Sedimentary Structures Mineral Composition Mudrocks % Sandstones % Clay minerals 60 5 Quartz 30 65 Feldspar 4 10 - 15 Carbonate minerals 3 <1 Organic matter, hematite, & others <3 <1 The longer a mineral is in the weathering and transportation cycles of sedimentary rock forming processes, the more likely it is to break down to a more stable mineral or disappear altogether. Thus, we can classify sediments on the basis to which they have achieved mineralogical maturity. Mineralogically mature sediments and sedimentary rocks consist entirely of minerals that are stable near the surface. Such sediment is considered to have been in the weathering and transportation cycle for a long amount of time. Mineralogically immature sediments and sedimentary rocks consist of a high proportion of unstable minerals. Because such minerals will not survive for a long time in the weathering and transportation cycles, sediments and rocks with high proportions of these minerals must not have been in the weathering/transportation cycle for long periods of time. Grain Size Clastic sediments and sedimentary rocks are classified on the basis of the predominant grain size of clasts in the rock.

The table below shows the common classification used based on grain size. Name of Particle Claystone, Mudstone, Shale Grain sorting Sorting refers to the uniformity of grain size in a sediment or sedimentary rock. Particles become sorted on the basis of density because of the energy of the transporting medium. High energy (high velocity) currents can carry larger fragments. As the energy or velocity 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, such as all grains of quartz, 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. Beach sands and dune sands tend to be well-sorted because the energy of the waves or wind is usually rather constant. The coarser grained sediment is not carried in because the wave or wind velocity is too low to carry such large fragments, and the finer grained sediment is kept in suspension by the waves or wind. Mountain streams, because they have many turbulent eddies where the velocity of the stream changes suddenly usually show poorly-sorted sediment on the bottom of the stream channel.

Similarly, glacial till, because it is deposited in place as glacial ice melts, and is not transported by water, tends to show poor sorting. 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, the degree of 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. Note that rounding is not the same a sphericity. Sphericity is controlled by the original shape of the grain. The longer the sediment is transported, the more time is available for grains to lose their rough edges and corners by abrasion. Structures of Sedimentary Rocks The process of deposition usually imparts variations in layering, bedforms, or other structures that give clues to environment in which deposition occurs. Such things as water depth, current velocity, and current direction can be sometimes be determined from sedimentary structures. Thus, it is important to recognize various sedimentary structures so that we can interpret the clues that they offer to these conditions.

Also, because sedimentary rocks can be deformed by folding and faulting long after the deposition process has ended, it is important to be able to determine which was up in the rock when it was originally deposited, especially if one is going to use the rocks to determine the sequence of events that occurred or interpret the geologic history of an area. Features that tell us which way was up are often referred to as top and bottom indicators. Stratification and Bedding 1) Layering (bedding) One of the most obvious features of sedimentary rocks and sediment is the layered structure which they exhibit. The layers are evident because of differences in mineralogy, clast size, degree of sorting, or color of the different layers. In rocks, these differences may be made more prominent by the differences in resistance to weathering or color changes brought out by weathering. Layering is usually described on the basis of layer thickness as per the table below. Other distinctive types of layering are described below. Layer Thickness Thinly laminated 2) Cross Bedding Consists of 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. Cross bedding is very common in beach deposits, sand dunes, and river deposited sediment. Individual beds within cross-bedded strata are useful indicators of current direction and tops and bottoms. Note how the beds become asymptotic to the lower boundary on which they were deposited. 3) Graded Bedding As current velocity decreases, the larger or more dense particles are deposited first, 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. This gives us a method for determining tops and bottoms of beds, since reverse grading will not be expected unless deposition occurs under unusual circumstances. Note that reverse graded bedding cannot occur as current velocity increases, because each layer will simply be removed as the current achieves a velocity high enough to carry sediment of a particular size. Surface Features These Sedimentary structures tell us about water currents, wind direction, and climate conditions. 1) Ripple Marks Ripple marks are characteristic of shallow water deposition. They are caused by waves or winds piling up the sediment into long ridges. Asymmetrical ripple marks can give an indication of current direction when formed in water, and when formed by wind, give wind direction.

Symmetrical ripples form when the water moves back and forth. Symmetrical ripple marks occur in environments where there is a steady back and forth movement of the water, such as tidal action. 2)Mudcracks These structures 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. In cross section the mudcracks tend to curl up, thus becoming a good top/bottom indicator. The presence of mudcracks indicates that the sediment was exposed at the surface shortly after deposition, since drying of the sediment would not occur beneath a body of water. Casts and Molds Any depression formed on the bottom of a body of water may become a mold for any sediment that later gets deposited into the depression. The body of sediment that takes on the shape of the mold is referred to as a cast. Load Casts - These are bulbous protrusions that are formed when compaction causes sediment to be pushed downward into softer sediment. Flute Casts (Sole Marks) - Flutes are elongated depressions that form on the bottom of a body water as the current erodes. The flutes may form a mold into which new sediment is deposited.

