عمرو
جيو صاعد
سجّل في : 19 أكتوبر 2007
عدد المساهمات : 26
university : ain shams university
 |  موضوع: Marine Environments  2007-10-22, 2:46 pm | |
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Detrital marine environments
1) Continental shelf environment
The continental shelf is a gently seaward-sloping surface bounded at its outer edge by a break at 200 meters depth.
Transportation and deposition are dominated by ocean, tidal, and storm-generated currents.
Subdivided into two zones:
a) Inner shelf: lies between fairweather wave base and storm wave base.
A high-energy environment due to waves and tidal currents.
Characterized by:
sands
marine fossils
b) Outer shelf: lies between storm wave base and the shelf edge.
A low-energy environment largely unaffected by tides and waves except during large storms.
Characterized by:
muds
marine fossils
2) Continental slope and rise environment
lies between the shelf edge and the deep ocean floor.
Sediments are transported and deposited by gravity processes such as:
turbidity currents: driven by the density difference between sediment-bearing water and the surrounding clear water.
Each deposits a single turbidite that consists of:
a scoured base marked by flute casts
graded bedding from gravel (base) to mud (top)
a cap of deep-sea muds
The turbidites accummulate as submarine fans at the base of the slope
3) Deep-ocean basin
Covered by laminated, fine-grained sediments composed of:
a) pelagic clays, derived from:
windblown dust
volcanic ash
meteorite dust
b) siliceous ooze: composed of shells of pelagic single-celled plants (diatoms).
c) calcareous oozes: composed of shells of pelagic single-celled animals (foraminifera).
Carbonate depositional environments
Most lime sediments are deposited in clear, warm, shallow seas on carbonate shelves where little clastic sediment is present.
Many limestones are detrital, consisting of:
skeletal grains
fecal pellets
oolites
micrite (carbonate mud).
Unlike clastic sediments, carbonate grains and micrite originate within the basin where they are deposited.
Reef-rimmed shelf
: fronted on its seaward side by a reef.
reef: a wave-resistant structure composed of a framework of corals, mollusks, sponges, or other skeletal organisms.
Subdivisions:
a) basin
Composed of horizontally bedded shales or micrite.
b) fore-reef
Composed of of reef-derived debris in beds that dip away from the reef
c) reef core
Consists of a massive, unbedded unit composed of cemented skeletons of reef-building organisms.
The seaward margin is at the depth limits of the reef-building organisms.
d) back-reef
A low-energy environment protected by the reef from wave energy.
Characterized by skeletal sands and muds
e) lagoon
f) tidal flats
Characterized by micrite-rich sediments, mudcracks, and evaporites
Evaporite depositional environments
Occur where evaporation rates are greater than precipitation and water inflow is limited. Usually limited to arid regions.
The common evaporite minerals are gypsum, anhydrite, and halite.
Form in:
some marine settings
Order of precipitation from evaporating seawater:
calcite > dolomite > gypsum > anhydrite > halite
saline lakes
playa lakes
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