I. Detrital or Clastic Sediments
These rocks are composed of fragments of pre-existing rock that have been eroded, transported and deposited in an accumulation that has subsequently been converted into a hard coherent sedimentary rock.  The erosion, transportation and deposition of material by moving water is an energy-controlled process that results in deposits of sediment that are typically well sorted in terms of the size of fragments.  As a river slows down on entering the ocean, for example, it will first deposit large materials, followed in turn by sand, silt and finally clay-sized particles.  This process leads to the classification of the detrital sediments based on the size of the fragments that make up the sedimentary rock.
 
 

Particle Size	Sediment	Rock Name
larger than 2 mm	boulders, cobbles and pebbles	Conglomerate (rounded)  Breccia (angular)
0.062 to 2 mm	sand	Sandstone
0.004 to 0.062 mm	silt	Siltstone
less than 0.004 mm	clay	Mudstone or Shale

 

The process of converting unconsolidated sediment into consolidated sedimentary rock is known as diagenesis or alternatively lithification.  This process involves compaction and squeezing as the sediments become buried. Typical muds, for example contain approximately 50-60% water by volume.  After compaction, shales contain 10-20% water and have volumes less than half that of the precursor mud.  Diagenesis also involves cementing of the individual grains together as water is either evaporated or squeezed out of the primary pore spaces.  An example of a typical sandstone is shown in the microscopic image to the right.  This rocks consists of relatively rounded grains of plagioclase (pl), pyroxene (px) and fragments of volcanic rock (v).  These grains are all cemented together by a cement consisting of calcite that was precipitated from the solution that originally occupied the pore spaces.
 
 
 
 
 
 

Certain distinctive sequences of detrital sedimentary rocks are found throughout the geologic record, and provide much information about the geologic history of regions containing former shorelines.  One of the most common sequences is that associated with marine transgression and regression.  As a result of the energy relationships resulting in sediment deposition in a near-shore environment, gravels will deposited in river channels inboard of the high-tide region.  These gravels will grade into sands at the shoreline, silts further outboard from the shoreline and finally muds will be deposited at some distance seaward of the shoreline.  During a marine transgression, sea level rises relative to the land surface, and the shoreline moves inland.  What was once a sandy beach, is now covered by silt and perhaps even mud.  As shown in the figure to the right, a marine transgression therefore results in successive layers of sedimentary rocks consisting of gravels, overlain by sands, which are in turn overlain by silts and finally fine muds.  A marine regression (lowering of sea level results in just the opposite cycle).

Another distinctive plate-tectonic environment for sedimentation is found in the trenches associated with subduction zones.  These deep trenches are commonly located at the base of the continental slope.  Sediment deposited on the continental shelf and the slope can be dislodged during earthquakes and flow down the continental slope in large submarine mudslides.  Owing to the fact that these slides occur underwater, they are well lubricated and thus can travel large distances.  Such slides bring sediment into the trenches that is chaotic, turbulent and unsorted.  When the slide reaches the bottom of the trench, it stops and deposits all of its sediment.  The first material to settle out of the turbulent flow consists of the coarsest grained material, followed by gradually finer-grained material.  The figure to the left shows a schematic view of such a deposit.  The lowermost layer is especially distinctive because of its gradual increase in grain size from top to bottom.  Such sedimentary layers are called graded beds.  One typical cycle consists of a graded bed at the bottom overlain by thin sands and finally fine muds at the top. These sedimentary sequences are known as turbidites.  Observation of turbidites indicates proximity to a tectonically active continental margin.
 

 II. Chemical Sediments
Chemical sediments form from the precipitation of minerals from water when the solution becomes saturated or supersaturated in that particular mineral.  Rivers deliver to the oceans elements dissolved within the water.  There is good geological evidence that the oceans have not significantly changed their chemical composition for several hundreds of million years.  This means that as new material is added to sea water, an equal amount of material is removed by precipitation.  The average length of time a particular element remains dissolved in sea water is known as its residence time, and can be calculated from the total amount of the element in sea water divided by the rate of input to the oceans from rivers.  The average concentrations and residence times of some major elements in the oceans are given in the table below:
 
 

Element	Concentration (gm/kg)	Residence Time (Ma)
Calcium	0.4	0.85
Magnesium	1.3	10.0
Sodium	10.8	48.0
Potassium	0.4	5.9
Sulfur	2.7	7.9

 
From this table, it is apparent that common elements reside in sea water a very long time before being precipitated as a sedimentary rock.

The precipitation process is controlled by chemical reactions such as shown in the following table:
 
 

Chemical Reaction	Mineral Precipitated	Rock Name
Ca2+ + CO32-  = CaCO3	calcite	limestone
Na+ + Cl-  = NaCl	halite	rock salt
Si4+ + O2-  = SiO2	silica or quartz	chert

 

These chemical reaction are controlled by physical parameters such as temperature, concentration of the dissolved species and by partial pressures of gaseous components involved in the reaction sequence.  As sea water is evaporated, for example, the solution becomes increasingly concentrated and reactions such as those shown are driven from left to right, resulting in the precipitation of minerals.

Very important and interesting chemical sediments are primarily composed of iron oxides and carbonates, and are known as iron formations.  The upper peninsula of Michigan has some of the largest sedimentary iron formations, and ore from these formations fueled the industrial revolution in the U.S.   Other large deposits occur in Australia, Brazil, South Africa and the former U.S.S.R.  Iron formations result from the precipitation of minerals such as FeCO₃ from sea water.  The interesting fact is the sea water currently contains very little dissolved iron (only 0.003 ppm).  The large iron formations are all PreCambrian in age, and in fact all appear to have formed around 3 Ga.  In contrast to the present situation, the early atmosphere (prior to roughly 3 Ga) contained insufficient oxygen to oxidize iron.  In its reduced state (Fe²⁺), iron was soluble in the oceans and apparently accumulated in considerable amount.  Once the atmosphere became sufficiently oxidizing, nearly all of the iron that was dissolved in sea water was precipitated to form the large iron formations.  None have formed since that point in the Earth's history.

III.  Biological Sediments
Purely biological sedimentary rocks are relatively rare but economically important.  The most important of these is coal -- the sedimentary rock that forms from the diagenesis of peat and other organic matter.

There is, of course, not a clear boundary between the biological and chemical sediments.  Many of the processes that produce limestone, for example, are actually biochemical in nature.  Many limestones are composed almost entirely from shells of marine critters that make their shells out of calcite.  Coral reefs are another good example of the interplay between chemical and biological processes.