Deposition (geology): Difference between revisions

Deposition occurs when the forces responsible for sediment transportation are no longer sufficient to overcome the forces of gravity and [[friction]], creating a resistance to motion; this is known as the null-point hypothesis. Deposition can also refer to the buildup of sediment from [[Organic matter|organically derived matter]] or [[chemical process]]es. For example, [[chalk]] is made up partly of the microscopic [[calcium carbonate]] skeletons of marine [[plankton]], the deposition of which has induced chemical processes ([[diagenesis]]) to deposit further calcium carbonate. Similarly, the formation of [[coal]] begins with the deposition of organic material, mainly from plants, in [[Hypoxia (environmental)|anaerobic]] conditions.

 

 

The null-point hypothesis explains how [[sediment]] is deposited throughout a shore profile according to its grain size. This is due to the influence of hydraulic energy, resulting in a seaward-fining of sediment particle size, or where fluid forcing equals gravity for each grain size.<ref name="Jolliffe">{{cite journal | first1=I. P. | last1=Jolliffe | title=Littoral and offshore sediment transport | journal=Progress in Physical Geography | year=1978 | volume=2 | issue=2 | pages=264–308 | issn=0309-1333 | doi=10.1177/030913337800200204| s2cid=128679961 }}</ref> The concept can also be explained as "sediment of a particular size may move across the profile to a position where it is in equilibrium with the wave and flows acting on that sediment grain".<ref name="Horn">{{cite journal | first1=Diane P | last1=Horn | title=A review and experimental assessment of equilibrium grain size and the ideal wave-graded profile | journal=Marine Geology | year=1992 | volume=108 | issue=2 | pages=161–174 | issn=0025-3227 | doi=10.1016/0025-3227(92)90170-M}}</ref> This sorting mechanism combines the influence of the down-slope gravitational force of the profile and forces due to flow asymmetry; the position where there is zero net transport is known as the null point and was first proposed by Cornaglia in 1889.<ref name="Horn"/> Figure 1 illustrates this relationship between sediment grain size and the depth of the marine environment.