Historical Geology/Sedimentary rocks
This article is 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.
Types of sedimentary rocks
Sedimentary rocks can be divided into three main classes:
- Clastic sedimentary rocks are formed from sediments created by rocks being broken down into small particles (clasts).
- 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 I 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 and hydroxides coming a distant third.
In the sections below we shall list the main types of sedimentary rock.
Clastic sedimentary rocks
As defined above, clastic sedimentary rocks are formed from broken pieces ("clasts") of pre-existing rocks.
Gravel is defined as clasts with diameter 2mm or more. We should 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: rounded gravel. Rock: conglomerate.
Breccia is like conglomerate except that the gravel is angular: that is, it 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: angular gravel. Rock: breccia.
Sand is defined as clasts less than 2mm and more than 1/16mm in diameter. Sandstone is sand that has been cemented together.
- Sediment: sand. Rock: sandstone
Most sandstone is quartz sandstone; that is, it consists of grains of quartz. This is because the process known as chemical weathering dissolves many rock-forming minerals, or, in the case of feldspar minerals, converts them to clay, leaving behind only the quartz from the original rock. We shall look at the process of chemical weathering in a subsequent article.
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 (the physical process of breaking rock into clasts) has predominated over the chemical weathering that would otherwise have converted the feldspar minerals to clay minerals.
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: silt and/or clay. Rock: mudrock.
Most mudstone and claystone is bedded. When it is, it is referred to as shale.
- Sediment: bedded mud or clay. Rock: shale.
Chemical sedimentary rocks
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.
- Sediment: salt. Rock: halite.
Calcium sulfate is another substance to be found dissolved in salt water, and gypsum, like halite, usually forms as an evaporite under pretty much the same circumstances.
- Sediment: hydrated calcium sulfate. Rock: gypsum.
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: silica. Rock: chert.
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 oolitic limestone.
- Sediment: calcium carbonate. Rock: limestone.
Most limestone, however, is biochemical sedimentary rock, formed from shells or coral.
Biochemical sedimentary rocks
Most limestone is formed from tiny hard parts of marine creatures which build their shells out of calcium carbonate; these settle on the sea floor to form calcareous ooze. 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 calcium carbonate. Rock: limestone.
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; these settle on the sea floor to form siliceous ooze which, when compacted and cemented, forms chert.
- Sediment: shells of silica. Rock: chert.
Peat is plant material laid down in oxygen-poor conditions, so that it doesn't entirely decompose. Pressure, and the higher temperatures which come with deep burial, can then convert it into coal.
- Sediment: peat. Rock: coal.
Modes of deposition
In the sections above we have principally divided sedimentary rocks by their composition. We can also classify them by their modes of deposition: for example, aeolian (deposited by the wind), or fluvial (deposited by rivers) and so forth. We shall have a lot more to say about this in subsequent articles.
Sedimentary rock often exhibits bedding: that is, the rock has distinct layers in it and is fissile: that is, it splits more easily at the divisions (bedding planes) between the layers. In cross-bedded rocks, the layers are not flat but lie at an angle to the horizontal, as a result of the original sediment being formed into dunes or ripples by the action of wind or water.
The picture to the right shows a particularly large-scale example of cross-bedding in sandstone.
How do we know?
How do we recognize sedimentary rocks as sedimentary? How do we recognize the sediments that compose them and the manner of their deposition?
Such questions will be answered later in this course one type of sediment at a time. At present we shall content ourselves with sketching out a general answer.
In the first place, the rocks look just like we would expect if sediments became lithified; for example sandstone looks like it's made of sand: everything about the size, the composition, and the erosion of the grains of which it's composed is in agreement with the idea that what we're looking at is grains of sand cemented together.
Secondly, we can drill down and take cores of sediments, and we can see, as depth increases, how (for example) sloppy muddy ooze on the surface grades into hard mudstone 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 of fossils, such as chalk). 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.
These considerations also allow us to figure out where the sediments were deposited: on land, in fresh-water, or in the sea.
The sedimentary 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. Again, consideration of these structures will allow us to make the deduction, not merely that the rock is sedimentary, but also about the method of its deposition; if, for example, we find sedimentary structures such as can only be formed by tidal action, we are forced to infer that we are looking at lithified nearshore sediments.
Also, we may observe the topographic patterns of deposition. For example, when we see sedimentary rocks which because of their structures and fossils we identify as terrestrial (i.e. associated with the land) divided from sedimentary rocks which because of their structures and fossils we associate with the sea by a long thin strip of sedimentary rocks which because of their structures and fossils we associate with the nearshore, then this observation confirms our identification of the rocks as being terrestrial, marine, and nearshore rocks. If, on the other hand, we found alternating bands of marine and nearshore rocks, this would tend to falsify our theories. The fact that the topography of sediments is always consistent with our theories is therefore a point in favor of their correctness.
So the conclusion that sedimentary rocks are, indeed, sedimentary in origin, is a safe one; and we are certainly not without clues as to the manner of their deposition.
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 clastic are sometimes called detrital.
As usual, I shall employ a consistent vocabulary in this text; these terms have only been supplied for the benefit of the reader who wishes to pursue a course of further reading.