5 Weathering, Erosion, and Sedimentary Rocks

The rock has a large cliff.
Light illuminates the sedimentary rocks of Notch Peak, in the House Range of western Utah.The House Range contains early Paleozoic marine rocks, highlighted by the Wheeler Formation, home to some of the best Cambrian fossils in Utah. Notch Peak contains one of the largest pure-vertical drops in North America at over 2000 feet.

5 Weathering, Erosion, and Sedimentary Rocks

KEY CONCEPTS

By the end of this chapter, students will be able to:

Sedimentary rocks and the processes that create them (which include weathering, erosion, and lithification) are an integral part of understanding Earth Science. This is true because the majority of the Earth’s surface is made up of sedimentary rocks and their common predecessor, sediments. Even though sedimentary rocks can form in drastically different ways, the vast majority of their origin and creation has one thing in common: water. We will start our investigation into sediments by first discussing the unique and fortunate properties of water.

5.1 Water’s Unique Chemical Properties

The hydrogen atoms are on one side, about 105° apart.
A model of a water molecule, showing the bonds between the hydrogen and oxygen.
The water molecule, two hydrogen atoms covalently bonded to one oxygen atom, has a specific and important geometry. Both hydrogen atoms are located on one side of the oxygen molecule, separated by an angle of about 105° . It is this arrangement, with the positively-charged hydrogen on one side and negatively-charged oxygen on the other side, the water molecule has a property called polarity. Much like a battery or a magnet, the molecule’s architecture allows it to have a positive and negative side, and this leads to a whole suite of properties.

The water drops are sticking to a spider's web
Dew on a spider’s web.
Due to its polarity, water has the ability to stick to other objects. This ability is called adhesion. Water is also attracted to itself, which leads to water’s most common form in the air: a droplet. This ability is called cohesion. Cohesion is also responsible for water’s surface tension, which allows various insects to “walk” on water, distributing their weight across the water surface.

The positive side of the water molecule is attracted to the negative side of the water molecule
Hydrogen bonding between water molecules.
The fact that water, in the liquid state, can be attracted to itself also leads to an extremely rare occurrence in the natural world: Water is less dense as a solid than a liquid. This allows ice to float on water, and a whole suite of life forms that can survive under the ice in the frozen polar regions of our globe. Ice is less dense due to the fact that the molecules in liquid water can sit very close to one another. Ice, as it crystallizes, the molecules of water actually move apart as they fit into the crystal lattice, causing water to expand as it freezes. Even more critical for life is the fact that liquid water exists over a very large range of temperatures. This also is due to the attraction molecules feel toward each other in the liquid state. It may seem normal to us, but the wide range of 180 °F (32°F-212°F)/100 °C(0-100°F) for which water remains a liquid is also very rare. Without water’s high boiling point, the water world we know would be much more constricted to certain temperature zones, instead of nearly universal from pole to pole. Both of these properties of water result from the polarity of water and to a result of that, a special weak bonding called hydrogen bonds. This allows molecules of liquid water to be more closely spaced than water ions in ice, and also a reason for the fact that water can absorb high amounts of energy before boiling.

The negative part of the water molecules surrounds the positively-charged sodium ion.
A sodium (Na) ion in solution.
Finally, water is known as a universal solvent. In layman’s terms, this means water can and will dissolve more substances than any other common, natural substance. The polar sides on the water molecules, using chemical attractions, can basically pry atoms off of solid crystals. Some things, like salt, dissolve quickly. But eventually, even the strongest rocks and minerals can succumb to the power of water’s dissolving properties.

All of these properties together are integral to all sediments and sedimentary rocks, as will be explained in the rest of this chapter. Water is one of the main agents to create minerals within in-situ in chemical sedimentary rocks, and in forming the grains via weathering and erosion that become detrital sedimentary rocks.

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5.2 Weathering and Erosion

The Earth’s outer layer, the crust, is made of solid crystalline rock called bedrock, by geologists, is what you would find as one dug down into the Earth, past the soil and sediment, which are derived from previously existing bedrock in the past. The process that turns bedrock into sediment or soil is called weathering. Weathering is achieved by both mechanical and chemical means.

5.2.1 Mechanical Weathering

Mechanical weathering is a process that physically breaks apart bedrock into smaller pieces. Gravity and associated processes of mass wasting (see chapter 10), are the primary and most important ways that bedrock is broken apart. Below are other ways that bedrock is weathered mechanically.

Pressure Expansion

Granite rock has a relatively thin layer that is peeling away
The outer layer of this granite is fractured and eroding away, known as exfoliation
All bedrock, in the process of forming the rock in question via the rock cycle (see chapter 1. 6), is buried within the Earth. This bedrock is under higher pressure and temperature due to the depth it has traveled within the Earth. As this rock is brought to the surface, the temperature changes slowly, while the pressure changes immediately. The release in pressure causes the rock to expand, resulting in a series of cracks. This series of cracks can be referred to as sheeting. These cracks can mimic the land surface, especially in homogeneous rocks, resulting in the rock unloading sheets of rock, which is known as exfoliation. A further extension of exfoliation is spheroidal weathering, where these homogeneous rocks weather chemically more quickly along the joints and produce rounded erosional features.

Frost Wedging

A crack in a rock gets progressively bigger as ice freezes, prying the crack open over time.
The process of frost wedging
Frost wedging (also called ice wedging) uses the power of expanding ice to break apart rocks. Water can work its way into various cracks, voids, and crevices in rocks. At night, the water can freeze. As the water freezes, it expands with great force, enough that it may widen any weakness that existed before. The next morning, the water may melt, and the liquid water can now move even further into the space that was widened. This can happen over and over, night after night, eventually prying rocks apart.

Root Wedging

The roots of the tree are breaking up the asphalt.
The roots of this tree are demonstrating the destructive power of root wedging. Though this picture is a man-made rock (asphalt), it works on typical rock as well.
Similar to frost wedging, root wedging is the process in which plants work themselves into cracks, prying the bedrock apart as the roots grow. Rhizolith is the term for these roots preserved in the rock record . Other biological agents that do similar weathering include tunneling organisms such as earthworms, termites, and ants.

Salt Expansion

The rock has many holes from the salt erosion.
Tafoni from Salt Point, California.
Also similar to frost wedging, areas with high evaporation or near-marine environments can produce various salts that grow and expand, similar to ice, and produce and extend cracking in bedrock. They are also one of the causes of tafoni, which are a series of holes in a rock. Once a tafoni is started, it becomes a location of increased mechanical and chemical weathering. Hopper crystal is the term for a square shape preserved in rock, commonly made by salt.

5.2.2 Chemical Weathering

The left side has one large cube, the middle has 8 medium cubes, the right side has 64 small cubes. Each group has the same overall volume.
Each of these three groups of cubes has an equal volume. However, their surface areas are vastly different. On the left, the single cube has a length, width, and height of 4 units, giving it a surface area of 6(4×4)=48 and a volume of 4^3=64. The middle eight cubes have a length, width, and height of 2, meaning a surface area of 8(6(2×2))=8×24=96. They also have a volume of 8(2^3)=8×8=64. The 64 cubes on the right have a length, width, and height of 1, leading to a surface area of 64(6(1×1))=64×6=384. The volume remains unchanged, because 64(1^3)=64×1=64. The surface area to volume ratio (SA:V), which is related to the amount of material available for reactions, changes for each as well. On the left, it is 48/64=0.75 or 3:4. The center has a SA/V of 96/64=1.5, or 3:2. On the right, the SA:V is 384/64=6, or 6:1.

