Sedimentary Rock:
Sediments are materials formed due to mechanical or chemical activity by the agents of denudation on preexisting rocks. They are deposited in a stratified fashion, layer after layer, at the surface of the lithosphere. Lithification of the sediments at relatively low temperatures and pressure leads to the formation of sedimentary rocks.
Nature of Sedimentary Rocks:
Sedimentary rocks are composed of
- Fragments of other rocks
- Chemical precipitates
- Organic matter
Sediment Sizes:
Detrital sediment is classified according to size. Detrital sedimentary rocks can contain various sizes, from mud to boulders.
Wentworth Grade scale
- boulder > 256 mm
- cobble 64 to 256 mm
- pebble 4 to 64 mm
- granule 2 to 4 mm
- sand 1/16 to 2 mm
- silt 1/256 to 1/16 mm
- clay < 1/256 mm
Common clastic sedimentary rocks:
- Conglomerate
- Sandstone
- Siltstone
- Mudrock or Claystone and
- Shale
Common non-clastic sedimentary rocks:
- Limestone
- Dolostone
- Chert
- Rock salt
- Rock gypsum
- Coal
Other Sedimentary Rocks:
- Phosphates. Nodules on the seafloor (ooze). Fossilized guano (bird crap).
- Iron formations. Sulfides. Bog iron. Laterites. Placer deposits. Oolites! Banded iron formation.
- Mn nodules on the ocean floor.
Sedimentary Processes:
Lithification is the general term for the processes that convert loose sediment into sedimentary rock. Most sedimentary rocks are lithified by a combination of
- Compaction
- Desiccation
- Cementation
- Crystallization
Compaction:
Compaction involves packing sediment grains together through burial, reducing sediment volume by up to 40%. This reduction generally results from the pressure or weight of overlying sediments.
Desiccation:
Desiccation involves water loss from sediment pore spaces, typically resulting from compaction and air evaporation.
Cementation:
Cementation is the process by which sediments or rock fragments are bound together after deposition. Cementing materials are deposited from the mineral-rich waters that percolate through the open pore space of the rock.
Cement:
Cement is the binding agent of grains in clastic rock. It is usually a chemical sediment (calcium carbonate, silica, or Fe oxides) that precipitates in pore spaces due to underground water moving through these remaining pore spaces.
Matrix:
The sedimentary rock matrix is a finer-grained sedimentary material, such as clay or silt, in which larger grains or clasts are embedded.
Crystallization:
Some sedimentary rocks form by crystallization, the development and growth of crystals by precipitation from solution at or near the Earth’s surface (the term is also used for igneous rocks that crystallize as magma cools). These rocks have a Crystalline texture, an arrangement of interlocking crystals that develops as crystals grow and interfere with each other.
Crystalline rocks lack cement, but the interlocking of crystals holds them together. Such stones have minimal pore space because the crystals typically grow until they fill all available space.
Different Types of Structure:
There are three inherent structures of sedimentary rocks
- Physical structures/ Primary/ Mechanical structures
- Chemical or secondary structures
- Organic structures
Physical structures/ Primary/ Mechanical structures:
Physical or primary structures are formed during sediment deposition. They are closely linked to deposition conditions, like the environment (river, ocean, or desert) and the energy levels of the transporting medium (wind, water, or ice). Common physical structures include:
- Being layered, or buried, within sedimentary rocks is one of their most striking features. Sediments are laid in layers, each indicating a particular environment or event. These beds can differ in thickness, composition, and color.
- Crossbedding happens when layers of sediment are tilted or slanted. It’s commonly seen in settings such as dunes or riverbeds, where the direction of sediment movement changes over time. Crossbedding offers insights into historical wind or water flow directions.
- In graded bedding, there is a gradual shift in particle size from the bottom to the top of a layer. This type of bedding usually occurs underwater during events like turbidity currents, with coarser materials settling first, followed by finer ones.
