Re: silt floats? then there are layers above and below the iridium ...
Get yourself some steel ball bearings in the same size as the marbles, also if you know someone who black powder shoots they may have lead balls in .5" (about 54 cal, 50 cal is .45" as a patch is put on the ball).
You can see the difference in the drop rate by S.G. in an aquarium, you don't even have to time it.
Now you've shown that things are sorted by size and specific gravity in water.
The number of places in the world where you can see that in a strat column or measured section that the rocks violate this basic sorting are probably countless. In fact I can't think of a sedimentary basin where I haven't seen this.
Therefore they could not have been deposited by a flood.
I saw this clearly shown in a video I have about placer gold mining. The difference between quartz sg about 2.65 and gold about 19 was amazing . The gold looked like it was dropping in air, while the quartz looked like it was floating down.
F=ma, but that force is a vector sum. If we ignore friction that leaves two forces. Gravity and the force of buoyancy in opposite directions. The force of buoyancy depends on area and specific gravity.
You know wood floats, so why aren't all the coal deposits right near the surface. That's what a flood would do.
Your suggestion sounds reasonable to me, but I'm not a geologist. This supposition would be much more solid if someone could find some experimental, observational data to back it up. Anyone?
Coal along with oil, gas and rocks with organic debris are reducing environments. U is mobile in oxidizing environments and precipitates out in reducing environments. Any ground water with U moving through an area will precipitate out the U if it crosses a reducing environment.
This is how a Uranium roll front deposit works. The ground water containing a very small amount of U keeps moving until it hits an area which is a reducing environment and precipitates out. Over millions of years you end up getting a mineable Uranium deposit. There are hundreds if not thousands of these in the western US
What all this means is that you cannot make the assumption the Uranium content of a coal has remained static since its formation since at any time ground/subsurface water containing U could have hit it and precipitate out addition U. This means a coal is totally worthless for dating. Seems to me RATE made a big deal out of a Cretaceous coal being much younger which doesn't surprise me since there are a lot of ash falls during the Tertiary which would add U to the system and would easily precipitate in the coals.
I've seen this more times than I can count in subsurface borehole geophysical logs where coals can have about any gamma ray reading from very low to very highdepending on how much U has moved through the system.. In oil and gas deposits GR is used to determine the amount of clay in a rock, it being assumed it comes from K40 which you have in most clays. If the GR reading comes from U and Th it screws everything up. Then you run a Schlumberger NGT log, (natural Gamma Ray spectroscopy) to see how much of your GR reading is from K40 vs U&Th.
Percy, A precipitate is a sediment and as such it is sedimentary. Limestone can be the result of chemical precipitation, biological activity, or clastic deposition or combinations of these. So salt, anhydrite, gypsum, limestone and dolomite are sedimentary.