[The section on Geology is contributed by Shri A. R. Sawarkar, Geologist (Junior) of the Geological Survey of India, Maharashtra Circle.]
No systematic geological mapping of the district has yet been taken up by the Geological Survey of India. Information available is only through the reports submitted by the officers of the Geological Survey of India in connection with their visits for studying ground-water position of certain areas or engineering geological aspects of some dam-sites. Lieutenant-Colonel Sykes (1833), in his contribution to the Geology of Western Area, makes brief mention of the general
geology of Ahmadnagar district. He indicates that throughout the district, the trap rock is distinctly stratified as in the rest of the Deccan.
The entire district is occupied by basaltic lava-flows erupted in the Cretaceo-Eocene age, which are popularly known as Deccan traps. These lava-flows are sometimes associated with inter-trappean beds such as limestone, sandstone, clay shales, red bole beds, porous ash or scoriaceous beds. Thin mantle of black cotton soil present almost everywhere on the basalts, river alluvium, sands, gravel, clays and silts represent the recent deposits in the district. Calcareous concretions and nodules, known as kankar, is commonly associated with the soil, more concentrated in the vicinity of the stream-courses.
Stratigraphic sequence: The stratigraphic sequence of the rocks in the district is given below: -
Black cotton soil, river alluvium sands, gravels, silts and calcareous kankar.
Intrusive basic dykes, basalt lava flows belonging to the Deccan trap volcanic episode, associated with inter-trappean beds, red bole beds, porous ash and scoriaceous matter, etc.
Description of rock types : Basalts: Every flow of the basalt is generally composed of two different units: (a) the lower and middle parts are hard and compact, dark grey to greenish grey or black in colour and almost non-vesicular or very minutely vesicular. They are traversed by numerous joints and fractures and on weathering at the surface give rise to spheroidal blocks with inter-spaces between the spheroids filled with soft weathered material, (b) the upper parts of the flows are comparatively soft and vesicular. They have reddish pink to purplish colour and the vesicles almost everywhere filled up by zeolites and at places by other secondary minerals like calcite, greenish chlorophaeite, quartz or ferruginous material.
For want of geological mapping in the area, flow pattern and fabric of basalts exposed in the district are not known. The thickness of the flows range from 3 metres to as much as 40 metres, the average thickness being around 15 metres. According to Roy (1948), the thickness of the volcanic rocks is around 900 metres.
The basalts are composed of plagioclase felspars and clinopyroxenes, usually augite or pigeonite, with iron ores and other minor constituents. They exhibit porphyritic texture wherein thin laths of plagioclase and felspars are set between the plates of augite. Such porphyritic trap having minor laminations or patches of pink, vesicular trap, is exposed in the Mula river bed near Ahmadnagar cantonment. The porphyritic trap here is fairly hard and compact but the vesicular
portions are comparatively soft and weathered. Climbing up the right bank-cliff, this porphyritic variety is traceable right upto 670 metres from the M.S.L. where it is over-lain by 0.6 metre thick red clay. followed in turn by a series of pink or green, highly amygdular lava beds, the total thickness of the section being around 20 metres. The amygdular varieties here are characterised by geodes or vesicles of various shapes, varying from round to elliptical bodies, sometimes in the form of mere stringers or veins, following the bedding or running irregularly in the trap. Their size varies from that of peas to bigger bodies which are usually not inter-connected. The removal of infillings, e.g., calcite, zeolite, quartz, etc. due to action of running water and weathering has brought about a marked porousness giving rise to a type of cellular or spongy traps at the river-banks. Sometimes at the junction of the lava-flows, one could see well-developed pipe amygdules indicating the bottom of the lava-flows.
Red bole beds: These are thin reddish horizons occurring in-between the successive flows of basalt and are exposed in discontinuous patches. They are very well exposed along the hill-sides.
Intrusive dykes: No details are available regarding true dykes in the district. Two trap dykes are reported to occur near Taskarwadi (47
I/3, 19°19': 74°18') near Mula dam site. One is about 1.2 metres thick and runs in the middle of the river for about 120 metres. The other is a large dyke about 10 metres thick and is seen on the left bank of the river.
