Sediments are usually formed from matter which falls to the bottom of oceans and lakes. The matter includes tiny pieces of other rocks, and dead animals, plants and microorganisms. The three most common sedimentary rocks are limestone, sandstone, and shale.
Sedimentary rocks cover 75–80% of the Earth’s land area, but they make up only 5% of the Earth’s crust. The relative abundances of the different types of sedimentary rocks are:
- Shale———-60%
- Sandstone—20%
- Carbonate—15%
- All others—–5%
Consolidation
Squeezed and compressed over time, the sediments become ‘consolidated’ (made solid) into layers of rock.
Dissolved minerals drip into the cracks. When the water evaporates, the minerals become part of the rock. It is the sticking together of sediment that forms a rock.
Consolidated rocks may be changed some more, by water seeping through, or by heat and extreme pressure.
Types of sedimentary rock
Sediments are formed by three processes:
- Solids swept down from the land
- Bits & pieces laid down in water, e.g. shells
- Chemicals in water
Sedimentary rocks may be found anywhere on Earth.
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Middle Triassic marginal marine sequence of siltstones (reddish layers at the cliff base) and limestones (brown rocks above), Virgin Formation, southwestern Utah, USA
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Sedimentary rocks on Mars, investigated by NASA’s Curiosity Mars rover
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Steeply dipping sedimentary rock strata along the Chalous Road in northern Iran
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Sedimentary rock with sandstone in Malta
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Outcrop of Ordovician oil shale (kukersite), northern Estonia
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A piece of a banded iron formation, a type of rock that consists of alternating layers with iron(III) oxide (red) and iron(II) oxide (grey). BIFs were mostly formed during the Precambrian, when the atmosphere was not yet rich in oxygen. Moories Group, Barberton Greenstone Belt, South Africa
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Fossil-rich layers in a sedimentary rock, Año Nuevo State Reserve, California
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Burrows in a turbidite, made by crustaceans, San Vincente Formation (early Eocene) of the Ainsa Basin, southern foreland of the Pyrenees
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Cross-bedding in a fluviatile sandstone, Middle Old Red Sandstone (Devonian) on Bressay, Shetland Islands
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Ripple marks formed by a current in a sandstone that was later tilted (Haßberge, Bavaria)
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Cyclic alternation of competent and less competent beds in the Blue Lias at Lyme Regis, southern England
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The Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah that makes up much of the famous prominent rock formations in protected areas such as Capitol Reef National Park and Canyonlands National Park. From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone. Picture from Glen Canyon National Recreation Area, Utah.
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Claystone deposited in Glacial Lake Missoula, Montana, United States. Note the very fine and flat bedding, common for distal lacustrine deposition.
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Lower Antelope Canyon was carved out of the surrounding sandstone by both mechanical weathering and chemical weathering. Wind, sand, and water from flash flooding are the primary weathering agents.
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Cross-bedding and scour in a fine sandstone; the Logan Formation (Mississippian) of Jackson County, Ohio
(Edited from source)
