Sedimentary Rocks (Origin & Formation)

Sedimentary Rocks Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water .

How sedimentary rocks are formed For thousands, even millions of years, little pieces of our earth have been eroded--broken down and worn away by wind and water. These little bits of our earth are washed downstream where they settle to the bottom of the rivers, lakes, and oceans. Layer after layer of eroded earth is deposited on top of each. These layers are pressed down more and more through time, until the bottom layers slowly turn into rock.

Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, mass movement or glaciers which are called agents of denudation.

Sedimentary rocks are deposited in layers as strata, forming a structure called bedding. The study of sedimentary rocks and rock strata provides information about the subsurface that is useful for civil engineering, for example in the construction of roads, houses, tunnels, canals or other constructions. Sedimentary rocks are also important sources of natural resources like coal, fossil fuels, drinking water or ores.

The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is both part of geology and physical geography and overlaps partly with other disciplines in the Earth sciences, such as pedology, geomorphology, geochemistry or structural geology.

Genetic Classification Schemes Based on the processes responsible for their formation, sedimentary rocks can be subdivided into four groups: Clastic sedimentary rocks Biochemical (or biogenic) sedimentary rocks Chemical sedimentary rocks ,and "other" sedimentary rocks formed by impacts, volcanism, and other minor processes.

Clastic sedimentary rocks Clastic sedimentary rocks are composed of silicate minerals and rock fragments that were transported by moving fluids and were deposited when these fluids came to rest. Clastic rocks are composed largely of quartz, feldspar, rock (lithic) fragments, clay minerals, and mica; numerous other minerals may be present as accessories.

Bio-Chemical Sedimentary Rocks Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Examples include: Most types of limestone are formed from the calcareous skeletons of organisms such as corals, mollusks, and foraminifera. Coal which forms as plants remove carbon from the atmosphere and combine with other elements to build their tissue.

Mollusks foraminifera Pillar coral ( a marine animal)

Chemical Sedimentary Rocks Chemical sedimentary rock forms when mineral constituents in solution become supersaturated and inorganically precipitate. Common chemical sedimentary rocks include oolitic limestone and rocks composed of evaporite minerals such as halite (rock salt), sylvite, barite and gypsum.

What is Structural Geology, and Why Study It?   Structural geology- Study of how the lithosphere is bent, broken, and deformed during plate tectonics. Structural geology is important for understanding: 1) The locations of earthquakes. 2) The formation of mountains. 3) The tectonic history of the earth. 4) How to safely building structures such as bridges and dams. 5) How to locate natural resources like oil and gold.

Bedding Planes Sedimentary rocks, occupy the greater part of the Earth's land surface; they occur essentially as layers or strata and are parts of the stratigraphical sequence of rocks. A single stratum may be of any thickness from a few millimetres to a metre or more, and the surfaces which separate it from the next stratum above or below are called bedding planes.

Stratification of sedimentary rock on the Rainbow Basin syncline near Barstow, Calif., U.S.

Consider a flat uniform stratum which is tilted out of the horizontal (Fig. 8. I). On its sloping surface there is one direction in which a horizontal line can be drawn, called the strike. It is a direction that can be measured on beds that are exposed to view and recorded as a compass bearing. Def:STRIKE Strike is the compass direction of a line marking the intersection of an inclined plane with a horizontal plane such as the Earth’s surface.

The directions for finding strike are: Place the side edge of the compass against the bedding plane. Use the bull's eye leveling bubble to make the compass horizontal (while maintaining step 1 above). The strike of the bedding plane is indicated by the needle direction. 

Dip Dip is the maximum angle between the inclined plane and the horizontal plane.  Dip is always perpendicular to strike, and has both a compass direction and an angle. The directions for finding dip are: 1) Determine the dip direction.  This is always perpendicular to the strike direction! 2) Use the inclinometer to measure the amount of dip in degrees (a plane lying flat along the horizontal as zero dip).

Reporting Strike and Dip on a Map Geologist frequently need to record strike and dip on a map (bird's eye view).  The procedure is quite simple: Draw the strike line with the correct orientation (0-360 degrees). Draw the dip line perpendicular to the strike line, and in the correct direction of the dip.  Record the amount of dip (0-90 degrees) at the tip of the dip line.

