Biogenic Sedimentary Rocks

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Biogenic (Organic) Sedimentary Rocks

Biogenic or organic sedimentary rocks are formed from the accumulation and lithification of plant and animal remains, primarily in marine or swampy environments. These rocks contain significant amounts of organic material or biologically produced minerals.

1. Coal
Composition: Carbon-rich plant material.

Formation Process:
1. Dead plant material accumulates in swampy environments.
2. Burial and compression reduce water content.
3. Heat and pressure convert organic matter into coal over time.

Depositional Environment: Swamps, peat bogs, and deltas.

Types of Coal (Based on Carbon Content & Maturity):

Peat – Loose, partially decomposed plant material.
Lignite – Low-rank, brown coal with high moisture content.
Bituminous Coal – Medium-rank, black, and commonly used for fuel.
Anthracite – High-rank, hard, shiny, and has the highest carbon content.

2. Chalk
Composition: Microscopic marine organisms (coccolithophores) made of calcium carbonate (CaCO₃).

Formation Process:
1. Accumulation of planktonic microorganisms in deep marine environments.

2. Compression and lithification form chalk.
Depositional Environment: Deep marine settings with low energy.
Key Feature: Soft, powdery texture; reacts with acid.

3. Coquina
Composition: Loosely cemented shell fragments (CaCO₃).

Formation Process:
1. Broken shells accumulate in high-energy marine environments.
2. Cementation binds fragments together.
Depositional Environment: Beaches, shallow marine areas.
Key Feature: Very porous and lightweight; often has visible shell fragments.

4. Fossiliferous Limestone
Composition: Calcium carbonate with visible fossils (corals, shells, skeletal remains).

Formation Process:
1. Accumulation of marine fossils in shallow seas.
2. Lithification turns sediments into limestone.
Depositional Environment: Coral reefs, shallow marine settings.
Key Feature: Contains well-preserved fossils, often used for paleontological studies.

5. Diatomite
Composition: Silica-rich microscopic diatoms (unicellular algae).

Formation Process:
1. Accumulation of diatom shells in lakes or oceans.


2. Compaction and lithification.
Depositional Environment: Deep lakes, ocean basins.

Key Feature: Lightweight, highly porous, used in filtration and abrasives.


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