Sedimentary Rocks

Rock Cycle: Sedimentary

Sedimentary rocks form when loose clasts (grains or fragments produced through erosion) are cemented together to become solid rock, a process known as cementation. Sedimentary rock formation is driven by solar energy and gravity. It is a largely exogenic process happening at and above Earth’s surface, and is commonly tied to the hydrologic cycle.

• Fun fact: sedimentary rock is where you will always find fossils! Fossils are formed when an organism is rapidly buried by clasts/sediment which eventually lithifies into rock. Check out this page about the Green River Formation Links to an external site., an approximately 50 million year old formation across parts of Wyoming, Colorado, and Utah states in the Western U.S. which contains a tremendous number of fossils.

Sedimentary rocks are divided into different categories based on their origins and how they are formed: clastic, chemical, biochemical, and organic. As you will see in the descriptions below, these are not mutually exclusive categories - a rock can fit into multiple categories at the same time!

Clastic Sedimentary Rock

Clastic sedimentary rocks form through the weathering and erosion of existing rock, which then cements together to form a new rock – sand into sandstone, for example. Clastic sedimentary rocks are formed through the following stages:

1. Weathering

The original, existing rocks are weathered through exposure to Earth’s atmosphere, precipitation, and other stressors over time. Eventually, the source rock disintegrates and dissolves into clasts or rock particles.

2. Erosion

The clasts are eroded away through water/streamflow, gravity, ice, and/or wind and carried across the landscape. At this point, the moving rock particles are called sediment.

3. Transportation

The sediment produced through erosion is transported from ‘high energy’ sites (where the carrying medium has the energy to pick up and move the sediment) to ‘low energy’ sites.

4. Deposition

As the energy of the transporting medium decreases, it can no longer carry the sediment. The sediment settles out of the transporting medium and is dropped along river channels, beaches, ocean bottoms, etc. It is eventually buried.

5. Lithification

After the sediment is buried, the loose sediment eventually lithifies, or is hardened into solid rock. Lithification happens through a combination of compaction and cementation. Compaction occurs when the weight of overlying material squeezes water and/or air out of the buried sediments. Cementation then follows as minerals fill the remaining spaces and fuse the clasts together into a coherent mass. The type of cement depends on the environment – calcium carbonate (CaCO3) is the most common cementation medium, followed by iron oxides and silica.

Clastic sedimentary rock occurs as eroded sediments are transported and deposited into water. Over time, these sediments are buried and compacted by new layers of material. These layers become cemented together (typically with CaCO3), becoming a sedimentary rock formation.

Different clast/sediment sizes and source materials create different kinds of sedimentary rocks. A few examples:

• Conglomerate: rocks made of gravel, pebbles, and even up to boulders
• Sandstone: rocks made of sand
• Siltstone, Mudstone: rocks made of silt
• Shale: rocks made of clay (microscopic particles)

Different sizes of sediments create different types of sedimentary rock - a process strongly influenced by the energy level of transportation and deposition. For instance, higher energy deposition results in rocks with larger-sized clasts such as conglomerate rock (left) made of pebbles, gravel, or even boulders. Meanwhile, lower energy deposition results in rocks made of smaller-sized clasts. For instance, shale (right) is made of microscopic clay particles.

(Bio)Chemical Sedimentary Rock

Chemical sedimentary rocks form through chemical precipitation, or the formation of a solid substance from a solution. In chemistry, a solution is a mixture of a substance dissolved into a liquid, creating a homogenous distribution of the substance. Chemical precipitation occurs when the solid substance forms and separates from the liquid medium in which it had been dissolved.

Consider what happens to a glass of salt water (a solution of salt and water) that is left out. Eventually the water will evaporate, but the salt will be left behind as a residue.

Chemical sedimentary rocks are produced through the chemical precipitation of dissolved minerals from a water solution.

Chemical sedimentary rocks are formed from dissolved minerals that precipitate out of water solutions and build up to form rock. The most common chemical sedimentary rock is limestone. Limestone can form purely as a form of chemical precipitation, as CaCO3 chemically precipitates out of groundwater that seeps to the surface. This process forms travertine, a type of limestone created through mineral deposits near natural hot springs and in caves where groundwater bubbles to Earth’s surface.

• For a fascinating read, check out this story Links to an external site. of a man who found an odd formation in his parents’ new travertine tile floor that turned out to be a fossilized human mandible!

Stalactites and stalagmites in limestone caves are formed as CaCO3 is chemically precipitated out of groundwater seeping through karst limestone environments. The water depicted on the stalactites above contains dissolved minerals like gypsum, which slowly precipitate out of the groundwater and build up, increasing the size of the stalactites over time.

However, limestone is more commonly formed as a biochemical sedimentary rock from marine organic origins. Many marine organisms extract dissolved CaCO3 from seawater to construct solid shells. When these organisms die, the solid shell material builds up on the ocean floor. Eventually, it is lithified to become limestone. For this reason, large biochemical limestone formations show us where shallow seas and oceans used to exist!

The Salem Formation is a limestone formation in southern Indiana and Kentucky, which is known for producing very high-quality limestone used for construction (including the U.S. Capital building in Washington, D.C.). The Salem Formation was formed during the Mississippian period (approx. 340 million years ago), when what we today know as much of North America was on a continental platform covered by a shallow sea.

Organic Sedimentary Rock

Organic sedimentary rocks are formed from organic debris. There is a lot of overlap between organic sedimentary rocks and other kinds of sedimentary rocks. For example, biochemical limestone is also considered organic sedimentary rock since it forms from marine organic shells.

Another example of an organic sedimentary rock is coquina, a clastic conglomerate sedimentary rock made of shell fragments.

Another organic sedimentary rock example is coal. Coal is formed from dead vegetation in a process called coalification. Approximately 90% of Earth’s coal beds were deposited in the Carboniferous and Permian geologic eras (~358.86-251.9 million years ago), periods when Earth had much higher atmospheric concentrations of carbon dioxide. In this time, Earth had many dense forests in low-lying areas and wetlands which sequestered this atmospheric carbon.

As plants died, the resulting biomass was quickly covered by mud or water, which protected it from oxygen exposure and slowing decomposition rates. This dead plant matter was eventually converted into peat, an accumulation of partially decayed vegetation and organic matter, forming peat bogs that trapped tremendous amounts of carbon sequestered by the dense forests.

• Fun fact: the lack of oxygen, low temperature, acidic water of peat bogs also can preserve organic matter. For instance, bog butter is ancient butter or animal fat found buried in peat bogs, usually in a wooden container. Burying bog butter for preservation dates back to at least 100 CE. Many ancient human cadavers have also been discovered in peat bogs, naturally mummified due to the unique characteristics of the bogs. There is a full list of all known discovered bog bodies Links to an external site. on Wikipedia!

Over millions of years, the heat and pressure of deep burial forced water and gas out of the buried matter (compaction), creating coal. Higher heat and pressure results in coal formations with higher proportions of carbon because more water and gas has been squeezed out.

Coal is an organic sedimentary rock that forms through millions of years of burial and compaction.