5 Weathering, Erosion, and Sedimentary Rocks

Learning Objectives

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

  • Describe how water is an integral part of all .
  • Explain how chemical and turn into .
  • Differentiate the two main categories of sedimentary rocks: rock formed from pieces of weathered ; and chemical rock that precipitates out of by organic or inorganic means.
  • Explain the importance of sedimentary structures and analysis of , and how they provide insight into the Earth’s history.

and the processes that create it, which include , , and , are an integral part of understanding Earth Science. This is because the majority of the Earth’s surface is made up of sedimentary rocks and their common predecessor, . Even though sedimentary rocks can form in drastically different ways, their origin and creation have one thing in common, water.

5.1 The Unique Properties of Water

Simple diagram of a water molecule with the letter O at the center and an individual letter H branching out on either side of the O. One letter H is to to the lower left of the letter O with a line connecting them and the other letter H is to the lower right of the letter O with a line connecting them. The angle between the two connecting lines is 104.45 degrees. Along one of the connecting lines is the label "95.84 pm."
Figure 5.1: A model of a water molecule, showing the bonds between the hydrogen and oxygen.

Water plays a role in the formation of most . It is one of the main agents involved in creating the in rock. It also is a and agent, producing the grains that become . Several special properties make water an especially unique substance, and integral to the production of and .

The water molecule consists of two hydrogen atoms covalently to one oxygen atom arranged in a specific and important geometry. The two hydrogen atoms are separated by an angle of about 105 degrees, and both are located to one side of the oxygen atom. This atomic arrangement, with the positively charged hydrogens on one side and negatively charged oxygen on the other side, gives the water molecule a property called . Resembling a battery or a magnet, the molecule’s positive-negative architecture leads to a whole suite of unique properties.

Spider web with detailed square structure. Water droplets stick to the web.
Figure 5.2: Dew on a spider’s web.

allows water molecules to stick to other substances. This is called . Water is also attracted to itself, a property called , which leads to water’s most common form in the air, a droplet. is responsible for creating surface , which various insects use to walk on water by distributing their weight across the surface.

3D model of four water molecules attaching to a single water molecule in the center; in each water molecule, the single larger oxygen atom is colored red with two smaller gray hydrogen atoms attached; two water molecules attach to the center one from a positively charged hydrogen atom to a negatively charged oxygen atom and two water molecules attach to the center one from a negatively charged oxygen atom to a positively charged hydrogen atom.
Figure 5.3: Hydrogen bonding between water molecules.

The fact that water is attracted to itself leads to another important property, one that is extremely rare in the natural world—the liquid form is denser than the solid form. The of water creates a special type of weak called . allow the molecules in liquid water to sit close together. Water is densest at 4°C and is less dense above and below that . As water solidifies into ice, the molecules must move apart in order to fit into the crystal lattice, causing water to expand and become less dense as it freezes. Because of this, ice floats and water at 4oC sinks, which keeps the oceans liquid and prevents them from freezing solid from the bottom up. This unique property of water keeps Earth, the water planet, habitable.

Diagram with a sodium atom at the center and six water molecules surrounding the sodium atom. The center atom is gray, labeled "Na+". The surrounding molecules each have one red oxygen and two gray hydrogen atoms with the Greek symbol delta labeling each atom; the hydrogen atoms each have delta+ while the oxygen atoms which all face the center sodium atom each have delta-.
Figure 5.4: A sodium (Na) ion in solution.

Even more critical for supporting life, water remains liquid over a very large range of temperatures, which is also a result of . Hydrogen allows liquid water can absorb high amounts of energy before turning into vapor or gas. The wide range across which water remains a liquid, 0°C-100°C (32°F-212°F), is rarely exhibited in other substances. Without this high boiling point, liquid water as we know it would be constricted to narrow zones on Earth, instead water is found from pole to pole. Further, water is the only substance that exists in all three phases, solid, liquid, and gas in Earth’s surface environments.

Water is a , meaning it dissolves more substances than any other commonly found, naturally occurring liquid. The water molecules use and to pry ions away from the crystal lattice. Water is such a powerful solvent, it can even the strongest rocks and given enough time.

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5.2 Weathering and Erosion

refers to the solid rock that makes up the Earth’s outer . is a process that turns into smaller particles, called . includes pressure expansion, , , and salt expansion. Chemical includes and , , and .

is a mechanical process, usually driven by water, wind, gravity, or ice, which transports (and ) from the place of . Liquid water is the main agent of . Gravity and processes (see chapter 10) move rocks and to new locations. Gravity and ice, in the form of (see chapter 14), move large rock fragments as well as fine .

resistance is important in the creation of distinctive geological features. This is well-demonstrated in the cliffs of the Grand Canyon. The cliffs are made of rock left standing after less resistant materials have weathered and eroded away. Rocks with different levels of resistance also create the unique-looking features called hoodoos in Bryce Canyon National Park and Goblin Valley State Park in Utah.

5.2.1 Mechanical Weathering

physically breaks into smaller pieces. The usual agents of are pressure, , freezing/thawing cycle of water, plant or animal activity, and salt evaporation.

Pressure Expansion

A layer of broken slabs of rock lying on top of a larger landscape of domed rock. The rocks are reddish-tan and gray.
Figure 5.5: The outer layer of this granite is fractured and eroding away, known as exfoliation.

buried deep within the Earth is under high pressure and . When uplift and brings to the surface, its drops slowly, while its pressure drops immediately. The sudden pressure drop causes the rock to rapidly expand and crack; this is called pressure expansion. Sheeting or is when the rock surface spalls off in layers. is a type of that produces rounded features and is caused when moves along in the .

Frost Wedging

Water seeps into cracks and fractures in rock. When the water freezes, it expands about 9% in volume, which wedges apart the rock. With repeated freeze/thaw cycles, rock breaks into pieces.
Figure 5.6: The process of frost wedging.

, also called , uses the power of expanding ice to break apart rocks. Water works its way into various cracks, voids, and crevices. As the water freezes, it expands with great force, exploiting any weaknesses. When ice melts, the liquid water moves further into the widened spaces. Repeated cycles of freezing and melting eventually pry the rocks apart. The cycles can occur daily when fluctuations of between day and night go from freezing to melting.

Root Wedging

Roots of a tree are visible rising and breaking through asphalt.
Figure 5.7: 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.

Like , happens when plant roots work themselves into cracks, prying the apart as they grow. Occasionally these roots may become fossilized. is the term for these roots preserved in the rock record. Tunneling organisms such as earthworms, termites, and ants are biological agents that induce similar to .

Salt Expansion

Tan rock with many holes resembling honeycomb with crashing ocean waves in the background.
Figure 5.8: Tafoni from Salt Point, California.

Salt expansion, which works similarly to , occurs in areas of high evaporation or near- environments. Evaporation causes salts to out of and grow and expand into cracks in rock. Salt expansion is one of the causes of , a series of holes in a rock. Tafonis, cracks, and holes are weak points that become susceptible to increased . Another phenomena that occurs when salt water evaporates can leave behind a square imprint preserved in a soft , called a hopper crystal.

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.
Figure 5.9: 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)=96 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=192. 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 96/64=0.75 or 3:2. The center has a SA/V of 192/64=1.5, or 3:1. On the right, the SA:V is 384/64=6, or 6:1.

is the dominate process in warm, humid environments. It happens when water, oxygen, and other reactants chemically degrade the components of and turn them into water-soluble ions which can then be transported by water. Higher temperatures accelerate rates.

Chemical and work hand-in-hand via a fundamental concept called surface-area-to-volume ratio. only occurs on rock surfaces because water and reactants cannot penetrate solid rock. penetrates , breaking large rocks into smaller pieces and creating new rock surfaces. This exposes more surface area to , enhancing its effects. In other words, higher surface-area-to-volume ratios produce higher rates of overall .

Carbonic Acid and Hydrolysis

Diagram showing hydrolysis; on the left hand side are two molecules, one of which is water; on the right hand side are two new molecules, resulting after water has broken the first molecule's chemical bond.
Figure 5.10: Generic hydrolysis diagram, where the bonds in mineral in question would represent the left side of the diagram.

(H2CO3) forms when carbon dioxide, the fifth-most abundant gas in the , dissolves in water. This happens naturally in clouds, which is why is normally slightly acidic. is an important agent in two reactions, and .

occurs via two types of reactions. In one reaction, water molecules ionize into positively charged H+1 and OH−1 ions and replace in the crystal lattice. In another type of , molecules react directly with , especially those containing silicon and aluminum (i.e. ), to form molecules of clay .

is the main process that breaks down rock and creates clay . The following is a reaction that occurs when silica-rich encounters to produce water-soluble clay and other ions:

+ (in water) → clay + metal (Fe++, Mg++, Ca++, Na+, etc.) + bicarbonate (HCO3-1) + silica (SiO2)

Clay are platy or phyllosilicates (see chapter 3) similar to micas, and are the main components of very fine-grained . The substances may later into rocks like and , as well as amorphous silica or nodules.

Dissolution

Chunk of brick red rock with white crystals; tiny gold flecks can be seen in void spaces of the rock; approximately 5 centimeters in size.
Figure 5.11: In this rock, a pyrite cube has dissolved (as seen with the negative “corner” impression in the rock), leaving behind small specks of gold.

is a reaction that dissolves in and leaves the ions in , usually in water. Some and , like salt and , are more prone to this reaction; however, all can be . Non-acidic water, having a neutral pH of 7, will any , although it may happen very slowly. Water with higher levels of acid, naturally or man-made, dissolves rocks at a higher rate. Liquid water is normally slightly acidic due to the presence of and free H+ ions. Natural rainwater can be highly acidic, with pH levels as low as 2. can be enhanced by a biological agent, such as when organisms like lichen and bacteria release organic acids onto the rocks they are attached to. Regions with high humidity (airborne moisture) and experience more due to greater contact time between rocks and water.

Photograph of a piece of basalt with a xenolith on top, sitting on a black and white scale with inches on the left and centimeters on the right. The xenolith consists of olive-green crystals and the basalt is gray-black. The entire sample is approximately 1.5 inches long.
Figure 5.12: 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.

The shows rates are associated to rankings in the (see chapter 4). at the top of the Bowen series crystallize under high temperatures and pressures, and chemically at a faster rate than ranked at the bottom. , a that crystallizes at 700°C, is very resistant to . High -point , such as and (1,250°C), relatively rapidly and more completely. and are rarely found as end products of because they tend to break down into elemental ions.

White-gray cliffside with numerous caves and holes throughout; the base and top of the cliffs are covered in green vegetation.
Figure 5.13: Eroded karst topography in Minerve, France.
Cream-colored rock formation attached to a cave ceiling that resembles a human heart.
Figure 5.14: A formation called The Great Heart of Timpanogos in Timpanogos Cave National Monument.

is also noteworthy for the special geological features it creates. In places with abundant , can produce a topography characterized by sinkholes or caves (see chapter 10).

Timpanogos Cave National Monument in Northern Utah is a well-known feature. The figure shows a cave created from followed by with seeped into the cavern, where evaporation caused the to out.

Oxidation

Chunk of dull tan rock with two dark brown glassy cubic crystals growing from it.
Figure 5.15: 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.

, the chemical reaction that causes rust in iron, occurs geologically when iron atoms in a with oxygen. Any containing iron can be oxidized. The resultant iron may permeate a rock if it is rich in iron . may also form a coating that covers rocks and grains of , or lines rock cavities and . If the are more susceptible to than the original , they may create void spaces inside the rock mass or hollows on exposed surfaces.

