19

Chapter –II

GEOLOGICAL STUDIES AND FIELD

CHARACTER OF BASALT FLOWS IN STUDY

AREA

INTRODUCTION:

The Sub-basin area is underlain by basaltic lava flows belonging to the Deccan

Traps of Upper Cretaceous to Eocene age alluvial pockets are observed along the major

river courses.

The entire Girja River sub-basin area 1398.07 Sq.km. is included in Deccan

Volcanic Provinces and comprises of volcanic formation. It is characterized by Deccan

volcanic Basalt. The multi basaltic lava flows are piled one above the other with

horizontal deposition. The lava flows generally be differentiated in two units viz. Massive

Unit (Non vesicular) and vesicular Unit. Vesicular and non vesicular flow unit may

alternate with each other at some places. The individual flows are generally separated by

a hydrothermally altered rock, very occasionally a green or brown coloured clay material

and some time red bole is also noticed and is known as green bole or brown bole. In

general, the area assumes step-like-topography or terrace appearance of outcrops. Almost

all the basaltic flows are flat with a gradient of 1 to 5 degree. The Deccan Trap covers

about 95% of total geographical area of Girja River basin of 1398.07 Sq.km.

Along the banks of river Girja and its main tributaries local shallow alluvial

deposits are observed in the form of patches consisting of sand, pebbles, boulders and

clay. The thickness of alluvial deposits varies from 10m. to 20m. They are recent to sub

recent overlying the Deccan Trap flow units. Deccan Traps consist of several lava flows

resting one above the other. However, the stratigraphic sequence of the flows over the

entire region occupied by them is not yet satisfactorily established. This is mainly

20

because diagnostic criteria to distinguish between different flows in the field could not be

determined. A single flow may show vitiation in texture, vesicle content, secondary

mineral assemblages in the vesicles, joints weathering etc. explain with geological

succession as flows.

Table No. 2 Geological Succession

Sr. No. Age Formation Lithology

1 Recent Soil Black Cotton Soil, kankar

2 Pliestocen Alluvium Alluvium comprising sand silt

etc.

3 Upper

Cretaceous to

Lower Eocene

Deccan Trap Basalt, red boles, lateritic and

inter trappean

Deccan Trap :

The term `Deccan Trap` was first coined by W.H. Sykes in 1833 after the Swedish

word Trapp/Trappa` meaning stair to describe the step-like or terrace-like topography

peculiar to the Deccan trap terrain. Deccan Trap is the second most extensive geological

formation in peninsular India, next only to the Archaean igneous and metamorphic

complex. It occupies large tracts between Kutch in the northwest, Bombay in the west,

belgaum in the south, Sarguja and Jashpur in the east and as far as Rajmahendri in the

southeast, covering parts of the States of Gujarat, Maharashtra, Madhya Pradesh and

21

Karnataka. The area occupied at present by the Trap is about 5,00,000 km2 but

considering the far off outliers from the present main mass the original extent could be

considered to be much more. Of this, two third areas are within Maharashtra. The present

day eastern margin of the main outcrop of the Deccan Trap in Maharashtra runs through

Nagpur in Vidarbha, east of Nanded, Aurangabad in Marathwada and east of Akkalkot in

southern Maharashtra. In the west the trap extends far beyond the west coast up to

Bombay High and even encountered in deep drill wells (Kaila, 1988). Thus, the western

extension could be considered to be perhaps delimited by a line joining Bombay High

and Ratnagiri off-shore in the Arabian sea. Thus, Maharashtra can be considered as the

home of Deccan Trap.

The total thickness of the Deccan Trap also varies in different parts because of the

undulating nature of the ground over which it is accumulated. The trap cover is thickest

along the west coast, near Bombay, where it is probably more than 2000m. The section at

matheran is over 850m. thick; at Mahabaleshwar in Sahyadri it is about 1700 m. The lava

pile in Western Ghat has a combined thickness of 2500 m. In the Melghat scarp north of

Achalpur in Amravati district the flows have a total thickness of 700 m. In the eastern

part of Vidarbha total thickness varies from 70 to 225 m. at Nagpur.

The nature of volcanic activity that produced the flows. Before doing this, however,

it is necessary to consider briefly how the field characters of flows will depend on the

nature of volcanic activity which produced them, and will therefore provide clues to its

nature. (Karmarkar 1974).

