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10.3: Identifying Sedimentary Rocks - Geosciences

10.3: Identifying Sedimentary Rocks - Geosciences


Overview

The classification of sedimentary rocks is largely based on differentiating the processes that lead to their formation. The biggest division in types of sedimentary rocks types is based on the primary type of weathering that leads to the material building the sedimentary rock. If the rock is largely made from broken pieces (called clasts) of rock that have been mechanically weathered the rocks are referred to as Detrital or Clastic Sedimentary Rocks. Simply put, these are rocks that are composed of the broken pieces of other rocks. In this case, the mineralogy of the clasts is not important, but instead, we need to note the properties of the sediment itself. Alternatively, if the rock is largely the product of chemical weathering the classification is then based on the composition of the material as well as the processes involved in the materials precipitation from solution. Chemical Sedimentary Rocks form from the inorganic precipitation of minerals from a fluid. If the ions present within a fluid (water) become very concentrated either by the addition of more ions or the removal of water (by freezing or evaporation), then crystals begin to form. In this case, the identification of the type of sedimentary rocks is based on the minerals present. If organisms facilitate the precipitation of these minerals from water, we refer to the rocks as Biochemical Sedimentary Rocks. An example of biochemical precipitation is the formation of skeletal minerals in many organisms: from starfish and clams that grow calcite to sponges that grow silica-based material, to humans that have bones made of hydroxyapatite. In many cases, it is hard to differentiate whether a mineral was formed organically or inorganically, so in the current lab, we will mostly group these two types of sedimentary rocks together. Rocks can also be formed from the carbon-based organic material produced by ancient life and are called Organic Sedimentary Rocks. Now we can discuss the identification and formation of particular sedimentary rocks.

Clastic Sedimentary Rocks

Weathering and erosion occur normally in areas that are at high elevation, such as mountains, while deposition occurs in lower areas such as valleys, lakes, or the ocean. The sediment is transported from the area of erosion to area of deposition by ice, water, or air. Not surprisingly, the sediment changes during its journey and we can recognize the amount of change and the distance the material has traveled, and the transport mechanism, by looking at its maturity (Figure 10.1). Maturity is defined as the texture and composition of a sedimentary rock resulting from varying amounts of erosion or sedimentary transport. Imagine a mountain composed of the igneous rock granite and let us explore how the sediment from this mountain changes as it makes the long-distance trek via the river to the ocean. The first process is just breaking the rock down into smaller pieces mechanically, which creates sediment that has large and small pieces, the pieces are jagged, and all of the minerals remain. The sizes of clasts in these rocks can range from large boulders, to cobbles, to pebbles, to the smallest particles, clay. As this sediment is transported in the river the pebbles collide with other pebbles and the rocks get smaller and the sharp edges are broken off. Also, as the slope of the land decreases the river slows leaving behind the large boulders and cobbles while carrying away the smaller particles. This results in sediments further from the source to be more uniform in size, which is a process called sorting. Chemical weathering also occurs, altering the feldspars into clay-sized particles. The end result of this process is the granite reduced from boulders and cobbles close to the mountain, to pebbles in the rivers, and finally to pure and uniform quartz sand at the beach and minuscule clay grains on the ocean floor. Therefore, different clastic rocks are found in different areas and have traveled different distances.

In this lab, we will look at three types of clastic rocks (Figure 10.1, Table 10.1), conglomerate, sandstone, and shale. Conglomerate is an immature sedimentary rock (rock that has been transported a short distance) that is a poorly sorted mixture of clay, sand, and rounded pebbles. The mineralogy of the sand and pebbles (also called clasts) can vary depending on its source. These rocks would be found on the continent in several types of deposits such as ancient landslides or pebble beds in rivers. Sandstone is defined as a clastic sedimentary rock that consists of sand-sized clasts. These clasts can vary from jagged to rounded as well as containing many minerals or just quartz. Therefore, sandstone ranges from being relatively immature to mature which makes sense because we can find layers of sand associated with mountain rivers to pure white quartz beaches. Last we have shale, which is composed of clay particles and has a finely layered or fissile appearance. This extremely mature sedimentary rock is made from the smallest particles that can be carried by wind or barely moving water and can be found thousands of miles away from the original source.

