Group (stratigraphy)

Rocks can form big sets.

Scientists call these sets groups. A group has many layers. These layers are called formations. One group can even form a supergroup.

Groups help us map the land. They show how layers relate. They help us study large areas.

Some groups have many named layers. Other groups have layers not yet named. We use rocks to name them. We do not use old fossils.

It is like a big book of stone.

Rocks form in many layers. Scientists study these layers. They call these layers formations. A group is a set of related formations.

Groups help people map the land. They show how different layers relate. They also help us study large areas. Some groups are very big. We call these supergroups.

One example is the Glen Canyon Group. It is in Utah. It has four parts. These parts are the Wingate Sandstone. Then there is the Moenave Formation. Next is the Kayenta Formation. The last part is the Navajo Sandstone.

These parts look different from each other. But they all formed in one big sand desert. This desert is called an erg. Not all parts are in every place. In New Mexico, there is the Vadito Group. Some parts have names. Other parts do not have names yet. Scientists must use the rock type to name a group. They do not use fossils to name them.

Geologists study layers of rock to understand Earth. They use a system called lithostratigraphy. This system organizes rocks into specific units. A group is a special type of unit. It is made of several related formations. Formations are the most basic units in this study. Sometimes, groups are even joined into supergroups. These groups help scientists map large areas of land. They also show how different rock layers relate to each other.

Groups work by grouping related rock layers together. Scientists look at the rock type to make these groups. This rock type is called lithology. Each formation in a group has its own lithology. This means they look and feel different from their neighbors. A group can be defined even if some parts are not yet split into formations. Geologists often define a group when exploring a new area. They might divide the layers into smaller formations later. This helps them organize their findings step by step.

One famous example is the Glen Canyon Group. This group is found in southeast Utah. It contains four distinct parts in order. First is the Wingate Sandstone. Next is the Moenave Formation. Then comes the Kayenta Formation. The final part is the Navajo Sandstone. All these layers formed in one huge sand desert. This type of desert is called an erg. Not every part of the group is in every area.

Other groups exist in different places too. The Vadito Group is in northern New Mexico. Some of its layers are named, like the Glenwoody Formation. Other lower layers are not yet divided into formations. In northwestern Europe, people used groups to track time. They used names like Rotliegend and Zechstein. They also used Buntsandstein, Muschelkalk, and Keuper. These groups belong to the Permian and Triassic ages. Some belong to the Jurassic age as well.

It is important to follow specific rules for naming. A group must be based on rock types. Scientists cannot use fossil types to define a group. This rule prevents confusion in the geologic timescale. The ICS does not use some European group names. This is because those names caused too much confusion. Using rock types keeps the science clear and steady. This helps everyone study the history of our planet.

In the science of geology, scientists use a system called lithostratigraphy to organize Earth's history. This system categorizes layers of rock into specific units. One such unit is known as a group. A group is a lithostratigraphic unit made of several related formations. Formations are considered the fundamental unit of stratigraphy. Sometimes, these groups are even combined into even larger units called supergroups.

Groups are essential tools for geologists working in the field. They are useful for showing the relationships between different formations. Scientists also use groups for small-scale mapping projects. They are helpful when studying the stratigraphy of very large regions. When geologists explore a brand new area, they might define a group immediately. They do this if they believe the layers can be divided into formations later. This happens during subsequent investigations of that specific area.

Not every part of a group is always divided into formations. It is possible for only some of the strata to be split up. For example, the Vadito Group is located in northern New Mexico. Many of its strata have been divided into formations, such as the Glenwoody Formation. However, other parts of the group remain undivided. Specifically, the lower part of the Vadito Group has not been split into formations. This shows that the classification of a group can evolve over time.

A primary way to distinguish layers is through their lithology. Lithology refers to the physical characteristics of the rock. In a group, each formation can be distinguished from its neighbor by its lithology. This means the rocks look or feel different from the layer above or below them. Even if they look different, they may share a common history. For instance, the Glen Canyon Group in southeast Utah is a famous example. It contains the Wingate Sandstone, the Moenave Formation, the Kayenta Formation, and the Navajo Sandstone.

The layers of the Glen Canyon Group tell a story of an ancient environment. All of these formations were deposited in the same vast erg. An erg is a large desert area covered in sand. While these formations share that history, they are not always seen together. Not all these formations are present in every area where the group exists. This variety helps geologists understand how environments change over time. The layers are listed in ascending order from the Wingate Sandstone up to the Navajo Sandstone.

In northwestern Europe, groups have a complicated history with timekeeping. In the past, groups like the Rotliegend and Zechstein were used. These two groups belong to the Permian age. Other groups include the Buntsandstein, Muschelkalk, and Keuper. These three belong to the Triassic age. There are also Lias, Dogger, and Malm groups from the Jurassic age. Because using these names caused confusion, the official geologic timescale changed. The ICS does not include these specific names in its official timescale.

There are strict rules that geologists must follow when defining these units. A group must be a lithostratigraphic unit. This means it must be defined by the rocks themselves. A group must not be defined by fossil taxonomy. Fossil taxonomy is the science of naming and classifying fossils. By focusing on rock types instead of fossils, scientists keep the system consistent. This ensures that lithostratigraphy remains a reliable way to map the Earth's crust.