Three commonly found are produced by iron- reactions: red or grey hematite, brown goethite (pronounced “GUR-tite”), and yellow limonite. These iron coat and bind grains together into sedimentary rocks in a process called , and often give these rocks a dominant color. They color the rock layers of the Colorado Plateau, as well as Zion, Arches, and Grand Canyon National Parks. These can permeate a rock that is rich in iron-bearing or can be a coating that forms in cavities or . When the replacing existing in are resistant to , iron concretions may occur in the rock. When is replaced by weaker , this process commonly results in void spaces and weakness throughout the rock mass and often leaves hollows on exposed rock surfaces.

5.2.3 Erosion

Tower of brick red rock with visible horizontal layers; the tower thins upward except for a larger, flat rock capping the top of the tower.
Figure 5.16: A hoodoo near Moab, Utah.

is a mechanical process, usually driven by water, gravity, (see chapter 10), wind, or ice (see chapter 14) that removes from the place of . Liquid water is the main agent of .

Canyon with many cliffs and slopes throughout; there is a visible band of whitish rock near the top of the canyon walls and the slopes are dark gray to black. The rest of the cliffs range in color from red to tan to brown.
Figure 5.17: Grand Canyon from Mather Point.

resistance is important in the creation of distinctive geological features. This is well demonstrated in the cliffs of the Grand Canyon. The cliffs are made of rock left standing after less resistant materials have weathered and eroded away. Rocks with different levels resistant also create the unique-looking features called hoodoos in Bryce Canyon National Park and Goblin Valley State Park in Utah.

5.2.4 Soil

Black and white cross sectional sketch of soil profiles: the top layer has plants on top with shallow roots and is filled in with small dots, labeled A; the layer below has small dots and larger grains drawn, labeled B; the layer below has more larger grains clustered along the base of the layer and also small dots throughout labeled C, and the bottom layer has a brick pattern labeled C. On the right of the sketch is a color photograph of those soil profiles in real life, with brown to tan soil matching each layer of the sketch.
Figure 5.18: Sketch and picture of soil.

is a combination of air, water, , and organic matter that forms at the transition between and . is made when breaks down and turns it into . If does not remove the significantly, organisms can access the content of the . These organisms turn , water, and atmospheric gases into organic substances that contribute to the .

is an important for organic components necessary for plants, animals, and microorganisms to live. The organic component of , called , is a rich source of bioavailable nitrogen. Nitrogen is the most common in the , but it exists in a form most life forms are unable to use. Special bacteria found only in provide most nitrogen compounds that are usable, bioavailable, by life forms.

A schematic diagram representing the nitrogen cycle; it consists of interconnected arrows and labeled boxes, showcasing the various stages and transformations of nitrogen. The cycle begins with atmospheric nitrogen (N2), which is converted into ammonium (NH4+) through nitrogen fixation, represented by an arrow pointing through nitrogen-fixing soil bacteria in the subsurface. The ammonium then undergoes nitrification, shown by arrows leading through nitrifying bacteria in the ground to a box labeled Nitrites (NO2-) and an arrow from that box leading through more nitrifying bacteria leading to a box labeled Nitrates (NO3-). From there, the nitrates are taken up by plants, indicated by an arrow labeled Assimilation, pointing towards a labeled plants box. The nitrogen can then move through the food chain as organisms consume the plants, depicted by an arrow leading to a rabbit. Plants and animals that die are consumed by Decomposers, represented by arrows from the Plants box and rabbit drawing that leads into the ground to a box labeled Decomposers. Denitrification, represented by an arrow leading from Nitrates, through denitrifying bacteria, and pointing back to atmospheric nitrogen, completes the cycle.
Figure 5.19: Schematic of the nitrogen cycle.

These nitrogen-fixing bacteria absorb nitrogen from the and convert it into nitrogen compounds. These compounds are absorbed by plants and used to make DNA, amino acids, and enzymes. Animals obtain bioavailable nitrogen by eating plants, and this is the source of most of the nitrogen used by life. That nitrogen is an essential component of proteins and DNA.  range from poor to rich, depending on the amount of they contain. productivity is determined by water and nutrient content. Freshly created , called andisols, and clay-rich that hold nutrients and water are examples of productive .

A mountain slope has been made into artificial steps for farming, covered in low green vegetation.
Figure 5.20: Agricultural terracing, as made by the Inca culture from the Andes, helps reduce erosion and promote soil formation, leading to better farming practices.

The nature of the , meaning its characteristics, is determined primarily by five components: 1) the mineralogy of the parent material; 2) topography, 3) , 4) climate, and 5) the organisms that inhabit the . For example, tends to erode more rapidly on steep slopes so layers in these areas may be thinner than in flood plains, where it tends to accumulate. The quantity and chemistry of organic matter of affects how much and what varieties of life it can sustain. and , two major agents, are dependent on climate. Fungi and bacteria contribute organic matter and the ability of to sustain life, interacting with plant roots to exchange nitrogen and other nutrients.

In well-formed , there is a discernable arrangement of distinct layers called . These soil horizons can be seen in road cuts that expose the layers at the edge of the cut. Soil horizons make up the . Each reflects climate, topography, and other soil-development factors, as well as its organic material and . The horizons are assigned names and letters. Differences in naming schemes depend on the area, type or research topic. The figure shows a simplified that uses commonly designated names and letters.

Block diagram of soil horizons: the top layer goes from 0 inches at the surface down to 2 inches into the ground and has plants on top; the layer is dark brown in color, labeled Organic: O. The layer below goes from 2 to 10 inches depth and has abundant plant roots branching down through the layer; the layer is gray in color, labeled Surface: A. The layer below goes from 10 to 30 inches depth and has fewer, deeper roots branching through the layer and also contains sparse larger rock clasts; the layer is dark tan in color, labeled Subsoil: B. The layer below goes from 30 to 48 inches depth and does not contain any plant roots but does have more abundant rock clasts than the layer above; the layer is light tan in color, labeled Substratum: C. The bottom layer goes below 48 inches depth and only consists of rock clasts; the layer is dark gray to black in color, labeled Bedrock: R.
Figure 5.21: A simplified soil profile, showing labeled layers.

O Horizon: The top horizon is a thin layer of predominantly organic material, such as leaves, twigs, and other plant parts that are actively decaying into .

A Horizon: The next layer, called , consists of mixed with . As soaks down through this layer, it leaches out soluble chemicals. In wet climates with heavy this leaching out produces a separate layer called horizon E, the leaching or eluviation zone.

B Horizon: Also called , this layer consists of mixed with removed from the upper layers. The is where is chemically weathered. The amount of organic material and degree of decrease with depth. The upper zone, called , is a porous mixture of and highly weathered . In the lower zone, saprolite, scant organic material is mixed with largely unaltered .

C Horizon: This is and is a zone of . Here, fragments are physically broken but not chemically altered. This layer contains no organic material.

R Horizon: The final layer consists of unweathered, parent and fragments.

Chunk of heavily weathered tan and porous rock that has unweathered elongated gray and black crystals in the center.
Figure 5.22: A sample of bauxite. Note the unweathered igneous rock in the center.

The United States governing body for agriculture, the USDA, uses a taxonomic classification to identify types, called orders. Xoxisols or laterite are nutrient-poor found in tropical regions. While poorly suited for growing crops, xosisols are home to most of the world’s mineable aluminum (). Ardisol forms in dry climates and can develop layers of hardened , called caliche. Andisols originate from deposits. Alfisols contain clay . These two orders are productive for farming due to their high content of nutrients. In general, color can be an important factor in understanding conditions. Black tend to be anoxic, red oxygen-rich, and green oxygen-poor (i.e. reduced). This is true for many sedimentary rocks as well.

Black and white photo of a few houses on flat land along with two people standing near a house; a giant wall of dust is seen approaching in the background of the photo.
Figure 5.23: A dust storm approaches Stratford, Texas in 1935.

Not only is essential to life in nature, but also human civilization via agriculture. Careless or uninformed human activity can seriously damage ’s life-supporting properties. A prime example is the famous Dust Bowl disaster of the 1930s, which affected the midwestern United States. The damage occurred because of large-scale attempts develop prairieland in southern Kansas, Colorado, western Texas, and Oklahoma into farmland. Poor understanding of the region’s geology, ecology, and climate led to farming practices that ruined the .

The prairie and plants are well adapted to a relatively dry climate. With government encouragement, settlers moved in to homestead the region. They plowed vast areas of prairie into long, straight rows and planted grain. The plowing broke up the stable and destroyed the natural grasses and plants, which had long roots that anchored the layers. The grains they planted had shallower root systems and were plowed up every year, which made the prone to . The plowed furrows were aligned in straight rows running downhill, which favored and loss of .

The local climate does not produce sufficient to support non- grain crops, so the farmers drilled wells and over-pumped water from the underground . The grain crops failed due to lack of water, leaving bare that was stripped from the ground by the prairie winds. Particles of midwestern prairie were deposited along the east coast and as far away as Europe. Huge dust storms called black blizzards made life unbearable, and the once-hopeful homesteaders left in droves. The setting for John Steinbeck’s famous novel and John Ford’s film, The Grapes of Wrath, is Oklahoma during this time. The lingering question is whether we have learned the lessons of the dust bowl, to avoid creating it again.

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5.3 Sedimentary Rocks

is classified into two main categories: and chemical. or sedimentary rocks are made from pieces of , , derived primarily by . rocks may also include chemically weathered . rocks are classified by grain shape, , and . Chemical sedimentary rocks are from water with . Chemical rocks are classified mainly by of in the rock.

5.3.1 Lithification and Diagenesis

On the right hand side is a series of dark brown rock layers that are vertical. As you look toward the left, the vertical layers are capped by gently-dipping reddish brown sedimentary layers. The ocean is seen in the background.
Figure 5.24: Geologic unconformity seen at Siccar Point on the east coast of Scotland.

turns loose grains, created by and transported by , into via three interconnected steps. happens when friction and gravity overcome the forces driving transport, allowing to accumulate. occurs when material continues to accumulate on top of the layer, squeezing the grains together and driving out water. The mechanical is aided by weak attractive forces between the smaller grains of . typically carries cementing agents into the . These , such as , amorphous silica, or , may have a different than the grains. is the process of cementing coating the grains and gluing them together into a fused rock.

The rock is in the shape of a large tree log with red and yellow minerals running throughout the rock. The surrounding landscape are gray and tan badlands.
Figure 5.25: Permineralization in petrified wood.

is an accompanying process to and is a low- form of rock (see chapter 6). During , are chemically altered by heat and pressure. A classic example is aragonite (CaCO3), a form of calcium that makes up most organic shells. When lithified aragonite undergoes , the aragonite reverts to (CaCO3), which has the same chemical formula but a different crystalline structure. In containing and magnesium (Mg), may the two into dolomite (CaMg(CO3)2). may also reduce the space, or open volume, between grains. The processes of , , and ultimately occur within the realm of , which includes the processes that turn organic material into .

5.3.2 Detrital Sedimentary Rocks (Clastic)

or sedimentary rocks consist of preexisting pieces that comes from weathered bedrock. Most of this is mechanically weathered sediment, although some clasts may be pieces of chemical rocks. This creates some overlap between the two categories, since sedimentary rocks may include chemical . or rocks are classified and named based on their .

