The rivers flowing on the land surface erode different types of rocks – igneous, sedimentary and metamorphic – and the eroded materials obtained from the disintegration and decomposition of these rocks are transported and dropped into the sea water.
The mechanical disintegration does not bring about any change in the chemical composition of the rocks. It simply breaks them into small pieces of different size. On the other hand, the decomposition involves chemical changes in the composition.
Some of the sediments are soluble, while others are insoluble. However, the weathering of rocks is dependant on two factors, namely, the climatic conditions and the nature of the rocks.
Let us remember that the finer rock materials have greater possibilities of reaching the sea. However, terrigenous sediment refers to material of a wide range of particle sizes that is derived directly from land sources and deposited rather rapidly on the continental shelf, continental slope, and even on the deep ocean floor as turbidities.
Turbidities are those marine deposits which consist of coarse materials carried down the continental slope to the deep ocean by turbidity currents.
There are many rivers of the world today that deposit their sediment in estuaries rather than carry it on to the continental shelf. For example, the continental margin of the Atlantic coast of North America is an area where most river sedimentation is presently taking place in estuaries.
In fact, a very small amount of sediment reaches the continental shelf. In such regions the sediments covering the continental shelf are 3000 to 7000 years old. It is estimated that such sediments presently cover about 70% of the world’s continental shelf.
Sediment that is carried on the continental shelf is sorted by current and wave action which leave the sand to be transported along the shore to form beach deposits. The finer silt and clay sized particles are carried further out to sea and deposited in areas where wave energy is at its minimum.
Where sediments are being laid down on the continental shelf, the rate at which they accumulate is generally more than 10 cm / 1000 year. These deposits generally contain materials that have not changed significantly since they weathered from the bedrock on the continent.
Despite the fact that the greatest percentage of sediment making up the continental margin was brought by the rivers, other types of sediments are also found in certain regions of the continental shelf.
In the terrigenous deposits the disintegrated rock materials comprise quartz, mica feldspar, pyroxene, amphibole and many other heavy minerals. The heavy minerals, however, are concentrated on the beaches by the sorting action of waves.
The common heavy minerals include limonite, magnetite, garnet, zircom, monazite and olivine. Terrigenous deposits are coarser than those found in deeper parts of the sea.
Remember that the texture of these deposits is largely controlled by the transporting agency and the character of the source material. On the basis of texture, Revelle has classified the sediments into the following categories:
It consists of more that 80 percent of the material coarser than 62 microns in diameter.
(ii) Salty sand:
Between 50 percent and 80 percent coarser than 62 microns
(iii) Sandy silt:
More than 50 percent coarser than 5 microns and more than 20 percent coarser than 62 microns.
(iv) Salty mud:
More than 50 percent coarser than 5 microns and less than 20 percent coarser than 62 microns.
(v) Clayey mud:
Less than 50 percent coarser than 5 microns.
It should be borne in mind that the grain size of the bottom deposits varies over a wide range – from diameter of stones, more than 20 mm., the grain size of gravel (20 – 2 mm.), coarse sand (2-0.5 mm), medium sand (0.5-0.2 mm), fine sand (0.2-0.1 mm), (2-0.1 mm), very fine sand (0.1-0.02 mm.), silt (0.02-0.002 mm), and to clay ( Less than 0.002 mm.).
On the floor of the continental shelf, particularly in areas which were formerly glaciated, there is a great variation in the distribution of grain size in different localities. On the contrary, on the deep sea floor distribution is comparatively more uniform.
Texture of sediment is determined by the grain size. This indicates the energy condition under which sediment is deposited.
The Wentworth Scale given in classifies particles into various categories ranging from boulders to colloids. In areas of strong wave action, areas of high energy, deposits that are laid down are composed mainly of larger particles, cobbles and boulders.
On the contrary, in areas where the energy level is low and the velocity of water movement is minimum, only the clay-sized smaller particles are deposited.
