GENERAL

GEOLOGY

[The section on Geology has been contributed by Shri A. R. Sawarkar and Shri S. Majumdar, Geologists (Junior) of the Geological Survey of India.]

No systematic geological mapping has yet been carried out in this district. The information available is only by passing references made to the local geology of some proposed dam sites and groundwater survey work pertaining to the same.

The district as a whole is monotonously covered by Deccan Trap basaltic lava flows, which, in turn, are covered by a thin mantle of soil almost everywhere. These flows, on account of differential weathering, give rise to undulating relief. There are no prominent hill ranges in the district and the region is characterised by typical Deccan trap geomorphology. The fine grained dark grey basaltic flows constitute the high country while the weathered vesicular and zeolitic basalts generally constitute the valleys in the area. The basalt of the district is just a part of the vast expanse of the Deccan lava flows which occupy around 5,18,000 square kilometres of the western and central parts of the country. The traps in the district probably represent middle traps in the three-fold classification of traps (the lower and upper being the other two classes) and attain a thickness of more than 1,200 metres.

The geological sequence in the district is as follows:-

Deccan Trap flows: The precise configuration of entire succession of basalt flows of the district is not known in as much as systematic geological mapping has not been carried out so far. During the course of survey in Akkalkot taluka for groundwater, in 1963-64, 6 flows in a vertical column of 84 metres confined between 472 and 556 metres, respectively from the M.S.L. have been recognised. The flows consist of dark grey fine grained, hard and compact basalts having red pinkish colour at the top portions. A few red bole beds, which are the ferruginous clayey substances of brick-red colour, have been noticed in between the successive flows. They usually conlain fragments of basalts and indicate the periods of diminished or no volcanic activity. The vesicles in basalt, which are around 4-5 mm. in diameters, are usually filled in by secondary minerals like zeolites, calcite, chlorophaeite, etc. There are textural variations within the same flow, both laterally and vertically. There is also a gradual variation in vesicularity from non-vesicular to a vesicular flow. The maximum thickness of a flow in the area, observed in 1963-64, is around 30 metres, while the average is generally of the order of 20 metres. These two types of massive and amygdalloidal basalts have also been met with in the sub-surface strata drilled for dam site exploration pertaining to Bhima Lift Irrigation Scheme near Ujani. Some of the drill holes encountered pockets of red bole and ash.

The basalts display typical spheroidal weathering, the depth of the weathered zone varying between 3 to 8 metres. Veins of kankar filling up the fractures of the vesicular traps are quite common. Such vein fillings are absent in the fine grained compact basalt.

Soil and Kankar: Almost everywhere in the district, black cotton soil ranging in thickness from 0.3 metre to 2.5 metres is found to cover the top of the different flows. This black cotton soil is the ultimate product of weathering of Deccan Trap. Calcareous kankar and nodules are commonly associated with these soils. The thickness of the soil cap along the river sections increases to as much as 3 to 4 metres.

Geological structure: The area is remarkably free from a bye-structural complexity. There are no evidences of any structural disturbances like folding or faulting. The trap flows exhibit a general horizontal disposition though low dips are noticed here and there which are exclusively a local phenomena. Such dips were observed (1963-64) near Setan Dudhani (56 C /7, 17°26': 76°16') and Maindargi (56 C/7, 17°28': 76°18'). The flows here show north-south strike with gentle dips towards east and west. The fine-grained basalts show the typical hexagonal columnar type of jointing. The vesicular traps at places exhibit north-south and east-west striking joints, with vertical dips. Closely-spaced sheet jointing gives the rock a sheared look.

Economic minerals: No mineral of economic importance has been reported from the district. The hard and compact basalts, however, are locally used as building material and for road metal.

Groundwater: It has been observed that in a trap-covered country the vesicular traps are the principal repositories of groundwater and when they occur below the water-table they serve as potential groundwater reservoirs under both confined and unconfined conditions, which observation has been further supported by the recent geophysical investigations. The inter-trappean sedimentary horizons also serve as good aquifers. But no such beds have been reported so far from the district. Infiltration of rainwater is the only means by which the annual re-charging of the groundwater body takes place. Hence the groundwater reserve in the district is entirely dependent on the amount and distribution of rainfall. Since the average annual rainfall of the district is very meagre being around 50-60 cm. consequently the annual re-charge of the groundwater body is also very scanty. Hence the general position of the groundwater in the district is not satisfactory. The individual trap flows in the southern part of Sholapur district have been tested for their yields in the partially penetrating open wells, where they are observed to yield a discharge of about 5 to 10 litres per second (i.e., 4,000 to 9,000 gallons per hour) for a draw down of 1 to 3 metres. The chemical quality of groundwater tapped from the vesicular zones is generally good and quite suitable for irrigation and domestic purposes. There is, therefore, adequate scope for effectively harnessing the groundwater in open wells by tapping for unconfined vesicular traps by fully penetrating them. Some of the State Government schemes such as constructions of dams and light irrigations in the Bhima basin and other areas have been helpful in easing the situation with regard to the domestic and irrigational requirement for groundwater in the hinterland.

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