Palaeontology and Stratigraphy
Palaeontology and Stratigraphy tests fossils, modes of preservation, relative and absolute dating, stratigraphic principles and the geological record of Karnataka. Expect questions on index fossils, unconformities and major divisions of geologic time.
Core rules
- Stratigraphic principles include superposition, original horizontality, lateral continuity, cross-cutting relationships and faunal succession.
- Index fossils are widespread, short-ranging and easily identified; they help correlate beds.
- Fossils preserve as body fossils, moulds, casts, impressions, petrifactions or trace fossils.
- Lithostratigraphy groups rocks by lithology; chronostratigraphy divides time.
- In Karnataka, Dharwar greenstones are largely unfossiliferous; Bhima Group limestones contain stromatolites, and the Deccan Traps cap older rocks in the north.
Worked example
A vertical section has conglomerate at the base, shale with brachiopods above it, and limestone with corals at the top. Which is oldest and what does the sequence suggest? Reasoning: by superposition, the conglomerate is oldest. Brachiopods and corals indicate marine conditions, so the sequence records a marine transgression over a conglomeratic shore deposit.
Traps
- A fossil's presence does not always give a precise age; reworked or long-ranging fossils mislead.
- An unconformity means time is missing, not that no rocks ever formed.
- Stromatolites are organosedimentary structures, not bones.
- Do not confuse lithological correlation with time correlation.
Karnataka link
Bhima and Kaladgi basin rocks are Proterozoic cover sequences; Dharwar schist belts are Archean greenstone sequences.