Chemical weathering dominates in warm, humid environments and is the process by which water, oxygen, and other reactants chemically break down components of bedrock into ions that dissolve into the water. This process goes hand-in-hand with mechanical weathering because of a fundamental concept called surface-area-to-volume ratio. Chemical weathering can only occur on the surface of the rock in question, not in the interior. As mechanical weathering breaks bedrock into smaller pieces each of which have surfaces, it creates more area for chemical weathering to occur, thus increasing the rate of chemical weathering overall. So, with more surface area, caused by mechanical weathering, chemical weathering is enhanced. In other words, a higher surface-area-to-volume ratio means a higher rate of overall weathering. Temperature also increases chemical weathering rates.

Carbonic Acid and Hydrolysis

The diagram on the left is before hydrolysis.
Generic hydrolysis diagram, where the bonds in mineral in question would represent the left side of the diagram.
Carbonic acid forms naturally when carbon dioxide (the 5th most common gas in the atmosphere) dissolves in water, having the formula H2CO3. It is important because of the reaction series called hydrolysis and dissolution. In hydrolysis, water molecules either ionize (turn into H+1 and OH−1) and replace cations (positive ions) in minerals, or, the H2CO3 molecule reacts with the mineral, especially minerals based on silicon and aluminum (like feldspars), to form clay minerals. This is the main process that breaks down silicate minerals and creates clay minerals. Hydrolysis also forms amorphous silica which can precipitate as chert nodules.

Dissolution

The rock is red.
In this rock, a pyrite cube has dissolved (as seen with the negative “corner” impression in the rock), leaving behind small specks of gold.

Dissolution is a reaction in which the mineral is dissolved and is now in a solution. Some minerals, such as evaporites (like salt) and carbonates (like calcite in limestone), are much more prone to this reaction, but all minerals are subject to dissolution. The natural acidic nature of water (due mostly to carbonic acid and free H+ ions) speeds up this reaction. Natural rainwater can be highly acidic, containing pH levels as low as 2 . Places that contain higher levels of acid, either naturally or man-made, can dissolve at a higher rate. Humid regions and places with more precipitation also have more dissolution.

The xenolith sits on top of a basalt rock. It has three sides like a pyramid; one of the sides is more altered to iddingsite.
This mantle xenolith containing olivine (green) is chemically weathering by hydrolysis and oxidation into the pseudo-mineral iddingsite, which is a complex of water, clay, and iron oxides. The more altered side of the rock has been exposed to the environment longer.

Even without acid present, water will naturally dissolve all minerals, though some at very slow rates. The natural rate at which minerals dissolve roughly mimics the Bowen’s Reaction Series (see Chapter 4), and is called the Goldich Dissolution Series . Minerals that form at conditions very dissimilar to the surface, namely high temperatures and pressures, are more likely to weather chemically. Minerals are more resistant (but not invincible) to dissolution if they formed at conditions more similar to the surface, namely lower temperature and pressure. Therefore, low-temperature quartz is very resistant to chemical weathering and high-temperature olivine and pyroxene are least resistant to weathering.

The rocks in this area are full of holes, formed from karst dissolution.
Eroded karst topography in Minevre, France.
Dissolution is also noteworthy for the special features that form because of it. In places with an abundance of carbonate bedrock which is susceptible to dissolution, a landscape is formed called karst topography, and is characterized by geographic features like sinkholes and caves. Dissolution can also have a biologic component, where organisms like lichen and bacteria aid in mineral dissolution with the release of organic acids, or even mineral components being used in metabolism.

Oxidation

Goethite is in cubes, though it usually is not. Pyrite is in cubes.
Pyrite cubes are oxidized, becoming a new mineral goethite. In this case, goethite is a pseudomorph after pyrite, meaning it has taken the form of another mineral.

Oxidation is the process that causes the rusting of metallic iron, but occurs geologically when iron within a mineral structure bonds with oxygen, and thus, changes the mineral’s formula. Any minerals that contain iron can be part of this reaction. Three new minerals are common results of this oxidation reaction: red or grey hematite, brown goethite (pronounced “GUR-tite”), and yellow limonite. These minerals are common cements binding grains together in sedimentary rocks that formed on the surface, and are found predominantly in the red rock colors of the strata on the Colorado Plateau, and famous protected lands within such as Zion, Arches, and Grand Canyon National Parks. This can permeate a rock that is rich in iron-bearing minerals, or can be a coating that forms in cavities or fractures. When these minerals replace existing bedrock, it commonly adds void space and weakness throughout, and often the oxidized minerals are weaker and softer.

5.2.3 Erosion

The rock is topped by a more resistant.
A hoodoo near Moab, Utah. The more resistant cap has protected the less resistant underlying layers.

Erosion is the process that moves sediment from the place of weathering. Water is also important in erosion, as it is the main agent of erosion. Wind, ice, and even gravity are also important in the transportation of sediments. See chapter 10 through chapter 14 for more details on these erosive processes.

The canyon has many cliffs and slopes.
Grand Canyon from Mather Point.
The primary adjective that is used in geologic studies of erosion is resistant. A rock that is more resistant to weathering and subsequent erosion will last longer than a less-resistant rock. This is perhaps best demonstrated in a place like the Grand Canyon. Any place in which a cliff forms indicates a resistant rock which is fighting erosion, and thus, making the cliff. Places with slopes are less resistant, and those rocks are being eroded more quickly. Erosion also creates features like hoodoos in places like Bryce Canyon National Park.

5.2.4. Soil

The soil is sketched and labeled.
Sketch and picture of soil.
Soil is formed at the transition between the biosphere and the geosphere. Soil is made as weathering breaks down bedrock and turns into sediment but does not move the sediment significantly by erosion, allowing life to access the mineral components they need. Soil is also a reservoir for organic material (e.g. nitrogen compounds) which are used by life. The organic material is a source for nitrogen, but not the only source in soils. Nitrogen is the most common element in the air, and yet, it is in a form that most like forms are unable to use. It is only special nitrogen-fixing bacteria (only found in soil) that can get the nitrogen usable by plants, and later, animals, and this is where the majority of nitrogen used by life comes from. That nitrogen is an essential component of proteins and DNA. Soil, in addition to mineral and organic material, also contains air and water.

The image shows the way nitrogen can move around, mostly in the soil
Schematic of the nitrogen cycle.
Soils range from poor to rich, that range being determined by the amount of organic matter contained within it called humus. Soil productivity is determined by nutrients in the soil: fresh volcanic soils (andisols) and clay-rich soils which prevent leaching of nutrients and water are examples of productive soils. Humus is decaying remains of plants and animals and represents a storehouse of nitrogen whose accessibility to other organisms is critical for the availability of nitrogen, and thus, sustainability of life on earth.

A mountain slope has been made into artificial steps form farming.
Agricultural terracing, as made by the Inca culture from the Andes, helps reduce erosion and promote soil formation, leading to better farming practices.
Topography, the nature of the parent material and how long the materials remain exposed to conditions of weathering and contributes to the nature of the soil. Where more active erosion on higher angle slopes removes weathering products more rapidly, soils may be thin.  Lower angle topography in valleys promotes thicker soil development. One of the most important determinant of soil formation is climate.  Temperature and precipitation exert the greatest influence on the nature of the soil.  Finally, the plants and animals that inhabit the soil contribute organic matter to the soil, and contribute to the ability of soil to sustain life.  Within the soil are fungi and bacteria and plant roots interact with them to exchange nitrogen and other nutrients .

