□ 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. (Ghodke et al., 1985; Godbole et al. 1996) and Giant Plagioclase basalts or GPB’s (Beane et al., 1986; Hooper et al., 1988; Bodas, 1985; Thorat and Subbarao, 1996).
The vesicular portion of flows is usually ash grey, purplish to reddish in colour. The vesicles are mostly rounded in pahoehoe flows and irregular and elongated in aa flows. The vesicles may be empty or filled and the fillings are mostly altered glass, zeolite, calcite and silica.
22 Red bole usually occurs as impersistent horizons at the top of flows but may occur even on flow units. It ranges in thickness from 10cm to a maximum of l-1.5mts. Where red bole horizons do not occur at the contact of flows, geomorphic break in slope has been used as criteria along with the change in petrography to separate the flows. To understand their spatial behaviour, the flow contacts were traced laterally at each successive section. Based on this systematic study of distribution of the flows in the entire area, a generalised stratigraphic succession for the area under investigation was prepared. This is shown in Table 2.3.
Twenty flows were identified in the area. Based on their field characteristics these flows were then grouped into four categories. Group I consists of flows 1 to 7, Group II is shown separately and consists of flow number 8. Flows 9 to 18 and 19 to 20 constitute Group III & Group IV respectively. A brief description of each of the four different groiq)s of flows is given below.
Group I The flow numbers 1 to 7 have been grouped together, since they are dominantly pahoehoe in nature with only thin aa and simple flows (flow numbers 4 & 6) which pinch off towards the east. Most of the flows are moderately to sparsely porphyritic varying in thickness from 15-lOOmts. The lower flows are non porphyritic. The flow No. 6 has been termed simple, because at places, it exhibits mixed characters (of both pahoehoe & aa) i.e. pipes at the base and fragmentary material towards the top as seen north of Khamundi.
This flow contains megaphenocrysts of plagioclase, ranging in size from 1.5 - 3.5cm. The thickness of this flow is maximum in the SE part of the area, attaining a maximum
23 TABLE23
STRAHGRAPHICSUCCESSIONOFIAVAFLOWS IN THE AREA STUDIED.
Flow Flow Characters Thickness No. type
20 Aa Topmost flow. Exposed only in the 20 mts. + western and South eastern most part Grey, Sparsely porphyritic, highly jointed a. 3 £ 19 Cph Exposed only in the western and 60mts. South Eastern part. Greenish grey. Highly porphyritic with medium size phenocrysts in medium grained ground mass
18 Aa Dk. Grey, Sparsely to moderately porphyritic. with fine to medium size phenocrysts 15 -40 mts. in a fine grained matrix.
17 Cph Greenish grey. Moderately porphyritic with medium size phenocrysts. Pinches west of 0-25 mts. Kanhur.
16 Aa Grey. Moderately to sparsely porphyritic with 10 -16 mts. phenocrysts in a fine garined ground mass
15 Cph Greenish grey. Mod. porphyritic with medium size phenocrysts. Exposed only in the 0-16 mts. south eastern part of the area.
B 14 Aa Dk. Grey. Mod. porphyritic with medium 15-45 mts. a, a to the size phenocrysts set in fine grained 2 ground mass. Highly jointed with crude O vertical joints.
13 Aa Dk. Grey. Mod. porphyritic with medium 15 -60 mts. size phenocrysts. Crude colimmar joints seen.
12 Cph. Grey. Moderately porphyritic medium grained. 0-35 mts. Pinches in hill section. 903 & south South west of Kaijule Harye in upgradient direction. Thickness increases in the east.
