(a) Describe stratigraphy and depositional environment of Gondwana Supergroup and discuss why only certain intervals of Gondwana…

(a) Gondwana Supergroup: stratigraphy, environment and coal

The Gondwana Supergroup (Late Carboniferous to Early Cretaceous) is a thick continental sequence deposited in fault-bounded troughs along the Damodar, Son–Mahanadi, Pranhita–Godavari and Satpura valleys.

Lower Gondwana (Permian):

• Talchir Formation (Early Permian): tillite, boulder bed and needle shale, glacial and glaciolacustrine.
• Karharbari and Barakar formations (Early Permian): sandstone, shale and thick coal, fluvial and swampy; the Barakar holds the main coal seams.
• Barren Measures (Kulti) (Late Permian): ironstone shale and sandstone, without coal.
• Raniganj Formation (Late Permian): sandstone, shale and coal, fluvial and swampy.

Upper Gondwana: Panchet (Early Triassic; Lystrosaurus), Mahadeva (Pachmarhi, Denwa, Bijori), Maleri and Tiki (Triassic) red beds, Kota and Jabalpur (Jurassic), Rajmahal basalts with Ptilophyllum flora (Jurassic–Early Cretaceous), and the Cretaceous Umia beds of Kuchh. Environments were fluvial, lacustrine, and in the Triassic, semi-arid.

Why only some intervals have coal: peat needs a warm, humid climate with abundant plants, as in the Permian after the glaciation, with the Glossopteris flora. It needs a basin that subsides at about the rate that peat accumulates, so that swamps persist, a condition met in Barakar and Raniganj times. Rapid burial and reducing, waterlogged conditions then preserve the organic matter. The Barren Measures reflect warm, drier, oxidising conditions with less vegetation and iron-rich sediment; the Triassic–Jurassic red beds reflect arid, oxidising climate, so they have little coal.

(b) Trace fossils

Trace fossils (ichnofossils) are records of the activity of organisms rather than their bodies: tracks, trails, burrows, borings, resting marks and coprolites.

Modes of preservation: as impressions or moulds on bedding surfaces (concave epirelief, or convex hyporelief casts on the underside of sandstone beds); as full-relief burrows, where the burrow is filled with sediment different from the host; as borings in shells, wood or hard rock; and as faecal pellets and coprolites. Their preservation needs a soft, cohesive sediment that is quickly covered by a different sediment.

Significance:

• They show the environment, as ichnofacies: Skolithos (high-energy, shallow), Cruziana (mid-energy, shelf), Zoophycos, and Nereites (deep sea).
• They record behaviour: feeding, dwelling, crawling.
• They indicate sedimentation rate, oxygen level and way-up of beds.
• They occur in rocks without body fossils, and Treptichnus pedum marks the base of the Cambrian.
• They are not transported, so they record the actual place of life; bioturbation also affects reservoir quality.

(c) Types of dams and geological conditions for sites

Dams are of four main types:

• Gravity dam: masonry or concrete, resisting water thrust by its own weight (Bhakra). It needs a strong, unfractured, unweathered rock foundation, such as granite, gneiss or massive basalt, with high bearing strength, no major faults or shear zones, and an impermeable base.
• Arch dam: curved upstream, passing thrust to the abutments (Idukki). It needs a narrow, V-shaped gorge with very strong, sound rock on both abutments, free from faults and joints that could slide.
• Buttress dam: a sloping deck on triangular supports. It needs fairly good rock, though the loads are lower than those under a gravity dam, and a foundation that is sound under the buttress.
• Embankment (earth or rockfill) dam: of compacted earth or rock (Tehri, Hirakud). It suits wide valleys and weaker foundations, but needs impermeable clay core material and rock quarry near the site, and cut-off treatment of permeable alluvium.

General conditions: the structure should dip upstream or be favourable, with no soluble limestone cavities, major faults or landslide-prone slopes at the reservoir rim, low seismicity risk, and a watertight reservoir.