Biochemical and Chemical Sedimentary Rocks

As mentioned before, biochemical and chemical sedimentary rocks either precipitated directly from water or by organisms. The most recognizable chemical sedimentary rocks are evaporites. These are minerals that are formed by the precipitation of minerals from the evaporation of water. You have already examined multiple examples of these minerals/rocks in a previous lab, such as halite and gypsum. In this current lab, we will focus on siliceous and carbonate biochemical sedimentary rocks. Chert is a rock composed of microcrystalline varieties of quartz, and thus it has properties that are associated with quartz itself, such as conchoidal fracturing and hardness greater than glass. Chert is often formed deep in the ocean from siliceous material that is either inorganic (silica clay) or biologic (skeletons of sponges and single-celled organisms) in origin. Carbonates are one of the most important groups of sedimentary rocks and as you have previously learned (Chapter 5), can result in distinctive landscapes (karst) and human hazards (sinkholes). Limestone is a sedimentary rock composed of the carbonate mineral calcite and can vary greatly in its appearance depending on how it is formed, but can easily be identified by its chemical weathering. Limestone composed of the mineral calcite undergoes dissolution in acids. In other words, it effervesces dramatically when we apply dilute HCl. As with chert, limestone can be formed inorganically from the supersaturation of calcium and carbonate ions in water in varying environments from caves to tropical beaches. Limestone that consists of crystals of calcite or microcrystalline masses of calcite is called crystalline limestone. Alternatively, limestone can be formed biologically with the most striking example called coquina, which are rocks made exclusively of fragmented carbonate (calcite or its polymorph aragonite) shells or coral. Lastly, we have dolostones, which are made from crystals (large or microscopic) of the mineral dolomite and are a carbonate that only weakly reacts to dilute HCl; you can scratch and powder dolostone to increase the surface area to see the reaction with acid. Dolostone is formed by the inorganic chemical alteration of limestones, therefore they are classified as chemical rather than biochemical sedimentary rocks.

Organic Sedimentary Rocks

Organic compounds are materials that contain a significant amount of the element carbon and are often associated with life. Organic sedimentary rocks are, therefore, rocks that consist mostly of carbon and are associated with significant biological activity. Other sedimentary rocks such as limestone and shale can contain carbon, but at much lower concentration (though shale can appear black from their carbon content). The most common organic sedimentary rock is coal, which is a very low density (light) black rock that has a dusty (sooty) or shiny appearance. It also produces a dark gray streak that can be seen both on a streak plate or a piece of paper. Coal is formed from the preservation and compaction of abundant plant material often in areas where oxygen is lacking, such as a swamp.


Sedimentary rocks are formed from deposits of pre-existing rocks or pieces of once-living organism that accumulate on the Earth's surface. If sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock. These rocks often have distinctive layering or bedding and create many of the picturesque views of the desert southwest. Sedimentary rocks are classified into three groups: Clastic, Biologic, and Chemical.

Key Terms

Cementation: The process by which clastic sediments become lithified or consolidated into hard, compact rocks, usually through deposition or precipitation of minerals in the spaces among the individual grains of the sediment.

Compaction: The process of consolidating fine-grained sediments into rock.

Lithification: The conversion of loose sediment into solid sedimentary rock. Several processes, including compaction of grains, filling of spaces between grains with mineral cement, and crystallization act to solidify sediment.

A typical clastic sedimentary rock, sandstone, appears red due to iron oxide.

Photo courtesy of Tina Kuhn


Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks.

Clastic sedimentary rocks form from the accumulation and lithification of mechanical weathering debris. Examples include: breccia, conglomerate, sandstone, siltstone, and shale.

Chemical sedimentary rocks form when dissolved materials preciptate from solution. Examples include: chert, some dolomites, flint, iron ore, limestones, and rock salt.

