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Chapter II GEOLOGICAL aETIING Introduction : In the coastal tract extending between Palghar and Thai tne dominant lithological type is the Deccan basalt which, at places, is associated with other volcanic types including rhyolites, trachytes and pyroclastics. The basalt flows are often separated byfchin horizons of clayey rock of reddish brown colour, which are referred to as ’red beds’, and occasionally by shales. The volcanics are intruded by dykes or email bodies of diorites, gabbros, anorthositic gabbros, dolerites, lamprophyres and trachytic agglomerates. Laterites are commonly developed at higher altitudes, especially above 330 metres and they form cappings on many ofjthe higher hills. In the coastal zone, the igneous rocks are covered by consolidated and/or unconsolidated Quaternary sediments, which range in thickness from 1 metre to 20 metres. Alluvial deposits occur in the river valleys and along the foot of the hills there is extensive development of colluvium. The distribution of these lithological units is shown in the geological maps (Fxgs.2.1, 2.2 and <:.3) INDEX TO FIG.2 Basalt 5 a ^ . x i 6 7 t ®a ' 8 9 10 A GAS HI N I 2 ® a s s e \ n __ I___ MILES 721SSE Fig-2-1 Geological map of Palghar and Bassein sectors. 18 axis of the Panvel flexure also follows the courses of these rivers (Fig.2.6). Hot springs are located in fault zones, ^.g. the hot springs of /Conner and Varai. In the hills to the east of Virar, two shear zon°s have been recognised which trend N 335°. Also along the w»3t^rn fringe of Tungar-Kamandurg hill range, a N 335° trending shear zone is recorded. A north-south trending fault to the west of Thane, which corresponds to the Thana creek, can be traced from Thane to Parol along the ^astern fringe of the Tungar-Kamandurg hill range. Further to the north, it follows the course of Tansa river in a N 45°W to N 50°W direction and, can be traced to near the mouth of the iiur river in the Kelve area (Plate 2.5, Photos 1 and 2). In Bombay sector, a couple of faults have been reported. One of the faults trend N 10° p to M 15° ? and follows the eastern fringe of the ^astern ridge of the Bombay and jalesette islands. Further north, this fault merges into the Thane-Kelve fault, described above. On* more fault which trends N-S follows a line joining Saki Naka-Kanheri Caves and Gaimukh Bandar. This fault can be traced further to the north of Ulhas riv-T along the Nagli Dongar hill range upto 3ativli village in a N 335° to N 345° direction. A Borivli Mahim Bay BOMBAY 6 MILES Fig • 2 • 2 Geological map of the Bombay sector 1MDEJL TO FIG.2.3 1 : Alluvium 2 : Mud 3 : Beach sand 4 : Beach rock 5 : Laterite 6 : Intertrappaan 7 : Andssite 6 : oolerite 9 : Basalt ' A ----- 1— t;— a— a ----- / ,A '----- (73'lodEr.--- >4-fQ _9 : A A A A| BOMBAY/ A ^ A A A >eU L._i A Panve\ cre ^ lA iv M .Panvel ELEPHANTA S-A::t'j \ 1 A / a 0 • - ~ i z r !7 ‘7 ’ ‘M / i i ? fw i l f / ' & ' V V BUTCHER / /A A, i A A| *--- ----- - <•---- -,s-i l\ I I » a t M ARABIA N SEA J8 50 11 and their important field characters are outlined in the following pages. Volcanic rocks j The volcanic rocks present in the area under study include basalts, andesites, rhyolites, rhyoaacites and tiachytes. a) Basalts j Basaltic flows related to Deccan volcanic activity occupy the greater part of the area covered by the present investigation. They vary in thickness from 5 metres to 135 metres and are seen to extend for considerable distance. Tha flow boundaries may be delineated on the basis of marked variation in the lithological characters, presence of red beds, volcanic breccia or zones of silicification or because of significant differences in the jointing and weathering patterns. The development of flat surfaces, corresponding with the presence of any of the above features has been taken as additional evidence of flow boundary, in many parts of the area studied flow boundaries may be demarcated on aerial photographs on the basis of tonal and structural variations. 12 In the area studied, the basalt flows comprise of both 'pahoehoo' and ’aa* types. The •pahoehoe* flows characteristically show a basal section with pipe amygdales, a middle section of dense basalt and a top highly vesicular section with spherical vesicles filled with secondary minerals like silica, calcite, apophyllite and seolites. The ’aa* flows, on the other hand, have a basal section of dense rock and a top section of fragmentary rocks. The'pahoehoe’ type being softer, is more easily eroded, it usually gives rise to conical hills with smooth slopes or to *Vf shaped valleys. In the dominantly ’pahoehoe* areas, flat-topped hills are rare and the topography is that of badlands. The uppermost, altered portions of ’aa* flows weather easily and give rise to finer fragments. The dense middle sections show spaeroxdal weathering and yield large boulders, in areas of *aa' flows, steep or vertical scraps are conspicuous. They form flat topped hills and broad »Uf shaped valleys. 