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Lecture 11: OCEAN SEDIMENTS

Introduction
Marine sedimentary provinces
Importance of particle size
Sedimentation from turbidity currents

Powerpoint Lecture Slides


INTRODUCTION
Sediment layers -- oldest at bottom, youngest at top
Distribution -- not uniform
very little on mid-ocean ridges
up to 10,000 m beneath continental rises
average = 500 m
Sedimentation rates -- also variable
Deep ocean (average): 0.5 - 1.0 cm/1000 yr
Continental margins: 10 - 50 cm/1000 yr
Deltas, some bays estuaries: as high as 500 cm/1000 yr
Sources of ocean sediments
Continents (weathering and erosion)
Biogenic particles
Volcanic ash
Chemical precipitates
Micro-meteorites
Importance of ocean sediments
Resources
Record of past processes and conditions on the earth

Marine sedimentary provinces
-- Environments of sediment accumulation

Inner continental margin (near-shore)
Type of sediment: Terrigenous
Source of sediment: Land-derived
 
Outer continental margin ("neritic")
Type of sediment: Neritic
Source of sediment: Land-derived and pelagic
 
Deep-sea
Pelagic
Deposition by sinking through the water column"
- - Biogenic particles
- - Wind-born particles from land and volcanic eruptions

Particle sizes of marine sediments

General classification

Particle diameter, mm, for the size classes:
Gravel > 2
Sand 0.06 - 2
Silt 0.004 - 0.06
Clay < 0.004

Note: "Mud" -- silt + clay
 
Proportions of different particle sizes in a sediment reflects
(1) distance from source
(2) mechanism of transport from source to deposition

How this works -- hydrodynamic principles:
(1) Large particles sink faster than small particles in "still" water.
(2) In moving water (rivers, ocean currents and waves) the amount and maximum size of particles carried along with the current or waves increases with the water velocity.
(3) Rapid currents transport coarse as well as fine sediments.
(4) Slow currents transport only fine sediments.

Implications for sediment distribution -- Particle size tends to decrease with increasing distance from source.
Coarsest: beaches, near-shore, inner shelf -- "fast" currents
Finest: edge of continental margin, deep ocean -- "slow" currents
Sediment "sorting": How well are coarse and fine particles "sorted out" from each other?
"Well sorted" - - uniform particle size (e.g., beach sand)
"Poorly sorted" - - range of particle sizes

Sorting occurs during transport and the continued action of moving water (e.g., "winnowing" of fine particles from sand by wave action and currents)

Sediments deposited by turbidity currents
-- Importance of particle-size distribution and sorting
Turbidity current: dense slurry of poorly sorted suspended sediment
Deposition as current slows (rise, abyssal plain)
Largest particles settle first
Finer particles settle next
Finest particles (mud) settle last

Result -- a "graded bed" characteristic of turbidity-flow deposits ("turbidities")

. . . . . . . . . . . .
. . . . . . . . . . . . Finest sediment particles
o o o o o o o o o
o o o o o o o o o
O O O O O O O
O O O O O O O
Coarsest sediment particles


(Detailed notes start here)

Introduction

The sea floor is covered by layers of sediment (except on youngest crust at mid-ocean ridges). The oldest sediment rests on oceanic crust (and is the same age as oceanic crust). The youngest sediment is at the top.

Sediment is not distributed uniformly on the sea floor. As we noted previously, there is very little sediment cover on mid-ocean ridges and up to 10,000 m beneath continental rises. The average thickness of sediments is about 500 m.

Rates of sediment accumulation (sedimentation rates) also vary. Sedimentation rates are typically measured in units of cm per 1,000 years.

Deep ocean (average): 0.5 - 1.0 cm/1000 yr.
Continental margins: 10 - 50 cm/1000 yr
Major river deltas, some bays and estuaries: as high as 500 cm/1000 yr

There are numerous sources of ocean sediments:
Continents -- products of weathering and erosion
Biogenic particles produced in the ocean
Volcanic ash
Chemical precipitates from sea water
Micro-meteorites

Ocean sediments are important for a variety of reasons:

(1) Ocean sediments contain valuable resources: petroleum and natural gas, minerals, etc.

(2) Sediments record evidence of past processes occurring on land and in the ocean. These include:
Rates of continental erosion and transport by rivers
Down-slope movements of turbidity currents
Biological activity in surface waters
Volcanic eruptions

Marine sedimentary provinces -- Environments of sediment accumulation

Land-derived, terrigenous sediment dominates in the inner continental margin -- that area of the continental shelf near to land.

Along the outer continental margin (outer shelf, slope, and rise) sediment sources are a mixture of terrigenous and pelagic -- sediment that is deposited by settling in the open ocean.

In the deep sea, away from continental influences, most of the sediment is pelagic. This includes biogenic particles produced in surface ocean waters and very fine-grained wind-blown particles from land and volcanoes.

Particle sizes of marine sediments

One way in which we classify marine sediment is by their particle size. Different common names are applied to specific size ranges (diameters): gravel, sand, silt, clay ("mud" is silt and clay).

Proportions of different particle sizes in a sediment reflects contains important information about the sediment:
(1) distance from source
(2) mechanism of transport to the site of deposition

How this information is recorded in sediments is explained by some simple hydrodynamic principles

(1) Particles sink through (stationary) water at different rates. Large particles sink faster than small particles.

(2) In moving water (rivers, ocean currents and waves) the capacity to transport particles in suspension or by bouncing along the bottom depends on velocity of water: The amount of sediment that can be carried as well as the maximum diameter of particles transported is directly proportional to "current" velocity.

(3) Thus, rapid currents transport coarse as well as fine sediments. Slow currents may only be capable of transporting very fine sediments.

Implications for sediment distribution -- Particle size tends to decrease with increasing distance from the source of the sediment.
Coarsest: beaches, near-shore, inner shelf -- where currents are strong
Finest: edge of continental margin, deep ocean -- where currents are weak

"Sorting" is another way in which we classify sediments
"Well sorted" - - uniform particle size (e.g., beach sand)
"Poorly sorted" - - range of particle sizes

Sorting occurs during transport and the continued action of moving water (e.g., winnowing of fines from sand by wave action and currents

Sediments deposited by turbidity currents demonstrate the importance of particle-size distribution and sorting

Recall that a turbidity current is a dense slurry of poorly sorted suspended sediment traveling at speeds of 20 -30 km/hr. As a turbidity corrent flows over more gentle slopes of the continental rise and abyssal plains, its speed decreases and therefore sediments are deposited.

The largest particles settle first from fast-moving "nose" of current. Finer particles settle next as main body of current slows. Finest particles (mud) settle last from slowest moving "tail."

This style of deposition produced what is termed a "graded bed," where the largest particles are at the base and the finest at the top. Graded beds are characteristic of turbidity-flow deposits, or "turbidities.

Succession of particle sizes from a turbidity flow

. . . . . . . . . . . .
. . . . . . . . . . . . Finest sediment particles
o o o o o o o o o
o o o o o o o o o
O O O O O O O
O O O O O O O
Coarsest sediment particles


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