Preservation of the overlying sediment as a cast result in flute casts, which are sometimes referred to as sole marks. Flute casts are excellent indicators of current direction and tops/bottoms of beds. Tracks and Trails These features result from organisms moving across the sediment as they walk, crawl, or drag their body parts through the sediment. Burrow Marks Any organism that burrows into soft sediment can disturb the sediment and destroy many of the structures discussed above. If burrowing is not extensive, the holes made by such organisms can later become filled with water that deposits new sediment in the holes. Such burrow marks can be excellent top and bottom indicators. Sedimentary Rocks | Pictures, Characteristics, Textures, Types Sedimentary Rocks Pictures of Sedimentary Rocks Photos of Common Clastic, Chemical, and Organic Sedimentary Rock Types. Breccia is a clastic sedimentary rock that is composed of large (over two-millimeter diameter) angular fragments. The spaces between the large fragments can be filled with a matrix of smaller particles or a mineral cement which binds the rock together. The specimen shown above is about two inches (five centimeters) across. What Are Sedimentary Rocks? Sedimentary rocks are formed by the accumulation of sediments.

There are three basic types of sedimentary rocks. Clastic sedimentary rocks such as breccia , conglomerate , sandstone , siltstone , and shale are formed from mechanical weathering debris. Chemical sedimentary rocks, such as rock salt , iron ore , chert , flint , some dolomites , and some limestones , form when dissolved materials precipitate from solution. Organic sedimentary rocks such as coal , some dolomites , and some limestones , form from the accumulation of plant or animal debris. Photos and brief descriptions of some common sedimentary rock types are shown on this page. Coal is an organic sedimentary rock that forms mainly from plant debris. The plant debris usually accumulates in a swamp environment. Coal is combustible and is often mined for use as a fuel. The specimen shown above is about two inches (five centimeters) across. Chert is a microcrystalline or cryptocrystalline sedimentary rock material composed of silicon dioxide (SiO2). It occurs as nodules and concretionary masses, and less frequently as a layered deposit. It breaks with a conchoidal fracture, often producing very sharp edges.

Early people took advantage of how chert breaks and used it to fashion cutting tools and weapons. The specimen shown above is about two inches (five centimeters) across. Conglomerate is a clastic sedimentary rock that contains large (greater than two millimeters in diameter) rounded particles. The space between the pebbles is generally filled with smaller particles and/or a chemical cement that binds the rock together. The specimen shown above is about two inches (five centimeters) across. Flint is a hard, tough, chemical or biochemical sedimentary rock that breaks with a conchoidal fracture. It is a form of microcrystalline quartz that is typically called “chert” by geologists. It often forms as nodules in sedimentary rocks such as chalk and marine limestones . Dolomite (also known as "dolostone" and "dolomite rock") is a chemical sedimentary rock that is very similar to limestone . It is thought to form when limestone or lime mud is modified by magnesium-rich ground water. The specimen shown above is about four inches (ten centimeters) across. Limestone is a rock that is composed primarily of calcium carbonate.

It can form organically from the accumulation of shell, coral, algal, and fecal debris. It can also form chemically from the precipitation of calcium carbonate from lake or ocean water. Limestone is used in many ways. Some of the most common are: production of cement, crushed stone , and acid neutralization. The specimen shown above is about two inches (five centimeters) across. Iron Ore is a chemical sedimentary rock that forms when iron and oxygen (and sometimes other substances) combine in solution and deposit as a sediment. Hematite (shown above) is the most common sedimentary iron ore mineral. The specimen shown above is about two inches (five centimeters) across. Rock Salt is a chemical sedimentary rock that forms from the evaporation of ocean or saline lake waters. It is also known by the mineral name " halite ." It is rarely found at Earth's surface, except in areas of very arid climate. It is often mined for use in the chemical industry or for use as a winter highway treatment. Some halite is processed for use as a seasoning for food. The specimen shown above is about two inches (five centimeters) across.

Oil Shale is a rock that contains significant amounts of organic material in the form of kerogen. Up to 1/3 of the rock can be solid organic material. Liquid and gaseous hydrocarbons can be extracted from the oil shale, but the rock must be heated and/or treated with solvents. This is usually much less efficient than drilling rocks that will yield oil or gas directly into a well. The processes used for hydrocarbon extraction also produce emissions and waste products that cause significant environmental concerns. Shale is a clastic sedimentary rock that is made up of clay-size (less than 1/256 millimeter in diameter) weathering debris. It typically breaks into thin flat pieces. The specimen shown above is about two inches (five centimeters) across. Sandstone is a clastic sedimentary rock made up mainly of sand-size (1/16 to 2 millimeter diameter) weathering debris. Environments where large amounts of sand can accumulate include beaches, deserts, flood plains, and deltas. The specimen shown above is about two inches (five centimeters) across. The best way to learn about rocks is to have a collection of specimens to examine while you study.

