Historical Geology/Nearshore sediments
In this article we shall discuss the sediments of the nearshore, their origin, characteristics, and sedimentary structures; and, as usual we shall discuss how we can identify sedimentary rocks as being lithified nearshore sediments.
Waves and the nearshore[edit | edit source]
We should first remind ourselves of certain facts about waves and tides. First, the reader should bear in mind that the energy of waves only goes down so far: the rule of thumb usually given is that their action affects the water beneath them to a depth about equal to half the wavelength (where the "wavelength" is the distance between two consecutive waves). Out at sea, the wave base (the lowest depth at which the wave has any effect) will be well above the sea-bed, and will move with a regular rolling motion known as swell. However, as waves come into shallower coastal waters, the wave base eventually hits the sea bed. This leaves the energy of the wave with no place to go but up. Therefore the wave rises higher and higher until it becomes unstable and breaks as surf. The consequence of this is that waves will have no effect on the marine sediments of the deep sea.
Tide-generated waves do not always move at right-angles to the beach: the result of this is to generate a longshore current which moves parallel to the beach. This is often confused with longshore drift, which is also caused by waves approaching the beach at an angle. As they wash up the beach, they travel at an angle, moving the sediment with them; but they tend to roll straight back down the beach, again taking the sediment with them. This means that the sediment on such a beach will be transported along the beach in a zigzag path.
A note on terminology[edit | edit source]
We can divide the sea and shore up into zones according to the action of the waves on the sea bed. Unfortunately, geologists do not do so consistently, so the same word may mean different things according to which geologist is using it. Indeed, I have seen one textbook in which the set of definitions supplied in the text contradicted the accompanying diagram intended to illustrate them. My advice to readers who wish to pursue their study of nearshore sediments further is that for each book they read they should pay careful attention to how each particular author defines his or her terms.
For the purposes of this article, I shall use the term nearshore to describe the zone in which the sea bed is affected by waves; the term foreshore to describe the part of the nearshore which is uncovered at low tide; and the term backshore to describe the area higher up the beach than the foreshore, i.e. that part of a beach which is above the high-water line. Other writers will differ, especially as to the proper meaning of the term "nearshore".
Varieties of nearshore environment[edit | edit source]
Nearshore sedimentary environments are very variable in their nature. The sediments can consist of mud or sand or pebbles, or any combination of the three, which may be mixed or sorted according to the action of the waves.
These sediments can be deposited by rivers; they can have their origin in clasts broken from a rocky coast; they can be carried around the coast by longshore currents and longshore drift; they can have their origin as broken fragments of shells or coral.
The sedimentary structures will depend on such factors as the energy of the tide in that locality, the slope of the nearshore, and whether or not there is a significant longshore current.
In short, a whole book could be written on this subject enumerating the various characteristics of, for example, a high-energy muddy nearshore with a longshore current; and a low-energy nearshore consisting mainly of sand and gravel with no longshore current; and so forth.
Because of this, this article can only be a first sketch of the subject.
Some nearshore sedimentary structures[edit | edit source]
The reader should note that because of the variability discussed in the previous section, we would not expect to find all the structures listed below in one single nearshore environment.
- Wave ripples. These are superficially similar to the ripples produced in desert sand, but they tend to be more symmetrical in cross-section, because they are formed by waves going in both directions (oscillatory flow).
- Interference ripples. These are formed when the tide goes out at, or nearly at, ninety degrees from the angle at which it comes in. The photograph to the right shows some interference ripples.
- Herringbone cross-bedding. This is one of the most distinctive types of coastal sediment. You should recall that cross-bedding is produced by the action of a current, and that the beds slope down in the direction of the current. But the current produced by a tide runs in two ways. The consequence of this is that the sets of cross-beds slope in opposite directions, producing a herringbone pattern.
