5.3: Sediments and Sedimentary Rocks
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Sedimentary rocks are arguably the most important to study and understand in this class as they contain a level of detail about Earth's history not always found in other rock types. They are also have the widest variation in their formation such that we classify them into four different types:
- clastic sedimentary rock (detrital)
- chemical sedimentary rock
- biochemical sedimentary rock
- organic sedimentary rock
Weathering
In most cases the process of sedimentary rock formation begins with the exposure of pre-existing rock to weathering and erosion. Weathering involves exposure of rock to Earth’s surface weather conditions. Wind, water and ice, plus gravity, play a role in the breakdown of the materials. Erosion is the removal of weathered rock material from its original location. The effects of weathering on pre-existing rock can be both physical and chemical.
Physical Weathering
Physical weathering is the process of physical disintegration: the breaking of solid rock into smaller pieces without changing its chemical composition, primarily caused by external forces such as temperature fluctuations, ice expansion, or abrasion by wind and water.
One example of physical weathering occurs when water in cracks in rock freezes and expands. This expansion places stress on the rock, and it breaks into smaller pieces. This process exposes an ever-increasing number of surfaces to additional weathering.
Chemical Weathering
Chemical weathering alters the chemistry of minerals in the pre-existing rock. These changes may result in new mineral formation by adding or subtracting elements from the original mineral structure, or in the further disintegration of pre-existing minerals through chemical decomposition.
The dominant agent involved in both physical and chemical weathering is water. Water acts as an agent of change, both introducing elements that can interfere with the chemistry of exposed minerals and physically removing weathered rock material and ions lost through decomposition.
Products of Weathering
Weathering produces particles of sediment (of various sizes) plus ions in water. The sediment is transported to some setting where it gets deposited. This depositional location may be very close to the origin of the sediment (such as a rock fall off a cliff face) or many hundreds (or even several thousand) of kilometers away (for example the beach along the continental shoreline). Following deposition, the sediment is buried, compacted, and cemented together to form solid rock. We call these processes “diagenesis,” and collectively they serve to lithify the sediment into a rock.
The ions in solution (weathered chemical elements) can be:
- used in cementing particles of sediment together.
- extracted from the water to support life, both biochemical processes (in plants or animals) and in the formation of skeletal material, such as shells.
- precipitated from the water through supersaturation triggered by evaporation or by changing physical conditions of the surrounding water.
The importance of these processes are discussed in more detail elsewhere in this text. The simple classification of sedimentary rock is presented below.
Clastic Sedimentary Rocks (Detrital)
Clastic sedimentary rock (also called ‘detrital’ sedimentary rock) consists of sediment pieces (clasts/rock detritus) that come from weathered (broken down) rock, cemented together to make a new rock. A more comprehensive explanation of this process is represented in the diagram immediately below in which source rock is weathered to create sediment, eroded (removed from its original location), transported (carried by wind, water, or ice), deposited (its journey ends), buried under other sediment such that it is exposed to small amounts of pressure and temperature resulting in the formation of the sedimentary rock.
Clastic (or detrital) rock is classified and named based on the size and composition of the clasts. Their various sizes, from boulders (large) to sand to clay (very small), is referred to as the sediment grain size. The classification and description of the various clastic rocks appears in the chart at the bottom of this page. The size, shape, and composition of the clasts varies widely based upon the source rock and the length of time the sediment has been moving through the system.
Common Clastic Sedimentary Rocks
Conglomerate is a coarse-grained clastic sedimentary rock composed of a substantial amount of rounded gravel-size (larger than 2 mm in diameter) clasts. Conglomerates form by the lithification (compaction and cementation) of gravel. Conglomerates are typically poorly sorted (contain many different grain sizes), having finer-grained sediment, e.g., either sand, silt, and/or clay in between the gravels.
Figure \(\PageIndex{4}\): Conglomerate. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.
Breccia is a coarse-grained clastic sedimentary rock composed of a substantial amount of angular gravel-size (larger than 2 mm in diameter) clasts. Breccias are typically poorly sorted (contain many different grain sizes), having finer-grained sediment, e.g., either sand, silt, and/or clay in between the gravels.
Figure \(\PageIndex{5}\): Breccia. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.
