Indian Sedimentary Basins and Sedimentary Structures
1. Sedimentary Basins
Definition: A sedimentary basin is a low-lying area of the Earth’s crust that undergoes subsidence and accumulates sediments over geological time.
Sediments may be clastic, chemical or biogenic.
Basins preserve information about tectonics, climate, provenance and resources.
2. Physiography of India and Basin Settings
India shows diverse physiographic divisions.
Major units:
- Himalayan orogenic belt
- Indo-Gangetic plain
- Peninsular craton
- Coastal plains
- Continental shelf and deep sea
Basin formation is controlled by:
- Tectonic movements and plate interactions
- Crustal flexure and rifting
- Gravity and topography (gravity and topography maps help identify basin geometry)
3. Types of Sedimentary Basins in India
(Based on tectonic setting)
Flexure Basins
Formed due to loading and bending of the lithosphere. Developed during Himalayan orogeny; also called Himalayan foreland basins.
Example: Indo-Gangetic Basin
Rift Basins
Formed due to tensional tectonics. Characterized by horst-and-graben structures.
Major Rift Basins of India:
- Satpura Basin
- Pranhita
- Godavari Rift
- Mahanadi Rift
- Narmada Rift
- Aravalli–Delhi trend
- Kutch Rift
- Jaisalmer Rift
Intracratonic Basins
Formed within stable continental interiors. Long-lived basins.
Major basins: Vindhyan Basin, Cuddapah Basin, Chhattisgarh Basin.
Minor basins: Kaladgi, Bhima, Pakhal, Penganga, Indravati, Khariar, Sabari, Kolhan.
Economic minerals: Limestone, dolomite, phosphorite, barite, pyrite, diamond and building stones.
Petroliferous Basins
Contain oil and natural gas.
India has 26 sedimentary basins covering about 3.35 million sq km.
Important basins: Krishna–Godavari, Pranhita–Godavari, Saurashtra, Kerala–Konkan, Bengal Basin.
4. Krishna–Godavari (KG) Basin
Location & extent: Eastern continental margin of India.
Onland: ~15,000 sq km. Offshore: ~25,000 sq km (up to 1000 m isobath).
Basin type: Passive margin, peri-cratonic basin.
Origin: Formed due to rifting during the Early Mesozoic. Basement faults aligned NE–SW (Eastern Ghats trend).
Basin evolution
- Rift stage: Permo-Triassic age — fluvial, lagoonal and brackish sediments.
- Syn-rift stage: Late Jurassic–Early Cretaceous — fault-controlled subsidence, thick sediment accumulation.
- Drift stage: Marine transgression in the Cretaceous; deposition of marine shale followed by deltaic Tirupati Sandstone.
- Late drift stage: Indian–Eurasian plate interaction; development of major faults.
- Neogene stage: Increased sediment supply, sea-level fall, formation of hydrocarbon-rich zones.
Economic importance: First gas discovery in 1983 (Razole-1 well). Major offshore gas discoveries by ONGC, Reliance, GSPC. Estimated gas reserves: ~34 TCF. Potential for tight oil and shale gas.
5. Pranhita–Godavari Basin
Gondwana rift basin with NNW–SSE trending grabens.
Length ~400 km, width ~100 km. Sediment thickness ~6000 m. Deposited over ~200 million years.
Stratigraphy
- Lower Gondwana: Talchir, Barakar, Barren Measures, Kamthi
- Upper Gondwana: Maleri, Kota, Gangapur, Chikiala
Economic importance
Major coal-bearing basin.
Important coalfields:
- Ramagundam
- Bellampalli
- Kothagudem
- Yellandu
- Manuguru
6. Saurashtra Basin
Location: Western continental margin of India, between Mumbai offshore and Kutch Basin. Onland part = Saurashtra Peninsula.
Stratigraphy
Sediment thickness ~6000 m.
Two major cycles:
- Jurassic marine transgression (carbonates & shales)
- Late Jurassic–Early Cretaceous regression (deltaic clastics)
Basin evolution
- Initiated during breakup of Gondwana
- Marine incursion in Early Jurassic
- Deltaic sedimentation in Mesozoic
- Covered by Deccan Trap volcanics
- Post-rift passive margin sedimentation in the Cenozoic
Economic importance
Hydrocarbon potential in Mesozoic rocks, but exploration is difficult due to basalt cover.