11 Aa Dk. grey. Mod. porphyritic with coarse size 0-40 mts. phenocrysts. Bouldery outcrop. Pinches east of Takli
24 Flow Flow Characters Thickness No. type
10 Aa Dk. Grey. Moderately porphyritic coarse size phenocryst. Bouldery outcrop. Pinches 0-20 mts. e in theAne ghat. s s 9 Cph. Grey. Moderately to sparsely porphyritic 0-20 mts. o Pinches ESE of Ane. Can be traced only upto West of Takli
8 Cph Grey to brownish grey. Mod. to highly M porphyritic Upper units are contain mega 5-50 mts. cus phenocrysts of plagioclase. ( M3 megacryst 2 marker flow of Godbole et al, 1996 & o Beane et al, 1986). Giravali GPB. Pinches in the South eastern part of the area
7 Cph Grey. Non porphyritic to highly porphyritic, glomeroporphyritic with 50- 100 mts. coarse phenocrysts of plagioclase.
6 Aa/ Bm. grey. Mod.to highly porphyritic with 0-50 mts. Simfde coarse and megaphenocrysts of plagioclase. In the western part it is tMn and shows mixed characters. The porphyritic nature and thickness increases to the east. ( Manchar GPB of beane et al, 1996) Pinches towards the east near Bokharwadi
5 Cph Greenish grey. Non to sparsely porphyritic 20 - 45 mts. with medium size phenocrysts. eu s Grey. Moderately porphyritic. Medium to fine 0-40 mts. 2 4 Aa O sized phenocrysts. Mafic phenocrysts also seea Pinches towards the South eastern part
3 Cph Greenish grey Non to sparsely porphyritic 35 - 100 mts.
2 Cph Grey. Moderately porphyritic. Exposed 10 mts. only in E part of the area in Mandohal Odha
1 Cph Dk. grey. Non porphyritic to sparsely 10 mts. + porphyritic. Base not seen
Abbreviations Aa : Aa flow Simp : Simple flow Cph : Compound palioehoe flow Pig : Plagioclase.
25 thickness of 50mts. The frequency distribution of the phenociysts, is sparse in the southern part, around Nagapur with reduction in size of phenocrysts giving rise to massive flow with bouldery outcrop. This flow his been used as a local marker. The flow pinches at Boknakwadi which is NNW of Takli. In the present study, this flow has been separated (hatched portion in Fig. 2.1) mainly because it helps as a marker to a certain extent, specially in the central & South eastern portion. This flow can be equated with the Manchar GPB of Beane et al. (1986) and Alkuti flow of Thorat & Subbarao (1996). This group can be correlated with Up. Ratangarh formation of Godbole (et al. 1996) & Thakurwadi & Bhimashankar formation of Beane et al. (1986) and Hooper (1988).
Group II The flow No. 8, a compound pahoehoe flow also contains meg^henocrysts of plagioclase ranging in size from 2 cm to a maximum of 3.5 to 4 cm. (Photo 2.1). The upper few units of this flow exhibit very large sized phenocrysts. The vesicularity, jointing & frequency distribution of the plagioclase laths is variable. In most sections this flow is capped by a 0.50 to 1 metre thick red bole or green bole, followed by a massive dark grey aa flow. Hence, this contact forms a prominent marker. This flow has therefore been used as a marker horizon, separating the dominantly aa flows above from the dominantly pahoehoe flows below. Ghodke et al. (1985) & Godbole et al. (1996) have designated this flow as M3 marker, separating the Up. Ratangarh formation below from the Indrayani formation above. Hooper (1988) has called this flow as Giravili GPB, separating the Bhimashankar formation from the upper Khandala formation.
26 Group III Flows 9 to 18 have been grouped together (Group III) due to the predominance of aa flows. The flows are moderately porphyritic and vary in thickness from 15 to a maximum of 60mts. Thin inter fingering compound pahoehoe flows 9,12,15,17 pinch towards the west in the upgradient direction and become thicker towards the east Flow nos 10 and 11 are aa flows and they pinch towards the east in the direction of the gradient. The aa flows have a bouldery appearance and are moderately to sparsely porphyritic and also exhibit crude columnar jointing. Within a single flow itself, 2 to 3 collanade and entabulature zones are seen, giving ^>pearance of different tiers. The pahoehoe flow No. 17 exposed in the areas north of Padli darya, Kanhur etc. has an extensive horizontal spread. This flow is fine grained, porphyritic with medium size phenocrysts and shows extensive weathering. This group of dominantly aa flows can be correlated with the Indrayani formation of Godbole et al. (1996) and Hooper et al. (1988)
Group IV Flow numbers 19 and 20 have been grouped together due to their limited exposure in the area. They are confined to die hill ranges in the north west and south eastern parts and occur above 950 -1000 ASL.The flows are porphyritic and exhibit crude jointing.
