Geology of Assam
Assam’s geology reflects its position at the eastern Himalayan foreland, with thick Tertiary sediments in valleys overlying Precambrian basement in hills/plateaus. Rich in hydrocarbons, it features geosynclinal deposits shaped by tectonics and erosion.
Geological Setting of Assam
Assam lies in the Assam Shelf and Assam-Arakan Basin, a foreland basin from Indian-Eurasian plate collision.
Thick Tertiary clastics (3.5–6.5 km) fill valleys; Archaean gneisses in hills.
Tectonics: Subduction zone influences; Naga Thrust, Haflong Thrust define structural highs/lows.
Major Geological Divisions
Brahmaputra Valley (Assam Shelf)
Holocene–Pleistocene alluvium (sand, silt, clay, pebbles) over 4 km Tertiary sediments.
Basement: Precambrian granitic/gneissic rocks; filled by geosynclinal Tethys sediments uplifted by Himalayan thrust.
Barak Valley (Surma Valley)
Similar alluvial cover; Oligocene–Miocene sediments (shales, sandstones) thinner than Brahmaputra.
Barail Range exposure: Folded Eocene–Oligocene rocks.
Karbi Anglong Plateau (Mikir Hills)
Archaean crystalline (gneiss, granite); extension of Shillong Plateau, senile topography (300–1,363 m).
Dissected by Kopili/Dhansiri; minor Gondwana (Talchir) patches.
Barail/North Cachar Hills
Tertiary sedimentary (Eocene limestone, Oligocene shales); folded anticlines-synclines N-S trend.
Faulted sedimentary rocks linking Patkai-Naga system.
Stratigraphy (Tertiary Sequence)
Dominant in plains/shelves; key for oil-gas.
| Formation/Group | Age | Lithology | Thickness | Significance |
|---|---|---|---|---|
| Alluvium/Dihing | Holocene–Plio | Sand, gravel, clay | Variable | Floodplains, chars |
| Tipam | Miocene | Coarse sandstone, grit | 1,500 m | Reservoir rock |
| Surma (Bokabil) | Oligocene | Shale, sandstone | 3,500 m | Source/cap rock |
| Barail | Eocene–Oligo | Coal-shale, sandstone | 2,000 m | Major source rock, coal |
| Kopili/Jaintia | Eocene | Shale, limestone | 200–400 m | Cap rock, foraminifera fossils |
| Sylhet/Prang | Paleocene–Eoc | Limestone, shale | 50–200 m | Basal marine transgression |
Mineral Resources
Petroleum: Upper Assam Shelf (Digboi, Sivasagar); Barail/Kopili source rocks, Tipam reservoirs; 7000 m sediments.
Coal: Barail Group (Makum, Nazira); low-ash, high-sulphur.
Limestone: Jaintia Hills (Sylhet Fm); cement raw material.
Others: Clay (alluvium), minor iron ore (hills).
Structural Features
Anticlines/Synclines: N-S trending in Naga Schuppen Belt; trap hydrocarbons.
Thrusts/Faults: Haflong Thrust (Barail), Naga Thrust; create horst-graben (Dhansiri Valley).
Pre-Tertiary: Gondwana rift-fill (limited); immature for HC generation.
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