Question

Identify the correct statements about Organic/Biochemical sedimentary rocks:Which of the statements is/are correct?

1. They are formed from organic remains.
2. Chalk is a classic example of this category.
3. They are formed by minerals precipitating directly from water.

• A. All
• B. i only
• C. i, ii
• D. ii, iii

Answer 1

Answer:

C. i, ii

Organic/Biochemical Sedimentary Rocks

Formation Processes:

• These rocks are primarily formed from the accumulation and lithification of organic matter, such as the remains of plants and animals.

• Alternatively, they can form when organisms extract chemical elements from seawater to create shells or skeletons, which then accumulate.

Key Characteristics:

• Organic Origin: The defining characteristic is their derivation from biological sources.

• Fossil Content: Often contain fossils, providing direct evidence of their biological origins.

Examples and Classification:

• Chalk: A quintessential example, composed mainly of the microscopic skeletal remains of marine organisms called coccolithophores. Its formation is a direct result of biological activity in marine environments.

• Coal: Formed from the compacted remains of plants that accumulated in ancient swamps or bogs. This is a classic example of a rock formed from organic debris.

• Limestone (Biogenic): Many types of limestone, like coquina (composed of shell fragments) and fossiliferous limestone, are formed from the accumulation of shells and skeletal fragments of marine life.

Distinction from Chemical Sedimentary Rocks:

• It is important to distinguish these from chemical sedimentary rocks. Chemical sedimentary rocks form when dissolved minerals precipitate directly from a water solution (e.g., rock salt from evaporating seawater, or some types of limestone formed by inorganic precipitation). Organic sedimentary rocks, while sometimes involving chemical precipitation by organisms, have a primary origin tied to organic matter or the biological processes of organisms.