Organic sedimentary rocks form from the accumulation of plant or animal debris. Examples include: chalk, coal, diatomite, some dolomites, and some limestones.

Photos and brief descriptions of some common sedimentary rock types are shown on this page.

Chalk is a type of limestone made up of the microscopic calcium carbonate shells of marine organisms. Chalk is soft, friable, porous, and effervesces vigorously in contact with hydrochloric acid. Because it is very porous, subsurface chalk units can serve as reservoirs for oil and natural gas.

Caliche is found in arid or semiarid climates around the world. It forms when rock fragments and sediment particles are cemented together by mineral matter, usually calcium carbonate, that precipitates and hardens between the particles. Over time, caliche can become very dense and durable, which explains why it is also called hardpan, calcrete, and duricrust. Some people might call it a puddingstone.

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.

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.

Diatomite is a sedimentary rock with many uses. It is made up of the siliceous skeletal remains of diatoms, which are tiny single-celled algae. Diatomite is crushed into a powder known as "diatomaceous earth". It is lightweight, porous, relatively inert, and has a small particle size along with a large surface area. These properties make diatomaceous earth useful as a filtration media, a lightweight aggregate, a lightweight filler, an effective absorbent, and more.

Coquina is a type of limestone composed of calcium carbonate shells, shell fragments, and other sand-sized fossil debris. It forms in the shallow waters of coastal areas with a tropical or subtropical climate. The particles are weakly cemented together, and therefore coquina is a very porous material that can function as an aquifer or a reservoir for oil and natural gas. Public Domain photo by Mark A. Wilson of the Department of Geology, The College of Wooster.

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. The specimen in the photo is about two inches (five centimeters) across.


Sedimentary Rocks

Sedimentary rocks (layered rocks) are made by the deposition of particles carried in air or water and by the precipitation of chemicals dissolved in water. These particles and chemicals come from the weathering (breaking apart in place) and erosion (carrying away and breaking apart while moving) of rocks on the Earth’s surface. Boulders, rocks, gravel, sand, silt, clay, and mud are carried by water currents in streams, rivers, lakes, and oceans. These particles are deposited in stream beds, shores, lake and ocean bottoms, and deltas where rivers empty into lakes and oceans. These particles are cemented together and hardened to form the sedimentary rocks called conglomerate, sandstone, siltstone, shale or claystone, and mudstone.

Chemicals that are leached or dissolved from other rocks are carried invisibly in streams and oceans. When these invisible chemicals reach a lake or ocean, and the water evaporates, the chemicals are left behind in evaporite deposits. Salt around salt lakes and limestone from sea bottoms are examples of these chemical sedimentary rocks. Plants and animals are sometimes buried and preserved in the finer grained sedimentary rocks, such as shale or limestone.

Sedimentary rocks are derived from pre‑existing rocks by weathering and erosion. The resulting particles settle out of water or air (clastic rocks such as sandstone and mudstone) or the resulting chemicals precipitate from concentrated solutions (non‑clastic rocks such as limestone and salt).

Examples of Sedimentary Rocks
Formation Characteristic Rock Name
particles course-angular Breccia
particles course-round Conglomerate
particles medium (less than 2 mm) Sandstone
particles fine (smooth) Shale
chemicals fizzes in dilute HCl acid Limestone
chemicals fizzes in acid only if scratched first Dolomite
chemicals salty taste Halite
chemicals scratched by a fingernail Gypsum
chemicals scratches glass, conchoidal fracture, like flint Chert

Clastic Sedimentary Rocks

How Clastic Sedimentary Rocks are Formed:

  1. Pre‑existing rock undergoes chemical and mechanical weathering by roots, acid rainwater, gravity, wind, and water.
  2. The broken particles are carried through water or air until they settle out in a lower area when the current wasn’t fast enough to carry the particles.
  3. Quartz is the most stable and has the greatest resistance to the mechanical and chemical abrasion during erosion, so most sand size grains are quartz.
  4. Feldspar alters to clay with chemical weathering and erosion, so arkose, which is sandstone with more than 25% feldspar, indicates the sediment was deposited close to the source rock and was not in transport long.
  5. Grain size and shape and composition can indicate the composition, distance, and height of the source rock.
  6. Textures & structures (ripple marks, cross‑bedding, sorting, etc.) and size, shape, and composition can indicate the environment of deposition.

