sedimentary basins

Sedimentary Basins

Sedimentary Basins

•Sedimentary basins are the subsiding areas where sediments accumulate to

form stratigraphic successions•The tectonic setting is the premier

criterion to distinguish different types of sedimentary basins

Types of sedimentary basins

• Extensional basins occur within or between plates and are associated with increased heat flow due to hot mantle plumes

• Collisional basins occur where plates collide, either characterized by subduction of an oceanic plate or continental collision

• Transtensional basins occur where plates move in a strike-slip fashion relative to each other

Passive margin

• A passive margin is the transition between oceanic and continental crust which is not an active plate margin. It is constructed by sedimentation above an ancient rift, now marked by transitional crust.

• Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-oceanic ridge and the locus of extension moves away from the continent-ocean boundary.

• The transition between the continental and oceanic crust that was originally created by rifting is known as a passive margin

Formation of passive margin

There are three main stages of formation of passive margins:

o Continental rift is established due to stretching and thinning of the crust and lithosphere by plate movement. This is the beginning of the continental crust subsidence. Drainage is usually away from the rift at this stage.

o The second stage leads to the formation of an oceanic basin similar to modern Red Sea. The subsiding continental crust undergoes normal faulting as transitional marine conditions are established. Areas with restricted sea water circulation coupled with arid climate create evaporite deposits.

o In last stage crustal stretching ceases and transitional crust and lithosphere subsides as a result of cooling and thickening (thermal subsidence). Drainage starts flowing towards the passive margin causing sediments to accumulate over it .

Morphology• Passive margins consist of both onshore coastal plain and offshore

continental shelf-slope-rise triads. Coastal plains are often dominated by fluvial processes, while the continental shelf is dominated by deltaic and longshore current processes.

• The great rivers (Amazon. Orinoco, Congo, Nile, Ganges, Yellow, Yangtze, and Mackenzie rivers) drain across passive margins.

• Extensive estuaries are common on mature passive margins. Although there are many kinds of passive margins, the morphologies of most passive margins are remarkably similar. Typically they consist of a continental shelf, continental slope, continental rise, and abyssal plain.

• The morphological expression of these features are largely defined by the underlying transitional crust and the sedimentation above it.

• Passive margins defined by a large fluvial sediment budget and those dominated by coral and other biogenous processes generally have a similar morphology.

Classification of passive margins

• There are four different perspectives needed to classify passive margins:

1. Map-view formation geometry (rifted, sheared and transtensional),

2. Nature of transitional crust (volcanic and non volcanic),3. Whether the transitional crust represents a continuous

change from normal continental to normal oceanic crust or this includes isolated rifts and stranded continental blocks( simple and complex), and

4. Sedimentation (carbonate-dominated, clastic-dominated, or sediment starved)

Global distribution

• Passive margins are found at every ocean and continent boundary that is marked by a strike-slip fault or subduction zone.

• Passive margins define the region around the Atlantic ocean, Arctic ocean, and western Indian ocean, and define the entire coast of Africa, Greenland, India, and Australia.

• They are also found on the east coast of North America and South America, in western Euorope and Antartica.

• East Asia also contains some passive margins.

Economic Significance• Passive margins are important reservoirs of oil and gas. Gasoline,

diesel oil, and kerosene all come increasingly from deposits buried beneath passive margins.

• A large proportion of oil and gas are found at passive margins. Mann et al. (2001) examined 592 giant oil fields (which contain proved reserves >500 million barrels of oil or >3 trillion cubic feet of natural gas) and are estimated to contain ~65% of oil. Mann et al. (2001) classified 592 giant oil fields into six basin and tectonic-setting categories, and noted that continental passive margins account for 31% of giants. Continental rifts (which are likely to evolve into passive margins with time) contain another 30% of the world's giant oil fields. Basins associated with collision zones and subduction zones are where most of the remaining giant oil fields are found.

• Passive margins are petroleum storehouses because these are associated with favorable conditions for accumulation and maturation of organic matter. Early continental rifting conditions led to the development of anoxic basins, large sediment and organic flux, and the preservation of organic matter that led to oil and gas deposits. Crude oil will form from these deposits. These are the localities in which petroleum resources are most profitable and productive. Productive fields are found in passive margins around the globe, including the Gulf of Mexico, western Scandinavia, and Western Australia.

References• "Diapir". Encyclopædia Britannica Online. Encyclopædia Britannica. 2007. • "Petroleum". Encyclopædia Britannica Online. Encyclopædia Britannica.

2007• A. Scrutton, ed (1982). Dynamics of Passive Margins. USA: American

Geophysical Union• Gernigon, L.; J.C Ringenbach, S. Planke, B. Le Gall (2004). "Deep

structures and breakup along volcanic rifted margins: Insights from integrated studies along the outer Vøring Basin (Norway)". Marine and Petroleum Geology 21-3: 363–372

• Bird, Dale (February 2001). "Shear Margins". The Leading Edge (Society of Exploration Geophysicists) 20 (2): 150–159.

• Fraser, S.I. (2007). "Return to rifts - the next wave: Fresh insights into the Petroleum geology of global rift basins". Petroleum Geoscience .

• www.google.com• http://www.nap.edu/catalog.php?record_id=1500#toc • North, F. K., Petroleum Geology.• Selley, R , C., Elements of Petroleum Geology. (2nd Edition)