Grain Size

Chart with the following columns from left to right: phi, mm, Fractional mm and Decimal Inches, Size Terms, Sieve Sizes, Intermediate diameters of natural grains equivalent to sieve size, Number of grains per mg, Settling Velocity (Quartz, 30 degrees C), and Threshold Velocity for traction cm/sed. Grain sizes are delineated using a log base 2 scale; the grain sizes in the boulder class are larger than 10.1 inches; the grain sizes in the cobble class are 2.52 to 10.1 inches; the grain sizes in the pebble class are 2.52 to 0.08 inches, which include the size terms very coarse, coarse, medium, fine, and very fine granules;  the grain sizes in the sand class are 1 mm to 1/16 mm, which include the size terms very coarse, coarse, medium, fine, and very fine; the grain sizes in the silt class are 1/16 mm to 1/256 mm, which include the size terms coarse, medium, fine, and very fine; and the grain sizes in the clay class are 1/256 mm to 1/1024 mm, which do not have specific size terms.
Figure 5.26: Size categories of sediments, known as the Wentworth scale.

rock is classified according to , which is from large to small on the Wentworth scale (see figure 5.26). is the average diameter of fragments in or rock. Grain sizes are delineated using a log base 2 scale. For example, the grain sizes in the pebble class are 2.52, 1.26, 0.63, 0.32, 0.16, and 0.08 inches, which correlate respectively to very coarse, coarse, medium, fine, and very fine granules. Large fragments, or clasts, include all grain sizes larger than 2 mm (5/64 in). These include, boulders, cobbles, granules, and gravel. Sand has a between 2 mm and 0.0625 mm, about the lower limit of the naked eye’s resolution. grains smaller than sand are called silt. Silt is unique; the grains can be felt with a finger or as grit between your teeth, but are too small to see with the naked eye.

Sorting and Rounding

The sediment on the left is all about the same size. The sediment on the right is many sizes.
Figure 5.27: A well-sorted sediment (left) and a poorly-sorted sediment (right).

describes the range of grain sizes within or . Geologists use the term “well sorted” to describe a narrow range of grain sizes, and “poorly sorted” for a wide range of grain sizes (see figure 5.27). It is important to note that engineers use similar terms with opposite definitions; well consists of a variety of grain sizes, and poorly has roughly the same grain sizes.

When reading the story told by rocks, geologists use to interpret or transport processes, as well as energy. For example, wind-blown sands are typically extremely well sorted, while deposits are typically poorly sorted. These characteristics help identify the type of process that occurred. Coarse-grained and poorly sorted rocks are usually found nearer to the source of , while fine are carried farther away. In a rapidly flowing mountain you would expect to see boulders and pebbles. In a lake fed by the , there should be sand and silt deposits. If you also find large boulders in the lake, this may indicate the involvement of another transport process, such as caused by ice- or root-wedging.

Series of three rows that each contain five drawn grain shapes; the top row is labeled High sphericity and contains rounded to more square-shaped grains while the bottom row is labeled Low sphericity and contains elongated to oval-shaped grains; the columns toward the left are labeled Angular and have pointed, jagged edges along each grain edge, while the columns toward the right are labeled Rounded and have smoothed, rounded corners along each grain edge.
Figure 5.28: Degree of rounding in sediments. Sphericity refers to the spherical nature of an object, a completely different measurement unrelated to rounding.

is created when angular corners of rock fragments are removed from a piece of due to abrasion during transport. Well-rounded grains are defined as being free of all sharp edges. Very angular retains the sharp corners. Most clast fragments start with some sharp edges due to the ’s crystalline structure, and those points are worn down during transport. More rounded grains imply a longer time or transport distance, or more energetic erosional process. is also a factor in .

Composition and Provenance

Two microscopic photos of the same sand grain made of basalt with a photo length of 0.25 mm. The top picture is plane-polarized light and shows a tan grain with numerous highlighted holes throughout it while the bottom is cross-polarized light and shows a black grain with numerous highlighted elongated grains throughout it.
Figure 5.29: 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.

describes the components found in or and may be influenced by local geology, like and hydrology. Other than clay, most components are easily determined by visual inspection (see chapter 3). The most commonly found is because of its low chemical reactivity and high , making it resistant to , and its ubiquitous occurrence in . Other commonly found grains include and lithic fragments. Lithic fragments are pieces of fine-grained , and include , clasts, or pieces of .

of produces Hawaii’s famous black () and green () sand beaches, which are rare elsewhere on Earth. This is because the local rock is almost entirely of and provides an abundant source of dark colored clasts loaded with mafic minerals. According to the Goldich Dissolution Series, clasts high in are more easily destroyed compared to clasts of like .

Beach cove with green sand and numerous people; surrounding the cove are black layers of volcanic rock.
Figure 5.30: Hawaiian beach composed of green olivine sand from weathering of nearby basaltic rock.

Geologists use to discern the original source of or . is determined by analyzing and types of present, as well as textural features like and . is important for describing history, visualizing paleogeographic , unraveling an area’s geologic history, or reconstructing past .

In , sometimes called (SiO2), may be determined using a rare, durable clast called (ZrSiO4). , or zirconium , contains traces of uranium, which can be used for age-dating the source that contributed to the lithified rock (see chapter 7).

Classification of Clastic Rocks

Cliff face alongside a curving gravel road; numerous large, angular dark gray boulder fragments are embedded throughout the white cliffside rock.
Figure 5.31: Megabreccia in Titus Canyon, Death Valley National Park, California.

rocks are classified according to the of their . Coarse-grained rocks contain clasts with a predominant larger than sand. Typically, smaller grains, collectively called or matrix, fill in much of the volume between the larger clasts, and hold the clasts together. are rocks containing coarse rounded clasts, and breccias contain angular clasts (see figure 5.31). Both and breccias are usually poorly sorted.

Close-up photo of unconsolidated amber-colored glassy grains with rounded edges; a scale bar at the lower right says 1.0 mm.
Figure 5.32: Enlarged image of frosted and rounded windblown sand grains.

Medium-grained rocks mainly of sand are called , or sometimes if well sorted. grains in can having a wide variety of compositions, roundness, and . Some names indicate the rock’s . contains predominantly grains. is with significant amounts of , usually greater than 25%. that contains , which weathers more quickly than , is useful for analyzing the local geologic history. Greywacke is a term with conflicting definitions. may refer to with a muddy matrix, or with many lithic fragments (small rock pieces).

Outcrop of gray rocks with a slope of broken rocks leading up to a vertical cliff of very thinly layered rocks; a group of six people stand at the top of the slope inspecting the rocks at the base of the cliff.
Figure 5.33: The Rochester Shale, New York. Note the thin fissility in the layers.

Fine-grained rocks include , , , and . is a general term for rocks made of grains smaller than sand (less than 2 mm). Rocks that are , meaning they separate into thin sheets, are called . Rocks exclusively of silt or clay , are called or , respectively. These last two rock types are rarer than or .

Light tan to gray chalky-looking rock cliff face with extremely thin laminations visible; a rock hammer leans against the outcrop for scale.
Figure 5.34: Claystone laminations from Glacial Lake Missoula.

Rock types found as a mixture between the main classifications, may be named using the less-common component as a descriptor. For example, a rock containing some silt but mostly rounded sand and gravel is called silty . Sand-rich rock containing minor amounts of clay is called clayey .

5.3.3 Chemical, Biochemical, and Organic

rocks are formed by processes that do not directly involve and . may contribute the materials in water that ultimately form these rocks. and organic are in the sense that they are made from pieces of organic material that is deposited, buried, and lithified; however, they are usually classified as being chemically produced.

Inorganic rocks are made of from ions in , and created without the aid of living organisms. Inorganic rocks form in environments where concentration, gasses, temperatures, or pressures are changing, which causes to crystallize.

sedimentary rocks are formed from shells and bodies of underwater organisms. The living organisms extract chemical components from the water and use them to build shells and other body parts. The components include aragonite, a similar to and commonly replaced by , and silica.

Organic sedimentary rocks come from organic material that has been deposited and lithified, usually underwater. The source materials are plant and animal remains that are transformed through burial and heat, and end up as , , and methane ().

Inorganic Chemical

A flat white expanse with a mountain range in the distant background.
Figure 5.35: Salt-covered plain known as the Bonneville Salt Flats, Utah.

Inorganic rocks are formed when out of an aqueous , usually due to water evaporation. The form various salts known as . For example, the Bonneville Salt Flats in Utah flood with winter rains and dry out every summer, leaving behind salts such as and . The order of deposit is opposite to their solubility order, i.e. as water evaporates and increases the concentration in , less soluble out sooner than the highly soluble . The order and percentages are depicted in the table, bearing in mind the process in nature may vary from laboratory derived values.

Mineral sequence Percent seawater remaining after evaporation
Calcite 50%
Gypsum/anhydrite 20%
Halite 10%
Various potassium and magnesium salts 5%

Table 5.1: Deposition order and saturation percentages.

Zoomed-in photo of a cluster of pearly white smooth, rounded grains; a scale bar at the lower right says 0.50 mm.
Figure 5.36: Ooids from Joulter’s Cay, The Bahamas.
Whitish gray limestone towers stick vertically out of the ground, resembling thin spires with rough rounded sides.
Figure 5.37: Limestone tufa towers along the shores of Mono Lake, California.

Calcium water precipitates porous masses of called . can form near degassing water and in saline lakes. Waterfalls downstream of springs often as the turbulent water enhances degassing of carbon dioxide, which makes less soluble and causes it to . Saline lakes concentrate calcium from a combination of wave action causing degassing, springs in the lakebed, and evaporation. In salty Mono Lake in California, towers were exposed after water was diverted and lowered the lake levels.

White and brown natural steps with steam rising near the back; the brown steps have a coating of liquid and organic material while the white steps are dry and chalky in appearance.
Figure 5.38: Travertine terraces of Mammoth Hot Springs, Yellowstone National Park, USA.

Cave deposits like stalactites and stalagmites are another form of chemical of , in a form called . slowly precipitates from water to form the , which often shows . This process is similar to the growth on faucets in your home sink or shower that comes from hard ( rich) water. also forms at hot springs such as Mammoth Hot Spring in Yellowstone National Park.

Zoomed-in photo of a slice of rock, showing red and brown layers with glittering dots throughout; a scale bar at the lower left says 5.0 mm.
Figure 5.39: Alternating bands of iron-rich and silica-rich mud, formed as oxygen combined with dissolved iron.

deposits commonly formed early in Earth’s history, but this type of rock is no longer being created. Oxygenation of the and oceans caused free iron ions, which are water-soluble, to become oxidized and out of . The iron was deposited, usually in alternating with layers of .

The flint is black, and the weathered crust around it is light tan. The overall shape is blobby, resembling the shape of a potato.
Figure 5.40: A type of chert, flint, shown with a lighter weathered crust.

, another commonly found rock, is usually produced from silica (SiO2) from . Silica is highly insoluble on the surface of Earth, which is why is so resistant to . Water deep underground is subjected to higher pressures and temperatures, which helps silica into an aqueous . As the rises toward or emerges at the surface the silica precipitates out, often as a cementing agent or into nodules. For example, the bases of the geysers in Yellowstone National Park are surrounded by silica deposits called geyserite or sinter. The silica is in water that is thermally heated by a relatively deep source. can also form biochemically and is discussed in the subsection. has many synonyms, some of which may have gem value such as jasper, flint, onyx, and agate, due to subtle differences in colors, striping, etc., but is the more general term used by geologists for the entire group.

Zoomed-in photo of a cluster of dull tan smooth, rounded grains, with a slightly larger star-shaped grain laying in the cluster as well; a scale bar at the lower right says 1.50 mm.
Figure 5.41: Ooids forming an oolite.

are among the few forms created by an inorganic chemical process, similar to what happens in . When water is oversaturated with , the precipitates out around a nucleus, a sand grain or shell fragment, and forms little spheres called (see figure 5.41). As evaporation continues, the continue building concentric layers of as they roll around in gentle currents.

