3.6 Sedimentary Rocks

Charlene Estrada

When a geologist encounters a sedimentary outcrop, they can reconstruct ancient landscapes – rapidly flowing mountain streams, frightening avalanches, or vast deserts – from these rocks. As the name suggests, Rocks are composed of sediments that have been and together, or , over long periods of time.

 

Deposition is the settling of clasts, compaction is the movement of clasts closer together, and cemention is the bonding of the clasts together.
Figure 3.6.1. The process of lithification to form sedimentary rocks: Deposition is the settling of clasts, compaction is the movement of clasts closer together, and cementation is the bonding of the clasts together.

Sediments may include:

  • Fragments of other rocks that often have been worn down into small pieces, such as sand, silt, or clay.
  • Organic materials, or the remains of once-living organisms.
  • Chemical precipitates, which are materials that get left behind after the water evaporates from a solution.

Both physical and chemical weathering processes that take place at the Earth’s surface are responsible for accumulating this source of sediment from preexisting rocks, rocks, and even other sedimentary rocks. breaks the rocks apart, while dissolves the less stable minerals. These original elements of the minerals end up in solution, and new minerals may form. Water, wind, ice, or gravity remove and transport sediments in a process called erosion.

Sandstone cliff formation near a beach in San Diego showing evidence of physical weathering.
Figure 3.6.2. Sandstone cliff formation near a beach in San Diego, showing evidence of physical weathering.

We can classify sedimentary rocks in two broad categories: clastic and chemical. sedimentary rocks are made from the fragments of eroded bedrock and sediment, which is usually derived from physical weathering. We classify clastic rocks by their grain size, shape, and sorting. Sedimentary rocks are precipitated from water that contains a very high concentration of dissolved elements, or s. We usually classify these chemical rocks based on the precipitated s or whether biological processes play a role in the rock’s formation.

Clastic Sedimentary Rocks

Clastic sedimentary rocks consist of preexisting rock fragments, or s, that come from weathered bedrock. The majority of this sediment has been physically weathered, but some of it may also be chemically weathered. We classify this type of sedimentary rock with three main identifiers: grain size, shape, and sorting.

Grain size

The size of the individual clasts making up a sedimentary rock is a strong indicator for the amount of energy a landscape had available for transporting the sediment. For example, extremely large fragments, such as boulders, might only be deposited by high energy processes, such as avalanches, whereas wind environments may be able to deposit smaller grains such as silt, mud and clay. Below is a diagram that gives an approximate size range for the different types of clasts that geologists encounter in clastic sedimentary rocks.

Grain Size Chart. Coarse grained clasts (pebbles to boulders) are between 2 mm to over 256 mm. Medium-grained sand is between 2mm down to 63 microns. Fine grained clasts (silt and clay) is between 63 micron to smaller than 2 microns.
Figure 3.6.3. Classification of grain sizes.

Grain shape

The general shape of the individual s within a sedimentary rock is also an excellent method to determine the type of geologic environment that may have deposited the sediment long ago. Clastic rocks with rounded grains were likely transported by water over long distances and with more energy, in which the individual sediments were and physically rounded as they moved along the riverbed (Fig. 3.6.5). By contrast, angular grains indicate the sediments were transported over a relatively short distance by a landslide or event, which did not involve water.

Transportation of sediment clasts by stream flow.
Figure 3.6.4. Transportation of clasts by stream flow. Water can move clasts over long distances and cause the abrasion of clasts that results in rounding. This abrasion happens by the movement of clasts on the stream bed.

Grain sorting

The third identifying property of clastic rocks is the sorting of the individual clasts. A well sorted clastic rock contains clasts that have the same general size in its matrix (Fig. 3.6.5). Such an arrangement means that the clasts were transported over longer distances and/or with greater energy. A poorly sorted clastic rock contains clasts that are a mixture of different sizes in its matrix. This type of clastic rock typically tells a story of sediment being transported over very short distances.

 

Sorting of clasts/grains from well-sorted to poorly sorted and the rounding of grains from angular to rounded.
Figure 3.6.5. Sorting of clasts/grains from well-sorted to poorly sorted and the rounding of grains from angular to rounded.

 

Chemical Sedimentary Rocks

sedimentary rocks are formed by processes that do not directly involve . When preexisting bedrock is weathered by chemical reactions that take place in water, the atmosphere, or the biosphere, that rock is broken down into its constituent s or s that are dissolved and transported in water.

