Within geology, there is a branch called stratigraphy that studies the superposition of strata and assigns an age to rocks. Within this branch, there is a more specialized subfield called biostratigraphy. Geologists have been able to establish relative ages of sedimentary rocks thanks to stratigraphic principles and the principle of uniformitarianism. However, to construct a global stratigraphic column, another tool is needed to establish the ages of all the different strata in different parts of the world and relate them to one another. This is the task of biostratigraphy.
In this article we are going to tell you everything you need to know about this branch of science.
What does biostratigraphy study
This branch of science arose to solve problems encountered when establishing the ages of rocks and the entire global stratigraphic column. Early geologists proposed the principle of faunal succession to explain stratigraphic sequences. This principle states that lithological units should contain a series of fossils characteristic of their age. These fossils are not only present in that unit but should also be repeated in subsequent units. The most characteristic fossils should be diverse and also appear in different types of sedimentary rocks.
The most representative fossils are the best ones that serve to capture the relative age of the rocks. These most important fossils are called index fossils. They are also known by the name of guide fossils. These fossils are presented in a way that geographically extends the entire area. They are more common and are usually well preserved. In addition, all species must appear in short periods of time. However, the genre can also occur in a long extended tense.
It's important to keep in mind that, to determine the age of these studies, we must refer to geological time . This geological timescale marks the era in which species appear and spread within a relatively short period. Throughout history and the geological periods that have occurred on our planet, there have been periods marked by large-scale global extinctions, as detailed in the article on all geological periods , including the Permian and Devonian.
We must compare the information provided by index fossils, the facies fossils that appear associated with a specific rock . These fossils have remained almost unchanged for a long period of time. Furthermore, the relationship of these fossils to the Miocene can offer valuable information about the geological past.
Biohorizons and biozones
These are two concepts established in the branch of science called biostratigraphy. This means that each fossil appears in a specific group of strata. No rock located either within or above the stratigraphic column should contain fossils of that species again. The lithological surfaces that define the presence of a fossil are called biohorizons . As the name suggests, they indicate the area where that fossil existed, independent of the surrounding rock.
There are two types of biohorizons. First-appearance biohorizons, and last-appearance biohorizons. Typically, a species evolves and gradually ceases to be present. The differentiations these species typically exhibit follow an evolutionary path. Analyzing these horizons reveals how diffuse they are. However, on some occasions, they were mass extinctions, as mentioned before, which caused many groups of both animal and plant species to be eliminated in a short period of time. An example of this is the great extinction of the dinosaurs that marks the end of the Cretaceous Period, an event also related to other geological events.
Biohorizons are those that mark mass extinctions and are much clearer. On the other hand, we have the biozones. These are those lithological units that present an index fossil or a paleontological content of great importance. We have some types of biozones:
- The biozones as a whole are those that represent the association of several fossils naturally within a stratigraphic section.
- tension biozones are those that correspond to the biozones that are expanding horizontally or vertically. They mark differences between strata.
- The peak biozones they are those that mark the maximum abundance of a species, genus and even family. These are more specialized.
- Interval biozones are those that represent the rocks between two biohorizons of different fossils.
Importance of geological time in biostratigraphy
This geological timescale is present in all stratigraphic studies. Biostratigraphy was the best tool that helped us to relatively estimate the age of rocks. All sedimentary rocks globally have been analyzed, and it also helped to construct the global stratigraphic section. All the data is relative and said nothing about the age of the Earth. Therefore, scientists will try to calculate this age using biostratigraphy.
Many experiments and scientists have offered diverse opinions on how to calculate the age of our planet. These experiments generated much controversy and debate, with some proposing that the Earth was only 75.000 years old. The matter was finally settled through research on radiation and radiometric dating experiments. This allowed scientists to study the content of radioactive elements and their decay into other elements. This was made possible by calculating the absolute age of volcanic rocks and their relationship to the Silurian period.
This calculated age is added to the relative scale, creating the geological timescale we know today. This scale marks the history of our planet from approximately 4.600 billion years ago to the appearance of the earliest surviving rocks, dating back about 3.600 billion years.
As you can see, fossils are a great tool for learning about the history of our planet. I hope that with this information you can learn more about the great utility of biostratigraphy.
