sedimentary rocks

Sedimentary Rocks

Contents of Entire Course

Introduction to Sedimentary rocks
Clastic Sediments
Textures of Clastic Sedimentary Rocks
Chemical Sediments and Sedimentary Rocks
Biogenic Sediments and Sedimentary Rocks
Features of Sedimentary Rocks That Give Clues to the Environment of Deposition
Stratification and Bedding
Sedimentary Facies
Common Sedimentary Environments

adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane University

 
 
 
Introduction to sedimentary rocks

 Rivers, oceans, winds, and rain runoff all have the ability to carry the particles washed off of eroding rocks. Such material, called detritus, consists of fragments of rocks and minerals. When the energy of the transporting current is not strong enough to carry these particles, the particles drop out in the process of sedimentation. This type of sedimentary deposition is referred to as clastic sedimentation. Another type of sedimentary deposition occurs when material is dissolved in water, and chemically precipitates from the water. This type of sedimentation is referred to as chemical sedimentation. A third process can occur, wherein living organisms extract  ions dissolved in water to make such things as shells and bones.  This type of sedimentation is called biogenic sedimentation.  Thus, there are three major types of sedimentary rocks: Clastic sedimentary rocks, chemical sedimentary rocks, and biogenic sedimentary rocks.


Clastic Sediments

Classification - Clastic sedimentary particles are classified in terms of size

 
Name of Particle  Size Range  Loose Sediment  Consolidated Rock 
Boulder  >256 mm  Gravel  Conglomerate or Breccia (depends on rounding) 
Cobble  64 - 256 mm  Gravel 
Pebble  2 - 64 mm  Gravel 
Sand  1/16 - 2mm  Sand  Sandstone 
Silt  1/256 - 1/16 mm  Silt  Siltstone 
Clay  <1/256 mm  Clay  Claystone, mudstone, and shale
 
The formation of a clastic sedimentary rock involves three processes:

Transportation-  Sediment can be transported by sliding down slopes, being picked up by the wind, or by being carried by running water in streams, rivers, or ocean currents. The distance the sediment is transported and the energy of the transporting medium all leave clues in the final sediment of the mode of transportation.

Deposition -  Sediment is deposited when the energy of the transporting medium becomes too low to continue the transport process. In other words, if the velocity of the transporting medium becomes to low to transport sediment, the sediment will fall out and become deposited. The final sediment thus reflects the energy of the transporting medium.

Diagenesis - Diagenesis is the process that turns sediment into rock. The first stage of the process is compaction. Compaction occurs as the weight of the overlying material increases. Compaction forces the grains closer together, reducing pore space and eliminating some of the contained water. Some of this water may carry mineral components in solution, and these constituents may later precipitate as new minerals in the pore spaces. This causes cementation, which will then start to bind the individual particles together. Further compaction and burial may cause recrystallization of the minerals to make the rock even harder.

Other conditions present during diagenesis, such as the presence of absence of free oxygen may cause other alterations to the original sediment. In an environment where there is excess oxygen (Oxidizing Environment) organic remains will be converted to carbon dioxide and water. Iron will change from Fe2+ to Fe3+, and will change the color of the sediment to a deep red (rust) color. In an environment where there is a depletion of oxygen (Reducing Environment), organic material may be transformed to solid carbon in the form of coal, or may be converted to hydrocarbons, the source of petroleum.
 


Textures of Clastic Sedimentary Rocks

When sediment is transported and deposited, it leaves clues to the mode of transport and deposition. For example, if the mode of transport is by sliding down a slope, the deposits that result are generally chaotic in nature, and show a wide variety of particle sizes. Grain size and the interrelation ship between grains gives the resulting sediment texture. Thus, we can use the texture of the resulting deposits to give us clues to the mode of transport and
 deposition.

sorting.gif

Sorting - The degree of uniformity of grain size. Particles become sorted on the basis of density, because of  the energy of the transporting medium.  High energy currents can carry larger fragments.  As the energy decreases, heavier particles are deposited and lighter fragments continue to be transported.  This results in sorting due to density.
If the particles have the same density, then the heavier particles will also be larger, so the sorting will take place on the basis of size.  We can classify this size sorting on a relative basis -  well sorted to poorly sorted. Sorting gives clues to the energy conditions of the transporting medium from which the sediment was deposited. Examples