Marine Sedimentation

Sediment in the Sea

• -Sediment is classified in two ways: Size and Origin

• -Size is based on the Wentworth grain-size scale

• -Size classification divides sediment by grain size into gravel, sand, and mud. Mud is a mixture of silt and clay.

Sediment Type Diameter in mm

Gravel Boulder Larger than 256.0

Cobble 65.0-256.0

Pebble 4.0-64.0

Granule 2.0-4.0

Sand Very Coarse 1.0-2.0

Coarse 0.50-1.0

Medium 0.25-0.50

Fine 0.125-0.25

Very Fine 0.0625-0.125

Mud Silt 0.0039-0.0625

Clay 0.0002-0.0039

Colloid Colloid Smaller than 0.0002

Wentworth Grain-Size Scale

Sediment in the Sea • Origin classification divides sediment

into five categories:

Sediment in the Sea –Terrigenous sediments – fine to coarse grains

that are produced by the weathering and erosion of rocks on land, typically sand and mud.

– Biogenous sediments – fine to coarse grains that are derived from the hard parts of organisms, such as shells and skeletal debris, typically lime (calcium carbonate) and siliceous mud (silicates)

Sediment in the Sea –Hydrogenous/Authigenic sediments – particles

that are precipitated by chemical or biochemical reactions in seawater near the sea-floor; manganese and phosphate nodules are examples.

Sediment in the Sea –Volcanogenic sediments – particles that are

ejected from volcanoes, like ash.

–Cosmogenic sediments – very tiny grains that originate from outer space and tend to be mixed into terrigenous and biogenic sediment. • Hard to distinguish from other sediments due to size!

B. Factors that control sedimentation • Particle size and the turbulence of the deposition

environment.

– So a larger, heavier sediment particle will not travel as far down a river as a lighter, smaller sediment particle.

– Heavier, larger particles require faster and stronger water currents to move them.

B. Factors that control sedimentation • Hjulstrom’s Diagram graphs the relationship between

particle size and energy for erosion, transportation, and deposition (when the sediment is too heavy to move).

C. Sedimentation in the Ocean

• Terrigenous sediments are found on the continental shelf.

• Seaward, water becomes deeper and more distant from a terrigenous source.

C. Sedimentation in the Ocean • - Worldwide distribution of recent shelf sediments by

composition is strongly related to latitude and climate.

– 1. Calcareous biogenous sediments dominate tropical shelves.

C. Sedimentation in the Ocean

–2. River-supplied sands and muds dominate temperate shelves.

C. Sedimentation in the Ocean

–3. Glacial till and ice-rafted sediments dominate polar shelves.

C. Sedimentation in the Ocean • If influx of terrigenous sediment is low and

the water is warm, carbonate sediments will dominate.

C. Sedimentation in the Ocean

• - Deep-sea Sedimentation has 4 main sources for sediment: terrigenous material from the land (found mostly on the shelf),

biogenous and hydrogenous from the sea, and volcanic from mid-ocean ridges and other active sites.

C. Sedimentation in the Ocean

• Major pelagic (open ocean) sediments in the ocean are red clay and biogenous oozes.

Sands on the ocean

floor are composed

of the shells

of living things.

C. Sedimentation in the Ocean

–Hydrogenous deposits are chemical and biochemical precipitates that form on the sea floor and include ferromanganese nodules and phosphorite.

C. Sedimentation in the Ocean

–Volcanic sediments come directly from volcanoes and are found primarily near mid-ocean ridges and island hotspots.

C. Sedimentation in the Ocean

• The distribution of sediments in the deep ocean varies greatly, but some parts of the ocean are dominated by certain types.

C. Sedimentation in the Ocean • New crust at the mid-ocean ridge has little sediment

covering it, where the sea floor close to the continents have great amounts of sediment, and in differing layers.

• These sedimentary layers create the great flat expanses of the Abyssal Plains.

C. Sedimentation in the Ocean • The distribution of sediments in the deep ocean varies greatly, but some parts of the ocean are

dominated by certain types.

• New crust at the mid-ocean ridge has little sediment covering it, where the sea floor close to the continents have great amounts of sediment, and in differing layers.

• These sedimentary layers create the great flat expanses of the Abyssal Plains.

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