Biochemical

Two photos of a piece of limestone full of small fossils; the top photo shows the top of the rock which is white to gray in color; the bottom photo shows a cross section of the rock which is tan to brown in cluster and filled with numerous layers of fossils; a scale bar at the lower right of the two photos says 1 cm.
Figure 5.42: 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.

sedimentary rocks are not that different from rocks; they are also formed from ions in . However, sedimentary rocks rely on biological processes to extract the materials out of the water. Most macroscopic organisms use , primarily aragonite (calcium ), to build hard parts such as shells. When organisms die the hard parts settle as , which become buried, compacted and cemented into rock.

This extraction and secretion is the main process for forming , the most commonly occurring, non- . is mostly made of (CaCO3) and sometimes includes dolomite (CaMgCO3), a close relative. Solid reacts with hydrochloric acid by effervescing or fizzing. Dolomite only reacts to hydrochloric acid when ground into a powder, which can be done by scratching the rock surface (see chapter 3).

Close-up photo of fragments of light tan broken shells; a scale bar at the lower right says 1.0 cm.
Figure 5.43: Close-up on coquina.

occurs in many forms, most of which originate from biological processes. Entire coral and their ecosystems can be preserved in exquisite detail in rock (see figure 5.42). contains many visible . A type of called originates from beach sands made predominantly of shells that were then lithified. is of loosely-cemented shells and shell fragments. You can find beaches like this in modern tropical environments, such as the Bahamas. contains high concentrations of shells from a microorganism called a coccolithophore. , also known as microscopic mud, is a very fine-grained containing microfossils that can only be seen using a microscope.

Biogenetic forms on the deep , created from made of microscopic organic shells. This , called ooze, may be calcareous (calcium based) or siliceous (silica-based) depending on the type of shells deposited. For example, the shells of radiolarians (zooplankton) and diatoms (phytoplankton) are made of silica, so they produce siliceous ooze.

Organic

Chunk of black, very shiny rock.
Figure 5.44: Anthracite coal, the highest grade of coal.

Under the right conditions, intact pieces of organic material or material derived from organic sources, is preserved in the geologic record. Although not derived from , this lithified organic material is associated with sedimentary and created by similar processes—burial, , and . C Deposits of these fuels develop in areas where organic material collects in large quantities. Lush swamplands can create conditions conducive to . Shallow-water, organic material-rich can become highly productive and deposits. See chapter 16 for a more in-depth look at these -derived energy sources.

Classification of Chemical Sedimentary Rocks

Cross sectional view of a rock with alternating thicker white and thinner brown layers.
Figure 5.45: Gyprock, a rock made of the mineral gypsum. From the Castle formation of New Mexico.

In contrast to , chemical, , and organic sedimentary rocks are classified based on . Most of these are monomineralic, of a single , so the rock name is usually associated with the identifying . rocks consisting of are called rock salt. Rocks made of () is an exception, having elaborate subclassifications and even two competing classification methods: Folk Classification and Dunham Classification. The Folk Classification deals with rock grains and usually requires a specialized, petrographic microscope. The Dunham Classification is based on rock , which is visible to the naked eye or using a hand lens and is easier for field applications. Most geologists use the Dunham system.

Inorganic clastic sedimentary rocks include Conglomerate, Breccia, Sandstone, Siltstone, and shale. They all have classic (fragmental) texture and a composition of mostly quartz, feldspar, and clay minerals; may contain fragments of other rocks and minerals. Conglomerate: rounded fragments; grain size are pebbles, cobbles, and/or boulders in a matrix of sand, silt, and/or clay. Breccia: angular fragments; grain size are pebbles, cobbles, and/or boulders in a matrix of sand, silt, and/or clay. Sandstone: fine to coarse in a variety of colors; grain size is sand (0.063-2mm). Siltstone: very fine grained, massive, usually dark; grain size is silt (0.039-0.063mm). Shale is compact, brittle, usually dark; grain size is clay (<0.0039mm). Chemically and/or organically formed sedimentary rocks include chert, rock salt, rock gypsum, dolostone, limestone, coal, and oil shale. Chert: crystalline texture, fine to coarse grains, quartz composition, chemical precipitates and evaporites. Rock salt: crystalline texture, fine to coarse grains, halite composition, chemical precipitates and evaporites. Rock gypsum: crystalline texture, fine to coarse grains, gypsum composition, chemical precipitates and evaporites. Dolostone: crystalline texture, fine to coarse grains, dolomite composition, chemical precipitates and evaporites. Limestone: crystalline or bioclastic texture, microscopic to very coarse grain size, calcite composition, biologic precipitates or cemented shell fragments. Coal: bioclastic texture, clay grain size, carbon composition, black and compacted plant remains. Oil shale: bioclastic texture, clay grain size, clay and kerogen composition, dark and may have oily smell or burn.
Figure 5.46: Sedimentary rock identification chart.

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5.4 Sedimentary Structures

Sedimentary structures are visible textures or arrangements of within a rock. Geologists use these structures to interpret the processes that made the rock and the environment in which it formed. They use to usually compare sedimentary structures formed in modern environments to lithified counterparts in ancient rocks. Below is a summary discussion of common sedimentary structures that are useful for interpretations in the rock record.

5.4.1 Bedding Planes

Landscape of flat layered reddish brown rocks in the foreground; in the background are slopes of gray and red rock fragments leading up to vertical reddish brown and tan cliffs above, consisting of flat-lying sedimentary rock layers.
Figure 5.47: Horizontal strata.

The most basic sedimentary structure is planes, the planes that separate the layers or in sedimentary and some rocks. Visible in exposed outcroppings, each plane indicates a change in conditions. This change may be subtle. For example, if a section of underlying firms up, this may be enough to create a form a layer that is dissimilar from the overlying . Each layer is called a , or stratum, the most basic unit of , the study of sedimentary layering.

Two students are looking at a cliff composed of layers of gray rock.
Figure 5.48: Students from the University of Wooster examine beds of Ordovician limestone in central Tennessee.

As would be expected, thickness can indicate quantity and timing. Technically, a is a plane thicker than 1 cm (0.4 in) and the smallest mappable unit. A layer thinner than 1 cm (0.4 in) is called a . Varves are planes created when and are deposited in repetitive cycles, typically daily or seasonally. Varves are valuable geologic records of climatic histories, especially those found in lakes and deposits.

5.4.2 Graded Bedding

Rock with the following layers labeled from bottom to top: layer labeled A is coarse-to fine-grained tan and gray sandstone; layer labeled B is finely laminated medium-to fine-grained gray and black sandstone; layer labeled C is black and gray rippled fine-grained sandstone; and the layer labeled D is finely laminated black siltstone grading to mudstone. There are also rounded cobbles lying around the described rock.
Figure 5.49: 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.

refers to a sequence of increasingly coarse- or fine-grained layers. often develops when occurs in an environment of decreasing energy. A is observed in rock called . are formed by gravity flows, which are underwater flows of . These subsea density flows begin when is stirred up by an energetic process and becomes a dense slurry of mixed grains. The flow courses downward through submarine channels and canyons due to gravity acting on the density difference between the denser slurry and less dense surrounding seawater. As the flow reaches deeper ocean basins it slows down, loses energy, and deposits in a of coarse grains first, followed by increasingly finer grains (see figure 5.49).

5.4.3 Flow Regime and Bedforms

Series of seven side-view diagrams; diagram 1 is labeled Typical ripple pattern and has a tan bed with six ridges along the top with a flat water line above the bed; diagram 2 is labeled Dunes with ripples superposed and has a tan bed with two elongated ridges along the top with a slightly curved water line above the bed and numerous curved arrows generally moving from left to right; diagram 3 is labeled Dunes and has a tan bed with two elongated, high ridges along the top with a more curved water line above the bed and numerous curved arrows generally moving from right to left with some swirls; diagram 4 is labeled Washed out dunes and has a tan bed with a gently sloping ridge along the top with a nearly flat water line above the bed and two arrows pointing from left to right; diagram 5 is labeled Plane bed and has a tan bed with three low, gently sloping ridges along the top with a flat water line above the bed; diagram 6 is labeled Antidune standing wave and has a tan bed with two elongated ridges along the top with a curved water line that matches the shape of the ridges above the bed; and diagram 7 is labeled Antidune breaking wave and has a tan bed with two domed ridges along the top with a curved water line that matches the shape of the ridges and also has a wave crashing on the right side.
Figure 5.50: Bedforms from under increasing flow velocities.

In fluid systems, such as moving water or wind, sand is the most easily transported and deposited grain. Smaller particles like silt and clay are less movable by fluid systems because the tiny grains are chemically attracted to each other and stick to the underlying . Under higher flow rates, the fine silt and clay tends to stay in place and the larger sand grains get picked up and moved.

Bedforms are sedimentary structures created by fluid systems working on sandy . , flow velocity, and or pattern interact to produce bedforms having unique, identifiable physical characteristics. Flow regimes are divided into upper and lower regimes, which are further divided into uppermost, upper, lower, and lowermost parts. The table below shows bedforms and their associated flow regimes. For example, the is created in the upper part of the lower .

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

Table 5.2: Bedforms and their associated flow regimes.

Plane Beds

There are slight grooves in a very grainy-looking whitish tan rock; a pocket knife rests on the rock for scale.
Figure 5.51: Subtle lines across this sandstone (trending from the lower left to upper right) are parting lineations.

Plane created in the lower are like planes, on a smaller scale. The flat, parallel layers form as sandy piles and move on top of layers below. Even non-flowing fluid systems, such as lakes, can produce plane . Plane in the upper are created by fast-flowing fluids. They may look identical to lower-flow-regime ; however, they typically show , slight alignments of grains in rows and swaths, caused by high transport rates that only occur in upper flow regimes.

Close-up photo of ripples in sand; there's a visible steeper side on the left of each sand ripple, and a more gentle slope on the right side of each ripple.
Figure 5.52: 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

are known by several names: marks, cross , or cross . The ridges or undulations in the are created as grains pile up on top of the . With the exception of , the scale of these is typically measured in centimeters. Occasionally, large flows like lake outbursts, can produce as tall as 20 m (66 ft).

Ripples in reddish-brown rock; each ripple looks to have symmetric slopes on either side; a pocket knife and hand lens rest on the rock for scale.
Figure 5.53: 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.

First scientifically described by Hertha Ayrton, shapes are determined by flow type and can be straight-crested, sinuous, or complex. Asymmetrical form in a unidirectional flow. Symmetrical are the result of an oscillating back-and-forth flow typical of intertidal swash zones. Climbing are created from high sedimentation rates and appear as overlapping layers of shapes (see figure 5.54).

Cross sectional view of rock containing a series of elongated ridges and troughs that appear to be ascending or climbing toward the upper right; a rock hammer rests at the base of the outcrop for scale.
Figure 5.54: Climbing ripple deposit from India.

Dunes

Tall cliff with a large variety of angles of tan to brown beds visible along the cliff face.
Figure 5.55: 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.

are very large and prominent versions of , and typical examples of large . happens when or pile atop one another, interrupting, and/or cutting into the underlying layers. Desert sand are probably the first image conjured up by this category of .

British geologist Agnold (1941) considered only Barchan and linear Seif as the only true forms. Other workers have recognized transverse and as well as parabolic and anchored by plants that are common in coastal areas as other types of .