Dip and Strike can be recorded as

Folding and Faulting

Fold is warps in rock strata due to ductile deformation Fold is warps in rock strata due to ductile deformation. Generally indicate horizontal compression. 1. FOLDS

Folds Anticline Syncline Tight Fold Overfold Recumbent Fold Nappe Fold There are six types of folds that may occur: Anticline Syncline Tight Fold Overfold Recumbent Fold Nappe Fold

Anticline An anticline occurs when a tectonic plate is compressed by movement of other plates. This causes the center of the compressed plate to bend in an upwards motion. Fold mountains are formed when the crust is pushed up as tectonic plates collide. To the top right is a picture of an anticline. Beneath is a picture of the Rocky Mountains.

Syncline A syncline is similar to an anticline, in that it is formed by the compression of a tectonic plate. However, a syncline occurs when the plate bends in a downward motion. The lowest part of the syncline is known as the trough. To the top right is a diagram of a syncline fold (The bottom of the fold center is the trough). Beneath, is an example of a syncline in California.

Tight Fold A tight fold is a sharp peaked anticline or syncline. It is just a regular anticline or syncline, but was compressed with a greater force causing the angle to be much smaller. Folds such as these occur to form steep mountain slopes like those in Whistler, British Columbia. To the left is a photo of a tight fold formed by extreme pressure on these rocks.

Overfold An overfold takes place when folding rock becomes bent or warped. Sometimes the folds can become so disfigured that they may even overlap each other. An example of overfolding is shown in the diagram below.

Recumbent Fold This type of fold is compressed so much that it is no longer vertical. There is a large extent of overlapping and it can take the form of an “s”. To the right is a diagram that shows the process of recumbent folding.

Nappe Folding This fold is similar to a recumbent fold because of the extent of folding and overlapping. However, nappe folding becomes so overturned that rock layers become fractured. To the right is a picture of someone standing under a fractured fold.

Faults In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the Earth's crust result from the action of tectonic forces. Energy release associated with rapid movement on active faults is the cause of most earthquakes. A fault is when tension and compression associated with plate movement is so great that blocks of rock fracture or break apart. This process can occur very rapidly, in the form of earthquakes. The damage caused by this event can be very destructive and cause severe changes to the earths surface. There are five types of faults that can occur: Normal Fault Reverse Fault Tear Fault Rift Valley Horst Fault

Normal Fault This occurs when rocks move away from each other due to the land moving apart. When the rocks move apart, the side with the less stable tectonic plate drops below the side with the more stable plate. On the top right is the movement of a normal fault. A picture is also shown below. Notice the displacement of the different types of rock on each side of the fault.

Reverse Fault Reverse faults are the opposite of normal faults. Rocks are compressed such that one plate moves up while the other descends below it. When plates compress and crack, usually the more dense one is forced under the less dense one. This is similar to the action of the continental crust colliding with the oceanic crust. Here the more dense crust, being the oceanic crust is forced under the continental crust. To the right is an animation of a reverse fault. Below that is a real picture of what a reverse fault looks like.

Tear Fault A tear fault, also known as a transform fault, occurs when two tectonic plates slide in a lateral motion past each other. This type of fault causes the most severe earthquakes because they grind against each other. These earthquakes can either be shallow or deep and cause tremors over a short or long period of time. Tear faults can occur frequently, especially along the coast of California.

Rift Valley A rift valley is when two normal faults occur parallel to each other and the land sinks between the faults. There are two major examples of this. One being the Great Rift Valley in North Africa and the other, the San Andreas Fault in California. The top right picture is San Andreas Fault and on the bottom right is a diagram of what a rift valley looks like.

Horst Fault A Horst is the opposite of a rift valley. The land between the parallel faults is forced upward because the two faults are being pushed together. This process can take a long time to occur because the average plate movement is one inch per year. There are examples of horst faults on the left.

Summary Folding and faulting has a major influence on the way the earth looks. Mountains form and disappear over time, as well as large rift valleys and other features. This has an impact on where and how we live.

The End