Sedimentary rock consists of sediment that in most cases has been lithified into solid rock by the processes of compaction and cementation. Sediment has two principal sources: (1) as detrital material, which originates and is transported as solid particles from both mechanical and chemical weathering, which, when lithified, forms detrital sedimentary rocks; and (2) from soluble material produced largely by chemical weathering, which, when precipitated, forms chemical sedimentary rocks.
Diagenesis refers to all of the physical, chemical, and biological changes that occur after sediments are deposited and during and after the time they are turned into sedimentary rock. Burial promotes diagenesis. Diagenesis includes lithification.
Conglomerate
Lithification refers to the processes by which unconsolidated sediments are transformed into solid sedimentary rock. Most sedimentary rocks are lithified by means of compaction and/or cementation. Compaction occurs when the weight of overlying materials compresses the deeper sediments. Cementation, the most important process by which sediments are converted to sedimentary rocks, occurs when soluble cementing materials, such as calcite, silica, and iron oxide, are precipitated onto sediment grains, fill open spaces, and join the particles. Although most sedimentary rocks are lithified by compaction or cementation, certain chemical rocks, such as the evaporites, initially form as solid masses of intergrown crystals.
Particle size is the primary basis for distinguishing among various detrital sedimentary rocks. The size of the particles in a detrital rock indicates the energy of the medium that transported them. For example, gravels are moved by swiftly flowing rivers, whereas less energy is required to transport sand. Common detrital sedimentary rocks include mudrocks (silt- and clay-size particles), sandstone, and conglomerate (rounded gravel-size particles) or breccia (angular gravel-size particles).
Precipitation of chemical sediments occurs in two ways: (1) by inorganic processes, such as evaporation and chemical activity; or by (2) organic processes of water-dwelling organisms that produce sediments of biochemical origin. Limestone, the most abundant chemical sedimentary rock, consists of the mineral calcite (CaCO3) and forms either by inorganic means or as the result of biochemical processes. Inorganic limestones include travertine, which is commonly seen in caves, and oolitic limestone, consisting of small spherical grains of calcium carbonate. Other common chemical sedimentary rocks include dolostone (composed of the calcium-magnesium carbonate mineral dolomite), chert (made of microcrystalline quartz), evaporites (such as rock salt and rock gypsum), and coal (lignite and bituminous).
Sandstone
Sedimentary rocks can be divided into two main groups: detrital and chemical. All detrital rocks have a clastic texture, which consists of discrete fragments and particles that are cemented and compacted together. The main criterion for subdividing the detrital rocks is particle size. Common detrital rocks include conglomerate, sandstone, and mudrocks. The primary basis for distinguishing among different rocks in the chemical group is their mineral composition. Some chemical rocks, such as those deposited when seawater evaporates, have a nonclastic texture in which the minerals form a pattern of interlocking crystals. However, in reality, many of the sedimentary rocks classified into the chemical group also contain at least small quantities of detrital sediment. Common chemical rocks include limestone, rock gypsum, and coal (e.g., lignite and bituminous).
Sedimentary environments are those places where sediment accumulates. They are grouped into continental, marine, and transitional (shoreline) environments. Each is characterized by certain physical, chemical, and biological conditions. Because sediment contains clues about the environment in which it was deposited, sedimentary rocks are important in the interpretation of Earth's history.