(a) Enumerate the different types of siwalik fauna and discuss their palaeoecology. (10 marks) (b) Define fossil and give…

(a) Siwalik fauna and its palaeoecology

The Siwalik Group (Miocene to Pleistocene) of the Himalayan foreland is divided into Lower, Middle and Upper Siwaliks, and its vertebrate fauna changes through them.

(i) Lower Siwalik (Kamlial and Chinji): early proboscideans such as Gomphotherium and Deinotherium, anthracotheres, tragulids, primitive giraffids and the ape Sivapithecus. Palaeoecology: warm, humid forest with rivers and swamps.

(ii) Middle Siwalik (Nagri and Dhok Pathan): Stegodon, Hipparion, Sivatherium, bovids, suids and the hippopotamus Hexaprotodon, with Sivapithecus persisting. Palaeoecology: a mosaic of woodland and expanding grassland as the climate became more seasonal, with C4 grasses spreading in the late Miocene.

(iii) Upper Siwalik (Tatrot and Pinjor): Elephas, Equus, Bos, camels and Sivatherium. Palaeoecology: open grassland and savanna under cooler, drier conditions.

The shift from forest to grassland follows Himalayan uplift, which strengthened monsoon seasonality and changed the drainage.

(b) Fossil and index fossils

A fossil is the remains, impression or trace of an organism of a past geological age, preserved in rock. An index fossil is widely distributed, abundant, easily identified and confined to a short time range, so it dates and correlates strata.

(i) Palaeozoic: Olenellus, a trilobite of the Early Cambrian (about 520–510 Ma), and Gangamopteris, a Gondwana seed fern of the Early Permian (about 299–270 Ma).

(ii) Mesozoic: Ceratites, an ammonoid of the Middle Triassic (about 247–237 Ma), and Baculites, an ammonite of the Late Cretaceous (about 84–66 Ma).

(iii) Cenozoic: Nummulites, a large foraminifer of the Eocene (about 56–34 Ma), and Hipparion, a three-toed horse of the late Miocene to Pliocene (about 11–2.6 Ma).

(c) Krol Formation: lithology, palaeoenvironment and age

The Krol Formation (Krol Group, divided into Krol A to E) forms the synclines of the Krol Belt of the Lesser Himalaya, as at Nainital, Mussoorie and Solan, and overlies the Blaini diamictite and Infra-Krol.

Lithology. It consists of limestone, dolomite and shale. The lower part has shale, marl and limestone; the middle part has stromatolitic limestone and dolomite; and the upper part has dolomite with chert and, locally, gypsum.

Palaeoenvironment. Shallow-marine carbonate shelf, with stromatolites showing intertidal to lagoonal conditions, and evaporites in the uppermost part indicating restricted, arid conditions.

Age. Late Neoproterozoic (Ediacaran). Acritarchs and carbon-isotope data place Krol A in the early Ediacaran. The Krol is overlain by the Tal Formation, which has Early Cambrian shelly fossils, so the Krol–Tal succession spans the Precambrian–Cambrian transition.

(d) Water-bearing properties of rocks

A rock bears water according to its porosity, which is the void fraction, and its permeability, which is the ability to transmit water.

(i) Primary porosity lies between grains, as in sands and sandstones; secondary porosity arises from joints, fractures, vesicles and solution cavities.

(ii) Specific yield is the water that drains under gravity, and specific retention is the water held by capillary forces; porosity is their sum.

(iii) Rocks are aquifers (sand, gravel, fractured or vesicular basalt, cavernous limestone), aquitards (silty clay) or aquicludes (clay, unfractured granite). Confined aquifers lie between impermeable beds and are under pressure.

In India, the Indo-Gangetic alluvium is the main aquifer. Weathered and fractured Deccan Trap basalt and Gondwana sandstones yield moderate water, while massive granite and gneiss yield little except in weathered zones and fractures.

(e) Engineering properties of rocks for building stone

A building stone should be strong, durable and workable.

(i) Compressive strength must exceed the load, and granite, basalt and quartzite are strong.

(ii) Low porosity and water absorption, preferably below about 1–5%, prevent frost and salt damage.

(iii) Hardness and abrasion resistance are needed for floors and roads.

(iv) Durability against chemical weathering, tested by soundness and slake-durability tests.

(v) Specific gravity and density, which relate to strength.

(vi) Workability, uniform texture and colour, and freedom from cleavage planes, shrinkage cracks and soluble minerals such as pyrite, which stains.

In India, Makrana marble is prized for polish, Kota stone for flooring, and Agra red sandstone and Chunar sandstone for monuments.