Lava flows are formed by the solidification of a liquid which has flowed out as a

sheet, and therefore normally have a form characterised by large lateral spread as

compared to thickness. However, the relation between lateral extent and thickness will

depend on the fluidity of the lava, the ratio, lateral spread/thickness increasing with the

fluidity of the lava. Also if the lava is fluid enough to flow freely its top surface will be a

horizontal plane. Therefore, though all lava flows will show a form more or less like that

of a sheet only flows resulting from fluid lavas will show regular sheet like form with

large lateral extent, comparatively large ratio and plane surfaces as tops. In an

undisturbed succession of flows of this type all flows will have horizontal plane surfaces

22

as tops and bottoms and will therefore show regular tabular form. Also, with an ample

supply of lava the sheets will attain considerable thickness and cover extensive areas.

On the other hand, if the lava is viscous, it will tend to pile up instead of flowing over

a large surface. The result will be a flow that, though elongated laterally, is thick as

compared to its lateral extent, and has low ratio. Such flows, because of the tendency of

the lava to pile up will have, instead of the regular tabular form of fluid flows irregular

forms such as lenticular and bulbous. The tops of such flows, instead of being horizontal

planes, will be quite irregular showing slopes in all directions because of pronounced

elevations and depressions. In detail also, the surfaces will not be smooth, but will show

irregularities typical of top surfaces of viscous flowing liquids. The tops of such flows

therefore may be ropy or billowy. As the nature of the bottom of a flow will be

determined by the nature of the surface on which it is outpoured the bottoms of such

flows in a succession will also be irregular.

If the supply of lava for each flow is also limited, the lateral extent of each flow

will be limited. By continued eruption of such flows will result a disorderly pile of small

flows having limited lateral extent, irregular form and tops and bottoms showing

conspicuous undulations and irregularities.

In the light of these considerations an attempt will now be made to determine the

nature of Deccan Trap volcanicity on the basis of the field characters of basalt flows

observed in the study area.

The most of the flows are of compact basalt which are thick and extensive having

high ratio. There are also a few flows of amygdaloidal basalt which have small thickness

and small lateral extent and low ratio. It means that these characters are governed by the

gas cavities.

The compact basalt flows are thick and extensive with high ratio and are always free

from gas cavities; whereas amygdaloidal flows have very much limited lateral extent and

low ratio and are vesicular, amygdaloidal for throughout their thickness.

23

Table No. 3 Average Composition of The Deccan Basalt Rock

Composition % Composition %

Sio2 50.61 Cao 9.45

Tio2 1.91 Na2o 2.60

Al2o2 13.58 K2o 0.72

Fe2o3 3.19 H2o+ 1.70

Feo 9.92 H2o- 0.43

Mno 0.16 P2o5 0.39

Mgo 5.46

Types of Flows Constitute The Deccan Trap Basalts.

The normal plagioclase-augite basalt is the predominant rock type, but olivine has

been altered to iddingsite occurring as reddish brown shining flakes, but many flows

contain olivine in fresh unaltered condition in abundance.

Completely aphanitic basalts are common. But porphyritic varieties with

phenocryst of plagioclase of different sizes are perhaps more abundant.

1. Compact Basalts (Aa Flow)

2. Amygdaloidal Basalt and Vesicular Basalt (Pahoehoe flow):

3. Tachylytic Basalt (Red bole)

4. Volcanic Breccia

5. Alluvial

1. Compact Basalts (Aa Flow):

24

Aa Flow a term of Hawaiian origin used in reference to basaltic lava that occurs in

flows with a fissured tough and jagged surface.

Compact basalt flows are always thick (fig 7) and extensive having tabular forms.

The thickness of flow varies from 2 meter to 60 meter. Its top surface is hydro-thermally

altered purple to green colour which is vesicular or amygdaloidal and free from jointing.

Due to jointing rock has dissected appearance. There may be variation in the pattern of

jointing and joint spacing. Joints may be closely spaced or broadly spaced and sometimes

may not be interconnected. (fig 8) Karmarkar B.M. (1974), Gupte R. B., et al (1974);

Kulkarni S. R. (1975); Gupte R. B. et al (1977); Marathe S. S. et al (1980)]

Top surface of the compact basalt flow:

The top portion of compact basalt flow is always vesicular, amygdaloidal,

unjointed and watertight in fresh condition. The vesicles are not interconnected and their

vesicular nature is obliterated since in the top portion of flow, most of these vesicles are

filled with zeolites and silica calcite. All these vesicles are independent from each other

and no interconnected therefore they do not provide avenue for the transmission and

circulation of the rain water.

