Travertine - Groundwater containing dissolve Calcium and bicarbonate ions can precipitate calcite to form a chemically precipitated limestone, called travertine. This can occur in lakes, hot springs, and caves. Dolostones - Limestone that have been chemically modified by Mg-rich fluids flowing through the rock are converted to dolostones. Chemical Cherts - Groundwater flowing through rock can precipitate SiO 2 to replace minerals that were present.
This produces a non-biogenic chert. There are many varsities of such chert that are given different names depending on their attributes, For example:. Flint — Black or gray from organic matter. Jasper — Red or yellow from Fe oxides.
Petrified wood — Wood grain preserved by silica. Agate — Concentrically layered rings. As mentioned previously, all stages of the sedimentary cycle leave clues to processes that were operating in the past. Perhaps the most easily observable clues are structures left by the depositional process. We here discuss sedimentary structures and the information that can be obtained from these structures.
Because sediment is deposited in low lying areas that often extend over wide areas, successive depositional events produce layers called bedding or stratification that is usually the most evident feature of sedimentary rocks.
The layering can be due to differences in color of the material, differences in grain size, or differences in mineral content or chemical composition. All of these differences can be related to differences in the environment present during the depositional events. A series of beds are referred to as strata. A sequence of strata that is sufficiently unique to be recognized on a regional scale is termed a formation.
A formation is the fundamental geologic mapping unit. See figure 7. Bedforms are linked to flow velocity and sediment size. Ripples are characteristic of shallow water deposition and can also be caused by wind. Sand dunes are similar, but on a larger scale. Ripples are commonly preserved in sedimentary rocks. Asymmetric ripples as shown above indicate flow direction,with the steep slope on the down - current direction.
Symmetric ripples form as a result of constant wave energy oscillating back and forth. If we look at various environments now present on Earth, we can find characteristics in the sediment that are unique to each environment. If we find those same characteristics in sedimentary rocks, it allows us to interpret the environment of the past.
Each environment has its own energy regime and sediment delivery, transport and depositional conditions that are reflected in the sediment deposited. Sedimentary Environments can be divided into the following. We will cover most of these environments in more detail later in the course. For now familiarize yourself with each of these by reading pages to in your text. Throughout geologic history sea level has risen and fallen by as much as a few hundred meters many times. These changes are the result of changes earth's climate or changes in the shape of the sea floor as a result of tectonics.
When sea level rises, the coast migrates inland. This is called a Transgression. Beach sand gets buried by marine sediments and the sea floor subsides due to the weight of the sediment. During a transgression, the beach sand forms an extensive layer, but does not all have the same age. When sea level falls, the coast migrates seaward. This is called a Regression. The sedimentary sequence then repeats itself in a vertical sense as the sedimentary environment migrates back and forth.
LIthification of sediment into sedimentary rocks takes place after the sediment has been deposited and buried. The processes by which the sediment becomes lithified into a hard sedimentary rock is called diagenesis and includes all physical, chemical and biological processes that act on the sediment.
The first step in diagenesis is the compaction of the sediment and loss of water as a result of the weight of the overlying sediment. Compaction and burial may cause recrystallization of the minerals to make the rock even harder. Fluids flowing through the rock and organisms may precipitate new minerals in the pore spaces between grains to form a cement that holds the sediment together.
Common cements include quartz, calcite, and hematite. Other conditions present during diagenesis, such as the presence of absence of free oxygen may cause other alterations to the original sediment. In an environment where there is excess oxygen Oxidizing Environmen t organic remains will be converted to carbon dioxide and water. In an environment where there is a depletion of oxygen Reducing Environment , organic material may be transformed to solid carbon in the form of coal, or may be converted to hydrocarbons, the source of petroleum.
Diagenesis is also a response to increasing the temperature and pressure as sediment gets buried deeper. Quartz is not easily dissolved or chemically altered, so it is resistant to breakdown by chemical reactions as well.
