Characteristics Of

Sedimentary RocksAn understanding of some of the basic characteristics of sedimentary rocks is

required to comprehend West Virginia’s geologic history. This presentation is not intended to be anything other than a very brief introduction to sedimentary rocks.

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Ancestry: The Origin of Earth’s Crustal RocksMaking Sedimentary Rocks:

Weathering, Erosion, Sediment, Deposition

, and LithificationDepositional Environments

and FaciesBedding of Sedimentary

Rocks

Response of Rocks to Stress

Ancestry: The Origin of Earth’s Crustal RocksWe have no record of the 500 million years following Earth’s formation. We speculate that it was a time of intense meteoroid bombardment when the mantle and the crust that surrounds the molten iron core were forming. Basaltic magmas rose to the surface and spread across the surface of the underlying mantle in a relatively thin layer that was eventually to become the oceanic crust. The surface of the crust during this primeval time was being formed and re-assimilated as underlying convection currents churned the outermost layer of Earth. The heat from the molten core, heat generated by the breakdown of radioactive elements, and the intense bombardment by meteoroids kept the temperature at Earth’s surface very high. Partial melting of the basaltic crustal rocks eventually began to generate masses of molten granitic rock. In time, these granitic masses would coalesce to form small continents. These micro-continents eventually converged and sutured together to form larger continents. Over time, the thermal activity within the crust began to slow as Earth cooled, the abundance of radioactive elements dwindled and the frequency of meteoroid impacts diminished.

Estimates indicate that by 2.5 billion years ago the present mass of continental crust had formed. At this point the processes of weathering and erosion assumed their roles in creating sediments that would eventually become sedimentary rock.

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Making Sedimentary Rocks: Weathering, Erosion, Sediment, Deposition, and Lithification

Making Sedimentary Rocks: Weathering, Erosion, Sediment, Deposition, and Lithification

Beginning about 2.5 billion years ago Earth’s surface had cooled sufficiently to allow precipitation. Water vapor, erupted by the world-wide volcanism and stored in the atmosphere, condensed and began to fall as rain. Once the rains started, the water filled the basaltic low area creating the oceans that cover 70% of Earth’s surface. The granitic high area above sea level became land.

The presence of precipitation initiated weathering and erosion. These processes produced sediment which was moved by wind and water and ice to areas of deposition where the sediment was lithified.

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Lithification

Deposition

Erosion

Sediments

Weathering

The destruction of rock into sediment due to the actions of wind, water, or ice. (Making smaller rocks out of bigger rocks!)

Varying sized pieces of rocks produced by the agents of weathering.

Transport of sediment to new locations by wind, water, or ice.

Accumulation of transported sediment in a variety of different environments and geographic settings.

Rock making process whereby deposited individual sediment grains are cemented together.

What do the following terms mean?Click on the terms and see if you’re right.

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Sediment is weathered (broken) pieces of older rock.

Sediment is classified by size ranging from clay to boulder.

Clay-sized sediment produces the rock shale. 70% of West Virginia sedimentary rocks are shale!

Sand-sized sediment becomes sandstone.

Limestone forms in its own unique way.

Particle Size

SedimentSediment

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Shale is a fissile rock composed of composed of oriented clay-sized particles. Fissile refers to the fact that a shale can be separated into layers.

Step 1:

Continuing deposition buries sediment. Weight of overlaying material begins to compress clay particles together. Water is squeezed out as particles become less randomly oriented.

Continuation of compression removes more water and produces highly aligned clay particles.

Majority of water removed and sediment particles are aligned producing the fissile rock we call shale.

Randomly arranged clay-size sediment is deposited as mud. Much water is located between individual particles.

Step 2:

Step 3:

Step 4:

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Click on Steps belowto view step descriptions:

Sand is a size term referring to sediment particles ranging from 1/16 to 2 millimeters in diameter. When the sized particles cement together into the rock called sandstone a entire suite of fine-grained, medium-grained, or coarse-grained sandstones may result.

