GENERAL

GEOLOGY

['The section on Geology is contributed by Shri A. R. Sawarkar, Geologist, (Jr.) of tbe Geological Survey of India, Maharashtra Circle. Pune.]

Akola district is bounded on the north by the southern foot-hill of the Gavilgarh range which, in turn forms a part of the Satpuda range of hills, whereas, on south, it is bounded by the Ajanta and Satmala hills. Another range comprising of steep hills runs across the middle of the southern tahsil; but for the above abruptly rising hills, the entire district is more or less a fertile alluvial tract drained by the Purna, Katepurna, Adan and Penganga rivers. The entire district remains geologically unmapped but for the inspection of a few particular sites carried out by the officers of the Geological Survey of India for studies on ground water problems and suitability of dam sites. A brief reference to the geology of the district is made by Blanford (1869) in his publication, "On the geology of the Taptee and Nerbudda valleys and some adjoining districts" wherein the salient features of the southern parts of the Gavilgarh range and the Purna plain are outlined. Engineering geology and ground water problems of specific areas have been dealt with in the unpublished reports of the Geological Survey of India by Jhingran 1950-1951-1952, Krishnaswamy (1958, 1959), Roy (1951) and Vaidyanath (1961, 1961-1962).

But for a small patch of the Upper Gondwana Sandstone (?) reported by Vaidyanath (1961), the entire district is occupied by Deccan basalt flows with intertrappean beds at places, river alluvia and soils. The stratigraphic sequence of these rocks is tabulated below:

Upper Gondwana sandstones.—The outcrops of the upper Gondwanas (?) have been reported by L. N. Vaidyanath (1961) about 0.3km. towards NE of village Wari (55 C/10, 21° 10'; 760 47') _ They comprise soft sandstones having a strike in N 80° E-S80⁰ W direction and dip of about 30° towards N 10° W. One more outcrop of this rock appears near the dam site of the Man river project, about 1.6 km south-east of Wari.

Deccan Traps.—The trap covers a major part of the district and is characterised by basalt lava flows which are generally dark grey, hard and compact. The tops of individual flows are usually vesicular and zeolitic whilst the middle zone is fairly compact and non-zeolitic. Amygdalloidal, porphyritic, or glomeroporphyritic textures are noticed locally. The vesicles are filled by secondary minerals like zeolites, quartz, calcite or some earthy or ferruginous material. Well developed columnar joints and spheroidal weathering are characteristic features of the massive basalts. Weathered zeolitised traps occur in the river beds and bluish grey, soft volcanic ash beds are encountered locally. The basalts are composed of laths of plagioclase felspars (andesine to labradorite), augite, pigeonite, glass, and minor amounts of opaque ores.

Inter-trappean beds.- These represent sedimentary, lacustrine or fluviatile deposits intercalated with the lava flows and presumably laid down during the interval between successive lava eruptions. Such beds have been reported at the following places in Akola tahsil: —

(i) From the hills, 9.6 km. south of Wahan village (20⁰29^,50'^/: 70°9'50") close to the Katepurna Dam site near the villages Vastapur (20⁰28'30": 70°81') and Khambora (20°36:77°12'), and,

(ii) In the hills close to the village Donad Buzurg (55 H/2, 20°34,: 77°10'30").

Conglomerates, Kankar, Alluvium and Soils:— In the alluvial tracts, a fairly thick alluvium is underlain by layers of false bedded sandy soils, conglomerates and yellowish, hard calcareous kankary beds. Near Donad Buzurg the Katepurna river flows over a bed of recent conglomerates and sands. The black soil or Megur is clayey or loamy. It is appreciably thick in certain areas but generally it occurs as a thin cover over the weathered basalt or mooram. The alluvial soils represent both the transported and residual soils. A generalised section of 36.6 metres deep well near Dahihanda (spelt as Dhyanda by Blanford, 20°52': 77°08') north of Akola, near the Purna river has been described by Blanford (1869) as below:

1. Ordinary brown alluvial clay.

2. Yellow sandy clay.

3. Reddish clay.

4. Gravel and sand of varying thickness.

5. Tenacious gravelly clay. This is the saliferous stratum, on lapping which the salt wafer rises with great force.

A large number of boreholes put in areas of Katepurna, Man, Nirguna and Ekburji dam sites in the Akola, Akot, Balapur and Basim tahsils of the district respectively have revealed that the thickness of soil and alluvium cover range from a few, metres to as much as 25 metres. This cover is admixed with boulders and pebbles of the country rock or Kankar together with some sandy and earthy material.

