GENERAL

GEOLOGY

[The section on Geology is contributed by the Geological Survey of India, Government of India, Calcutta.]

The principal hill ranges are the Satpudas, runing east-west, dividing the valleys of Tapi and Narmada. These include the plateau near Toranmal, called Chincholi, which is about 914 metres above mean sea level and about 41 km.2 in area. Between Dhulia and Gujarat on the west is the northern extremity of the Sahyadri range. Spurs rise from the plain for the most part at right angle to the main ranges. The Arva and Galna hills divide Dhulia from Nasik.

The chief drainage system is river Tapi, entering at the south eastern corner of the district, flowing through a broad fertile plain in a westerly direction dividing the district into two unequal parts. The larger part lies in the south and is drained by the tributaries Bori, Panjhra, Buray, Amaravati and Siva. The tributaries of Tapi to north are Aner, Baler, Arunavati and Gomai. Towards north and west, the plain rises into difficult, rugged and quickly wooded country. During rainy season Tapi is not fordable. The river Narmada skirts the north-west corner of the district. The tributaries of Narmada, in the district, are Udai and Khat draining the northern slopes of the Satpudas.

Geological Formations.

The Geological formations met with in the district are:

Name of the formation

Age

Alluvium

Recent.

Deccan Basalt

Eocene.

Bagh Beds

Upper Cretaceous.

The oldest geological formation met with in the north-western portion of the district are the Bagh Beds of upper Cretaceous age. They are conspicuously exposed on the banks of the Devaganga and its tributaries, along which sections of great thickness are seen. The hills east of the river, towards Attior Arithi, are entirely composed of sandstone, with shale beds appearing at places near the summits. Here the thickness is more than 300 metres. The beds have a quaquavarsal dip, sloping away in every direction. The sandstones are much hardened and frequently cut up by dykes and large irregular intrusions of trap. East of Devaganga, near Surpan, a small area is occupied by the cretaceous beds, the top of the high range called 'Bawagupnyo' being trap. Near Warwee or Vami, shale with limestone and oyster beds occur, and in the calcareous shale, just below the trap, on the western spur of Bawagupnyo hill, shark's teeth abound in calcareous (limestone) black rock, containing irregular siliceous masses. To the west, the Cretaceous rocks go beneath the traps, whereas the north and south boundaries are faulted. The eastern boundary shows an abruptly denuded termination, as if the lower lava flows had been consolidated against a preexisting sandstone cliff, which is apparent north of Babasiraj hill, where it is suddenly over-lapped by the trap. Looking westwards from the top of the Babasiraj hill, the general anticlinal feature of the rocks with an east-west axis may be inferred, the trap of the hills dipping to north and south. To the east of the hill, the rocks dip to the south and south-east, which explains the absence of cretaceous rocks in Akrani. Between the overlying Traps and the Baga Beds there is a slight but distinct unconformity.

Trap Rocks.

The close 0f the mesozoic era was marked by the outpouring of voluminous lava flows which spread over vast areas. The flows are called 'traps' because of the steplike or terraced appearance of their outcrops, the term being of Scandinavian origin. The traps or the Deccan Basalts have plagioclase felspar labrodorife and pyroxene augite as the main minerals. They are grouped into vesicular and non-vesicular varieties. The amygdoloidal traps are characterised by vesicles filled with quarts, chalcedony and zeolites. A major part of the district is occupied by these trap flows. Contrary to the general rule in the case of the traphills, the ridges lying west of Toranmal are rarely flat topped, and frequently extremely craggy. The traps of the hills along the northern boundary of the district have generally a low northerly dip, but it is not very regular. The traps of Toranmal itself are nearly horizontal. Spheroidal exfoliation is a characteristic feature of weathering in trap. It produces thin eon-centric shells or layers which become soft and fall off gradually, the weathering starting along well developed joints. The interior of the spheroidal masses however, remain quite fresh.

South of Hamp or Hap, as in the hills north of the river Narmada, ash breccias are very abundant amongst the rocks exposed in the river section. Some of these rocks have evidently originally been pymice, the vesicles now being, in some cases at least, filled with agate.

A few trap dykes occur in Akrani. Around Durgam, however, there are several parallel dykes of small sire. They occur every 180 to 270 metres and strike North 60-75 East. Some of these dykes are coarse grained and composed of felspar and granular greenish augite.

Alluvium and soils.

The alluvium along the Tapi thins out westwards. It is not more than 50 metres thick towards east, near Savalda. The soil derived from the trap is black cotton soil and is very fertile as it is rich in plant nutrients such as lime, magnesia, iron and alkalies.

Ground Water.

The traps being massive and fine grained, are unsuitable for groundwater storage. It is through joints and fissures in them, that the rain water percolates and accumulates in the weathered portions which are the most reliable sources of groundwater in traps. However, the baked and/or highly vesicular portions between trap flows also provide passage or channel ways for percolating rain water. As it is un-economical to put down tubewells in these hard rocks, the alluvial tract of the Tapi was explored for groundwater utilisation during the All-India Exploratory Tubewell drilling programme.

In all four boreholes were drilled at Torkhed, Jhiribeda, Taloda and Savalda. All the boreholes ended at shallow depth in trap, the deepest hole being at Savalda, where the alluvium is about 50 metres thick. All the boreholes proved unproductive as no aquifer was encountered. However, small scale groundwater development is possible around Taloda and Savalda. The quality of groundwater is suitable for irrigation. The content of dissolved solids and chlorides are low and the percentage of sodium, although a little high, is not likely to produce soil alkalinisation. The hardness mostly due to bicarbonates is not detrimental to soil or crops. To augment groundwater supply over trap country, the existing dug wells should be deepened and small diameter bores drilled from the bottom of the well.

Earthquakes.

The district lies in the stable peninsular shield area, but occasionally receives minor tremors due to earthquakes originating in the extra peninsular region of India which is more vulnerable to frequent earthquakes. The recorded tremors felt in the district were of the Kutch earthquakes on 16th June, 1819 and 31st December, 1858 and that of the Bihar-Nepal earthquake on 15th January, 1934. The last tremor had the acceleration of 10-25 km. per sec.

Economic Geology.

No minerals of economic importance, are so far recorded in the district. Being dense, bard, and durable the trap is used fairly extensively, as building stone, road metal, railway ballast and as aggregate in cement concrete.

 

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