EAE3311 Lecture 5 Flashcards
How is Sediment in the Ocean produced?
2 points.
- Produced by the weathering of rocks such as granite and basalt into particles that can be moved by air, water or ice
- Accumulation of shells of dead organisms
Mostly made up of a mixture of both
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What is the sediment at the equator?
Broad band of biogenic sediment straddles the equator, extending into the subtropics (coral reefs, clams, snails, calcareous algae): warmer waters
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What is the sediment in middle & temperat latitudes?
Middle and temperate latitude shelves are covered by river-supplied terrigenous deposits
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How does the Earth’s surface change?
Most erosion of rocks occurs on Land
Most deposition of sediments occurs in the ocean
The net effect of these processes (erosion and deposition) evens out Earth’s surface (not accounting for plate tectonics which lifts it up again)
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How is ocean sediment classified?
1) Based on origin or mode of formation
2) Based on grain size
e.g. “Biogenous mud”
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What is the Wentworth Grain Size Scale?
Phi = a logarithmic scale unit
𝜙=−𝑙𝑜𝑔₂(𝐷/𝐷₀)
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What is the Wentworth Grain Size for mixed samples?
In mixed samples we describe the Characteristics of the Distribution:
D₅₀ = median grain size
Dₓ = proportion of grains up to size x
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What are the most important factors that control the nature of the sediment besides distance from source?
Particle-size distribution → Settling velocity
Energy conditions at the site of deposition → Transport/Resuspension
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Describe
Shelf Sedimentation
Waves are the dominant processes affecting the sea bottom of the shelf
Energy for eroding and transporting sediment grains is provided by tides and wind-generated waves and currents
The energy of the wave, measured at the sea bottom, is the bottom energy (It decreases as water depth increases)
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What is the impact of the shorline on shelf sedimentation.
As waves break at the shoreline, high energy conditions lead to the resuspension and transport of fine sediments (leaving sand and gravel on the beach)
Systematic decrease in grain size with depth (distance from shore, changes in sea level…)
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What are relict sediments?
Sediments deposited during past sea conditions
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What is an example of rate of beach movement?
15 ky before present US Eastern seaboard beaches were about 100 km offshore
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Describe
Shelf sediments
Most of the sediment on the shelf (60-70%) is relict deposited during various previous low stands of sea level
Recent sediments (Holocene) are confined mainly to the inner shelf
The coarse grains are not in equilibrium with the bottom energy conditions
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What is Bulk Emplacement?
Large quantities of sediment are transported to the deep-sea floor as a mass rather than grain by grain (slumps, turbidity currents, ice rafting)
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Describe
Bulk Emplacement?
Sediment laden slurries
Self accelerating (picking on more sediments along the way thus increasing the density of the slurry)
As they reach the flat abyss, the energy is spread out and dissipated, slowing the current
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