GENERAL

GEOLOGY

[The section on Geology has been contributed by Shri S.S, Ghodke, Geologist (Sr.) of the Geological Survey of India.]

The district of Buldhana provides little of geological interest except for the unique occurrence of picturesque ' Lonar crater ' the only such in the great basaltic province of India. Though several prominent workers have been associated in some way or other with the Lonar lake investigation work since early 19th century, the geology of Buldhana district as a whole had been studied only in a cursory manner, and except for Wynne's account (1867) on the Purna valley and some reports on the damsite and ground water investigations, by the Geological Survey of India carried out in recent years, no detailed account of the geology of Buldhana district is available.

By far the greater part of the district is covered by rocks of the Deccan volcanics of Creteceo-Eocene age, and a few alluvium patches of the Purna and Penganga basin, respectively. The trap rocks are usually fine to coarse-grained, dark grey to greenish-black basalts of vesicular and massive types (Venkatesh 1967). The hard compact massive flows are generally noticed on the hill tops, e.g., Melghat section whereas comparatively soft and amygdular varieties usually occupy the flanks of the hill or valley floors. Spheroidal exfoliation is a characteristic feature of weathering in the traps. Besides vertical and inclined jointing, columnar jointing is also well seen in more massive types. The vesicular and non-vesicular flows are at places separated by thin beds of ash or scoriae, but typical inter-trappean sedimentary rocks have not been recognised in the area. The amygdular varieties of flows carry secondary minerals like zeolites (mostly heulandite), calcite and chalcedony. No dykes have been found associated with the trap flows in the district where a lava pile of approximately 800 metres is preserved.

Alluvium.—The Purna valley alluvium occupies an extensive stretch of low lying ground between Paturda and the confluence of Purna river with that of Tapi in Jalgaon district. In the river valleys and where superficial rain-wash has accumulated, a mixture of black cotton soil associated with sub-recent conglomeratic formation or light brown laterite material is noticeable at places, but otherwise, as aptly described by Wynne (1867) there is little variation in the nature and extent of soil or any variety of geological interest. The alluvium of the plains is usually of considerable depth sometimes, exceeding upto 50 metres as noticed near Pimprala or Malkapur. At places in the river alluvium, calcareous fragmentary bones or teeth ruminants are found sporadically. Much on the alluvium produces effervescence of sodium salts. Majority of the wells sunk in the area have brackish water.

Lonar lake area: The Lonar [For details see section on Lakes.] lake is one of the few large isolated caldera-like depression that has given rise to a good deal of controversy regarding its origin. A number of scientific accounts about the lake and its alkalinity have been published since early 19th Century, but all these accounts are based on the data collected from surface examination of the lake area only. More recently the prospecting department of Tata Iron and Steel Company Limited had carried out drilling operations in the lake area (1960) with regard to the evaluation of soda contents in the brine and silts of the lake, whereas some preliminary geo-physical and geo-chemical surveys had been undertaken by the Geological Survey with a view to ascertaining the possible nickel and cobalt contents, if any, in the Lonar crater formation (1964).

The Lonar structure variously described as a hollow, depression or crater, is situated about a kilometre to the south-west of the village of Lonar (19° 58'45"; 76° 34'00"), and the circular feature measuring 2,000 metres across and about 135 metres in depth has a shallow saline lake. Maximum depth of the brine is about 5.5 metres. The general elevation of the surrounding country is 550 to 600 metres above mean sea level and the depression has fairly steep walls sloping at an angle of 25°—33°. A raised rim or bank, up to 100 m in width and at places 10 to 15 m high surrounds the hollow. This inland lake with no effluent is fed by the seasonal drainage mainly confined to its periphery and also by a number of springs such as Dhar, Sitanahani, Ramgaya, etc.

The country rock is andesitic lavas (Nandy and Deo, 1961) of the Deccan trap facies of late crataceous period. The rock is usually cryptocrastalline and fine to medium-grained, generally containing amygdules of agate, green earth and zeolites. Intensive weathering has rendered these trap rocks very friable at some places. Small pockets of ash beds, have been reported from the cliff sections, but intertrappean sedimentary rocks have not been recognised so far in the area. Rocks at the periphery of the lake show quaquaversal dips varying from 14°—27°. However, faulting, intense shattering and brecciation, shatter cones, dykes, agglomerates, fused or glassy material of any kind, etc., have so far not been reported. The rocks at the floor of the lake, as found from bore hole cores, are similar in composition to those exposed in cliff sections (Nandy and Deo, 1961). A generalised vertical section of the lake as revealed by the deepest bore hole No. 10 of Tata Iron and Steel Company Limited (1961) from top downwards is given below: —

The remarkable shape, size and lone occurrence of this structure have evoked controversies regarding its origin and salinity. Various theories so far proposed can be grouped generally in two schools of thought viz., one ascribing the origin to a crater of an extinct volcano (Bradley, 1853, G. Smith, 1857, Blanford, 1870, R. D. Oldham 1906, etc.) and the other suggesting that the depression was caused due to the collapse of the roof of a large ' blister ' elevated by the inrush of vapour or molten lava from below (Orlebar, 1839, Newbold, La Touche, 1912, etc.).

