Chapter 3 Geology of the Area Chapter 3 Geology of the Area In order to understand the lithologic input to the soils, knowledge on bedrock geology is most essential. This chapter therefore deals with the regional to local geology of the study area (Fig. 3.1). The study area essentially lies in the regional extent of the Deccan traps covering almost entire state of Maharashtra and the west central part of India. It lies between N latitudes 15"45': 22"0', and E longitudes 43"()": SO^SQ' and can be physiographically divides as below: 1. The Maharashtra Plateau 2. The Western Ghats 3. The Coastal region of Konkan 3.1. The Maharashtra Plateau It extends from the eastern border of the state, through Vidarbha and Marathwada regions upto the Ghats covering about 75% of the state. The region is more or less an upland plain and is traversed by almost east-west or northwest to southwest trending river valleys and hill ranges with more or less similar trends (Fig. 3.1). The eastern region covering Bhandara, Gadchiroli, Nagpur districts is occupied by narrow alluvial plains of Painganga and Wardha rivers with the maximum altitude of 300 m. The alluvials of Tapi River valley, with east-west trend and maximum altitude of 300 m above MSL occupies the northern part of the Amravati- Akola and Dhule districts of Khandesh. The remaining part of Plateau is occupied by the valleys of the Godavari, Bhima and Krishna rivers. The altitude of these plains ranges from 300 to 600 m above MSL. In general the altitudes of the plateau increases from east to west. There are some hill ranges in this area which are connected to the Western Ghats on the west. Their altitude ranges between 600 to 900 m above MSL. 3.2. The Western Ghats These constitute the ranges of Sahyadri Mountains. Trending almost north-south they arc parallel to the west coast of India along the entire western border of Maharashtra. They 26 4= H cS o u s-< Cu u M c3 ^O t/i ^o Lo. <u A+-» T3 o -(-» -o "e3n c •5 § O <u o9> 5 o ^o c o ? <+- 0 o c; <D XI crt O> nS u X! "3 S' o '3) s% L> e* ^^/a c •? nj 4o3 CO CS ts 43 «cd ^ 4= cS O o s are about 1600 km in length with average elevation between 1000-1300 m. Among all the ranges Kalsubai is the highest peak in the Sahyadri mountains with the maximum elessvation of 1650m. The Sahyadri is an area of spatial elevation having been formed by tectonic processes of the western margin further modified by the differential weathering and erosion. The ranges are composed of piled one over the other with individual flows varying in thickness from a few meters to upto 36m. Some major rivers of Maharashtra have their source in the Sahyadri whereas some of the tributaries of these rivers originate in the east-west trending ranges. 3.3. The Coastal Region of Maharashtra The narrow strips of land west of Sahyadri upto the Arabian Sea or the west coast of India is the coastal region or the Konkan coast. There are small and narrow plateaus at different elevations in this region at places covered by the laterites. The coastal region display various geomorphic features including the rocky and sandy beaches, wave cut platforms, uplifted terraces, dunes, creeks and mangroves swamp. Drainage The drainage system of Maharashtra can be divided into three parts viz. Konkan, Central Maharashtra and eastern Maharashtra systems. The Sahayadri forms the major divide for the drainage system in the Konkan and Central Maharashtra regions. The rivers in the Konkan region flow in the west direction whereas the rivers in the central Maharashtra flow in the eastern direction towards the Bay of Bengal (e.g., Godavari, Bhima and Krishna rivers). Godavari is the largest and longest river in the Indian Peninsula, with a total length of 1500 km, about 500km of which lies in the Maharashtra. The Bhima River originates from the Bhimashankar in Sahyadri. Bhima basin lies between the Mahadeva hills in south and Balaghat range in the north. The Krishna River rises in Sahyadri near Mahabalcshwar and it occupies the southern triangular portion of the state. In contrast to these major rivers, the Tapi River, which flows in the east-west trending basin occupy the north central part of the state and debouch in the Arabian Sea. The rivers in 27 the eastern part oF Maharashtra have their courses in ahnost N-S direction. The major river in this area is Vainganga. Climate Weather conditions in the Maharashtra are variable. Three distinct seasons are recognized viz. summer, rainy and winter. In the summer the highest temperature ranges between 40" and 48° C over the greater part of state. In the Konkan region the weather is hot and humid. In Ghats, temperature is comparatively low. In the region east to the Ghats, weather is mostly hot and dry in the Vidarbha region and at some places like Akola, Nagpur and Chandrapur maximum