I am not sure you yourself will stay on topic. We will see:
Fossi sequence is determined by rock sequence.
After a sequence of fossils is established, then ...
Rock sequence is determined by fossil sequence.
If that is not a circular reasoning, what is it?
(Don't try to touch the issue of radiometric dating. It does not belong to the OP)
That is the best description of circular reasoning as described in the OP I have seen so far. However, radiometric dating does belong in the OP because it destroys the circular reasoning argument.
First of all understand what an index fossil is. An index fossil defines a specific range of geologic time. That is, they are only found in specific layers of strata within specific time periods and nowhere else. Thus specific geologic periods and their age can be recognized from an index fossil. This is what is called "Relative" dating, because a relative age spans a block to time, or time between an upper and lower age. Conversely, "Absolute" dating does provide a specific age along with a stated statistical margin of error (+/- "x" amount of years).
What destroys the circular reasoning argument is that the strata is dated either through radiometric or non-radiometric means to give an absolute age in which an index fossil is found. So, by knowing the absolute age of the strata, the age of the index fossil is then known, but only within the span of the existence of the index fossil.
That is not circular reasoning in the context described in the YEC literature.
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