That is why beach sand is often more rich in quartz than any other mineral. Although feldspar is a fairly hard mineral, it does cleave split apart and is chemically reactive, especially in the presence of water. The most abundant product of chemical reaction of feldspar and water is clay minerals. During the erosion-to-deposition process, clastic sediments lose feldspar and gain a larger proportion of clay.
Other minerals such as amphiboles, micas, and carbonates are relatively soft and chemically reactive and tend to be scarce or absent as sediment grains in mature clastic sediments, although calcite may be present in clastic sedimentary rocks as a secondary, cementing mineral that grew during lithification.
Minerals in chemical sedimentary rocks precipitate from water and usually remain in place or are not transported far before lithification. Such mineral sediments are subjected to little, if any, erosion and transportation.
Therefore, the minerals in chemical sedimentary rocks are not winnowed during the weathering-to-deposition process as are the minerals in clastic sedimentary rocks. In some cases, during the formation of chemical sediments, the minerals may change as a result of chemical reactions. For example, dolostone is a chemical sedimentary rock that forms in certain coastal environments by alteration of precipitated calcite to dolomite.
Besides minerals, mineraloid solids occur in some chemical sedimentary rocks. For example, the carbonaceous material in coal is an organic mineraloid rather than a mineral. Another example, opal, is a chemical sedimentary rock that does not have a fully developed crystal lattice and therefore is a mineraloid.
Figure 2. Sandstone is one of the common types of sedimentary rocks that form from sediments. There are many other types. Sediments may include:. Rocks at the surface undergo mechanical and chemical weathering.
These physical and chemical processes break rock into smaller pieces. Sediments are removed and transported by water, wind, ice, or gravity in a process called erosion Figure 2.
Streams carry huge amounts of sediment Figure 3. The more energy the water has, the larger the particle it can carry. A rushing river on a steep slope might be able to carry boulders. As this stream slows down, it no longer has the energy to carry large sediments and will drop them. A slower moving stream will only carry smaller particles. Sediments are deposited on beaches and deserts, at the bottom of oceans, and in lakes, ponds, rivers, marshes, and swamps.
Avalanches drop large piles of sediment. Glaciers leave large piles of sediments, too. Wind can only transport sand and smaller particles. The type of sediment that is deposited will determine the type of sedimentary rock that can form. Different colors of sedimentary rock are determined by the environment where they are deposited. Red rocks form where oxygen is present.
Darker sediments form when the environment is oxygen poor. Figure 4. Dissolution is a form of weathering—chemical weathering. With this process, water that is slightly acidic slowly wears away stone. These three processes create the raw materials for new, sedimentary rocks. Precipitation and lithification are processes that build new rocks or minerals.
Precipitation is the formation of rocks and minerals from chemicals that precipitate from water. Finally, lithification is the process by which clay, sand, and other sediments on the bottom of the ocean or other bodies of water are slowly compacted into rocks from the weight of overlying sediments.
Sedimentary rocks can be organized into two categories. The first is detrital rock , which comes from the erosion and accumulation of rock fragments, sediment , or other materials—categorized in total as detritus, or debris. The other is chemical rock, produced from the dissolution and precipitation of minerals. Detritus can be either organic or inorganic. Organic detrital rocks form when parts of plants and animals decay in the ground, leaving behind biological material that is compressed and becomes rock.
Coal is a sedimentary rock formed over millions of years from compressed plants. Inorganic detrital rocks, on the other hand, are formed from broken up pieces of other rocks, not from living things. These rocks are often called clastic sedimentary rocks. One of the best-known clastic sedimentary rocks is sandstone. He works as a research guide for the U. Geological Survey. Updated September 28, Featured Video.
Cite this Article Format. Alden, Andrew. Get to Know 24 Types of Sedimentary Rock. How to Identify the 3 Major Types of Rocks. Conglomerate Rock: Geology, Composition, Uses. Mechanical Weathering Through Physical Processes. Rock Provenance by Petrologic Methods. Geologic Maps of the 50 United States.
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