3tep-like topograpnic terraces are also common. Over much of the area covered by the present study and especially in the SAhyadri, the flows are largely of the 'panoehoe' type. fAa» flows occur in the Bombay 13 group of islands and in the western part of the Bassein and Thai sectors. An idea about the relative proportions of these types and about their physical characteristics may be had from the description of flows in different areas given below. in the Kaldurg hills of Palghar sector thirteen flows, varying in thickness from 10 metres to 110 metres, have been recognised. Most of these flows 3how characteristics of 'pahoehoe1 type. In the Tungar hills of the Bassein sector, seventeen flows varying in thickness from 7 metres to 110 metres are recorded. Out of these, eleven flows are of faaf type. In the Mumbra-Belapur hill range of the Panvel sector, six 'pahoehoe* flows, varying in thickness from 30 metres to 132 metres, have been recognised. Out of these six fpahoehoef flows, two flows are porphyritic and the remaining four are non-porphyritic in nature. On Sheva island of the Panvel sector, two *pahoehoe* flows have been reported, the lower flow exposed upto 20 metres above AaL is a compound 'pahoeboe' flow, comprising of two flow units. The upper flow having a thickness of about 25 metres is fine grained and dense, sparsely porphyritic in nature, and exhibits close 14 spaced jointing. ;even and five flows of ’pahoehoa1 type are present in the hills of Uran and Elephanta island respectively. The thickness of these flows varies from 20 metres to 70 metres. In the Mira Dongar hills of Pen sector, fifteen flows have baan recognised. Most of these are of *pahoehoe' type and five of them are porphyritic in nature. In tha Kankeshwar hills of the Thai sector, nine flows exhibiting characteristics of ’aa ’ type are present. In Bombay group of islands both fpahoehoe* and *aa' types of flows are present, but tha 'aa' types of flows dominate in most parts of the islands. As an illustration, the succession of flows observed in tha Karlakhind hill is given in Table 2.1 and tfte spatial distribution of these flows is shown in Fig.2.4* A geological map of Thai sector showing different flows, prepared on the basis of aerial photograph study ia presented in Pig.2.5. The colour of basalts ranges fro a greenish to dark gray. The^asalts are generally fine to medium grained rocks. Some basaltic flows are porphyritic. Tha phenocrysts of plagioclase feldspar range in length from 3 to 5 centimetres. At many places, there is development Table 2.1 t Succession of flows in tha Karlakhind hill, Thai sector. Flow No, 11 t Gray, fine grained basalt ( 25 metres ) Flow Mo. 10 t Compact, medium grained. ( 30 metres) grey basalt. Flow No, 9 t Compact, gray coloured* ( 20 metres) sparsely amygdaloidal basalt. Flow No, 8 i Compact dark gray basalt ( 40 metres) with amygdalae. Flow No, 7 t Compact dark basalt with ( 33 metres) fine white amygdalae. Flow No. 6 i Hard compact, derk basalt. ( 25 metres ) Shows characteristic spheroidal weathering. Flow No. 5 t Grey, medium grained ( 20 metres ) basalt. Flow No, 4 i Dark, compact basalt with ( 33 metres ) less number of amygdales. small vesicles ar* also present. Flow No, 3 t Compact, gray basalt. ( 13 metres ) Flow No, 2 t Compact, black to gray ( 6 metres ) basalt. Flow No, 1 « Gray basalt, fine grained. ( 7 ) with large amygdalae. o o 4- + - Late rite Flow No-9 + + Flow No- 6 f- low No • 3 fo o Flow No-11 ■* • * J Flow No-8 m . Flow No- 5 Flow No 2 AI luvium Flow No %vl Flow No. Flow No -101-V__U-J 7 -1 Fig.7-L Geological map of the Karlakhind hill, Thai sector. - 8 N 50 0 Kms. ]8 > A 5' 70 > > z LO m > 18 lo ) M $ r v " 1. ; a , m s * '::m f ' M ud ■- v-_*. Beach sands Beach rock/Alluvium Basaltic flows Fig-2-5 Lithological map of Thai sector, based on aerial photographs. 15 of spectacular columnar jointings; tha columns being upto 40m»tres in height. Such columnar joints are present in the hills of Andheri, Ghatkopar and Borivli area of the Bombay sector (Plate 2.1, Photos 1 and 2).

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