The tan sand dune is rippled on the left, steeper side and smooth on the right side.
Figure 5.56: Modern sand dune in Morocco.

are the most common sedimentary structure found within channelized flows of air or water. The biggest difference between and air-formed (desert) is the depth of fluid . Since the ’s depth is immense when compared to a channel, desert are much taller than those found in . Some famous air-formed landscapes include the Sahara Desert, Death Valley, and the Gobi Desert.

As airflow moves along, the grains accumulate on the ’s windward surface (facing the wind). The angle of the windward side is typically shallower than the leeward (downwind) side, which has grains falling down over it. This difference in slopes can be seen in a cross-section and indicates the direction of the flow in the past. There are typically two styles of : the more common cross with curved windward surfaces, and rarer planar cross with flat windward surfaces.

In tidal locations with strong in-and-out flows, can develop in opposite directions. This produces a feature called herringbone .

A cross sectional view of thin cross beds that create a V-shape with the V opening toward the left of the photo.
Figure 5.57: Herringbone cross-bedding from the Mazomanie Formation, upper Cambrian of Minnesota.
Tan rock with visible bumpy and undulating lines in the rock; a pencil rests vertically against the rock face for scale.
Figure 5.58: 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 occurs when very strong, hurricane-strength, winds agitate parts of the usually undisturbed seafloor. These are called and have a 3D architecture of hills and valleys, with inclined and declined layering that matches the shapes.

Antidunes

A shallow stream of water flows over a sandy bank toward a larger pool; the water stream has small waves in it as it travels over depressions in the sand.
Figure 5.59: Antidunes forming in Urdaibai, Spain.

are so named because they share similar characteristics with , but are formed by a different, opposing process. While form in lower flow regimes, come from fast-flowing upper flow regimes. In certain conditions of high flow rates, accumulates upstream of a subtle instead of traveling downstream (see figure 5.59). form in phase with the flow; in they are marked by rapids in the current. are rarely preserved in the rock record because the high flow rates needed to produce the also accelerate .

5.4.4 Bioturbation

Slab of gray rock with lighter gray ovals and lines visible in darker gray matrix; a scale bar in the upper right says 1 cm.
Figure 5.60: Bioturbated dolomitic siltstone from Kentucky.

is the result of organisms burrowing through soft , which disrupts the layers. These tunnels are backfilled and eventually preserved when the becomes rock. happens most commonly in shallow, environments, and can be used to indicate water depth.

5.4.5 Mudcracks

Grayish brown rock that has filled-in cracks that go in several directions, forming polygonal shapes; a pair of hiking boots is seen on top of the rock.
Figure 5.61: Lithified mudcracks from Maryland.

occur in clay-rich that is submerged underwater and later dries out. Water fills voids in the clay’s crystalline structure, causing the grains to swell. When this waterlogged begins to dry out, the clay grains shrink. The layer forms deep polygonal cracks with tapered openings toward the surface, which can be seen in profile. The cracks fill with new and become visible veins running through the lithified rock. These dried-out clay are a major source of , small fragments of mud or , which commonly become in and . What makes this sedimentary structure so important to geologists, is they only form in certain —such as that form underwater and are later exposed to air. Syneresis cracks are similar in appearance to but much rarer; they are formed when subaqueous (underwater) clay shrinks.

5.4.6 Sole Marks

A view of the bottom of a tan rock layer with a bulge sticking out of the base toward the observer; a pen is next to the rock for scale.
Figure 5.62: 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.

are small features typically found in deposits. They form at the base of a , the sole, and on top of the underlying . They can indicate several things about the conditions, such as flow direction or up-direction (see Geopetal Structures section). or are grooves carved out by the forces of fluid flow and loads. The upstream part of the flow creates steep grooves and downstream the grooves are shallower. The grooves subsequently become filled by overlying , creating a of the original hollow.

A view of the bottom of a tan and gray rock with visible narrow, parallel ridges that go across the base of the rock.
Figure 5.63: Groove casts at the base of a turbidite deposit in Italy.

Formed similarly to but with a more regular and aligned shape, are produced by larger clasts or debris carried along in the water that scrape across the layer. come from objects like sticks carried in the fluid downstream or embossed into the layer, leaving a depression that later fills with new .

A cylindrical piece of gray rock; light-colored gray rock with sand-sized grains is sticking down into dark gray rock with mud-sized grains.
Figure 5.64: A drill core showing a load cast showing light-colored sand sticking down into dark mud.

Load , an example of soft- , are small indentations made by an overlying layer of coarse grains or clasts intruding into a softer, finer-grained layer.

5.4.7 Raindrop Impressions

Gray rock with round circle impressions dotted along the surface; a scale bar at the upper right says 3 cm.
Figure 5.65: Mississippian raindrop impressions over wave ripples from Nova Scotia.

Like their name implies, are small pits or bumps found in soft . While they are generally believed to be created by rainfall, they may be caused by other agents such as escaping gas bubbles.

5.4.8 Imbrication

Tan to brown conglomerate with rounded cobbles embedded parallel and stacked on each other.
Figure 5.66: 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.

is a of large and usually flat clasts—cobbles, gravels, , etc.—that are aligned in the direction of fluid flow. The clasts may be stacked in rows, with their edges dipping down and flat surfaces aligned to face the flow (see figure 5.66). Or their flat surfaces may be parallel to the layer and long axes aligned with flow. Imbrications are useful for analyzing paleocurrents, or currents found in the geologic past, especially in deposits.

5.4.9 Geopetal Structures

Gray rock that contains a cross sectional view of a clam fossil that is partially filled with tan sediment and partially filled with white calcite; the line between the sediment and calcite is roughly horizontal; a person's finger points to the fossil.
Figure 5.67: 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.

structures, also called up-direction indicators, are used to identify which way was up when the layers were originally formed. This is especially important in places where the rock layers have been deformed, tilted, or overturned. Well preserved , , and can be used to determine up direction. Other useful structures include:

  • Tan rock with a cast of a dinosaur footprint that has three elongated toes; a scale bar at the upper right says 5.0 cm.
    Figure 5.68: Eubrontes trace fossil from Utah, showing the geopetal direction is into the image.

    Vugs: Small voids in the rock that usually become filled during . If the void is partially filled or filled in stages, it serves as a permanent record of a level bubble, frozen in time.

  • – In places where or pile on top of one another, where one interrupts and/or cuts another below, this shows a cross-cutting relationship that indicates up direction.
  • , : Sometimes the are preserved well enough to differentiate between the crests (top) and troughs (bottom).
  • : Body in life position, meaning the body parts are not scattered or broken, and like footprints (see figure 5.68) can provide an up direction. Intact fossilized coral are excellent up indicators because of their large size and easily distinguishable top and bottom. , such as ammonites, can be used to age date and determine up direction based on relative rock ages.
  • Vesicles – flows eliminate gas upwards. An increase of vesicles toward the top of the flow indicates up.

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5.5 Depositional Environments

Block diagram of various depositional environments; there is a mountain range with two environments labeled Alluvial and Glacial; at the base of the mountains is flat land with four environments labeled Aeolian, Fluvial, Evaporite, and Lacustrine; the flat land ends at the shoreline which has four environments labeled Lagoonal, Beach, Deltaic, and Tidal; from the shoreline into the ocean there's a continental shelf, the top of which has two environments labeled Reef and Shallow water marine; and at the base of the continental shelf is a deep ocean basin with one environment labeled Deep water marine.
Figure 5.69: A representation of common depositional environments.

The ultimate goal of many studies is to understand the original . Knowing where and how a particular was formed can help geologists paint a picture of past environments—such as a mountain , gentle , dry desert, or deep-sea . The study of is a complex endeavor; the table shows a simplified version of what to look for in the rock record.

Location Sediment Common rock types Typical fossils Sedimentary structures
Abyssal Very fine muds and oozes, diatomaceous Earth Chert Diatoms Few
Submarine fan Graded Bouma sequences, alternating sand/mud Clastic rocks Rare Channels, fan shape
Continental slope Mud, possible sand, countourites Shale, siltstone, limestone Rare Swaths
Lower shoreface Laminated sand Sandstone Bioturbation Hummocky cross beds
Upper shoreface Planar sand Sandstone Bioturbation Plane beds, cross beds
Littoral (beach) Very well sorted sand Sandstone Bioturbation Few
Tidal flat Mud and sand with channels Shale, mudstone, siltstone Bioturbation Mudcracks, symmetric ripples
Reef Lime mud with coral Limestone Many, commonly coral Few
Lagoon Laminated mud Shale Many, bioturbation Laminations
Delta Channelized sand with mud, ±swamp Clastic rocks Many to few Cross beds
Fluvial (river) Sand and mud, can have larger sediments Sandstone, conglomerate Bone beds (rare) Cross beds, channels, asymmetric ripples
Alluvial Mud to boulders, poorly sorted Clastic rocks Rare Channels, mud cracks
Lacustrine (lake) Fine-grained laminations Shale Invertebrates, rare (deep) bone beds Laminations
Paludal (swamp) Plant material Coal Plant debris Rare
Aeolian (dunes) Very well sorted sand and silt Sandstone Rare Cross beds (large)
Glacial Mud to boulders, poorly sorted Conglomerate (tillite) Striations, drop stones

Table 5.3: Rock record and depositional environments.

5.5.1 Marine

 are completely and constantly submerged in seawater. Their depositional characteristics are largely dependent on the depth of water with two notable exceptions, and .

Abyssal

World map with ocean basins color-coded according to the sediment thickness on the ocean floor; the thickest sediment is tan in color with a darker gradient for thinner sediment, while the thinnest sediment id deep blue in color with a lighter gradient for thicker sediment. The sediment is thickest around the ocean margins near the continents while the sediment is thinnest away from the continents in the deepest ocean basins and trenches.
Figure 5.70: Marine sediment thickness. Note the lack of sediment away from the continents.

sedimentary rocks form on the . The plain encompasses relatively flat with some minor topographical features, called hills. These small seafloor mounts range 100 m to 20 km in diameter, and are possibly created by . Most plains do not experience significant fluid movement, so formed there are very fine grained.

There are three categories of . Calcareous oozes consist of -rich plankton shells that have fallen to the . An example of this type of is . Siliceous oozes are also made of plankton debris, but these organisms build their shells using silica or hydrated silica. In some cases such as with diatomaceous earth, is deposited below the , a depth where solubility increases. Any -based shells are , leaving only silica-based shells. is another common rock formed from these types of . These two types of are also classified as in origin (see biochemical section).

Chunk of white, fine-grained, powdery-looking rock.
Figure 5.71: Diatomaceous earth.

The third type is pelagic clay. Very fine-grained clay particles, typically brown or red, descend through the water column very slowly. Pelagic clay occurs in areas of remote open ocean, where there is little plankton accumulation.

Block diagram showing continental shelf sloping down toward deep ocean water with underwater avalanches of mud and sand labeled on the slope and lumps at the base labeled Turbidite Deposits.
Figure 5.72: Turbidites inter-deposited within submarine fans.

Two notable exceptions to the fine-grained nature of are and deposits. occur at the base of large systems. They are initiated during times of low sea level, as strong currents carve into the . When sea levels rise, accumulates on the shelf typically forming large, fan-shaped floodplains called deltas. Periodically, the is disturbed creating dense slurries that flush down the underwater canyons in large gravity-induced events called . The is formed by a network of that deposit their loads as the slope decreases, much like what happens above-water at fans and deltas. This sudden flushing transports coarser to the where they are otherwise uncommon. are also the typical origin of sequences. (see chapter 5).

