Ocean Sediments-Lecture Notes

Sediments are everywhere on the seafloor, but there are many different types. In order to recognize environments, and then to interpret changes in those environments, we need to come up with a set of guideline to communicate about sediments.

Descriptions of Sediments:Type, Size, and Shape/Sorting

!!Click on the link for a picture of the sediment!!

Type-Sediments classified in four broad categories: Biogenous, Terrigenous, Authigenic, and Cosmogenous.

Biogenous-Formed from living organisms, primarily by forming a hard skeleton. Included in this category are skeletons of critters ranging from one-celled algae to whales. The skeletal material is either calcium carbonate (CaCO3), silica (SiO2), or phosphatic. It is important to note that calcareous skeletons cannot be deposited below the "Calcium Carbonate Compensation Depth" or "CCD." In most parts of the ocean, the CCD is at about 4500 m; the CCD increases toward the surface at the poles. Below this level, the ocean water is too cold and the pressure too high for calcium carbonate to be stable. Therefore, the biogenous sediments are primarily siliceous. Main sources of biogenous sediment are: DIATOMS, RADIOLARIANS, FORAMINIFERA, PTEROPODS, COCCOLITHOPHORES, SPONGES, and a host of invertebrates.

Terrigenous-Broad category including all of the sediment derived from erosion of the land. Once in the "oceans," this sediment can take on a variety of depositional forms such as sand bars, deep sea avalanches, and cobble beaches. Besides sediment eroded from the continents, TERRIGENOUS sediment also includes volcanic debris, such as wind-borne ash and erosion of volcanic islands.

Authigenic-Class of sediment that forms "in place" from the seawater. By this, we mean the evaporate minerals, such as HALITE (table salt), GYPSUM, and ANHYDRITE. This grouping also includes the sulfide minerals that come out of solution near deepsea vents and along spreading centers. Sometimes, we also include minerals that form from seawater, but maybe because of biological activity. This includes calcium carbonate mud and MANGANESE NODULES.

Cosmogenous-Life may not come from outer space (yet), but dirt does! Specifically, cosmic dust is constant raining down on Earth and can be preserved in oceanic sediments. Besides these micrometeors, cosmogenic sediments also include MICROTEKTITES which form from the impact of an asteroid against the Earth, causing material to melt. These sediments are very rare on the ocean floor.

Size-Different catagories based on size of the grains. This classification scheme differentiates sediments solely on size, as follows:

Particle Sizs and Settling Rate in Sediment
Type of Particle	Diameter	Settling Velocity	Time to Settle 4 km (2.5 mi)
Boulder	>256 mm (10 in.)	-	-
Cobble	64-256 mm(>2.5 in.)	-	-
Pebble	4-64 mm(1/6 - 2 1/2in.)	-	-
Granule	2-4 mm(1/12 - 1/6 in.)	-	-
Sand	0.062-2 mm	2.5 cm/sec ( 1in/sec)	1.8 days
Silt	0.004-0.062 mm	0.025 cm/sec ( 1/100 in/sec)	6 months
Clay	<0.004 mm	0.00025 cm/sec	50 years*
*Though the theoretical settling time for individual clay particles is usually very long, under certain conditions clay particles in the ocean can interact chemically with seawater, clump together, and fall at a faster rate. Small biogenous particles are often compressed by organisms into fecal pellets that can fall more rapidly than would otherwise be possible.

Shape/Sorting-We also classify sediments by how round or angular the grains are (somewhat subjectively) and on how well sorted the sediment is. By sorting, I mean are all the grains of a similar type? Are they all about the same roundness? The more similar and the more rounded the grains are, the more mature, or well-sorted the sediment. Likewise, the less similar, the more immature the sediment.

Sediments listed by environment:
On the first pass, we can broadly define sedimentary environments by whether they exist on the continental shelves (neritic) or slopes and deep sea basins (pelagic). Note that this is very simplified and each environment can be further subdivided:

 

Neritic: Lagoon Beach Reef Bay Estuary Tidal flat Delta

Pelagic: Trench Deep sea fan Deep ocean Deep, DEEP ocean (Abyssal plain) Mid-ocean ridges Around volcanic islands, seamounts, and guyots

WEB SITES:

http://www.paccd.cc.ca.us/instadmn/physcidv/geol_dp/dndougla/SAND/SANDHP.htm
The ULTIMATE sand page!

http://www.guilford.edu/academic/geology/marseds.html
Another informative site on sediments.

http://home.interpath.net/lj/index.htm
Ok, ok, it is a business site but she sells BEACH GLASS!!