Stage (stratigraphy)

In chronostratigraphy, a stage is a group of rock layers formed during one specific time period, called an age, on the geologic timescale. These layers usually build up over millions of years. The same name and boundaries are used for both the rock stage and the matching time age.

Rock series are split into stages, similar to how geological epochs break into ages. Stages can be divided into smaller parts known as chronozones or substages. When put together, these smaller parts form larger units called superstages.

Sometimes, people use the term faunal stage. This name comes from the fact that the same kinds of fauna, or animals, are found throughout these rock layers. This helps scientists understand how long ago these animals lived and how the Earth changed over time.

Definition

Stages help us understand how old rocks are by looking at special fossils or magnetic patterns in the rocks. Scientists use certain fossils, like trilobites, that are found all over the world to tell which stage a rock belongs to. For example, finding pieces of a trilobite called Olenellus means the rock is from the Waucoban Stage.

In the past, stages were a main way to figure out the age of rocks before we had tools like seismology and radioactive dating. Looking closely at tiny parts of rocks can also help confirm their age. At first, stages were only used in certain areas, but now we use names mostly from Europe to describe the same time periods around the world.

International standardization

Boundaries and names are set by the International Commission on Stratigraphy (ICS) of the International Union of Geological Sciences. By 2008, the ICS had almost finished a task started in 1974. This task was to divide the Phanerozoic eonothem into stages that everyone in the world agrees on. They use two types of markers to show where these stages begin and end. For more recent stages, they use a Global Boundary Stratotype Section and Point (GSSP). This is a specific spot in rocks that clearly shows where one stage ends and another begins. For older stages, they use a Global Standard Stratigraphic Age (GSSA), which is an exact date. These markers help scientists compare their results with more confidence.

In many places, scientists still use their own local ways to divide and classify rocks. But once the international system is fully finished, it is expected that these local systems will no longer be used.

Stages and lithostratigraphy

Stages can include many different types of rock layers, such as formations, beds, and members. These layers form in various environments at the same time. Similarly, a rock layer can contain several stages or parts of stages.