The dissolved ions in the planet’s water supply will eventually precipitate as solid, chemical sedimentary rocks. The type of chemical sedimentary rocks that form broadly varies based on the type of elements that precipitate the rock, as well as whether organic materials contribute to the process. Chemical sedimentary rocks may contain , ions (sulfate and chloride), and .

We can divide chemical sedimentary rocks into two general categories: inorganic and biochemical/organic. are precipitated from only dissolved ions in the water. However, if a chemical rock requires microscopic fossils, shells, or organic material to precipitate, it is called a or . As you will discover with some examples in the field guide below, a sedimentary rock can sometimes be both inorganic and organic!

Sedimentary Rock Field Guide

Conglomerate

Conglomerate Interactive Image
Figure 3.6.6. Conglomerate. Click this image to go to an interactive model by Sara Carena CC-BY.

“CON-GLOM-ER-AT”

Most commonly confused with:

A . is poorly sorted with well-rounded s that are larger than sand (cobble or pebble-sized) within a fine-grained matrix. In a conglomerate, the rock fragments within are sub-rounded to rounded, implying that they traveled along a high energy water source such as a steep stream or river. To tell conglomerate apart from , take a careful look at the individual clasts that compose it. If most of the clasts are rounded, then your rock sample is a conglomerate.

Conglomerate often contains large rock fragments within a very fine-grained matrix, and as a result, these larger clasts are typically the first to dislodge under .

Breccia

Breccia interactive model
Figure 3.6.7. Breccia. Click this image to go to an interactive model of breccia by Sara Carena CC-BY.

“BRECH-AH”

Most commonly confused with:

A . is a poorly sorted rock with angular clasts that are larger than sand. These clasts are cemented within a much finer-grained matrix. Breccia usually indicated that the clasts were transported over a short distance by a high energy event, such as a landslide or the movement of a glacier across the Earth’s surface. To tell breccia apart from , look at the individual clasts within the rock. If the edges of the clasts are angular or sharp, then the rock sample is breccia.

Like conglomerate, breccia contains very large rock fragments within a fine-grained matrix, and these clasts have a tendency to dislodge first under physical weathering.

Sandstone

Sandstone Interactive Model
Figure 3.6.8. Sandstone. Click this image to go to an interactive model by Sara Carena CC-BY.

“SAND-STONE”

Most commonly confused with: ()

A . contains sand-sized s, is most easily identified by its “sandpaper” feel. Sandstone usually appears as a uniform accumulation of sand, and it can vary in color as pink, gray, or beige. Sandstone is reliably deposited by desert environments as well as beaches.

Sandstone can sometimes be confused with the metamorphic rock and for good reason, sandstone is quartzite’s parent rock before it is subjected to high temperatures and pressures! (See section 3.7). Sandstone is typically rougher to the touch and contains smaller individual grains of quartz, whereas quartzite has larger, recrystallized grains of quartz throughout its matrix.

Light tan varieties of sandstone with well-sorted grains of are among the most resistant sedimentary rocks to and . Sandstone is often used as bricks and other construction materials for its durable properties.

Shale

Shale interactive model
Figure 3.6.9. Shale. Click this image to go to an interactive model by Dr. Parvinder Sethi CC-BY.

“SHAIL”

Most commonly confused with: , ,

A . is well-sorted with silt, mud, or clay-sized clasts that are tightly packed into a solid matrix. It is most easily identified by its tendency to split into thin planes, which is a property called . Shale derives from very low energy environments, in which fine-grained sediment can slowly settle from still water and accumulate over time. Examples of these environments are lagoons, lakes, and continental shelves.

“Shale” is sometimes an umbrella term for fine-grained clastic rocks and the term “mudstone”, which refers to rocks made of clasts smaller than sand, also applies to shale. For the purposes of this course, “shale” and “mudstone” are interchangeable. Shale is often composed of , , and minerals, although its color can significantly vary depending on the presence of other minerals. Shale can be pigmented red, black, green, gray, brown, etc., by different minerals.

Shale can easily be confused with counterpart, slate, which is also fine-grained and separates into thin planes. However, with slate, the breakage is much more pronounced and lines can be observed. Light and dark gray varieties of shale are similar in appearance to , but shale does not contain significant quantities of calcium carbonate, and it will not react to dilute hydrochloric acid.

Shale is very susceptible to physical weathering due to the ease with which it breaks into planes. However, when buried under the Earth’s surface, shale’s fine-grained matrix can prevent liquids from seeping past, and consequently, shale has held important roles in natural gas and oil exploration.

