Model Answer
0 min readIntroduction
The Spiti Basin, nestled in the cold desert of Himachal Pradesh, stands as a remarkable geological archive, preserving one of the most continuous and fossil-rich Palaeozoic sedimentary sequences in the entire Himalaya. This sequence, spanning from the Cambrian to the Permian periods (approximately 541 to 252 million years ago), offers invaluable insights into the ancient Tethyan realm, providing critical data for understanding stratigraphy, palaeontology, and palaeogeography of the Indian subcontinent prior to its collision with the Asian plate. The well-exposed and folded marine deposits of the Spiti valley are often referred to as a "Museum of Indian Geology" due to their exceptional preservation and diverse fossil assemblages.
Cambrian Period (approx. 541 - 485 Ma)
The Cambrian sequence in Spiti is represented by the Haimanta Group, which is further subdivided:
- Batal Formation (Lower Cambrian): Predominantly argillaceous (shales and slates), often unfossiliferous in its lower parts.
- Kunzam La Formation (Upper Cambrian): Characterized by a change to arenaceous facies, comprising thinly bedded quartzites, slates, and phyllites.
Fossil Content:
- Rich in trace fossils like Cruziana and Diplocraterion.
- Early trilobites such as Redlichia, diverse brachiopods, and algal structures in dolomitic bands.
The age is well-constrained by the trilobite fauna and trace fossils, marking the beginning of the Palaeozoic marine life in the region.
Ordovician Period (approx. 485 - 443 Ma)
The Ordovician succession in Spiti is mainly represented by the Thango Formation, which conformably overlies the Cambrian sequence.
- Lithology: Consists of quartzites and conglomerates, often poorly fossiliferous in its lower parts.
Fossil Content:
- Contains characteristic brachiopods (e.g., Orthis).
- Graptolites and trilobites, particularly in the overlying beds which help in fixing the upper and lower age limits.
An Upper Ordovician to Lower Silurian age is suggested for this formation, and it is considered nearly equivalent to the Gugaldhar Formation of Kashmir Basin.
Silurian Period (approx. 443 - 419 Ma)
The Silurian is marked by the Takche Formation.
- Lithology: Comprises limestones with minor shales and dolomites.
Fossil Content:
- Rich in reef-building organisms such as Rugose and Tabulate corals, indicating a marine transgression that began in the Ordovician.
- Brachiopods and other marine invertebrates.
The Silurian age is confirmed by the fossil record, particularly the presence of characteristic Silurian fauna.
Devonian Period (approx. 419 - 359 Ma)
The Devonian period is predominantly represented by the Muth Formation (Muth Quartzites).
- Lithology: Characterized by hard, white to light green quartzites, often thickly bedded sandstones, with some limestones, dolomites, and shales in the upper parts. It overlies the Takche Formation with a sharp contact.
Fossil Content:
- Largely unfossiliferous, but some trace fossils and stromatolites are known.
- Brachiopods of the genus Dalmanella have been reported from equivalent formations in Kashmir (Muth Quartzite).
The age is generally assigned as Early Devonian, with a phase of non-deposition for a greater part of the Devonian period occurring between the Muth and Lipak formations.
Carboniferous Period (approx. 359 - 299 Ma)
The Carboniferous sequence is encompassed within the Kanawar Group, divided into two main formations:
- Lipak Formation (Late Devonian - Early Carboniferous):
- Lithology: Consists of grey to dark blue limestones, interbedded with shales and quartzites.
- Fossil Content: Contains ostracods and Carboniferous brachiopods (e.g., Syringothyris, Spirifer, Linoproductus). It is correlated with the Syringothyris Limestone of Kashmir.
- Po Formation (Lower to Upper Carboniferous):
- Lithology: Overlying the Lipak Formation, it comprises a thick sequence of interbedded shales and quartzites with siltstones, exhibiting sedimentary structures like cross-bedding and ripple marks.
- Fossil Content: The lower Thabo member yields plant fossils (e.g., Sphenopteridium furcillatum and Racopteris ovata) of Lower Carboniferous age. The upper part contains brachiopods (e.g., Spirifer, Linoproductus), bryozoans (e.g., Fenestella, Protoretepora), and a rich assemblage of trace fossils. This suggests glacially influenced sedimentation towards the upper part.