Structure.—The entire district, forming part of the Purna valley, is a basin shaped depression in the Deccan Traps filled by river deposit. The northern edge of the basin in contact with the Satpuda hills is supposed to be a fault line (Roy, 1951), and is covered by boulders and debris derived from the disintegration of the traps. The basalts in the Narnala and other hills in more or less east-west alignment display dips towards north. The flows in other areas are almost horizontal in disposition. In general, the valley is free from structural disturbances of any significance; but local faults have been reported from a few places. For example, the conglomerate bed in the Katepurna river near Donad Buzurg is suspected to indicate a fault plane (Krishnaswamy, 1959). Since the district has not yet been geologically mapped, likelihood of encountering such local disturbances cannot be ruled out. The basalt exposures on the high hills as well as the rocks met with in boreholes show a large number of joints, both vertical as well as basal. Columnar joints are more conspicuous in massive basalts. The boreholes also encountered jointed basalt. A small patch of Upper Gondwana Sandstone (?) having a fairly high dip of 30° towards N 10° W appears to be an inlier; this also indicates possibility of existence of such inliers in the area which may be deciphered on systematic geological mapping of the district.

Groundwater. From the hydrological point of view the parts of the Purna valley falling in the district could be divided in two categories, viz., (1) the fresh water tract and, (2) the saline tract.

Fresh Water Tract.—This tract occupies the northern part of the district, close to the Gavilgarh range covered by the boulders and debris. Here abundant supplies of fresh water are available at a depth of 3 to 5 metres from the surface. Roy (1951) has estimated the annual rate of replenishment of groundwater in the fresh water tract to be of the order of 20 million cubic feet per sq. mile (0.22 million cubic metre per sq. kilometre). The groundwater is potable with chloride content varying between 8 and 50 parts per million.

Saline tract. -The salinity map of the area reveals that the chloride content increases in general towards the Purna river from either edge of the Purna valley. In the inner regions of the valley the chloride content is between 2,000 and 5,000 p. p. m., the maximum being 12,917 p. p. m. (Roy, 1951). The upper clays do not contain appreciable salt; but at places, the shallow wells also encounter brackish water. Occasionally the carbonate of soda forms an efflorescence upon it but the chloride of sodium is obtained from beds below the gravels and calcareous conglomerates. This salt being stratum is interpreted by Blanford (1869) to be "something distinct from the upper fresh water alluvium, and of older geological age. The presence of common salt in large quantities may also indicate that the clays containing it are of marine origin. But the absence of marine remains is opposed to the idea of these plains having been delta accumulations on a sea coast." Wells are sunk on both sides of Purna river for obtaining brine.

Since the district does not enjoy copious annual precipitation (annual incidence ranging between 750 and 1000 mm.) most of the small streams go dry in summer. The dams constructed across the various rivers might be helpful in partly meeting the irrigational and drinking water requirements of the tahsils. In general, the contacts of successive flows and those of the dykes with the country basalt having discordant relationship may be more suitable sites for tapping underground water.

Economic Geology.

Building material and road metal.—There is no dearth of dense, hard and compact basalts in the district for utilisation as building material, these rocks being quite durable and excellent.

Salt.—The entire belt along the Purna river is exceptionally rich in sodium chloride and a large number of wells are reported to have been sunk in the past to procure salt out of the brine. Wells are a little over 0.76 metre wide. Each well produces about 725 kg of salt per month. The brine is very strong and is evaporated by solar heat alone in shallow pans about 7.62 metre long and 1.32 metre wide.

There does not appear to be any good prospect of the occurrences of minerals of major economic importance in the area.

TOP