Other minerals: Lignite, bauxite.
7. Kerala–Konkan Basin
Location: Western margin of India, from Goa to Cape Comorin. Area ~580,000 sq km.
Basin evolution
- Early rift phase
- Post-rift passive margin phase
Sediment thickness >5 km.
Stratigraphy
Continental → transitional → marine deposition, interrupted by Deccan Trap volcanism. Cretaceous to Recent sediments.
Structural elements
- Shelfal horst-graben system
- Alleppey Plateau
- Lakshadweep Depression
- Lakshadweep Ridge
- Arabian Abyssal Plain (formed by sea-floor spreading)
8. Bengal Basin
Location: Northeastern India (West Bengal). Total area ~89,000 sq km.
Origin
Formed due to subduction of the Indian plate and rifting of the eastern margin. Large oceanic-type basin.
Stratigraphy
Initiated in the Middle–Upper Cretaceous. Early fluvial deposits (Bolpur Formation), shelf deposits (Ghatal Formation). Sediment thickness up to ~20 km.
Environment changes from brackish (shelf) to marine (deep basin).
Economic importance
- Hydrocarbons
- Coal and peat
- Heavy minerals
- Limestone
- Glass sand
- China clay
4. Krishna–Godavari (KG) Basin
Location & extent: Eastern continental margin of India.
Onland: ~15,000 sq km. Offshore: ~25,000 sq km (up to 1000 m isobath).
Basin type: Passive margin, peri-cratonic basin.
Formed due to rifting during the Early Mesozoic. Basement faults aligned NE–SW (Eastern Ghats trend).
Basin evolution:
(a) Rift Stage — Permo-Triassic age: fluvial, lagoonal and brackish sediments.
(b) Syn-rift Stage — Late Jurassic–Early Cretaceous: fault-controlled subsidence, thick sediment accumulation.
(c) Drift Stage — Marine transgression in the Cretaceous: deposition of marine shale followed by deltaic Tirupati Sandstone.
(d) Late Drift Stage — Indian–Eurasian plate interaction and development of major faults.
(e) Neogene Stage — Increased sediment supply, sea-level fall, formation of hydrocarbon-rich zones.
Economic importance: First gas discovery in 1983 (Razole-1 well). Major offshore gas discoveries by ONGC, Reliance, GSPC. Estimated gas reserves: ~34 TCF. Potential for tight oil and shale gas.
5. Pranhita–Godavari Basin
Gondwana rift basin. NNW–SSE trending grabens. Length ~400 km, width ~100 km. Sediment thickness ~6000 m. Deposited over ~200 million years.
Stratigraphy
Lower Gondwana: Talchir, Barakar, Barren Measures, Kamthi.
Upper Gondwana: Maleri, Kota, Gangapur, Chikiala.
Economic importance
Major coal-bearing basin.
Important coalfields: Ramagundam, Bellampalli, Kothagudem, Yellandu, Manuguru.
6. Saurashtra Basin
Location: Western continental margin of India, between Mumbai offshore and Kutch Basin. Onland part = Saurashtra Peninsula.
Stratigraphy: Sediment thickness ~6000 m.
Two major cycles: Jurassic marine transgression (carbonates & shales); Late Jurassic–Early Cretaceous regression (deltaic clastics).
Basin evolution: Initiated during breakup of Gondwana. Marine incursion in Early Jurassic; deltaic sedimentation in Mesozoic. Covered by Deccan Trap volcanics. Post-rift passive margin sedimentation in the Cenozoic.
Economic importance: Hydrocarbon potential in Mesozoic rocks. Exploration difficulty due to basalt cover. Other minerals: lignite, bauxite.
7. Kerala–Konkan Basin
Location: Western margin of India from Goa to Cape Comorin. Area ~580,000 sq km.
Basin evolution: Early rift phase, post-rift passive margin phase. Sediment thickness >5 km.
Stratigraphy: Continental → transitional → marine deposition, interrupted by Deccan Trap volcanism. Cretaceous to Recent sediments.
Structural elements: Shelfal horst-graben system, Alleppey Plateau, Lakshadweep Depression, Lakshadweep Ridge, Arabian Abyssal Plain (formed by sea-floor spreading).