Seeing and handling the rocks will help you understand their composition and texture much better than reading about them on a website or in a book. The Geology.com store offers inexpensive rock collections that can be mailed anywhere in the United States or U.S. Territories. Mineral collections and instructive books are also available. Siltstone is a clastic sedimentary rock that forms from silt-size (between 1/256 and 1/16 millimeter diameter) weathering debris. Specimens in the photo are about two inches (five centimeters) across. More Rocks Sedimentary Rocks Sedimentary 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 biochemical sedimentation. The accumulation of plant matter, such as at the bottom of a swamp, is referred to as organic sedimentation. Thus, there are 4 major types of sedimentary rocks: Clastic Sedimentary Rocks, Chemical Sedimentary Rocks, Biochemical Sedimentary Rocks, and Organic Sedimentary Rocks. Clastic Sediments and Sedimentary Rocks The formation of a clastic sediment and sedimentary rocks involves five processes: Weathering - The first step is transforming solid rock into smaller fragments or dissolved ions by physical and chemical weathering as discussed in the last lecture. Erosion - Erosion is actually many process which act together to lower the surface of the earth. In terms of producing sediment, erosion begins the transpiration process by moving the weathered products from their original location. This can take place by gravity (mass wasting events like landslides or rock falls), by running water. by wind, or by moving ice. Erosion overlaps with transpiration.

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 about 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 too low to transport sediment, the sediment will fall out and become deposited. The final sediment thus reflects the energy of the transporting medium. Lithification (Diagenesis) - Lithification 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.

Classification - Clastic sedimentary particles and sedimentary rocks are classified in terms of grain size and shape, among other factors. Name of Particle Conglomerate or Breccia (depends on rounding) Cobble In general, the coarser sediment gets left behind by the transportation process. Thus, coarse sediment is usually found closer to its source and fine grained sediment is found farther from the source. 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 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 Beach deposits and wind blown deposits generally show good sorting because the energy of the transporting medium is usually constant. Stream deposits are usually poorly sorted because the energy (velocity) in a stream varies with position in the stream and time. 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. Sediment Maturity Sediment Maturity refers to the length of time that the sediment has been in the sedimentary cycle. Texturally mature sediment is sediment that is well rounded, (as rounding increases with transport distance and time) and well sorted (as sorting gets better as larger clasts are left behind and smaller clasts are carried away.

Because the weathering processes continues during sediment transport, mineral grains that are unstable near the surface become less common as the distance of transport or time in the cycle increases. Thus compositionally mature sediment is composed of only the most stable minerals. For example a poorly sediment containing glassy angular volcanic fragments, olivine crystals and plagioclase is texturally immature because the fragments are angular, indicating they have not been transported very far and the sediment is poorly sorted, indicating that little time has been involved in separating larger fragments from smaller fragments. It is compositionally immature because it contains unstable glass along with minerals that are not very stable near the surface - olivine and plagioclase. On the other hand a well sorted beach sand consisting mainly of well rounded quartz grains is texturally mature because the grains are rounded, indicating a long time in the transportation cycle, and the sediment is well sorted, also indicative of the long time required to separate the coarser grained material and finer grained material from the sand. The beach sand is compositionally mature because it is made up only of quartz which is very stable at the earth's surface. Types of Clastic Sedimentary Rocks We next look at various clastic sedimentary rocks that result from lithification of sediment.

Conglomerates and Breccias Conglomerate and Breccia are rocks that contain an abundance of coarse grained clasts (pebbles, cobbles, or boulders). In a conglomerate, the coarse grained clasts are well rounded, indicating that they spent considerable time in the transportation process and were ultimately deposited in a high energy environment capable of carrying the large clasts. In a breccia, the coarse grained clasts are very angular, indicating the the clasts spent little time in the transportation cycle. Sandstones A Sandstone is made of sand-sized particles and forms in many different depositional settings. Texture and composition permit historic interpretation of the transport and depositional cycle and sometimes allows determination of the source. Quartz is, by far, the dominant mineral in sandstones. Still there are other varieties. A Quartz arenite – is nearly 100% quartz grains. An Arkose contains abundant feldspar. In a lithic sandstone, the grains are mostly small rock fragments. A Wacke is a sandstone that contains more than 15% mud (silt and clay sized grains).. Sandstones are one of the most common types of sedimentary rocks.