temperature reaches upto 48" C. In the rainy season from June to October, the state receives rainfall from SW monsoon. The rainfall in the state varies from 500 mm in the central semi-arid part to over 6000 mm in the Western Ghats. The coastal region receives 2000-3000 mm annual rainfall, the Western Ghats zone receives 4000-6000 mm, the central arid part receives 450-550 mm and the Vidarbha region receives 1200-1500 mm. The central zone has been identified as the drought-prone areas in the state. In the winter the weather is generally pleasant with the maximum temperature reaching upto 30"C and the minimum temperature of 4"-5"C. Soils Major part of the Maharashtra, right from the Western Ghats up to Nagpur, is covered with the black soil or "Regur' which is the ultimate product of the weathering and erosion of the Deccan Traps. It is black in colour, porous and swells when water is added and on drying contracts producing cracks. In Konkan region and the Western Ghats, soil is mainly lateritic. It is produced from laterite which is the alteration product of basalt. In the eastern part of Vidarbha region the soil is usually reddish generally formed by the alteration of granitic, gneissic, and other metamorphic rocks underlying this region. Apart from this, adjoining the Tapi River, a thick sequence of Late Quaternary alluvium is observed. The borehole data indicate that the thickness of this soil is more than 200m. Bank failure and gulling in the alluvium track is common during large floods and this has given rise to "Badland" topography. 28 3.4. Regional Stratigraphic Distribution The oldest rocks having an age of about 3500 Ma are mostly biotite gneisses. These form the basement over which the younger sediments were deposited and metamorphosed. They were involved in several orogenic cycles and hence are intensely deformed and mixed up. The unclassified basement gneisses are overlain by the meta-sediments belonging to Amgaon, Sakoli and Sausar groups (Table 3.1). They are represented by different types of schists, gneisses, marbles, amphibolites and hornblende schist's with intercalated bands of iron and manganese ore. All these rock fonnations are then intruded by the doleritic dykes. After the intrusive activity there was a long period of quiescence during which the Archean rocks were subjected to weathering. This period is recognized as Eparchaean unconformity or period of no sedimentation. In Maharashtra, Proterozoic rocks are exposed in Yavatmal, Chandrapur, Gadchiroli and Ratnagiri districts in the fonn of isolated outcrops. During the period between Cambrian and upper Carboniferous (ca. 300 Ma) there is no record of any sedimentation in the southern India, this may be because this part of Indian Peninsula was concealed below ice cover, being the part of the giant continental mass called Gondwanaland. After the cessation of sedimentation of the Gondwana basin around 200 m.y. ago a marine incursion marked by the deposition of Bagh and Lamela beds occurred in parts of Nagpur and Chandrapur districts. The major event after this was during the upper Cretaceous duuring which there was voluminious outpouring of lava through extensive fissures. The eruption was mostly silent, not of explosive type and the lava spread over extensive areas in the form of horizontal (lows of varying thickness. Hundreds of such flows are found piled one above the other and some individual flows can be traced for several hundred kilometers. The flows occupy almost the entire area of Maharashtra expecting some parts of Nagpur, Vidarbha region, and Ratnagari and Sindhudurg districts of Konkan. The lava flows of Deccan traps are intruded by the number of dolerite dykes which shows almost N-S trend parallel to the western coast. The last episode in the geological history of the state was the formation of the laterite and the older and younger alluvia in Kolhapur, Satara, Sangli, Kolaba and Thane district and some districts of eastern Maharashtra. A concise sequence of the stratigraphy in the Maharashtra region is given in Table 3. Table 3.1. Stratigraphic sequence in the Maharashtra region (Deshpande, 1998) Age Stratigraphic Representative in ml Geographic Distribution Sequence Rock-Formation years Younger and older alluvium in Nagpur, Bhandara, Chandrapur, Wardha, Yavatmal, Alluvium, Akola, Thane, Kolhapur, Satara, Sangli Pleistocene- Latcrite, <2.5 districts. River terraces of Vainganga and Recent Sand, Soils. Painganga rivers, raised beaches along west coast. Miocene- Tertiary sediments, 2.5-23.5 Ratnagari and Sindhudurg district. Pliocene lignite, shales. Basalt flows cover most of the state from the Nagpur upto the Arabian sea coast Dcccan trap basalt ilows excepting in the eastern parts of Nagpur, Eocene-Upper with intertrappeans and Bhandara, and Ratnagari districts.