- Ganmachidam Formation (Upper Carboniferous - Lower Permian):
- Lithology: The uppermost part of the Kanawar Group, consisting dominantly of polymictic conglomerates, quartzites, siltstones, and shales.
- Fossil Content: Poorly fossiliferous, yielding remains of brachiopods, bryozoans, and bivalves.
Permian Period (approx. 299 - 252 Ma)
The Permian rocks are designated as the Kuling Group, further subdivided:
- Gechang Formation (Lower Permian):
- Lithology: Comprises sandstones with thin bands of shales and conglomerates at the base.
- Fossil Content: Dated based on diagnostic species of bivalves (e.g., Eurydesma) and corals (e.g., Waagenophyllum).
- Gungri Shale / Productus Shales (Upper Permian):
- Lithology: Dark grey to black shales, rich in carbonaceous matter and often capped by an iron-rich pebble-sand layer ("ferruginous layer") marking the Permian-Triassic boundary.
- Fossil Content: Highly fossiliferous with diverse Productid brachiopods (e.g., Productus, Marginifera), ammonoids (e.g., Cyclolobus), and bivalves, providing excellent biostratigraphic markers for the Late Permian.
The Spiti Basin thus offers a nearly continuous and comprehensive record of the Palaeozoic era, making it a critical locality for understanding the geological and palaeontological history of the Himalayas.
The following table summarizes the lithology, fossil content, and age of the Palaeozoic sequence in the Spiti Basin:
| Period | Formation/Group | Approx. Age (Ma) | Lithology | Key Fossil Content |
|---|---|---|---|---|
| Cambrian | Haimanta Group (Batal & Kunzam La Fms) | 541 - 485 | Slates, Phyllites, Quartzites, Dolomites | Cruziana, Diplocraterion (trace fossils), Redlichia (trilobites), brachiopods, algal structures |
| Ordovician | Thango Formation | 485 - 443 | Quartzites, Conglomerates | Orthis (brachiopods), graptolites, trilobites |
| Silurian | Takche Formation | 443 - 419 | Limestones, Shales, Dolomites | Rugose and Tabulate corals, brachiopods |
| Devonian | Muth Formation (Muth Quartzites) | 419 - 359 | Hard, white to light green Quartzites | Trace fossils, stromatolites, Dalmanella (brachiopods in correlative units) |
| Carboniferous | Kanawar Group (Lipak Formation) | 359 - ~323 | Limestones, Shales, Quartzites | Ostracods, Syringothyris, Spirifer (brachiopods) |
| Kanawar Group (Po Formation) | ~323 - ~300 | Shales, Quartzites, Siltstones | Plant fossils (Sphenopteridium, Racopteris), Fenestella (bryozoans), brachiopods | |
| Kanawar Group (Ganmachidam Fm) | ~300 - ~290 | Conglomerates, Quartzites, Shales | Brachiopods, bryozoans, bivalves (poorly fossiliferous) | |
| Permian | Kuling Group (Gechang Formation) | 299 - ~270 | Sandstones, Shales, Conglomerates | Eurydesma (bivalves), Waagenophyllum (corals) |
| Kuling Group (Gungri Shale / Productus Shales) | ~270 - 252 | Dark Shales (carbonaceous) | Productus, Marginifera (brachiopods), Cyclolobus (ammonoids), bivalves |
Conclusion
The Palaeozoic sequence of the Spiti Basin in Himachal Pradesh stands as a paramount geological record, offering an unparalleled continuous stratigraphic section from the Cambrian to the Permian. Its distinctive lithology, ranging from quartzites and shales to limestones and conglomerates, coupled with an exceptionally rich and diverse fossil content, provides critical insights into the ancient Tethyan Sea and the evolutionary trajectory of life during this crucial era. The meticulous study of these rocks aids in global stratigraphic correlations and helps reconstruct palaeogeographic settings and tectonic events that shaped the Indian subcontinent before the Himalayan collision. This basin truly serves as a natural laboratory for understanding Earth's deep history.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.