The image shows 5 soil layers, ranging from highly altered at the top, to unaltered at the bottom.
A simplified soil profile, showing labeled layers.
Where soils are well formed, the processes of formation causes a noticeable sequence called a soil profile. Within a soil profile, there are discernible layers which are known as a soil horizon .  While the completeness of the profile in a given setting depends on climate, topography, and the other factors of soil development, these layers reflect the processes involved (see figure). A series of letters are commonly assigned to the different layers. The following is a simplified lettering, though many differences occur in these naming schemes depending on soil type or topic of research. The top layer, O, is a thin layer of only organic material composed of leaves, twigs, and other plant matter in the process of decaying into humus. The next layer, A, also known as the topsoil, is organic material mixed with minor mineral material. As precipitation soaks down through this material, it leaches out soluble chemicals. In wetter climates, this zone is recognizable and is referred to as the zone of leaching or eluviation.  Materials removed from upper layers are carried downward and deposited in a zone of accumulation referred to as the B layer, or subsoil.  The subsoil is the zone where parent rock is starting to be chemically weathered and grades down into unaltered parent rock.  The upper part of this, is called the regolith. This layer is a porous layer of decreasing humus mixed with chemically-weathered rock.  Below this is saprolite, and it is less weathered than the regolith and more like the bedrock. Soil profiles can often be seen in road cuts where recent removal of slope material has exposed the soil at the top of the cut. The next layer, C, is also known as the substratum, is a layer where the bedrock has been broken apart, but has not been chemically altered. The final layer, R, is very minor to unweathered bedrock that has little signs of surficial processes.

The outside of the rock is tan and weathered, the inside is grey.
A sample of bauxite. Note the unweathered igneous rock in the center.
Soil types are separated as soil orders in the United States by the USDA, have a taxonomic classification similar to organisms. There are many soil orders, and they include: Oxisols (also known as laterite), which can create valuable bauxite deposits, home to most of the world’s mineable aluminum. Ardisol forms in dry climates and can lead to caliche layers, a hard build up of calcite. Andisols are productive due to their high nutrient content originating in volcanic ash. In general, color can be an important factor in understanding soil conditions. Black soils tend to be anoxic, red oxygen-rich, and green oxygen-poor (i.e. reduced). This is true for many sedimentary rocks as well.

The black and white photo shows a giant wall of dust.
A dust storm approaches Stratford, Texas in 1935.
Soils are not only especially precious to terrestrial life, but also to human civilization, as agriculture is dependent on them. Careless or uninformed human activity has done considerable damage to soils.  A prime example is the famous midwestern United States Dust Bowl of the 1930s. The damage occurred with an attempt to turn the southern prairies of Kansas, Colorado, West Texas and Oklahoma into farmland .  The prairie soils and life of the region were well adjusted to the climate, but with government encouragement, settlers moved in to homestead there, plowed the prairie and planted fields of grain.  Loss of natural grass and straight furrows running downhill caused a loss of topsoil.  Lacking sufficient precipitation for the non-native crops, resulting in bare ground which began to be stripped of soil by the wind.  Dust storms called “black blizzards” made life unbearable, and the once hopeful homesteaders left in droves, a fact made famous by John Steinbeck’s book and John Ford’s film The Grapes of Wrath.  Poor planning on the part of government officials and poor understanding of the natural systems of the region led to poor farming practices that ruined the soils.  The prairie winds then removed what was left of them.  Particles of midwestern prairie soil were deposited along the east coast and were detected as far away as Europe.  The question that lingers is whether we have learned the lessons of the dust bowl, in order to avoid doing it again .

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5.3 Types of sedimentary rocks

Sedimentary rocks are classified  in two main categories. Clastic (or detrital) sedimentary rocks are made of pieces (sediment) derived from mechanical  weathering of bedrock. Chemical sedimentary rocks come directly from water via precipitation. These two categories are also classified in different ways, with clastic rocks being classified by grain shape, size, and sorting, and chemical rocks mainly by composition of minerals in the rock.

5.3.1 Lithification and Diagenesis

It shows two different rock types at different angles
Siccar point, showing the lithified layers that allowed James Hutton to ponder deep geologic time.
Lithification is the process of taking loose sediment (created by weathering and erosion) and turning it into a clastic sedimentary rock. It typically involves three interconnected steps. Deposition happens when the processes that transport sediment are overcome by friction and gravity, and thus, movement is no longer feasible, is the piling together of sediment into a grouping. Compaction occurs when sediment is squeezed together as it gets buried. It is aided in smaller sediments by weak attractive forces between grains. Cementation is when minerals, typically quartz, calcite, amorphous silica, or oxides, are used to glue sediments together into a fused rock. Depending on the depositional history, the cement may be very different in composition to the sediments.

Photo of log of petrified wood showing structures of the original wood
Permineralization in petrified wood
Diagenesis is the process, somewhat akin to a very mild metamorphism, where minerals can be altered, moved, and otherwise changed within a rock that is lithifying. A classic example is aragonite, a polymorph of CaCO3, reverts to calcite during burial alongside deposition, compaction, and cementation. Calcite also changes to dolomite (CaMg(CO3)2) under certain conditions. The processes of cementation, compaction, and ultimately lithification occur within the realm of diagenesis, and are roughly the same processes that turn organic material into fossils. Pore space, the amount of volume in a sedimentary rock which is not rock, can be reduced within diagenesis.

5.3.2 Clastic Sedimentary Rocks

Clastic sedimentary rocks, also known as detrital, are made from the pieces, namely sediment, that come from previously-existing bedrock. Clastic rocks are classified and named based on their grain size.

Grain Size

Chart with sizes ranging from clay to boulders
Size categories of sediments, known as the Wentworth scale.
Grain size is a measure of the average diameter of a piece of sediment, from fine grained (small) to coarse grained (big). The delineations of sediment sizes are based on a log-base 2 scale . Large clasts are called boulders, cobbles, granules, and gravel, defined as larger than 2 mm. Sand is defined as sediment between 2 mm and 0.0625 mm; with the smaller limit at the naked eye’s resolution. Particles just smaller than sand are called silt. Silt is unique in that it can be felt, with a finger or as grit between your teeth, but it is not seen. Clay is the smallest size class, being so small that typical minerals, like quartz and feldspar, do not even normally exist due to their tendency to chemically weather at that size. Interestingly, the term “clay” can not only refer to the size, but also to a group of minerals called clay minerals. They are layered sheet silicates similar to mica, but are chemically stable at small sizes. Famous examples include kaolinite, smectite, and montmorillonite. A sediment that has a mixture of silt and clay is referred to as mud. The common definition of mud, which includes water, is not part of the sedimentary definition.

Sorting and Rounding

The sediment on the left is all about the same size. The sediment on the right is many sizes.
A well-sorted sediment (left) and a poorly-sorted sediment (right).
Sorting describes the distribution of grain sizes within a sediment or sedimentary rock. Geologists use the term well sorted to refer to a sediment or rock with a very narrow range of sizes, and poorly sorted for a rock/sediment with a wide range of grain sizes (see figure) . Please note that soil engineers use similar terms in somewhat opposite ways, with well graded meaning lots of sizes, and poorly graded meaning a narrow range of sediment sizes. Sorting can be indicative of certain processes of transportation, erosion, grain size, and energy of deposition. For example, wind-blown sands are typically extremely well sorted, while glacial deposits are typically poorly sorted.