As in shales, once the sand sediment is deposited, the removal of water initiates the rock forming sequence.

Question: Without something to hold the individual sand grains together, the rock would not exist. It would just be a pile of sand. What process is required to form the rock called sandstone that we have not yet talked about?

Click here to find out.Click here to find out.

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Answer = CEMENT

Sand-sized particles are cemented together when iron (Fe2O3), silica (SiO2), or calcium carbonate (CaCO3) precipitate from ground water moving through or being forced out of the sediment particles as they are being compacted.

Water between sand-sized

sediment grains contains

cementing agents that are dissolved

in water.

Some of the iron, silica, or calcium carbonate precipitates and is left behind (brown areas). This material becomes the cement holding individual grains together to form the rock called sandstone.

The open spaces between the sand grains are called pores. Pores provide places for the accumulation of water, oil, and natural gas to collect.

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Limestone is predominately calcium carbonate (CaCO3).

Calcium carbonate is dissolved in water and can be removed and turned into a carbonate-rich mud either chemically or biologically. In either case, the CaCO3 is both the accumulating sediment and the cementing agent holding the sediment together to form the rock called limestone.

Aqueous environments where carbonate-rich sediment accumulates over a large area is sometimes referred to as a “carbonate shelf.”

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Depositional Environments and Facies

The two basic depositional environments are terrestrial and marine.

Terrestrial environments are located on land with the most important being those associated with streams; such deposits are referred to as fluvial deposits.

Marine deposits are those that accumulate in ocean/sea environments. Unique nearshore brackish areas such as estuaries, lagoons, and tidal basins are referred to as non-marine.

Deposition occurs simultaneously at different locations and, therefore, the elements of time and location must be considered. Geologists call this concept “facies.”

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Fluvial deposits include sediments accumulated in

stream channels, as levees, or on floodplains.

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A typical facies scenario would occur where three types of sediment are being simultaneously deposited in bands parallel to a shoreline. In this scenario sand-sized particles accumulate in the near-shore environment and beach area while clay-sized materials accumulate in slightly deeper water because they are separated from the sand by ocean currents and then carried farther offshore. At the same time, if the water is relatively warm, limestone (also called carbonate) would accumulate in still deeper ocean depths even further offshore.

Upon lithification, a single layer of sedimentary rock would be created that would, in various locations, be composed of three different rock types: sandstone, shale, and limestone. These rocks would be seen to change into each other laterally. Each rock type would represent a particular depositional environment and would be referred to as a facies of the overall unit. The lateral change from one facies to another is called a facies change. Note that within an individual layer, the order of facies change points either landward (limestone to shale to sandstone) or seaward (sandstone to shale to limestone).

Facies and FaciesChanges

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The basic characteristic of all sedimentary rocks is that they are bedded, that is, they are made up of layers called beds. Bedding is the result of the original sediments having been deposited in essentially horizontal layers, be it on an ocean floor, a lake bottom, or a floodplain, a relationship summarized during the early days of geology in the law of original horizontality.

The significance of the law of original horizontality is this…

Should you see sedimentary beds in anything other than a horizontal position it means that the rocks were subjected to some degree of deformation after they formed.

Bedding of Sedimentary Rocks

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The way any material responds to an applied force depends on the kind of force, the ability of the material to withstand external forces and the physical nature of the material. The general term for all external forces is stress. The ability of a material to resist external stress is called strength. The way a material responds to stress is referred to as strain or, more commonly, deformation with deformation being defined as any change in either shape or size.

STRESS STRENGTHSTRAIN

STRENGTH

STRENGTH

STRAINSTRAINSTRESSSTRESS

Response of Rocks to Stress

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In compression, the forces are directed toward each other. Compressional force can be oriented either directly opposite each other (non-rotational compression) or along parallel paths (rotational compression). This is a “PUSH TOGETHER” or “SLIDING PAST” motion.

