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

Sedimentary Rocks Classification

Sedimentary rocks are classified based on various criteria such as their mineralogical composition, mode of formation, environment of deposition, and textural features. This classification helps us understand the different types of sedimentary rocks that exist.

1. 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 include sandstone, shale, and conglomerate.

2. Non-clastic Rocks

• Non-clastic rocks are formed from chemical or organic processes rather than the accumulation of rock fragments.
• These rocks may include limestone, coal, and chert.

By understanding the classification of sedimentary rocks, geologists can interpret the history of the Earth and the environments in which these rocks were formed.

Clastic Rocks

• Sediments of clastic rocks originate from the breakdown of existing rocks.
• The grain size of these sediments varies widely, ranging from clay, silt, and sand to larger materials like pebbles, cobbles, and boulders.
• These sediment particles, known as clasts, are transported by forces such as gravity, running water, wind, and glaciers.
• They are deposited in different types of basins like marine, fluvial, aeolian, lacustrine, etc., based on the prevailing temperature and pressure conditions.

Examples of Clastic Rocks:

• Conglomerate: A rock formed from the deposition of a mix of rounded gravel-sized clasts.
• Sandstone: Comprised of sand-sized grains cemented together.
• Shale: Consists of very fine-grained clay and silt particles compacted together.

Composition of Clastic Rocks:

• Clastic rocks are made up of grains such as quartz, feldspar, mica, clay minerals, iron oxides, and various rock fragments.
• Lithic fragments found in clastic rocks may include pieces of limestone, mudrock, plutonic/volcanic rock, and chert, among others.
• Additionally, clastic rocks can contain plant debris, shell and skeletal materials, as well as secondary cementing minerals like calcite and amorphous silica.
• A small amount of heavy minerals like zircon, tourmaline, and rutile may also be present in clastic rocks.
• Quartz is the most abundant mineral in clastic rocks due to its resistance to weathering, while feldspars are common and tend to alter to clay minerals during chemical weathering.
• Other chemically reactive minerals, such as olivine, amphiboles, pyroxenes, micas, and carbonates, are typically absent in mature clastic sediments.

Texture of Clastic Rocks:

• Clastic sedimentary rocks exhibit clastic texture, characterized by a framework of mineral grains of varying sizes cemented by chemical precipitates or matrix.
• Texture in clastic rocks includes considerations of grain size, grain shape, and sorting of sediments, which are crucial for interpreting the geological history of the rock.

Sedimentary Rocks Characteristics

Softness

• Sedimentary rocks are softer compared to igneous rocks. They can be easily scratched with a nail.

Layering

• Sedimentary rocks are often found in layers or beds that can range from a few millimeters to 100 feet in thickness. The typical thickness is around 1-5 feet.

Texture

• These rocks have a granular and gritty texture if they are composed of sand and silt-sized particles. Sandstones often contain rounded or sometimes angular grains.

Sedimentary Structures

• In hand specimens, sedimentary structures like cross-bedding, mud cracks, ripple marks, worm trails, burrows, and fossil shells are usually not visible. However, they can be observed in outcrops or exposed rock formations.

Color

• The color of sedimentary rocks is typically not significant. Even a small amount of hematite (iron oxide) can give the rock a rich red color. Some pinkish sandstones derive their color from feldspar.

Fossils

• Fossils are more commonly found in shales compared to sandstones. This is because shales preserve fossils better due to lower current activity, while the higher energy environments of sandstones may destroy fossils.

Non-Clastic Rocks

• Definition: Non-clastic sedimentary rocks are composed of fragments of interlocking minerals that are formed through chemical processes like evaporation, precipitation, crystallization, or biochemical processes.
• Types of Non-Clastic Rocks:
  • Chemical Sedimentary Rocks: These rocks form when minerals precipitate out of a solution, often through processes like evaporation. For example, rocks like limestone and rock salt are formed through chemical processes.
  • Biochemical / Organic Sedimentary Rocks: These rocks are formed by the accumulation of remains of hard parts of organisms. For instance, rocks like chalk and coal are examples of biochemical sedimentary rocks.

Chemical Sedimentary Rocks

• The dissolved mineral products resulting from weathering processes are carried in solution and deposited in a basin through chemical precipitation.
• Chemical sedimentary rocks form through chemical precipitation, such as limestone, dolomite, ironstone, and phosphates.
• Some rocks are created through the chemical precipitation and evaporation of substances dissolved in seawater within enclosed basins.
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  • Examples of such rocks include evaporites like gypsum (CaSO4.2H2O) and rock salt (NaCl).
• This type of sedimentation is known as chemical sedimentation.

Biochemical / Organic Sedimentary Rocks

• Organic Sedimentary Rocks Composition:
  • Organic and biogenic sedimentary rocks are primarily composed of carbonaceous materials such as plant debris and skeletal remains of organisms.
• Examples of Organic Sedimentary Rocks:
  • Coal Deposits: Formed by the accumulation of plant debris in swamps or bogs, coal is a significant example of organic sedimentary rocks.
  • Coquina and Chalk Deposits: These rocks are deposited by the accumulation of macro and microorganisms, showcasing the diverse origins of organic sedimentary rocks.

Characteristics of Soft Rocks

• Soft rocks are primarily composed of minerals like halite, gypsum, and calcite, making them susceptible to easy scratching by materials like steel or a copper penny.
• They are typically fine-grained and homogeneous in nature.
• Fossils are frequently found embedded in limestone, a type of soft rock.
• Limestone displays effervescence when exposed to dilute hydrochloric acid (HCl) due to its composition of the mineral calcite (CaCO3).
• Chert, a very hard silica akin to flint, is sometimes present in limestone. It weathers to a brown hue on the surface, often occurring in nodules and occasionally replacing fossil shells.

Examples and Explanation

Soft rocks, such as limestone, are easily scratched because they consist of minerals like calcite. For instance, when you run a steel tool or a copper penny across their surface, visible marks are left behind due to the softness of these minerals.

Significance of Fossils in Soft Rocks

Fossils found within soft rocks like limestone provide crucial insights into past ecosystems. By studying these fossils, scientists can reconstruct ancient environments and understand the evolution of life on Earth.