Quartz sandstone is a clastic sedimentary rock composed mainly of sand-sized (1/16 to 2 mm in diameter) quartz mineral grains. They are most commonly white or red in color. The red sandstones have iron oxide (essentially rust) cementing the quartz grains together. The shape of the sand grains (rounded or angular) and the sorting (all one size or variable in size) varies and helps geologists determine in what geologic setting (beach vs sand dune vs river) the sand was collected.
Figure \(\PageIndex{6}\): Quartz Sandstone. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.
Arkose sandstone is a clastic sedimentary rock composed mainly of sand-sized (1/16 to 2 mm in diameter) feldspar mineral grains. Pink (potassium feldspar) and white (plagioclase feldspar) are most common. The sand grains tend to be angular and they tend to be poorly sorted (contain many different grain sizes).
Figure \(\PageIndex{7}\): Arkose Sandstone. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.0 Attribution.
Greywacke is a clastic sedimentary rock that is a variety of sandstone. It is generally characterized by its dark color, and poorly sorted (contain many different grain sizes), angular grains of quartz, feldspar, and small rock fragments set in a matrix of clay.
Figure \(\PageIndex{8}\): Greywacke. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.
Siltstone is a clastic sedimentary rock primarily composed of silt-sized (1/16-1/256 mm in diameter) particles.
Figure \(\PageIndex{9}\): Siltstone. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.
Shale (with shell and plant fossils in the sample above), which is the most common sedimentary rock, is a clastic sedimentary rock primarily composed of silt- and clay-sized (< 1/16 mm) minerals. Shale is characterized by breaks along parallel layering or bedding less than one centimeter in thickness, called fissility.
Figure \(\PageIndex{10}\): Shale. Photo credit: CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.
Claystone is a clastic sedimentary rock composed largely of clay, which measures less than 1/256 of a millimeter in particle size (not visible with the naked eye). Claystone is non-fissile as it does not break easily into layers like shale.
Chemical, Biochemical, and Organic Sedimentary Rocks
Chemical sedimentary rock is formed by processes that do not directly involve mechanical weathering and erosion. Chemical weathering contributes the dissolved ions in water that ultimately form the various rock types. Inorganic chemical sedimentary rock is made of minerals precipitated from ions dissolved in solution. Inorganic chemical sedimentary rock forms in environments where ion concentration, dissolved gasses, temperatures, or pressures are changing, which causes minerals to crystallize, such as through the process of evaporation.
Biochemical sedimentary rock is formed from shells and bodies of underwater organisms (such as coral). The living organisms extract chemical components from the water and use them to build shells and other body parts. Biochemical sedimentary rocks are commonly composed of calcite, from a wide variety of sea life, or silica, largely from the single-celled microorganisms called radiolarian.
Organic sedimentary rock contains the remains of organic material that has undergone the lithification process. The source materials are plant and animal remains that are transformed through burial and heat, and end up as coal, oil, and methane (natural gas).
Common Chemical, Biochemical and Organic Sedimentary Rocks
Fossiliferous limestone is any type of limestone, made mostly of calcium carbonate (\(\ce{CaCO3}\)) in the form of the mineral calcite, that contains an abundance of fossils or fossil traces. The fossils in these rocks may be of macroscopic or microscopic size. This particular sample contains an abundance of mollusc and gastropod fossils.
Figure \(\PageIndex{12}\): Fossiliferous Limestone. Photo credit: CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.
Oolitic Limestone is a type of limestone formed from ooids, small, spherical grains composed internally of calcium carbonate (\(\ce{CaCO3}\) – calcite) in concentric layers.
Figure \(\PageIndex{13}\): Oolitic Limestone. Photo credit: CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.
Micrite is a limestone formed of calcareous (composed of calcite) particles ranging in diameter up to four microns. It is formed by the recrystallization of calcareous mud.
Figure \(\PageIndex{14}\): Micrite. “Fossiliferous dismicrite (Nineveh Limestone, Lower Permian; Clark Hill section, Long Ridge, Monroe County, Ohio, USA) 1” by James St. John is licensed under CC BY 2.0 Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.Chalk is a soft, white, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells (coccoliths) shed from micro-organisms called coccolithophores.
Figure \(\PageIndex{15}\): Chalk. Photo credit: CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.
