Sedimentary rock is formed at or near the Earth’s surface by the accumulation and lithification of sediment (detrital rock) or by the precipitation from solution at normal surface temperatures (chemical rock). Sedimentary rocks are the most common rocks exposed on Earth’s surface but are only a minor constituent of the entire crust, which is dominated by igneous and metamorphic rocks.
Those rocks are produced by the weathering of preexisting rocks and the subsequent transportation and deposition of the weathering products. Weathering refers to the various processes of physical disintegration and chemical decomposition that occurs when rocks at Earth’s surface are exposed to the atmosphere (mainly in the form of rainfall) and the hydrosphere.
These processes produce soil, unconsolidated rock detritus, and components dissolved in groundwater and runoff. Erosion is the process by which weathering products are transported away from the weathering site, either as solid material or as dissolved components, eventually to be deposited as sediment.
Any unconsolidated deposit of solid weathered material constitutes sediment. It can form as the result of deposition of grains from moving bodies of water or wind, from the melting of glacial ice, and from the downslope slumping (sliding) of rock and soil masses in response to gravity, as well as by precipitation of the dissolved products of weathering under the conditions of low temperature and pressure that prevail at or near Earth’s surface.
Sedimentary rocks are the lithified equivalents of sediments. They typically are produced by cementing, compacting, and otherwise solidifying preexisting unconsolidated sediments. Some varieties of rocks, however, are precipitated directly into their solid sedimentary form and exhibit no intervening existence as sediment. Organic reefs and bedded evaporites are examples of such rocks.
Because of the processes of physical (mechanical) weathering and chemical weathering are significantly different, they generate markedly distinct products and two fundamentally different kinds of sediment and sedimentary rock: (1) terrigenous clastic sedimentary rocks and (2) allochemical and orthochemical sedimentary rocks.
Classification of sedimentary rocks
Sedimentary rocks are primarily grouped into two main types according to how they were formed. Clastic terrigenous (Detrital) sedimentary rocks were formed through physical means (pressure) while chemical sedimentary rocks were chemically formed.
Clastic sedimentary rock
Clastic terrigenous sedimentary rocks consist of rock and mineral grains, or clasts, of varying size, ranging from clay-, silt-, and sand-size up to pebble-, cobble-, and boulder-size materials. These clasts are transported by gravity, mudflows, running water, glaciers, and wind and eventually, are deposited in various settings (e.g., in desert dunes, on alluvial fans, across continental shelves, and in river deltas).
Because of the agents of transportation commonly sort out discrete particles by clast size, terrigenous clastic sedimentary rocks are further subdivided on the basis of average clast diameter. Coarse pebbles, cobbles, and boulder-size gravels lithify to form conglomerate and breccia; sand becomes sandstone; and silt and clay form siltstone, claystone, mudrock, and shale.
Chemical sedimentary rockmass
Chemical sedimentary rocks form by chemical and organic reprecipitation of the dissolved products of chemical weathering that are removed from the weathering site.
Allochemical sedimentary rocks, such as many limestones and cherts, consist of solid precipitated nondetrital fragments (allochems) that undergo a brief history of transport and abrasion prior to deposition as nonterrigenous clasts.
Examples are calcareous or siliceous shell fragments and oöids, which are concentrically layered spherical grains of calcium carbonate. Orthochemical sedimentary rocks, on the other hand, consist of dissolved constituents that are directly precipitated as solid sedimentary rock and thus do not undergo transportation. Orthochemical sedimentary rocks include some limestones, bedded evaporite deposits of halite, gypsum, and anhydrite, and banded iron formations.
Presence of sedimentary rockmass
Sediments are confined to Earth’s crust, which is the thin, light outer solid skin of Earth ranging in thickness from 40–100 km (25–62 miles) in the continental blocks to 4–10 km (2.5–6 miles) in the ocean basins. Igneous and metamorphic rocks constitute the bulk of the crust.
The total volume of sediment can be either directly measured using exposed rock sequences, drill-hole data, and seismic profiles or indirectly estimated by comparing the chemistry of major sedimentary rock types to the overall chemistry of the crust from which they are weathered.
Both methods indicate that Earth’s sediment-sedimentary rock shell forms only about 5 percent by volume of the terrestrial crust, which in turn accounts for less than 1 percent of Earth’s total volume. On the other hand, the area of outcrop and exposure of sediments comprises 75 percent of the land surface and well over 90 percent of the ocean basins and continental margins.
In other words, 80–90 percent of Earth’s surface area is mantled with sediments rather than with igneous or metamorphic varieties.
The sediment shell forms only a thin superficial layer. The mean shell thickness in continental areas is 1.8 km (about 1 mile); the sediment shell in the ocean basins is roughly 0.3 km (0.2 mile).
Rearranging this shell as a globally encircling layer (and depending on the raw estimates incorporated into the model), the shell thickness would be roughly 1–3 km (0.6–2 miles).