Sediment Diagenesis

Sediments form from the breakdown of rocks.  This process is called diagenesis, and has three stages:  weathering, erosion & transport, and deposition.

Weathering - The breakdown of rocks into smaller pieces, called clasts, is called weathering. Breakdown may occur via mechanical weathering, the physical breaking apart of Earth materials, or by chemical weathering, where water (or other processes) alters the internal structure of minerals.

In general, mechanical weathering speeds chemical weathering.  Mechanical weathering breaks the rock into smaller and smaller pieces, increasing the surface area of the clast being broken. An increase in surface area accelerates chemical attack.

Grain or particle size is determined by the diameter of the particle.  The Wentworth grain size classification assigns size limits to each class.  A grain with a diameter of 10 mm would be classified as a medium-grained gravel, for example. 

You've now got broken bits of rock.  Now what?

Erosion & Transport

The sediments are moved from one place to another via erosion & transport. Dispersal of sediments occurs due to wind, water, and ice.  The size of the particle will determine how far and how fast it moves.

• The larger and heavier the particle, the harder it is to move. Large fragments are moved down slope via gravity.
• The smaller and lighter the particle, the easier it is to move.  Smaller fragments are carried away by water or wind.

This animation illustrates how sediments move in a fluvial (river) environment.  Heavier particles tend to stay on the stream bed (alluvium) while medium sized particles move downstream via rolling or jumping (saltation).  Both of these particle sizes comprise the bed load.  Finer particles, such as silt and clay, and dissolved particles flow downstream in the water, forming the dissolved load.

Click to view animated version

When a particle first breaks off, it is large and angular.  On it's journey towards it's final place of deposition, it is broken into smaller pieces and abraded into a rounder shape.  Thus, the further a clast travels from it's source, the smaller, more round, and more spherical it becomes. 

Sorting - the variety of shapes, sizes, and compositions - changes with transport as well.  Some minerals (feldspars, hornblende, and augite) weather and alter faster than others and will be removed from the sediment as distance increases.  Quartz is extremely durable and tends to survive the transport process better than other minerals.  Sediments that are comprised of all (or mostly all) the same shape, size, sphericity, and composition are said to be mature sediments. 

Deposition

All good things must come to an end.  When transport stops, sediment accumulates, or is deposited, where it settles.  Sometimes, this process can happen several times before the sediment ends up in its final place of deposition. 

Sediments are deposited due to a reduction in energy levels of the medium transporting them.  A geographic setting where sediment is accumulating is called the depositional environment.  The depositional location determines the nature of the sediments that accumulate.  

Non-Marine Environments

Deposition in non-marine environments is associated with rivers (fluvial), lakes (lacustrine), wind (eolian), or glaciers.  Rivers have variable energy levels, however, they tend to be faster and more energetic in mountainous regions, and slower and less energetic in flatlands.  Therefore, the sediment sizes will vary from large and angular to fine and rounded accordingly.  Lacustrine, eolian, and glacial environments are also called quiet-water environments as the energy levels are so low that there is little to know movement.  Sediment sizes in these areas tend to be in the silt to clay range.

Transitional Environments

As the name implies, these are geographic locations where the non-marine environment transitions to the marine one.  Thus, transitional environments have characteristics of both.  Deposition associated with tidal flats, lagoons, and deltas.  These are all quiet-water environments where silts and clays dominate.

Marine Environments

These environments, not surprisingly, are in the ocean.  The ocean is a vast place, and therefore environments vary from the energetic beaches to the quiet waters of the deep sea floor.  Coarser sediments (sands) are located close to the shore and on the continental margins, while finer sediments (silts, muds, and clays) are found in the deep ocean.