Travertine is a form of limestone deposited by mineral springs, especially hot springs. Travertine is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot spring or in a limestone cave. In the latter, it can form stalactites, stalagmites, and other speleothems (as in the 3D model, below. Grab and rotate the sample to see it in 3D). Travertine can be white, tan, cream-colored, or somewhat rusty in color.
Crystalline limestone is a carbonate sedimentary rock that is composed of the precipitation of the mineral calcite (\(\ce{CaCO3}\)) from saturated sea water. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (\(\ce{CaCO3}\)). About 10% of sedimentary rocks are limestones.
Figure \(\PageIndex{17}\): Crystalline Limestone. Photo credit: Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, licensed under a Creative Commons Attribution 3.0 United States License. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.Dolostone is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite, \(\ce{CaMg(CO3)2}\). Dolomite contains nearly equal amounts of magnesium and calcium. Most dolomites formed as a magnesium replacement of limestone or lime mud before lithification. Dolomite is less soluble than limestone and, for identification purposes, must first be scratched into a small power before it will effervesce with dilute hydrochloric acid (whereas limestones effervesce without being powdered).
Figure \(\PageIndex{18}\): Dolostone. Photo credit: “Dolostone (Silurian; midwestern USA)” by James St. John is licensed under CC BY 2.0. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.Chert is a hard, fine-grained sedimentary rock composed of crystals of quartz (silica \(\ce{SiO2}\)) that are very small (microcrystalline or cryptocrystalline). It often breaks with a conchoidal fracture and was used to make sharp objects like knives and arrowheads in the prehistoric era. Chert is often of biological origin (organic) but may also occur inorganically as a chemical precipitate or a diagenetic replacement (e.g., petrified wood). Geologists use chert as a generic name for any type of microcrystalline or cryptocrystalline quartz. Varieties dark in color are called flint; red varieties are known as jasper; multi-colored and banded varieties are known as agate.
Figure \(\PageIndex{19}\): Chert. Photo credit: Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, licensed under a Creative Commons Attribution 3.0 United States License. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.Rock gypsum is composed of the very soft mineral gypsum which defining characteristic is that it can be scratched with a fingernail. It is typically white to very pale pink or tan. Gypsum rock forms within layers of sedimentary rock from the slow evaporation of seawater.
Figure \(\PageIndex{20}\): Rock Gypsum. James Petts From: Wikimedia is licensed under: Creative Commons Attribution-Share Alike 3.0 license.Rock salt is composed of the mineral halite (sodium chloride, NaCl, common table salt) which defining characteristics include that it tastes salty and will dissolve readily in water. It is typically clear to white to pale pink or tan. Rock salt forms within layers of sedimentary rock from the slow evaporation of seawater.
Figure \(\PageIndex{21}\): Rock Salt. Photo credit: CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.
Bituminous coal is an organic sedimentary rock formed by diagenesis of peat bog material. Bituminous coal or black coal is a relatively soft coal containing a tar-like substance called bitumen. Its coloration can be black or sometimes dark brown; hard varieties may shine with an iridescent effect. The carbon content of bituminous coal is around 60–80%; the rest is composed of water, air, hydrogen, sulfur, and heavy metals such as lead, mercury, nickel, tin, cadmium, antimony, and arsenic, as well as radio isotopes of thorium and strontium. Consequently, there is no such thing as “clean coal".