Middle compact basalt flow:

The middle and lower portion of compact basalt flow is black in colour,

nonvascular, aphanitic or porphyritic in nature. It is in the dissected condition

25

26

due to development of joints which are contraction cracks formed during cooling and

consolidation of the lava. Variation in the jointing pattern is observed in the middle and

lower portion of the compact basalt flows.

Bottom portion of the compact basalt flows:

Immediately above the bottom surface of the compact basalt flow, a narrow strip

of the vesicles and the amygdales occur, which is parallel to the bottom of the flow.

Lithologically two compact basalts flows may be similar but there is distinct

difference in the jointing pattern. One flow may show broadly jointing, where as the other

may have closely paced or inconsistent jointing.

With appearance of weathered surfaces of joint blocks, to flows can be demarcated

from each other. The compact aphanitic basalt always developed smooth surfaces where

as porphyritic verity of basalts developed rough and pitted (appearance) surfaces on

weathering, because of dislodging of plagioclase phenocryst.

Giant-phenocryst basalts:

Normally the laths of plagioclase occurring as phenocrysts are only upto 1 cm in

length, but in some porphyritic basalts the laths are very large, from 2 cm to 10 cm in

length. Such basalts are called giant-phenocryst basalt, and are very common all over the

area. They actually are predominant rock types in some districts of Maharashtra. In some

of the giant phenocryst basalts there are granules of iddingsite indicating there phenocryst

of olivine were also formed.

Hydrothermal alteration:

Further variety is produced by the different degrees of hydrothermal alteration

produced in the basalts by magmatic gases. Fresh unaltered basalt have a dark blue or

black colour. The colour is changed by hydrothermal alteration to shades of brown, pink,

red, purple or green, and basalts showing these colours are widespread.

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2. Amygdaloidal Basalt and Vesicular Basalt (Pahoehoe flow) :

On the basis of the thickness and lateral extent amygdaloidal basalt flows are

mainly grouped into two categories viz. thin irregular amygdaloidal basalt flows having

maximum thickness upto 6 meters and lateral extent upto few kilometers and thicker

amygdaloidal basalt flows having thickness upto 100 meters and lateral extent upto few

kilometers and average thickness between 10 to 20 meters. (fig 9)

Some geologist terms this flow as ‘Compound Pahoehoe Flow’ (Nicholas 1936).

Both these flows are characterized by bulbous irregular form, rapidly pinching out, and

presence of ropy top surface and pipe amygdales along the bottom of the flow. However,

these are un-jointed when fresh and amygdaloidal throughout its thickness. Sometimes

middle and bottom portions of these flows are free from vesicles and amygdales in which

jointing is developed. Volcanic Breccia it is heterogeneous rock in which angular

fragment of different. Types of basalt are held together either in red Tachylytic matrix or

in zeolites matrix or in grey lava matrix. It has limited field occurrence in the terrain.

In the Deccan Traps vesicular basalts are rare as most of them have been

converted into amygdaloidal basalts by the filling of vesicles by secondary mineral. True

vesicular basalts with open gas cavities are rare and the amygdaloidal basalt are

widespread. As the vesicular basalt is unjointed fresh and compact they are quite

impervious.

i) Pipe amygdales

Pipe amygdales generally occur in a row along bottom of the Amygdaloidal basalt

flows which help in demarcating the junction between the flows.

ii) Top surface of the flow

The top surface of Amygdaloidal basalt flow is generally Tachylytic and reddish

brown in colour. Therefore in vertical cut the top surface of the flow is seen as reddish

brown shining line. As Tachylytic is unstable under atmospheric conditions, a groove is

formed the along the flow junction due to removal of Tachylytic material.

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iii) Ropy surface

On top of the Amygdaloidal basaltic flow, ropy and wrinkled surfaces are

observed. take figure from sir On the basis of thickness and the lateral extent,

Amygdaloidal basalt flows are mainly grouped into two categories, some scholars have

called these flows as compound, and Pahoehoe flows (Nichols, 1936). a) Thin irregular,

amygdaloidal basalt flows and, b) Thick, irregular, amygdaloidal basalt flows. a) Thin

irregular, amygdaloidal basalt flows: Their thickness varies from a few cm to 6 meter.

