Sedimentary rock

In this section we will deal with sedimentary rock and its place in the rock cycle. Sediments are classed as originating from (Plummer and McGeary 1994 p.304)


 * 1. Weathering and eroding of any existing rock, igneous, metamorphic or sedimentary (shale, gabbro, marble etc).
 * 2. Any chemical precipitation from solution, salts or organic secretions.

These types of rock formations are generally porus, in that fluids can flow in between the rock particles and the binding matrics. This is the basis of ground water as opposed to a cavity within a rock substrate.

Rock size
All types of sediment are classed as unconsoldated in that they can move indivdually and are not bound to one another. There are three different classifications of sediment and these are classed according to size in millimeters in diameter, with sub-categories.


 * Gravel: this type of sediment is classified into boulders at over 256 mm, cobbles at between 64-256 mm and pebbles at 2-64 mm, called either rudites or psephites.


 * Sand has no sub-categories but is sized at 1/16 (0.0625) to 2 mm, called either arenites or psammites.


 * Mud is the last category and includes silt at between 1/256-1/16 of a mmm, and clay at below 1/256 of a mm, called either lutite (Latin) or a pelite (Greek)..

Transportation
Transportation in the geological sense is the mass movement of unconsolidated material, using the force of gravity the material can be shifted from from a high elevation to a lower elevation, and sometines in the reverse. This movement is not precise and smaller fragments can be moved a greater distance than larger material. This is because the energy required to move a large piece of say rock is greater and can be overcome quicker than the energy required to move silt.

A fragment of weathered rock would in the right circumstance be transported via a river system, glacier or waves down to a lower elevation. During this transportation the size of the rock would be sorted into the size categorization above. When the transportation body has more energy the size and amount of the material carried would increase.

During the process, the material goes through a process called Rounding. This is the grinding away of sharp edges of a rock to form rounded corners. The process involves a rock material hitting another equally hard or harder rock surface. This occurs in sand or gravel.

Sorting is the process where the sediment is selected and seperated into different size, grain shape and specific gravity. Shallow marine shoreline systems are the most effective at sorting material, as the sediment is continuously reworked. High energy shoreline systems can move larger sediemnt and may therefore produce rock, pebble and sand beaches. Sediment from beach environments are characteristically well sorted, well rounded and are composed of 90% or more quartz. Glaciers are poor at sorting the material because of their high viscosity and manner of flow, as all size rocks are trasnported together till the terminus of glacier is reached and once it has retreated form terminal moraine

Deposition
When a transporting fluid body like wind, a river, a wave or a glacier looses momentum, it looses energy. Once the kinetic energy decreases below that required to sustain a certain weight and/or friction, a stone over the energy weight to sustain its' suspention in the transporting body will drop out of circulation down to surface level. Also deposition can refer to organic matter, as it falls and accumulates onto a surface area. Another type of deposition is the precipitation os salts out of a solution. There is deposition that depends on the shape or specific gravity of the object, that can also be affected by kinetic energy of the transporting body.

Lithification
This is the group of processes that turns loose sediment into sedimentary rock. First there is the deposition of loose sedimentary grains, during this process the material is compacted as more sediment is put on top. The initial pore space for slow moving sediment can be about 40-50 percent of the sediment, in faster moving sediment this is reduced. Compaction reduces this area by decreasing the pore space as it squeezes water or air from the matrix. After compaction cementation usually occurs and makes the sedimentary rock harder and stable. The most prominant cement is calcite (CaCO3) which may be produced from chemical weathering of the sediment itself, or transported from a distant site from water percolating through the rock.

Types of Sedimentary rocks
Clastic rocks are grained rock which may or may not be filled with a matrics. The largest of these include coarse grainedBreccia and Conglomerate types of rocks, usually filled in with 15% or less of matrics of sand and finer particles. As with the nature and size of the material, the source rock was close to the locality of deposition. For Breccia as there is little rounding of the rock fragments, the source material was in the immediate vacinity. Deposition could have been the base of a cliff or another locality where transportation from the immediate area is not likely to occure.

For conglomerates the material has suffered rounding indicating transportation. As the size of the material is quiet large, the area of deposition is still close to the area of erosion, generally no more than a few kilometres.

Sandstone rocks are made of mainly quartz and/or feldspar minerals. Because of the size of the material, there is consderable transportion from the parient material. This material is more dominate as sediment in slow flowing rivers, lakes, streams the that have a lower energy factor. But the sand dominance depends on the energy contained within the transporting water course. Sand is also a major consitution of beach material along with organic rocks.

Carbonate or organic rocks
These are rocks that are made mainly via organic processes, that is by living organisms, the exception would be the Oolitic limestone. They include the various types of limestone and dolomite.

Major types of rock inclue those made by the accumulation of foraminifera, coral, bivalves and brachiopods, crinoids just to name a few. Generally vertebrate remains do not consolidate into rock alone, usually sand, clay etc or other organism makes up the rest of the matrixs and/or cement.

Chemical rocks
A chemical rock is one that has been precipitated from a solution, these include rock salts and some limestones. As a water body has a reduced inflow of water, through friction and evaporation it tends to loose that water. As the concentration of suspended chemicals and other material increases it tends to perculate out of the solution if concentrations of that chamical become oversaturated. During this chemical formation into crystals, some other material eg,. plant remains, shell or silt can become trapped within the crystals that are forming. This accumulation of crystals and other material over a long period, or even if it only occured once and covered with another type sedimentary material can form a layer within a rock formation.

Readings

 * Basic information on different sedimentary rocks from Geology.com

Resource

 * Sedimentary Rock from Professor Stephen A. Nelson of the Tulane University


 * The Petrology of Sedimentary Rocks by R.L. Folk