Figure \(\PageIndex{22}\): Bituminous Coal. Photo credit: Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, licensed under a Creative Commons Attribution 3.0 United States License. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.Sedimentary Rock Identification Chart
| Step 1: Determine makeup | Step 2: Determine Grain Size | Step 3: Rock Description | Step 4: Rock Name |
|---|---|---|---|
| CLASTIC: Composed of pieces of rocks and minerals. | Gravel (> 2 mm) |
Rounded rock or mineral fragments; usually poorly sorted. |
Conglomerate |
|
Angular rock or minerals fragments; usually poorly sorted. |
Breccia |
||
| Sand (0.06 mm to 2 mm) |
Mostly quartz grains; sorting and rounding variable. |
Quartz Sandstone |
|
|
Mostly quartz with at least 25% feldspar; rock fragments common; usually poorly sorted with angular grains. |
Arkose |
||
|
Clay, quartz, feldspars, and rock fragments; usually poorly sorted with angular grains; often dark color. |
Graywacke |
||
| Silt (< 0.06 mm) |
Silt-sized particles are too small to identify; no layering making it not fissile. |
Siltstone |
|
|
Silt and clay-sized particles are too small to identify; layering makes rock break in planes making if fissile. |
Shale |
||
| Clay (< 0.004 mm) |
Clay-sized particles are too small to see. |
Claystone |
|
| Step 1: Determine makeup |
Step 2: Composition |
Step 3: Rock Description |
Step 4: Rock Name |
| CHEMICAL, BIOCHEMICAL & ORGANIC: Made of minerals that have crystallized together, or biological fragments of shells of plants. | Calcite - All fizz with acid |
Abundant fossils. Possibly in micrite. |
Fossilliferous Limestone1 |
| Abundant ooids - coarse, sand. sized spheres with concentric internal layers |
Oolitic Limestone |
||
|
Microcrystalline2; breaks with conchoidal fracture. |
Micrite |
||
|
Shell fragments loosely cemented with a high porosity. |
Coqina |
||
|
Microscopic fossil fragments; chalky; white color; soft. |
Chalk |
||
|
Microcrystalline2; color banding of browns, grays, whites, and blacks. |
Travertine |
||
|
Coarse crystals easily visible. |
Crystalline Limestone |
||
| Dolomite |
Fizzes with acid only when powdered; fine to coarsely crystalline; may contain fossils. |
Dolostone |
|
| Microcrystalline Quartz |
Microcrystalline2; conchoidal fracture; hardness of 7 so steel nail leaves a metal streak on the chert surface; sharp edges used for pre-historic spearheads and knives; synonymous with flint. |
Chert |
|
| Gypsum |
Crystalline; very soft with a hardness <2, white, gray, or pink color. |
Rock Gypsum |
|
| Halite |
Crystalline; salty taste, white or gray color; fairly soft hardness of 2.5. |
Rock Salt |
|
| Plant Material |
Plant fragments; low density; brown to black color; often crumbly. |
Bituminous Coal |
1Fossiliferous Limestone can be further subdivided into Grainstone, Packstone, Wackestone and Mudstone depending on the amount of mud with the lithified lime mud. Grainstone = less than 10% mud \(\rightarrow\) Mudstone = less than 10% fossil organisms.
2Microcrystalline - crystals that are visible only through a high-powered microscope.
- arkose sandstone - clastic sedimentary rock composed of sand-sized feldspar particles
- biochemical sedimentary rock - sedimentary rock formed as shells and the body parts of underwater organisms lithify
- bituminous coal - sedimentary rock formed by the breakdown of peat bog material
- breccia - poorly sorted clastic sedimentary rock composed of angular gravel-sized particles
- chemical sedimentary rock - sedimentary rock formed as minerals precipitate from ions dissolved in solution
- chalk - sedimentary rock made of very small shells made of calcite
- chert - sedimentary rock made of very small crystals of quartz
- clastic sedimentary rock - sedimentary rock composed of sediment compacted and cemented to form a new rock
- claystone - clastic sedimentary rock composed of clay-sized particles
- conglomerate - poorly sorted clastic sedimentary rock composed of rounded gravel-sized particles
- crystalline limestone - a sedimentary rock formed as calcite precipitates
- dolostone - a sedimentary rocks formed as magnesium replaces the calcium in limestone before lithification
- erosion - the removal of weathered rock material from its original location
- fossiliferous limestone - sedimentary rock composed of calcite that contains an abundance of fossils
- greywacke - clastic sedimentary rock composed of sand-sized particles that vary in their composition
- micrite - sedimentary rock made of very fine grains of calcite
- oolitic limestone - sedimentary rock formed from small grains of calcite
- organic sedimentary rock - sedimentary rock formed as the remains of organic materials undergo lithification
- quartz sandstone - clastic sedimentary rock composed of sand-sized quartz particles
- rock gypsum - sedimentary rock made of gypsum
- rock salt - sedimentary rock made of halite
- shale - clastic sedimentary rock composed of silt- and clay-sized particles that breaks into sheets
- siltstone - clastic sedimentary rock composed of silt-sized particles
- travertine - form of limestone deposited by mineral springs
- weathering - breakdown of materials due to exposure to the atmosphere and water