They can be traced laterally up to distance of 40 meter. Such small irregular flows are

Amygdaloidal through out there thickness. These are unjointed when they are fresh.

29

4. TachylyticBasalts :

30

Tachylytic basalts (fig.10) are basaltic glass formed due to rapid cooling and

chilling of the lava. It has very much closely spaced mutually perpendicular three sets of

joints. Tachylytic basalts have different shades of colour, such as red, green, brown and

black. However, red colour is very common. (fig 11 & 12)

They occur as thin band and pockets and lenses between the lava flows and also as

irregular, thin intrusion in the top portion of the flow, when Tachylytic bands occur

between the flows their thickness varies from a few centimeters to sometimes upto 3 m.

The peculiarity of the Tachylytic basalts is that, in confined condition, below the cover of

rock, it occurs as quite hard, red colour rock. But only on exposure to atmospheric

conditions, it disintegrates and crumples into powder like material due to opening of

joints. (A. V. Tejankar 2002)

7. Volcanic Breccia

Volcanic Breccias with Tachylytic matrix and grey lava matrix occur as water

tight rock. Therefore no rain water percolates through them. In volcanic breccias in which

rock fragments are held together by zeolites, some voids occur, therefore small quantity

of water may percolates through them.

As mentioned previously, middle and lower portions of compact basalt flows are

jointed. It jointed portions of Compact basalt flow are exposed at the surface; rain water

percolates through joints forming ground water. However, quantity of percolation of

water depends upon joint spacing and pattern of jointing.

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In closely spaced jointed basalt considerable quantity of rain water may percolate

through the joints. But if joints are broadly spaced limited quantity of water percolates

32

through them. Basalt having inconsistent jointing occurs as watertight mass even though

it is jointed. Joints generally open out at the surface but gradually, at the lower level, they

become tight and occur as only weak planes.

Weathering in Basalts:

Weathering is brought about by water containing oxygen and carbon dioxide in solution,

and rocks in which water can circulate freely are more quickly weathered. As water

circulates freely in the joint cracks, well jointed rocks are easily weathered.

Decomposition starts at the joints and gradually eats inwards. As a result closely spaced

jointed basalts show deep weathering. The Deccan Trap basalts commonly show

spheroidal weathering. On weathering the rock splits up in to concentric shells along

spherical surfaces with a core of fresh undecomposed rock at the centre.

Variation in thickness :

On the basis of the thicknesses measured in the field, it is observed that the thickness

of compact basalt flows varies from 3m to 54m. few flows have thickness varying from

3m. to 15m. of which 7 flows are of compact aphanites Basalt, some flows are of

compact porphyritic basalt and one flow is of giant phenocryst basalt. few Flows have

thickness between 16m. and 50m. in which very few flows are of compact aphanitic

basalt, few flows are of compact porphyritic basalt. Only two flows are much thicker

having thickness 50m. and

54m., in which some flow is of compact porphyritic basalt and other is of giant

phenocryst basalt. This indicates that there is a large variation in the thickness of compact

basalt flows.

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Alluvium:

Alluvial soil is formed by accumulated sediments transferred by the rivers and

rapids, thus, it is amongst the most fertile soils. It is a finegrained fertile soil deposited in

river beds or by water flowing over flood plains. They generally lack humus and

nitrogen. Alluvial soil is composed of alluvium deposits by the rivers. While slowing

down a river loses its potential to hold the large soil particles in a suspended state and

these particles thus settli down on the riverbed. Futher slowing down in the speed of river

enables smaller particles in the water to settle. Thus the river with its slow pace holds

only highly fine particals in suspension and these fine particles are lastly deposited at the

river`s mouth, where they form the Deltas comprising finely graine soil.

Along the banks of rivers, alluvial deposits occur in area as disconnected linear

patches of limited thickness. It comprises of admixture of gravel, intercalations of course

semi consolidated sand and clays, derived mostly from traps and resting directly over the

weathered or massive basaltic lava flows. Such patches are noticeable along Girja River.

