Geopix UK - Sedimentation

Sedimentary Structures

By comparing the textures of modern sediments with those of consolidated sedimentary rocks, we can suggest the conditions under which these ancient sediments were deposited and build up a picture of the geography of the area at that time. This is one small step which leads to the larger picture of continental drift and plate tectonics. For example, by locating ancient mountain chains, we can suggest thast they lie along an ancient destructive plate boundary.

Detail is added to this interpretration by using characteristic sedimentary structures which are formed as the sediment is deposited, and which indicate the conditions of deposition. For this, we assume that the Principle of Uniformitarianism applies - that the sediments we see being deposited in one set of conditions nowadays will have the same features of sediments deposited under the same set of conditions millions of years ago.

Bedding

Deposition of sediment in water normally results in horizontal bedding - layers laid down parallel with the surface of the water and at right angles to the direction of gravity.

If sedimentation continues uninterrupted, the beds will be thick. If the sediment arrives in pulses, it will be deposited in laminations.

Variations to this general rule - sedimentary structures - are more interesting.
Examples of sedimentary structures include:

Current bedding

Formed by flowing river water.

From which direction did the current come?
From this you can work out where the high
land was at the time.

Dune bedding

Formed by wind.

From which direction did the wind blow?
From this you can show the direction of the prevailing wind and therefore within which global wind belt the dunes were formed.

Ripple marks

Formed by flowing water, either as a current or in a wave form - what difference would it make to the shape of the ripples?

If you can identify wave action, you know where the coastline used to be.

Mud cracks

Formed as wet mud dries out. The hard crust shrinks and cracks into polygonal flakes.

From this, you know that the deposit was formed in a hot desert (here in Britain!) with occasional heavy showers.

Graded bedding

Formed when a batch of mixed sediment is carried into calm water; the heaviest grains settle first; the lightest settle last.

This suggests a submarine area, perhaps on the continental slope, with turbidity currents set off by earthquakes.