2.5: Sedimentary Rocks
- Page ID
- 21442
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Clastic (Detrital) Sedimentary Rocks
Sedimentary rock forms near Earth’s surface following the accumulation of fragments of rocks and minerals that have been weathered and eroded from outcrops, transported by gravity, rivers, waves, wind, or glacial ice, and then deposited as sediment (Figure \(\PageIndex{1}\)). The fragments of rock and minerals, or detritus, are also known as clasts, and the resulting sedimentary rocks are called clastic sedimentary rocks or detrital sedimentary rocks if they are composed mostly of such grains.
Sediment Size
The size of clasts is related to sediment transport. In general, the longer and/or farther the clast has been transported, the smaller its size. Clasts can range in size from tiny (invisible) clay fragments to boulders the size of buildings (Table \(\PageIndex{1}\)) and we classify clastic sedimentary rock on the basis of clast size. There is no upper or lower limitation on the size of clasts. The key piece of information to remember is that anything sand-sized or larger can be seen with the unaided eye. Sand grains range in size from 1/16th mm to 2 mm. Very fine sand will feel gritty (not slippery) between your fingertips.
Clasts smaller than sand or 1/16th mm are classified as silt and clay. Clasts larger than 2 mm are granules, pebbles, cobbles, and boulders (in order of increasing size). Granules and larger fragments can typically only be transported and deposited by fast-flowing water, and so they are commonly deposited in high-energy parts of streams. Sand grains can be transported by rivers with medium flow, by strong winds, and by waves, and so sand deposits tend to accumulate in rivers, deltas, deserts, and on beaches. Silt and clay can be transported in similar environments, but they tend not to be deposited unless the transport medium slows significantly, so they will usually be deposited in water moving more slowly such as in lakes and the deep ocean basins. Any of these sediments must then become buried beneath other layers of sediment, compressed and cemented before they can become sedimentary rock.
| Type | Description | Size range (millimeters) | Size range (microns) |
|---|---|---|---|
| Boulder | large | 1024 and up | |
| medium | 512 to 1024 | ||
| small | 256 to 512 | ||
| Cobble | large | 128 to 256 | |
| small | 64 to 128 | ||
| Pebble (Granule) | very coarse | 32 to 64 | |
| coarse | 16 to 32 | ||
| medium | 8 to 16 | ||
| fine | 4 to 8 | ||
| very fine | 2 to 4 | ||
| Sand | very coarse | 1 to 2 | 1000 to 2000 |
| coarse | 0.5 to 1 | 500 to 1000 | |
| medium | 0.25 to 0.5 (1/4 to 1/2 mm) | 250 to 500 | |
| fine | 0.125 to 0.25 (1/8th to 1/4 mm) | 125 to 250 | |
| very fine | 0.063 to 0.125 (or 1/16th to 1/8th mm) | 63 to 125 | |
| Silt | very coarse | 32 to 63 | |
| coarse | 16 to 32 | ||
| medium | 8 to 16 | ||
| fine | 4 to 8 | ||
| very fine | 2 to 4 | ||
| Clay | clay | 0 to 2 |
Other Sediment Characteristics
While size is the main characteristic used to classify sediment, other observed characteristics are also important to the description of clasts and analysis of sedimentary environments. Roundness or angularity of the clasts is another important observed characteristic. Roundness is a measure of the smoothness of the clasts. When clasts are well-rounded, they are very smooth. In contrast, when clasts are angular they are not at all smooth and have a jagged, rough shape (Figure \(\PageIndex{2}\)). As with size, the longer/farther the transport, generally the more rounded the clasts.
The other major characteristic of sediment is sorting. Sorting is a measure of the range of clast size within the sediment or the sedimentary rock. Well sorted sediment is mostly the same size and the more variation in size the more poorly sorted the sediment (Figure \(\PageIndex{3}\)). As with size and roundness, the longer/farther the transport, the better sorted the sediment. The sediment in Figure \(\PageIndex{1}\) is an example of moderate sorting.
Clastic Sediment to Sedimentary Rock
Clastic sedimentary rocks are primarily named by the size of the largest clasts present in the rock (Table \(\PageIndex{2}\)). Rocks with clasts smaller than sand-sized are subdivided by the presence or absence of bedding, or fine layering within the rock (Figure \(\PageIndex{4}\)). Rocks with sand-sized clasts are subdivided by the composition of the clasts (e.g. quartz, rock/lithic fragments; Figure \(\PageIndex{5}\)). Rock with clasts larger than sand-sized are subdivided by the angularity/roundness of the clasts (e.g. conglomerate and breccia; Figure \(\PageIndex{6}\)).
| Group | Examples | Characteristics |
|---|---|---|
| Mudrock | mudstone | Greater than 75% silt and clay, not bedded |
| shale | Greater than 75% silt and clay, finely bedded (laminations) | |
| Sandstone | quartz sandstone | Dominated by sand, greater than 90% quartz |
| arkose | Dominated by sand, greater than 10% feldspar | |
| lithic wacke (aka "graywacke") | Dominated by sand, greater than 10% rock fragments, greater than 15% silt and clay | |
| Conglomerate | Dominated by rounded clasts, granule size and larger | |
| Breccia | Dominated by angular clasts, granule size and larger |
Chemical and Biochemical Sedimentary Rock
Sedimentary rock can also form from the crystallization of ions that are dissolved in water. These rocks are known as chemical sedimentary rocks because they form by a chemical process. For example, when minerals crystallize from the evaporation of water in an inland sea or lake, layers of rock salt (halite) or rock gypsum can form.
