Historical Geology/Marine sediments
This article contains a short general discussion of marine sediments and some important terms and concepts relating to this field.
We need not in this article ask and answer our usual question: "How do we know?" since in the case of the facts given in this article the answer is simply and uniformly: "We looked at the bottom of the sea."
The sea in profile
Before we discuss marine sediments, we should introduce a few terms from oceanography. The diagram to the right shows a cross-section of the sea and land. Though it has been simplified and stylized, it is based on actual data. Do note, however, that the vertical scale has been exaggerated by a factor of 25 compared to the horizontal scale, and that this consequently exaggerates all the angles of slope.
Features to note are:
- The continental shelf. This may be regarded as the part of the continent which just happens to be underwater. Its true angle of slope is rarely more that half a degree.
- The continental slope. As you can see, this is formed of land-derived sediment which has piled up at the foot of the continental crust. Its angle of slope rarely exceeds ten degrees, and is more typically around four degrees.
- The continental rise. This has an average inclination of about half a degree from vertical, and flattens out into ...
- The abyssal plain. As you can see from the diagram, this tends to be flat (hence the name), because all but the most prominent topographical features are obliterated by sediment.
The shelf, slope, and rise are known collectively as the continental margin.
Distribution of marine sediments
The map to the right shows which marine sediments are deposited where. The key is as follows:
- Gray: land.
- White: Sediments of the continental margin.
- Blue: glacial sediments.
- Orange: land-formed sediments.
- Brown: pelagic clay.
- Green: siliceous sediments.
- Yellow: calcareous sediments.
The nature of these sediments will be discussed in more detail below.
There are a few ways in which this map may be misleading. First, note that this is a Mercator projection map, since, for technical reasons, this was the easiest map projection for me to use. All map projections distort reality, since they involve representing a spherical surface as a flat plane. In the case of the Mercator projection, its besetting fault is that it exaggerates north-south distances near the poles.
The second way in which the map does not truly represent reality is that it divides the sea bed into distinct regions of carbonates, siliceous ooze, pelagic clay, and so forth. In fact, the sediments will rarely be pure: the sediments that form pelagic clay, for example, get just about everywhere. So really every region represents a mixture of sediments, of which the color used on the map represents the one that predominates. This inaccuracy will, of course, be especially severe at the borders between regions, where one sediment type will grade into another.
Thirdly, note that this map represents sediments on top of the sea bed: they are not the same all the way down, as we shall discuss in the final section of this article.
Types of marine sediment
- Sediments of the continental margins. These include sand and mud from rivers; material eroded from cliffs; in some latitudes, material deposited by glaciers during ice ages; material carried down to the continental rise by turbidity currents; and, in some cases, calcareous ooze (see below).
- Land-formed (or "terragenic") sediments. Technically, both glacial sediments and the sediments of the continental margin can be considered land-formed. However, the areas marked as such on the map are regions where the abyssal plain has been covered by land-formed sediments either borne down to the abyssal plain by turbidity currents, or borne out to sea by river currents powerful enough to carry the sediment beyond the continental shelf.
- Siliceous ooze. This is formed from the silica shells of microscopic organisms: diatoms and radiolarians. These are common only in the most nutrient-rich and biologically productive parts of the ocean, such as the polar oceans and upwelling zones near the equator. If lithifed, this would by definition be chert.
- Calcareous ooze. This is formed from the remains of tiny organisms such as foraminifera, coccolithophores, and pteropods. Its peculiar pattern of deposition can be explained by the fact that this sediment tends to dissolve in deep cold water. Chemically, it is composed of calcium carbonate; hence when lithified it would by definition be limestone.
- Pelagic clay (also called "red clay" or "brown clay"). This consists chiefly of particles fine and light enough to be borne out to sea by currents of wind or water. As such, you might expect these sediments to get pretty much everywhere, and you would be right. The areas on our map showing pelagic clay are not so much areas in which lots of this sediment are deposited as areas in which nothing much else is deposited.
Marine sediments and plate tectonics
We mentioned above that the sediments found in a given region of the sea floor are not the same all the way down.
This would be puzzling if the ocean bed stayed still: for in that case, why should not calcareous ooze (for example) be deposited on the same spot for ever?
But in light of the theory of plate tectonics, this observation will turn out later in this textbook to make perfect sense. Given that the sea floor moves, and has been moving for a long time, we should expect it to become layered as the same bit of continental crust passes under successive different regions of deposition. Indeed, given that we know how the sea floor is moving, and given a map such as the one presented above, we can say what pattern of layers we should expect to see: and this is confirmed by observation.
We shall discuss this point in more detail in the articles on plate tectonics.