The sediments show various stages of rounding and sphericity, from high to low.
Degree of rounding in sediments. Sphericity refers to the spherical nature of an object, a completely different measurement unrelated to rounding.
Rounding is the amount which angular corners are removed from a piece of sediment, caused by weathering and erosion, due to abrasion. Since most mineral grains naturally start with some sharp edges, during transport, those sharp points would be worn away. Therefore, a more rounded sediment usually implies a longer time in erosional processes, longer distance travelled, or more energetic process from the source of the sediment, though the hardness of the clast is a factor as well. Rounding ranges from well-rounded (if free of all edges) to very angular (if the sediment maintains original points from formation).

Composition and provenance

The grain is round and has vesicles.
A sand grain made of basalt, known as a microlitic volcanic lithic fragment. Box is 0.25 mm. Top picture is plane-polarized light, bottom is cross-polarized light.

Composition describes the types of mineral or rock components found in a sediment or sedimentary rock. Besides clay, the components of most sediments can be easily determined visually (see chapter 3). Most commonly, this includes quartz, because of its ubiquitous nature in bedrock, low chemical reactivity, and high hardness. Other common sediments include feldspar and lithic fragments, which are pieces of fine-grained bedrock . This could include mud chips, volcanic clasts, or pieces of slate. Composition can also be influenced by local geology. Weathering products of volcanic rocks create the famous black and green sand beaches in Hawai’i. If only basalt or olivine are available as a source, then it is easy to make beach sand from that component. Normally, these components are too weak and are easily destroyed in sedimentary processes in favor for more durable clasts like quartz . The process of discerning where a sediment or sedimentary rock comes from is called provenance. Provenance can be important for describing tectonic history , paleogeography , or even reconstructing past supercontinents . Even in a pure quartz sandstone (sometimes called a quartz arenite), provenance is commonly found using a rare, but equally durable clast: zircon. Zircon is useful because it contains trace uranium, making age dating of the grains possible. This would not give the age of the sandstone, but would give the age ranges of different bedrock units that contributed to the sandstone.

Classification of Clastic Rocks

The rock is full of round rocks.
Conglomerate from the Carmelo Formation, Point Lobos, California.
The grey rock is broken and angular within the larger rock.
Megabreccia in Titus Canyon, Death Valley National Park, California.
Classification of clastic rocks is based on grain size . Rocks that have a predominant grain size larger than sand are classified as coarse-grained. These rock types are called conglomerates the grains are rounded due to abrasion, and breccia if the grains are angular. The part of these rocks that are smaller in size is usually called the groundmass or matrix, which act to hold larger clasts together. Both conglomerates and breccias are poorly sorted.

Windblown sand grains showing rounding and frosted surfaces due to transport b wind.
Enlarged image of frosted and rounded windblown sand grains
Medium grained rocks made mainly of sand are called sandstones, also sometimes referred to as arenites if well sorted. At times, they can have the main compositional component as a descriptive adjective (e.g. quartz sandstone). They have a wide variety of composition, roundness, sorting, and even size classes within sandstone. Sandstones with a noticeable feldspar component (>25%) are called called arkose. A term which has conflicting definitions is a greywacke . One version of greywacke is a sandstone with a muddy matrix, another is a sandstone with many lithic fragments.

The rock breaks apart in very thin layers.
The Rochester Shale, New York. Note the thin fissility in the layers.
Fine-grained mudstone is a general term for rocks that are chiefly made of sediments smaller than sand. If the rock is fissile, which means it separates in thin sheets, then it is called shale. Although more rare than mudstone or shale, if a rock is found to have only silt or clay, then it is called siltstone or claystone, respectively.

The light grey layers are very thin.
Claystone laminations from Glacial Lake Missoula.

Any of the above rock types, if found as a mixture between the main classifications listed, can use the name of the less-common component as an adjective. For example, a rock with some silt but more rounded gravel can be named a silty conglomerate; whereas a sand-rich rock with minor clay can be called a clayey sandstone.

5.3.3. Chemical, Biochemical, and Organic

Chemical, biochemical, and organic sedimentary rocks are formed in a very different manner than clastic rocks. Weathering and erosion are not directly involved with the process to make them. They are created where they are formed with no need for transport.

Inorganic/chemical

The ground is white and flat for a long distance.
Salt-covered plain known as the Bonneville Salt Flats, Utah.
Chemical (or inorganic) sedimentary rocks are formed by precipitation from solution. This commonly occurs when the water evaporates. At this point, various salts known as evaporites are produced. For example, the Bonneville Salt Flats in Utah, floods with winter rains and drys out every summer, leaving behind salts such as gypsum and halite. The order that evaporites deposit is in opposite order of their solubility, i.e. more soluble minerals will stay in solution longer. Though the entire sequence is not always naturally present, or in the correct proportions based on laboratory studies , the general order is:

Mineral(s) Percent Seawater remaining after evaporation
Calcite 50
Gypsum/anhydrite 20
Halite 10
Various potassium and magnesium salts 5

Table after .

The ooids are very smooth and round
Ooids from Joulter’s Cay, The Bahamas
The grey limestone towers vertically stick out of the ground.
Limestone tufa towers along the shores of Mono Lake, California.
Evaporite minerals generally precipitate with their specific crystal habits, but can precipitate in a variety of ways. The Great Salt Lake, also in Utah, is so salty that the mineral calcite chemically precipitates out as ooids (sand-sized spheres of concentric calcite growth) and tufa (chemical precipitation of calcite from springs in porous masses). Tufa towers have been exposed in the salty Mono Lake, California after the lake level dropped due to water diversions.

The white and brown natural steps show the formation of travertine.
Travertine terraces of Mammoth Hot Springs, Yellowstone National Park, USA

Cave deposits like stalactites and stalagmites are another common form of chemical precipitation of calcite, in a form called travertine. Travertine forms when calcite is slowly precipitated from water, often with banding. This process is similar to mineral growth on faucets from hard water in your home sink or shower. Travertine also forms at hot springs such as Mammoth Hot Spring in Yellowstone National Park.

The rock shows red and brown layering.
Alternating bands of iron-rich and silica-rich mud.
A chemical sedimentary rock that is no longer deposited, but was very common early in Earth’s history, is banded iron formation. These rocks are a result of the oxygenation of the atmosphere and the oceans, which caused iron to no longer be transported aqueously, but instead, iron was deposited in bands, alternating with chert.

The flint is dark brown/grey, and the weathered crust is light tan. The overall shape is blobby.
A type of chert, flint, shown with a lighter weathered crust.
Another common chemical sedimentary rock is chert, which is chemically-precipitated silica, SiO2, from groundwater. Although silica is very insoluble at the surface of earth (hence quartz being very resistant to chemical weathering), deep underground the higher pressures and temperatures allow for silica to be dissolved and re-precipitated often as cement or nodules. For example, the deposits around the base of geysers in Yellowstone National Park are silica deposits (called geyserite or sinter) from silica being dissolved by the hot water under the surface. In other cases, chert forms on the deep ocean floor from microscopic shells of radiolaria (zooplankton) and diatoms (phytoplankton). Chert has many synonyms, like jasper, flint, onyx, and agate, due to subtle differences in colors, striping, etc., but chert is a more general term used for the entire group by geologists.

