SedimentsI. Sedimenta. Sediment is particles of organic or inorganic matter that accumulate in a loose, unconsolidated form.i. The particles originate from1. the weathering and erosion of rocks, 2. the activity of living organisms3. volcanic eruptions4. chemical processes within the water itself5. spaceii. Marine sediments occur in a broad range of sizes and types1. Beach sand is sediment2. Mud in quiet bays is sedimentiii. The origin and distribution of materials depends on a combination of biological, chemical and physical processesb. What do sediments look like?i. Surface1. Could be smooth2. May be rippled where there is a strong currentii. Color1. Biological sediments are white or cream-colored2. Clays range in color from tan to chocolate brown3. Nearshore sediments are dark and contain organic matter and smell of hydrogen sulfide (rotten eggs)c. Where do you find sediments?i. Almost everywhereii. If an area looks like it doesn’t have sediments it’s because the current there is moving the smaller particles away and leaving behind only the larger onesd. Classifying Sediment by Particle Sizei. Particle size is frequently used to classify sediments1. Most marine sediments are made of sand, silt and clayii. The smaller the particle the more easily it can be transported by streams, waves and currents.1. As sediment is transported it tends to be sorted by sizea. Coarser grains, which are moved only by strong, turbulent flow, tend not to travel as far as finer grains.2. Clays, particles < 0.004 millimeter in diameter, can remain suspended for very long periods and may be transported great distances by ocean currents before they are deposited.iii. A layer of sediment can contain particles of similar size, or it can be a mixture of different-sized particles1. Sorting of sizes depends on the energy of the environment2. Well-sorted sediments are composed of particles of one size and occur in environments where energy fluctuates within narrow limitsa. Sediments from the deep ocean tend to be well-sorted3. Poorly-sorted sediments have a mixture of sizes and are found in environments where energy fluctuates over a widespectruma. The mix of rubble at the base of rapidly eroding shore cliffs is an example of poorly-sorted e. Classifying Sediments by Sourcei. Can also classify sediments by their origin1. Terrigenous Sediments are the most abundant and originate on the continents or islands near them.a. The most familiar continental igneous rock is granite, the source of quartz and clay, the two most common components of terrigenous sedimentsi. Quartz, an important mineral in granite, is hard, durable and cans withstand weatheringand long transport.1. Quartz sands are important components of sediments along continental marginsb. Estimated that 15 billion metric tons of terrigenous sediments is transported in rivers to the sea each yeari. 100 million metric tons is transported by air.2. Biogenous Sediments are the next most abundant and they come from the living organismsa. The siliceous and calcareous compounds used by the organisms originally came from rivers or mid ocean ridgesb. The plants and animals precipitated these minerals and used them to form their shells and skeletons. c. Most abundant in areas of high productivityi. Over millions of years they can form oil andnatural gas3. Hydrogenous Sediments are minerals that have precipitated directly from seawatera. The sources of dissolved minerals include submerged rock and sediment, leaching of the fresh crust at oceanic ridges, material issuing from hydrothermal vents and substances flowing to the ocean in river runoff.b. Also called authigenic sediments because they were formed in the place they currently occupy.4. Cosmogenous Sediments are of extraterrestrial origin and are the least abundanta. Typically greatly diluted by other sediment components. b. Come from two major sourcesi. Interplanetary dust 1. Silt and sand sized micrometeoroids that come from asteroids and comets or from collisions between asteroids2. About 15,000 to 30,000 metric tons of interplanetary dust enters earth’s atmosphere every yearii. Rare impacts by large asteroids and comets1. Very rare2. 65 million years ago an asteroid 10 km in diameter struck earth on what is now the northern coast of the Yucatan.ii. Sediment Mixtures1. Sediments on the ocean floor only rarely come form a single source; most are a mixture of biogenous and terrigenous particlesf. Distribution of Marine Sedimentsi. Sediments on continental shelves are often different from those on the deeper basin floors1. Continental shelf sediments, neritic sediments, consist primarily of terrigenous material. a. 72% of all marine sediment is on continental slopes and rises2. Deep ocean floors are covered by finer sediments than those of the continental margins.a. Pelagic sediments come from the seaii. Sediments of the Continental Margins1. In general, coarser sediments are found closer to land, and they become finer as one moves farther off the coast. a. Exceptions occur during glaciations when glaciers deposit poorly sorted sedimentsb. Also, when sea levels were lower rivers carried sediments right to the shelf’s edge2. Biological productivity is also high along continental shelves, thus adding a lot of biogenous sediments as well.3. Sediments can undergo lithification, and be converted into sedimentary rock if they get deep enougha. This type of rock can be found at the top of Everest or on the Colorado plateauiii. Sediments of the Deep-Ocean Basins1. Sediment thickness vary greatly from place to placea. Atlantic = 1 km deepb. Pacific = 0.5 km deepc. Three reasonsi. Atlantic is smaller in areaii. Atlantic is fed by a greater number of rivers laden with sedimentiii. Pacific has many trench that trap sediments moving toward basinsd. Sediments are thickest on the abyssal plains and thinnest (or absent) on ridges2. Turbiditesa. Underwater avalanches of thick muddy fluid help cut canyons and end up transporting continental sediments onto the abyssal plain.b. The deposits are called turbidites and are graded layers of terrigenous sand interbedded with the finersediments of the deep sea floor.3. Claysa. About 38% of the deep-sea is covered by clays and other fine terrigenous particlesb. Terrigenous sediment accumulation on the deep-ocean floor is typically about 2 mm/thousand years4. Oozesa. Seafloor samples taken farther from land usually show a greater proportion of biogenous sediments than those