(a) What are megafossils? Give names of any three age-diagnostic megafossils indicating their significance in Indian…

(a) Megafossils and age-diagnostic examples

Megafossils are fossils large enough to be seen and studied with the naked eye or a hand lens, in contrast to microfossils. Fossils that lived for a short time over a wide area serve as index fossils, which date and correlate rocks.

Examples in Indian stratigraphy:

• Glossopteris (a seed fern): the index fossil of the Permian Lower Gondwana (Talchir to Raniganj), with Gangamopteris; it dates the coal-bearing beds and proves the link of India with the other Gondwana continents.
• Ammonites (such as Perisphinctes and Macrocephalites): index fossils of the Jurassic of Kuchh, used to subdivide the Jurassic into zones and date marine beds; Cretaceous ammonites do the same in the Trichinopoly area.
• Nummulites (large foraminifera): index of the Eocene of Kuchh, Rajasthan and Assam, marking a shallow warm sea.

Significance: they give relative age, allow correlation between basins, show the environment of deposition and past climate, and help in tracing evolution and in locating oil-bearing and coal-bearing horizons.

(b) Palaeogeography and tools of reconstruction

Palaeogeography is the study of the geography of the past: the distribution of land and sea, mountains, rivers, shorelines, climate and life at a given time in earth history. It is shown on palaeogeographic maps.

Tools:

• Lithofacies and isopach maps, which show the type and thickness of sediments and so the source and basin shape.
• Sedimentary structures and palaeocurrent data, which give the direction of flow and slopes.
• Fossils and faunal provinces, which show the environment, climate and past connection of regions.
• Palaeomagnetism, giving past latitudes and plate positions.
• Climatic indicators: glacial tillites, evaporites, coal and reef rocks.
• Stable isotopes (oxygen and carbon) for temperature and sea level.
• Plate-tectonic reconstruction, geochronology, seismic stratigraphy and well logs, now handled in GIS.

(c) Lithostratigraphy and diagnostic properties

Lithostratigraphy is the classification and naming of rock strata into units according to their lithology (rock type) and position, without reference to age. The formal units, in descending rank, are supergroup, group, formation, member and bed; the formation is the basic unit.

Diagnostic properties: lithology and mineral composition, colour, grain size and texture, bedding and sedimentary structures, thickness and lateral extent, mappability at surface or in the subsurface, geophysical-log character, stratigraphic position bounded by contacts or unconformities, and fossil content only as a lithological feature.

Indian examples: the Vindhyan Supergroup (Semri, Kaimur, Rewa and Bhander groups); the Gondwana Supergroup, where the Talchir Formation (tillite and shale) is distinct from the coal-bearing Barakar Formation (sandstone and coal); the Siwalik Group (Lower, Middle, Upper Siwaliks); and the Deccan Traps, a basalt group made of many formations.

(d) Confined and unconfined aquifers

An unconfined aquifer has its upper surface at the water table, which is at atmospheric pressure and open to the surface through permeable rock above. Characteristics: it is recharged directly by rain over the area; the water level in a well is the water table itself and rises and falls with season; storage comes from drainage of pore spaces, specific yield about 0.1–0.3; it is shallow and easily polluted.

A confined aquifer is bounded above and below by impermeable or low-permeability layers (aquicludes or aquitards). Characteristics: water is under pressure greater than atmospheric, so the level in a well rises above the top of the aquifer to the piezometric (potentiometric) surface, and a well may flow without pumping (artesian well); recharge is through a distant outcrop zone; storage comes from compression of the aquifer and expansion of water, with very small storage coefficient, 10⁻⁵ to 10⁻³; it is deeper, better protected from pollution, and slow to recharge.

(e) Earthquake-resistant structures

An earthquake-resistant structure resists ground shaking without collapse, so that lives are saved.

Characteristics:

• A simple, symmetrical, compact plan and regular elevation, with uniform mass and stiffness, avoiding soft storeys and large overhangs.
• Low height and light weight, since the earthquake force is proportional to mass.
• Strong, ductile materials and detailing, with well-confined reinforcement, strong joints and "strong column, weak beam" design.
• Lateral resistance by shear walls, bracing and box action, with a rigid roof diaphragm.
• Seismic separation joints between blocks, and isolation or dampers where needed.
• Sound foundation: firm rock or compact soil, a single type of foundation, and avoiding liquefiable soil, faults and unstable slopes.
• Design to the codes: IS 1893 and IS 4326, IS 13920 for ductile detailing.