Sediments of the Ocean
Chapter 5
Sediments of the Oceans
Are particles of organic or inorganic matter that accumulates in a loose unconsolidated form Originates from: Weathering Erosion Living organism
Volcanic eruptions Chemical process Outer space
Why study Sediments
Sediments are the history book of the planet recording: Movement of plates Ocean currents Ancient climates and past ocean temperatures Mountain building and continental collision
Evolution of life Volcanic eruptions Meteorite and asteroid impacts Sediments are also the waste products of both the continents and oceans.
Weathering processes
Mechanical weathering is the physical breakdown of rock – biological sediments Chemical weathering is breakdown of rock by chemical processes Igneous rock + water = what type of rock? Sedimentary
Erosion, Transportation, and Deposition
Water delivers the vast majority of particulate and dissolved material to the sea. Wind transports only a small fraction of the sediment reaching the oceans Glaciers: In polar regions, they deliver considerable quantities of coarse sediment.
Sediment: Rounding & Sorting
As material is transported, it is sorted by size, since current velocity will set a maximum on the size of particle that can be transported. Collisions between particles, particularly the larger ones, and with the bottom of a stream, will result in rounding of the particles.
All 4 particles types have 4 properties:
1. Texture 2. Porosity 3. Permeability 4. Composition
1. TEXTURE
sizes of particles (Table 5-1) boulders, cobbles, pebbles, granules, sand, silt, clay, colloids. Silt + clay = mud) sorting (how much of each size particle: all same size = well-sorted; mix of mud, gravel, sand = poorly sorted)
Larger particles require more energy to be moved (fig. 5.5) The energy is supplied by moving water or wind. The faster the water or wind, the larger size particle can be picked up.
2. POROSITY
the spaces between particles highest porosity occurs in clay
3. PERMEABILITY
how inter-connected the pores are highest permeability is in sand; lowest is in clay
4. COMPOSITION
A) Minerals (naturally-occurring, inorganic compounds with a specific chemical composition and a crystalline structure examples: halite, calcite, silicate minerals
minerals from rocks: basalt on the left contains black silicate minerals; granite, on the right, contains mostly white silicate minerals with a few black ones scattered through.
minerals from living organisms, calcite from corals
B) Non-minerals a. Organic: contains C (carbon) and H (hydrogen), Ex.:plant matter, algal matter (oil)
b. Inorganic: lacks the crystalline structure (= glass) examples: volcanic ash; the silica (SiO2) of diatom shells
Classifying Sediments
Terrigenous or Detrital
Detrital sediments consist of particles which arise from weathering on the continents; chemical sediments precipitate from seawater. Most sediments are a mixture of various components, but often one component is dominant Sediment are classified according to this dominant component.
Lithogenous or Terrigenous (cont.)
Rivers and volcanic eruptions are the main source Igneous rocks, sands, mud of the continental shelf
Volcanic Ash layers Glacial Marine: sediment delivered to the oceans by glaciers and carried by icebergs.
Biogenous
Biogenous, or biogenic, sediments are the remains of organisms; most abundant, often the shells of single cell organisms living in the surface waters. Calcareous Oozes (consist of >30% CaCO3 shells; eventually forms chalk or limestone)
Foraminferal or Globigerina Oozes (protozoans).
Pteropod Oozes (swimming snails; rare)
Coccolith Oozes (algae)
Siliceous Oozes (consist of >30% SiO2 shells
Diatom Oozes (algae)
Radiolarian Oozes (protozoans)
Coral Reefs: reefs grow in shallow water; carbonate skeletons of coral and cemented debris
Hydrogenous (Chemical)
Precipitated from the water Evaporites are an important group of hydrogenous deposits Form when sea water evaporates
Typical evaporite, It doesn't look like table salt yet, because it has red clay and black organic matter in it
Manganese (Mn) nodules grow by precipitation of iron and manganese hydroxides from seawater (fig. 5.12) Require very low sedimentation rates. Phosphate Nodules: grow by precipitation from sediment
Cosmogenic
micrometeorites are an identifiable component in slowly accumulating sediments. Microtektites in the shape of a drop can be observed on cosmogenic sediments (fig. 5.7)
Distribution of Marine Sediments
Near-shore Deposition: coarsest materials are deposited close to source and close to shore sediment on shelves tends to be coarser (larger) in grain size (gravel, sand, and silt), while sediment in the deep areas tends to be finer (smaller) in grain size (silt and clay)
Turbidites
Turbidity Currents: play an important role in transporting sediment to deep ocean Turbidity Currents are density currents: a flowing mixture of sediment and water which is denser than surrounding water.
image of a turbidity current moving down the continental slope off California
Like rivers, they seek lowest point. Characteristics of Turbidite deposits: graded bedding Turbidity currents occur catastrophically. Associated features: submarine canyons, fans, levees
Clays
Abyssal Clays (Brown Clays): In general, these are very fine-grained products of continental weathering Gray clays: often distal ends of turbidity currents. Red clays (contain hydrogenous component): wind-blown material is most important constituent.
calcareous oozes (carbonate) accumulates in shallower areas of the deep sea, along mid-ocean ridges, siliceous oozes accumulate in cold water areas (around Antarctica and off the coast of Alaska) and siliceous oozes accumulate where there is a source of rich nutrients (along the equator in the Pacific Ocean)
Calcite (calcareous) particles dissolve in deeper ocean water. The pteropods go first, followed by the forams, and, lastly, the coccoliths. The water depth at which all of the forams dissolve is called the lysocline.
The water depth at which the coccoliths dissolve is called the Calcium Compensation Depth, the CCD (fig. 5.9)
red clay fills in the rest of the area 'H' stands for 'hydrogenous', which is the same as 'authigenic'. These occur where sedimentation rates are very low (in red clay areas), and where deep ocean currents move swiftly (Cape of Good Hope, southern Africa; also between Florida and the Bahamas)