Biochemical

Rock has many fossils throughout
Fossiliferous limestone (with brachiopods and bryozoans) from the Kope Formation of Ohio. Lower image is a section of the rock that has been etched with acid to emphasize the fossils.
Biochemical sedimentary rocks are not that different than chemical sedimentary rocks, since they still take elements dissolved in solution to form new minerals. Biochemical sedimentary rocks use biological processes to create hard parts such as shells. Most macroscopic marine organisms use calcium carbonate (in the form of aragonite) to build their living structures. When the organism dies, these hard parts can be incorporated in the geologic record (and act as a carbon sink). This is the main method of formation for the most common non-clastic sedimentary rock: limestone. Limestone is mostly made of the mineral calcite, CaCO3, but sometimes includes a close relative, dolomite (CaMgCO3). Calcite reacts  when hydrochloric acid (HCl) is applied to its surface. Dolomite only reacts when ground into a powder, usually done by scratching its surface . Entire coral reefs and their associated ecosystems can be preserved in exquisite detail in the rock record. In modern tropical locations, like the Bahamas, entire beaches of shell material can be found.

Rock is broken shells
Close-up on coquina.
There are many different types of limestone. Some of the more common forms include those that are more biologic in origin such as fossiliferous limestone (made of many visible fossils), micrite (microscopic calcite mud, with or without microfossils), coquina (composed of loosely-cemented shells and shell fragments), and chalk (high concentration of shells of a microorganism called coccolithophores). Some limestones, like oolites, are not biological in origin, but formed inorganically. These form when the solution is oversaturated with respect to calcite, similar to evaporite deposition discussed above.

Organic

It is very black and shiny.
Anthracite coal, the highest grade of coal.
With the right conditions, the actual organic material, or material that is derived from the original organic material, can be preserved in the geologic record. This material, though not technically a sediment or a sedimentary rock, usually is associated with sedimentary strata, and thus, will be mentioned here. Organic sedimentary “rocks” have been a highly important source of energy for human society. Any area in which organic material piled up could be concentrated enough that the viability of the resource could be used. This includes swamps, which create the conditions of coal formation, and highly productive shallow marine sediments, which create oil (petroleum) and natural gas. See chapter 16.2 for a more in-depth look at these energy sources.

Classification of Chemical Sedimentary Rocks

The rock has many light-colored layers.
Gyprock, a rock made of the mineral gypsum. From the Castle formation of New Mexico.
All chemical, biochemical, and organic sediments are classified based on their composition. Most are monomineralic, so simply identifying the mineral they contain can usually narrow down the rock name. For example, rocks with halite are called “rock salt” or by the same name of the mineral, halite. Rocks with unorganized silica, usually originating from ocean-floor oozes, are called chert. Limestones, rocks made from calcite, have the most elaborate subclassifications, with even two competing methods of classifying them in detail .

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5.4 Sedimentary Structures

Sedimentary structures are textures or arrangements of sediments within a rock that indicate some of the processes that made the rock and allow interpretations of the environment in which it formed. This is usually done by comparing modern environments that form sedimentary structures and comparing them to lithified counterparts. Below is partial discussion of sedimentary structures that are common and useful for interpretations in the rock record.

5.4.1. Bedding Planes

Photo of strata in Utah lying horizontal
Horizontal strata in southern Utah.

The simplest and most basic sedimentary structure are bedding planes, the lines that exist in sedimentary (and some volcanic) rocks and define strata. Each such bedding plane in the rock indicates a (sometimes subtle) change in the conditions of deposition. If the underlying layer had time to set, even that alone may be enough to cause a layer to form. As would be expected, the thickness of layers can indicate important information about quantity and timing of depositional events. Each layer is called a bed, or stratum, the most basic unit of stratigraphy, the study of sedimentary layering. 

Two students are looking at the layers of rock.
Students from the University of Wooster examine beds of Ordovician limestone in central Tennessee.
When a bedding plane is larger than 1 cm, it is called a bed (the smallest mappable unit); when it is smaller than 1 cm, it is called a laminae . When laminae or beds are placed down in a rhythmic, cyclical, and typically seasonal manner, they are called varves . Varves are important climatic histories, especially in lakes and glacial deposits.

5.4.2. Graded Bedding

Rock shows layers described.
Image of the classic Bouma sequence. A=coarse- to fine-grained sandstone, possibly with an erosive base. B=laminated medium- to fine-grained sandstone. C=rippled fine-grained sandstone. D=laminated siltstone grading to mudstone.
Graded bedding indicates a sequence of beds that either get coarser or finer. A famous example of this is a Bouma sequence , formed in off shore gravity flows called turbidites. As these subsea flows of sediment are channelized in submarine canyons  into the deeper ocean basins, it  loses energy below storm wave base and creates a coarse-to-fining upward sequence.

5.4.3. Flow Regime and Bedforms

There are 7 images of increasing velocity.
Bedforms from under increasing flow velocities.
In fluid systems (flowing water and wind, such as the Bouma sequence above), the easiest sediment to move, erode, and therefore deposit, is fine sand. Smaller particles, like silt and clay, actually are harder to erode and move. Even with faster flow that can pick up larger particles, the smaller silt and clay is chemically attracted to underlying sediment. Larger particles, obviously, require a faster flow to move them. With typical grain sizes and depths of flow, and as flow velocity (called flow regime) increases, sandy sediments take on certain characteristic sedimentary structures, called bedforms . There are two flow regimes, upper and lower, and those are further divided into upper, middle, and lower part of each flow regime. The table below shows the different flow regimes and their associated idealized bedforms. Reading from this table, the proper description would be, for example, “the upper part of the lower flow regime creates dunes.”

Flow Regime (part) Bedform Description
Lower (lowest) Plane bed Lower plane bed, flat laminations
Lower (lower) Ripples Small (with respect to flow) inclined layers dipping downflow
Lower (upper) Dunes Larger inclined cross beds, ±ripples, dipping downflow
Upper (lower) Plane bed Flat layers, can include lined-up grains (parting lineations)
Upper (upper) Antidunes Hard to preserve reverse dunes dipping shallowly upflow
Upper (uppermost) Chutes/pools (rare) Erosional, not really a bedform; rarely found preserved

Plane Beds

There are slight groves in the rock.
Subtle lines across this sandstone (trending from the lower left to upper right) are parting lineations.

Plane beds in the lower flow regime are similar to bedding planes, but on a smaller scale. They are flat, parallel layers formed as sandy sediment piles and moves on top of layers below. Even still fluids, such as sediments deposited in lakes, can produce plane bedsPlane beds in the upper flow regime, however, are a feature of fast-flowing fluids. Initially, they may look identical to their lower-flow-regime counterparts. However, their high amounts of sediment transport and fast flows typically result in parting lineations. These are slight alignments of grains in rows and swaths that only happens with upper flow regime plane beds.

Ripples

The sand has a steep side on the left of the ripple, and a more gentle slope on the right.
Modern current ripple in sand from the Netherlands. The flow creates a steep side down current. In this image, the flow is from right to left.
Ripples, also known as ripple marks, ripple cross beds or ripple cross laminations, are small ridges or undulations that have height into the flow path as the sediment piles on top of itself and climbs out from the plane bed. First scientifically described by Hertha Ayrton , they change shape based on the type of flow, from straight-crested, sinuous, to more complex.

This brown rock has symmetry in its ripples.
A bidirectional flow creates this symmetrical wave ripple. From rocks in Nomgon, Mongolia. Note the crests of the ripples have been eroded away by subsequent flows in places.

They can be asymmetrical when formed in an unidirectional flow, or symmetrical with an oscillating back-and-forth flow typical of the intertidal swash zone.