Calcareous Kankar: These are either associated with soil or occurring as small nodules or concretions either associated with soil or as infillings in cracks, fissures and joints in rocks. Whitish kankary clay are commonly seen in the cliff at both the banks of Mula river near Ahmadnagar Cantonment.
Soil and alluvium : Black cotton soil is the ultimate product of decomposition and weathering of basalt. This is highly absorbant and cracks and crumbles after a period of long drought. Recent alluvium and consolidated gravels have attained a thickness of 24-30 metres on the right bank of the Mula river near the dam-site.
Geological structure : The lava-flows are almost horizontal in disposition but local gentle tilting, undulations and minor flexures are rarely seen. But for these, no major faulting or folding is seen in the area. The rocks are, however, traversed by a large number of vertical, high angled or sub-horizontal joints; the last ones when closely-spaced, dissect the basalt into finely laminated or slabby beds with bedding thickness of a few cm. to a metre. At places the trap jointing is most irregular. Alteration of traps into spheroidal blocks is very common. Columnar jointing is also common as noticed at (19°20': 74°13') Nandur Khadarmal. The flat-topped hills at the Mula valley
flanks near Ahmadnagar Cantonment are formed of horizontal trap flows, in the form of fine laminations upto a few metres in thickness. This feature has given rise to a marked slabbiness sometimes resulting in "terminal creep" at the cliff edges abutting the valley, particularly at the right bank. The traps are fractured near the surface as compared to the sub-surface strata near the Mula dam site. The intensive trap dykes mentioned elsewhere in the note have caused lot of crushing and cleavages around them. The river sands and clays near Digras (19°22': 74°37') occasionally show current bedding.
Economic minerals: No minerals of economic importance are available in the district. The hard and compact basalts, however, are used extensively as building material and road metal. The pinkish-coloured vesicular variety is amenable to cutting into blocks of desired sizes. The calcareous kankar is burnt in lime kilns and is responsible for a regular "lime industry" around Ahmadnagar town.
Ground-water: The geological setting of Ahmadnagar district has a decisive role in determining the ground-water possibilities in different parts of the district. The district is mainly occupied by basaltic lava flows and alluvium of the Godavari and the Pravara rivers. Ground-water occurs under water table conditions in different units of basaltic flows and the Godavari alluvium. The massive trap units being less permeable as compared to the vesicular units are likely to act as confining beds for the underlying vesicular zones and hence possibility of confined aquifers at depth cannot be ruled out.
Aquifer performance tests on wells tapping vesicular and massive traps indicate that the former have comparatively much higher yields than the latter. The yields in the former case are of the order of 1,000 to 3,000 klpd for a depression of 1 to 4 metres, while in the case of massive traps it is of the order of 100 to 1,000 klpd for a depression of 2 to 6 metres. In the case of alluvium, the safe yields are of the order of 700 to 2,000 klpd for 1 to 2.5 metres of depression.
As regards the confined aquifers at depths, lack of adequate and reliable data prevents from giving any firm opinion on their true or apparent potential at this stage. Certain areas have been demarcated for exploratory drilling in parts of Shrigonda and Karjat talukas based upon recent studies carried out by the Geological Survey of India. However, presence of successful tube-wells in the adjoining areas of Pune district leaves scope for cautious optimism.
Study of the chemical quality of water samples from different wells shows that the water is generally good for both domestic and irrigation purposes except for occasional pockets of brackishness. Care has to be taken to see that in areas irrigated by canals, there is also simultaneous development of ground-water to avoid problems of water-logging and consequent quality deterioration, as noticed in the adjoining parts of Pune district.
Data on the surface flow of Godavari and Bhima rivers indicates that there is high discharge even in peak summer months. This is naturally attributable to the ground-water leakage (run-off) from the water-table aquifer feeding these rivers. The leakage has, therefore, to be effectively checked and adequately utilised. This can be done by construction of surface and sub-surface check weirs in stream beds at suitable places selected on the basis of detailed geohydrological studies. Contour bunding would also be an important measure in addition to this to check the surface run-off. The water thus impounded would considerably increase the recharge to the ground-water body.