Historical Geology/Pelagic clay
Origin and distribution of pelagic clay
The bulk of pelagic clay has its origin as very fine sediment (which is therefore clay in the sense of having small grain size) wind-blown from the land, chiefly fine particles of quartz and of clay in the mineralogical sense; volcanic ash also contributes, and even cosmic dust, i.e. micrometeorites fallen from outer space.
We should not be surprised to find sediment from the land being deposited in the middle of the ocean, since the fine dust raised by duststorms can be carried right across the ocean from continent to continent; dust storms in the Sahara, for example, can trigger smog warnings in Florida, and are also the main source of mineral nutrients for the Amazon Basin.
This dust is spread liberally across the surface of the ocean, but as you can see from the map to the right, not all of the ocean bed is covered with pelagic clay (represented by the brown areas on the map).
This is not because the clay is not deposited in other regions, but because in other areas other sediments are also deposited to the extent that the resulting sediment is considered to be nearshore, siliceous, calcareous, terrigenous, glacial, or turbidite in origin. Pelagic clay is therefore the "none-of-the-above" sediment; it is what you get where there are insufficient nutrients for coccolithophores and foraminiferans, or the sea floor is below the CCD; and where there are insufficient nutrients for diatoms and radiolarians, or the sea floor is below the OCD; and which is sufficiently far from the ablation of glaciers into the sea that glacial sediment is not deposited; and ... et cetera, et cetera.
Pelagic clay is the most slowly deposited of all sediments, typically accumulating at less than 10mm per thousand years; hence it is easily swamped by other sources of sediment if these are present.
The color of pelagic clay varies from place to place, depending on the source of the dust that composes it. Rich browns or reds are common, these colors being produced by iron-based minerals.
In texture it is of course fine-grained, since only the finest and lightest materials can be wind-borne so far out to sea.
As it is compacted by burial, the flakes of which the clay is composed reorient themselves to lie flat and parallel to one another.
Compared with other muds, pelagic clay is remarkable for having very little in the way of sedimentary structure. Because of the extremely gentle mode of deposition, pelagic clay will exhibit no ripple marks, sole marks, cross-bedding, grading by size, or other sedimentary structures; the most we may see are the faintest suggestions of extremely fine laminae.
Lithified pelagic clay: how do we know?
The resulting rock can be recognized as having its origin as pelagic clay by virtue of its mineral composition, its extremely fine-grained and dense texture, and by its lack of sedimentary structure.
Furthermore, pelagic claystone will be rich in ichthyoliths: disjointed parts of fish, usually teeth and scales, testifying to its marine origin. It may also contain the occasional siliceous or calcareous test; or in some cases it will be interbedded between layers of limestone or chert, serving as another indication of its origin.