Short essay on The Dharwar System

The term Dharwar has been derived from Dharwad district of Karnataka was these rocks were first discovered. The Dharwar system repre­sents those rocks that were formed from the sediments derived from the denudation of pristine Achaean gneisses and schist’s.

These rocks are para-gneisses and para-schists and generally overlie the Achaean rocks. But at other places they are largely interbed- ded with gneisses and in some cases even older than the latter. The Dharwar system consists of metamor­phosed Achaean sediments, including all the schistose series below the eparchaean unconformity.

These rocks are not found in their original horizontal man­ner, rather these have been greatly deformed by tectonic activities (folding, faulting, tilting etc.),’ have lost all traces of their sedimentary nature and have acquired crystalline and schistose structure hardly to be distinguished from the underlying gneisses and schist’s. Besides, these rocks are extensively intruded by granitic bosses and dolerite dykes.

While geologists like D.N. Wadia believe that the weathering of the pristine Archaean gneisses and schists yielded the earliest sediments which were deposited on die bed of the sea, and formed the oldest sedimentary strata called the Dharwar system (D.N. Wadia, 1975, p. 89); others consider their deposition in the low valleys on the land instead of the sea.

Lithology

The Dharwar system is a complex of rocks- like classic sediments, chemically precipitated rocks, volcanic and plutonic rocks-which have been highly metamorphosed. The main rocks are schist, phyllites and slates. The schists are of various types : horn­blende schist, chlorite schist, magnetite schist, haema­tite schist, felspathic schist which are interbedded withquartzites, marbles, ophicalcites, banded jespars, beds of steatite, iron ores etc. These rocks are very much intruded by plutonic masses like nepheline- syenites, sodalite-syenites, tourmaline-granites, dunites etc. Their pagmatite veins are rich in miner­als like muscovite, beryl, pitch-blende, samarskite, columbite etc. The Dharwar rocks occur in narrow elongated synclinal outcrops and are rich in miner­als. The structure of the Dharwar rocks shows no Iayerings and these are mostly found in tabular form.

Distribution

The Dharwar system of rocks occurs in scat­tered patches in: (i) southern Deccan, (ii) central and eastern parts of the Peninsula, (iii) die north- western region, and (iv) the Himalayan region.

(i) Southern Deccan

This includes the Dharwar region of Karnataka where these rocks have been studied in detail. It is here that Brucefoot first named these rocks in the last part of the 19th century which were later on given due attention by W.M. Smith (1975) and B. Ramarao. These rocks occupy 15,540 sq. km. of area in Dharwad and Bellary districts of Karnataka and extend up to the Nilgiris, Madurai and Sri Lanka. Ramarao has identified 5 main areas of their occurrence : eastern, eastern-central, western-central, central and west­ern belts. Earlier these rocks were considered to be of igneous origin but recent studies have indicated their sedimentary origin. These rocks are divided into lower, middle, upper and post-Dharwar strata (Table 2.II). Amongst these the lower.

Amphibolites and green stone Source: R.C. Mehdiratta, p. 53. Dharwar are largely igneous, varying from acidic to basic. These are overlain by middle Dharwars con­sisting of quartzite’s, slates, conglomerates, horn­blende-schist, talc-schist, chart, iron-stone, marble and also some granite and porphyries. Numerous veins of quartz, some auriferous or containing gold (Kolar gold reef) traverse the rocks. The upper Dharwars are sedimentary rocks, ferruginous, cal­careous, siliceous and argillaceous, containing large quantities of iron and manganese ores.

Champion Gneiss-it is named after the Champion reef in the Kolar gold-field. It is the oldest gneiss in Karnataka. It is dull grey to pink, greasy looking, consisting of quartz, oligoclase, andesine and muscovite with apatite and magnetite as neces­sary minerals.

Closepet Granite-it is a pink or grey col­oured coarse-textured biotite-granite extensively exposed near Channapetnna and Closepet in Karnataka. It is intensive in nature and contains xenoliths of older rocks.

Rocks belonging to the Dharwar system are also found in Andhra Pradesh (Hyderabad, Guntur, Nellore), and Tamil Nadu (Arcot, Tinichchirappalli). The Nellore granite is rich in white mica and beryl.

