Sedimentologia

Prof. Stephen A. Nelson | EENS 111 |
Tulane University | Physical Geology |
Sedimentary Rocks |
This page last updated on 26-Dec-2006
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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 strongenough 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 livingorganisms 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 SedimentsClassification - Clastic sedimentary particles are classified in terms of size |

Name ofParticle  | 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 |

Theformation of a clastic sedimentary rock involves three processes:   1. 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 that tell us something aboutthe mode of transportation. |

2. 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 too low to transport sediment, the sediment will fall out and become deposited. The final sediment thus reflects the energy of the transporting medium. 3.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 asnew 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 wherethere 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 tohydrocarbons, 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 interrelationship between grains gives...