Open Adit™ - Sedimentary Rocks

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Sedimentary rocks are those rocks which form at or near the earth's surface at relatively low temperatures and pressures primarily by either;

Deposition by water, wind or ice
Precipitation from solution
Growth in position by organic processes

Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth’s surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock.

Sedimentary rocks are generally stratified, fine-grained or composed of fragments of older rocks from which these were derived, such as pebbles, sand, angular fragments of older rocks, broken shells, rounded mineral grains and alteration minerals such as clays. Limestones are easily identified because they effervesce in dilute hydrochloric acid. Many sedimentary rocks also contain fossils.

Processes:

The key to understanding sedimentary rocks is to realize that all sedimentary processes of weathering, transportation, and deposition are aimed at one goal - reaching the three final end products of all sedimentary processes, quartz sand, shale (clay), and limestone (CaCO₃). Sedimentary systems work this way because of two processes:

Weathering:

Weathering is the breakdown of one mineral/rock into another. Of the eight igneous rock forming minerals all are subject to degradation (weathering into something else), except quartz. Quartz, for all intents and purposes, does not weather and will survive in the system relatively unscathed.
The remaining seven rock forming minerals all dissolve to make the sea salty, or decompose to form new minerals stable at the earth's surface. Orthoclase, for example, breaks down to form clay, and the calcium in Ca plagioclase goes into solution to form CaCO₃. There are lots of other weathering products, of course, but they are just details. The simple, ideal model predicts three end products, quartz sand, shale, and limestone, which all together compose the vast majority of sedimentary rocks.
Transportation and Sorting:

The second process is sorting during transportation. The sand and clay, beginning as a poorly sorted mixture, are separated more and more as they travel down stream away from the source. Quartz sand, which rolls and bounces along the bottom, does not transport as easily as clay which travels in suspension. The CaCO₃ is dissolved and therefore just travels with the water. The result is, during transportation these three weathering products do not transport equally well, and become separated. The final separation takes place at the ocean shoreline (image to right). Here we see river transported sediment entering the ocean. Waves crashing on the beach keep the sediment continuously stirred up. The quartz, being relatively heavy, settles quickly to the bottom, the clay remains in suspension until it drifts to the quieter near shelf, where if finally settles to the bottom to form shale. Finally, the dissolved CaCO₃ precipitates out of suspension in the far shelf, beyond the range of sand and clay to form limestone. The calcite is deposited because plants and animals extract it from sea water and use it to build their skeletons. After death their calcite skeletons form the limestone sediment.

Classification & Identification:

Many sedimentary rock classification strategies have been proposed over the decades. The diversity of strategies reflects, in part, the complication of sedimentary rocks, but also the fact that different classifications accomplish different purposes. Often when a geologist works on a problem it is only a small part of the realm of sedimentary processes they are interested in, not the total diversity of rocks, so special, more narrow schemes are devised that make the problem easier to solve. Below are listed three generalized strategies of classification.

Clastic:

Clastic sedimentary rocks are the group of rocks most people think of when they think of sedimentary rocks. Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering, then transported to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock. Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their clast or grain size. The smallest grains are called clay, then silt, then sand. Grains larger that 2 millimeters are called pebbles. Shale is a rock made mostly of clay, siltstone is made up of silt-sized grains, sandstone is made of sand-sized clasts, and conglomerate is made of pebbles surrounded by a matrix of sand or mud.
Biologic:

Biologic sedimentary rocks form when large numbers of living things die, pile up, and are compressed and cemented to form rock. Accumulated carbon-rich plant material may form coal. Deposits made mostly of animal shells may form limestone, coquina, or chert.
Chemical:

Chemical sedimentary rocks are formed by chemical precipitation. The stalactites and stalagmites you see in caves form this way, so does the rock salt that table salt comes from. This process begins when water traveling through rock dissolves some of the minerals, carrying them away from their source. Eventually these minerals can be redeposited, or precipitated, when the water evaporates away or when the water becomes over- saturated with minerals.

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