Continental Slope

2D seismic diagram with two-way traveltime on the vertical axis increasing downward and distance on the horizontal axis increasing toward the right, from ESE on the left to WNW on the right. Thin layers are visible in the image, sloping shallowly toward the right; the ground surface above the layers also slopes down toward the right with a moat visible near the far right and then a steep upward slope past that.
Figure 5.73: Contourite drift deposit imaged with seismic waves.

deposits are not common in the rock record. The most notable type of deposits are contourites. Contourites form on the slope between the and deep . Deep-water ocean currents deposit into smooth drifts of various architectures, sometimes interwoven with .

Lower Shoreface

Cross sectional diagram of sand sloping upward toward the right with shallowing ocean water above; there is a horizontal line between two successive ocean wave crests labeled Wave Length; at the top of the diagram it says "Wave base is the depth to which a passing wave will cause water motion." There are circular arrows stacked vertically at the wave trough with the largest circle at the top and smaller and smaller circles toward the bottom until there are no more circles; at the bottom of the circular arrows is the label "Wave Base = 1/2 Wave Length." Along the sloping sand are two labels: on the side dipping below the vertical stack of arrows, the sand is smooth and there is the label "Ocean bottom undisturbed by waves (Lower Shoreface);" on the side dipping upward above the vertical stack of arrows, the sand is rippled and there is the label "Ocean bottom agitated and rippled by waves (Upper Shoreface)."
Figure 5.74: Diagram describing wavebase.

The lower lies below the normal depth of wave agitation, so the is not subject to daily winnowing and . These layers are typically finely laminated, and may contain hummocky cross-stratification. Lower are affected by larger waves, such those as generated by hurricanes and other large storms.

Upper Shoreface

Block diagram of shoreline zones: on land is the Coast; there is gently sloping sand coming from the bases of cliffs along the coast; this includes the labels Beach, Backshore, and Berms; in the shallowest part of the ocean water are the labels Foreshore, Terrace, Surf zone, and Breakers. A line labeled Shoreline points to the contact between the edge of the ocean water and the beach. In deeper water is the label Nearshore and then in the deepest water is the label Offshore. The high tide and low tide line are drawn on the diagram as well, and there is a deposit on the continental slope labeled Bar. Across the entire diagram is the label Littoral zone. The deepest part of the water is about 60 m.
Figure 5.75: Diagram of zones of the shoreline.

The upper contains within the zone of normal wave action, but still submerged below the beach environment. These usually consist of very well sorted, fine sand. The main sedimentary structure is planar consistent with the lower part of the upper , but it can also contain created by .

5.5.2 Transitional Coastline Environments

Two cross sectional diagrams; the top diagram shows onlap with layers of sediments being deposited on top of each other, with each successive upper layer being deposited toward inland; there is an arrow pointing toward the right-hand side labeled transgression; the bottom diagram shows offlap with layers of sediments being deposited on top of each other, with each successive upper layer being deposited toward the ocean; there is an arrow pointing toward the left-hand side labeled regression.
Figure 5.76: The rising sea levels of transgressions create onlapping sediments, regressions create offlapping.

Transitional environments, more often called or environments, are zones of complex interactions caused by ocean water hitting land. The preservation potential is very high in these environments because often occurs on the and underwater. environments are an important source of hydrocarbon deposits (, ).

The study of is called . examines depositional changes and 3D architectures associated with rising and falling sea levels, which is the main force at work in deposits. These sea-level fluctuations come from the daily tides, as well as climate changes and . A steady rise in sea level relative to the is called . is the opposite, a relative drop in sea level. Some common components of environments are zones, , , , and deltas. For a more in-depth look at these environments, see chapter 12.

Littorals

Side view of a sand deposit that has layers of tan sand interbedded with layers of gray and black sand; a penny is stuck into the sand for scale.
Figure 5.77: Lithified heavy mineral sand (dark layers) from a beach deposit in India.

The zone, better known as the beach, consists of highly weathered, homogeneous, well-sorted sand grains made mostly of . There are black sand and other types of sand beaches, but they tend to be unique exceptions rather than the rule. Because beach sands, past or present, are so highly evolved, the amount grain can be discerned using the , tourmaline, and rutile. This tool is called the ZTR (, tourmaline, rutile) index. The ZTR index is higher in more weathered beaches, because these relatively rare and -resistant become in older beaches. In some beaches, the ZTR index is so high the sand can be harvested as an economically viable source of these . The beach environment has no sedimentary structures, due to the constant bombardment of wave energy delivered by surf action. Beach is moved around via multiple processes. Some beaches with high supplies develop nearby.

Tidal Flats

Generalized map view of a tidal flat with labeled and color-coded features: deep littoral is colored blue and found to the upper left of the diagram; lower littoral (subtidal) is colored cyan and next to the deep littoral zone; littoral (beach) is colored marigold and is a strip alongside the lower littoral zone, however the beach has a gap in it; in the gap of the beach there are tidal bars colored turquoise to match the intertidal label; other turquoise intertidal zones are found inland from the beach; supratidal is colored lime green and surrounds the intertidal zones behind the beach; and continental is colored olive green and surrounds the supratidal zone as well as the areas behind the beach along the edges of the diagram.
Figure 5.78: General diagram of a tidal flat and associated features.

, or , are sedimentary environments that are regularly flooded and drained by ocean tides. have large areas of fine-grained but may also contain coarser sands. deposits typically contain gradational and may include multi-directional marks. are also commonly seen due to the being regularly exposed to air during low tides; the combination of and marks is distinctive to .

Tidal water carries in , sometimes focusing the flow through a narrow opening called a tidal inlet. Tidal channels, creek channels influenced by tides, can also tidally-induced flow. Areas of higher flow like inlets and tidal channels feature coarser grain sizes and larger , which in some cases can develop into .

Reefs

Aerial view of a long, thin whitish ridge that runs through the surrounding tan to brown landscape.
Figure 5.79: Waterpocket fold, Capitol Reef National Park, Utah.

, which most people would immediately associate with tropical coral found in the oceans, are not only made by living things. Natural buildups of sand or rock can also create , similar to . Geologically speaking, a is any topographically-elevated feature on the , located oceanward of and separate from the beach. The term can also be applied to (atop the ) features. Capitol Reef National Park in Utah contains a topographic barrier, a , called the Waterpocket Fold.

Colorful coral reef underwater in light blue, shallow water.
Figure 5.80: A modern coral reef.

Most , now and in the geologic past, originate from the biological processes of living organisms. The growth habits of coral provide geologists important information about the past. The hard structures in coral are built by soft-bodied organisms, which continually add new material and enlarge the over time. Under certain conditions, when the land beneath a is subsiding, the coral may grow around and through existing , holding the in place, and thus preserving the record of environmental and geological condition around it.

Satellite image of an elongate dark green island that's surrounded by deep blue water. A reef can be seen as a lighter blue ring outside of the island.
Figure 5.81: 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.

found in coral is typically fine-grained, mostly , and tends to deposit between the intact coral skeletons. Water with high levels of silt or clay particles can inhibit the growth because coral organisms require sunlight to thrive; they host symbiotic algae called zooxanthellae that provide the coral with nourishment via photosynthesis. Inorganic structures have much more variable compositions. have a big impact on in environments since they are natural storm breaks, wave and storm buffers, which allows fine grains to settle and accumulate.

The left side of the map shows part of Australia and Asia while the right side of the map shows the west-southwestern coast of North America. The Pacific Ocean is white, with numerous red dots, ranging from tiny to small, scattered throughout the ocean; guyots are shown as slightly larger green dots and are less abundant than seamounts.
Figure 5.82: Seamounts and guyots in the North Pacific.

are found around shorelines and islands; coral are particularly common in tropical locations. are also found around features known as , which is the base of an ocean island left standing underwater after the upper part is eroded away by waves. Examples include the Emperor Seamounts, formed millions of years ago over the Hawaiian . live and grow along the upper edge of these flat-topped . If the builds up above sea level and completely encircles the top of the , it is called a coral-ringed atoll. If the is submerged, due to , , or sea level rise, the structure is called a guyot.

Lagoons

Satellite image of a brown, barren landscape with a large blue rounded body of water on the land; to the left of that body of water is a deeper blue larger water body, the Caspian Sea.
Figure 5.83: Kara-Bogaz Gol lagoon, Turkmenistan.

are small bodies of seawater located inland from the or isolated by another geographic feature, such as a or . Because they are protected from the action of tides, currents, and waves, environments typically have very fine grained . , as well as , are ecosystems with high biological productivity. Rocks from these environments often includes marks or deposits. Around where evaporation exceeds water inflow, salt flats, also known as sabkhas, and sand fields may develop at or above the high line.

Deltas

Satellite view of a land-sea boundary with a green fan-shaped delta between the edge of tan land and deep blue ocean, widening toward the ocean.
Figure 5.84: The Nile delta, in Egypt.
Satellite view of light blue water with a green dendritic or birdfoot-shaped delta branching out into the water.
Figure 5.85: Birdfoot river-dominated delta of the Mississippi River.

Deltas form where enter lakes or oceans and are of three basic shapes: -dominated deltas, wave-dominated deltas, and -dominated deltas. The name comes from the Greek letter Δ (, uppercase), which resembles the triangular shape of the Nile . The velocity of water flow is dependent on riverbed slope or , which becomes shallower as the descends from the mountains. At the point where a enters an ocean or lake, its slope angle drops to zero degrees (0°). The flow velocity quickly drops as well, and is deposited, from coarse clasts, to fine sand, and mud to form the . As one part of the becomes overwhelmed by , the slow-moving flow gets diverted back and forth, over and over, and forms a spread out network of smaller distributary channels.

Satellite view of numerous roughly parallel tan-colored rivers leading to a larger body of tan water that becomes deep blue away from the mouths of the rivers.
Figure 5.86: Tidal delta of the Ganges River.

Deltas are organized by the dominant process that controls their shape: -dominated, wave-dominated, or -dominated. Wave-dominated deltas generally have smooth coastlines and beach-ridges on the land that represent previous shorelines. The Nile River delta is a wave-dominated type (see figure 5.84).

The Mississippi River delta is a -dominated . shaped by levees along the and its distributaries that confine the flow forming a shape called a birdfoot . Other times the tides or the waves can be a bigger factor, and can reshape the in various ways.

A -dominated is dominated by tidal currents. During flood stages when have lots of water available, it develops distributaries that are separated by sand bars and sand ridges. The tidal of the Ganges River is the largest in the world.

5.5.3 Terrestrial

are diverse. Water is a major factor in these environments, in liquid or frozen states, or even when it is lacking (arid conditions).

Fluvial

Sinuous tan river running through roughly flat green terrain.
Figure 5.87: The Cauto River in Cuba. Note the sinuosity in the river, which is meandering.

() systems are formed by water flowing in channels over the land. They generally come in two main varieties: or . In , the flow carries grains via a single channel that wanders back and forth across the . The away from the channel is mostly fine grained material that only gets deposited during floods.

Many channels that braid back and forth among each other; between channels are tan sediment deposits.
Figure 5.88: The braided Waimakariri river in New Zealand.

systems generally contain coarser grains, and form a complicated series of intertwined channels that flow around gravel and sand bars (see chapter 11).

Alluvial

Landscape with a broad, tan valley dotted with low scrubby vegetation; a river from distant ridges splits and runs in numerous branches down the broad valley.
Figure 5.89: An alluvial fan spreads out into a broad alluvial plain. From Red Rock Canyon State Park, California.