Mudrocks Mudrocks are made of fine grained clasts (silt and clay sized) . A siltstone is one variety that consists of silt-sized fragments. A shale is composed of clay sized particles and is a rock that tends to break into thin flat fragments (See figure 7.6e in your text). A mudstone is similar to a shale, but does not break into thin flat fragments. Organic-rich shales are the source of petroleum. Fine grained clastics are deposited in non-agitated water, calm water, where there is little energy to continue to transport the small grains. Thus mudrocks form in deep water ocean basins and lakes. Biochemical and Organic Sediments and Sedimentary Rocks Biochemical and Organic sediments and sedimentary rocks are those derived from living organisms. When the organism dies, the remains can accumulate to become sediment or sedimentary rock. Among the types of rock produced by this process are: Biochemical 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.

Sometimes the fossilized remains of the organism are preserved in the rock, other times recrystallization during lithification has destroyed the remains. Limestones are very common sedimentary rocks. Biochemical Chert - Tiny silica secreting planktonic organism like Radiolaria and Diatoms can accumulate on the sea floor and recrystallize during lithification to form biochemical chert. The recrystallization results in a hard rock that is usually seen as thin beds (see figure 7.8a in your test). Diatomite - When diatoms accumulate and do not undergo recrystallization, they form a white rock called diatomite as seen in the White Cliffs of Dover (see figure 7.22b in your text). Coal - Coal is an organic rock made from organic carbon that is the remains of fossil plant matter. It accumulates in lush tropical wetland settings and requires deposition in absence of Oxygen. It is high in carbon and can easily be burned to obtain energy. Chemical Sediments and Sedimentary Rocks Dissolved ions released into water by the weathering process are carried in streams or groundwater. Eventually these dissolved ions end in up in the ocean, explaining why sea water is salty.

When water evaporates or the concentration of the ions get too high as a result of some other process, the ions recombine by chemical precipitation to form minerals that can accumulate to become chemical sediments and chemical sedimentary rocks. Among these are: 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. This can occur in lakes that have no outlets (like the Great Salt Lake) or restricted ocean basins, like has happened in the Mediterranean Sea or the Gulf of Mexico in the past. Travertine - Groundwater containing dissolve Calcium and bicarbonate ions can precipitate calcite to form a chemically precipitated limestone, called travertine. This can occur in lakes, hot springs, and caves. Dolostones - Limestone that have been chemically modified by Mg-rich fluids flowing through the rock are converted to dolostones. CaCO3 is recrystallized to a new mineral dolomite CaMg(CO3)2. Chemical Cherts - Groundwater flowing through rock can precipitate SiO2 to replace minerals that were present.

This produces a non-biogenic chert. There are many varsities of such chert that are given different names depending on their attributes, For example: Flint – Black or gray from organic matter. Jasper – Red or yellow from Fe oxides. Petrified wood – Wood grain preserved by silica. Agate – Concentrically layered rings Ripple Marks - Water flowing over loose sediment creates bedforms by moving sediment with the flow. Bedforms are linked to flow velocity and sediment size. Ripples are characteristic of shallow water deposition and can also be caused by wind. blowing over the surface. Sand dunes are similar, but on a larger scale. Ripples are commonly preserved in sedimentary rocks. Asymmetric ripples (as shown above) indicate flow direction,with the steep slope on the down - current direction. Ripples persevered in ancient rocks can also be indicators of up/down direction in the original sediment. Symmetric ripples form as a result of constant wave energy oscillating back and forth. 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.

When present in rock, they indicate that the surface was exposed at the earth's surface and then rapidly buried. Sole Marks - Flutes are troughs eroded in soft sediment that can become filled with mud. Both the flutes and the resulting casts (called flute casts) can be preserved in rock. 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 just prior to burial. Fossils - Remains of once living organisms. Probably the most important indicator of the environment of deposition. Different species usually inhabit specific environments. Because life has evolved - fossils give clues to relative age of the sediment. Can also be important indicators of past climates. Rock Color 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 and is often indicative of deposition in a non-marine environment. Such red colored rocks are often referred to as red beds. Sedimentary Environments If we look at various environments now present on Earth, we can find characteristics in the sediment that are unique to each environment.

If we find those same characteristics in sedimentary rocks, it allows us to interpret the environment of the past. Each environment has its own energy regime and sediment delivery, transport and depositional conditions that are reflected in the sediment deposited. Sedimentary Environments can be divided into the following Terrestrial (Non-marine) environments Deep Marine We will cover most of these environments in more detail later in the course. For now familiarize yourself with each of these by reading pages 220 to 227 in your text. Transgressions and Regressions Throughout geologic history sea level has risen and fallen by as much as a few hundred meters many times. These changes are the result of changes earth's climate or changes in the shape of the sea floor as a result of tectonics. When sea level rises, the coast migrates inland. This is called a Transgression. Beach sand gets buried by marine sediments and the sea floor subsides due to the weight of the sediment. During a transgression, the beach sand forms an extensive layer, but does not all have the same age. When sea level falls, the coast migrates seaward. This is called a Regression.

