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Re: Why non-avian dinosaurs weren't able to survive
> Subject: Re: Why non-avian dinosaurs weren't able to survive
> >> Additionally, there was a great physical destruction. Not much
> >> sediment.>Exactly: you only get fossils in a depositional
> environment. The effects of the Chicxulub impact didn't increase rates of
> sedimentation (well, except for tsunami deposits along the
> Gulf...), so you don't get any more chance of fossilization than other times.
> >Terrestrial deposition is very episodic and spotty: it isn't like deep sea
> >or lake sedimentation. You only get overbank deposits when
> the river floods; you only get channel deposits when the channel is RIGHT at
> that spot, etc.
> The uppermost Hell Creek (towards the K-Pg) is actually a period of high
> accommodation and significant deposition. The reason why
> the iridium layer & various ashes are preserved at the K-Pg (and a few metres
> above and below) is because there is deposition
> occurring. There is little evidence for any significant hiatus through the
> K-Pg in the Hell Creek.
>High for a terrestrial deposit, yes. But as a once-paleoceanographer (not
>entirely be choice...) I can tell you that with the exception of lakes, no
>terrestrial form of deposition shows the kind of nearly-continuous
>sedimentation you have in some marine environments.
The reason why the iridium spike is preserved is because there was deposition
occurring exactly at that point in time, so you cannot suggest that there might
not have been deposition then (ie that terrestrial deposition is potty),
because the presence of the boundary layer proves that there was! (and I use
prove" here in its true sense). Of course terrestrial deposition is less
'constant' than varves or marine carbonates, but the ~50ky period either side
of the K-Pg is a period of high deposition rates and good preservation in most
of the Hell Creek. The fact that there are any sediments at all in the 300ky
prior to the K-Pg boundary is testament to the fact that deposition was
occurring. Sparseness of fossils in the upper 3m is not because of lack of
deposition, indeed the opposite may be true.
Anyway, the original point was implying that there might be a significant
(?10-100ky?) hiatus at the K-Pg, and while this is true in many global
sections, it isn't true in the Hell Creek.
Fowler Paleontology & Geology Consulting