8. Bengal Basin
Location: Northeastern India (West Bengal). Total area ~89,000 sq km.
Origin: Formed due to subduction of the Indian plate and rifting of the eastern margin. Large oceanic-type basin.
Stratigraphy: Initiated in the Middle–Upper Cretaceous. Early fluvial deposits (Bolpur Formation), shelf deposits (Ghatal Formation). Sediment thickness up to ~20 km.
Environment changes: Brackish (shelf) → Marine (deep basin).
Economic importance: Hydrocarbons, coal and peat, heavy minerals, limestone, glass sand, china clay.
Sedimentary Structures
Sedimentary structures are physical features formed during or just after deposition of sediments. They give information about depositional environment, direction of current, energy conditions and younging direction of strata.
1. Bedding
Definition: Bedding refers to layering in sedimentary rocks. Each layer is called a bed. Beds form when one layer of sediment is deposited over another. Bedding planes separate beds of different composition, texture or color. Thickness of beds is usually more than 1 cm.
Importance: Indicates changes in depositional conditions and helps in stratigraphic correlation.
2. Dip and Strike
Dip: The angle at which a bed is inclined from the horizontal. Measured in the direction of maximum slope and expressed in degrees.
Strike: The direction of the line of intersection between a bedding plane and a horizontal plane. Measured as a compass direction.
Significance: Helps in understanding tectonic deformation and is important for geological mapping.
3. Lamination
Laminations are very thin layers within sedimentary rocks (thickness ≤ 1 cm). They are finer and less prominent than bedding.
Formation: Formed due to variation in sediment supply, seasonal changes, and low-energy conditions.
Types: Laminated beds, non-laminated beds.
4. Cross-Bedding (Cross-Stratification)
Inclined layers formed within a single bed. These layers are not parallel to the main bedding plane.
Formation: Caused by migration of ripples and dunes under current action. Sediments are deposited on the lee side of bedforms.
Types: Planar cross-bedding, trough cross-bedding, herringbone cross-bedding (bidirectional flow – tidal environment).
Significance: Indicates direction of paleocurrent; common in river, desert and shallow marine environments.
5. Graded Bedding
Shows systematic change in grain size within a bed.
Normal graded bedding: Coarse grains at the base and fine grains at the top (most common).
Reverse graded bedding: Fine grains at the base and coarse grains at the top (less common).
Formation: Due to settling of particles from suspension and decrease in flow velocity with time.
Significance: Important way-up indicator; common in deep marine deposits.
6. Turbidity Currents
Gravity-driven flows of sediment-laden water that occur mainly in deep marine environments. They are highly turbulent with high Reynolds numbers.
Deposits: Produce turbidites and show graded bedding.
7. Bouma Sequence
A typical vertical sequence formed by turbidity currents, divided into five units:
- Ta: Massive or graded sand
- Tb: Parallel lamination
- Tc: Ripple lamination
- Td: Fine parallel lamination
- Te: Pelagic mud
Significance: Indicates deep-sea sedimentation and is used in paleogeographic reconstruction.
8. Ripple Marks
(a) Current ripples: Formed by unidirectional flow (river, wind). Asymmetrical shape with a gentle stoss side and a steep lee side. Indicate current direction.
(b) Wave ripples: Formed by oscillatory motion of waves. Symmetrical shape; crests are long and straight. Common in shallow marine environments.
9. Dunes
Large-scale current ripples with greater wavelength and height. Form under strong current conditions.
10. Mud Cracks
Form in clay-rich sediments due to drying and shrinkage of mud. Cracks are later filled by younger sediments.
Significance: Indicate subaerial exposure; typical of floodplains, tidal flats, deserts. Excellent way-up indicator.
11. Rain Imprints
Small, circular, crater-like pits on soft sediment. Diameter usually < 1 cm. Formed by raindrop impact.
Significance: Indicates exposure of sediment surface to the atmosphere.
12. Sole Marks
Erosional structures formed on the base of beds.
(a) Scour marks: Produced by turbulent flow; include flute casts and obstacle scours.
(b) Tool marks: Formed by objects dragged along the bed. Types: groove marks, prod marks, skip or bounce marks.