A distinctive characteristic of systems is the intermittent flow of water. deposits are common in arid places with little development. Lithified are the primary -filling rock found throughout the region of the western United States. The most distinctive sedimentary deposit is the fan, a large cone of formed by flowing out of dry mountain valleys into a wider and more open dry area. are typically poorly sorted and coarse grained, and often found near lakes or deposits (see chapter 13).

Lacustrine

A large hole in the top of a mountain that is filled with a lake. There is also an island in the lake.
Figure 5.90: Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama.

Lake systems and deposits, called , form via processes somewhat similar to deposits, but on a much smaller scale. deposits are found in lakes in a wide variety of locations. Lake Baikal in southeast Siberia (Russia) is in a . Crater Lake (Oregon) sits in a . The Great Lakes (northern United States) came from glacially carved and deposited . Ancient Lake Bonneville (Utah) formed in a pluvial setting that during a climate that was relatively wetter and cooler than that of modern Utah. lakes, named for their curved shape, originated in floodplains. tends to be very fine grained and thinly laminated, with only minor contributions from wind-blown, current, and tidal deposits. When lakes dry out or evaporation outpaces , form. deposits resemble those of normal lake deposits but contain more . Certain can have -type deposits as well.

Paludal

systems include bogs, marshes, swamps, or other wetlands, and usually contain lots of organic matter. systems typically develop in coastal environments, but are common occur in humid, low-lying, low-, warm zones with large volumes of flowing water. A characteristic deposit is a peat bog, a deposit rich in organic matter that can be converted into when lithified. environments may be associated with tidal, deltaic, , and/or .

Aeolian

Six block diagrams of various dune shapes: blowout dunes which are sandy depressions; parabolic dunes which are U-shaped with the wind blowing in the direction of the curve of the U; dome dunes which are dome-shaped rising from the land; barchan dunes which are a narrow U-shape with the wind blowing in the direction of the arms of the U; barchanoid ridges which are elongate wavy ridges with the wind blowing from the shallowly sloping side toward the steeper sloped side; and transverse ridges which are linear ridges with the wind blowing from the shallowly sloping side toward the steeper sloped side.
Figure 5.91: Formation and types of dunes.

, sometimes spelled or œolian, are deposits of windblown . Since wind has a much lower carrying capacity than water, deposits typically consists of clast sizes from fine dust to sand. Fine silt and clay can cross very long distances, even entire oceans suspended in air.

With sufficient influx, systems can potentially form large in dry or wet conditions. The figure shows features and various types. British geologist Ralph A. Bagnold (1896-1990) considered only Barchan and linear Seif as the only true forms. Other scientists recognize transverse, star, parabolic, and types. Parabolic and grow from sand anchored by plants and are common in coastal areas.

Tan landscape of plateaus and canyons with buildings carved into the stone.
Figure 5.92: Loess Plateau in China. The loess is so highly compacted that buildings and homes have been carved in it.

Compacted layers of wind-blown is known as . commonly starts as finely ground up rock flour created by . Such deposits cover thousands of square miles in the Midwestern United States. may also form in desert regions (see chapter 13). Silt for the Plateau in China came from the Gobi Desert in China and Mongolia.

Glacial

Along the edge of glacial ice, numerous broken pieces of tan, gray, and black rocks lay along the edge, ranging in size from large boulders to tiny bits.
Figure 5.93: Wide range of sediments near Athabaska Glacier, Jasper National Park, Alberta, Canada.

sedimentation is very diverse, and generally consists of the most poorly-sorted deposits found in nature. The main clast type is called , which literally means two sizes, referring to the unsorted mix of large and small rock fragments found in deposits. Many , glacially derived , include very finely-pulverized rock flour along with giant boulders. The surfaces of larger clasts typically have striations from the rubbing, scraping, and polishing of surfaces by abrasion during the movement of ice. systems are so large and produce so much , they frequently create multiple, individualized , such as , deltaic, , pluvial, , and/or (see chapter 14).

5.5.4 Facies

In addition to and process, geologists also classify by its depositional characteristics, collectively called or lithofacies. Sedimentary consist of physical, chemical, and/or biological properties, including relative changes in these properties in adjacent of the same layer or geological age. Geologists analyze to interpret the original environment, as well as disruptive geological events that may have occurred after the rock layers were established.

It boggles the imagination to think of all the sedimentary environments working next to each other, at the same time, in any particular region on Earth. The resulting develop characteristics reflecting contemporaneous conditions at the time of , which later may become preserved into the rock record. For example in the Grand Canyon, rock of the same geologic age includes many different : beach sand, silt, mud, and farther . In other words, each sedimentary or presents recognizable characteristics that reflect specific, and different, that were present at the same time.

may also reflect depositional changes in the same location over time. During of rising sea level, called , the moves inland as seawater covers what was originally dry land and creates new . When these turn into , the vertical sequence reveals beach lithofacies buried by lithofacies.

Biological are remnants (, diatomaceous earth) or evidence () of living organisms. , fossilized life forms specific to a particular environment and/or geologic time , are an example of biological . The horizontal assemblage and vertical distribution of are particularly useful for studying species evolution because , , burial, and processes happen over a considerable geologic time range.

assemblages that show evolutionary changes greatly enhance our interpretation of Earth’s ancient history by illustrating the between sequence and geologic time scale. During the Middle (see chapter 7), regions around the Grand Canyon experienced in a southeasterly direction (relative to current maps). This shift of the is reflected in the Tapeats Sandstone beach facies, Bright Angle Shale near-offshore mud , and Muav Limestone far-offshore facies. organisms had plenty of time to evolve and adapt to their slowly changing environment; these changes are reflected in the biological , which show older life forms in the western regions of the canyon and younger life forms in the east.

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Summary

Sedimentary rocks are grouped into two main categories: () and chemical. () rocks are made of clasts or that lithifies into solid material. is produced by the mechanical or of and transported away from the source via . that is deposited, buried, compacted, and sometimes cemented becomes rock. rocks are classified by ; for example is made of sand-sized particles. rocks comes from out an aqueous and is classified according to . The rock is made of calcium . Sedimentary structures have textures and shapes that give insight on depositional histories. depend mainly on fluid transport systems and encompass a wide variety of underwater and above ground conditions. Geologists analyze depositional conditions, sedimentary structures, and rock records to interpret the paleogeographic history of a region.

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Text References

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Figure References

Figure 5.1: A model of a water molecule, showing the bonds between the hydrogen and oxygen. Dan Craggs. 2009. Public domain. https://commons.wikimedia.org/wiki/File:H2O_2D_labelled.svg

Figure 5.2: Dew on a spider’s web. Luc Viatour. 2007. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Spider_web_Luc_Viatour.jpg

Figure 5.3: Hydrogen bonding between water molecules. Qwerter. 2011. Public domain. https://commons.wikimedia.org/wiki/File:3D_model_hydrogen_bonds_in_water.svg

Figure 5.4: A sodium (Na) ion in solution. Taxman. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Na%2BH2O.svg

Figure 5.5: The outer layer of this granite is fractured and eroding away, known as exfoliation. Wing-Chi Poon. 2005. CC BY-SA 2.5. https://en.m.wikipedia.org/wiki/File:GeologicalExfoliationOfGraniteRock.jpg

Figure 5.6: The process of frost wedging. Julie Sandeen. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Mechanical_weathering.png

Figure 5.7: The roots of this tree are demonstrating the destructive power of root wedging. Arseny Khakhalin. 2006. CC BY 3.0. https://commons.wikimedia.org/wiki/File:Pine-tree_roots_digging_through_the_asphalt_-_panoramio.jpg

Figure 5.8: Tafoni from Salt Point, California. Dawn Endico. 2005. CC BY-SA 2.0. https://commons.wikimedia.org/wiki/File:Tafoni_03.jpg

Figure 5.9: Each of these three groups of cubes has an equal volume. Kindred Grey. 2022. CC BY 4.0.

Figure 5.10: Generic hydrolysis diagram, where the bonds in mineral in question would represent the left side of the diagram. Unknown author. 2014. CC BY-SA 4.0. https://www.wikiwand.com/en/Hydrolysis#Media/File:Hydrolysis.png

Figure 5.11: In this rock, a pyrite cube has dissolved (as seen with the negative “corner” impression in the rock), leaving behind small specks of gold. Matt Affolter (QFL247). 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:GoldinPyriteDrainage_acide.JPG

Figure 5.12: 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. Matt Affolter. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Iddingsite.JPG

Figure 5.13: Eroded karst topography in Minerve, France. Hugo Soria. 2005. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Karst_minerve.jpg

Figure 5.14: A formation called The Great Heart of Timpanogos in Timpanogos Cave National Monument. Sleeping Bear Dunes National Lakeshore. 2012. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Tica_(7563200350).jpg

Figure 5.15: Pyrite cubes are oxidized, becoming a new mineral goethite. Matt Affolter (QFL247). 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:PyOx.JPG

Figure 5.16: A hoodoo near Moab, Utah. Qfl247. 2010. GNU Free Documentation License. https://en.wikipedia.org/wiki/File:MoabHoodoo.JPG

Figure 5.17: Grand Canyon from Mather Point. Szumyk. 2004. Public domain. https://commons.wikimedia.org/wiki/File:Grand_Canyon-Mather_point.jpg

Figure 5.18: Sketch and picture of soil. Carlosblh. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Estructura-suelo.jpg

Figure 5.19: Schematic of the nitrogen cycle. Johann Dréo. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Nitrogen_Cycle.svg

Figure 5.20: Agricultural terracing, as made by the Inca culture from the Andes, helps reduce erosion and promote soil formation, leading to better farming practices. Unknown author. 2007. Public domain. https://www.wikiwand.com/en/Andean_civilizations#Media/File:Pisac006.jpg

Figure 5.21: A simplified soil profile, showing labeled layers. Wilsonbiggs. 2021. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Soil_Horizons.svg

Figure 5.22: A sample of bauxite. Werner Schellmann. 1965. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Bauxite_with_unweathered_rock_core._C_021.jpg

Figure 5.23: A dust storm approaches Stratford, Texas in 1935. George E. Marsh Album via NOAA. 1935. Public domain. https://commons.wikimedia.org/wiki/File:Dust_Storm_Texas_1935.jpg

Figure 5.24: Geologic unconformity seen at Siccar Point on the east coast of Scotland. dave souza. 2008. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Siccar_Point_red_capstone_closeup.jpg

Figure 5.25: Permineralization in petrified wood. Moondigger. 2005. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Petrified_forest_log_2_md.jpg

Figure 5.26: Size categories of sediments, known as the Wentworth scale. Jeffress Williams, Matthew A. Arsenault, Brian J. Buczkowski, Jane A. Reid, James G. Flocks, Mark A. Kulp, Shea Penland, and Chris J. Jenkins via USGS. 2011. Public domain. https://commons.wikimedia.org/wiki/File:Wentworth_scale.png

Figure 5.27: A well-sorted sediment (left) and a poorly-sorted sediment (right). Woudloper. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Sorting_in_sediment.svg

Figure 5.28: Degree of rounding in sediments. Woudloper. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Rounding_%26_sphericity_EN.svg

Figure 5.29: A sand grain made of basalt, known as a microlitic volcanic lithic fragment. Matt Affolter (QFL247). 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:LvMS-Lvm.jpg

Figure 5.30: Hawaiian beach composed of green olivine sand from weathering of nearby basaltic rock. Aren Elliott. 2018. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Panorama_of_Papakolea_green_sand_beach_in_Hawaii_2.jpg

Figure 5.31: Megabreccia in Titus Canyon, Death Valley National Park, California. NPS. Unknown date. Public domain. https://commons.wikimedia.org/wiki/File:Titus_Canyon_Narrows.jpg

