scale was carried out and a generalised geological map on 1: 250,000 scale was prepared (Fig: 2.1). Besides this detailed studies were carried out in selected localities, like Bote, Gurewadi, Chilewadi, Kotul etc. The studies were aimed at understanding the influence of lithology (type of flow) and structure in the formation of various anomalous geomorphological features. 20 The lava flows in the area have been classified as compound/pahoehoe and simple/ aa flows since they bear resemblance to the Hawaiian types, though they may differ slightly from the ones seen in Hawaii (Walker, 1969). The same terminology is followed by the Geological Survey of India. During the present study, the terminology aa, simple and pahoehoe/compound flows was used for identification of flows in the field. The general characteristics of the aa and pahoehoe flows, on the basis of which the field identification has been done is given below. The aa flows have a fragmented top or a zone of irregular vesicles towards the top of dense basalt. The fragmentary material varies in thickness from hn to a maximum of 5mts and may be absent at places. The fragmentary material consists of angular to subangular blocks ranging in size from a few centimetres to as much as a metre. Size stratification is not generally observed. Fragments are made up mostly of scoraceous basalt, which are often found immediately below the fragmentary layer. Small fragments of zeolites, calcite, and silica occupy the intervening space between the fragments. The top and the bottom of fragmentary layer are undulatory. At a few places, a thin impersistent fragmentary layer termed basal clinker is seen at the base of the flow. This can be distinguished from the fragmentary top of a lower aa flow, only where red bole separates the two flows. The pahoehoe flows unlike aa flows are constituted of numerous flow units and may also be referred to as compound flows. The individual unit varies in thickness from a few centimetres to as much as 5mts. Occassionally units may even be 10 - 15mts thick. Each small unit has pipe vesicles or amygdules at the base. A reddened and glassy crust envelops the unit tops and separates one unit from the other. Ropy 21 structure, squeeze ups, toes etc. can be seen on the glassy crust of the flow units. The middle of a pahoehoe flow comprises of massive basalt which grades into vesicular, scoraceous and amygdular varieties towards the top. The pahoehoe flows occur as thin units of limited extent and thickness and therefore several units are grouped into a single compound flow, while the aa flows represent a single volcanic surge (Walker, 1969). The thickness of the individual flows varies from 15-350 mts. The middle part of the aa flows and pahoehoe flow unit is compact, dark grey to greenish grey, non porphyritic to porphyritic and often jointed. The porphyritic basalts of the area have been classified into highly porphyritic to sparsely porphyritic on the basis of frequency of phenocrysts. Based on the size of the phenocryst they are termed megaphenocryst basalt (2 - 3.5cm long plagioclase laths), coarse phenocryst basalt (1 - 1.5cm long plagioclase laths) medium phenocryst basalt (lesser than 1cm). The mega phenocryst basalt because of its striking megascopic ^pearance has been used as a marker horizon. Such flows have been termed variedly as megacryst flows.