Brief notes on the Classification of Marine Sediments
Marine sediments are derived from many sources. There are four basic sources of such sediments-rocks, organic materials, water, and the cosmos. In fact, the fundamental classification for marine sediments is based on their origin. The following classification of the marine sediments is, therefore, based on the basic sources:
(a) Lithogenous Sediment (c) Hydrogenous Sediment
(b) Biogenous Sediment (d) Cosmogenous Sediment
(a) Lithogenous Sediment:
The word ‘lithogenous’ means derived from rocks. Thus, the sources of lithogenous sediments are the rocks of the earth’s crust. There are two processes which are responsible for the weathering of the rocks, whether of igneous or sedimentary type. The two processes are disintegration and decomposition.
Disintegration means the mechanical breakdown of existing rocks. However, in this process there is no change in the composition of the breaking material. On the contrary, decomposition involves chemical changes in the rocks.
The process of weathering itself depends upon the composition of the rocks and various climatic conditions to which the rocks are exposed.
Some of the constituents of the weathered rock materials are soluble and pass into solution. The rivers carry them to the ocean in a dissolved form. However a larger part of the weathered rock material is insoluble and is in the form of rock – pieces of different size ranging from a fine clay particle to a big boulder.
Thus when the rocks break up they are carried to the ocean by transporting agencies like the river, wind or glacier and deposited there. Even though by far the largest quantity of lithogenous material is deposited on the continental shelf and slope, the finer materials may be carried to a greater distance by waves and currents.
It is true that there are no parts of the ocean basins where traces of the lithogenous sediment cannot be found. Besides, the volcanic islands in the open ocean are also suppliers of lithogenous sediment.
(b) Biogenous Sediment:
Biogenous sediment comprises the insoluble remains of organisms, for example, bones and teeth of animals and the protective shells of animals and coverings of plants that are deposited on the ocean bottom.
Calcium carbonate (CaC03) and silica (Si02) are the most common chemical compounds found in this type of sediment. Calcarious sediments contain the most common particles of the protective coverings of foraminifera, coccolithophores, and pteropods.
On the other hand, the siliceous sediment consists of diatom and radiolaria protective coverings. Biogenous sediments are, therefore, of organic origin. They are divided into two types: benthonic and planktonic. Benthonic sediments comprise remains of animals and plants living on the ocean floor.
Since the existence of bottom flora and fauna is rendered impossible on the floor of the dark deep oceans, these sediments are found only in coastal shallow water deposits.
The planktonic sediments are formed on the floor of the deep ocean basins from the remains of animal and plant planktons which actually live in the subsurface layer of the ocean in which the sun light is able to penetrate.
(c) Hydrogenous Sediment:
The word ‘hydrogenous’ means derived from water. This type of sediment results from the chemical reaction occurring within the seawater. Manganese deposits, phosphorite, and glauconite are minerals that form by chemical precipitation from water. However, the rate of accumulation of this sediment is rather very slow.
(d) Cosmogenous Sediment:
Cosmogenous sediment is derived from meteoric dust falling from the outer space. Magnetic spherules rich in iron and ranging in size from 10 to 640 micrometers are typical particles of cosmic origin present in marine sediments. These sediments are less important than the inorganic precipitates.