Characteristics:

  1. Soft, compared to igneous rocks.
  2. Occur in layers or beds from a few millimeters thick to 100 feet thick, most commonly 1‑5 ft. thick.
  3. Granular and gritty if composed of sand and silt‑sized particles sand is often rounded, sometimes angular.
  4. Sedimentary structures (cross‑bedding, mud cracks, ripple marks, worm trails and burrows, fossil shells) are not usually visible in hand specimens, but are noticeable in outcrops.
  5. Color is not usually significant, because as little as 3% hematite (iron oxide) gives a rich red color. Some pinkish sandstones get their color from feldspar.
  6. Fossils are more common in shales than sandstones, because of the higher current activity in sandstones.

Non-Clastic Sedimentary Rocks

How Non-Clastic Sedimentary Rocks are Formed:

Formed by chemical precipitation from a concentrated solution in water as salt, gypsum, or limestone.


(f). Characteristics of Sedimentary Rocks

Sedimentary rocks can be categorized into three groups based on sediment type. Most sedimentary rocks are formed by the lithification of weathered rock debris that has been physically transported and deposited. During the transport process, the particles that make up these rocks often become rounded due to abrasion or can become highly sorted. Examples of this type of sedimentary rock include conglomerate and sandstone. Scientists sometimes call this general group of sedimentary rocks clastic. The remaining types of sedimentary rocks are created either from chemical precipitation and crystallization, or by the lithification of once living organic matter. We identify these sedimentary rocks as being non-clastic.

Figure 10f-1 : Conglomerate.

Figure 10f-2 : Sandstone.

All sedimentary rocks are lithified into some collective mass. Lithification is any process that turns raw rock sediment into consolidated sedimentary rock. The process of lithification usually produces identifiable layering in these type of rocks (Figure 10f-3). Lithification can occur by way of:


Grain Size Usual Color Other Composition Rock Type
fine dark glassy appearance lava glass Obsidian
fine light many small bubbles lava froth from sticky lava Pumice
fine dark many large bubbles lava froth from fluid lava Scoria
fine or mixed light contains quartz high-silica lava Felsite
fine or mixed medium between felsite and basalt medium-silica lava Andesite
fine or mixed dark has no quartz low-silica lava Basalt
mixed any color large grains in fine-grained matrix large grains of feldspar, quartz, pyroxene or olivine Porphyry
coarse light wide range of color and grain size feldspar and quartz with minor mica, amphibole or pyroxene Granite
coarse light like granite but without quartz feldspar with minor mica, amphibole or pyroxene Syenite
coarse light to medium little or no alkali feldspar plagioclase and quartz with dark minerals Tonalite
coarse medium to dark little or no quartz low-calcium plagioclase and dark minerals Diorite
coarse medium to dark no quartz may have olivine high-calcium plagioclase and dark minerals Gabbro
coarse dark dense always has olivine olivine with amphibole and/or pyroxene Peridotite
coarse dark dense mostly pyroxene with olivine and amphibole Pyroxenite
coarse green dense at least 90 percent olivine Dunite
very coarse any color usually in small intrusive bodies typically granitic Pegmatite
Hardness Grain Size Composition Other Rock Type
hard coarse clean quartz white to brown Sandstone
hard coarse quartz and feldspar usually very coarse Arkose
hard or soft mixed mixed sediment with rock grains and clay gray or dark and "dirty" Wacke/
Graywacke
hard or soft mixed mixed rocks and sediment round rocks in finer sediment matrix Conglomerate
hard or
soft
mixed mixed rocks and sediment sharp pieces in finer sediment matrix Breccia
hard fine very fine sand no clay feels gritty on teeth Siltstone
hard fine chalcedony no fizzing with acid Chert
soft fine clay minerals splits in layers Shale
soft fine carbon black burns with tarry smoke Coal
soft fine calcite fizzes with acid Limestone
soft coarse or fine dolomite no fizzing with acid unless powdered Dolomite rock
soft coarse fossil shells mostly pieces Coquina
very soft coarse halite salt taste Rock Salt
very soft coarse gypsum white, tan or pink Rock Gypsum