Detailed Geology As mentioned earlier the generalised geology & stratigr^hic succession in the area was arrived at by carrying out systematic geological mjqjping at different sections
27 in the area. Four localities with prominent geomorphic features were selected for detailed studies. These were; i near Bote, Kas nadi. ii near Gurewadi, Mandohal odha iii near Chilewadi, Mandvi nadi iv near Kotul, Mula nadi At each of this location, wherever possible, the flows/flow units on either side of the river, forming the gorge were studied with tq)e & brunton and supplemented with geological maps on 1:50,000 scale wherever necessary. The details of the geology in these localities are enumerated in brief in the following paragraphs.
Geology off the area around Bote The basalt flows e?q)osed at Kas Nadi SE & E of Bote (Fig 2.2) are dominantly of pahoehoe type with a single aa flow exposed almost at the base which is close to the present stream bed level. These flows form part of flow No. 3 of Group I (Table 2.3). The lowermost con^und pahoehoe flow is moderately poiphyritic and medium graiued with 2 to 3 units. The overlying aa flow is greyish in colour and has a 1 metre thick fragmentary material towards its top. The fragmentary material being softer is easily removed resulting in caving in at this junction (Fig. 2.2).
The aa flow is overlain by a thick sequence of compound pahoehoe flows. The lowest flow of this group of flows is 12 -13 mts thick, with non-porphyritic to sparsely porphyritic units. This flow is less vesicular and more compact. Close spaced sheet jointing is seen towards the top and bottom of the flow. Vertical joints, coupled with the compact nature of the flow must have resulted in the near vertical
28 LEGEND
Silt with gravel/pebble lenses & calcareous noduies/kankar Squeeze ups
Sheet joints Simple flow / aa flow
Vertical joints How brcceia
*- *- Porphyritic flow unit with coarse VesicJes / Amygdules phenocrysts
Red bole / Reddened erust Porphyritic flow unit with medium size phenocry sts
Ropy structure Non-porphyritie basalt flow unit
\ >, Pipe vesicles / Amygdules
Fig 2.2 : SECTION ACROSS KAS NADI NEAR BOTE walls. The pahoehoe flow sequence that overlies this jointed flow is composed of highly vesicular and amygdular flows, with abundant pipes, squeeze ups, ropes etc. The units vary in texture from non-porphyritic to highly porphyritic. These flows are more weathered as compared to lower ones and hence form a smooth concave slope. Spheroidal weathering is more common in these flows. Potholes are formed in this compound flow sequence. The different flows shown in the section across Kas nadi constitute one single flow on 1:50,000 scale (Fig 2.3). This flow is the lowermost one in the area . The flat planar surface at 720 mts is composed of pahoehoe flows.
A basic dyke intrudes the lava pile north of Kelwadi and south of Bote, trending N85°E - S 85°W. This is parallel to the fracture trending N80®E - S80°W and to the direction of the knick point SE of Bote present on a tributary of Kas Nadi. The cfyke is fine to medium grained and doleritic in composition (Photo 2.2). East and southeast of Bote, Quaternary alluvial deposits rest on the basalt on the 680 - 700m surface on either banks of the Kas nadi. Alluvium is also seen filling the gorge from the bend north of Kelwadi to its confluence with Mula. The quaternary deposits are described in detail in the later part of this chapter.
Geology of the area around Gurewadi In the section e?qx)sed at Mandohal odha, a sequence of eight flows have been delineated (Fig 2.4). Flows 1 to 5 are of pahoehoe nature, varying in texture from non-porphyritic to moderately porphyritic. The lower four flows are highly vesicular and amygdular. Ropes and squeeze ups are abundant on the flow unit tops. Sheet