Several Examples of Sedimentary Rocks

Sedimentary rocks are rocks that are created through sediment which is pressurized and heated as it pushes deep into the earth.  The sediment for these rocks are pieces from other rocks, which can be sedimentary, igneous, or metamorphic in nature.  Additionally organic material can wind up in the mix for the creation of sedimentary rocks.

Sedimentary rocks are classified into three types; Organic (Biogenic), Chemical, and Clastic.  For more information on sedimentary rocks, what they are and how they are formed read this article “What Are Sedimentary Rocks”.  Some additional reading on the “Diagram of The Rock Cycle Diagram Explained” can also help in identifying and understanding the sedimentary rock cycle

With the many different examples of sedimentary rocks on our earth, we can note that some are more popular than others.  In this article we will list out several known examples of sedimentary rocks as well as some that are not so well known.  We will go into some detail into the three types of sedimentary rocks as well so as to give you a feel for the sedimentary rock types.

Some examples of sedimentary rocks:

• Coal
• Limestone
• Dolomite
• Chert
• Flint
• Gypsum
• Rock Salt
• Breccia
• Conglomerate
• Shale
• Sandstone

Organic sedimentary rocks

Organic sedimentary rocks are rocks that contain greater amounts of organic life.  Plants and the remains of life are much of what you would find as organic in these types of rocks.  There are other forms of organic life found in sedimentary rocks such as fossilized remains of fish or mammals, however those rocks may not be considered as an organic.

For a sedimentary rock to be classified as organic, it has to contain greater than 3% of organic carbon.  This organic carbon is distributed throughout the rock and at times will not be in any sort of recognizable form when viewed.

Organic sedimentary rocks are formed much like other sedimentary rocks.  Organic life such as plants and animals collect on the floor of larger bodies of water.   With great amounts of pressure and heat from other layers on top of it, the organic life will solidify to create the sedimentary rock.     

Some good examples of organic sedimentary rocks can include some forms of limestone, some forms of dolomite, coal, and shale.

Coal

This sedimentary rock is found in seams between other rock.  For the most part, coal is comprised of carbon but can have some other elements as well.  This rock is formed when dead plants decay and initially form as layers of peat.  As time goes on, burial of this decaying plant matter is pressurized and heated to form coal.  Coal is, for the most part, used as fuel for energy and electricity. 

Some Forms of Limestone

Some limestones can contain large amounts of plant and animal life.  This organic life can will be distributed throughout the host rock and mixed in with other sediment.  Some limestones will show many small pieces of shells, scales, and other remains of life.  Other limestones will not express the organic contents in it by visual sight but would have to be tested.

Some Forms of Dolomites

Dolomite is a sedimentary rock that contains calcium magnesium carbonate.  It is said that dolomite is an alteration of limestone, and that alteration was brought about by the magnesium within the rock likely interacting with the amounts of organic material available.  Some believe that sulfate-reducing bacteria can play a part in some formations of dolomite.  Magnesium in water interacts with surrounding limestone and creates a chemical reaction to alter limestone into dolomite.  Some call this transformation dolomitization.  

Chemical sedimentary rocks

Chemical sedimentary rocks are formed when minerals are precipitated from water.  This precipitation of minerals from water can be done through the process of evaporation or a process of temperature change and increased acidity. 

When the water evaporates for a chemical sedimentary rock, you have a high concentration of what is left over.  For a temperature changing example the water does not disappear, but will precipitate in the water, no longer capable of holding the elements within it.

Some good examples of chemical sedimentary rocks are; Chert, Flint, Gypsum, Limestone, and Rock salt.

Chert

Chert is a chemical sedimentary rock that seems like it could be considered organic at times.  It can contain remains of what is called siliceous ooze, or very small organic ocean life remains.  Chert is very fine grained and will consist of very tiny grains of quartz.  Chert is very similar to flint and is capable of generating a spark when struck with steel.   For more information on chert you can read “Chert Rock – What is Chert?”