Rock salt

Rock Salt/Halite Interactive Model
Figure 3.6.10. Rock Salt/Halite. Click on this image to go to a 3D interactive model by Dexter Perkins (CC BY-NC)

Most commonly confused with: and calcite.

A . Almost every variety of precipitates inorganically from excess sodium (Na+) and chloride (Cl) s in water. Rock salt is one of the few rocks that is composed of a single mineral, (NaCl), and as such, it has a predictable chemical formula and structure. As with halite, rock salt is typically white or colorless with a cubic shape or clusters of cubic crystals that have a distinctive, salty taste.

The presence of rock salt usually indicates that a wet environment has or is undergoing significant drought. Designated as an , rock salt will commonly precipitate along the edges of an evaporating lake; as water evaporates into the atmosphere, the sodium and chloride ions remaining in the dwindling water become more concentrated until rock salt forms. Rock salt may also be common to dry plains that receive periodic rainfall or regions with briny water.

The formation of evaporite sedimentary rocks. As a closed off body of water, such as a lake, evaporates over time, minerals will precipitate in the following order: calcite, gypsum, halite.
Figure 3.6.11. The formation of evaporite sedimentary rocks. As a closed off body of water, such as a lake, evaporates over time, minerals will precipitate in the following order: calcite, gypsum, halite.

Rock salt is sometimes confused with another clear, single-mineral sedimentary rock called . The primary difference between these two rocks can be found in the shape of the crystals. Rock salt has cubic-shaped crystals, whereas rock gypsum can have rhomb-shaped crystals. And, you cannot scratch rock salt with a fingernail.

Rock Salt has been popularly used in the food industry, city-planning (de-icing frozen streets), and agriculture. This rock can completely dissolve in freshwater, and it has a of 3. Therefore, it is very susceptible to both and , and often causes sinkholes if dissolved underground.

Rock gypsum

Rock Gypsum Interactive Model
Figure 3.6.12. Rock gypsum. Click on this image to go to a 3D interactive model by EDUROCK – Educational virtual rock collection (CC BY)

“ROCK JIP-SOME”

Most commonly confused with: ,

A . Almost every variety of precipitates inorganically from excess calcium (Ca2+) and sulfate (SO42-) ions in water. Rock gypsum is an rock that is composed of a single mineral, (CaSO· 2H2O). Therefore, this rock has a predictable chemical formula and atomic structure. Rock gypsum is usually white or colorless with rhomb-shaped crystals or sometimes prismatic crystals. Rock gypsum is also very soft, and its surface can be scratched by a fingernail.

Similar to , the presence of rock gypsum in an environment or outcrop tells you that it was deposited at an arid region with either a briny or evaporating water source. These environments are often evaporating lakes or salt flats that accumulate a high concentration of calcium . It is not unusual to find both rock gypsum, rock salt, and in a sequence in the same outcrop. An evaporating body of water will usually precipitate calcite first, followed by rock gypsum, and then rock salt.

Rock gypsum does not react to dilute hydrochloric acid, unlike the mineral calcite, and its crystals are usually prismatic or rhomb-shaped, unlike rock salt. Rock gypsum is softer than both calcite and halite, and therefore more susceptible to . Rock gypsum has been traditionally used in the construction industry as a building material, such as drywall.

Chert

Chert interactive model
Figure 3.6.13. Chert. Click on this image to go to a 3D interactive model by rocksandminerals (CC BY).

CHURT”

Most commonly confused with: ,

A . Chert can be both inorganically and biochemically precipitated in groundwater or the ocean, and it is usually composed of . varieties of chert form exclusively from dissolved silica in high-temperature, high-pressure groundwater. When this geothermal water source rises to the surface, it can no longer keep the silica in a dissolved state, and it precipitates as solid chert.

varieties are formed when silica-based skeletons such as s and s accumulate on the seafloor after they die. That sediment cements together to form biogenic chert.

 

Microscopic silica-based organisms that make up the ocean. They have intricately built skeletons that are rounded and detailed like snowflakes.
Figure 3.6.14. Various diatoms (left) and radiolarian (right) phytoplankton.

We call chert “jasper”, “flint”, “onyx”, and “agate”, which reflect the wide variety of hues and colors that this rock can have. However, chert is most easily identified by the curved pattern it displays, which is also present in and . But, chert is not as glassy or reflective as these rocks and minerals.

Because of its high silica content, chert is very durable and resistant to weathering. This property has made chert desirable in the construction trade as a road material and gravel. Like onyx, it has been traditionally used as a weapon in spearheads and arrowhead for thousands of years.

