Tertiary Rocks and Himalayan Uplift

The Tertiary Rock System represents the transition from the Mesozoic Era to the Cenozoic Era, spanning from approximately 65 million years to 2.5 million years ago. This geological period is the most critical era for the physical geography of India, as it witnessed the final breakup of Gondwanaland, the rapid northward drift of the Indian plate, and its subsequent collision with the Eurasian plate. This collision caused the marine sediments of the ancient Tethys Geosyncline to fold and uplift, forming the Himalayan mountain chain.

The Tethys Geosyncline and Plate Tectonics

• The Geosynclinal Basin: Throughout the Mesozoic Era, a shallow, elongated marine sea called the Tethys Sea existed between the Angaraland (Eurasian plate) to the north and Gondwanaland (Indian plate) to the south. Rivers from both landmasses deposited immense thicknesses of sedimentary strata into this basin.
• First Collision Phase: As the Indian plate collided with the Eurasian plate during the Upper Cretaceous to Eocene periods, the compressive forces compressed, squeezed, and buckled these accumulated marine sediments, initiating the first phase of Himalayan uplift.

Phases of Himalayan Uplift and Stratigraphy

The uplift of the Himalayas did not occur in a single tectonic event. It took place in three distinct, sequential phases of mountain-building (orogenies), each corresponding to a specific stratigraphic layer of the Tertiary Rock System.

Phase 1: The Great Himalayas (Greater Himalayas / Himadri)

• Geological Epoch: Upper Eocene to Oligocene epochs (approx. 40 to 30 million years ago).
• Tectonic Mechanism: The primary push of the Indian shield against the Angaraland crust smashed the deep-seated sediments. It was accompanied by extensive intrusions of granitic magma into the core of the folds.
• Stratigraphic Units: Represented by the Barail Group in northeastern India and highly metamorphosed crystalline rocks (schists, gneisses) forming the central axis of the Great Himalayas.

Phase 2: The Lesser Himalayas (Middle Himalayas / Himachal)

• Geological Epoch: Middle Miocene epoch (approx. 25 to 15 million years ago).
• Tectonic Mechanism: A second major pulse of tectonic compression intensified the folding. This event was characterized by massive thrust faulting, where older rock layers were pushed over younger rock layers along the Main Boundary Thrust (MBT).
• Stratigraphic Units: Represented by the Murree Group in Jammu and Kashmir and the Dagshai-Kasauli series in Himachal Pradesh, which consist of gray sandstones, red clays, and brackish-water shales.

Phase 3: The Outer Himalayas (Shiwalik Range)

• Geological Epoch: Post-Miocene to Pliocene/Pleistocene epochs (approx. 7 million to 1.5 million years ago).
• Tectonic Mechanism: The rising Greater and Lesser Himalayas were aggressively eroded by antecedent rivers. These rivers deposited vast amounts of alluvial debris, sand, and gravel into a shallow foredeep (a long structural depression) running along the southern foot of the mountains. A final tectonic pulse compressed these loose, young sediments, uplifting them into the southernmost foothill zone.
• Stratigraphic Units: The Siwalik System, which is composed of loosely consolidated sandstones, conglomerates, clay, and silt.

Stratigraphic Correlation Table

Geographic Distribution of Tertiary Rocks in India

Tertiary rock formations are widely distributed across two distinct geographical provinces of India: the Extra-Peninsular region (Himalayan belt) and the Peninsular margins.

Extra-Peninsular Distribution

• The Himalayan Arc: A continuous, 2,400-km-long belt stretching from Jammu and Kashmir, through Himachal Pradesh, Uttarakhand, Nepal, Sikkim, and Bhutan, up to Arunachal Pradesh.
• The Purvanchal Hills: Tertiary rocks curve sharply southward beyond the Dihang gorge (Syntaxial bend) to form the Patkai Bum, Naga Hills, Manipur Hills, and Mizo Hills. The Surma and Tipam Series dominate this northeastern frontier.

Peninsular Distribution

• Western Coast Outliers: Tertiary sediments occur as marine and estuarine intercalations along the western edge of the peninsula. Notable locations include the Dwarka and Gaj beds in Kathiawar (Gujarat), and the Quilon and Varkala beds in Kerala.
• Eastern Coast Outliers: Found as narrow patches stretching along the Coromandel coast. The most prominent is the Cuddalore Series in Tamil Nadu, which extends into parts of Andhra Pradesh and Odisha (Baripada beds).

Relief and Topographical Structural Adjustments

The structural relief of Northern India is defined by the ongoing tectonic stresses of the Tertiary uplift, leaving distinct physiographic features.

Longitudinal Structural Valleys (Duns)

Between the Lesser Himalayas and the Shiwalik foothills lie flat-bottomed structural valleys called Duns (e.g., Dehradun, Kotah Dun, Patli Dun). These were originally temporary lakes formed when the rising Shiwalik range blocked the paths of rivers flowing from the Lesser Himalayas. The rivers deposited silt into these lakes, which eventually breached their barriers, leaving behind fertile, flat valley floors.

Syntaxial Bends

The Himalayan ranges terminate abruptly at both western and eastern extremities, bending sharply southward. The Western Syntaxial Bend occurs near Nanga Parbat (Kashmir), where the geological structures pivot sharply toward Pakistan. The Eastern Syntaxial Bend occurs near Namcha Barwa (Arunachal Pradesh), where the structural trends pivot sharply into Myanmar. These bends were caused by the wedges of the underlying Indian peninsular shield exerting uneven pressure during the collision.

Ongoing Tectonic Activity

Because the Indian plate continues to move northward into the Eurasian plate at a rate of roughly 4 to 5 centimeters per year, the entire Tertiary relief remains unstable. This ongoing compression makes the region highly vulnerable to major earthquakes and triggers frequent landslides along its fault zones.

Economic Significance and Natural Resources

The Tertiary Rock System is highly valuable for its energy resources and building materials.

Petroleum and Natural Gas Reserves

• Source Rocks: Tertiary sedimentary rocks are the primary reservoir for hydrocarbons in India. Marine and lacustrine sediments from the Eocene and Miocene epochs contained abundant organic material that converted into crude oil and natural gas over time.
• Major Oil Fields: * Assam Valley: The oldest oilfields in India, located within the Digboi, Nahorkatiya, and Moran-Hugrijan fields (Digboi belongs to the Tipam sandstone series).
  • Cambay and Gujarat Basins: Major oil structures located in Ankleshwar, Kalol, and Sanand.
  • Offshore Basins: The Mumbai High structure, located in the continental shelf off the Maharashtra coast, consists of oil-bearing Tertiary limestone layers.

Tertiary Coal and Lignite

• Quality: Unlike the older, high-grade Paleozoic Gondwana coal, Tertiary coal is younger and categorized as low-grade Lignite or brown coal. It features a higher moisture, sulphur, and volatile matter content.
• Key Deposits:
  • Neyveli Lignite: Located in Tamil Nadu (Cuddalore Series), it is the largest lignite deposit in India.
  • Northeastern Deposits: Found in Makum (Assam), Namchik-Namphuk (Arunachal Pradesh), and Palana (Rajasthan).

Industrial Raw Materials

• Limestone: Extensive Tertiary limestone deposits (especially Eocene-era Nummulitic limestone) are found in Meghalaya (Cherrapunji plateau), Gujarat, and Rajasthan, serving as the foundational raw material for India’s cement manufacturing industry.
• Gypsum and Nodule Clays: Found within the sub-Himalayan Tertiary beds, these minerals are widely extracted for chemical fertilizers and plaster production.