V. Ball (1881) and Smythe (1884) offered an entirely new theory of collapse due to solution of underlying strata. However, the hypothesis of La Touche (1912) is more or less similar to the now well-known rationale for explaining the ' cryptovolcanic' structures (Venkatesh, 1964 P. 10). In recent years, encouraged and inspired by the increasing evidence of meteorite impact for the Barrington crater of Arizona, USA and also lunar craters, there has been considerable enthusiasm to ascribe several of the cryptovolcanic structures to meteorite impact and Dietz (1959) further suggests that most of the structures of doubtful origin could be covered by the term ' cryptoexplosion ' structures or ' astroblems '. Large craters attributed to impact and explosion of meteorities involve a huge mass of extraterrestrial matter striking the earth's surface at very high velocities, resulting in relatively greater concentration of nickel, cobalt and other metals in which the iron meteorities are very high (Ehman, 1962, Taylor and Kolbe, 1965, Ginsburg, 1960). In the Lonar area, where conditions are most favourable for high concentrations of the metals to occur, the geo-chemical samples show only normal metal values which is considered, prima facie, inconsistent with a meteoritic origin for the crater (Venkatesh, 1964). Since the geo-chemical approach had so far not provided any reliable criterion for recognising meteorite crater, like Lonar (Venkatesh, 1967) or Ganyon Diablo in Arizona (Hawkes, 1966), perhaps new data in other established lines of research would be necessary to arrive at some reasonable and acceptable solution on the origin of this Lonar depression, the only of its kind in India.

Salts in the Brine and Silt.—In a closed basin like Lonar, it is not unnatural for the accumulation and concentration of some saline constituents in the lake water. The Lonar lake water is characterised by a high concentration of chlorides, carbonates and sodium with Sio2, R4, S2, O3, So4, Ca, Mg and K in minor amounts, besides traces of boron, as shown below:—

ANALYSIS OF LONAR LAKE WATER

The water of the Lonar lake for its carbonate and chloride contents has been classed by Clarke (1924) under the group ' carbonate-chloride '. The origin of the salinity of the lake has also been subjected to some controversy and Christie (1912) has summarised these various theories regarding its alkalinity. Amongst the earliest observers, Malcolmson (1840) accounted for the occurrences of sodium carbonate as a result of an interaction between sodium chloride, calcium carbonate and water. This view was more or less supported by Smith (1857), Mackenzie (1870) and Plymen (1909); whereas Blanford (1870) believed that evaporation of the water in the absence of any exit was responsible for the alkalinity of the lake water. Christie (1912), supporting Blanford's theory, however opined that water and unlimited supply of carbonic acid from atmosphere are directly responsible, besides some part played by the atmospheric agents, viz., wind and rain, to carry windborne salt from sea to the lake water. More recently, Jhingran and Rao (1954) suggested that there must be some juvenile springs in the area, a fact borne out by the presence of traces of boron in the lake waters, which admixed with varying meteoric water would account for the chlorine and sodium salts in the lake. In thier opinion, the alkalinity of the lake is the result of simultaneous operation of a number of reactions, the chief of them being the conversion of sulphate to carbonate through the intermediate sulphide formation and the concentration of lake water. Tata Iron and Steel Company's recent drilling operations in the area (Nandy and Deo, 1960) indicate that the brine of the lake shows practically the same composition laterally and at depth. Comparative analysis of trap rock, sprung water, brine and silt from Lonar area are given below:—

(Jhingran and Rao, 1954) (Nandy and Deo, 1960).

Based on the drilling data, Nandy and Deo (1960) suggest that the alkalinity of the lake is due to the meteoritic waters feeding the lake. These influents while passing through the volcanic rocks, rich in sodium salts, dissolve some of these salts and bring them to the lake. Evaporation and interactions of these salts in the lake water aided by the inorganic compounds and enhanced by the atmospheric agents and bacteral action, have resulted in the concentration of salts found today in this closed basin.

Economic Geology.—The district has not been endowed with any economic mineral deposits, excepting some minor activity with regard to the exploitation of Lonar lake soda deposit. The evaporates formed from the lake water are extracted and sold into five different types depending on the stages at which they separate out in the evaporation process and the relative percentages of respective carbonates and alkalies. These products differ widely in composition, as seen from the statement below. The Lonar soda deposit is, however, considered to be similar to the soda lakes of the world and found to correspond to the ' Trona' or straited soda (NaCo3. NaHCO3 2H2O) from the lakes of Fezzan in Tripoli (Jhingran and Rao, 1954).

The natural occurrence of bicarbonate of soda in such large quantity with common salts is of great economic interest. In ancient times, there was a local demand for the alkaline lake products as well as for the salt. In recent years the possibility of reviving this trade by establishing a large scale consorted industry at Lonar was again examined by the Tata Chemicals Ltd., but owing to the limited extent of this deposit and for a number of other reasons this scheme has so far not been implemented.

The dense, massive basalts, which occur in extensive quantity, are good as constructional or building material.

Ground Water.— Buldhana forms the westernmost district of Vidarbha with an average annual rainfall of 70 to 85 centimetres. Excepting for the east-west stretch of the western extremity of the fertile Purna valley in the north-central portion of the district, the entire district is occupied by the trappean hill ranges of Gawali-garh in the north and the Ajanta in the south with a general depression towards the valley of Purna from either side. In the northern portion of the valley belt an east-west stretch extending for over 30 km, with an average width of 6 km is the potable fresh water belt with a discharge of 8 to 12 litres per second within a depth range of 40 metres. In the southern part of the valley south of the river Purna there is acute scarcity with the river alluvium; but the major part of the alluvial portion of the district is generally free from it. In the basaltic terrain the groundwater occurs partly in the western mantle and partly in the vesicular basalt and intertrappean beds, and with lack of favourable conditions there is often acute scarcity felt, particularly in peak summer months. The Lonar crater lake is a saline water lake and there is also some amount of salinity in the direction of the Purna saline tract.

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