Limestone

Limestone outcrop interactive model
Figure 3.6.15. Outcrop of gray limestone that shows weathering. Click on this image to go to a 3D interactive model by Théobald GUFFON (CC BY-NC-SA)

Most commonly confused with: ,

A . can be both inorganically and biochemically precipitated in seawater, and it primarily composed of calcium . limestone forms in the deep ocean when seawater reaches colder depths that cause the dissolved ions calcium (Ca2+) and carbonate (CO32-) to precipitate as solid rock. There are several varieties of limestone that incorporate both the ions and organic material from marine organisms when forming a solid matrix. These types of limestone, described in more detail below, are s.

Limestone is often light to dark gray, or tan, and it can be scratched by a penny. Limestone is composed of calcium carbonate (, (CaCO3) and (CaMg(CO3)2). This rock will strongly fizz when exposed to dilute hydrochloric acid (remember the properties of calcite in video 3.2.3). This reaction clearly distinguishes limestone from other dark gray rocks such as and .

Limestone is very susceptible to by as rainwater and groundwater are both slightly acidic. Consequently, regions with underlying limestone often have caves and experience sinkholes. Despite its tendency to weather over long periods of time, limestone has been (and remains) a popular construction material for thousands of years, dating back to the Great Pyramid of Giza in Ancient Egypt.

Fossiliferous Limestone

Fossiliferous limestone interactive model.
Figure 3.6.16. Fossiliferous limestone. Click on this image to go to a 3D interactive model by rocksandminerals (CC BY)

“FOSSIL-LIFF-ER-RUS LIME-STONE”

Most commonly confused with:

A . specifies a type of that includes visible fossils within the rock’s matrix, and therefore, it is biochemically precipitated. Fossiliferous limestone may contain the remains of shells, s, s, plants or many other types of other animals.

Fossiliferous limestone can form in the deep ocean or near coral reefs, which are composed primarily of calcium . The surrounding matrix has the same properties as limestone (see above), and this rock will react when exposed to dilute hydrochloric acid. However, macrofossils within the rock distinguish fossiliferous limestone from inorganically precipitated limestone.

Coquina

 

Coquina Interactive Model
Figure 3.6.17. Coquina. Click on the image to go to an interactive model by the Digital Atlas of Ancient Life (CC BY-SA)

“CO-KEEN-AH”

Most commonly confused with: n/a

A . is a biochemical variety of limestone that is composed of shells, fossils, and sand that have been poorly together. Coquina is often tan in color with shells that are easily visible to the naked eye. Most of the material within coquina is composed of calcium carbonate, and it fizzes in contact with dilute hydrochloric acid.

Coquina can often be found along beaches or tidal pools in which there are abundant shelly creatures. The leftover shells, sand, and fossils are eventually clustered together by wave action and cemented together upon burial. Coquina is usually not well-cemented together, and it is easily weathered and reworked by and processes.

Chalk

 

Chalk interactive model
Figure 3.6.18. Chalk. Click the image to open an interactive model by PalomarESES CC BY-ND.

Most commonly confused with: n/a

A , sometimes also called marlstone or marl. is a variety of that is composed of microscopic shells from an oceanic organism called a or coccolith. When these organisms die, their calcium carbonate-based shells accumulate along the bottom of the ocean as an ooze-like sediment, which eventually cements as chalk.

Chalk is distinctively white, powdery, and extremely soft to the point of crumbling when touched. It is easily weathered on the Earth’s surface and typically is collected by mining. Chalk has traditionally been used on school boards, but its use has since been widely discontinued in favor of .

Coal

Coal Interactive Model
Figure 3.6.19. Coal. Click on the image to go to an interactive model by the Byrd Polar & Climate Research Center (CC BY-ND)

Most commonly confused with:

An . is one of the few organic sedimentary rocks that is composed of dead plant matter. When enormous amounts of plant matter decay and accumulate from an environment, such as swamplands or dense forests, it eventually becomes buried within the Earth’s surface. There, over millions of years and at high temperatures and pressures, this matter will transform into solid, black rock.

Coal is easily identified by its low , dark black color, and soft, crumbling surface. Because it has been formed by organic matter, it is mostly made of carbon (C). Although is also black and shines like coal sometimes does, obsidian is much harder and more resistant to breaking.

Coal is combustible, and it has reshaped how societies are powered by electricity. Byproducts of burning this rock are commonly called , and they have contributed to significant climate problems that the world is currently facing.