6.2 Classification of sedimentary rocks
4 min read•july 22, 2024
Sedimentary rocks tell Earth's story through their composition and structure. From sandy beaches to ancient swamps, these rocks form in diverse environments. Understanding their classification helps geologists decipher past landscapes and climates.
Clastic, chemical, and organic-rich sedimentary rocks each have unique characteristics. By examining , mineral content, and textures, we can unravel the conditions under which these rocks formed, revealing Earth's dynamic history.
Classification of Sedimentary Rocks
Classification of clastic sedimentary rocks
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- Grain size
- Gravel (> 2 mm)
- Conglomerate formed from rounded gravel (river rocks, beach pebbles)
- Breccia composed of angular gravel (talus slopes, fault zones)
- Sand (1/16 mm to 2 mm)
- Sandstone common in beaches, rivers, and deserts (Navajo Sandstone)
- Silt (1/256 mm to 1/16 mm)
- Siltstone forms from compacted silt (loess deposits, river floodplains)
- Clay (< 1/256 mm)
- Claystone made of compacted clay particles (mud flats, deep marine)
- Shale is a fissile claystone that splits into thin layers (offshore marine)
- Gravel (> 2 mm)
- Composition
- -rich rocks like quartz arenite and quartzite contain over 90% quartz (beach sands)
- -rich arkose forms from rapidly eroded granitic terranes (fault-bounded mountains)
- Lithic fragments in lithic arenite and graywacke reflect nearby volcanic sources
- Texture
- describes grain size uniformity from well-sorted to poorly-sorted (beach vs. glacial till)
- of grains ranges from angular to rounded, indicating transport distance
- Matrix-supported rocks have more fine-grained matrix than larger clasts (mudflow deposits)
- Clast-supported rocks contain more gravel-sized clasts that support each other (river channel)
Types of chemical sedimentary rocks
- Limestone
- Composed of (CaCO₃) from precipitated minerals or marine organisms (corals, shells)
- Reacts readily with dilute hydrochloric acid (HCl) to produce bubbles (carbon dioxide gas)
- Common in shallow tropical seas (Bahamas) and as cave formations (stalactites, stalagmites)
- Dolostone
- Composed of dolomite (CaMg(CO₃)₂) formed when magnesium replaces some calcium in limestone
- Slightly harder than limestone and reacts more slowly with dilute HCl
- Often associated with evaporative tidal flat environments (sabkhas of Persian Gulf)
- Evaporites
- Form in restricted basins by evaporation of saline waters in arid climates (Great Salt Lake)
- Gypsum (CaSO₄·2H₂O) is a soft mineral that forms swallow-tail twins and desert roses
- Halite (NaCl) is rock salt that tastes salty and dissolves in water
- Sylvite (KCl) is an important potassium source often mined from evaporite deposits
Organic-rich sedimentary rock formation
- Coal
- Plant material accumulates in swamps or marshes and is rapidly buried (Carboniferous coal forests)
- With increasing burial depth and temperature, peat is converted to lignite (brown coal)
- Further burial and heating transforms lignite into bituminous coal (most common rank)
- At the highest temperatures and pressures, bituminous coal becomes anthracite (highest carbon)
- Coal ranks reflect carbon content and energy value, which increase with burial
- Oil shale
- Fine-grained sedimentary rock rich in solid, insoluble organic matter called kerogen
- Deposited in anoxic environments with abundant organic input (algal blooms, marine upwelling)
- When heated sufficiently, kerogen can generate oil and gas (Green River Formation)
- Extracted by mining and heating the shale (retorting) to release hydrocarbons
Identification of sedimentary rocks
- Grain size
- Use a grain size card or comparator to determine average grain diameters
- Gravel (> 2 mm), sand (1/16 to 2 mm), silt (1/256 to 1/16 mm), or clay (< 1/256 mm)
- Composition
- Identify primary mineral components like quartz, feldspar, and rock fragments
- Note any fossils (shells, coral), organic matter (plant debris), or unusual minerals (evaporites)
- Texture
- Well-sorted rocks have uniform grain sizes, while poorly-sorted contain a mixture
- Angular grains indicate little transport, rounded grains suggest significant abrasion
- Matrix-supported if fine sediment is more abundant than larger clasts, clast-supported if the opposite
- Chemical and
- Limestone fizzes vigorously with dilute HCl and may contain visible fossils
- Dolostone reacts more slowly with acid and tends to be harder and more resistant than limestone
- Evaporites like gypsum are soft (easily scratched) and halite tastes salty
- Organic-rich rocks
- Coal is black, low-density, and combustible, often breaking along cleats (fractures)
- Oil shale is fine-grained, dark colored, and may emit a petroliferous odor when broken
Sedimentary Rock Formation and Environments
Depositional environment indicators
- Clastic sedimentary rocks
- Grain size and sorting
- Coarse-grained, poorly-sorted sediments indicate proximity to source and high transport energy (alluvial fans, braided rivers)
- Fine-grained, well-sorted sediments suggest far from source in low-energy settings (offshore marine, deep lakes)
