Sedimentary Rocks


Types of Sedimentary Rocks
The three types of sedimentary rocks, based on their formation process, are clastic, chemical, and organic.

Sedimentary rocks are one of the three major types of rocks found on Earth, alongside igneous and metamorphic rocks. They are unique in their formation process, which involves the deposition, compaction, and cementation of sediment. This contrasts significantly with igneous rocks, which form from the cooling and solidification of magma or lava, and metamorphic rocks, which are formed through the transformation of existing rock types under high pressure and temperature.

Common Examples of Sedimentary Rocks

Sedimentary rocks are incredibly diverse, each type offering a unique glimpse into Earth’s geological and environmental history. Here are some common examples:

  • Sandstone: Forms from cemented sand-sized particles. You often find sandstone in desert or beach environments.
  • Shale: Made from compacted clay, shale is typically fine-grained and readily splits into thin layers.
  • Limestone: Consists primarily of calcite, often from marine organisms’ skeletal fragments.
  • Conglomerate: Consists of rounded gravel-sized clasts cemented together, indicating a high-energy environment like a fast-flowing river.
  • Chert: Made of microcrystalline quartz, often occurring as nodules in limestone.
  • Coal: An organic sedimentary rock formed from accumulated plant debris, typically in swamp environments.
  • Gypsum: A soft mineral rock formed from the evaporation of seawater.
  • Dolostone: Similar to limestone but containing the mineral dolomite.
  • Siltstone: Similar to shale but with slightly coarser grains, formed from silt-sized particles.

Properties of Sedimentary Rocks

The properties of sedimentary rocks depend on their mode of formation and the materials they consist of. Key properties include:

  1. Stratification or Layering: Most sedimentary rocks exhibit visible layers or beds, a result of different periods or conditions of sediment deposition.
  2. Fossils: Many sedimentary rocks contain fossils, the preserved remains of plants, animals, or microorganisms.
  3. Porosity and Permeability: These rocks often have spaces between grains, making them porous and sometimes permeable to fluids like water and oil.
  4. Variability in Grain Size: Sedimentary rocks range from fine-grained (like clay in shale) to coarse-grained (like pebbles in conglomerate).
  5. Clastic Texture: This is a chunky or pebbly texture in rocks like sandstone and shale, where the rock contains fragments of pre-existing rocks.
  6. Biogenic and Chemical Textures: Some sedimentary rocks, like limestone and chert, form from biological materials or chemical precipitates, giving them unique textural properties.
  7. Color Variations: The color ranges widely based on the minerals present and the environment of formation. For instance, iron oxides impart a red or yellow hue, while organic materials often lead to darker colors.
  8. Softness: Compared to igneous and metamorphic rocks, sedimentary rocks are generally softer and less dense.

Formation Processes of Sedimentary Rocks

The formation of sedimentary rocks involves several key processes:

  1. Weathering: Weathering breaks down existing rocks into smaller particles via chemical and physical processes. Freezing and thawing, salt, water, thermal stress, acid, and the action of plants and animals all alter rocks.
  2. Erosion: This involves the transportation of weathered products away from their physical location, by rock falls, wind, running water, gravity, or landslides.
  3. Transportation: This is the movement of these particles by wind, water, or ice.
  4. Deposition: Deposition is the settling of particles in new locations.
  5. Compaction and Cementation (Lithification or Diagenesis): Particles gradually accumulate and bind together over time.

Classification and Types of Sedimentary Rocks

Sedimentary rocks are primarily classified into three categories based on their origin:

Clastic Sedimentary Rocks

Clastic sedimentary rocks form from the accumulation of clasts, or fragments of other rocks and minerals. In the United States, Arches National Park and the Grand Canyon are clastic formations. Examples of clastic sedimentary rocks include:

  • Sandstone: Consists of sand-sized particles, often quartz, indicating deposition by water or wind.
  • Shale: Very fine-grained, composed mostly of clay minerals; splits easily into thin layers.
  • Conglomerate: Contains rounded clasts, typically pebble-sized or larger, cemented together.
  • Breccia: Similar to conglomerate but with angular instead of rounded clasts.
  • Siltstone: Composed of silt-sized particles, finer than sandstone but coarser than shale.
  • Mudstone: Similar to shale but does not split into thin layers; composed of a mix of silt and clay.
  • Greywacke: A type of sandstone with a muddy matrix, often found in submarine settings.

