(a) Describe the stratigraphic sequence of Dharwar Supergroup and add a note on its economic importance. (20 marks) (b) Elucidate…

(a) Stratigraphic sequence of the Dharwar Supergroup and its economic importance

The Dharwar Supergroup is a sequence of volcano-sedimentary rocks in the greenstone (schist) belts of the Dharwar Craton in southern India. It lies unconformably on the Peninsular Gneissic Complex, which is 3.4–2.6 Ga, and is intruded by late granites. The craton is divided into the older Western Dharwar (Bababudan, Shimoga, Chitradurga belts) and the younger, more granitic Eastern Dharwar (Kolar, Hutti, Ramagiri belts).

Sequence, from older to younger:

(i) Sargur Group (about 3.3–3.0 Ga), treated as older than the Dharwar and sometimes kept separate. It is found as enclaves in the gneiss, with high-grade metasediments, quartzite, marble, ultramafic rocks and komatiite.

(ii) Bababudan Group (lower Dharwar, about 2.9–2.7 Ga). It has a basal quartz-pebble conglomerate and quartzite, then metabasalt with amygdales and banded iron formation.

(iii) Chitradurga Group (upper Dharwar, about 2.7–2.6 Ga). It has the Talya conglomerate, greywacke, phyllite, limestone and dolomite with stromatolites, manganese and iron formations, and felsic to mafic volcanics.

Metamorphism is greenschist to amphibolite facies, rising to granulite in the south.

Economic importance.

(i) Gold occurs in shear-controlled quartz veins of the Kolar (closed in 2001) and Hutti belts, and in the Ramagiri belt.

(ii) Iron ore of haematite and magnetite from banded iron formation in the Bababudan Hills, Kudremukh, Hospet–Sandur and Chitradurga belts.

(iii) Manganese from the Shimoga and Sandur belts.

(iv) Chromite from the ultramafic rocks of the Sargur Group, at Nuggihalli.

(v) Copper at Ingaldhal, and dimension stone, limestone, kyanite and corundum.

(b) Types of microfossils, their composition and applications

Microfossils are fossils so small, generally under 1 mm, that they are studied under a microscope. They are abundant in a small sample of rock, which is why they are used in drilled cores.

(i) Calcareous: foraminifera, coccoliths (nannofossils), ostracods, made of calcite or aragonite.

(ii) Siliceous: radiolaria and diatoms, made of opaline silica.

(iii) Phosphatic: conodonts, made of calcium phosphate (apatite).

(iv) Organic-walled: spores, pollen, acritarchs, dinoflagellate cysts and chitinozoans, made of resistant organic matter such as sporopollenin.

Applications.

(i) Biostratigraphy and correlation, especially in boreholes, using foraminifera, conodonts and palynomorphs.

(ii) Petroleum exploration: dating of reservoir rocks, palaeoenvironment, and thermal maturity, using the conodont alteration index and spore colour.

(iii) Palaeoenvironment and palaeoclimate: water depth, salinity and temperature, and oxygen and carbon isotopes in foraminifera.

(iv) Palaeoceanography and past vegetation from pollen.

(c) How an earthquake occurs and the pattern of earthquake-resistant construction

An earthquake is ground shaking produced by the sudden release of strain energy in rock. According to Reid's elastic rebound theory, plate motion builds strain in rock on either side of a locked fault until it exceeds the strength of the rock, and the fault then slips. The energy spreads as P, S and surface waves from the focus, and the point on the surface above it is the epicentre. Most occur at plate boundaries, such as the Himalayan collision zone. Intraplate events also occur in peninsular India, as at Latur in 1993 and Bhuj in 2001, and reservoir-induced events like Koyna in 1967.

Earthquake-resistant construction.

(i) Site and foundation: avoid soft soil, liquefiable sand and active faults, and prefer firm rock.

(ii) Plan: a simple, symmetrical, regular plan and low, light structures, with no soft storey.

(iii) Structural system: ductile reinforced-concrete frames with strong columns and weak beams, shear walls and bracing. For masonry, horizontal bands at plinth, lintel and roof levels with vertical steel.

(iv) Seismic joints between parts of different height.

(v) Base isolation and dampers for important buildings.

The Indian codes are IS 1893 (design criteria), IS 4326 (construction) and IS 13920 (ductile detailing). Kashmir's dhajji dewari and Kutch's bhunga are traditional forms.