Sedimentary Rocks
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[edit] Introduction
The article constitutes a brief introduction to the various kinds of sedimentary rocks. Further information about the sources of sediment, its transport, and its deposition, will be covered in further articles; indeed, in much of the rest of this textbook.
Sedimentary rocks can be divided into three main classes:
- Detrital sedimentary rocks are formed from sediments created by rocks being physically broken down into small particles (detritus).
- Chemical sedimentary rocks are formed from sediments created by dissolved chemicals being precipitated out of the water they're dissolved in.
- Biochemical sedimentary rocks are formed from sediments consisting of dead organisms, or parts of dead organisms.
In some schemes of classification, biochemical sediments are treated as a sub-class of chemical sediments, but this leaves one with the awkward question of what to call chemical sediments which aren't biochemical. For this reason we shall treat them as two non-overlapping classes.
Before we review the main types of sedimentary rocks, it is worth mentioning the process by which they turn into rock: this is known as lithification. In some cases, such as shale, mere compaction, along with the resulting loss of water, is sufficient. Coarser sediments, such as sandstone, are both compacted and cemented, as can be seen under a microscope. The cements are minerals precipitated out of the water in which they are dissolved: silica and calcium carbonate are the commonest forms of cement, with iron oxides/hydroxides coming a distant third.
In the sections below we shall list the main types of sedimentary rock.
[edit] Detrital sedimentary rocks
Detrital sedimentary rocks are formed from broken pieces (clasts) of pre-existing rocks.
- Sediment: rounded gravel. Rock: conglomerate.
Gravel is defined as clasts with diameter 2mm or more. Conglomerate is formed of rounded gravel. Note that when geologists speak of rounded clasts, they do not necessarily mean that they are round like a ball, but merely that the sharp corners and edges have been worn off them by erosion. Conglomerates are rocks formed mainly from rounded gravel which has been compacted and cemented together.
- Sediment: angular gravel. Rock: breccia.
Breccia is like conglomerate except that the gravel has not been rounded. This reflects a different history, since gravel that has been transported any appreciable distance by water, or which has been rolled about by waves on a beach, will quickly have its corners and edges worn away.
- Sediment: sand. Rock: sandstone
Sand is defined as clasts less than 2mm and more than 1/16mm in diameter. Sandstone is sand that has been cemented together.
Arkose sandstone is sandstone with an appreciable proportion of feldspar minerals in it. This reflects a somewhat different history to quartz sandstone, in that it must have been formed when mechanical weathering and erosion have predominated over the chemical weathering that would otherwise have converted the feldspar minerals to clay minerals.
Greywacke is sandstone that contains quartz, feldspar, and small fragments of rock, all set in a clay matrix. It is produced by turbidity currents. See our main article on turbidites for further a discussion.
Sandstone often exhibits bedding: the sandstone has distinct layers (beds) in it and is fissile: that is, it splits more easily at the divisions (bedding planes) between these two layers. In cross-bedded sandstone, the layers are not flat, but form oblique shapes as a result of the original sand being formed into dunes or ripples by the action of wind or water.
- Sediment: silt and/or clay. Rock: mudrock.
Silt is defined as clasts between 1/16mm and 1/256mm in diameter.
The term "clay" is a little ambiguous. In the classification of sediments, it is defined as particles less than 1/256mm in diameter. However, in mineralogy, clay is a class of minerals (technically, hydrous aluminosilicates). In practice, this need cause no confusion, because what is clay by size will be overwhelmingly clay by composition.
Mudrocks can then be divided into siltstone (formed from silt sediments); mudstone (from sediments that are a mixture of silt and clay); and claystone (from clay sediments).
- Sediment: bedded mud or clay. Rock: shale.
Most mudstone and claystone is bedded (see the entry above on sandstone for a definition of this term). When it is, it is referred to as shale.
[edit] Chemical sedimentary rocks
- Sediment: salt. Rock: halite.
Halite, also known as rock salt, is an evaporite, formed by the evaporation of salt water.
It can be formed by complete evaporation of salt water, as seen, for example, in desert salt flats. However, complete evaporation is not necessary; it is sufficient that enough water should evaporate that the remaining water can't hold all of the salt in solution; so halite can also form in shallow seas or salt lakes in a hot environment. See our main article on marine evaporites for further discussion.
- Sediment: hydrated calcium sulfate. Rock: gypsum.
This is another substance to be found in salt water, and, like halite, usually forms as an evaporite under pretty much the same circumstances. See our main article on marine evaporites for further discussion.
- Sediment: silica. Rock: chert.
Dissolved silica can precipitate out of the water in which it's dissolved to form chert. Note, however, that chert is more usually formed as a biochemical sedimentary rock.
- Sediment: calcium carbonate. Rock: limestone.
Calcium carbonate, like silica, can precipitate out of water to form limestone. Sometimes it forms tiny spheres called ooids, which form around grains of sand or fragments of shell, which are then cemented together by the further precipitation of calcium carbonate.; such limestone is known as ooilitic limestone.