The sedimentary sequence then repeats itself in a vertical sense as the sedimentary environment migrates back and forth. See figure 7.24 in your text. Diagenesis LIthification of sediment into sedimentary rocks takes place after the sediment has been deposited and buried. The processes by which the sediment becomes lithified into a hard sedimentary rock is called diagenesis and includes all physical, chemical and biological processes that act on the sediment. The first step in diagenesis is the compaction of the sediment and loss of water as a result of the weight of the overlying sediment. Compaction and burial may cause recrystallization of the minerals to make the rock even harder. Fluids flowing through the rock and organisms may precipitate new minerals in the pore spaces between grains to form a cement that holds the sediment together. Common cements include quartz, calcite, and hematite. 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. Diagenesis is also a response to increasing the temperature and pressure as sediment gets buried deeper. As temperature increases beyond about 200oC, we enter the realm of metamorphism, the subject of our next discussion. Questions on this material that might be asked on an exam What are the four types of sedimentary rocks.? Give some examples of each. How are clastic sedimentary rocks classified? What characteristics of sediment would tell you that the sediment is texturally and compositionally mature or immature? Define the following (a) evaporites, (b) coal, (c) travertine, (d) varves, (e) fossils, (f) transgression, (g) regression.

What information can be obtained about the depositional processes when one finds the following features in sediment or sedimentary rocks (a) cross-beds, (b) ripple marks, (c) mudcracks, (d) sole marks? What is the significance of red colored sediment? What is the significance of black colored sediment with high amounts of organic material? How does sediment turn into hard sedimentary rock? Sedimentary Rocks Sedimentary Rocks Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay First-time Visitors: Please visit Site Map and Disclaimer . Use "Back" to return here.

What Rocks Tell Us Deposited on or Near Surface of Earth by Mechanical or Chemical Processes Clastic Rocks Environmental Clues in Sedimentary Rocks Grain Size - Power of Transport Medium Grading - Often Due to Floods Rounding Cross-bedding-wind, Wave or Current Action Fossils Black Shale - Oxygen Poor, Often Deep Water Bedding or Stratification Almost Always Present in Sedimentary Rocks Originally Horizontal Tilting by Earth Forces Later Variations in Conditions of Deposition Size of Beds (Thickness) Can Range From Microscopic to 50m Clastic Rocks Sediment Sizes and Clastic Rock Types Shale - Clay less than 0.001 Mm Siltstone - Silt .001-0.1 Mm Sandstone - Sand .01-1 Mm Conglomerate - Gravel 1mm + Sedimentary rocks made of silt- and clay-sized particles are collectively called mudrocks, and are the most abundant sedimentary rocks. Some Special Clastic Rock Types Arkose - Feldspar - Rich Removal of Unstable Ingredients - Mechanical Working Diagenesis Example: Ca(sol'n) + SO4 (Sol'n) = CaSO4 Gypsum Fossil Fuels Coal Coal is a slam-dunk.

It's carbonized wood. We know that because the actual wood fragments are easily visible in low-grade varieties of coal, fossilized wood is often found in adjacent rocks, the overall environment is typical of coastal swamp or delta settings, and ancient soils are sometimes found beneath the coal beds. Organic matter goes through a variety of changes as it becomes coal: Peat Compacted and partially decomposed organic matter. About 50% carbon. Lignite Brown or gray brittle coal with lots of impurities, and often with easily visible plant fragments. About 80% carbon. Bituminous Black with banding. Some bands are dull, others shiny. These bands reflect different types of processed plant matter, which are still visible under the microscope. About 90% carbon. Anthracite Black or dark gray, metallic luster and conchoidal fracture. A true metamorphic rock, since it's heated beyond the temperatures found in normal sedimentary burial. About 95% carbon Graphite Dark gray and metallic, 100% carbon but unburnable in normal flames. Diamond Contrary to popular misconception, diamond is NOT the final stage in coal metamorphism! Coal is never buried deeply enough to reach the pressures needed to form diamond.

Diamond form in the earth's mantle from carbon that was always in the earth's interior. Anthracite is the purest and best form of coal. Unfortunately, the temperature difference between bituminous coal and graphite is small, so anthracite is uncommon. Also, unlike flat-lying bituminous, anthracite often occurs in folded rocks, making it hard to mine. Finally, since it burns so hot, it requires special furnaces. So despite its desirable properties, anthracite use has declined substantially. Petroleum The problem with petroleum is that it's a fluid and moves, so it may migrate far from its source. A typical petroleum molecule looks like this: The above molecule has eight carbon atoms in a chain and is called octane. Molecules with 1-4 carbons are called methane, ethane, propane and butane, respectively. From then on, they are named for the number of carbons in the chain: pentane, hexane, heptane, octane, etc. Octane makes a good motor fuel, so fuel that burns as well as pure octane is termed 100 octane.