Figure 5.32: Enlarged image of frosted and rounded windblown sand grains. Wilson44691. 2008. Public domain. https://commons.wikimedia.org/wiki/File:CoralPinkSandDunesSand.JPG

Figure 5.33: The Rochester Shale, New York. Wilson44691. 2015. Public domain. https://en.m.wikipedia.org/wiki/File:Rochester_Shale_Niagara_Gorge.jpg

Figure 5.34: Claystone laminations from Glacial Lake Missoula. Matt Affolter. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:GLMsed.jpg

Figure 5.35: Salt-covered plain known as the Bonneville Salt Flats, Utah. Michael Pätzold. 2008. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Bonneville_salt_flats_pilot_peak.jpg

Figure 5.36: Ooids from Joulter’s Cay, The Bahamas. Wilson44691. 2010. Public domain. https://commons.wikimedia.org/wiki/File:Ooids,_Joulter_Cays,_Bahamas.jpg

Figure 5.37: Limestone tufa towers along the shores of Mono Lake, California. Yukinobu Zengame. 2005. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Limestone_towers_at_Mono_Lake,_California.jpg

Figure 5.38: Travertine terraces of Mammoth Hot Springs, Yellowstone National Park, USA. Frank Schulenburg. 2016. CC BY-SA 4.0. https://en.m.wikipedia.org/wiki/File:Mammoth_Terraces.jpg

Figure 5.39: Alternating bands of iron-rich and silica-rich mud, formed as oxygen combined with dissolved iron. Wilson44691. 2008. Public domain. https://commons.wikimedia.org/wiki/File:MichiganBIF.jpg

Figure 5.40: A type of chert, flint, shown with a lighter weathered crust. Ra’ike. 2014. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Flint_with_weathered_crust.JPG

Figure 5.41: Ooids forming an oolite. Unknown author. 2008. Public domain. https://www.wikiwand.com/en/Oolite#Media/File:OoidSurface01.jpg

Figure 5.42: Fossiliferous limestone (with brachiopods and bryozoans) from the Kope Formation of Ohio. Jim Stuby. 2010. Public domain. https://commons.wikimedia.org/wiki/File:Limestone_etched_section_KopeFm_new.jpg

Figure 5.43: Close-up on coquina. Wilson44691. 2008. Public domain. https://commons.wikimedia.org/wiki/File:CoquinaClose.jpg

Figure 5.44: Anthracite coal, the highest grade of coal. USGS. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Coal_anthracite.jpg

Figure 5.45: Gyprock, a rock made of the mineral gypsum. James St. John. 2016. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Gyprock_(Castile_Formation,_Upper_Permian_Eddy_County,_New_Mexico.jpg

Figure 5.46: Sedimentary rock identification chart. Virginia Sisson. Unknown date. CC BY-NC-SA 4.0. Figure 2.13 from https://uhlibraries.pressbooks.pub/historicalgeologylab/chapter/chapter02-earthmaterials/

Figure 5.47: Horizontal strata. Matt Affolter (QFL247). 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:SEUtahStrat.JPG

Figure 5.48: Students from the University of Wooster examine beds of Ordovician limestone in central Tennessee. Wilson44691. 2007. Public domain. https://commons.wikimedia.org/wiki/File:OrdOutcropTN.JPG

Figure 5.49: Image of the classic Bouma sequence. Mikesclark. 2013. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Turbidite_from_Pigeon_Pt_Fm_at_Pescadero_Beach,_CA.jpg

Figure 5.50: Bedforms from under increasing flow velocities. US Dept. of Transportation Federal Highway Administration. 2013. Public domain. https://commons.wikimedia.org/wiki/File:Bedforms_under_various_flow_regimes.pdf

Figure 5.51: Subtle lines across this sandstone (trending from the lower left to upper right) are parting lineations. Matt Affolter (a.k.a. QFL247). 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:PartingLineation.JPG

Figure 5.52: Modern current ripple in sand from the Netherlands. Heinz-Josef Lücking. 2007. CC BY-SA 3.0 DE. https://commons.wikimedia.org/wiki/File:Rippelbildungen_am_Strand_von_Spiekeroog.JPG

Figure 5.53: A bidirectional flow creates this symmetrical wave ripple. Matt Affolter (QFL247). 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:WaveRipple.JPG

Figure 5.54: Climbing ripple deposit from India. DanHobley. 2011. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Climbing_ripples.JPG

Figure 5.55: Lithified cross-bedded dunes from the high country of Zion National Park, Utah. Dr. Igor Smolyar, NOAA/NESDIS/NODC. 2010. Public domain. https://commons.wikimedia.org/wiki/File:Cross-bedding_Of_Sandstone_Near_Mt_Carmel_Road_Zion_Canyon_Utah.jpg

Figure 5.56: Modern sand dune in Morocco. Rosino. 2005. CC BY-SA 2.0. https://en.wikipedia.org/wiki/File:Morocco_Africa_Flickr_Rosino_December_2005_84514010.jpg

Figure 5.57: Herringbone cross-bedding from the Mazomanie Formation, upper Cambrian of Minnesota. James St. John. 2015. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Herringbone_cross-stratified_quartzose_sandstones_with_trace_fossils_(Mazomanie_Formation,_Upper_Cambrian;_riverside_cliff,_western_side_of_the_St._Croix_River,_northeast_of_Lookout_Point,_Minnesota,_USA)_4_(18812079220).jpg

Figure 5.58: Hummocky-cross stratification, seen as wavy lines throughout the middle of this rock face. Matt Affolter (QFL247). 2010. CC BY-SA 3.0. https://www.wikiwand.com/en/Hummocky_cross-stratification#Media/File:HumXSec.JPG

Figure 5.59: Antidunes forming in Urdaibai, Spain. Kol35. 2011. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Antidunes_(Urdaibai_estuary).JPG

Figure 5.60: Bioturbated dolomitic siltstone from Kentucky. Jstuby. 2010. Public domain. https://en.wikipedia.org/wiki/File:Saluda_bioturbation.jpg

Figure 5.61: Lithified mudcracks from Maryland. Jstuby. 2009. Public domain. https://en.wikipedia.org/w/index.php?title=File:Mudcracks_roundtop_hill_MD.jpg

Figure 5.62: 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. Matt Affolter (QFL247). 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:FluteCast.JPG

Figure 5.63: Groove casts at the base of a turbidite deposit in Italy. Mikenorton. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Groove_casts.JPG

Figure 5.64: A drill core showing a load cast showing light-colored sand sticking down into dark mud. Matt Affolter. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:LoadCast.JPG

Figure 5.65: Mississippian raindrop impressions over wave ripples from Nova Scotia. Rygel, M.C. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Raindrop_impressions_mcr1.jpg

Figure 5.66: 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. Verisimilus. 2008. CC BY 3.0. https://commons.wikimedia.org/wiki/File:Imbricated_fabric.jpg

Figure 5.67: This bivalve (clam) fossil was partially filled with tan sediment, partially empty. Matt Affolter. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Geopetal.JPG

Figure 5.68: Eubrontes trace fossil from Utah, showing the geopetal direction is into the image. Wilson44691. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Eubrontes01.JPG

Figure 5.69: A representation of common depositional environments. Mikenorton. 2008. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:SedimentaryEnvironment.jpg

Figure 5.70: Marine sediment thickness. Note the lack of sediment away from the continents. NOAA. 2019. Public domain. https://ngdc.noaa.gov/mgg/sedthick/

Figure 5.71: Diatomaceous earth. James St. John. 2013. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Diatomite_(diatomaceous_earth)_Monterey_Formation_at_a_diatomite_quarry_just_south_of_Lompoc.jpg

Figure 5.72: Turbidites inter-deposited within submarine fans. Oggmus. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Turbidite_formation.jpg

Figure 5.73: Contourite drift deposit imaged with seismic waves. Integrated Ocean Drilling Program. 2012. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Contourite_sparker_seismic_elongate_drift.png

Figure 5.74: Diagram describing wavebase. GregBenson. 2004. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Wavebase.jpg

Figure 5.75: Diagram of zones of the shoreline. US government. 2012. Public domain. https://commons.wikimedia.org/wiki/File:Littoral_Zones.jpg

Figure 5.76: The rising sea levels of transgressions create onlapping sediments, regressions create offlapping. Woudloper. 2009. CC BY-SA 1.0. https://commons.wikimedia.org/wiki/File:Offlap_%26_onlap_EN.svg

Figure 5.77: Lithified heavy mineral sand (dark layers) from a beach deposit in India. Mark A. Wilson. 2008. Public domain. https://commons.wikimedia.org/wiki/File:HeavyMineralsBeachSand.jpg

Figure 5.78: General diagram of a tidal flat and associated features. Foxbat deinos. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Tidal_flat_general_sketch.png

Figure 5.79: Waterpocket fold, Capitol Reef National Park, Utah. Bobak Ha’Eri. 2008. CC BY 3.0. https://commons.wikimedia.org/wiki/File:2008-0914-CapitolReef-WaterpocketFold1.jpg

Figure 5.80: A modern coral reef. Toby Hudson. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Coral_Outcrop_Flynn_Reef.jpg

Figure 5.81: The light blue reef is fringing the island of Vanatinai. NASA image by Jesse Allen and Rob Simmon, using data provided by the United States Geological Survey. 2002. Public domain. https://en.m.wikipedia.org/wiki/File:Vanatinai,_Louisiade_Archipelago.jpg

Figure 5.82: Seamounts and guyots in the North Pacific. PeterTHarris. 2015. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Distribution_of_seamounts_and_guyots_in_the_North_Pacific.pdf

Figure 5.83: Kara-Bogaz Gol lagoon, Turkmenistan. NASA. 1995. Public domain. https://commons.wikimedia.org/wiki/File:Kara-Bogaz_Gol_from_space,_September_1995.jpg

Figure 5.84: The Nile delta, in Egypt. Jacques Descloitres, MODIS Rapid Response Team, NASA/sh. 2003. Public domain. https://commons.wikimedia.org/wiki/File:Nile_River_and_delta_from_orbit.jpg

Figure 5.85: Birdfoot river-dominated delta of the Mississippi River. NASA. 2001. Public domain. https://commons.wikimedia.org/wiki/File:Mississippi_delta_from_space.jpg

Figure 5.86: Tidal delta of the Ganges River. NASA. 1994. Public domain. https://commons.wikimedia.org/wiki/File:Ganges_River_Delta,_Bangladesh,_India.jpg

Figure 5.87: The Cauto River in Cuba. Not home~commonswiki. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Rio-cauto-cuba.JPG

Figure 5.88: The braided Waimakariri river in New Zealand. Greg O’Beirne. 2016. CC BY 2.5. https://www.wikiwand.com/simple/Braided_river#Media/File:Waimakariri01_gobeirne.jpg

Figure 5.89: An alluvial fan spreads out into a broad alluvial plain. Matt Affolter. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:AlluvialPlain.JPG

Figure 5.90: Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama. Zainubrazvi. 2006. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Crater_lake_oregon.jpg

Figure 5.91: Formation and types of dunes. NPS Natural Resources. 2016. Public domain. https://flic.kr/p/GAn1Pj

Figure 5.92: Loess Plateau in China. Till Niermann. 1987. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Loess_landscape_china.jpg

Figure 5.93: Wide range of sediments near Athabaska Glacier, Jasper National Park, Alberta, Canada. Wing-Chi Poon. 2006. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Glacial_Transportation_and_Deposition.jpg

 

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