Stage (stratigraphy)

In the study of chronostratigraphy, a stage is a vital unit of time. It represents a succession of rock strata. These layers were laid down during a single age on the geologic timescale. This process usually spans millions of years of deposition. By scientific convention, a specific stage of rock and its corresponding age share the same name. They also share the same boundaries. Stages serve as the building blocks for larger divisions. For instance, rock series are divided into stages. This is similar to how geological epochs are divided into ages.

Stages can be broken down into even smaller parts. These smaller stratigraphic units are called chronozones or substages. When scientists group many stages together, they create superstages. It is important to note that stages and rock types are not always the same. A single stage might include many lithostratigraphic units. These units include things like formations, beds, or members. These different rock types might have formed in different environments at the same time. Conversely, one lithostratigraphic unit might contain several different stages or parts of stages.

Scientists use specific methods to define a stage. They often rely on biostratigraphy, which is the study of fossils. A stage is defined by a consistent set of fossils found within the rock. They may also use paleomagnetism, which looks at consistent magnetic polarity. Another method is petrology, the microscopic analysis of the rock itself. This can help confirm if a segment of rock belongs to a particular age. In the past, some researchers used the term "faunal stage." This referred to the fact that the same fauna, or animals, were found throughout the layer. However, the term "faunal" is now often dropped as global correlations have become more certain.

To find the start of a stage, scientists look for index fossils. An index fossil is a very useful tool for dating. These fossils must be common and easily recognized. They should be found worldwide. Most importantly, they must be limited to only a single stage or a few stages. For example, in North America, paleontologists use specific trilobites to identify time. Finding fragments of the trilobite Olenellus identifies the beds as the Waucoban Stage. If they find the later trilobite Elrathia, they know they are in the Albertan Stage.

The way we use stages has changed over time. In the 19th and early 20th centuries, stages were the primary tool for dating. Scientists used them to correlate rock units across different areas. At that time, they did not have modern technology. They lacked seismology and radioactive dating, which were developed in the second half of the 20th century. Originally, faunal stages were only defined for specific regions. As new geochronologic tools were created, stages were defined over much broader areas. There was even a tendency to use European or Asian names for stages worldwide. This happened even if the animals in other regions were very different.

Today, international standardization is managed by the International Commission on Stratigraphy, or ICS. The ICS is part of the International Union of Geological Sciences. They work to establish boundaries and names that everyone accepts. Since 1974, the ICS has been working to subdivide the Phanerozoic eonothem. They use two different types of benchmarks to ensure accuracy. For younger stages, they use a Global Boundary Stratotype Section and Point, known as a GSSP. A GSSP is a physical outcrop that clearly demonstrates the boundary. For much older stages, they use a Global Standard Stratigraphic Age, or GSSA. A GSSA provides an absolute date.

These benchmarks are essential for modern science. They allow researchers to compare results with great confidence. This certainty provides a much wider scope than using only local knowledge. While many regions still use local subdivisions and classification criteria, the world is moving toward a uniform system. As international research becomes more complete, these local systems are expected to be abandoned. This global system helps all geologists speak the same language when discussing the history of the Earth.