Sedimentary Rocks Are Stories

You can see that each type of sedimentary rock has a story behind it. The beauty of sedimentary rocks is that their strata are full of clues to what the past world was like. Those clues might be fossils or sedimentary structures such as marks left by water currents, mud cracks or more subtle features seen under the microscope or in the lab.

From these clues we know that most sedimentary rocks are of marine origin, usually forming in shallow seas. But some sedimentary rocks formed on land: clastic rocks made on the bottoms of large freshwater lakes or as accumulations of desert sand, organic rocks in peat bogs or lake beds, and evaporites in playas. These are called continental or terrigenous (land-formed) sedimentary rocks.

Sedimentary rocks are rich in geologic history of a special kind. While igneous and metamorphic rocks also have stories, they involve the deep Earth and require intensive work to decipher. But in sedimentary rocks, you can recognize, in very direct ways, what the world was like in the geologic past.


4.3.1 Clastic Sedimentary Rocks

Weathering and erosion occur normally in areas that are at high elevation, such as mountains, while deposition occurs in lower areas such as valleys, lakes, or the ocean. The sediment is transported from the area of erosion (e.g. the “source”) to area of deposition (e.g. the “sink”) by ice, water, or air. The sediment changes during its journey we can recognize the amount of change, the distance traveled, and the transport mechanism, by looking at its maturity (e.g. Figure 4.1). Maturity is defined as the texture and composition of a sedimentary rock resulting from varying amounts of erosion or sedimentary transport. Imagine a mountain composed of granite, and let us explore how the sediment from this mountain changes as it makes the long journey via river to the ocean. First, the rock is mechanically breaking down into smaller pieces, which creates sediment that has jagged large and small clasts, and all of the minerals remain. The sizes of clasts in these rocks can range from large boulders, to cobbles, to pebbles, to the smallest particles, clay. As this sediment is transported in the river, the clasts collide with each other and the rocks get smaller and the sharp edges are broken off. Also, as the slope of the land decreases the river slows, leaving behind the large boulders and cobbles, while carrying away the smaller particles. Now, sediments further from the source will be more uniform in size, which is a process called sorting. Chemical weathering also occurs, altering the feldspars into clay-sized particles. In the end, the granite is reduced from boulders and cobbles close to the mountain, to pebbles in the rivers, to pure and uniform quartz sand at the beach, and miniscule clay grains on the ocean floor. Therefore, different clastic rocks are found in different areas and have traveled different distances.

In the lab, we will look at four types of clastic rocks, conglomerate, breccia, sandstone, and shale. Conglomerate is an immature sedimentary rock (e.g. it has been transported a short distance) that is a poorly sorted mixture of clay, sand, and rounded pebbles. The mineralogy of the sand and pebbles (also called clasts) can vary depending on its source. These rocks would be found on the continent in several types of deposits, such as ancient landslides or pebble beds in rivers. Breccia is an immature sedimentary rock that is a poorly sorted mixture of clay, sand, and angular pebbles. The mineralogy of the clasts can vary depending on its source. These rocks would be found on the continent in several types of deposits, such as next to fault zones and debris flows. Sandstone is defined as a clastic sedimentary rock that consists of sand-sized clasts. These clasts can vary from jagged to rounded, and can contain many minerals or just quartz. Therefore, sandstone ranges from being relatively immature to mature, which makes sense, since we can find layers of sand associated with mountain rivers, to pure white quartz beaches. Last we have shale, which is composed of clay particles and has a finely layered or fissile appearance. This extremely mature sedimentary rock is made from the smallest particles that can be carried by wind or barely moving water and can be found thousands of miles away from the original source.

