Introduction tosedimentary 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 RockBoulder / >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 - 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
o Beach deposits and wind blown deposits generally show good sorting because the energy of the transporting medium is usually constant.
o Stream deposits are usually poorly sorted because the energy (velocity) in a stream varies with position in the stream.
Rounding- During the transportation process, grains may be reduced in size due to abrasion. Random abrasion results in the eventual rounding off of the sharp corners and edges of grains. Thus, rounding of grains gives us clues to the amount of time a sediment has been in the transportation cycle. Rounding is classified on relative terms as well.
Chemical Sediments and Sedimentary Rocks
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Cherts - chemically precipitated SiO2
Evaporites - formed by evaporation of sea water or lake water. Produces halite (salt) and gypsum deposits by chemical precipitation as concentration of solids increases due to water loss by evaporation.
Biogenic Sediments and Sedimentary Rocks
Limestone - calcite (CaCO3) is precipitated by organisms usually to form a shell or other skeletal structure. Accumulation of these skeletal remains results in a limestone.
Diatomite - Siliceous ooze consisting of the remains of radiolarian or diatoms can form a light colored soft rock called diatomite.
Coal - accumulation of dead plant matter in large abundance in a reducing environment (lack of oxygen).
Oil Shale - actually a clastic sedimentary rock that contains a high abundance of organic material that is converted to petroleum during diagenesis.
Features of Sedimentary Rocks That Give Clues to the Environment of Deposition
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Stratification and Bedding
Rhythmic Layering - Alternating parallel layers having different properties. Sometimes caused by seasonal changes in deposition (Varves). i.e. lake deposits wherein coarse sediment is deposited in summer months and fine sediment is deposited in the winter when the surface of the lake is frozen.
Cross Bedding - Sets of beds that are inclined relative to one another. The beds are inclined in the direction that the wind or water was moving at the time of deposition. Boundaries between sets of cross beds usually represent an erosional surface. Very common in beach deposits, sand dunes, and river deposited sediment.
Graded Bedding - As current velocity decreases, first the larger or more dense particles are deposited followed by smaller particles. This results in bedding showing a decrease in grain size from the bottom of the bed to the top of the bed.
Non-sorted Sediment - Sediment showing a mixture of grain sizes results from such things as rockfalls, debris flows, mudflows, and deposition from melting ice.
Ripple Marks - Characteristic of shallow water deposition. Caused by waves or winds
Mudcracks - result from the drying out of wet sediment at the surface of the Earth. The cracks form due to shrinkage of the sediment as it dries.
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Raindrop Marks- pits (or tiny craters) created by falling rain. If present, this suggests that the sediment was exposed to the surface of the Earth.
Fossils - Remains of once living organisms. Probably the most important indicator of the environment of deposition.
o Different species usually inhabit specific environments.
o Because life has evolved - fossils give clues to relative age of the sediment.
o Can also be important indicators of past climates.
Color
· Iron oxides and sulfides along with buried organic matter give rocks a dark color. Indicates deposition in a reducing environment.
· Deposition in oxidizing environment produces red colored iron oxides.
Sedimentary Facies
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A sedimentary facies is a group of characteristics which reflect a sedimentary environment different from those elsewhere in the same deposit. Thus, facies may change vertically through a sequence as a result of changing environments through time. Also, facies may change laterally through a deposit as a result of changing environments with distance at the same time.
Common Sedimentary Environments
· Non-marine environments
o Stream sediments
o Lake sediments
o Glacial (ice deposited) sediments
o Eolian (wind deposited) sediments
· Continental Shelf sediments
o Estuarine sediments
o Deltaic sediments
o Beach sediments
o Carbonate shelf sediments
o Marine evaporite sediments
· Continental slope and rise sediments
o Turbidites
o Deep Sea Fans
o Sediment drifts
· Deep Sea Sediments
o Deep -Sea oozes
· Land-derived sediments
Within the Earth rocks are continually being subjected to forces that tend to bend them, twist them, or fracture them. When rocks bend, twist or fracture we say that they deform (change shape or size). The forces that cause deformation of rock are referred to as stresses (Force/unit area). So, to understand rock deformation we must first explore these forces or stresses.