The watershed wise Geology can be given as follows,

There are 07 watersheds In Girja river basin viz; GP-14-B, GP-15, GP-14, GP-7,

GP-20 GP-8, GP-3. All these watersheds have been critically examined with respect to

their geological and Geohydrological characters.

. While demarcating the basalt flows exposed in the study area the concept of

“basalt flows” is used as defined by Karmarkar (1974), who defined lava flows as, “a

mass of lava which, irrespective of size and shape, is seen in the field to have been

outpoured in a single uninterrupted eruption, which having come after the solidification

of at least the upper portions of the previous eruptions and not being followed by another

34

eruption before consolidation of at least its topper portions, can be clearly demarcated in

the field from overlying and underlying masses of lava.”

Geology of GP-14-B Watershed Area:

Total area covered by this watershed is 9764 Ha. It has minimum elevation of 525

m. msl. Near Jodwalsa village at the confluence of Purna and Girja rivers and its

maximum elevation 573 m.msl. This is comparatively low lying area of the basin. The

flows scaling the watershed are follows.

Flow No. 1:

This is compact aphanitic basalt, occurring in the well section of the low-lying

area of Hasnabad. The flow is observed from 573m to 576m. The lower part of the flow

shows closely spaced jointing and highly permeable. The top portion of the flow is

hydrothermally altered and has becomes amygdaloidal basalt by filling the vesicles (fig.

13) having thickness of 2.00m to 2.5m which show limited percolation due to less

jointing developed.

Flow No. 2:

This is jointed compact porphyritic basalt (fig. 14) is occurring in the well section

from560m to 575m having a thickness of 15m. The flow shows change in the jointing

pattern in the middle part is closely jointed and in the lower part, it shows broadly spaced

jointing and top portion is thick hydrothermally altered which clearly seen in dugwell(fig.

15).

Flow No. 3:

This is thin band of red Tachylytic basalt having 1 to 1.5m thickness (fig. 16). It is

seen at limited places in the well section of Northwest part of Latifpur and Northern part

of Pimpri village area. This play role as a better for downward percolation of ground

35

water. Thickness of red Tachylytic basalt is not uniform at some places it is zero. Which

is suitable area for downward percolation.

Flow No.4:

It is compact porphyritic basalt with small to medium sized plagioclase phenocryst

which are white in colour. The lower and middle parts of the flow from 568m to 582m

show favorable conditions for the percolation.

The top portion of this flow has become hydrothermally altered and

amygdaloidal which is watertight.

36

37

Flow No.5:

38

This flow starts appearing along the road level. It is compact porphyritic basalt

with small plagioclase phenocryst which are crowded in the flow. On weathering due to

removal of thesephenocryst, the flow has acquired very rough and rugged appearance. It

show broadly spaced jointing and has developed spheroidal weathering. The top portion

of this flow has become purple and vesicular amygdaloidal with large amygdales filled

with silica and zeolites. From there are no outcrops of the flow as it is canceled under soil

cover. The road rises above this flow

Geology of GP-15 Watershed:

GP-15 watershed covers in area of 11050 Ha. It has minimum elevation at Padila

587m.s.l. and maximum elevation at Pirboda 641m.s.l. The area falls in the terrain of

Deccan Trap basalt consisting of two major types of basaltic flows viz. compact basalt

and amygdaloidal basalt, compact basalt is a thick and extensive flow. It may be

aphanitic or porphyritic in nature. Jointing pattern in the compact basalt plays important

role in the percolation of water. But jointing pattern shows variation even within short

distances. Joints may be inconsistent, broadly spaced or closely spaced. Top of the

compact basalt is almost always hydrothermally altered and becomes amygdaloidal.

The Volcanic Breccia present in this watershed is in highly weathered condition

with the development of sheet jointing throughout its thickness therefore percolation

along the sheet jointing has been observed. The percolation is lateral through the

weathered zone and not within the volcanic breccia.

Flow No. 1:

This is unjointed amygdaloidal basalt (fig. 17) in which fine to medium size

amygdales are filled with zeolites and silica. The flow is fresh, hard and massive

amygdaloidal basalt flow hence impermeable. It is observed in Girja River.

Flow No. 2:

39

This is thin compact porphyritic basalt (fig. 18). Broadly spaced jointing has

developed in the flow. The joints at some place are interconnected permitting limited

percolation. At other places joints are not interconnected hence permeability is very less

as in case of Southwest part Goshagon. Hence dugwell in this area commonly dry and it

is water available deep dugwell which very limited which is not used for irrigation.

