Clastic and Non-Clastic Rocks | Geology Optional for UPSC PDF Download

Classification of Sedimentary Rocks

Sedimentary rocks are classified based on their mineralogical composition, mode of formation, environment of deposition, and textural features. This classification provides insights into the different types of sedimentary rocks and their significance in Earth's history.

Clastic Rocks

Clastic rocks are formed from the accumulation of fragments or clasts of pre-existing rocks. These fragments are transported and deposited by wind, water, ice, or gravity.

Examples of Clastic Rocks

• Conglomerate: Composed of a mix of rounded gravel-sized clasts.

• Sandstone: Consists of sand-sized grains cemented together.

• Shale: Made of very fine-grained clay and silt particles compacted together.

Sediments and Formation

• Clastic rock sediments originate from the breakdown of existing rocks.

• The grain size varies widely, ranging from clay, silt, and sand to pebbles, cobbles, and boulders.

• Clasts are transported by forces such as gravity, running water, wind, and glaciers.

• Deposited in different types of basins like marine, fluvial, aeolian, and lacustrine under specific temperature and pressure conditions.

Composition of Clastic Rocks

• Minerals: Quartz, feldspar, mica, clay minerals, iron oxides, and various rock fragments.

• Lithic Fragments: Include limestone, mudrock, plutonic/volcanic rock, and chert.

• Cementing Minerals: Calcite and amorphous silica.

• Heavy Minerals: Zircon, tourmaline, and rutile may be present in small amounts.

• Quartz is the most abundant mineral due to its resistance to weathering, while feldspars often alter to clay minerals during weathering.

Texture of Clastic Rocks

• Clastic Texture: Characterized by a framework of mineral grains of varying sizes cemented by chemical precipitates or matrix.

• Factors: Grain size, shape, and sorting of sediments provide clues to the geological history.

Non-Clastic Rocks

Non-clastic rocks are formed from chemical or organic processes rather than the accumulation of rock fragments.

Types of Non-Clastic Rocks

Chemical Sedimentary Rocks

• Formed when minerals precipitate out of a solution through evaporation or crystallization.
• Examples: Limestone, dolomite, rock salt, and gypsum.

Biochemical/Organic Sedimentary Rocks

• Formed by the accumulation of organic materials such as plant debris or skeletal remains.
• Examples: Chalk, coal, and coquina.

Chemical Sedimentary Rocks

• Created through chemical precipitation and evaporation of substances dissolved in water within enclosed basins.
• Examples: Evaporites like gypsum (CaSO4.2H2O) and rock salt (NaCl).
• This type of sedimentation is known as chemical sedimentation.

Biochemical/Organic Sedimentary Rocks

• Composition: Composed of carbonaceous materials such as plant debris and skeletal remains.
• Examples:
• Coal Deposits: Formed by the accumulation of plant debris in swamps or bogs.
• Coquina and Chalk: Deposited by macro and microorganisms.

Characteristics of Sedimentary Rocks

• Softness: Sedimentary rocks are softer compared to igneous rocks and can be easily scratched with a nail.
• Layering: Found in layers or beds ranging from millimeters to 100 feet in thickness, typically around 1-5 feet.
• Texture: Granular and gritty if composed of sand and silt-sized particles. Sandstones may contain rounded or angular grains.
• Sedimentary Structures: Features like cross-bedding, mud cracks, ripple marks, worm trails, burrows, and fossil shells may be visible in outcrops.
• Color: Affected by the presence of minerals like hematite, which gives rocks a red hue, and feldspar, which can create a pinkish appearance.
• Fossils: Common in shales due to low-energy depositional environments. Fossils are less preserved in sandstones due to higher energy conditions.

Significance of Fossils in Sedimentary Rocks

• Fossils found in sedimentary rocks like limestone provide critical insights into ancient ecosystems and evolutionary history.

Soft Rocks Characteristics

• Composed of minerals like halite, gypsum, and calcite, making them easily scratched by steel or a copper penny.
• Fine-grained and homogeneous.
• Fossils often embedded in soft rocks such as limestone, which reacts to dilute HCl due to calcite (CaCO3).
• Chert, a silica-rich material, can weather to a brown surface hue and is sometimes found replacing fossil shells.

Conclusion

The classification of sedimentary rocks based on clastic and non-clastic types, their composition, texture, and depositional environment, provides geologists with valuable tools for interpreting Earth's geological history and environmental changes over time.