Besides, the ashfall deposits may not have been a single continuous event. There is layering within the deposits.
How can you know that is the case?
I am not sure what you mean, but in edge's shot you can see a horizontal band and in many other deposits, I have seen clear layering bands. I do not know what the layering represents as far as the deposition event(s). Sorry if I implied I did.
What if Eleanor Roosevelt had wings? -- Monty Python
One important characteristic of a theory is that is has survived repeated attempts to falsify it. Contrary to your understanding, all available evidence confirms it. --Subbie
If evolution is shown to be false, it will be at the hands of things that are true, not made up. --percy
Because there are sandstones and mudstones interbedded with the ash flows. You can see the layering with more fluvial sediments in this picture. The reddish beds are more oxidized than the green-gray reduced beds. The tan beds are more resistant sandstone.
I've also seen burnt coal beds mixed in with the ash. Looking for a picture.
You've mentioned coal seams several times. How would a coal seam get formed?
In between eruptions, in a swamp-type of environment. Remember there are sandstone and shale interbedded with the ash as well. Some of the sand is pretty clearly fluvial and it's only a short step from there to a swamp or bog.
And here is a picture of fluviatile sediments associated with coal in a New Mexico coal mine.
Coal occurs at the bottom of the mine, with sandstone and mudstone layers above it. You can see a sandstone channel at the top of the highwall on the left edge of the image.
Now, just imagine this sequence being deposited during a volcanic episode of rather large proportion, derived from one or more of the larger volcanic fields in the region.
Perhaps I'm not explaining very well here, but I know that as we drilled for deep oil fields in the region, we encountered a lot of coal on the way down through the clay beds that you've seen in the these photos. I've also seen layers of 'clinker', the term for baked clay related to coal seam fires, interbedded with the ash; certainly ancient in timing.