Flow No. 3:

It is compact aphanitic basalt. With broadly spaced jointing pattern. Its top portion

hydrothermally altered amygdaloidal top portion which is watertight. In general the flow

shows poor percolation. Dugwell of this area are dry or low yielding.

Flow No. 4:

It starts appearing form along the road level. It is black compact porphyritic basalt,

with small plagioclase phenocryst scattered throughout the flow. In the lower portions of

this flow horizontal jointing is prominently developed. The top portions of this flow have

become vesicular and amygdaloidal. Due to steep gradient the road rises above this flow.

40

Geology of Watershed GP-14:

41

GP-14 watershed spans an area of 8284 Ha. It has minimum elevation at 550 m.s.l.

BorgoneArj and maximum elevation at 630m.s.l. Aland. The Aland area falls in the

terrain of Deccan Trap basalt consisting of two major types of basaltic flows viz. compact

basalt and amygdaloidal basalt, compact basalt is a thick and extensive flow. It may be

aphanitic or porphyritic in nature. Jointing pattern in the compact basalt plays important

role in the percolation of water. Jointing pattern shows variation even within short

distances. Joints may be inconsistent, broadly spaced or closely spaced.

In the well at BorgoneArj, top portion is hydrothermally altered Amygdaloidal

basalt and below it is compact Aphanitic basalt. Compact aphanitic basalt is fresh and

undecomposed. Both these rocks are impermeable because of which there is no

groundwater in the well.

Flow No. 1:

This is compact porphyritic basalt, (fig. 19) occurring in the wells section of the

low-lying villages such as pimpri and shivta. The flow is observed from 570m to 576m.

The lower part of the flow shows medium spaced jointing and is permeable. The top

portion of the flow is hydrothermally altered and has becomes amygdaloidal having

thickness 2.00m to 2.5m which is impermeable as seen in the well section.

Flow No. 2:

This is amygdaloidal basalt with medium size amygdales filled with zeolites and

quartz. Chlorophaeite is abundant occurring in the flow and the flow shows high degree

of weathering in the south and Western part of the area in top portion of flow.

Flow No. 3:

This is a thin band of red Tachylytic basalt having 1 to 1.5m thickness. It is seen

only at limited places in the well section of Northwest part of Satala and Northern part

Aland.

Flow No. 4:

42

It is compact porphyritic basalt with small to medium sized plagioclase

phenocryst. The lower and middle portions of the flow from 568m to 582m show

inconsistent jointed pattern (fig. 20) and joints are not interconnected. The top portion of

this flow has become hydrothermally altered amygdaloidal and is watertight.

Geology of Watershed Gp-7:

GP-7 watershed encompasses and area of 20529 Ha. total area It has minimum

elevation at Pal 618m.s.l. and maximum elevation at Nidhona 685m.s.l. Nidhona village

is surrounded by hillock. It is having an altitude of 685 m depicted in topo-sheet no 46P/8

which shows different basaltic flow.

Geology of the area: the area falls in the terrain of Deccan Trap Basalt consisting

of two major types of basaltic flows viz. Compact Basalt and Amygdaloidal Basalt.

Compact Basalt is thick extensive flow. It may aphanitic or porphyritic in nature.

Amygdaloidal Basalt is a massive homogeneous rock which is impermeable in

nature. It becomes permeable due the development of sheet jointing which is an

intermediate stage of weathering. It also shows limited permeability when the amygdales

are scanty and the rock develops broadly spaced jointing pattern. Because of all these

variations there are limitations in the occurrence of groundwater.

43

Flow No. 1:

44

It is a compact, aphanitic thick basaltic flow which is unjointed and show smooth

appearance on the surface due to glassy nature flow is very thick having thickness of

about 15mt. (fig. 21) Rock is hard compact unjointed and fresh.

Flow No. 2:

It is compact, porphyritic basalt with medium sized (about 2.9mm) plagioclase

phenocryst. Due to removal of plagioclase phenocryst on weathering the rock has

acquired rough and pitted appearance. In the flow, jointing is not consistently developed.

At the bottom flow shows closely spaced jointing whereas, in the middle part broadly

spaced joints are developed. In the weathered portion of the flow spheroidal weathering is

developed and large sized spheroids are seen protruding out of the weathered surrounding

rock (fig.22).

