Sedimentary rocks - rocks formed from the deposition, compaction, and/or cementation of mineral and rock particles, often including organic material. They may be classified as terrigenous (derived from pre-existing rock), organic (produced directly or indirectly by organic processes), chemical (produced by precipitation from water), or volcanogenic (such as pyroclasts).
Sedimentary cycle - a process which involves the weathering of existing rock and erosion of minerals, followed by transportation and deposition, then burial. First-cycle sediments are characterized by more angular grains and less resistant minerals and rock particles. If this material is reworked in subsequent cycles, the more mature the rock becomes and is dominated by well-rounded, more resistant minerals.
Brief Description of Sediment Deposition
Continuing research into areas of active sedimentation (including rivers, deltaic environments, and oceans) indicate that the formation of sedimentary rocks is governed by four general processes:
During these processes, eroded sediments are sorted according to size and composition by the actions of the transporting mechanism (such as wind or water). This is called sediment differentiation.
Rivers - generally have an erosive region at the headwaters and a depositional region at base level, however, erosion and deposition can and do occur along the length of the river in river channels, river bars, and flood plains. Sediments are deposited wherever the forces of stream turbulence, viscosity and bouyancy that keep particles suspended in the water are over come by gravity (low-flow regimes).
Lakes - frequently, rivers and streams empty into lakes, bringing their load of suspended sediments with them. In these lower flow environments, suspended load is deposited, often as varved silt or clay. Additionally, organic-rich peat bogs and swamps may be present, contributing large amounts of organics to trapped sediments. In some regions, (most notably in arid climates) evaporite deposits may form from precipitated salts.
Deserts - infrequent, torrential rainfall leads to a tremendous amount of erosion and sediment transportation. Often, the turbulence of these events are reflected in the very poorly-sorted and bedded nature of such deposits. Wind is another great factor in erosion and sediment deposition in these areas. Sand-sized particles are usually deposited wherever surface irregularities exist, but the finest of grains are typically transported over great distances, some ending up as loess or deep ocean deposits.
Non-Fluvial Glacial Environments - glacial till (rock debris scoured by the glacier) accumulates at the snout and base of the glacier, to be either deposited in situ during periods of recession, or over-ridden to form ground moraines. Often, these sediments are further reworked by streams and lakes associated with the recession of the glacier.
Deltas - are formed where high-flow rivers and streams encounter low-flow standing water (such as marine shorelines). The rapid deposition of large amounts of suspended river sediments results in frequent river channel divergence.
Beaches - a high-energy environment where ocean sediments are frequently deposited due to wave action. These sediments are typically well-sorted sands and silts with some organics.
Carbonate Sediments - shallow marine environments where organisms form shells made of CaCO3, or by inorganic chemical precipitation of CaCO3.
Deep Marine Sediments - fine dusts eroded from the great deserts of the world and transported by wind are deposited as red/brown clays in a region near 30-degrees North and South latitude. Carbonate oozes are generally found near the equator, while siliceous oozes predominate in more northerly and southerly latitudes according to the environmental requirements of associated micro-organisms.
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