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Sediment

The term sediment is used to describe any particulate matter that can be transported by fluid flow and which is eventually deposited as a layer of solid particles at the base of a body of water or other liquid.

In particular, oceans and lakes accumulate sediment over time. Those sediments are the source of sedimentary rocks, which sometimes contain fossils. Lakebed sediments that have not solidified into rocks can be used to determine past climate conditions.

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Sediment transport

Rivers and streams

If a fluid, such as water, is flowing, it can carry suspended particles. The settling velocity is the minimum velocity a flow must have in order to transport, rather than deposit, sediments, and is given by Stoke's Law[?]:

<math>w=\frac{(\rho_p-\rho_f)gd^2}{18\mu}</math>
where w is the settling velocity, ρ is density (the subscripts p and f indicate particle and fluid respectively), g is the acceleration due to gravity, d is the diameter of the particle and μ is the dynamic viscosity of the fluid.

If the flow velocity is greater than the settling velocity, sediment will be transported downstream as suspended load. As there will always be a range of different particle sizes in the flow, some will have sufficiently large diameters that they settle on the river or stream bed, but still move downstream. This is known as bed load and the particles are transported via such mechanisms as saltation (jumping up into the flow, being transported a short distance then settling again), rolling and sliding. Saltation marks are often preserved in solid rocks and can be used to estimate the flow rate of the rivers that originally deposited the sediments.

Fluvial Bedforms

Any particle that is larger in diameter than approximately 0.7 mm will form visible topographic features on the river or stream beds. These are known as bedforms and include ripples, dunes, plane beds and antidunes. See bedforms[?] for more detail. Again, bedforms are often preserved in sedimentary rocks and can be used to estimate the direction and magnitude of the depositing flow.

Key depositional environments

The major fluvial (river and stream) environments for deposition of sediments include:
  1. Deltas (arguably an intermediate environment between fluvial and marine)
  2. Point-bars
  3. Alluvial fans
  4. Braided streams[?]
  5. Oxbow lakes[?]
  6. Levees

Shores and shallow seas

The second major environment where sediment may be suspended in a fluid is in seas and oceans. The sediment itsel will consist of both terrigenous sediment supplied by nearby rivers and streams, and reworked marine sediments (e.g. sands). Arguably, living organisms can also be thought of as sediments when they die and are deposited at the sea floor.

The key transport mechanism is long-shore drift, driven by river outputs, coastal erosion and marine currents.

Marine Bedforms

Marine environments also see the formation of bedforms, whose characteristics are particularly influenced by the Earth's tides.

Key depositional environments

The major areas for deposition of sediments in the marine environment include:
  1. Littoral sands (e.g. beach sands, coastal bars and spits, largely clastic with little faunal content)
  2. The shelf (silty clays, increasing marine faunal content).
  3. The shelf margin (low terrigenous supply, mostly calcareous faunal skeletons)
  4. The shelf slope (much more fine-grained silts and clays)

One further depositional environment which is a mixture of fluvial and marine is the turbidite[?] system, which is a major source of sediment to the ocean shelf and basins.



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