Wentworth Scale of grain size for sediments
Wentworth (1992) after Udden (1898)
The particle size of the sediments controls the two processes i.e. the fall velocity and the transportation of the particles along the bottom by ocean currents. It means that the grain size controls the erosion and sedimentation and, therefore, the bottom topography.
The Wentworth Scale (Table 49.1) classifies particles into categories ranging from boulders to colloids. Between these two extremes are cobbles, pebbles, granules, sand, silt, and clay.
As stated above, silt consists of very fine particles of rocks whose diameter, ranges from 3.9 to 3.12 microns. However, particles smaller than silt are called mud. Actually the mud includes very fine particles of minerals forming different types of rocks, out of these minerals quartz is the most important.
The mud includes finest particles of clay as well which is also derived from various rocks found on the land. The rivers bring these particles in suspension and pour them out in the sea water. These particles mix with sea water and make it muddy.
In due course of time these microscopically small particles are deposited on the sea floor. Because of the size of these particles, calm water provides the most favourable condition for their deposition.
The rate of deposition of such particles is undoubtedly very slow. The major part of these particles is deposited at a depth which varies from 100 to 1000 fathoms.
The mud being of different colours, Murray has classified them in the following categories:
(i) Blue mud:
The blue mud occupies the largest area on the sea floor. Most of this mud is found in deep seas in the vicinity of continents and the partially enclosed seas. Blue mud is formed from the sediments obtained from the disintegration and decomposition of such rocks which include iron sulphide as well as organic substances.
Such muds are black in colour with bluish tinge. ‘Due to the presence of iron oxide the upper layer of the mud is reddish. It includes about 35% lime.
The deposits of this mud are mostly found on the floor of the Atlantic Ocean, the Mediterranean Sea and the Arctic Ocean. However, it is also present in certain parts of the Pacific and Indian Oceans.
(ii) Red mud:
Because of the predominance of iron oxide certain type of mud is red in colour. The area occupied by red mud is rather limited. The percentage of calcium carbonate in it is roughly about 32.
However, this mud is conspicuous by the absence of siliceous organisms like the radiolarian and diatoms. The iron content found in this group of mud is in the most oxidized form which accounts for its red colour. Large areas in the Yellow Sea, the coastal sea of Brazil and the Atlantic Ocean are covered with this type of mud.
(iii) Green mud:
The green colour of this category of mud is mainly due to the presence of particles of a mineral called glauconite. In fact, glauconite is the silicate of iron which forms due to the decomposition of organic substances.
This particular mineral is found in the outer coverings of foraminifera. After the shells of these tiny organisms get dissolved in sea water, the residua of very small round particles of glauconite are left out.
Sometimes the abundance of the glauconite particles lends green colour to the sand as well. The depth at which red mud is found varies from 100 to 900 fathoms. It is found in those areas which are characterized by the absence of large rivers flowing into the sea, and the sediments deposited by them.
The green mud is also found in those areas where the warm and cool ocean currents meet together. This type of mud is found in abundance on the sea floor near the Cape of Good Hope at the southern tip of Africa. It includes 7.8% glauconite and 0.56% calcium.
The continental shelves of the Atlantic and Pacific Oceans around North America, and those of Australia and South Africa are well-known for the deposits of green mud.
Organic and Carbonic Deposits:
There are certain continental shelves on the bottom of which various kinds of vegetation and animals are found. So on the floors of these shelves are found the deposits of the remains of these vegetations and organisms.
As we are aware, the tropical seas are characterized by the abundance of coral polyps and calcareous algae. After death their skeletal remains are broken down by the action of waves and converted into sand and silt which are very rich in calcium carbonate.
On the contrary, the deposits formed by terrigenous substances and their sediments are deficient in calcium carbonate. On the floor of the seas near the West Indies the aforementioned organic deposits are found in abundance.
Bahama Islands are formed by the sediments of the shells of marine animals and corals carried by the winds. The bottoms of the nearby seas are covered by the deposits of such sediments and muds. Such organic deposits have also been discovered on the floor of the Gulf of Mexico and the Caribbean Sea.