Flow No. 3:

The fresh exposures show that the flow is compact, porphyritic basalt with

medium sized (3.4 mm) plagioclase phenocryst. Phenocryst have fallen down from the

ground mass due to weathering and rock has acquired pitted and rough appearance. The

top portion of the flow is vesicular, amygdaloidal and has become purple due to

hydrothermal alteration.

45

Flow No. 4:

46

It starts appearing along the road cut. It is compact basalt showing broadly spaced

jointing. The flow is covered by soil and thick vegetation. In this stretch weathered

portion of the flow is fresh and unjointed. (fig. 23)

The top portion is vesicular amygdaloidal and has become purple due to

hydrothermal alteration. Top surface of the flow is very irregular and it abruptly goes

down below the road. Thickness of the flow is 19m.

Geology of Watershed Gp-20:

GP-20 watershed has an area of 27637 Ha. having minimum elevation at Latifpur

574m. and maximum elevation at Dhabadi 596m. The Dhbadiarea falls in the terrain of

Deccan Trap basalt consisting of two major types of basaltic flows viz. compact basalt

and amygdaloidal basalt, compact basalt is a thick and extensive flow. It may be

aphanitic or porphyritic in nature. Jointing pattern in the compact basalt plays important

role in the percolation of water. But jointing pattern shows variation even within short

distances. Joints may be inconsistent, broadly spaced or closely spaced.

Flow No. 1:

The lower portion of this flow is clearly exposed in dug well. It is compact

porphyritic basalt with medium sized plagioclase phenocryst. The flow has developed

closely spaced jointing.

The amygdaloidal and purple due to hydrothermal alteration, is the top portion of

the flow. In this top portion of the flow thin, almost vertical, red

47

Tachylytic intrusions are seen, which are feeding the overlying flow No.-2. (fig. 24)

48

Flow No. 2:

It is a green and red Tachylytic basalt flow.It shows limited lateral extent only in

few dug well. It pinches out on both sides of the well its maximum thickness of is 1 m.

Flow No. 3:

This flow starts appearing on flow No.-2. It is compact aphanitic basalt with

medium sized, milky white plagioclase phenocryst. It has developed broadly spaced

jointing in which vertical joints are prominent. Its thickness is 30m. (fig. 25)

Geology of Watershed Gp-8:

GP-8 watershed has an area of 27825 Ha. having minimum elevation at Pathri

615m.s.l. and maximum elevation at Chawka 693m.s.l. Chawka is depleted in topo-sheet

no.46P/8 and its altitude is 693 m. It shows the variation in basaltic flows. There are

volcanic vents, having the diameter about 2 to 3 m. Exposed on the road cut. Near village

the vent rock is volcanic breccias with angular fragments of varieties of basalts of

different sizes and shapes, held together by Zeolitization. The vent is surrounded radial

jointing developed around the vent.

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Flow No. 1:

50

At the lower level in the nala bed, the lower portion of this flow is clearly exposed

in near nala. It is compact porphyritic basalt with medium sized plagioclase phenocryst.

The flow has developed closely spaced jointing. (fig. 26) show suitable recharge

condition.

Flow No. 2:

It is a red Tachylytic basalt flow. It shows limited lateral extent. It pinches out on

both sides of the cut; its maximum thickness is 1 m. (fig . 27) This flow observed in very

few dugwell.

Flow No. 3:

This flow starts appearing above flow No.-2. It is compact porphyritic basalt with

medium sized, milky white plagioclase phenocryst. It has developed closely spaced

jointing in which vertical joints are prominent. Its thickness is 30m. (fig . 28)

Flow No. 4:

It starts appearing from the top of green Tachylytic basalt i.e. flow No. 4. It is a

black compact porphyritic basalt with small plagioclase phenocryst. At places vesicles

filled with green chlorophaeite are occurring in this flow.

