Sedimentary Rocks Formation and Fossils
A sedimentary rock is just what it sounds like: it is a rock that consists of sediment! They can consist of sand, clay, chalk and fossils. As a marine geologist, I find sedimentary rocks very fascinating!
Some may think that sedimentary rocks are a bit dull since they aren't created by violent and exciting volcano eruptions from the Earth's mantle like the igneous rocks. No, sedimentary rocks have another type of fascinating origin, and every single rock tells a story if you just know how to "read" them! That is one of the fascinating things with sedimentary rocks.
The other exciting part with sedimentary rocks is that they tell us about Earth's history! I will tell you a little about how to do read the rocks, and I hope it will help you to see sedimentary rocks in nature in a new way.
Every single particle in a sedimentary rock initially comes from a rock or as soil on land. Over time, the rock is broken down into small particles by weathering and the small particles are transported away. Sometimes the transportation distance is long and sometimes shorter. Most sedimentary rocks consist of small particles that have a long and fascinating story to tell from their long journey behind them. Read on and you will know why and how.
First we need to make clear what sediment is! Sediment is material that occurs natural and is broken down by processes like weathering and erosion. Sediment are also transported in some way by water or wind, by ice, and/or transported by gravity from the particles itself.
This means that sedimentary rocks can consists of all the materials on earth and take a minute to think about the breathtaking fact that every single particle in a sedimentary rock have been transported and shaped by transportation in more than one media, and finally, that particle has settled down upon the deep ocean floor long, long time ago. It gets even more fascinating to think about that we can actually see and walk on former ocean floor that looks amazing in many places on earth. I have some pictures from such a place further down in the article. And then, when the ocean floor becomes rocks on land the weathering starts again. It is like an ongoing transportation of particles that never ends.
I think you all know what weathering is but I include the definition anyway. Weathering occurs when a rock is fragmented by mechanical forces or are decomposed by chemical alteration.
Mechanical weathering is done by water, wind, frost wedging, heat, ice, biological activity like roots, and since it is only mechanical influence, there is no change in the ingoing parts of the rock since the rock's mineral composition is the same. It is only broken it down in smaller pieces. The end result is many small pieces from a single large one.
Chemical weathering means that the rock goes through a transformation chemically into one or more new compounds. Since water is a great solvent water is a major force in chemical weathering. But rocks are also weathered in other ways such as through dissolution, oxidation and hydrolysis that occurs in water.
How Are Sedimentary Rocks Formed?
All these single particles of sand, rocks, mud and clay becomes sedimentary rocks mainly by through two major ways of lithification.
Lithification means a process where sediments are transformed into sedimentary rocks. Cementation and compaction are both lithification processes that transform sediments into sedimentary rocks. The necessary compaction is created by the accumulation of sediment that accumulate over already deposited sediment. By time, the weight and the heat increases and the grains are pressed closer and closer together. The compaction reduces the pore space between the particles and can in this way transform fine grained particles into more or less solid rocks.
For rocks with bigger particles, the transformation to a rock comes from cementation that is created by smaller particles that fills the pore spaces between the bigger particles.
There are two major groups of sedimentary rocks: chemical sedimentary rocks and detrital sedimentary rocks.
Detrital Sedimentary Rocks
The most common minerals in detrital sedimentary rocks are clay minerals and quartz but they can also consist of feldspars and micas.
Detrital rock are distinguished by particle size as you can see in the table above. Apart from distinguish different sedimentary rocks the particle size can also tell us useful information about the environment where the particle once deposited. Big particle size needs stronger currents to move them and so big particle size implies that the particle was settled in a stronger current. Less energy is required to transport small particles as clay and sand, so then we know that fine sand can be transported by wind and smaller water currents and it takes a very calm environment for clay to settle and deposit. This is very interesting and by thinking about it we can figure out where a sedimentary rock was created.
Shale is a very common sedimentary rock that consists of clay and silt sized particles. Since the particles are so small they can not be seen without magnification. The particle size is very small and this means that it must have been deposited in a relatively calm environment such as deep-ocean basins or in lakes with not so strong currents. Other places where shale can form are lagoons and river floodplains. The special with shale is that this sedimentary rock has the ability to split into thin layers. This is because silt and clay particles in shale are so closely packed and the particles are also positioned parallel alignment to each other. Although shale is the most common of sedimentary rock it is not so well known as the sandstone. The reason for that is probably that shale isn't so visible and many time the shale are covered with soil or are overgrown by vegetation. The soil comes from the shale itself since shale decompose easily. This is very obvious in places where shale and sandstone are present. In such places you can see sandstone that have dramatic forms with steep edges and the shale that has much less steep slopes and shale is also often the areas where vegetation are visible.