(ii) Central and Eastern Peninsula

This is a large area spreading over the parts of the country in the states like Madhya Pradesh, (Balaghat, Chhindwara, Jabalpur), Maharashtra (Nagpur, Bhandara), Jharkliand (Ranchi, Singhbhum, Hazaribag), Orissa (Sambalpur, Keonjhar, Mayurbhanj) and West Bengal. Here the rock sys­tem consists of following main series:

Chilpi Series-it occupies parts of Balaghat and Chhindwara districts of Madhya Pradesh. The series consists of grits, phylites, quartzites, green stones and magniferous rocks.

Sausar Series-it spreads over Nagpur, Bhandara (Maharashtra), and Chhindwara (M.P.) districts. It largely consists of quartzites, schists, granulites, mica schists, marble and magniferous rocks. Here the marble is of metasomatic origin.

Sakoli Series-Overlying the Sausar series are chlorites, schists, phyllites and banded ferruginous rocks named as the Sakoli series. In Jabalpur and Rewa districts the Dharwar system is known as Gondite series which consists of phyllites, mica, schists, dolomites and marbles. The famous marble rock belongs to this category.

Khondalite Series-it occupies a large area in the Eastern Ghats from the northern extremity to the valley of Krishna. Here the principal rock types are khondalites, kodurites, chamockites and gneisses.

Iron Ore Series-it consists of shale’s, and banded jasper with plenty of iron-ores. Iron ore deposits occur at Singhbhum, Bonai, Mayurbhanj and Keonjhar in a range of hills 65 kilometers in length and a reserve of over 3,000 million tons of metallic iron. This is the richest deposit of iron in the world.

Other important series of the region include the Kolhan series (shales), the Gangpur series (gondites, marble, mica schist and phyllite), the Dalma trap series of Singhbhum, the newer dolerite series (quartz-dolerite), the Peridotite series, and die older metamorphic series (mica schist with horn­blende).

(iii) North-Western Region

The Dharwar rocks occupy a wide surface area in Rajasthan and Gujarat states including the Aravalli Hills. These consist of the Aravalli system, the Raialo series and die Champaner series.

The Aravalli System-It is equivalent to die Dharwar system of soudi India. It occupies a long and wide synclinorium over the basement schistose gneisses. Heron has named die lower division as die Aravalli system and the upper as the Raialo series which are separated by a zone of unconformity. The lower Aravalli system consists of arkose, gritstones, phyllites, slates, clays, siliceous limestones and ba­sic volcanic rocks. It rests with a great erosional unconformity on die finely schistose and banded gneiss (the Bundelkliand gneiss). It contains metal­liferous ores, rare minerals, gemstones and odier economic minerals.

The Raialo Series-The Raialo series comes above die Aravallis widi a pronounced unconformity. This series is rich in crystalline limestones, associ­ated with quartzites, grits and schistose rocks. The famous marbles of Makrana, Rajnagar and Bhagwanupra belong to tliis series. Recent observa­tions have revealed that the Raialo series are younger than die Dharwars and slighly older than the overly­ing Delhi system belonging to die Cuddapahs. It infact belongs to die time of Eparcliaean unconformity.

The Champaner Series-It is an outlier of the Aravalli system in the vicinity of Vadodara. It con­sists of quartzites, conglomerates, phyllites, slates and marbles. An attractive green variety of marble is obtained from dis series near Motipura.

(iv) Extra-Peninsula

Outside Peninsula the outcrops of the Dharwar rocks are also found in the Meghalaya Plateau (die Shillong series in Kliasi and Jaintia hills), die Hima­layas (die Salkhala series in Gilgit, Baltistan, Ladakli, Keran and Kishtwar consisting of carbonaceous and graphitic shales, slates, phyllites, quartzites, schists, gneisses and marble; die Vaikrita series of mica- schists, phyllites and slates in the Spiti valley; die Jutogh and Chail series of die Shimla Himalayas; die Janusar series of the Garhwal Himalayas; and die

Dating series of Daijeeling region), and the Mog6 series of Myanmar.

Life

The Dharwar rocks are all azoic or unfossiliferous. It is due to non-existence of life during this period, intense metamorphism obliterat­ing the traces of fossils and extensive intrusion of magmatic masses.

Economics

The Dharwar systems of rocks are highly met­alliferous. They bear out gold, iron ore, manganese mica, cobalt, chromium, copper, tungsten, lead, nickel, precious stones and budding stones.