Significance: Indicate flow direction and are important for paleocurrent analysis.
13. Bioturbation
Disturbance of sediments by organisms. Includes burrows, tracks and trails. Results in destruction of primary structures.
Importance: Indicates biological activity; common in marine and shallow-water environments.
Paleocurrent Analysis
Definition
Paleocurrent is the direction of sediment transport at the time of deposition.
Indicators
- Cross-bedding
- Ripple marks
- Groove marks
- Pebble imbrication
- Flute casts
Method
Orientation data are collected and plotted on rose diagrams.
Types
- Unimodal – river
- Bimodal – tidal
- Multimodal – aeolian
Importance
Helps in provenance studies, paleogeographic reconstruction and climate interpretation.
Way-up Indicators
Definition
Features that help determine the younging direction of strata.
Need
Tectonic activity may tilt or overturn beds, so way-up indicators are essential to establish the original top and bottom of strata.
Common way-up indicators
- Graded bedding
- Mud cracks
- Ripple marks
- Cross-bedding
- Sole marks
Significance: Helps in relative age dating and is essential in structural geology.
5. Graded Bedding (repeated)
Shows systematic change in grain size within a bed. Normal graded bedding: coarse grains at the base, fine grains at the top (most common). Reverse graded bedding: fine grains at the base, coarse grains at the top (less common).
Formation: Due to settling of particles from suspension and decrease in flow velocity with time.
Significance: Important way-up indicator; common in deep marine deposits.
6. Turbidity Currents (repeated)
Gravity-driven flows of sediment-laden water that occur mainly in deep marine environments. Highly turbulent with high Reynolds numbers.
Deposits: Produce turbidites and show graded bedding.
7. Bouma Sequence (repeated)
A typical vertical sequence formed by turbidity currents. Divided into five units:
- Ta: Massive or graded sand
- Tb: Parallel lamination
- Tc: Ripple lamination
- Td: Fine parallel lamination
- Te: Pelagic mud
Significance: Indicates deep-sea sedimentation and is used in paleogeographic reconstruction.
8. Ripple Marks (repeated)
(a) Current ripples: formed by unidirectional flow (river, wind). Asymmetrical shape: gentle stoss side, steep lee side. Indicate current direction.
(b) Wave ripples: formed by oscillatory motion of waves. Symmetrical shape. Crests are long and straight. Common in shallow marine environments.
9. Dunes (repeated)
Large-scale current ripples with greater wavelength and height. Form under strong current conditions.
10. Mud Cracks (repeated)
Form in clay-rich sediments due to drying and shrinkage of mud. Cracks are later filled by younger sediments.
Significance: Indicates subaerial exposure. Typical of floodplains, tidal flats, deserts. Excellent way-up indicator.
11. Rain Imprints (repeated)
Small, circular, crater-like pits on soft sediment. Diameter usually < 1 cm. Formed by raindrop impact. Significance: indicates exposure of sediment surface to atmosphere.
12. Sole Marks (repeated)
Erosional structures formed on the base of beds.
(a) Scour marks: produced by turbulent flow (include flute casts and obstacle scours).
(b) Tool marks: formed by objects dragged along the bed. Types: groove marks, prod marks, skip or bounce marks. Significance: indicate flow direction and are important for paleocurrent analysis.
13. Bioturbation (repeated)
Disturbance of sediments by organisms; includes burrows, tracks, trails. Results in destruction of primary structures. Importance: indicates biological activity. Common in marine and shallow-water environments.
Paleocurrent Analysis (repeated)
Definition: Paleocurrent is the direction of sediment transport at the time of deposition.
Indicators: Cross-bedding, ripple marks, groove marks, pebble imbrication, flute casts.
Method: Orientation data collected and plotted on rose diagrams.
Types: Unimodal – river; Bimodal – tidal; Multimodal – aeolian.
Importance: Helps in provenance studies, paleogeographic reconstruction and climate interpretation.
Way-up Indicators (repeated)
Definition: Features that help determine the younging direction of strata.
Need: Tectonic activity may tilt or overturn beds.
Common way-up indicators: Graded bedding, mud cracks, ripple marks, cross-bedding, sole marks.
Significance: Helps in relative age dating and is essential in structural geology.