The clasts in clastic rocks get classified according to grain size, shape, and composition. From largest to smallest, clasts are boulder (>256 mm), cobble (64 – 256 mm), pebble (2 – 64 mm), sand (1/16 – 2mm), silt (1/256 – 1/16 mm), and clay (<1/256 mm). Loose boulder, cobble, and pebble grains are gravel, while sand, silt, and clay are names for both particles in rocks and loose sediment.

Chemical Sedimentary Rocks

Chemical sedimentary rocks form from mineral precipitates from solution. Cave formations, such as Carlsbad Caverns, typically contain chemical sedimentary rocks. Examples of specific rocks include:

  • Limestone: Primarily consists of calcite, often from marine shells and skeletons. Because of its origins, limestone often is considered to be a biochemical rock, or a combination of chemical and organic.
  • Dolostone: Similar to limestone but composed of the mineral dolomite.
  • Chert: Composed of microcrystalline quartz; often forms as nodules within limestone.
  • Halite: Rock salt that forms from the evaporation of seawater or saline lake water.
  • Gypsum: Another evaporite mineral, softer than halite.
  • Travertine: A form of limestone deposited by mineral springs.
  • Banded Iron Formation (BIF): Consists of layers of iron-rich minerals and chert; significant for studying the Earth’s early atmosphere.

Organic Sedimentary Rocks

Organic sedimentary rocks, also called biologic sedimentary rocks, form from the accumulation of plant or animal debris.

  • Coal: Forms from compressed plant matter, typically in swamp environments.
  • Oil Shale: Contains significant amounts of organic material (kerogen) that can produce oil upon heating.
  • Peat: Precursor to coal, composed of partially decayed plant material in wet environments.
  • Lignite: A soft brownish coal showing traces of plants; an intermediate step in coal formation.
  • Chalk: Consists of calcite from the microscopic marine organisms (like plankton).
  • Diatomite: Forms when diatoms accumulate. The White Cliffs of Dover are an excellent example.

Biscayne Bay National Park in Florida and Guadalupe Mountains National Park in Texas are good sources of organic sedimentary rocks.

Properties of Sedimentary Rocks

Sedimentary rocks display distinct properties, such as:

  • Layering or Stratification: Visible layers due to different sediment deposition events.
  • Fossils: Often contain remnants of plants, animals, or microorganisms.
  • Porosity: Often has pores that can hold water or oil.
  • Variability in Grain Size: Ranging from fine clay to coarse gravel.

Textures of Sedimentary Rocks

The texture of sedimentary rocks varies greatly and offers a clue to its formation environment:

  • Clastic Textures: Size, shape, and arrangement of the sediment grains.
  • Biogenic Textures: Resulting from the accumulation of biological materials.
  • Chemical Textures: Indicative of the chemical environment of formation, like the crystalline texture in evaporites.

Where to Find Sedimentary Rocks

Sedimentary rocks commonly form in aquatic or windy environments such as:

  • Rivers
  • Floodplains
  • Deltas
  • Lakes
  • Oceans
  • Deserts
  • Glacial regions
  • Beaches

These rocks form when there are processes for transporting and depositing rock fragments and other particles.

Tips for Identifying Sedimentary Rocks

How do you know whether a rock you pick up is sedimentary? Here are some tips:

  1. Look for Layers or Strata: This is a hallmark of sedimentary rocks.
  2. Check for Fossils: The presence of fossils is a strong indicator.
  3. Observe the Grain Size and Composition: This helps in distinguishing clastic sedimentary rocks.
  4. Consider the Color and Texture: These properties indicate the type of sediment and environment of formation.
  5. Use Location Clues: Understanding the local geology provides hints.
  6. Compare with Igneous and Metamorphic Rocks: Sedimentary rocks are generally softer and less dense.

References

  • Blatt, Harvey; Tracy, Robert J. (1996). Petrology (2nd ed.). W.H. Freeman. ISBN 978-0-7167-2438-4.
  • Boggs, S. Jr. (2006). Principles of Sedimentology and Stratigraphy (4th ed.). Upper Saddle River, NJ: Pearson Prentice Hall. ISBN 978-0-13-154728-5.
  • Haldar, S. K. (2013). Introduction to Mineralogy and Petrology. Elsevier Science. ISBN 9780124167100.
  • Picard, Aude; Kappler, Andreas; et al. (2015). “Experimental diagenesis of organo-mineral structures formed by microaerophilic Fe(II)-oxidizing bacteria”. Nature Communications. 6 (1): 6277. doi:10.1038/ncomms7277
  • Stow, D. A. V. (2005). Sedimentary Rocks in the Field. Burlington, MA: Academic Press. ISBN 978-1-874545-69-9.