Sedimentary rocks are one of the three primary rock types, formed at or near the Earth’s surface through the accumulation and lithification of various materials. These materials can be fragments of pre-existing rocks, remnants of organisms, or minerals precipitated directly from water. Lithification involves the compaction of sediment under pressure and the cementation of grains by dissolved minerals, transforming loose deposits into solid rock. Sedimentary rocks are important because their layered structure, known as bedding, preserves a detailed record of Earth’s history, including past climates and environments. Geologists classify these rocks based on their diverse origins and formation environments.
The Three Foundational Categories
Classification begins with the rock’s genetic origin—the process by which the material was created and deposited. This establishes three primary categories.
The first group is Clastic, or Detrital, sedimentary rocks, defined by their composition of physically weathered fragments (clasts) of other rocks and minerals. These particles are transported by water, wind, or ice, then deposited and cemented together.
The second category consists of Chemical sedimentary rocks, which form through purely inorganic processes. These rocks crystallize when dissolved ions in water become oversaturated and precipitate out of the solution, often due to the evaporation of the water body in restricted basins or arid environments.
The third major grouping is Biochemical or Organic sedimentary rocks, where life processes determine the sediment’s formation. Organisms extract dissolved ions to construct shells or skeletons, which accumulate after death. This category also includes rocks formed from the burial and alteration of accumulated plant matter.
Detailed Classification of Clastic Rocks
Clastic rocks (like sandstone and shale) are classified primarily based on the physical characteristics of their constituent clasts. Four metrics are used:
Grain Size
Grain size is the most defining characteristic, indicating the energy level of the depositional environment. Particles are categorized from the finest clay to the coarsest boulders.
Gravel-sized particles (greater than two millimeters) form conglomerate (if rounded) or breccia (if angular).
Sand-sized grains (1/16 to two millimeters) form sandstone.
Finer materials are silt-sized (forming siltstone) or clay-sized (forming shale or claystone).
Grain Composition
This refers to the mineral or rock fragments making up the grains. Sandstones are sub-classified based on the abundance of quartz, feldspar, and rock fragments. For example, a quartz arenite is almost entirely quartz, indicating extensive weathering. An arkose contains significant feldspar, suggesting shorter transport and less chemical breakdown.
Sorting
Sorting is the uniformity of grain size, which provides information about the transport history. Well-sorted rocks, where grains are similar in size, result from prolonged transport by selective agents like wind or waves. Poorly sorted rocks, containing a wide mixture of sizes, indicate rapid deposition.
Shape or Angularity
This metric is used for coarse-grained rocks. The degree of rounding reflects the abrasion the clasts experienced during transport. Highly rounded grains, characteristic of a conglomerate, signify a long journey. Sharp, angular fragments in a breccia suggest the material was deposited close to its origin.
Detailed Classification of Chemical and Biochemical Rocks
Classification of these rocks relies heavily on mineral composition and source material, as their formation involves crystallization or biological accumulation rather than particle size.
Chemical Rocks
Chemical rocks form through inorganic precipitation and are named for the dominant mineral that crystallized from the solution. Evaporites are a major sub-group, formed by the evaporation of water.
- Rock salt (halite) and rock gypsum (gypsum) precipitate as water bodies shrink and dissolved ion concentration increases.
- Siliceous chemical rocks, like some chert, are composed of microcrystalline quartz precipitated inorganically from silica-rich waters.
Biochemical and Organic Rocks
These rocks are classified based on the specific source material derived from organisms.
- Limestone is the most common, composed primarily of calcite derived from the skeletal remains or shells of marine organisms. Coquina is a type of limestone made almost entirely of shell fragments.
- Organic rocks, such as coal, are classified by the degree of transformation of accumulated plant matter in oxygen-poor environments.
- Some siliceous rocks, like radiolarian chert, are biochemical, formed from the accumulation of silica-based microfossils.