Sedimentary Structures & Rocks: Types, Indicators and Depositional Features
Unit 3a: Sedimentary Structures
Definition
Sedimentary structures are physical features formed during or just after deposition of sediments.
They give information about:
- Depositional environment
- Direction of current
- Energy conditions
- Younging direction of strata
1. Bedding
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
- Helps in stratigraphic correlation
2. Dip and Strike
Dip
Dip is the angle at which a bed is inclined from the horizontal. It is measured in the direction of maximum slope and expressed in degrees.
Strike
Strike is the direction of the line of intersection between a bedding plane and a horizontal plane. It is measured as a compass direction.
Significance
- Helps in understanding tectonic deformation
- Important for geological mapping
3. Lamination
Laminations are very thin layers within sedimentary rocks, with thickness less than or equal to 1 cm. They are finer and less prominent than bedding.
Formation
Formed due to:
- Variation in sediment supply
- Seasonal changes
- Low-energy conditions
Types
- Laminated beds
- Non-laminated beds
4. Cross-Bedding (Cross-Stratification)
Cross-bedding consists of 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
Graded bedding shows a systematic change in grain size within a bed.
Normal Graded Bedding
- Coarse grains at the base
- Fine grains at the top
- Most common type
Reverse Graded Bedding
- Fine grains at the base
- Coarse grains at the top
- Less common
Formation
Due to:
- Settling of particles from suspension
- 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. Occur mainly in deep marine environments. Highly turbulent with high Reynolds number.
Deposits
- Produce turbidites
- Show graded bedding
7. Bouma Sequence
A typical vertical sequence formed by turbidity currents. Divided into five units:
Division and Characteristics
- Ta — Massive or graded sand
- Tb — Parallel lamination
- Tc — Ripple lamination
- Td — Fine parallel lamination
- Te — Pelagic mud
Significance
- Indicates deep sea sedimentation
- Used in paleogeographic reconstruction
8. Ripple Marks
(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
Large-scale current ripples with greater wavelength and height. Form under strong current conditions.
10. Mud Cracks
Form in clay-rich sediments. Develop 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
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
- 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
- Important for paleocurrent analysis
13. Bioturbation
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
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
- Climate interpretation
Way-up Indicators
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
- Essential in structural geology
Unit 3b: Sedimentary Rocks
1. What are Sedimentary Rocks?
Sedimentary rocks are formed by:
- Weathering of pre-existing rocks
- Transportation of sediments
- Deposition in basins
- Compaction and cementation (lithification)
They cover ~75% of Earth’s surface but form only ~5% of Earth’s crust by volume.
2. Broad Classification of Sedimentary Rocks
Sedimentary rocks are classified into four major groups:
- Clastic (Detrital) rocks
- Carbonate rocks
- Evaporites
- Other sedimentary rocks
PART – A
3. Clastic (Detrital) Sedimentary Rocks
Definition
Clastic rocks are formed from fragments (clasts) of pre-existing rocks. These fragments are produced by mechanical weathering.
Characteristics
- Made of solid particles
- Transported by water, wind, ice
- Deposited when energy decreases
Examples
- Conglomerate
- Breccia
- Sandstone
- Shale
4. Size Range of Clasts
Clasts vary greatly in size:
- Clay — < 0.004 mm
- Silt — 0.004 – 0.062 mm
- Sand — 0.062 – 2 mm
- Gravel — > 2 mm
- Boulder — metres
📌 Mudrocks (shale, claystone) form ~60%
📌 Sandstone & conglomerate form ~20–25%
5. Size-Based Classification of Clastic Rocks
- Gravel (>2 mm) — Conglomerate / Breccia
- Sand — Sandstone
- Silt — Siltstone
- Clay — Shale / Claystone
6. Conglomerate vs Breccia
Conglomerate → Rounded clasts. Breccia → Angular clasts. Shape indicates distance of transport.
7. Triangular Plots (Gravel–Sand–Mud)
Used to:
- Classify clastic rocks
- Understand proportion of grain sizes
📌 Important in sedimentology & basin analysis
8. Sandstone Classification
Sandstones are classified based on texture and composition.
9. Textural Maturity of Clastic Rocks
Textural maturity depends on four factors:
- Mud content — more mud → less mature
- Sorting — well sorted → more mature
- Grain size uniformity
- Grain shape — rounded grains → more mature
📌 High energy environments → more mature sediments
10. Compositional Maturity
Based on mineral stability:
- Unstable minerals (feldspar, rock fragments) → less mature
- Stable minerals (quartz) → more mature
➡ Quartz-rich sandstone = most mature
11. Depositional Environment & Maturity
- River, delta → moderate maturity
- Desert, beach → high maturity
- Glacial → low maturity
PART – B
12. Carbonate Rocks
Definition
Carbonate rocks contain >50% calcium carbonate (CaCO₃).
Important Note
Calcareous → rich in CaCO₃. Carbonaceous → rich in carbon (coal).
📌 Carbonates form 10–15% of sedimentary rocks.
13. Sources of Carbonates
- Biogenic — shells, skeletal remains
- Chemical precipitation — direct from seawater
14. Important Carbonate Minerals
(a) Calcite
- Composition: CaCO₃
- Crystal system: Trigonal
- Hardness: 3
- Reacts strongly with acid
(b) Aragonite
- Same composition as calcite
- Crystal system: Orthorhombic
- More dense than calcite
(c) Dolomite
- Composition: CaMg(CO₃)₂
- Forms by dolomitisation — Mg replaces Ca in limestone
- Reacts weakly with acid
15. Petrography of Limestones
Under microscope: high relief, two cleavages, hardness ~3. 📌 Dolomite can be stained, calcite cannot.
16. Bioclastic Limestones
Characterized by type of fossils (shells, corals), shape of bioclasts, and non-biogenic grains (ooids, pellets).
17. Dunham’s Classification of Limestone
Based on depositional texture:
- Mudstone
- Wackestone
- Packstone
- Grainstone
- Boundstone
📌 Very important for exams.
PART – C
18. Evaporites
Definition
Evaporites form when salts precipitate due to evaporation of water.
19. Common Evaporite Rocks
(a) Gypsum
CaSO₄·2H₂O — hydrous, soft.
(b) Anhydrite
CaSO₄ — no water, harder than gypsum.
(c) Halite
NaCl — rock salt.
(d) Sylvite
KCl — bitter-salty taste.
PART – D
20. Other Sedimentary Rocks
21. Ironstones
Contain iron minerals like hematite, magnetite, goethite, limonite, pyrite, siderite.
📌 Deposited mostly in shallow marine environments.
22. Banded Iron Formation (BIF)
Precambrian age; alternating layers of hematite and chert / siltstone. Not forming today.
23. Ferromanganese Deposits
Nodules on the ocean floor formed authigenically.
24. Carbonaceous Deposits
Contain high organic matter:
- Mudrock — >2% organic content
- Limestone — >0.2% organic content
- Sandstone — >0.05% organic content
25. Coal
Organic matter >65%. Formed from plant remains. Progression: Peat → Lignite → Bituminous → Anthracite.
26. Volcaniclastic Rocks
Formed from volcanic eruptions. Pyroclastic material includes ash, lapilli, bombs — collectively called tephra.
27. Tuff
Consolidated volcanic ash, classified based on grain size.
28. Volcanic Agglomerate
Coarse volcanic fragments found near volcanic vents.
29. Final Summary (VERY IMPORTANT)
This unit covers:
- Sedimentary rock classification
- Clastic rocks & sandstone classification
- Carbonate rocks & Dunham’s classification
- Evaporites
- Ironstone, coal & volcaniclastic rocks