A distinct layer arising due to unusual circumstances that did not exist before, or after, the time that the layer was created in the geologic record would, bas

A distinct layer arising due to unusual circumstances that did not exist before, or after, the time that the layer was created in the geologic record would, basically by definition, make it a geological era. If we were studying an extraterrestrial planet and discovered that there had been intelligent creatures in the deep past of that world that then went extinct, who had altered their world while they were around as much as we have, we'd probably select it as the #1 dividing line in the geological record for that planet. And not just for intelligence-centric reasons. It would be a logical way to segment it. For example, you're gonna have long-lived synthetic isotopes in the layers above, and not in the layers below.