The ripples are on top, slightly offset, from each other.
Climbing ripple deposit from India.
Ripples can also be laid out on top of themselves when sedimentation is high, and these are called climbing ripples. Even though the scale of these are typically on the centimeter scale, large flows, like glacial lake outbursts, can produce ripples on the scale of 20 meters high.

Dunes

The mountain has a large variety of angles of beds, resulting from dunes moving in all directions.
Lithified cross-bedded dunes from the high country of Zion National Park, Utah. The complexity of bedding planes results from the three-dimensional network of ancient dune flows.
Dunes, which are typically what is referred to when referring to large cross bedding, are a larger and more prominent version of ripples . In the case of dunes, the features are large enough to be a significant player in the overall flow of the fluid. The word dune probably first conjures ideas of desert sand dunes, which are included in this category. In channelized flow, like in rivers, dunes are most likely the most common sedimentary structure found within the channel. The biggest difference between river dunes and air-formed dunes is the depth of fluid.

The red dune sand is rippled on one side (the steep side) and smooth on the other.
Modern sand dune in Morocco.
Since the atmosphere’s depth is immense when compared to the ground, it can create much taller dunes than the typical river flow, and can lead to famous dune landscapes in places like the Sahara, Death Valley, and the Gobi desert When dunes form, sediment piles up onto the lee side of the dune, as the flow brings the sediment up. The lee side is typically a more shallow angle than the stoss side, which has grains falling down. This is clearly seen within a cross bed, and can indicate the direction of flow in the past. Dunes typically have two styles: the more rare planar cross beds, and the more common trough cross beds that have a curved surface.

The flow is to the left on the bottom, and the right on the top.
Herringbone cross-bedding from the Mazomanie Formation, upper Cambrian of Minnesota.
In tidal locations with strong flows, dunes can develop in opposite directions as the tide goes in and out. This produces a feature called herringbone cross bedding.

The up and down waves are famous from hummocky-cross stratification.
Hummocky-cross stratification, seen as wavy lines throughout the middle of this rock face. Best example is just above the pencil in the center.

Another variant on dune formation occurs with very strong (hurricane-size) storms agitate parts of the seafloor that are usually undisturbed. These are called hummocky cross stratification, and have a 3D architecture of hills and valleys, with inclined and declined layering to match that shape.

Antidunes

The large waves are in place.
Antidunes forming in Urdaibai, Spain.
Antidunes are called “anti” because they have similar characteristics, but are formed in an opposite manner to dunes. As the flow increases, sediment are added upstream, causing the feature to subtly dip upstream, instead of downstream like their dune counterparts. They also form in phase with the flow, and can be seen as rapids on the fluid (river) surface. Because of the high speed of flow of formation, and common erosion at those speeds, antidunes are much more rare in the rock record.

5.4.4. Bioturbation

There are several ovals and lines representing places where organisms crawled through the sediment.
Bioturbated dolomitic siltstone from Kentucky.
Bioturbation is the action of burrowing organisms through soft sediments that gets preserved in the rocks. This happens more commonly in shallow environments, typically in marine settings. It disrupts bedding throughout the rock. Different types of bioturbation can indicate water depth .

5.4.5. Mudcracks

The cracks are in several directions, forming squares, triangles, and other polygonal shapes.
Lithified mudcracks from Maryland.
Mudcracks occur when clay-rich sediments are underwater and swell as water fills voids within the clays’ structures. Later, when the sediments are no longer submerged, and the clays dry out, they shrink and form polygonal cracks , with tapered openings toward the surface as seen in profile. They are also a major source of mud chips, small fragments of mud (and later shale) that are common inclusions in sandstones and conglomerates. What makes this sedimentary structure so important is that only certain depositional environments are alternately underwater and exposed to air. Similar but rare syneresis cracks occur with shrinking of clays subaqueously .

5.4.6. Sole Marks

The bulge is sticking out of a rock layer above the head of the observer.
This flute cast shows a flow direction toward the upper right of the image, as seen by the bulge sticking down out of the layer above. The flute cast would have been scoured into a rock layer below that has been removed by erosion, leaving the sandy layer above to fill in the flute cast.

Sole marks are small features found at the base of one overlying bed (and therefore at the top of an underlying bed), typically in river deposits. They can indicate several things about the deposition at the time. Flute casts or scour marks are grooves carved out by the force of the flow and sediment load of the fluid . They are typically steep upstream and shallow downstream.

The rock is filled with narrow, parallel ridges.
Groove casts at the base of a turbidite deposit in Italy.
Similarly formed, but more regular and aligned are groove casts. Tool marks are carvings made by objects like sticks carried in the fluid downstream.

The drill core is cylindrical.
A drill core showing a load cast showing light-colored sand sticking down into dark mud.

Load casts, an example of soft-sediment deformation, are small indentations made by a stronger, coarser sediment intruding into a more water-laden, softer, and finer sediment below .

5.4.7. Raindrop Impressions

This grey rock has round circles left by raindrops
Mississippian raindrop impressions over wave ripples from Nova Scotia.
Like the name implies, raindrop impressions are small pits or bumps found in soft sediments. While it is generally believed rainfall causes them, other things like gas bubbles escaping from the sediments may also cause them .

5.4.8. Imbrication

The rocks in this conglomerate are tilted, leaning toward the right.
Cobbles in this conglomerate are positioned in a way that they are stacked on each other, which occurred as flow went from left to right.
Imbrication is a stack of usually flat and large clasts (cobbles, gravels, etc.) that are aligned in the direction of flow. It can be stacking, with the clasts oriented dipping down flow, or it can be an alignment of the long axes of the clasts, which can be statistically shown to align with flow . These are common for determining paleocurrents, or past currents found in the geologic past, especially in alluvial deposits.

5.4.9. Geopetal Structures

Line is horizontal in picture as well.
This bivalve (clam) fossil was partially filled with tan sediment, partially empty. Later fluids filled in the fossil with white calcite minerals. The line between the sediment and the later calcite is paleo-horizontal.

Geopetal structures , also called way-up indicators, are a series of geological items that can tell the investigator which way was up in the past. This is especially important in places where the bed of rock has been tilted or overturned. Examples are listed below:

  • Vugs – Small voids in rocks. These are usually later filled by diagenesis, and if partially filled or filled in stages, can act as a permanent level bubble.
  • Cross bedding – In places where ripples or dunes pile on top of one another, where one cross bed interrupts and/or cuts another below, this shows a cross-cutting relationship that indicates up direction. Sometimes the ripple itself is preserved well enough to see the crest and trough and differentiate between them.
  • This footprint of a dinosaur is three toes.
    Eubrontes trace fossil from Utah, showing the geopetal direction is within the image, toward the reader.
    Vesicles – Lava flows degas upwards, and can show more vesicles toward the top of the flow.
  • Fossils – Both body fossils in life position (e.g. corals) and trace fossils like footprints can give an up direction.
  • Several of the sedimentary structures listed above, when preserved well, can give an up direction, including mudcracks, sole marks, and raindrop impressions.

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5.5 Depositional Environments

Many different environments are representative environments from high elevation to deep under water.
A representation of common depositional environments.
The ultimate goal of many studies into sedimentary rocks is to describe the depositional environment. Knowing if rocks were formed high in a mountain glacier, on a gentle floodplain leading to the ocean, in a dry desert, or at the bottom of the ocean, can give great insight into painting a picture of the past. The study of depositional environments is a complex endeavor, and below is a quick and simplified version of what to look for in the rock record.

5.5.1. Marine

Marine environments are completely submerged by ocean water at all times. They are largely dependent on the depth of water for their depositional style, with some notable exceptions that will be covered.