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Flow No. 5:

52

This flow starts appearing along the Pal road level. It is compact porphyritic basalt

with small plagioclase phenocryst which is crowded in the flow. On weathering due to

removal of this phenocryst, the flow has acquired very rough and rugged appearance. It

show broadly spaced jointing and has developed spheroidal weathering. The top portion

of this flow has become purple and vesicular amygdaloidal with large amygdales filled

with silica and zeolites. From there are no outcrops of the flow as it is canceled under soil

cover. The road rises above this flow

Geology of Watershed GP-3:

Miasmal is a height elevated portion in Ajanta ranges having maximum elevation

903m. It has been depicted in topo-sheet no. 46p/4 with latitude 20004’50 longitude is

75011’15 and altitude 903.m. The lowest altitude is at Bazar Sawaing 662m.s.l. There are

discontinuous patches of thin, irregular, amygdaloidal basalt flows. However, as the

major part of the hill is nearly on flat terrain, the individual thin irregular amygdaloidal

basalt flows, have not been demarcated and numbered separately. (fig. 31) In the actual

ascent of the Ghat and along the road cats on the top of thin irregular amygdaloidal basalt

flows, a flow of green, Tachylytic basalt flow is exposed for a very short distance.

(fig.32)

Flow No. 1:

It is black, compact porphyritic Basalt with medium sized plagioclase phenocryst.

These phenocryst are scattered throughout the flow at some places along with the

plagioclase phenocryst some copper red colourediddingsite flakes altered after olivine are

seen. At some places in this flow towards the top, horizontal stringers of vesicles are

seen. In the lower portions, the flow shows broadly spaced jointing with roughly three

sets of joints which are almost perpendicular to each other. Along these three sets of

joints, spheroidal weathering has developed. (fig . 29)In the middle portion. In these

portions the number of phenocryst has become smaller and the flow appears to be

aphanites basaltic.

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In the top portions of this flow, green and red, nearly vertical and steeply inclined

thin, intrusions of Tachylytic basalts are seen. The top portion of this flow has become

hydrothermally altered, purple in colour vesicular amygdaloidal with medium sized

amygdales filled with chlorophaeite and heulandites.

Flow No. 2:

It is a compact porphyritic basalt with small plagioclase phenocryst which are

crowded in the flow. On weathering due to removal of thesephenocryst, the flow has

acquired very rough and rugged appearance (fig.30). It show broadly spaced jointing and

has developed spheroidal weathering. The top portion of this flow has become purple and

vesicular amygdaloidal with large amygdales filled with silica and zeolites.

Flow No. 3:

It is black compact porphyritic basalt, with small plagioclase phenocryst scattered

throughout the flow. In the lower portions of this flow horizontal jointing is prominently

developed. (fig .33) The top portions of the flow have become vesicular and

amygdaloidal.

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55

Flow No. 4 to 11:

56

It is composed of numerous thin, irregular, amygdaloidal basalt flows. Their petro

logical description is given along with the sketch. Similarly five flows (7 to -11)

The junction between these thin, irregular, amygdaloidal basalt flows and above

lying compact porphyritic basalt flow No.-12 is very irregular. The junction is 2 m. above

the road level but within a distance of 25 m. the junction nearly comes to the road level

and again it goes up by 3 m. at a higher level. Above the road with a distance of 20 m.

flow No. -12 is exposed which is compact porphyritic basalt with small plagioclase

phenocryst. The flow is broadly spaced jointed in the bottom portions. At higher level it

has become closely spaced jointed. (fig . 34)

Flow No. 12:

On the top of flow No. 12 there occurs a thin flow of green Tachylytic basalt

which is 60 cm. thick. It pinches out within a short distance.

Flow No. 14:

It starts appearing on the top of green Tachylytic basalt i.e. flow No. -13. It is a

black compact porphyritic basalt with small plagioclase phenocryst. At places vesicles

filled with green chlorophaeite are occurring in top portion of flow which is

hydrothermally altered amygdaloidal basalt. (fig . 35)

57

58

Flow No. 15:

59

The flow is thin green Tachylytic basalt having 30 cm. thickness. It pinches out

within a short distance. (fig. 36)

Flow No. 16:

It starts appearing in the red Tachylytic basalt, (fig.37) flow No.-15. This flow

No.-16 is compact porphyritic basalt. The top of the flow is very thick exposed in the

road cut showing deep weathering.

Flow No. 17:

It is exposed on the top of flow No.-16 with pipe amygdales at its base. On this

flow an extensive plateau is developed on which rest house is located where the traverse

ends. The flow No.-17 is a back, compact, porphyritic basalt. It shows closely spaced

jointing. (fig . 38)

60