Stratigraphic unit

Rocks come in many layers.

Some layers look the same. They have special colors. They can have old bones inside.

Scientists group these layers. They use the look of the rock. They can also use old bones.

Small layers are called beds. Larger layers are called formations. A group is many formations together.

These layers help us map the land. We can see them in the ground.

Rocks often form in many layers. Scientists call these layers stratigraphic units. Each unit has a special look. They might have a certain color. They might have a certain type of rock.

Some units are defined by fossils. These are called biostratigraphic units. People often call them biozones. A biozone can be based on many fossils living together. This is called an assemblage zone. Other biozones look for one specific fossil. These are called range zones. Some zones show where many fossils of one kind are found. This is an abundance zone.

Scientists also group rocks by their parts. They use a scale for these groups. A bed is the smallest unit. A member is a part of a formation. A formation is a main unit. These can be very big or very small. A group is two or more formations. A supergroup is a set of groups.

Geological surveys give these units formal names. They often name them after the place where they were found. This helps people map the land.

Rocks often form in many different layers. Scientists call these layers stratigraphic units. A unit is a volume of rock with a specific origin. It also has a specific age range. Each unit must be easy to map and recognize. You can tell them apart by their look or their fossils. These special features are called facies.

There are different ways to define these rock units. One way is by the type of rock used. For example, a unit might be defined if sandstone is more than 75% of it. Another way is to use fossils. These fossil-based units are called biostratigraphic units. People often call them biozones. You can find five main types of biozones. An assemblage zone has three or more fossils living together. A range zone shows where one specific fossil is found.

Scientists use a hierarchy to organize these units. This means they group them from smallest to largest. A bed is the smallest unit you can recognize. It is a distinct layer within a larger part. A member is a named part of a formation. A formation is the main unit used for mapping. These can be very small or many kilometers wide. A group is a set of two or more formations. A supergroup is a set of groups.

Geological surveys give these units their formal names. They usually name them after the place where they were first found. The name often includes the rank or the rock type. In Utah, you can see many famous rock formations. These are in places like Capitol Reef National Park. You can see the Navajo Sandstone and the Kayenta Formation there. You can also see the Wingate Sandstone and the Chinle Formation. There is also the Moenkopi Formation and the Cutler Formation.

Understanding these layers helps us see the history of Earth. It is like looking at the pages of a very old book. Each layer tells a story about what happened long ago. Some layers might show where animals lived. Other layers show how the land changed over time. You can see how a group might thin out into a formation. This is called a pinch out. This helps scientists map the world more accurately.

A stratigraphic unit is a specific volume of rock. It has an identifiable origin and a certain age range. Geologists define these units using distinctive features called facies. These facies are recognizable patterns in the rock's composition or its fossils. A unit must be easy to map and distinct from other units. However, the boundary between two units does not always have to be sharp. For example, a unit might be defined if its sandstone content exceeds 75%.

Geologists use a hierarchy to organize these units by their rank. This system moves from the smallest units to the largest. The main lithostratigraphic ranks include the bed, member, formation, group, and supergroup. Higher rank units usually contain two or more units of a lower rank. A unit can change its rank depending on the location. A group might thin out to become a formation in a different region. A formation might also reduce to a member or a bed when it "pinches out."

The smallest recognizable unit is called a bed. A bed is a lithologically distinct layer found within a member or a formation. Most beds do not have formal names. They are only named if they serve as a marker horizon. A member is a named, distinct part of a formation. Not all formations are divided into members. Even when they are, a member might only make up part of the total formation. Members do not always have to be mappable at the same scale as a formation.

Formations are the primary units used to subdivide a sequence of rocks. They can vary greatly in scale. A formation might be only tens of centimeters thick or many kilometers wide. To be formally recognized, a formation must be large enough to be useful for mapping. Groups consist of two or more formations that share lithological characteristics. A group can include different formations in different geographical areas. Individual formations can also appear in more than one group. Sometimes, groups are divided into subgroups, though this is rare in the North American Stratigraphic Code.

The largest category in this hierarchy is the supergroup. A supergroup is a collection of two or more associated groups or formations. These units share certain lithological characteristics. Like groups, supergroups can be made of different units from various geographical locations. Formal names for these units are assigned by geological surveys. Usually, a unit of formation rank or higher is named after its type location. The formal name often includes the unit's rank or its lithology.

Scientists also use fossils to define rock sequences. These are called biostratigraphic units, or biozones. There are five common types of biozones used in study. An assemblage zone features three or more coexisting fossil taxa. A range zone shows the entire range where a specific fossil taxon is found. An abundance zone is where a specific taxon appears in much higher numbers than nearby areas. An interval zone is defined by the first and last appearance of two different taxa. Finally, a lineage zone contains fossils that show parts of an evolutionary lineage.

In the United States, the Colorado Plateau provides a clear example of these principles. The Permian through Jurassic strata in southeastern Utah show many different layers. These rocks form famous landmarks in Capitol Reef National Park and Canyonlands National Park.

Moving from top to bottom, you can see the tan Navajo Sandstone. Below that is the red Kayenta Formation. Then comes the vertically jointed Wingate Sandstone. The purplish Chinle Formation follows, then the lighter-red Moenkopi Formation. Finally, the white, layered Cutler Formation sandstone sits at the bottom.