- Flaser deposits. Since the action of the tide is weaker at high and low tide than in between, sediments affected by the tide can in effect be in a high-energy environment and a low-energy environment alternately. This can result in a situation where, during the high-energy period of the tidal cycle, the waves shape sand into ripples, and during the low energy period of the cycle, the waves deposit mud in the depressions of the ripples. The resulting pattern of sediment is known as a flaser deposit. The photograph to the right shows a lithified example of flaser beds.
- Deltas. A river disgorging into the sea will often form a delta: a fan of shifting bars and channels. There is enough to say about these structures that we have already covered them in a separate article on deltas. The reader should recall that we can tell a marine delta from the case where a river is flowing into fresh water by measuring the slope of its foreset beds.
- Longshore bars. At the point on the shoreface where the landward force exerted by the wave base is equal to the seaward force of the backwash of breakers, sand will accumulate in longshore bars: that is, bars of sediment running parallel to the beach. On the seaward side these bars typically have layers of crossbeds sloping gently seaward; on the landward side they exhibit ripple-formed laminae and trough cross-bedding (that is, cross-bedding formed by waves repeatedly scouring out and filling in troughs in the bar). Broken shells are collected in the longshore bar by the same mechanism that accumulates the sand itself, and so longshore bars are typically abundant in layers of broken shells.
- Sand dunes. As the wind tends to blow off the sea, a sandy beach will have sand dunes pile up at the back of it. These will be similar to the wind-formed dunes found in deserts. However, unless the beach borders on a desert, there is no reason that it shouldn't get rain, and so beach dunes will tend to have plants growing in them, and often animals such as crustaceans burrowing in them; these will leave root traces and traces of burrows visible in the geological record, if the dunes are preserved. Another difference between these dunes and those of a sandy desert is of course that a desert will be spread across a large area; beach dunes may extend a long way along a coastline, but will form only a thin strip.
- Bioturbation. The fauna that burrows in the shoreface leave burrows of characteristic shapes. Indeed, the inhabitants of different zones of the shoreface leave different traces.
- Pebbles. The action of the waves and abrasion by other sediments will rapidly convert gravel to smooth pebbles. This can happen in other environments, such as rivers, but there is a tendency for nearshore pebbles to be flat on one axis.
Lithified nearshore sediment: how do we know?[edit | edit source]
We can identify sedimentary rocks that originated as nearshore sediments using a number of criteria.
First, many of the sedimentary structures that occur on the nearshore are unique: there is, for example, no other way of producing flaser bedding. Others are very rare in any other context: for example, interference ripples can very occasionally be produced by the flooding of rivers, but they cannot be continuously produced except in a tidal environment.
The picture to the right, for example, shows a slab of sandstone exhibiting the ripples characteristic of a beach. Certainly it looks exactly like a lithified section of beach to anyone who has ever been to the seaside. The reasonable conclusion is that that's because that's exactly what it is.
Second, there is the fossil fauna of these sediments. The fauna of the nearshore is quite different both from the fauna of the land and the fauna of the deeper sea. For example, on the foreshore we find those creatures which can survive some exposure to the open air, when the tide is out, but cannot permanently survive such conditions.
Another consideration is the topography of the sediments. The coastline is a line, so if geologists are right in identifying certain kinds of rocks as lithified nearshore sediments, then we should expect to find these rocks in long thin strips running between rocks formed by the sorts of sediments we find on land and the sort of sediments we find further off shore: and this is indeed what we find, confirming the theory that these are nearshore sediments.
Furthermore, we should expect the different sorts of sedimentary structures and fossils to be arranged within each strip as they are in nearshore sediments. For example, if we find sedimentary rocks which we identify as backshore dunes, they should form a still narrower strip on the landward side; on the seaward side we should then find the characteristic fauna and bioturbation of the foreshore, followed by those of the deeper nearshore.
Finally, there are chemical clues, as explained here, for example. Coastal sediments have consistent chemical differences from those of deeper waters, and these differences are preserved in sedimentary rocks.