Abyssal

The thickness is low in the abyssal plain.
Marine sediment thickness. Note the lack of sediment away from the continents.

Abyssal rocks formed on the abyssal plain, which is the relatively flat ocean floor. Some of the only topography that does exist along the abyssal plain includes abyssal hills, small (100 m – 20 km) features possibly related to extension Most of the abyssal plain is far from any significant source of fluid movement, and thus, the rocks here are most very fine grained . These sediments typically fall in three categories. First, there are calcareous oozes, which consist of calcite plankton shells that have fallen to the ocean floor. An example of this type of sediment chalk. Second are siliceous oozes, which also are made of plankton debris, but in this case, the shells are from organisms that use silica or hydrated silica to create their shells.

The rock is powdery and white.
Diatomaceous earth
Diatomaceous Earth are deposits from these oozes, and they usually occur below the calcite compensation depth, a depth where the solubility of calcite increases, so that only silica-based shells remain. Chert is another common rock formed from these types of sediments. The third sediment type is pelagic clay. This is very fine grained clay which falls through the water column very slowly, and is typically brown or red. Pelagic clay occurs in remote open ocean, where even plankton are not common enough to accumulate.

The canyon allows stacking of these deposits on the ocean floor.
Turbidites inter-deposited within submarine fans.
One notable exception to the fine-grained nature of abyssal sediments are turbidite and submarine fan deposits . Submarine fans occur at the base of large river systems and emerge from submarine canyons on continental shelves. These are initiated during times of low sea level as rivers cut through the continental shelf. As sea levels return, and sediment accumulates normally on the shelf, typically in deltas, and sediments periodically flush down the canyon in large gravity-induced events called turbidites. The fan is a network of turbidites deposited together due to a decrease in slope (much like what happens at alluvial fans and deltas). This process transports relatively coarse material to the ocean floor where they are uncommon. It is also the typical origin of Bouma sequences.

Continental Slope

The deposit is a large, dipping pile of sediment
Contourite drift deposit imaged with seismic waves.

Continental slope deposits are not very common or distinctive in the rock record. The most notable type of rock that forms at the slope area are contourites . Contourites are deposits formed from deep-water currents on the slope or even deeper. These create smooth drifts and swaths of sediment that have many different architectures, even interwoven with turbidites.

Lower shoreface

The diagram shows that wavebase is 1/2 the wavelength of waves of water.
Diagram describing wavebase.

Lower shoreface is the part of the ocean which is below the depth of normal wave agitation and therefore is not subject to the day-to-day winnowing and deposition associated with waves, but are affected by larger storms, such as hurricanes. Lower shoreface sediments are typically finely laminated, and may contain hummocky cross-stratification .

Upper shoreface

The image shows the many complexities of the shoreline described in the text.
Diagram of zones of the coastline.
Upper shoreface are sediments that are within the zone of wave action, but below the beach environment. They are usually very well sorted fine sand and have planar beds (lower part of the upper flow regime) as their main sedimentary structure, but can also contain cross bedding from longshore currents .

5.5.2. Transitional coastline environments

Onlap is sediments moving toward the land. Offlap is moving away.
The rising sea levels of transgressions create onlapping sediments, regressions create offlapping.
Transitional environments, more often called shoreline or coastline environments, are the complex interactions where ocean water hits the land, mostly occurring on top of the continental shelf. Because these environments are often on the continent, the preservation potential is very high. This environment is also very important for hydrocarbon deposits, and has spawned its own branch of geology called sequence stratigraphy. Sequence stratigraphy is the study of changing shoreline deposits and their 3-dimensional architecture as sea level changes. The main force at work in shoreline deposits is the rising and falling of sea level, both in the daily work of tides and the changes in sea level that come from changes in climate and plate tectonics. Transgression is the geologic term for a steady sea level rise, relative to the shoreline. Regression is the opposite, a relative drop in sea level to shore. Below are common components of shoreline deposits. For a more in-depth look at these environments, see Chapter 12.

Littoral

The tan rock has dark streaks of minerals.
Lithified heavy mineral sand (dark layers) from a beach deposit in India.
The littoral zone, better known as the beach, is famous for having highly weathered (i.e. generally mostly quartz), homogeneous, well-sorted sand without any sedimentary structures. This is due to the extreme energy that is involved with the constant bombardment the surf delivers to the beach. Yes, there are black sand and other beaches that have unique characteristics, but those are the exceptions, not the rule. In fact, some beach sands (both in the past and currently) are so highly evolved, that a tool has been developed to discern this called the ZTR (zircon, tourmaline, rutile) index . The ZTR index is higher in more weathered beaches, because these relatively rare minerals can be concentrated in beach sands due to their resistance to weathering, even to the point where harvesting them can be economically viable. Beach processes have a multitude of ways in which they move this sediment around. Some beaches have such high sediment supply that even dunes can develop nearby.

Tidal Flats

The tidal flat it a network of channels.
General diagram of a tidal flat and associated features.
Tidal flats, or mud flats, are areas with sufficient tidal range that are flooded and drained regularly, about twice daily. Tidal flats can have large areas of fine grained sediment, but also contain coarser sands. As the tidal water washes in sediment, sometimes flow can be focused though a small opening, called a tidal inlet. These flows are typically gradual, and can have multi-directional ripple marks. In areas that have concentrated flow, the grain size is typically coarser, with dunes present. This is called a tidal channel. Mudcracks are also common within tidal flats due to the exposure to air .

Reefs

The fold is a long ridge.
Waterpocket Fold, Capitol Reef National Park, Utah.
The term reef, which most people would immediately imagine tropical coral reefs, are not only made by living things. Natural buildups of sand or rock can also create reefs, similar to barrier islands. Therefore, a reef is any topographically-elevated feature on the continental shelf, oceanward from the beach, but separated from the beach. A more general definition extends to non-ocean features that are topographic barriers, and inspired the name of Capitol Reef National Park, with the Waterpocket Fold acting as the reef.

The reef has many intricacies.
A modern coral reef.
Even with non-living examples, the majority of reefs currently and in the geologic past are biologic in origin . Coral reefs are important in the geologic record due to their growing habit. Corals are animals, yet they need sunlight because of the photosynthetic algae called zooxanthellae that live with them symbiotically and provide nourishment. In places with rapid subsidence, corals continue to grow up, around, and through sediments, all the while holding in place, and thus preserving, the geologic record around them.

The reef is offshore of the island proper.
The light blue reef is fringing the island of Vanatinai. As the island erodes away, only the reef will remain, forming a reef-bound seamount.
Sediments found in coral reefs are typically fine-grained carbonates between the coral skeletons due to the fact that even silty or clayey water can inhibit the growth of the animals. Inorganic reef structures are much more variable in their composition. Reefs of both types have a big impact on deposition in lagoons. Reefs naturally act as wave and storm buffers and allow fine-grain sediments to accumulate.

The map shows locations.
Seamounts and guyots in the North Pacific.
Reefs are commonly found around shorelines and islands, especially in tropical locations. Reefs are also found around features known as seamounts. When an ocean island erodes away, its base still exists just below the waves. Reefs can still occupy these features long after the rocks of the island are below the sea. Eventually, the seamount can turn into a coral-ringed atoll if the reef completely rings and encircles the seamount. When further subsidence or erosion make the coral no longer able to reach the surface, it is called a guyot. Examples include the Emperor Seamounts, formed by the Hawaiian Hotspot.

