This system derives its name from the Vindhyan hills where its rocks are prominently exposed. It forms a dividing line between the Ganga Plain and the Deccan. The system covers an extensive area of over 103,600 square km. from Chittaurgarh in Rajasthan to Sasaram in Bihar. It contains an enor­mous deposit of over 4,270 m of thickness with sedimentary rocks like sandstones, shales and lime­stones. At places Vindhyan rocks have been buried under the Deccan lava.

The Vindhyan system repre­sents the epeirogenic upheaval of shallow-water sediments deposited over the denuded and uneven surface of the Cuddapah rocks. It took a long period of formation with a marked period of interval be­tween the lower and the upper Vindhyans. In addi­tion, there is a distinct line of unconformity created by the rise of the Aravallis.

Lithology

The main rocks of the Vindhyan system are coarse, medium and fine textured sandstones, shales and limestones. While the lower Vindhyan rocks are mostly calcareous and argillaceous of a marine ori­gin, the upper Vindhyan rocks largely consist of sandstones of a fluviatile and estuarine origin. The lower Vindhyan depicts extrusions of igneous mat­ter in the form of hyalites and tuff while sandstones of the upper Vindhyan contain two layers of con­glomerates both diamondiferous. The rocks of the Vindhyan system have retained their original horizontality except in a narrow area of south-east of Aravallis where some folding has been observed. They also do not show any signs of metamorphism as a result of which sedimentary rocks like shales, limestones and sandstones have retained their origi­nal characteristics.

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The rocks exhibit ripple marks, sun-cracked surfaces, current-bedding etc proving their shalow water formation. The junction-region of the Vindhyan rocks with the older Aravallis is very much faulted which makes a major fault of about 800 km. and is known as the Great Boundary Fault. This fault extends from Beawar in Rajasthan to Khed-Brahma in Maharashtra.

Classification

The Vindhyan system has been divided into the Lower and Upper divisions which are separated by a zone of unconformity. The rocks of the lower Vindhyan system have been deposited in the bed of the sea and depict maritime influences. On the other hand, rocks of the upper Vindhyan system are riverine and estuarine in origin. The Lower Vindhyans show tectonic deformation by folding movements, while the upper Vindhyans are generally found in undis­turbed horizontal strata. Table 2.IV presents the Lower and Upper division of the Vindhyan System and their important series. Similarly figure 2.2.D gives a distribution of the Vindhyan rocks.

The Lower Vindhyan

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The Lower Vindhyan system is well devel­oped in the spectacular escarpments of the Vindhyan hills north of the Narmada valley. It has obtained various local names on the basis of its place of occurrence, i.e. Semri series, Kurnool series, Bhima series, Malani series etc.

The Semri Series-this is the main series of the Lower Vindhyan rocks in the Sun Valley region whose detailed study has been made by J.B. Auden. He has identified 900 m thick layer of limestones, shales and sandstones with interbedded porcellanites (silicified ash and tuffs) and intrusive dolerites and basalt. Its upper most stage, known as the Rohtas Stage (150-215 m), is composed of limestone’s, and shale’s which provide raw material to the cement industry of the region.

The Kurnool Series-Outcrops of this series are found in Kurnool district of Andhra Pradesh. It contains limestone, shale and sandstone. Some bands of sandstones containing conglomerates are called Benaganapalli beds which are diamondiferous.

The Bhima Series-The Lower Vindhyan rocks found in the valley of the Bhima in Gulbarga and Bijapur (Karnataka) districts are called the Bhima series. The series is divided into three stages, the lower-arenaceous, the middle-calcareous and argillaceous and the upper-argillaceous. The lower part is composed of quartzites and grits while the upper part consists of shales and limestone’s.

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The Malani Series-It occupies parts of the western Rajasthan in Malani region (Jodhpur). It consists of rhyolites, felsites and a good deal of

Stages

Upper Bhander sandstone Sirbu shales

Lower Bhander sandstone Bhander limestone Ganurgarh shales Conglomerate-bed (diamondiferous)! Upper Rewa sandstone, Jhiri shales Lower Rewa sandstone Panna shales

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Conglomerate-bed(diamondiferous) Upper Kaimur sandstone Kaimur conglomerate Bijaigarh shales Lower Kaimur sandstone Suket shales

Limestone, shales Shales, sandstone Shales, limestone, quartzite, grits Rhyolites, tuffs Granite bosses pyroclastic material. In majority of cases the lava has undergone such an amount of devitrification that it appears almost as felsite, the glassy ground-mass having completely disappeared. An outcrop of the Malani series is found near Sangla and Kirana in Punjab plains.

