II- Review of the Nature Principles (cont.)

f) Gravitation

Everything is curved. Linearity exists only locally.

To picture the solar system think of space-time as a rubber sheet with the Sun as a heavy ball and planets as a marble place on it. This is Einstein’s model of the way matter curves space-time and helps visualize the force of gravity between masses.

Gravity Anomalies: the geoid is the most graphical representation of the Earth gravity field. The largest positive geoid heights are associated with subduction zones and hotspots and have no simple relationship to other high regions such as continents and ridges. Subsequently, as sea level is not uniform, a global rise of 10 meters, induced for instance by a deglaciation, does not give the same relative consequences all around the Earth. Probably, in sequence stratigraphy, the majority of unconformities have a regional extend but not global.

All clastic sediments are deposit by gravity.

Fluvial, marine, eolian, pyroclastic and glacial deposits are all controlled by gravity.

Eustasy is supposed to be the major reason of the cyclicity of the sedimentary deposits. However, sediments must be transported before being deposited, so gravity plays also an important role. In this model, the vertical exaggeration is almost 100 x. Each black line corresponds to a chronostratigraphic line. Their spacing is 100k years. Terrigeneous influx is constant (area between two consecutive chronostratigraphic lines is the same). The red lines correspond to unconformities (erosional surfaces). The green dots underline the successive positions of the shelf breaks.

This seismic line, in which two majors intervals can be individualize (an aggradational and a progradational), illustrates the importance of gravity in clastics deposits. However, it does not depicts the Vail's model for clastic deposition. Actually, on this line, the majority of the seismic intervals correspond to continental encroachment sub-cycles and not to stratigraphic sequence cycles (see Duval et al., 1998).

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