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Bug Creek erosion (Re: extinction)
"James R. Cunningham" <firstname.lastname@example.org> writes:
> how much water was actually placed into the atmosphere, and
> how long a time was the hydrologic cycle disrupted by the reduced
> vegetative cover and increased erosion?
And increased rainfall and erosion has the added effect of creating
multiple layers of iridium out of [formerly] one layer of iridium.
Air-fall that was deposited in the highlands gets eroded and washed down
into the basins, where it is re-deposited over the same air-fall layer.
Voila, multiple Ir spikes.
Here's an additional problem for K-T sedimentologists:
Increased erosion also causes increased deposition, and this ephemeral
phenomenon means that calculations of "average sedimentation rate" near
the K-T boundary, and hence estimates of the duration of the "event" at
the K-T boundary, may be inaccurate. I am very suspicious of some
published claims that the iridium "smear" encompasses a time span of
"500,000 - 600,000 years". One cannot extrapolate "average sedimentation
rate" for a formation down to small-scale involving individual beds or
laminations within beds. The "average sedimentation rate" for a
stratigraphic unit is just that: an average. If one is arm-waving about
the geologic history of the entire Hell Creek Formation, then mentioning
the "average sedimentation rate" for the formation is okay. But if a
researcher is studying a particular couple-meter interval within the
formation, then useage of the "average sedimentation rate" to estimate a
time span is fool-hearty.
On a slightly different K-T boundary topic:
Want to see *huge* fossil stream channels? Do a Google search on the
"Bug Creek" fossil beds. Curiously, these massive channel deposits are
right at the top of the Hell Creek Formation (they are probably earliest
Paleocene in age). The beds are unusual both in occurence and in size
for both the Hell Creek Formation and the Tullock Formation, and they
contain Cretaceous fossils that were ripped-up from the old Cretaceous
landscape by the streams and then mixed with Paleocene fauna. No
channels of a similar massive scale existed during Hell Creek time, and
nothing similar existed later in Tullock time. These were relatively
high-energy Missouri River-volume river(s) that were suddenly created and
then existed for a geologically-short time period.
The occurrence of the Bug Creek beds just above the K-T boundary could be
a coincidence. Nothing similar is found above the Lance Formation
further to the south. The B.C. channels are very limited in areal extent,
occupying only a square mile or so of Montana real estate. Therefore
most of the K-T boundary in the area is of the boring non-erosional
stuff. And the B.C. beds are entirely Paleocene in age, because
Paleocene fauna are present in the channels, meaning that the channels
*cannot* be synchronous with the K-T Boundary.
But you gotta wonder what the hydrologic cycle was like during the
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