General Oceanography Exam 2: Continental Margins

What are continental margins and how do they form?

edges of the continent. Transition from granitic to basaltic crust. Can be on the edge of a lithospheric plate or in the interior of a plate.

Consists of

• Continental shelf

• continental slope

• continental rise

formed through divergence/rifting of continents and seafloor spreading that create ocean basins, margin moves away sinking lower into mantle then eventually accumulates sediment over years.

Underlying geology of continental margins

• Normal Faults

• Evaporites (salt)

• remnant reef

• sediment in fill basin

Explain the the process of salt domes

formed by faulting due to rifting apart from Pangea, which left troughs and ridges parallel to shore. Troughs filled periodically with seawater, then isolated, allowing evaporation to leave behind salt deposits (evaporites). Salt has lower density than surrounding sediment, so creates dome. Folds and faults create pockets for collection of oil. Oil migrates up salt column; trapped by impermeable layers.

continental rise

accumulations of sediments at base of slope

______ discharge to continental margins is largely impacted by ______ controlling ____ patterns

sediment, tectonically built mountain belts, drainage

_____ deposition rates during episodes of uplift

faster

What is Amero-Trailing Edge coast?

passive margin with collision coast on opposite side of the continent

• rivers flow from mountains, across continent, and discharge sediment on passive margin

• well-developed coastal plain

• low wave energy

• high sediment transport mainly during storms

What are Afro-Trailing Edge Coasts?

no active margin on opposite side of continent, thus no large scale organization of drainage basin

• lower sediment supply

• diverse morphology of coast

What are Neo-trailing edge coasts?

recent continental rifting

• linear seas

• narrow to no coastal plain

• little sediment

• earthquakes, but no volcanoes

Terrigenous sediment

• largely trapped on shelves

• slumps and slides down the slope

• only deposits sediment on the rise not the abyssal plain

Submarine Canyons

conduits of sediment to rise and abyssal plains

• formed by submarine erosion by turbidity currents

• some extend offshore from rivers on land, but many don’t

Turbidity currents

density driven, starts with an event to suspend sediments, higher density of sediment and water mixture causes flow downhill, current suspends more sediment

Terrigenous sediment are characterized by:

• sorting

• angularity

• grain size

terrigenous clays that are products of weathering:

• Kaolinite: chemical weathering of feldspars

• Chlorite: common in high latitudes where physical weathering dominates

• Illite: most wide spread

• Montmorillonite: weathering of volcanic ash on seafloor

Go in depth on quarts and its significance

• only common silicate composed entirely of oxygen and silicate

• hard and resistant to weathering

• conchoidal fracture

Relict sands

• relict sands are remains from last ice age when sea level was lower

• most common sands is quartz, feldspar, rock fragments, glauconite, and phosphorite

• in relict sands, volcanic ash from eruptions can be present in sediments cores, giving insight on the chronologic order marine sediment.

Authigenic (hydrogenous) deposits:

• Evaporites: salts that precipitate out of water

• Phosphorites: high productivity regions, pavements at redox boundaries

• Manganese nodules: used heavily in steelmaking to improve strength and durability

• Glauconite: green silicate common in shallow shelf, where sedimentation rates are slow

Biogenic ooze

sediment with at least 30% shell debris

Limestone

rock comprised of >50% of CaCOs; 10-15% of sedimentary rock

Primary sources of biogenic sediment:

• invertebrate shells (mollusks, gastropods, pteropods)

• coccolithophores (phytoplankton, algae)

• foraminifera

Tropical sources of calcium carbonates:

• coral reefs

• calcareous algae

• ooids and oolites: chemically precipitated spheres

What are organic-rich sediments and where do they come from?

occur where circulation is restricted and impedes oxygen supply and/or where there are very high rates of organic matter supply to sediments (2%)

• Sapropel: derived from organic micro-algal production

• Peat: of higher plant origin (salt marshes, mangroves)

• black shales: organic mud rock, significant as hydrocarbon reservoirs

• siliceous deposits: derived from marine organisms with siliceous hard parts

organisms that make up siliceous sediment

• diatoms

• radiolarians: zooplankton, common in productive warmer water

• silicoflagellates: algal

• sponge spicules