Stress and Strain
Stress is a force applied over an area. One type of stress that we are all used to is a uniform stress, called pressure. A uniform stress is a stress wherein the forces act equally from all directions. In the Earth the pressure due to the weight of overlying rocks is a uniform stress, and is sometimes referred to as confining stress.
If stress is not equal from all directions then we say that the stress is a differential stress. Three kinds of differential stress occur.
- Tensional stress (or extensional stress), which stretches rock;
- Compressional stress, which squeezes rock; and
- Shear stress, which result in slippage and translation.
When rocks deform they are said to strain. A strain is a change in size, shape, or volume of a material.
Stages of Deformation
When a rock is subjected to increasing stress it passes through 3 successive stages of deformation.
- Elastic Deformation -- wherein the strain is reversible.
- Ductile Deformation -- wherein the strain is irreversible.
Fracture - irreversible strain wherein the material breaks.
We can divide materials into two classes that depend on their relative behavior under stress.
o Brittle materials have a small or large region of elastic behavior but only a small region of ductile behavior before they fracture.
Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they fracture.
How a material behaves will depend on several factors. Among them are:
o Temperature - At high temperature molecules and their bonds can stretch and move, thus materials will behave in more ductile manner. At low Temperature, materials are brittle.
o Confining Pressure - At high confining pressure materials are less likely to fracture because the pressure of the surroundings tends to hinder the formation of fractures. At low confining stress, material will be brittle and tend to fracture sooner.
o Strain rate -- At high strain rates material tends to fracture. At low strain rates more time is available for individual atoms to move and therefore ductile behavior is favored.
Composition -- Some minerals, like quartz, olivine, and feldspars are very brittle. Others, like clay minerals, micas, and calcite are more ductile This is due to the chemical bond types that hold them together. Thus, the mineralogical composition of the rock will be a factor in determining the deformational behavior of the rock. Another aspect is presence or absence of water. Water appears to weaken the chemical bonds and forms films around mineral grains along which slippage can take place. Thus wet rock tends to behave in ductile manner, while dry rocks tend to behave in brittle manner.
Brittle-Ductile Properties of the Lithosphere
We all know that rocks near the surface of the Earth behave in a brittle manner. Crustal rocks are composed of minerals like quartz and feldspar which have high strength, particularly at low pressure and temperature. As we go deeper in the Earth the strength of these rocks initially increases. At a depth of about 15 km we reach a point called the brittle-ductile transition zone. Below this point rock strength decreases because fractures become closed and the temperature is higher, making the rocks behave in a ductile manner. At the base of the crust the rock type changes to peridotite which is rich in olivine. Olivine is stronger than the minerals that make up most crustal rocks, so the upper part of the mantle is again strong. But, just as in the crust, increasing temperature eventually predominates and at a depth of about 40 km the brittle-ductile transition zone in the mantle occurs. Below this point rocks behave in an increasingly ductile manner
Deformation in Progress
Only in a few cases does deformation of rocks occur at a rate that is observable on human time scales. Abrupt deformation along faults, usually associated with earthquakes caused by the fracture of rocks occurs on a time scale of minutes or seconds. Gradual deformation along faults or in areas of uplift or subsidence can be measured over periods of months to years with sensitive measuring instruments.
Evidence of Former Deformation
Evidence of deformation that has occurred in the past is very evident in crustal rocks. For example, sedimentary strata and lava flows generally follow the law of original horizontality. Thus, when we see such strata inclined instead of horizontal, evidence of an episode of deformation is present. In order to uniquely define the orientation of a planar feature we first need to define two terms - strike and dip
For an inclined plane the strike is the compass direction of any horizontal line on the plane. The dip is the angle between a horizontal plane and the inclined plane, measured perpendicular to the direction of strike
In recording strike and dip measurements on a geologic map, a symbol is used that has a long line oriented parallel to the compass direction of the strike. A short tick mark is placed in the center of the line on the side to which the inclined plane dips, and the angle of dip is recorded next to the strike and dip symbol as shown above. For beds with a 900 dip (vertical) the short line crosses the strike line, and for beds with no dip (horizontal) a circle with a cross inside is used as shown below.