[The section on Geology has been contributed by Shri A. M. Kulkarni, Assistant Geologist of the Geological Survey of India.]
The sequence of geological formations exposed in the districtis tabulated below.-
Cotton soil, Kankar, etc
Recent and sub-recent.
Deccan traps with inter-trappean sediments
Lower Eocene to upper cretaceous
Apart from the extensive spread of Purna alluvium, much of the district is occupied by Deccan lava flows, though Lameta, Gondwana and other metamorphic rocks occur as minor inliers in the northern part of the district.
Description of Rock Units.
The oldest formations comprising unclassified metamorphics are exposed the southern margin of Gondwana Sandstones, forming the southern scarp of Gawilgad hills north north-east of Achalpur, apparently brought up due to faulting.
Gondwana rocks crop out as small isolated inliers within the
Deccan traps along the northern border of the district. A fault trending east, north-east, west, south-west with a downthrow to the south is noticed to the north and north-east of Achalpur. Along the upthrow side of this fault and along the base of the hills, sedimentary beds crop out from beneath the traps.
The Gondwana rocks are mainly fine to medium grained feldspathic sandstones, some of which lithologically simulate the typical. coal-bearing Barakar sandstones. These beds of white clay varying in thickness from 1.5 to 3 metres extending for about 450 metres are noticed interbedded with sandstones near Balkher. Several small pockets of clay are also present in them west of Pandhri between Khakarsima Pir and Bairam Ghat. In the upper horizons the sequence passes into conglomerates containing red jasper pebbles. This sequence is apparently conformably overlain by Lameta beds.
The sediments resting apparently conformably on the
Gondwanas and underlaying the Deccan traps may be regarded as equivalent to the Lametas. They comprise thick purple clays followed by purple grey limestonea containing fossil fragments of Mollusca. The outcrops are noticed associated with Gondwanas only.
Deccan Basalts (Traps).
Much of the area is occupied by Deccan lava flow of basaltic
composition. They generally form flat-topped hills and display
step-like terraces; terrated topography being due to variation in
hardness of different units of the flows. These deccan traps are
emplaced through long narrow fissures or cracks in the earth's
crust and are of the nature of fissure eruptions. They are
horizontally disposed with a very gentle tilt to the west, northwest up to about 2 degrees.
Two varieties are generally noticed, the hard compact grey to dark grey and the vesicular. The latter forming the top of the flow with numerous zeolitic and calcitic infillings. The flows may be separated by thin volcanic ash or scoriae or a clayey bed popularly known as red bole bed as seen on Gawilgad hills. The timelag between two successive flows is also demarcated at places by laccustrine sediments known as Intertrappean beds as seen along the Purna river bed. On the Gawilgad range of hills the Deccan traps are composed of hard basalts showing excellent system of columnar joints.
Laterites generally cap the Deccan trap plateaux and are noticed south of Rithpur, about 6 km. north of Amravati, around Chikhaldara and at a few other places. It is a weathering product of Deccan basalts under the tropical conditions and is a porous, pitted, clay-like rock exhibiting various shades of red and brown colours. It is often pisolitic, the pisolites having a concentric structure and being cemented together by ferruginous matrix.
Alluvium with Kankar and associated with sub-recent calcarious conglomerate under-lying the black cotton soil cover extensive areas in the district, the conglomerate being more persistent along the river courses. Dazzling white, finely laminated sandy deposits extending to a thickness of about one metre is noted in the alluvial banks of the Purna river near Paruth. Much of this Purna alluvium produces afflorescence of salts, of soda chiefly and the wells sunk in the vicinity yield brackish water.
This extensive accumulation of the alluvium in the Purna valley is homotaxial with Narmada and Tapi valley alluvium and are considered to have formed in the Pliestocene period.
Decomposition of Deccan traps has given rise to either deep
brown to rich red or black cotton soil (regur) and is widespread in the district. The regur is rich in plant nutrients such as lime, magnesia and low in nitrogen and phosphorus. It is generally porous and swells considerably on addition of water and dries up with distinct cracks on loosing the moisture.
Useful Minerals and Rocks.
Being hard, compact, durable and wear-resisting, the fine grained basaltic rocks are extensively used as building stones. At places the Gondwana sandstones, clays and laterite are also used as building material. Zeolites occurring in the traps may find use in water-softening process. A few translucent varieties of amorphous and crystalline silica, viz., chalcedony, agate, etc., may serve as raw material for lapidary industry. Sandstones suitable for fine carving may be obtained from Kamthi division
of lower Gondwanas near Achalpur. White clays associated with
sandstones are locally used for white-washing village huts.
Deccan traps and the overlying alluvium cover extensive areas in the district and the ground water conditions depend on the porosity and permeability in the traps. It is generally found that the flowlines, the porous decomposed upper portions of the flows and the system of joints in them are responsible for the percolation of the sub-surface water in the region. The Intertrappean beds which are highly porous are the places very thick along the river bed and allow water to pass freely. This has given rise to springs and seepages which may be seen at intervals in the bed of the Purna river.
The perennial source of water in the district is the Purna and Maru rivers with large catchment areas in precipitous country with high rainfall and impervious bed rock.