Back when Detroit was turning out mammoth Klingon cruisers, some fuels were over 100 octane (outperformed pure octane). Nowadays fuels are 85-90 octane. So if petroleum is the remains of living things, what sorts of organisms make these molecules? Answer: NONE. If it were that easy, we wouldn't have to look for oil, we'd just toss our garbage into a vat of the right microbes and skim off the petroleum. But lots of organisms make molecules that look like this: This is called a fatty acid (octanoic acid to be exact). Most petroleum occurs in marine sedimentary rocks, so we want organisms rich in fatty acids that live in the sea, in huge quantities. And we have them. They're called plankton. Marine plankton, not dinosaurs, are the precursors of petroleum. Being fluid, petroleum moves, and since it's lighter than water, it floats upward. Left unconfined, it will reach the surface and evaporate or be oxidized. So it has to be confined somehow. Contrary to the popular term "oil pool," oil does not collect in pockets in the rock.

It floats upward on water until it either reaches the surface or is trapped from above. The most important economic application of an understanding of rocks in three dimensions is the search for petroleum traps. In the diagram, light blue represents water-soaked porous rock, dark gray represents petroleum and light gray represents natural gas. Like water in an aquifer, the petroleum and natural gas fill pore spaces in the rocks. All other colors represent impervious rocks. Traps that result from deformation of the rocks by outside forces. Startigraphic traps (bottom) Traps formed by variations within the sedimentary rocks themselves. Since oil weighs a lot less than rock, the oil in a well weighs far less than the same volume of rock next to the well. Thus in many cases oil is still under pressure when it reaches the surface. In old-time movies, it was common to see the climax come when oil drillers on the verge of quitting hit oil, got a "gusher" and celebrated in the resulting rain of oil. Fact: the absolute last thing anyone in the oil business ever wants is a gusher. They are incredibly dangerous to get under control.

In fact, when PBS did a series on oil, they could not locate a sound recording of a gusher anywhere and had to interview a few surviving old-timers who could remember what one sounded like. It's been that long since one happened. However, that pressure is extremely valuable because not only does it get the oil to the surface, but it helps move oil through the rocks to the well. Get greedy and drill too many wells, and you bleed off the pressure, and in the long run you get less oil, not more. Gas Hydrates Natural gas hydrates are a curious kind of chemical compound where two dissimilar molecules are mechanically intermingled but not truly chemically bonded. Instead one molecule forms a framework that traps the other molecule. Natural gas hydrates can be considered modified ice structures enclosing methane and other hydrocarbons, but they can melt at temperatures well above normal ice. At 30 atmospheres pressure, methane hydrate begins to be stable at temperatures above 0 C and at 100 atmospheres it is stable at 15 C. This behavior has two important practical implications. First, it's a nuisance to the gas company.

They have to dehydrate natural gas thoroughly to prevent methane hydrates from forming in high pressure gas lines. Second, methane hydrates will be stable on the sea floor at depths below a few hundred meters and will be solid within sea floor sediments. Masses of methane hydrate "yellow ice" have been photographed on the sea floor. Chunks occasionally break loose and float to the surface, where they are unstable and effervesce as they decompose. The stability of methane hydrates on the sea floor has a whole raft of implications. First, they may constitute a huge energy resource. Second, natural disturbances (and man-made ones, if we exploit gas hydrates and aren't careful) might suddenly destabilize sea floor methane hydrates, triggering submarine landslides and huge releases of methane. Finally, methane is a ferociously effective greenhouse gas, and large methane releases may explain sudden episodes of climatic warming in the geologic past. The methane would oxidize fairly quickly in the atmosphere, but could cause enough warming that other mechanisms (for example, release of carbon dioxide from carbonate rocks and decaying biomass) could keep the temperatures elevated. In the diagram at left, each vertex is occupied by an oxygen atom and the midpoint of each edge is a hydrogen atom.

This atom is attached to one oxygen as part of a water molecule and hydrogen bonded to the other. In the diagram at left one cage is shown with oxygen atoms in blue and hydrogen in red. A methane molecule is shown inside one of the cage skeletons. Facies Changes Sedimentary rocks change laterally. These changes reflect the different environments where the rocks formed. Environment During Deposition Landforms Associated with Sedimentary Rocks Mesa Sedimentary Rocks Lesson #13 Sedimentary Rocks Lesson #13 The land around you, no matter where you live, is made of rock. If you live in a place that has good rich soil, the soil itself is finely broken down or weathered rock. People that live in a desert region can easily find rocks on the surface. These rocks lay on a surface of clay that is also a product of weathering rock. Weathering is the process of breaking down rocks and minerals into smaller pieces by water, wind, and ice. Sedimentary rocks are formed from the breaking apart of other rocks (igneous, metamorphic, or sedimentary rocks) and the cementation, compaction and recrystallization of these broken pieces of rock.