Lagoon

The lagoon is just inside the coastline.
Kara-Bogaz Gol lagoon, Turkmenistan.

Lagoons are small bodies of water inland from the shore or other geographic feature, such as a reef or barrier island. Because they are disconnected from the strength of wave action, and instead are flooded by seawater, they typically have very fine grained sediments . Lagoonal areas, as well as similar features like estuaries, can have very high biological productivity, and commonly have bioturbation or even coal forming as a result. Salt flats (also known as sabkhas) and sand dune fields can also develop in lagoons at or above the high tide line, where evaporation exceeds water input.

Deltas

The river, as it flows north and enters the sea, spreads out.
The Nile delta, in Egypt.
Deltas, which are named after the greek letter, are triangular-shaped bodies of sediment that form where rivers enter the ocean or other quiet body of water . All rivers have a slope, and this is partly where the speed of the water comes from. Where the river enters the ocean, now it has zero slope, and the speed instantly stops. This lowers the carrying capacity of the river, resulting in less ability to carry sediment. A pile of coarse to fine sand results, and this forms the delta. As one part of the delta becomes overwhelmed by sediment, soon another part is relatively advantageous to receive the flow and its sediment. It switches back and forth, over and over, resulting in the triangular shape. Some deltas have a more prominent shape, where the river has the greatest energy, other times the tides or the waves can be a bigger factor, and can reshape the delta in various ways.

5.5.3. Terrestrial

Terrestrial depositional environments are diverse. Water is still a big part of their deposition, even when frozen, or when the lack of water is a factor.

Fluvial

The river wiggles back and forth.
The Cauto River in Cuba. Note the sinuosity in the river, which is meandering.
Fluvial (or river) systems are formed from flowing channelized water over the land. They generally come in two main varieties: meandering or braidedMeandering channels have the flow of water concentrated in a slow, swooping channel in which most of the coarse sediment is found. Away from the channel are mostly fine grained materials that are only deposited in large overflow events.

The river has many inter-braided channels.
The braided Waimakariri river in New Zealand.
These areas are known as flood plains. Braided fluvial systems are more overall composed of coarser sediments, and form a complicated series of switching channels . Both of these will be discussed further in chapter 11.

Alluvial

This broad valley in the desert has alluvial deposition.
An alluvial fan spreads out into a broad alluvial plain. From Red Rock Canyon State Park, California.

Alluvial sediments are most distinctive in their intermittent flow of water. Alluvial sediments are common in arid places with little soil development, and are the primary basin filling rock found in valleys throughout the Basin and Range in the western United States. The most distinctive alluvial sedimentary deposit is the alluvial fan, a large cone of sediment out of dry mountain valleys. Alluvial sediments are typically poorly sorted and coarse grained, and are often nearby playa lakes or aeolian deposits . For more information on alluvial deposits in deserts, see chapter 13.

Lacustrine

The mountain has a large hole in the center that is filled with the lake.
Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama.
Lake systems and deposits, called lacustrine, are somewhat similar to marine deposits, just on a much smaller scale. There are a wide variety of causes of lacustrine deposits, including tectonics (e.g. Lake Baikal), volcanic (e.g. Crater Lake, Oregon), glacial (e.g. Great Lakes), pluvial (increased precipitation in climate swings; e.g. ancient Lake Bonneville), or fluvial (e.g. oxbow lakes). In general, lacustrine sediments are very fine grained and thinly laminated, with only minor wind-blown, current, and tidal deposits . When lakes dry out, or evaporation outpaces precipitation, playa lakes form. Their deposits are similar to normal lakes, but with more evaporite minerals. Certain tidal flats can have playa-type deposition as well.

Paludal

Paludal is deposition in bogs, marshes, swamps, or other wetlands. These typically form near coastal environment, but are also common in humid, low-lying, low-latitude, warm zones with large fluxes of water. One of the most characteristic depositional features is organic-rich peat, which can convert into coal under geologic conditions. Paludal environments typically are associated with tidal, deltaic, lacustrine, and/or fluvial deposition.

Aeolian

The chart has the way dunes are made and four dune types.
Formation and types of dunes.

Aeolian, sometimes spelled eolian or œolian, are wind blown sediments. They form potentially large dunes in areas with proper sediment influx, not only in dry conditions. Since wind has a much lower carrying capacity than water, typically aeolian sediments are fine sand and dust at the coarsest . Bagnold (1941) considered only Barchan and linear Seif dunes as the only true dune forms.  Other workers have recognized transverse and star dunes as well as parabolic and linear dunes anchored by plants that are common in coastal areas as other types of dunes. Fine silt and clay can cross very long distances, even entire oceans suspended in air. This accumulation of wind-blown sediment is known as loess, which can commonly start as a glacial sediment, and is common in the midwestern United States. For more information on aeolian deposits in deserts, see chapter 13.

Glacial

Large boulders and smaller sand are seen together.
Wide range of sediments near Athabaska Glacier, Jasper National Park, Alberta, Canada.
Glacial sedimentation is very diverse, and in general, they are known as some of the most poorly-sorted sediments in existence. The main glacial rock type is called a diamictite, which literally means “two sizes,” referring to the larger and smaller grains common in glacial deposits and not other rocks . Many glacial tills (glacially derived diamictites) contain very finely-pulverized rock flour along side giant erratic boulders. Larger clasts typically do have striations from glacial rubbing, scraping, and polishing of surfaces by abrasion. Glaciers are so large and produce so much sediment, that they create their own individualized sedimentary assemblages, complete with fluvial, deltaic, lacustrine/pluvial, alluvial, and/or aeolian sedimentation. See chapter 14 for a more detailed look at glacial processes.

5.5.4. Facies

Photo of the Grand Canyon showing expanse of canyon and the various rock layers
The Grand Canyon of Arizona has many facies, mostly in the marine realm.

In a sequence of rocks, such as the Grand Canyon, many different depositional environments may be seen: beach sands, offshore muds, farther offshore limestones. The rocks formed in these different contemporaneous environments are called facies [zotpressInText item=”{5UX65ZW2}” format=”%num%” brackets=”yes”]. Facies represent different rocks grouped by similar characteristics. As relative sea level changed and the sea transgressed over the land with the shoreline shifting inland, the offshore lithofacies or stratigraphic facies were deposited later on top of the beach facies. In a transgressive event, the sequence of lateral facies is repeated vertically, meaning the beach facies might be repeated several times in a vertical rock column. Fossils of life forms present in these different environments and characteristic of them are then biologic facies. It takes considerable geologic time for transgressive processes to happen and the vertical distribution of biologic facies may show evolution and evolving fossil assemblages. Such evolving fossil assemblages enhance the correlation of the sequence to the Geologic Time Scale. Facies can also be based on composition or organic material.

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Summary

Sedimentary rocks are formed in two main categories: chemical, which are precipitated from an aqueous solution, and detrital, which are made out of sediment. Sediment is formed as bedrock weathers into smaller pieces, either by chemical or mechanical means. The sediment can then be moved via erosion, deposited, and then lithified into a detrital sedimentary rock. Chemical sedimentary rocks are defined based on their composition (e.g. limestone is made out of calcium carbonate), while detrital sedimentary rocks are defined mostly by their grain size (e.g. sandstone is made out of sand-sized particles). Sedimentary rocks also have sedimentary structures, which give insights on depositional histories. For example, moderate channelized flows make cross bedding, and mudcracks from after a mud was underwater, then exposed to air. Depositional environments take the lithologies and sedimentary structures together to place the rock into a greater paleogeographic context.

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References