Marine Sedimentation Flashcards
Ocean sediment
Various materials settled through the water column and accumulated on the ocean floor
Origin of lithegenous sediment
- Weathering
- Transportation
- Deposition
Lithogenous sediment occurs as:
- Neritic (nearshore) deposits
e.g. beaches, continental shelves, turbidites, glacial rafted debris - Pelagic (deep ocean floor) deposits e.g. abyssal clay
Lithogenous sediment composition
- majority= quartz (chemically stable and abundant)
Factors controlling sedimentation
- Particle size
-Turbulence of deposition environment - Rate of erosion
Terrigenous sediment
Sediments derived from erosion of rocks on land ( derived from terrestrial not marine environments ) and carried by rivers, wind and glaciers to the sea
What does average grain size reflect?
Energy of depositional environment
Lithogenous sediment texture over time…
- Maturity increases
- Degree of sorting increases
- Clay content decreases
- Rounding of and particles increases
- Time increases
Hjulstrom’s diagram
Graphs relationship between particle size and energy for :
- Erosion
- Transportation
- Deposition
Shelf sedimentation is controlled by:
Tides, Waves and Currents
–> Influence of these factors decreases with water depth
Shoreline turbulence
Prevents small particles from settling
- Transports them seaward where they are deposited in deep water
Past fluctuation of sea level
Cause coarse (relict) sediment across the shelf (includes most areas where only fine sediments are deposited today)
What controls worldwide distribution of recent shelf sediments ?
Latitude and climate
What type of sediment dominates tropical shelves?
Calcareous biogenous sediments
What type of sediment dominates temperate shelves?
River-supplied sands and muds
What type of sediment dominate polar shelves?
Glacial till and ice rafter sediments
For a time frame of up to 1000 years:
Waves, currents and tides control sedimentation
For a time frame of up to 1,000,000 years:
Sea level lowered by glaciation controls sedimentation and causes rivers to deposit their sediments at the shelf edge onto the upper continental slope
For a time frame of up to 100,000,000 years:
Plate tectonics determine the type of margin that develops and controls sedimentation
How does hydrogenous sediment form?
Forms when dissolved materials come out of solution (precipitate)
Cause of precipitation:
- Changes in temperature
- Changes in pressure
- Addition of chemically active fluids
Types of hydrogenous sediment:
- Manganese nodules/crusts
- Phosphates
- Carbonates
- Metal sulfides
- Evaporite salts
What are hydrogenous desposits?
Chemical and biogeochemical precipitates that form on the sea floor. Including:
- ferromanganese nodules
- phosphorite
Cosmogenous sediment
Composed of material derived from outer space (insignificant proportion of ocean debris)
Two types of cosmogenous sediment
- Microscopic space dust
- Macroscopic meteor debris
Methods of sampling deep sea sediments
- Bottom dredges (wire bag) used to scrape up sediments
- Grab samplers
- Box corer ( up to 60cm sediment)
Grab samplers
Take a ‘bite’ out of the sediment covering bottom (up to 50cm)
Gravity Corer
This corer is a weighted tool with steel tubes attached, weighing from 100kg to 1000kg. It collects sediment samples with one to four-meter-long tubes fitted with a catcher. These tubes have a clear 63.5mm polycarbonate liner. To collect a sample, it’s lowered to the seabed on a wire, lowered into the sediment at a set speed, then raised to dismantle and remove the sample liner.
Piston corer
The piston corer collects longer samples (90mm or 110mm diameter) than the gravity corer, using weighted steel tubes, a PVC liner, and piston mechanism.
Collecting cores of ocean sediment
- Specially designed ships collect cores by rotary drilling
-Cores allow scientists to analyse ocean sediment - Sediment sequences of hundred of meters can be taken
2 main sources of deep sea sedimentation
- External - terrigenous material from the land
- Internal - biogenous and hydrogenous from the sea
Major pelagic sediments in the ocean?
- Red clay
- Biogenic oozes
Origin of biogenous sediment
- organisms producing hard parts when they die
- Material rains down on ocean floor and accumulates as :
-> macroscopic shells, bones,teeth
-> Microscopic tests (shells)
Biogenous ooze
Formed if biogenous sediment is formed of at least 30% test sediment
2 main chemical compounds in microscopic biogenous tests
- Silica (including opal)
-> diatoms -> Radiolarians (protozoan) - Calcium carbonate or calcite (CaCO3)
-> Coccolithophores (algae)
-> Foraminifers (protazoan)
Examples of silica- secreting organisms
- Diatom
- Radiolarian
Siliceous ooze
Silica-secreting organisms accumulate to form siliceous ooze(>30% siliceous test material)
Example of calcite- secreting microscopic organisms
- Coccolithophores
- Foraminifers
Calcareous ooze
Calcite-secreting organisms accumulate to form calcareous ooze(>30% calcareous test material)
Biogenous ooze –> rock
When biogenous ooze hardens and lithifies it can form:
- Diatomaceous earth (diatom-rich ooze)
- Chalk ( coccolith-rich ooze)
What’s reflected in the distribution of sediments in the deep ocean?
- Latitude
- Distance from landmasses
- CaCO3 compensation depth
Glacial marine sediments
Occur in high latitudes
Pelagic clays
Occur far from land and in deepest water
Occur above the calcium carbonate composition depth
Calcareous oozes
Distribution of biogenous ooze
Most found as pelagic deposits
Factors affecting the distribution of biogenous ooze:
- Productivity (amount of organisms in surface waters)
- Destruction (dissolving at depth)
- Dilution (mixing with lithogenous clays)
Distribution of siliceous ooze
- Silica slowly but steadily dissolves in seawater
- Siliceous ooze found where it accumulates faster than it dissolves
Distribution on calcareous ooze
- Calcite dissolves beneath the calcite compensation depth (CCD) at 4.5 km
- Calcareous ooze can be found below the CCD if it is buried and transported to deep water
Deep- sea stratigraphy
Refers to the broad-scale layering of sediments that cover the basaltic crust
The stratigraphy of the deep sea
Strongly influenced by sea floor spreading
Stratigraphy of the Atlantic Basin
Contains a ‘two layer cake’ stratigraphy- a thick basal layer of carbonate ooze overlain by a thick layer of mud
Stratigraphy of Pacific basin
‘four-layer cake’ stratigraphy
- Crosses the equator where the CCD is lowered to the ocean bottom
Composition of red clay
- Aeolian dust from land
- Clay minerals -product of weathering
- Volcanic ash
- Minor biogenic material
What ocean basin are red clays formed in ?
Central North Pacific Ocean