- Composition
- Quartz-rich sands form on stable cratons with intense chemical weathering or from recycled sediments (passive margins, beaches)
- Feldspar-rich arkoses derive from rapid erosion of uplifted metamorphic and igneous terranes (fault-block mountains)
- Lithic-rich sediments occur in active tectonic settings with nearby volcanic sources (island arcs, continental rifts)
- Grain size and sorting
- Chemical and biochemical sedimentary rocks
- Limestone forms in shallow, warm marine environments (coral reefs, carbonate platforms like the Bahamas)
- Dolostone is associated with evaporative tidal flats and restricted lagoons (sabkhas of Persian Gulf)
- Evaporites accumulate in restricted basins with arid climates and high evaporation rates (salt flats of Death Valley)
- Organic-rich sedimentary rocks
- Coal originates in swamps and marshes with humid climates and rapid burial of plant material (Carboniferous coal forests)
- Oil shale forms in anoxic marine environments with high organic productivity (algal blooms in upwelling zones, stagnant lagoons)
Key Terms to Review (24)
Biochemical rocks: Biochemical rocks are a type of sedimentary rock formed from the accumulation of organic material, typically the remains of plants and animals, as well as the activities of microorganisms. These rocks often contain significant amounts of calcite or silica and can provide important insights into past environments and biological processes. They are categorized based on their composition and the processes that lead to their formation.
Calcite: Calcite is a common mineral composed of calcium carbonate (CaCO₃) and is a major component of sedimentary rocks such as limestone and marble. Its presence in the rock cycle highlights its role in the transformation of sediments into solid rock, as well as its significance in various geological processes, including weathering and sedimentation. Calcite is also notable for its unique optical properties and reaction with acids, making it an important mineral in both geological studies and practical applications.
Cementation: Cementation is the process by which dissolved minerals precipitate from water and fill the spaces between sediment particles, binding them together to form solid rock. This essential step in the formation of sedimentary rocks is crucial for transforming loose sediments into a coherent structure, influencing the rock's durability and characteristics.
Chemical precipitates: Chemical precipitates are solid substances that form from a solution when the concentration of certain dissolved ions exceeds their solubility. This process often occurs during chemical reactions or changes in environmental conditions, leading to the formation of sedimentary minerals. These precipitates play a key role in the formation and classification of sedimentary rocks, particularly in identifying their composition and origin.
Chemical Rocks: Chemical rocks are a type of sedimentary rock formed from the precipitation of minerals from solution, primarily through chemical processes. They result from the evaporation of water or changes in pressure and temperature, leading to the crystallization of dissolved minerals. Understanding chemical rocks is crucial for recognizing how mineral content varies based on environmental conditions and influences geological formations.
Clastic rocks: Clastic rocks are a type of sedimentary rock that are formed from the accumulation and cementation of mineral fragments or particles derived from the weathering and erosion of pre-existing rocks. They are characterized by their granular texture, which is determined by the size and composition of the clasts, and they can provide valuable information about the geological history of an area.
Compaction: Compaction is the process by which sediments are squeezed together under pressure, reducing their volume and expelling water. This key process plays a crucial role in the formation of sedimentary rocks as it transforms loose sediments into more solid structures. As layers of sediment accumulate over time, the weight of the overlying materials exerts pressure, causing the grains to pack closer together, which leads to lithification—the transformation of sediments into rock.
Cross-bedding: Cross-bedding is a sedimentary structure characterized by the arrangement of sediment layers at an angle to the main bedding plane. This feature often indicates the flow direction of water or wind, providing important clues about the depositional environment. Cross-bedding is significant in the classification of sedimentary rocks as it helps geologists interpret past environmental conditions and sediment transport processes.
Delta: A delta is a landform that forms at the mouth of a river, where it meets a body of water, such as an ocean, sea, or lake. It is characterized by the deposition of sediment carried by the river as it slows down and spreads out upon entering the standing water. This process creates a fan-shaped area of sediment that can support diverse ecosystems and affect both sedimentary rock formation and coastal landscapes.
