Sedimentary rock is one of the three main rock groups (along with igneous and metamorphic rocks) and is formed in four main ways: by the deposition of the weathered remains of other rocks (known as ‘clastic’ sedimentary rocks); by the accumulation and the consolidation of sediments; by the deposition of the results of biogenic activity; and by precipitation from solution.
Sedimentary rocks form through the process of consolidation of loose materials called debris. Formation of loose debris through disintegration of solid rocks is a common process of rock weathering and erosion. After detachment from the parent rock masses, the debris (or the loose materials) is transported by agencies like streams, rivers, and glaciers to distant places. These are finally deposited in some water bodies like sea, lake, or flood plains of a river. The places of deposition of sediments are called ‘basins’ or more precisely the sedimentary basins. The deposited sediment get compacted consolidated and finally transformed into a cohesive solid mass. That is a sedimentary rock.
Sedimentary rocks are broadly grouped into two classes:
- Clastic rocks (Formed mechanically)
- Non-clastic rocks (Formed organically and chemically)
1.Clastic sedimentary rocks:- Consist of rock and mineral grains, or clasts, of varying size, ranging from clay-, silt-, and sand- up to pebble-, cobble-, and boulder-size materials. These clasts are transported by gravity, mud-flows, 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 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 etc.
2.Chemical sedimentary rocks:– form by chemical and organic reprecipitation of the dissolved products of chemical weathering that are removed from the weathering site. Chemical sedimentary rocks, such as many limestones and cherts, consist of solid precipitated nondetrital fragments that undergo a brief history of transport and abrasion prior to deposition as nonterrigenous clasts. Examples are calcareous or siliceous shell fragments 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 etc.