The description of the Miocene is the same kind of circular reasoning that all of the rest of the geological timescale is built on. If sediment doesn’t have an characteristics of the Miocene, it’s not assigned to the Miocene.
Radiometric dating is pseudoscience, therefore it’s irrelevant.
We all have the same evidence. Our choice of paradigm determines what we think it’s evidence of.– Matty’s Razor
These assemblages of fossils are assigned to the Miocene because they’re significantly different from other deposits, and therefore the Miocene is notable for the assemblage of fossils which are assigned to the Miocene. It’s a specific habitat type or ecosystem, an assemblage of plant an animal communities that adapted to specific conditions of aspect and altitude.
“Marked world-wide continental uplifts and a major orogeny resulting in the rise of the Alps. Climatic cooling and restriction of broad-leaved evergreens to lower latitudes. Establishment of present-day forest associations“. This is what’s really interesting: Notice that Stewart puts together the establishment of present-day forest associations with the rise of the Alps. Did you get that? A major mountain chain which defines continental Europe was formed in the present-day. The Alps are clearly more recent than the vast amount of fossil bearing strata of which they are made. This means that tectonic breakup of Pangaea was after Noah’s flood as we have elsewhere stated. It’s Hypothesis 35.
And it happened in the present-day. Like all the other strata.