Figure 4.1 | Maturity in clastic sedimentary rocks showing how the sediments change as they are eroded further from their source.
Source: Bradley Deline (2015) CC BY-SA 3.0 view source

Activity 11B: Identifying Sedimentary Rocks

Download and complete this fillable PDF . You will need a program like Acrobat Reader (free!) to access the file. Once the file is downloaded, name and save the file in the following format: LastName_FirstName_AssignmentTitle. Once completed, save again and upload your file to this assignment. You will also find the assignment text below.

For this section, you will need the following materials before you begin:

  • Sedimentary rock samples (or kit) selected by your instructor,
  • Glass plate, streak plate, and a hand lens (or magnifying glass)
  • Bottle of diluted HCl or white vinegar (optional)
  1. Your instructor will provide you with a variety of sedimentary rock samples. Use the Sedimentary Image Index to order your samples to their corresponding sample numbers.
  2. Using the kit provided, begin by separating the rocks into two piles, a clastic and a non-clastic pile. Remember, sedimentary rocks must be categorized as either clastic OR non-clastic a sample cannot be both.For each sample number, indicate whether the sample is clastic or non-clastic by placing an X in the appropriate column below.

Using the same samples, and your answers from above, fill out Table 11.7.

  1. If the sample is clastic, determine the grain size, shape, and overall sorting.
  2. If the sample is non-clastic, indicate whether the sample is organic (O) or chemical/biochemical (C/BC). Then determine the dominant composition of the rock.
  3. Using the characteristics of each rock, identify each sample by name. Add the rock name to the final column of the table below.
  1. Chert and limestone are both fine-grained sedimentary rocks. What are two tests you can use to tell them apart?
  2. What characteristic distinguishes sedimentary breccia from conglomerate?
  3. How can sorting, rounding, and grain size be useful for understanding depositional environments? Use examples in your explanation.

Extra Help for Select Rocks:

Sedimentary Rock 1 in vinegar/weak acid. Does it react and effervesce?

Sedimentary Rock 2 in vinegar/weak acid. Does it react and effervesce?

Sedimentary Rock 4 in vinegar/weak acid. Does it react and effervesce?

Sedimentary Rock 5 in vinegar/weak acid. Does it react and effervesce?

Sedimentary Rock 6 in vinegar/weak acid. Does it react and effervesce?

Sedimentary Rock 7 in vinegar/weak acid. Does it react and effervesce?

Sedimentary Rock 9 in vinegar/weak acid. Does it react and effervesce?

Sedimentary Rock 10 in vinegar/weak acid. Does it react and effervesce?


Sedimentary Rock Textures

In clastic sediments the sedimentary texture includes the grain size, rounding, and sorting of the grains, all of which are related to what happened to the sediment during the weathering-to-deposition process.Because the processes that lead to the formation of chemical sedimentary rocks do not involve the weather-to-deposition process, there is no widely agreed-upon texture scheme that applies to chemical sedimentary rocks.

Clastic Textures

Clastic sedimentary textures are described in terms of the size of the sediment grains, how round they are, and how well they are sorted.

Grain Characteristics

The diameter or width of a clastic sediment grain determines its grain size. Specific ranges of grain size have specific names.

  • Gravel is an overall name for large sediment grain size, which includes boulder, cobble, and pebble.
  • Sand includes sediment grains ranging in size from 2mm to 0.625 mm.
  • Silt is the name of a sediment grain that range in size from 0.625 mm to 0.0039 mm.
  • Mud is the smallest grain size and is also known as clay. It is important to distinguish the grain size called clay from the mineral called clay. Clay sized grains are too small to see individually without the aid of a microscope.

Rounding

Clastic sediment grains can be round, angular, or in-between (subangular or subrounded). Breccia is a clastic sedimentary rock that by definition consists largely of angular grains of pebble size or larger. Conglomerate, another sedimentary rock, consists largely of rounded grains of pebble size or larger. The schematic diagram below shows classes of rounding, from the left: angular, subangular, subrounded, rounded. Not shown are very angular and well-rounded, which are less common.

