Coal deposit thickness and how this could form over millions of years without contamination from slow sedimentation

[Paraleptopecten (Timothy Campbell)]

@St.Roymond

mudstone topped by shale topped by sandstone. The first two can be plausibly explained by a mild event where mud was being deposited in a way that stirred up the really fine particles to settle on top of the coarser (relative to the fine particles) ones; the particles in shale are so small that they will be last to settle in any single event. Then sandstone on top of shale requires very mild deposition conditions that don’t start until the particles in the shale are not just settled but clinging together so that the new flow bearing sand won’t disturb the top of the preceding layer – and it’s not hard to tell the difference since if the shale particles are disturbed while sand is being deposited you get ‘dirty’ sandstone.

Completely unsorted deposits are also a problem–everything from shells larger than my hand down to fine clay that takes weeks to settle out of still water (let alone alternating between sandier and muddier layers over and over in a single deposit) is impossible to get out of a single flood event.