in Hawaii that will, over time, and with the help of bacteria, break down into fertile soil. Sedimentary rocks are formed from broken pieces of rocks. These broken pieces of rock are called sediments. The word "Sedimentary" comes from the root word "Sediment". Sedimentary rocks are usually formed in water. Streams and rivers carry sediments in their current. When the current slows around a bend or the river empties into a lake, or ocean, or another river the sediments fall out because of gravity. The larger sediments fall out first and the lightest sediments fall out last. The diagram above shows layers of sediment that were laid down in a lake. In the spring the lake receives an influx of water from the mountain snow melt. This snow melt carries with it a large amount of sediment that becomes suspended in the lake water. As the sediment settles out during the summer and especially in the winter, if the lake becomes frozen over, the sediments come to rest on the bottom. The heaviest and largest particles settle out first and the lightest sediments such as silts and clays settle out last.

The number 1 shows sediment that would have been laid down during 1994, number 2 in 1995, and number 3 would have been laid down in 1996. The gray area above the 3 would be the latest layer being laid down at the present time. This laying down of rock-forming material by a natural agent is called deposition. Natural agents of deposition are water, ice, gravity, and wind. Sediment is deposited in flat, horizontal layers with the oldest layers on the bottom and the younger layers laying on and over the older layers. Geologists use this knowledge to read layers of sedimentary rock like the pages in a book. They can date layers by the fossils that are found in them. If a layer has a fossil in it that is known to be 50 million years old the layer itself must be at least 50 million years old and the layers below it have to be older than 50 million years. The size of sediment is defined by the size of the particles that make up the sediment. The largest sediment size is called a boulder. Boulders have a diameter that is larger than 256 millimeters (about 10 inches).

Cobbles are the next largest sediment, they are 64 - 256 mm in diameter (about 3-10 inches). Pebbles are next in size and are 4-64 mm in diameter (about 1/6-3 inches). The next sizes of sediments are very small, granules are 2-4 mm, sand 1/16-2mm, silt 1/256-1/16 mm, and the smallest sediment size is clay which is less than 1/256 of a millimeter in diameter. Sedimentary rocks are formed in three ways from these different sized sediments. A sedimentary rock is a layered rock that is formed from the compaction, cementation, and the recrystallization of sediments. Compaction is the squeezing together of layers of sediment due to the great weight of overlying layers of rock. This squeezing of the layer results in reducing the thickness of the original layer. When the layers are reduced in thickness the pore spaces around the sediments are also reduced, which leads to a tighter packing of the layers.

Cementation is the changing of sediment into rock by filling spaces around the sediments with chemical precipitates of minerals. binding the sediments, and forming solid rock. Calcite and are common minerals that cement the sediments together. Recrystallization is the third way that sedimentary rocks are formed. Recrystallization is the of new mineral grains that are larger than the original grains. As the sediments recrystallize they arrange themselves in a series of interlocking crystals that connect the other grains together into a solid rock. The photo above shows layers of sedimentary rocks that were deposited in flat horizontal layers. These layers were then uplifted and bent by mountain building. Sedimentary rocks form a thin layer of rock over 75 per cent of the Earth's surface. They are the site of very important resources such as ground water, coal, oil, and soil. Shale, sandstone, and limestone are the most common types of sedimentary rocks. They are formed by the most common mineral that is found on or near the surface of the Earth. The mineral that forms these sedimentary rocks is feldspar. Running water, such as the mountain stream above, sorts and transports more sediment than any other agent of deposition.

Clastic sedimentary rocks are made of pieces of rock or mineral grains that have been broken from preexisting rock. These particles and grains have become solid rock by the processes of compaction or cementation of sediments. Some clastic rocks are conglomerate, shale, breccia, gray and red sandstone, siltstone, and graywacke. Non-clastic sedimentary rocks form from the precipitation (Precipitation is the separating of a solid from a solution) of minerals from ocean water or from the breakdown of the shells and bones of sea creatures. Sea animals such as coral produce calcium carbonate solutions that harden to form rock. As the chemicals, that comes from the mineral or biological precipitation, mix with sediments on the floor of the ocean or lake they crystallize and grow in the spaces around the sediment. When these crystals grow large enough to fill the spaces they harden and form a solid rock. Some non-clastic rocks are limestone, chert, dolostone, gypsum, halite (rock salt), diatomite, and chalk. Organic sedimentary rocks form from the build up and decay of plant and animal material.