Sorting

The extent to which all the grains are the same size is known as sorting. If all the grains are the same size, they are well sorted. Some sandstones are well sorted, and some are not. Most conglomerates are poorly sorted, and consist of a mixture of grain sizes ranging from sand to pebble. The schematic diagrams below represent poorly-sorted, moderately-sorted, and well-sorted sediments.

Other Aspects of Texture

Other aspects of clastic sedimentary texture include the packing of the grains, the porosity of the rock, and the hardness of the rock as a whole.

The packing of the grains applies only to poorly sorted sediments in which the finer clasts form a matrix around the coarser clasts. If the large grains are touching each other, the packing is said to be clast-supported. If the coarse grains are separate and not touching each other, with the finer-grained sediment between them, the packing is said to be matrix-supported.

The porosity of a rock or sediment is the amount of empty space between the grains of sediment.

The hardness of the rock refers to how easily it breaks apart. Shale is harder than claystone, for example.


Sedimentary Rocks

Sedimentary rocks are one of three main types of rocks, along with igneous and metamorphic. They are formed on or near the Earth&rsquos surface from the compression of ocean sediments or other processes.

Sedimentary Rock

An example of a sedimentary rock, which is, by definition, composed of many, smaller rocks.

Photo courtesy of Alamy Stock Photo

Sedimentary rocks are formed on or near the Earth&rsquos surface, in contrast to metamorphic and igneous rocks, which are formed deep within the Earth. The most important geological processes that lead to the creation of sedimentary rocks are erosion, weathering, dissolution, precipitation, and lithification.

Erosion and weathering include the effects of wind and rain, which slowly break down large rocks into smaller ones. Erosion and weathering transform boulders and even mountains into sediments, such as sand or mud. Dissolution is a form of weathering&mdashchemical weathering. With this process, water that is slightly acidic slowly wears away stone. These three processes create the raw materials for new, sedimentary rocks.

Precipitation and lithification are processes that build new rocks or minerals. Precipitation is the formation of rocks and minerals from chemicals that precipitate from water. For example, as a lake dries up over many thousands of years, it leaves behind mineral deposits this is what happened in California&rsquos Death Valley. Finally, lithification is the process by which clay, sand, and other sediments on the bottom of the ocean or other bodies of water are slowly compacted into rocks from the weight of overlying sediments.

Sedimentary rocks can be organized into two categories. The first is detrital rock, which comes from the erosion and accumulation of rock fragments, sediment, or other materials&mdashcategorized in total as detritus, or debris. The other is chemical rock, produced from the dissolution and precipitation of minerals.

Detritus can be either organic or inorganic. Organic detrital rocks form when parts of plants and animals decay in the ground, leaving behind biological material that is compressed and becomes rock. Coal is a sedimentary rock formed over millions of years from compressed plants. Inorganic detrital rocks, on the other hand, are formed from broken up pieces of other rocks, not from living things. These rocks are often called clastic sedimentary rocks. One of the best-known clastic sedimentary rocks is sandstone. Sandstone is formed from layers of sandy sediment that is compacted and lithified.

Chemical sedimentary rocks can be found in many places, from the ocean to deserts to caves. For instance, most limestone forms at the bottom of the ocean from the precipitation of calcium carbonate and the remains of marine animals with shells. If limestone is found on land, it can be assumed that the area used to be under water. Cave formations are also sedimentary rocks, but they are produced very differently. Stalagmites and stalactites form when water passes through bedrock and picks up calcium and carbonate ions. When the chemical-rich water makes its way into a cave, the water evaporates and leaves behind calcium carbonate on the ceiling, forming a stalactite, or on the floor of the cave, creating a stalagmite.

An example of a sedimentary rock, which is, by definition, composed of many, smaller rocks.


Watch the video: Sedimentary Rock Examples