Most limestone, however, is biochemical sedimentary rock, formed from shells. For more on limestone, see our main article on marine carbonates.
[edit] Biochemical sedimentary rocks
- Sediment: shells of calcium carbonate. Rock: limestone.
Most limestone is formed from tiny hard parts of marine creatures which build their shells out of calcium carbonate. Chalk is an example of such a rock: the tiny fossils that compose it can be clearly seen and identified under a microscope. The hard parts of coral reefs are sometimes preserved intact, giving us reef limestone.
- Sediment: shells of silica. Rock: chert.
While calcium carbonate is the most popular substance to make shells out of, some organisms such as diatoms and radiolarians build their shells out of silica; when compacted and cemented, these tiny shells form chert.
- Sediment: peat. Rock: coal.
Peat is plant material laid down in oxygen-poor conditions, so that it doesn't decompose. Pressure, and the higher temperatures which come with deep burial can then progressively convert the peat into lignite; then sub-bituminous coal; and finally bituminous coal. For more on coal, see our main article on coal.
[edit] Volcanic ash
- Sediment: volcanic ash. Rock: tuff.
Strictly speaking, volcanic ash is igneous. However, when it's deposited, it behaves a lot like a sedimentary rock; as geologists say: "it's igneous when it goes up, and sedimentary when it comes down". For this reason we have included it in our articles on sedimentology.
[edit] How do we know?
How do we know that sedimentary rocks are sedimentary? After all, the process of lithification takes some time and usually takes place at some depth below the surface: we cannot expect to be able to watch mud turning into shale.
In the first place, the rocks look just like we would expect if sediments became lithified; sandstone looks like it's made of sand. In some cases, this is blatantly true, as when microscopic examination of chalk shows the shells of recognizable types of organism.
Secondly, we can drill down and take cores of sediments, and we can see, as depth increases, how sloppy muddy oozes on the surface grade into hard shale with no sharp dividing line between them; similarly we can see calcareous ooze grade into solid limestone.
Then again, all types of sedimentary rocks can contain fossils (including, as we have remarked, those rocks which consist entirely of fossils). This is consistent with the processes of burial of organic remains in sediment which we can see going on today.
Trace fossils are also a strong argument: when we find, in shale, the recognizable fossil footprints of land animals, it is hard not to conclude that what we are looking at once lay on the surface and was soft enough to take impressions such as we can see being made in mud today.
The structures within the rocks, such as bedding and cross-bedding can be seen today in oozes forming on the sea floor; in sand-dunes; in ripples caused by tidal action, and so forth.
Finally, we may observe the patterns of deposition. When we look horizontally at a series of different sedimentary rock types (facies) and see first of all mudstone containing fossils of land animals and fossil footprints; then sandstone with the cross-bedded structures we see in modern sand-dunes, and containing fossil shells; then sandstone with ripple marks and containing marine organisms of the type we associate with shallow waters; then shale with bedding structures such as we find in marine mud, plus more marine fossils; and finally chalk composed of the shells of free-floating marine organisms --- when we see all this, it is hard not to think that we are looking at the lithified remains of a progression from land to sea. What else could we be looking at? A vast coincidence? A prank played by a malicious deity?
So the conclusion that sedimentary rocks are, indeed, sedimentary in origin, is a safe one.
In this section we have given an overview of the types of evidence that lead us to identify sedimentary rocks as sedimentary. In further articles, we shall spend a good deal of time looking at the specific evidence connecting the different sorts of sedimentary rocks to the sediments from which they were formed. For further information, see our articles on glaciers and glacial drift; deserts and aeolian sandstone; rivers and the deposits associated with them; deltas and the deposits associated with them; swamps, peat, and coal formation; volcanic ash and volcanic tuff; soils and paleosols; nearshore sediments; turbidity currents and turbidites; marine sediments; marine evaporites; marine carbonates; and, indeed, much of the remainder of our articles on geology.
[edit] Summary table of sedimentary rocks
| Sediment | Rock |
| Rounded gravel | Conglomerate |
| Angular gravel | Breccia |
| Sand | Sandstone |
| Silt | Siltstone |
| Mud (silt and clay) | Mudstone |
| Clay | Claystone |
| Bedded mud or clay | Shale |
| Precipitated calcium carbonate | Limestone |
| Precipitated silica | Chert |
| Precipitated salt | Halite |
| Precipitated calcium sulfate | Gypsum |
| Calcareous shells | Limestone |
| Siliceous shells | Chert |
| Peat | Coal |
| Volcanic ash | Tuff |
[edit] Note on vocabulary
Conglomerates and breccias are sometimes called rudaceous rocks; sandstones are sometimes called arenaceous rocks or arenites; and mudrocks are sometimes called argillaceous rocks.
The rocks which we have called detrital are sometimes called clastic, since they are formed out of clasts.
As usual, we shall employ a consistent vocabulary in this text; these terms have been supplied for the benefit of the reader who wishes to pursue a course of further reading.