Flint

Flint will be found in chalk or marly limestone which is primary how many will distinguish flint from chert.  It will typically be found as nodules within these other sedimentary rocks.  Along with chert, flint will also have a glassy look to it and can be fairly smooth on the surface.  This rock has been used in the past for tools and weapons much like obsidian and chert.

Gypsum

Gypsum is a calcium sulfate dihydrate.  That means two water molecules included for its chemical formula.  This mineral can come in some interesting forms such as what is called a desert rose or selenite.  It is often used as fertilizer and various forms of plaster and drywall.  Gypsum is considered to be an evaporate and is very common mineral on our earth.  It occurs by evaporation of water and forms at the bottom of dried out bodies of water.   For more information on gypsum you can read “All About Gypsum – Uses, Properties, Color, and Worth”

Limestone

Limestone can be considered organic or chemical sedimentary rock.  It is commonly made up of oceanic remains, calcite, and aragonite.  It is very similar to dolomite however the big difference is that it dolomite contains magnesium.  Limestone is used most commonly in architecture.  This can include pyramids, buildings, and sculptures.  Its ability to be easily cut, makes it great for these types of applications.  You can often find limestone in products such as paint, paper, plastics, and even in your toothpaste.  Fossils can also be found in some limestones.  These limestones are capable of being broken, or split, on a horizontal plane revealing the long ago life captured within its layers.

Rock Salt

Rock salt, also known as Halite, is a type of salt containing sodium chloride.  It will be found within other sedimentary rocks as an evaporate from very salty water.  Rock salt will be found in large concentrations and is often mined for the purposes of flavoring our food.  Food preservation is often a use of rock salt.

Clastic sedimentary rocks

Clastic sedimentary rocks are what you would think of when it comes to the basic sedimentary rocks in the sedimentary process as part of the rock cycle.  Various different sized rocks, or clasts, will break down and find their way into large bodies of water to form as layers.  These layers are solidified by pressure and heat and depending on the grain size of the rocks contained within, will determine what clastic sedimentary rock it is.

Some examples of clastic sedimentary rocks will include; Breccia, Conglomerate, Sandstone, and Shale.

Breccia

Breccia is a clastic sedimentary rock that has larger grains sizes within it.  This clastic sedimentary rock will form from larger events that will break off bigger pieces of rock, such as landslides.  These larger rocks will get caught up in the process of the sedimentary rock cycle and will often be rocks that are angular and misshapen rather than rounded like conglomerate rock.  Some Breccia can be igneous in origin, coming from a giant bubble of magma as seen in above photo.

Conglomerate

Conglomerate is a clastic sedimentary rock that is composed of more rounded rocks from smaller grain sizes to larger sizes.  The smaller sized grains, which can consist of sands and clay, will make up the surrounding rock for the larger clasts.  With larger more rounded clasts, it might indicate those rocks may have traveled in rivers and streams or just downhill.  Those rocks would be smoothed out a bit before it got to its final destination for inclusion as conglomerate.   

Shale

Shale is a very fine grained clastic sedimentary rock consisting of mud, clay, and other fine grained debris and minerals such as quartz.  This rock will break into thin slabs if split on their horizontal plane.  Shale is the most common sedimentary rock available on our earth.   Though shale rock contains roughly 95% of organic matter in all existing sedimentary rocks, those organic amounts within the rock per mass, are less than the 3% keep it from being an organic sedimentary rock.   One of the wonders of sedimentary rocks like shale is that there can be fossils found in between its layers at times.

Sandstone

Sandstone is a clastic sedimentary rock that contains a little bit larger grain sizes than shale.  This well know sedimentary stone contains mostly quartz or feldspar.  Sandstone can have some interesting and colorful layers within the rock which has made this sedimentary rock quite popular.  This rock will contain what is known as pore space.  Pore space is simply the space between grains of sediment.  For sandstone the pore space is a bit more open giving it more porosity as it is said.  For further reading and information on sandstone you can read this article “What is Sandstone”