Creationists don't understand basic biostratigraphy

Aside from denying biological reality (like evolutionary biology or even the basic meaning of terms within this field on which I elaborated in What does 'evolution' even mean?), creationists also support other forms of pseudoscience. This is especially the case in Young Earth Creationist circles where a large portion of science denial is also dedicated towards geology. While creationists mostly focus on radiometric dating itself or the supposed evidence of the Biblical flood, there are also creationists that misunderstand how biostratigraphy is used in geological dating to prove that the logic of "atheists"/"evolutionists" is supposedly circular.

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A simplified representation of the biostratigraphic method

Before we begin I shall define what biostratigraphy is in the first place. Biostratigraphy is typically defined as the branch of stratigraphy in which rock strata are identified and organized (correlated and have a relative age assigned to them) based on their fossil content. It is especially useful in determining the time period of the sediments in places where age may be difficult to determine based on sediments due to local sedimentary environmental variation. The concept began to be introduced in the early 1800s. Now it's time to take a look at the argument made about this field by creationists. It usually goes something like: "EVOLUTIONISTS' (or atheists' etc.) thinking about the age of the Earth is circular because they use a fossil to determine the age of the rocks to be millions of years old and then they use the age of the rock to prove the age of the fossil from these rocks to be millions of years old! It's so circular grrrrrrr!!!!". As you can see it is problematic for a couple of reasons:
1. the method of biostratigraphy was introduced decades before Darwin presented his original theory (let alone before it became widely accepted and modern theories of evolution were conceived)

2. biostratigraphy was not (and in a sense still isn't) used to determine the absolute age of sedimentary layers. Instead, it is used to determine the relative age of rocks. This means that biostratigraphy allows to determine the age of a given layer in correlation to another layer. In other words: if we find a new sedimentary section with fossils typically found in the layer C that is below (and thus older than) the layer D this means that this newly found section belongs to the layer C and thus it was formed before the layer D was formed. The only way that we are now able to use biostratigraphy to tell what the estimated age of a given rock layer is, is due to our ability to use absolute dating methods (like radiometric dating using radioactive isotope decay or thermoluminescence).

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A fossilized trilobite. Trilobite fossils are commonly used in biostratigraphy.

The way that biostratigraphy may help in determining the absolute age of sedimentary layers is much different than what many creationists think. For example, if we determine the age of sedimentary layers in which fossil X is found using, for example, radiometric dating to be between 128 and 120 million years old this means that fossil X is of the same age. Once we find new rocks with fossil X we can safely conclude that these newly found sediments are probably between 128 and 120 million years old (unless later analyses prove otherwise as the temporary range of fossil taxa may be narrowed or expanded). This means that the age of the fossils is not retroactively used in determining the age of the rock because it was already determined by absolute dating methods. Nobody would leverage biostratigraphy to claim that a given layer is millions of years of age if modern (or other) absolute dating methods have not been developed. Biostratigraphy is simply used as a shortcut alternative to radiometric dating as it is much less time- and resource-consuming. Once radiometric dating has determined the age of given fossils, these fossils may then be used to determine the age of sedimentary rocks found/explored later, bypassing more costly methods. Furthermore, biostratigraphic method is also calibrated with radioactive isotope ages.

Usage of this misguided argument is thus even more unfortunate as basics of biostratigraphy and absolute dating methods are frequently explained as early as in high school (which was certainly the case with me). This makes the claim of creationists that "evolutionists" are usually not educated enough to understand why evolution is supposedly totally not true even more ironic because creationists clearly show that they themselves frequently can't comprehend basic principles of biostratigraphy and other forms of dating used in geology.