Deep Marine Basin Environments

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Main deep sea sediment is carbonate ooze.

Deep basins consist of:

continental shelf is next to land and extends out to top of continental slope

continental slope has a steeper gradient and in may places is cut by submarine canyons eroded by turbidity currents

continental rise has gentle gradient because it is where turbidity currents deposit much of their load

abyssal plain continues out across ocean floor

convergent plate margins there is a trench where sediments carried by turbidity currents are deposited.

Ocean Margins

Most deposition on abyssal plains is from turbidity currents. Sediment brought to sea by rivers accumulates on the continental shelf. If an earthquake causes the material to slip down the continental slope, it will become a turbidity current.

Density of turbidity flow means that it flows with high velocity down the gentle gradients. A turbidity current transports huge volumes of clastic material from continental shelf.

Includes coarse and medium grained particules as well as mud. Much of the sediment is deposited in submarine fans on the continental rise or in ocean trenches. However some can be spread over abyssal plain.

Heavier coarse grains deposited first. As current loses energy, finer sediment settles out. During times when turbidity currents do not flow, fine mud is deposited from suspension to form shale. Overall product is an upward fining sequence called a turbidite.

Sedimentary structures in turbidite deposits

graded bedding is very common structure in greywacke due to coarse particles settling out faster

flute casts are found at base of greywacke beds

Thickness of deposits gets thinner with distance over abyssal plain, as most material is deposited at foot of slope.

Grain size decreases away from source of sediment as fine sediment is less heavy so carried further away. Ratio of sand to mud decreases.

Ocean Basins

Abyssal plains are 3-5km below sea level.

Wind blown sediment is deposited to form pelagic clay. Many of sediments are biogenic, including calcareous and siliceous oozes. When planktonic organisms die, their tests slowly sink to sea floor to be preserved as mircofossils.

Carbonate oozes contain skeletal remains of coccolithophores and foraminifera. Preserved at depths shallower than 4km - known as carbonate compensation depth. Depth varies across the ocean and has varied in the Earth's history. Below CCD any carbonate material dissolves before it reaches the sea floor, due to higher CO2 content and lower temperature of water.

Siliceous oozes form from skeletons of diatoms in deposits nearer the poles and radiolaria in deposits near equator. Silica dissolves at slow rate in sea water. Oozes accumulate where the rate of deposition is greater than the rate of solution. Sedimentation rates on the abyssal plains are estimated at 1mm per 1000 years, up to 3cm per 1000years.

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