Disclaimer -- I have come NO WHERE NEAR reading all the posts on this thread.
I saw this and it jumped out at me.
it's absurd to think that one sediment would deposit exclusively for twenty million years and then abruptly with no transition whatever but a knife-edge thin demarcation between the two, be covered by an entirely different sediment that proceeds to deposit exclusively for another umpteen million years.
It sounds like you are imagining a scenario is static climate where gravel is laid down, then suddenly sand, or something else.
What I would suggest is that you aren't picturing the changes that take place in the world.
Imagine an area at the bottom of a body of water (let's use Great Salt Lake). I would assume that below GSL is a rather thick layer of a silt/salt mix.
Thousands of years pass, maybe millions and, for whatever, reason the water that feeds GSL dries up. The lake itself then dries up leaving a barren wasteland, which hardens.
Over a relatively short period of time plants which can survive in that nasty soil colonize the area. They grow and die. Their leaves deteriorate into soil and more plants grow. Over a thousand years a praire of hardy grasses builds up. This praire lasts for a million years.
Then, another Ice Age descends, completely burying the praire. The ice, full of the gravel it's collected in it's journey south, eventually melts, leaving the stones behind.
The run off from the melting ice creates a new lake, this time with Fresh water. Plants and animals thrive, and the stony bottom is covered in a black muck.
etc.
This scenario is not hard to believe at all, and from a geological perspective, the line between the salt clay and the grasses would be "knife like" since it likely took less than a hundred years to get started.
Likewise, the line between the praire soil and the ice age gravel would be just as abrupt, as would the life between the gravel and the black muck of the fresh water sea.
While this scenario examines a relatively small area, it would hold true on a larger scale. I doubt that most geological levels are completely consistant across their entire form.
Also, I didn't bring up fast events, like volcanic ash or a landslide, both of which would be even more abrupt than my examples.
I hope this helps explain the process