This usually forms in swamp regions in which there is an abundant supply of growing vegetation and low amounts of oxygen. The vegetation builds so quickly that new layers of vegetation bury the dead and decaying material very quickly. The bacteria that decay the vegetation need oxygen to survive. Because these decaying layers are buried so fast the bacteria use up what oxygen there is available and can not finish the decomposition of the vegetation. The overlaying layers become so heavy that they squeeze out the water and other compounds that aid in decay. This compressed vegetation forms coal. The longer and deeper that coal is buried makes it of higher quality. Peat is the first stage of coal formation. Lignite is the next grade of coal followed by bituminous and the highest grade, anthracite. Anthracite is actually a metamorphic rock. It forms during mountain building when compaction and friction are extremely high. This form of coal burns very hot and almost smokeless. It is used in the production of high grade steel. Shale is one of the most common sedimentary rocks. It is composed of silt or clay that has been compacted or squeezed together to form a solid rock. Shale is usually found in thin layers.

The silt or clay that composes shale is made of very small pieces of weathered rock. The pieces are from 1/16 to 1/256 of a millimeter in diameter. The color of a sample of shale is that of the clay or silt that it was formed from. Sandstone is a clastic sedimentary rock that forms from the cementing together of sand sized grains forming a solid rock. Quartz is the most abundant mineral that forms sandstone. Calcium carbonate, silica, or iron has been added to the water that is in contact with the sand grains. These minerals grow crystals in the spaces around the sand grains. As the crystals fill the gaps the individual sand grains are now transformed into a solid rock. Limestone is the most abundant of the non-clastic sedimentary rocks. Limestone is produced from the mineral calcite (calcium carbonate) and sediment. The main source of limestone is the limy ooze formed in the ocean. The calcium carbonate can be precipitated from ocean water or it can be formed from sea creatures that secrete lime such as algae and coral. Chalk is another type of limestone that is made up of very small single-celled organisms.

Chalk is usually white or gray in color. Limestone can easily be dissolved by acids. If you drop vinegar on limestone it will fizz. Put a limestone rock into a jar and cover it with vinegar. Cover the jar and watch the bubbling of the calcium carbonate and also the disintegration of the rock over a few days. Limestone caves are an interesting geological feature. They form because the limestone deposits located under the ground are chemically dissolved by moving ground water. The ground water contains minerals that make the water slightly acidic. When an acid comes into contact with a rock that is composed of calcium carbonate a chemical reaction takes place. The acid "eats" the limestone. The calcium carbonate then goes into the ground water which moves down farther into the cave. The water will find its way into small crack and crevasses. The dripping water will create formations called stalactites and stalagmites. Stalactites (they grow from the ceiling)and stalagmites (they grow from the floor) are not technically limestone. They form in caves because as the limestone is dissolved calcium carbonate is put into solution in the ground water. This solution drips through crack and slowly forms stalactites and stalagmites.

The photo above shows both stalactites and stalagmites growing together in Jewel Cave National Park in South Dakota. Conglomerate is a clastic sedimentary rock that forms the cementing of rounded cobble and pebble sized rock fragments. Conglomerate is formed by river movement or ocean wave action. The cementing agents that fill the spaces to form the solid rock conglomerate are silica, calcite, or iron oxides. Notice in the photo above the rounded rock particles in the conglomerate. These rounded particles make conglomerate different from the next rock you are about to study, breccia. Breccia is formed in a very similar fashion to conglomerate. The difference between the two rocks is that breccia's rock fragments are very sharp and angular. These rock fragments have not been transported by water, wind, or glaciers long enough to be rounded and smoothed like in the conglomerate. The cementing agents silica, calcite (CaCO3), and iron oxides are the same as in conglomerate. Write the answers to the following questions in complete sentences on a piece of paper. In your own words describe the process of weathering. 2.

USGS Geology and Geophysics Sedimentary rocks Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock. Clastic sedimentary rock Clastic sedimentary rocks are the group of rocks most people think of when they think of sedimentary rocks. Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering , then transported to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented , forming sedimentary rock. Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their clast or grain size. The smallest grains are called clay , then silt , then sand . Grains larger that 2 millimeters are called pebbles.

Shale is a rock made mostly of clay, siltstone is made up of silt-sized grains, sandstone is made of sand-sized clasts, and conglomerate is made of pebbles surrounded by a matrix of sand or mud. Biologic sedimentary rock Biologic sedimentary rocks form when large numbers of living things die, pile up, and are compressed and cemented to form rock. Accumulated carbon-rich plant material may form coal. Deposits made mostly of animal shells may form limestone, coquina, or chert. Chemical sedimentary rock Chemical sedimentary rocks are formed by chemical precipitation. The stalactites and stalagmites you see in caves form this way, so does the rock salt that table salt comes from. This process begins when water traveling through rock dissolves some of the minerals, carrying them away from their source. Eventually these minerals can be redeposited, or precipitated, when the water evaporates away or when the water becomes over- saturated with minerals. Use the back button on your browser to return to the previous page.

Question: Of these sedimentary rocks, which one is not organic, being instead 'clastic'?
Answer: Shale