Marine basins are the main depositional sites where most of the material obtained by the wear and tear of the rock masses on the land through various natural agencies like river, glacier, wind etc. accumulate.
Besides, the material produced through the processes of erosion by the sea also form a part of the marine deposits. Apart from the fragmental material derived from the land, the skeletal remains of the or ganisms dwelling in the sea basin as well as the material transported in the form of true and colloidal solutions are also involved in the formation of marine deposits. Thus marine sediments are derived from the following sources:
(a) detrital material of immediate terrigenous origin (i.e. material derived from the land).
(b) products of subaenal and submarine volcanism;
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(c) inorganic precipitates from the sea water;
(d) products of chemical transformations taking place in the sea;
(e) skeletal remains of organisms and organic matter;
(f) extra-terrestrial materials (meteoric dust, meteorites etc.)
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The waves and currents have a general tendency to move sediment seaward from the shore and to deposit coarser particles near the shore and finer sediments more in the seaward direction.
This type of separation and sorting of the grains according to their size and specific gravity are effectively carried out by the joint action of waves and currents. While the waves move the rock-waste towards the shore (i.e. the upswash), the returning undertow current drags them towards the sea.
This to and fro motion of the materials give rise to well-sorted marine sediments. In certain zones of marine deposits, a definite sequence of accumulation is noticed, which is as follows:-
Terrigenous, Biogenic, Chemogenic, Volcanic and Polygenic. Depending on the distance of the deposits from the shore, the depth of a basin, the movement of the water, the organic activities, etc, marine deposits are of various character. Accordingly they are classified into four main groups, such as:
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1. Littoral deposits.
2. Neritic or Shallow-water deposits.
3. Bathyal or Deep-sea deposits.
4. Abyssal or Very-deep water deposits.
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1. Littoral Deposits
These are the shore-zone deposits. The littoral zone includes the area between the extreme levels of the high-tides and low-tides. This zone is submerged during high tides and emerge during low tides and the deposits here are found to be unstable with respect to time.
Its width may vary depending on the nature and structure of the coast Due to the proximity of this zone to the land, from which the materials are derived, the deposits are mainly terrigenous and consist primarily of boulders, gravel, sand and mud. Mollusca, crustacea and echinoids are found mixed in these materials.
Depositional shore features include beaches, longshore bars, spits, beach barriers, barrier island and tombolos.
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(a) Beaches
As defined by Longwell and Flint (Introduction of Physical Geology, 2nd Edition, John Wiley & Sons, Inc.. New York) a beach is “the wave-washed sediments along a coast extending throughout the surf zone”.
As we know most of the sediments entering the shore-zone of the breaking waves are mainly brought by the streams and are derived from the wave erosion or breaking down of rock units exposed along the shore.
Besides, glaciers, wind and volcanoes may also contribute to these sediments only locally. The accumulation of these material between the submerged wave-cut bench and the marine cliff, gives rise to a beach.
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Beaches are composed of unconsolidated materials ranging in size from pebbles to sand and mud. The sediments in a beach tend to shift along a shore as well as back and forth as the wave advances and retreats.
Thus any given beach tends to change its dimension from one season to the next. This impermanence is characteristic of beach deposits.
(b) Bars
These are ridge-like deposits formed along the coast and are composed of sand, pebble and shell accumulations. Sometimes they are hundreds of kilometres long, 20-30 km wide and as high as 15-30 metres.
Bars often confine a portion of the sea, separating it form the main body of water. This separated part of the sea is called a lagoon. When the bar lies far offshore and is completely detached from the main land, it is known as an off shore bar. It is also termed as beach barrier or Barrier Island.
The term ‘bar* is mostly used for those depositional features which are generally submerged below sea-level and emerge at low tides.
(c) Spits
These are often considered as the most common types of bar. These are ridge-shaped deposits of sand and gravel which projects out from the land mass in to the sea. Thus its one end is attached to the land and the other end terminates amidst the sea. Sometimes spits may reach a considerable length.
(d) Tombolo
It is a form of bar which connects the main land mass with an island or sometimes connects the neighbouring islands. These are also known as ‘connecting bars’.
As far as the beach barriers and barrier islands are concerned, they are elongated sand ridges generally parallel to the shore, often encloses a lagoon between the ridge and the shore.
Behind the beach barriers in the near-shore zone conditions are sometimes favourable for the development of peat-bogs. Besides, in some places of the littoral zone there are accumulations of fractured shells of various sea-organisms.
2. Nerilic deposits
These are the shallow water deposits formed on the continental shelf and at similar depths in the flanking regions of oceanic islands. In this zone, the sediments which are found are of terrigenous, chemogenic and biogenic nature.
By far the greater part of the deposits on the continental-shelf zone are composed of terrigenous sediments. Due to intense wave action which causes sorting of the grains according to size, the coarser fragments are laid down close to the shore, whereas the finer particles are carried away to a considerable distance out to sea.
Accordingly, we find rudaceous sediments (psephites) consisting of boulders, pebbles, cobbles, gravels etc. followed by arenaceous sediments (psam- mites) consisting mainly of sands and then silty sediments and finally argillaceous sediments (pellites) where clayey particles predominate.
In addition to the terrigenous sediments, the neritic deposits also contain chemicalsediments. As we know, various mineral substances migrate from the land in to the marine basins solely as solutions, and precipitate under favourable physico-chemical conditions giving rise to calcareous, ferruginous, manganous’ aid sediments of other compounds.
Mineral matter brought in to the marine basins in colloidal solutions get coagulated by the electrolytes of the sea water and drop out of the solution and precipitate forming chemical sediments. Thus in the neritic zone, deposits of calcium caibonate, iron and manganese hydroxides are found.
Along with terrigenous and chemogenic sediments, the shelf-zone deposits also contain biogenic sediments. An abundance of nutrients and availability of sufficient light in the shelf-zone create favourable conditions for the development of organic life.
The neritic deposits, therefore, contain organic shells and skeletons of various animals. These organisms mostly assimilate calcium carbonate (lime) dissolved in the sea water to build their skeletal parts. Deep-water neritic deposits occur around the oceanic islands. Here, the coral reefs are of special importance.