Chapter 9 Main Ideas
9.1 Clastic Sedimentary Rocks
Clastic sedimentary rocks are formed from rock and mineral particles that are cemented together. The naming system for these rocks depends on grain size, sorting, composition, and shape. Five common types of clastic sedimentary rocks are conglomerate, breccia, sandstone, shale, and mudstone. Sandstones are further organized according to the abundance of fine particles they contain, and the composition of their sand-sized grains.
9.2 Chemical and Biochemical Sedimentary Rocks
Chemical and biochemical sedimentary rocks form from ions that were transported in solution, and then converted into minerals by chemical and/or biological processes. The most common biochemical rock, limestone, typically forms in shallow tropical marine environments, where biological activity is a very important factor. Chert and banded iron formations can be from deep-ocean environments. Evaporites form where the waters of lakes and inland seas become supersaturated due to evaporation.
Extra! Compare & Contrast
Do you know your clastic, chemical, and biochemical rocks well enough to be able to tell them apart? How would you distinguish each of the following rocks from each other?
Start by thinking about which general sedimentary rock type they are. When you have it figured out, click each link to check your answer.
- Conglomerate vs. breccia: Both conglomerate and breccia are coarse-grained and poorly sorted clastic sedimentary rocks. However, the grains in conglomerate are rounded whereas the grains in breccia are angular.
- Sandstone vs. shale: These are both clastic rocks. Sandstone is medium-grained so you may be able to see individual sand grains. Shale is fine grained, so individual grains will be too small to see and it will feel smoother than the sandstone. Also, shale is distinctive in that it breaks apart in layers.
- Travertine vs. limestone formed in a reef: Travertine and limestone are both made of the mineral calcite. One way to distinguish between these rocks is to look for fossils of reef-building organisms in the biochemical limestone. Travertine is a chemical sedimentary rock, so it won’t be built from the shells of marine organisms.
- Chert vs. gypsum, an evaporite rock: Both are chemical sedimentary rocks. Chert is made of silica (the same composition as quartz) and gypsum is a mineral as well as a rock. You could test the mineral properties of the samples (chert will scratch glass, but you can probably scratch gypsum with your fingernail), but you could also add water to the samples. Gypsum will dissolve but chert will not. (But don’t do that if you want to keep your gypsum sample safe.)
9.3 Organic Sedimentary Rocks
Organic sedimentary rocks contain abundant organic carbon molecules (molecules with carbon-hydrogen bonds). An example is coal, which forms when dead plant material is preserved in stagnant swamp water, and later compressed and heated.
9.4 Depositional Environments and Sedimentary Basins
There is a wide range of depositional environments, both on land (including glaciers, lakes, and rivers) and in the ocean (including deltas, reefs, shelves, and the deep-ocean floor). In order to be preserved, sediments must accumulate in sedimentary basins, many of which form through plate-tectonic processes.
9.5 Sedimentary Structures and Fossils
Sedimentary rocks can have distinctive structures that are important in determining their depositional environments. Fossils are useful for determining the age of a rock, the depositional environment, and the climate at the time of deposition.
Mysterious Rocks on Mars
The image below was taken by NASA’s Curiosity Rover (field of view is approximately 1.2 m across.) The image shows a slab of red stone surrounded by sand. The slab is covered in a complex network of interconnected lines.
Which sedimentary structure might the lines represent?
The pattern of lines on the slab of rock is most likely from mud cracks.
What would that sedimentary structure imply about past environments on Mars?
The presence of mud cracks is evidence that Mars had abundant liquid water in the past.
Read more about this image.
Jan. 17, 2017
The network of cracks in this Martian rock slab called “Old Soaker” may have formed from the drying of a mud layer more than 3 billion years ago. The view spans about 4 feet (1.2 meters) left-to-right and combines three images taken by the Mars Hand Lens Imager (MAHLI) camera on the arm of NASA’s Curiosity Mars rover.
Mud cracks would be evidence of a time when dry intervals interrupted wetter periods that supported lakes in the area. Curiosity has found evidence of ancient lakes in older, lower-lying rock layers and also in younger mudstone that is above Old Soaker.
MAHLI was positioned about 3 feet (90 centimeters) above the surface when it took the component images on Dec. 31, 2016, during the 1,566th Martian day, or sol, of Curiosity’s work on Mars. This observation was planned as part of assessing a hypothesis that the target preserves evidence of drying mud. The location is within an exposure of Murray formation mudstone on lower Mount Sharp inside Gale Crater.
The slab bears a network of four- and five-sided polygons about half an inch to 1 inch (1 to 2 centimeters) across, which matches the pattern commonly formed when a thin layer of mud dries. Some edges of the polygons are ridges of material the same color as the surrounding rock. This could result from a three-step process after cracks form due to drying: Wind-blown sediments accumulate in the open cracks. Later, these sediments and the dried mud become rock under the pressure of multiple younger layers that accumulate on top of them. Most recently, after the overlying layers were eroded away by wind, the vein-filling material resists erosion better than the once-muddy material, so the pattern that began as cracks appears as ridges.
Note that some of the cracks contain material much brighter than the surrounding rock. These are mineral veins. Curiosity has found such bright veins of calcium sulfate in many rock layers the rover has investigated. These veins form from circulation of mineral-laden groundwater through underground cracks. Rover-team scientists suggest that a likely scenario for the history of Old Soaker is more than one generation of fracturing: mud cracks first, with sediment accumulating in them, then a later episode of underground fracturing and vein forming.
The target rock’s name comes from the name of an island off the coast of Maine. The names informally assigned by the rover team to features in the area of lower Mount Sharp that includes this slab are from a list of islands, hills and other sites in or near Maine’s Bar Harbor.
Malin Space Science Systems, San Diego, built and operates MAHLI. NASA’s Jet Propulsion Laboratory, a division of the Caltech in Pasadena, California, manages the Mars Science Laboratory Project for the NASA Science Mission Directorate, Washington, and built the project’s Curiosity rover. More information about Curiosity is online at http://www.nasa.gov/msl and http://mars.jpl.nasa.gov/msl/.
9.6 Groups, Formations, and Members
Sedimentary sequences are classified into formations so that they can be mapped easily and without confusion. Formations can be combined into groups, or broken down into members for more detail.
Key Term Check
What key term from Chapter 9 is each card describing? Turn the card to check your answer.