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In this article we will discuss about:- 1. Introduction to Ocean Deposits 2. Classification of Ocean Deposits 3. Distribution.
Introduction to Ocean Deposits:
The unconsolidated sediments, derived from various sources, deposited at the sea floors are included in ocean deposits. The study of ocean (marine) deposits includes the consideration of types of sediments, their sources, methods of their transportation, horizontal distribution, lithological successions or vertical variations in their distribution and composition etc.
The sediments derived from weathering and erosion of continental rocks are transported to the oceans by rivers, winds etc. Volcanic eruptions also provide sediments. Besides, the decay and decomposition of marine organisms (both plants and animals) also contribute sediments to ocean deposits.
Classification of Ocean Deposits:
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Ocean deposits are classified on different bases e.g.:
(1) On the basis of location,
(2) On the basis of depth,
(3) On the basis of the origin of sediments etc.
1. On the Basis of Location:
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This classification is based on typical locations of particular marine sediment. Though several scientists have attempted to classify ocean deposits on the basis of their locations, the classifications of Sir John Murray and J.T. Jenkins are widely acclaimed.
(i) Classification of Murray:
Sir John Murray has classified the ocean deposits into two broad categories viz.:
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(a) terrigenous deposits and
(b) pelagic deposits.
Terrigenous deposits are found mainly on the continental shelves and slopes whereas pelagic deposits predominate on the deep sea floor. Terrigenous deposits are composed of coarser materials and are derived from the continents through weathering and erosional processes and are transported to the oceans by various agencies.
Their colour may be blue, yellow, grey or red. Pelagic deposits consist of fine materials formed of skeletons and shells of marine organisms and a few inorganic substances. They are generally blue, grey or red in colour.
(ii) Classification of Jenkins:
Jenkins has divided marine deposits into three groups viz.:
(a) Deep sea deposits,
(b) Shallow water deposits, and
(c) Littoral deposits.
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The following is the detailed classification of Jenkins:
(A) Pelagic deposits:
(1) Red clay,
(2) Radiolarian ooze,
(3) Diatom ooze,
(4) Globigerina ooze, and
(5) Pteropod ooze.
(B) Terrigenous deposits:
(1) Blue mud,
(2) Red mud,
(3) Green mud,
(4) Coral mud,
(5) Volcanic mud,
(6) Gravel, and
(7) Sand.
2. On the Basis of Depth:
(A) Deep sea deposits (Below 100 fathoms):
(a) Pelagic Deposits:
(1) Red clay,
(2) Radiolarian ooze,
(3) Diatom ooze,
(4) Globigerina ooze, and
(5) Pteropod ooze.
(b) Terrigenous Deposits:
(1) Blue mud,
(2) Red mud,
(3) Green mud,
(4) Coral mud, and
(5) Volcanic mud.
(B) Shallow sea deposits (between low tide water and 100 fathoms):
(1) Gravels,
(2) Sands, and
(3) Mud.
(c) Littoral deposits (Between high and low tide water):
(1) Gravels,
(2) Sands,
(3) Mud.
2. General Classification:
(1) Terrigenous Deposits:
i. Littoral deposits,
ii. Shallow water deposits, and
iii. Terrigenous mud.
(2) Neritic Deposits:
i. Shallow water neritic deposits,
ii. Deep sea water neritic deposits, and
iii. Pelagic deposits.
3. Classification on the Basis of Origin of Sediments:
(1) Littoral deposits (derived from land)):
(i) Shore deposits.
(ii) Shelf deposits.
(2) Hemipclagic deposits (Partly from land and partly from marine origin):
(i) Green mud.
(ii) Volcanic mud.
(iii) Coral mud.
(3) Eupelagic deposits (Of marine and cosmic origin):
(i) Red clay.
(ii) Radiolarian ooze.
(iii) Globigerina ooze.
(iv) Pteropod ooze.
Distribution of Ocean Deposits:
Terrigenous Deposits:
Terrigenous deposits include gravels, sands, muds, volcanic materials etc. which are derived through weathering and erosion of continental rocks by various denudational processes. There is marked gradation of these sediments when they are deposited in the oceans. The sequence of these materials from the coast towards the sea is gravel, sand, silt, clay and mud. The ocean currents and waves very often disturb the gradation and sequence of sediments.
Terrigenous deposits are classified into 3 categories on the basis of location and depth:
(1) Littoral deposits are generally found on the continental shelves mainly near the coastal margins upto the depth of 100 fathoms (600 feet) but they have been also traced upto the depth of 1000m-2000m. Littoral deposits consist of gravels, sands, silt, clays and muds.
(2) Shallow water deposits include terrigenous sediments deposited between low tide water and 100- fathom depth. These deposits consist of gravels, sands, silt and clays of varying proportions. Sea waves and tidal waves help in the gradation and sorting of sediments but landslides, slumping, strong storm waves, and storms sometimes disturb the vertical stratification of sediments.
(3) Deep water deposits include the sediments deposited below the depth of 100 fathoms. There is marked gradation of sediments in vertical succession where the sequence of sediments with increasing depths is blue mud, red mud, green mud, coral mud and volcanic mud.
Pelagic Deposits:
Pelagic deposits consisting of remains of marine plants and animals in the form of different types of oozes cover about 75.5 per cent of the ocean areas. Pteropod, diatom and radiolarian oozes cover 0.4, 6.4 and 3.4 per cent areas of all the oceanic deposits respectively. Red clay constitutes 31.1 per cent of the total ocean deposits.
Pteropod oozes are found over an area of 12,90,000 km . Globigerina oozes cover larger areas in the Pacific (64.5 million km2), the Atlantic (37.9 million km2) and the Indian (31.4 million km2) oceans (figs. 27.3, 4 and 5). Radiolarian oozes are found over an area of 5.16 million km2 in the Pacific and Indian oceans, Diatom oozes arc spread over an area of 1,03,000 km2 in the North Pacific Ocean and 27.6 million km2 in the southern oceans. Red clay is distributed over an area of 129 million km2 of all the oceans.
Philippi has described a vertical stratification of different pelagic sediments wherein the sequence from top to the bottom includes pteropod ooze, globigerina ooze, radiolarian ooze, diatom ooze and red clay.
It is apparent from the figures that terrigenous deposits are found along the coasts mainly on continental shelves but they cover greater extent near the East Indies, in the North Pacific and along the Labrador coast. Globigerina ooze, red clay and diatom ooze dominate in the western, eastern and southern parts of the Indian Ocean whereas it contains maximum areal extent in the Pacific Ocean.