Apart from these areas the Lower Vindhyan rocks are also found in scattered patches near Sullavai in the Godavari valley (called Sullavai series- sandstones and quartzites); near Hyderabad (the Palnad series-limestones, shales and quartzites). The Upper Vindhyan System.The upper Vindhyan system is separated from the Lower Vindhyan by the unconformity which denotes the change in the physical geography of the Peninsula. The sea-basin in which the shales and
limestones of the lower Vindhyan system were laid down was driven away by earth-movements and continental conditions prevailed for a long time during which the flu vatic rocks such as sandstones, shales and conglomerates were laid down. This period was also marked by dry climatic conditions which are evidenced by the reddish colour of the sediments, perfect rounding of quartz-grains in sandstones and occasional presence of gypsum in the Bhandcr shales. The Upper Vindhyan system is divided into three series, viz., the Kaimur, Rewa and Bhandcr. These series are divided from each other by conglomerate beds, containing rolled fragments of diamonds derived from the erosion of dykes of older igneous rocks.

The Kaimur Series-this scries extends over the eastern parts of the Chotanagpur region. The main rocks in this series are sandstones, conglomer­ates and shales. The shales at the base of the series are known as the Sukel shales. The sandstones arc divided into two stages, viz., the lower Kaimur sandstone and the upper Kaimur sandstone. The shales in between are called the Bijaigarh Shales. The red sandstones of the series depict clear layerings because the area has not suffered any type of folding and faulting. The region is also characterised by radial drainage.

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Rewa Series-Rewa series lies north-west of the Kaimur scries. The railway line from Katni to Allahabad passes through this series. It consists of shales at the base called the Panna shales. The sandstones lying over the Panna shales are divided into two stages, viz. the lower and upper Rewa sandstones which are separated from each other by the Jhiri shales. The sandstones found in the series have coarse grains indicating the effect of arid climate. Sandstones are utilised for cement manu­facturing and glass making (especially silica sand­stone). They also contain inferior quality of iron and manganese. In Panna distrct of this series diamonds are also mined.

Bhander Series-this series is spread over the western part of the Vindhyan ranges. It is arenaceous as well as argillaceous. At places the thickness of rocks goes to 500 m indicating some sort of sinking during sedimentation. The main rocks of the series are sandstones, shales and limestones. The sandstones are divided into lower Bhander and upper Bhander stages with shales known as the Sirbu Shales in between them. During the process of its formation the Bhander series has suffered tilting from below which created rift valleys presently occupied by the Narmada and Tapi rivers. The series provides good quality of building material besides the diamond- mines.

Extra-Peninsular Vindhyan

Outcrops of the Vindhyan system are also found in the extra peninsula where these are desig­nated by various names in different localities. Here the main rocks are slates with some sandstones and limestones. In Himalayan region the Vindhyan rocks occupy the area between the central and outer Hima­layas. Here mention may be made of the Dogra slates of the Kashmir Himalayas, the Shimla slates and the Chail series of the Shimla Himalayas and the Attock slates near Peshawar (Pakistan). Similarly traces of the Vindhyan rocks are also found in Meghalaya and Afghanistan region. In contrast to their peninsular counterpart the Vindhyan rocks in the extra-peninsular region are unfossiliferous and have been subjected to folding and metamorphic activities.

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Life

The Vindhyan System is generally unfossiliferous. The presence of some carbonaceous shales at the base of the Kaimur series is the only indication of the existence of some plant life during the Vindhyan times. Here Mr. H.C. Jones has dis­covered horny discs, 1-3 mm of some fossil organ­ism embedded in black Sucket shales. Stromatoliths of algal origin have been discovered from various Vindhyan horizons. Similarly fucoid markings, be­longing to indistinguishable thallaphytic plants, are usually seen on the ripple marked and sun-cracked surfaces of sandstones and shales.

Economics

The Vindhyan system of rocks bear out pre­cious stones, ornamental stones, diamonds, building materials, and raw materials for cement, lime, glass and chemical industries. In certain places these rocks yield inferior quality of iron ore and manganese. The well known diamond mining areas (Golconda and Panna) of the country are associated with the Vindhyan conglomerates. Similarly many historic buildings like Red Fort of Delhi, the Fort at Agra, the Buddhist

Stupa at Sarnath, and the Jama Masjid have been built by Vindhyan sandstones. The limestone’s of the system is being extensively used for the manufac­ture of portland cement and also for lime making in the region. Coarser sandstones have been used as grindstones and millstones.