Stress is defined as a directionally applied force of which there

are basically only two kinds:

• Compression• Tension

Tensional forces are always oriented directly opposite each other. This is a “PULL APART” motion.

Compression Compression

Compression

Compression

Tension

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Once strength is exceeded, materials deform in one of three ways: 1) elastic, 2) plastic, or 3) brittle. The type of strain can be determined using the decision tree.

TIP: It is important to know that a material may begin to deform plastically and then break. The reason being that only so much energy can be

absorbed and internally consumed (plastic deformation). The remaining energy must be

released by breaking (brittle deformation).

NO

YES

Plastic Or Brittle

Deformation

ElasticDeformation

YES

NO

BrittleDeformation

PlasticDeformation

Did the material break?

START HEREDid the material return to its original

shape when the stress was removed?

Strain

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Click on terms below for more:

Strength

Every material has an inherent strength which is simply the ability of a material to absorb stress without deformation. Strength can be looked upon as an invisible wall that stands between stress (applied force) and

strain (deformation). Before any material deforms under stress, the strength of the material must be exceeded. Once strength is exceeded, a material will deform.

Rocks are very weak under tension but very strong under compression. The best example involves a bit of ancient history, in particular, the architecture of ancient Greece and of Rome demonstrates basic

engineering methods for dealing with the inherent strength of rocks.

Inherent Strength of Rocks

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Greek engineering construction techniques

The architecture of ancient Greece is characterized by the use of many columns as is beautifully exemplified by the Parthenon. The reason why the Greeks used so many columns is because they were never able to overcome the inherent weakness of rocks under tension. The ancient Greeks spanned the distance between columns with rock slabs called lintels. Whenever the distance between columns became too great, the lintels would fail by collapse. The reason for the failure was that with the lintels supported from their ends and loaded from above, the forces generated within the lintel were tensional. When the span between adjacent columns became too great, the rock failed under the tensional forces. This, of course, necessitated the use of many columns.

Compressive and tensile forces in columns of the

Greek Parthenon.

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The Romans spanned the space between columns with an arch. They discovered that by placing a keystone at the apex of an arch, all of the forces within the structure experienced non-rotational compression. Under this situation rocks are very strong. The result of their discovery resulted in all of the vaulted ceilings and domes of all the cathedrals as well as all of the arched structures such as the bridge over the New River gorge.

Roman Engineering Construction Techniques

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Hitting a baseball with a bat is a great example of elastic deformation. At the moment of impact the bat bends and the ball flattens) on one side. Both bat and ball are deformed. However, both return to their original shape. This is the basic definition of elastic deformation. Kicking a football or soccer ball also demonstrates elastic deformation because both your foot and the ball return to their original shape.

Elastic Deformation

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Brittle failure of rocks under compression and tension creates faults. Faults are breaks in Earth’s crust along which there is movement. In both compression and tension, the break occurs at an angle to the direction of stress; the difference being in the relative movement of the rock mass on opposite sides of the fault.

Faulting: The Brittle Deformation of RocksFaulting: The Brittle Deformation of Rocks

Historically, the mass of rock above the fault is called the hanging wall while the mass below the fault is called the foot wall. These are terms originated by miners following beds of ore that encountered faults in the mine. Faults formed under tensional forces are called normal faults and are described as a fault where the hanging wall has moved down relative to the foot wall. Faults formed under non-rotational compression are called thrust or reverse faults, where, in both cases, the hanging wall has moved up relative to the foot wall.

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Plastic deformation of rocks does not break the rocks but folds them. Folds do not form at or near Earth’s surface where rocks are most brittle but rather at depth where increased temperatures and pressures result in a more plastic response. There are three types of folds: 1) monoclines, 2) anticlines, and 3) synclines.

A monocline is a regional steepening of an otherwise uniform dip

Anticlines and synclines usually occur together and are the result of compressive forces. Anticlines are upwarps

in Earth’s crust while synclines are downwarps.

The limb (side) of an anticline is also the limb of the adjacent syncline!

Folding: The Plastic Deformation of Rocks

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