Sandstone is a rock that contains of sand sized grains and sandstone is the second most common sedimentary rock on earth and probably the most known. The history and origin of a sandstone can often be told by the sorting of the grains, by the particle size, the particles roundness and mineral composition. For example, if the grains are rounded we can tell that the particle has been transported some distance by water. There are many different types of sandstones and the difference between them is due to the minerals in the stone.
Grand Canyon, Arizona is a place where both Sandstone and Shale can be seen.
Conglomerate and Breccia
Conglomerate consists mostly of gravels. It can consist of large boulders and smaller gravel. The particles with large grain size can be seen visually and the spaces between the gravel are often filled with sand and mud. With the aid from the big particle size in conglomerate we can tell that there are indications of that the deposition have occurred in an environment with strong currents and/or slopes.
Breccia is pretty much the same as a conglomerate but in a breccia the particles have angular shape instead of rounded shape. The angular shape tells us that the gravel hasn't been transported very long distance from where it was deposited.
Gravel (<2 mm)
Rounded rock fragments
Gravel (<2 mm)
Angular rock fragments
Sand (1/16 mm)
Sand (1/16 mm)
Quartz with considerable feldspar
Sand (1/16 mm)
Dark color; quartz, feldspar, clay
Mud (<1/16 mm)
Splits into thin layers
Mud (<1/16 mm)
Breaks into clumps and blocks
Chemical Sedimentary Rock
Limestone is the most abundant chemical sedimentary rock and consists mostly of calcite. Most of them origins from a marine environment and consists of the skeletons of dead organism. One example are coral reefs, and the most well-known coral reef is the Australian Great Barrier Reef. But there are also limestones that have an inorganic origin and consist of calcite that that was formed by chemical changes or high water temperatures.
In 2002 I participated in an Excursion on Cyprus conducted by the Department of Earth Sciences, University of Gothenburg. It was 14 wonderful days on the lovely island Cyprus and I learn so much by using marine geology practically in field. One of the tasks that was given to us was to describe and interpret the formation and also date the limestone called the Lefkara formation at Petounda Point in southern Cyprus. You can see the impressive formation in the photo. Although this formation is very spectacular and special, there are so many places where you can see fantastic limestones on Earth.
Chart is made of silica that is very compact and hard. Examples of Chart are Flint and Agate. Chart can be found in limestone and as layers in rock. The silica in charts can be originated from organism with silica skeletons or from volcanic ash.
Dolostone is composed of dolomite that are a calcium, magnesium carbonate mineral. They are formed in seawater and is related to limestone.
Evaporites are formed when seawater evaporates. And everywhere evaporites can be found today there has been a basin submerged under seawater during Earths history. The minerals in seawater evaporates in different pace according to their solubility. Gypsum and halite are not so soluble and evaporates first and later comes potassium and magnesium salt.
Coal is made of organic matter such as leaves, wood and bark and other plant materials. It takes millions of years to form coal and it can only develop in a oxygen- poor swamp where the bacteria can not fulfill the decomposition of the plants.
Dolomite, CaMg (Co3)2
Microchrystalline Quartz SiO2
Gypsym, Ca So4 2 H2O
Altered Plant Remains
Fossils and Dating
Dating fossils can for example be done by dating the rock itself or by dating the rock from its fossil content. Many times both methods are used for correlation.
Through many years of research on rocks and fossils scientists has developed a geological time scale for Earth. This geologic scale has been thouroughly verified by matching rocks of similar age in different regions.
Dating rocks from a rocks physical criteria can easily be done by correlation when it comes to short distances since we can find similar layers in the rocks from one place to another. But when it comes to correlation of rocks in widely separated areas it is better to correlate the layers in rock by its fossil content. This can be done because research has shown that fossil organisms succeed one another in a definite and determinable order. This means that any time period in Earth history can be recognized by its fossil content. This is known as the "principal of fossil succession."
Some fossils are more useful for dating and correlation than other fossils and these fossils are called index fossils. Index fossils are fossils that was spread over big parts of the Earth at a certain time and these fossils are therefor great time indicators.
Dating fossils and rocks isn't done by using only one dating method. Instead many different methods are being used to correlate in order to be sure of the dating. It would take me several more hubs to explain in detail the methods and how it is done, and maybe I will extend this hub later with dating methods.