When chemical processes are assisted by biological organisms, the rocks that form are called biochemical sedimentary rocks. For example, marine organisms extract bicarbonate and calcium ions (HCO3– and Ca2+) from ocean-water to make calcite shells (CaCO3) which then accumulate on the seafloor (typically in tropical areas around reefs) to form calcite mud and sand that later gets buried and becomes limestone. Some organisms make their shells out of silica, and those can accumulate on the seafloor to make the rock chert. Chemical and biochemical sedimentary rocks are classified and named according to their composition.
Carbonate Rocks
Carbonate rocks are usually formed from the carbonate minerals calcite, aragonite, or dolomite. The most common carbonate rocks are limestones and dolostones.
Limestone is usually formed from the mineral calcite, CaCO3, or may be formed by its polymorph, aragonite. Limestone is also one of the few sedimentary rocks that can form as either a clastic, chemical, or biochemical sedimentary rock. Frequently limestone is formed from marine organisms, but limestones form in many different ways and often have names based on their formation. For example, fossiliferous limestone is formed from the fossilized remains of organisms (Figure \(\PageIndex{7}\)), tufa forms at springs (\(\PageIndex{8}\)), and travertine can form in caves or at hot springs (Figure \(\PageIndex{9}\)). Limestone is of economic importance because it is a raw material used in the formation of concrete and also used in steel production.
Dolostone is the carbonate rock made of the mineral dolomite (CaMg(CO3)2). Dolostone is common in some parts of the world, for example there is a whole Italian mountain range named after it (the Dolomite Mountains). This is surprising because marine organisms do not directly precipitate dolomite. Dolomite forms through dolomitization, a process involving chemical reactions between magnesium-rich water percolating through rocks, and sediments containing calcite.
Calcite and dolomite can be distinguished from one another by applying a drop of weak acid to the rock; calcite will react with weak acid, whereas dolomite will not. Also, when dolomite weathers, it tends to turn buff (tan) in color, whereas calcite tends to turn either gray or white.
Chert
Chert is made of silica (SiO2). It has the same chemical formula as quartz, but is cryptocrystalline, meaning that the quartz crystals comprising chert are so small that it is difficult to see them even under a microscope. Chert can be a chemical sedimentary rock, often forming as beds within limestone, or as irregular lenses or blobs (nodules).
Chert can also be biochemical. Some tiny marine organisms, such as diatoms and radiolaria, make their tests from silica. When they die their tiny shells settle slowly to the bottom of the lake or ocean, where they accumulate and are transformed into chert (Figure \(\PageIndex{10}\)).
Evaporites
In arid regions, lakes and inland seas typically have no stream outlet, and the water that flows into them is removed only by evaporation. Under these conditions, the water becomes increasingly concentrated with dissolved salts, and eventually some of these salts may reach saturation levels and start to crystallize (Figure \(\PageIndex{11}\)).
Although all evaporite deposits are unique because of differences in the chemistry of the water, in most cases minor amounts of carbonates start to precipitate when the solution is reduced to about 50% of its original volume. The mineral gypsum (CaSO4·2H2O) precipitates at about 20% of the original volume, and halite (NaCl) precipitates at 10%. Other important evaporite minerals include sylvite (KCl) and borax (Na2B4O7·10H2O). In California, sylvite occurs in dry lake beds in Inyo and Imperial Counties.
Depositional Environments
Sedimentary rocks and sediments reflect the environments in which they are deposited. This makes them incredibly useful when trying to determine the geologic history of an area. They are classified by their distinctive physical and chemical indicators caused by whether they are on land (terrestrial), transitional between the land and the sea (coastal), or marine. The physical and chemical indicators in terrestrial environments in particular are useful for helping to determine the paleoclimate of areas over time.
| Environment | Key Transport Process(es) | Depositional Setting(s) | Typical Sediments |
|---|---|---|---|
| Glacial | Gravity, moving ice, moving water | Valleys, plains, streams, lakes | Glacial till, gravel, sand, silt, clay |
| Alluvial | Gravity, moving water | Where steep-sided valleys meet plains | Coarse angular fragments |
| Fluvial | Moving water | Streams | Gravel, sand, silt, organic matter |
| Aeolian | Wind | Deserts and coastal regions | Sand, silt |
| Lacustrine | Moving Water | Lakes | Sand, silt, clay, organic matter |
| Evaporite | Still water | Lakes in arid regions | Salts, clay |
| Environment | Key Transport Process(es) | Depositional Setting(s) | Typical Sediments |
|---|---|---|---|
| Deltaic | Moving water | Deltas | Sand, silt, clay, organic matter |
| Beach | Waves, long-shore currents | Beaches, spits, sand bars | gravel, sand |
| Tidal | Tidal currents | Tidal flats | Fine-grained sand, silt, clay |
| Reef | Waves, tidal currents | Reefs and adjacent basins | Carbonates |
| Shallow marine | Waves, tidal currents | Shelves, slopes, lagoons | Carbonates in tropical climates; sand/silt/clay elsewhere. |
| Lagoonal | Little transportation | Lagoon bottom | Carbonates in tropical climates, silt, clay |
| Submarine fan | Underwater gravity flows (turbitity currents) | Continental slopes, abyssal plains | Gravel, sand, silt, clay |
| Deep water | Ocean currents | Deep-ocean abyssal plains | Clay, carbonate mud, silica mud |
References
- Earle, S. (2019). Physical Geology – 2nd Edition. Victoria, B.C.: BCcampus. Retrieved from https://opentextbc.ca/ March 2024