Types of Clastic (Detrital) Sedimentary Rock

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Types of Clastic (Detrital) Sedimentary Rocks

Clastic sedimentary rocks are formed from the accumulation, compaction, and cementation of fragments of pre-existing rocks and minerals. They are classified based on grain size, composition, and depositional environment.


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1. Conglomerate

Grain Size: Coarse-grained (> 2 mm)

Composition: Rounded pebbles, cobbles, and boulders cemented together by sand, silt, or clay.

Depositional Environment: High-energy environments such as fast-flowing rivers, alluvial fans, and beaches.

Key Feature: Rounded clasts indicate prolonged transport and erosion.

2. Breccia
Grain Size: Coarse-grained (> 2 mm)
Composition: Angular fragments of rock cemented together.
Depositional Environment: Found in landslide deposits, fault zones, and debris flows.

Key Feature: Angular clasts suggest minimal transport before lithification.

3. Sandstone
Grain Size: Medium-grained (0.0625 – 2 mm)
Composition: Mostly quartz, feldspar, and rock fragments.
Depositional Environment: Rivers, deserts, beaches, deltas, and shallow marine environments.

Key Feature: Porous and often well-sorted, making it a good reservoir for water, oil, and gas.


Types of Sandstone
1. Quartz Arenite – Mostly quartz, well-sorted, formed in stable environments (e.g., beaches, deserts).

2. Arkose – Contains at least 25% feldspar, often found near granitic source rocks.

3. Graywacke – Poorly sorted, contains rock fragments and clay, formed in deep-sea environments.

4. Siltstone
Grain Size: Fine-grained (0.004 – 0.0625 mm)
Composition: Quartz and clay minerals.
Depositional Environment: Floodplains, deltas, shallow marine, and lake beds.

Key Feature: Softer and more cohesive than sandstone but more resistant than shale.

5. Shale
Grain Size: Very fine-grained (< 0.004 mm)
Composition: Mostly clay minerals and organic material.
Depositional Environment: Deep marine, lake beds, lagoons, and floodplains.

Key Feature: Thinly layered (fissile), splits into sheets, and often rich in fossils.


Related Fine-Grained Rocks
Mudstone – Similar to shale but lacks fissility (does not split easily).
Claystone – Dominated by clay minerals, very smooth and compact.

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