Marine Sedimentation n The central topics of this chapter are the nature and significance of the tremendous amount of sediment that blankets the deep sea floor and the edges of the continents. We will consider n Sediment n Sedimentation n coastal n pelagic

Sediment in the Sea: Overview n The final character of any deposit is determined by several factors: n 1. Delivery (supply, usually from the surface) n 2. Dissolution (applies mostly to biogenic stuff) n 3. Dissemination (I needed a “d” word for erosion) n 4. Dilution (even if supply of a given component is high, if the supply of something else is higher, it will dominate) n 5. Diagenetic alteration (both chemical and mineralogical changes without any actual loss)

Sediment in the Sea: Overview n Here is how these factors work: n 1. Delivery n 2. Dissolution n 3. Dissemination n 4. Dilution n 5. Diagenetic alteration Atmospheric Biogenic Rivers

Sediment in the Sea: Overview n Each type of sediment particle will be affected differently by each factor n 1. Delivery (different factors affect production vs erosion) n 2. Dissolution (different chemistries dissolve more under different conditions) n 3. Dissemination (some particles are eroded much more easily than others) n 4. Dilution (eg. For clay, the only time it dominates is when nothing else does) n 5. Diagenetic alteration (some particles are stable, some are transformed into other minerals after burial) n We will look at all of these by the time we’re through

Classification of Marine Sediment n Marine sediments can be classified on the basis of: n Size n Mode of formation/origin n chemistry

Sediment Size n The Wentworth grain-size scale is used to distinguish particles. n s=2 -  (s in mm) n Subdivision  Units microns n Gravel ,000 n Granule ‑ 12,000 n Very Coarse Sand01,000 n Coarse Sand1 500 n Medium Sand2250 n Fine Sand3125 n Very Fine Sand462 n Silt84 n Mud (Clay)>8 8<4 n Most of the sediments in the ocean are sands and muds

Mode of Formation/Origins n Sediments in the ocean originate from one of 5 sources. n Terrigenous n Biogenous n Authogenous n Volcanic n Cosmogenous

Mode of Formation/Origins (cont). n Terrigenous sediments are derived from the land (terra) n rocks weather to small particles n these particles are delivered to the ocean n Much of it is deposited in river deltas like this one (the Mississippi)

Mode of Formation/Origins (cont). n Biogenous sediments are the skeletal remains of living organisms n only the “hard parts” are preserved n this is a small fraction of what’s living out there n These skeletons dominate the sediment in some places

Mode of Formation/Origins (cont). n Four of the most common are: Coccolithophorids foraminifera Radiolarians Diatoms Foraminifera

Mode of Formation/Origins (cont). n Authigenic deposits include n Biochemical precipitates n Ferromanganese nodules n irregular or sphere-shaped n concentric layers of metal oxides n major components are iron and manganese n contain other economically important metals n Metal deposits at hydrothermal vents phosphorite

Mode of Formation/Origins (cont). n Authigenous sediments are formed in place n the most common are ferro- manganese nodules n these have large potential enconomic value These are normally fist-sized

Deep-sea Sedimentation (Cont.) Distribution of manganese nodules.

Authigenous Sediments n Hydrothermal deposits are formed at midocean ridges as part of the spreading process n water circulates though cracks in the crust, dissolving minerals and bringing them to the top

Authigenous Sediments n When this water mixes with the cold ocean water n The water cools n the minerals precipitate out n These precipitates form mineral-rich sediments

Mode of Formation/Origins (cont). n Cosmogenic particles come from “outer space” n meteorites and impacts n very small amount n important tracers of “events”

Sedimentation n The principal factors that control sedimentation are: n Particle size / Energy conditions at the site of deposition n chemistry / production n These are different for different particles and for different environments

Sediment in the Sea: Overview n Here is how these factors work: n 1. Delivery n 2. Dissolution n 3. Dissemination n 4. Dilution n 5. Diagenetic alteration Atmospheric Biogenic Rivers

Energy and Sedimentation n Generally the particle size of a deposit is proportional to the energy level present at the time of deposition. Thus, n High energy beaches are composed of coarse sands. n Quiet lagoons are composed of muds.

Energy and Sedimentation Hjulstrom’s Diagram. little particles are easier to erode, but little particles are easier to erode, but the tinest particles (clay) stick together and are hard to pick up the tinest particles (clay) stick together and are hard to pick up

Energy and Sedimentation Clay is hard to erode, but easy to transport Sand is easy to erode but harder to transport

Continental Shelf Sedimentation n Wave energy decreases with depth below the surface n In deep water, no energy reaches the seafloor

Continental Shelf Sedimentation n Water depth increases with distance offshore, so: n The energy at the bottom decreases with increasing distance offshore. n Hence grain size diminishes with increasing distance offshore.

Continental Shelf Sedimentation n The plate tectonic setting and history of a continental margin controls the character of the coastline n Characteristics of the western North American boundary n Subduction zone n Compressional boundary n Narrow shelf

Continental Shelf Sedimentation n In a subduction zone, sediment accumulates through: n erosion from volcanic mountains n accretion from scraped-off marine sediments

Continental Shelf Sedimentation n Things are much different at the eastern North American boundary n Passive margin (NO subduction zone!) n Sedimentation n Sea floor sinking from sediment buildup n Remains shallow because sedimentation balances sinking

Summary n Sediment comes from a variety of sources: n biogenic, lithogenic, authigenic, volcanogenic, cosmogenic n Sediment record determined by 5 “D’s”: n Delivery, Dissolution,Dissemination, Dilution, Diagenetic alteration n Sediment can be characterized by: n size, chemistry/mineralogy, origin n Sedimentation on the continental shelf is controlled by wave energy because of the shallow water