Deposition: Deposition is the geological process in which sediments, soil, and other particles settle out of a fluid, such as water or air, and accumulate in a new location. This process is crucial in the formation of sedimentary rocks and shapes various landforms by contributing to the layering of materials over time.
Detrital sediments: Detrital sediments are fragments of pre-existing rocks and minerals that have been weathered and eroded into smaller particles. These sediments are typically transported by water, wind, or ice and eventually deposited in various environments, contributing to the formation of sedimentary rocks. Their classification primarily depends on the size and composition of the particles, which can provide insights into the geological history and processes of the area.
Feldspar: Feldspar is a group of rock-forming minerals that make up about 60% of the Earth's crust, characterized by their aluminum silicate composition. They are crucial in identifying and classifying various rock types due to their abundance and properties, impacting both igneous and sedimentary formations in the Earth's geology.
Folk classification: Folk classification refers to the traditional and intuitive system of categorizing and naming sedimentary rocks based on observable characteristics, such as color, texture, and mineral composition. This system often reflects local knowledge and cultural practices, providing a way for communities to communicate about their geological environment without formal scientific terminology.
Graded bedding: Graded bedding is a sedimentary structure characterized by a gradual change in grain size within a single layer, typically transitioning from coarser material at the bottom to finer material at the top. This phenomenon often indicates the energy conditions during deposition, revealing important information about past environments and sedimentary processes. Graded bedding is commonly associated with turbidity currents and can help classify sedimentary rocks based on their formation processes and depositional settings.
Grain size: Grain size refers to the diameter of individual particles or crystals in sedimentary rocks and sediments, which plays a crucial role in their formation and classification. The size of grains can indicate the energy of the environment in which the sediments were deposited, helping to interpret geological history. Understanding grain size is essential for differentiating between types of sedimentary rocks and their depositional environments.
Ocean floor: The ocean floor refers to the bottom surface of the ocean, encompassing various geological features such as trenches, ridges, and plains. This area is crucial for understanding sedimentary processes, as sediments accumulate over time and can reveal information about Earth's history, climate changes, and marine ecosystems.
Organic sediments: Organic sediments are types of sedimentary deposits that are primarily composed of the remains of living organisms, such as plants and animals. These sediments play a crucial role in the formation of certain sedimentary rocks, particularly those that contain a significant amount of organic material, which can eventually become fossil fuels or contribute to soil formation. The presence of organic sediments often indicates a rich biological environment, making them essential for understanding past ecological conditions.
Quartz: Quartz is a widely abundant mineral composed of silicon dioxide (SiO2) that forms in various geological environments. Known for its hardness and resistance to weathering, quartz plays a crucial role in the formation of many types of rocks and is an essential component in various industrial applications.
Ripples: Ripples are small, wave-like features that form on the surface of sediment, typically in response to the movement of water or wind. These structures can provide important information about the conditions under which the sediment was deposited, such as flow direction and energy levels in aquatic environments. They are key indicators used in the classification of sedimentary rocks and help geologists interpret past environments.
River basin: A river basin is the land area that drains into a specific river and its tributaries, collecting precipitation and surface runoff. This area encompasses not only the river itself but also all the surrounding land that contributes water to it, which can greatly influence sediment transport and deposition processes, making it vital for understanding sedimentary rock formation.
Rounding: Rounding refers to the process of smoothing the edges of sediment particles through physical abrasion as they are transported by wind, water, or ice. This process not only affects the shape and size of the particles but also influences the characteristics of sedimentary rocks formed from these materials, contributing to their texture and classification.
Sorting: Sorting refers to the process by which sediment is separated based on size, shape, and density during transportation by wind, water, or ice. This process is crucial in understanding sedimentary processes as it directly influences the characteristics of sedimentary rocks that form from these sediments, affecting their composition, texture, and layering.
Transportation: Transportation refers to the processes through which sediments are moved from one location to another by various natural agents, including water, wind, and ice. This movement is crucial for sedimentary rock formation, as it determines how sediments are deposited and accumulated over time. The dynamics of transportation impact the characteristics of sedimentary rocks, influencing their composition and texture, and play a significant role in shaping fluvial landforms through erosion and deposition in river systems.
Wentworth Scale: The Wentworth Scale is a system used to classify sediment based on particle size, ranging from clay to boulders. This scale is crucial in geology as it helps in understanding the composition and characteristics of sedimentary rocks, guiding geologists in the identification of depositional environments and sediment transport processes.