- Ripple Marks are small, wavelike structures that form due to the influence of water or wind on sandy surfaces. They can be symmetric, indicating wave action, or asymmetric, suggesting current flow in a particular direction.
- Mud Cracks: When muddy sediments dry and contract, they form polygonal patterns. These patterns are commonly seen in lake beds, floodplains, or other areas where water evaporates after depositing fine sediments.
Chemical Structures (Secondary Structures):
After the sediments have been deposited, chemical or secondary structures start to form, usually during diagenesis – the process through which sediments are transformed into rock. These structures emerge due to chemical reactions, mineral crystallization, or alterations in sediment composition. Examples of chemical structures encompass:
- Concretions are rugged, compact masses created when mineral-rich groundwater deposits minerals into specific areas within the sediment. They typically have a different composition and texture compared to the surrounding rock and can vary in size from small pebbles to large boulders.
- Geodes: Hollow, spherical stones lined with crystals on the inside. These form when mineral-rich liquid seeps into cavities in sediments, leaving behind crystals as the water evaporates.
- Stylolite comprises jagged, irregular surfaces that pop up when sediments are put under pressure, causing them to dissolve. You’ll usually spot stylolite as these dark, wavy lines within the rock come about due to the minerals dissolving along weak zones.
- Silicification occurs when fluids rich in silica replace the original minerals in sedimentary rocks. This process results in the formation of chert or flint.
Organic Structures:
Organic formations develop through the gathering and solidifying of organic matter, often from plant or animal debris. These formations play a crucial role in comprehending ancient life and ecosystems. Some typical organic formations found in sedimentary rocks are:
- Fossils: Fossils are perhaps the most striking feature of sedimentary rocks. They are the preserved remains or traces of prehistoric organisms. They provide valuable insight into the biological history of the Earth and past environmental conditions.
- Coal Seams: These are created through the buildup and compression of plant matter, usually in marshy areas. As time passes, this organic material changes into coal in strata within sedimentary rock formations.
- Oil shales are formed in environments where organic material accumulates faster than it can decompose, often in low-oxygen conditions. They are rich in organic matter and serve as important sources of hydrocarbons.
- Some sedimentary rocks, like specific types of limestone, are created from the buildup of shells, coral, and other biological matter. These rocks frequently exhibit preserved traces of marine life.
Sorting:
- Sorting is the degree of similarity in particle size in a sedimentary rock.
- It is the arrangement of particles in a particular order according to their type or to separate particles of type from other
- The sediment sorting is primarily a result of water’s variable carrying capacity, which is a vital function of current velocity.
- These currents are common in most rivers. Strong currents (> 50 cm/s) occasionally move gravel and boulders. Moderate currents (20-50 cm/s) move sand and smaller clasts. Weaker currents can only transport silts and clays, the smallest size class.
- Desert winds also move sand. Due to their low viscosity, wind can usually only move small clasts like sand and clay particles, but strong wind can carry larger grains like pebbles, cobbles, etc.
- Glaciers move very slowly compared to liquid water, but due to the much higher viscosity of ice, they have enormous carrying capacity.
- Sediments of mixed size are sorted by carrying capacity. If a stream’s capacity decreases, its larger clasts will no longer be transported. The suspended load then consists only of fines.
Relative Abundance Sedimentary & Igneous Rocks:
Sedimentary rocks cover only 5% of the Earth’s crust but cover about 75% of its surface.
Conclusion:
Sedimentary rocks provide a unique record of Earth’s history, and their structures offer critical clues to the processes that formed them. From the physical structures that reflect the environment of deposition to chemical structures that reveal post-depositional changes and organic structures that preserve the remnants of past life, sedimentary rocks are a vital resource for understanding the geological past.
By studying these structures, geologists can reconstruct ancient landscapes, climates, and even life forms that once thrived on Earth.
This diversity of sedimentary rock structures showcases the dynamic processes in Earth’s surface environments. Understanding them enriches our knowledge of how the planet has evolved over millions of years.
