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It is easy to make two or more sedimentary layers out of one layer by the
process of redeposition. I have seen places in central Washington State
where stream sedimentation has eroded the 1980 Mout St. Helens air-fall
ash and pumice and then redeposited it down-slope. This creates multiple
ash layers along the stream banks. Two or more ash layers, clearly
separated by years of intervening sedimentation, yet each layer came from
one air-fall event from May 18, 1980.
One can replicate such a scenario in a contraption called a "sediment
box". A "sediment box" is essentially a 10-20-gallon glass aquarium tank
that is filled with 2 layers of very fine sand of 2 different colors.
The aquarium is placed on a tiltable board and is rocked back and forth.
Or a steady current is induced on one side of the tank and allowed to
drain out the other end. I have demonstrated this phenomenon to
students. It never ceases to be a show-stopper.
Now try the same experiment, but use 2 colored sediments of different
sizes and densities. The above effect is magnified. By creating various
currents, you can make one layer into a zig-zag pattern that maintains
its continuity up-section. In outcrop, if this pattern is of a large
enough scale, and if a geologist has a limited amount of outcrop to
study, the zig-zag morphology wouldn't be obvious and it would appear to
be two or more distinct layers.
Another experimental device called a "wave tank" could also be used.
K-T boundary sedimentation processes would make a neat little Senior
Thesis project, and the empirical data collected from sediment box and
wave tank experiments has the potential to make a real contribution.
Since the iridium is usually found in a clay layer, the student should
use two clays of different colors.
Before the multiple impact hypothesis is widely accepted, the
single-impact/redepostion hypothesis should first be falsified. This
would involve studying the *micro*stratigraphy of K-T outcrops where the
layer(s) are well-defined, looking for evidence of current-imposed
micro-structures and textures. Obviously, this is more along the lines
of Masters- or PhD-level work.
On Thu, 8 Jan 2004 11:09:01 -0500 "Dewey M. McLean" <email@example.com>
> Response to David Marjanovic's post of Thu, 08 Jan 2004 12:08:43
> (below the dashed line).
> Perceptive questions, David, and I'll do my best to respond to
> The longer I research the K-T extinctions, the more I despair that
> much of anything will be "proven" concerning cause of the
> for long into the future. The reason is that nearly every aspect of
> the K-T geobiological record is controversial. Many new
> multidisciplinary data, and interpretations that most scientists
> agree upon, are requisite to meaningful progress in discovering the
> actual cause(s) of the extinctions.
> Your point on transport of iridium is one such controversial topic.
> recall how at the first Snowbird I extinctions conference in 1981,
> the impactors claimed that iridium was refractory. Later, they had
> iridium migrating to where it best served their purposes. So,
> individual multiple "spikes" in place, or enrichments via migration
> of the iridium? Based on my communications with other scientists
> through the years, I prefer to think of individual spikes in place.
> But, others feel differently.
> Just because scientific papers are several years old does not
> invalidate them unless they have been shown to be incorrect. Nor,
> does it excuse scientists for ignoring the data and interpretations
> in them.
> Dewey McLean
> >> In fact, multiple iridium spikes do occur at localities other
> than China.
> > Hm. These are still rather few -- though the wide geographic
> > could mean something --, so that, without knowing the local
> > I wonder if some bioturbation and groundwater percolation could
> > multiplied the peaks or at least smeared them out. It's also
> > that all 3 papers are quite old. Is there a reason why this topic
> > seemingly disappeared from the literature?
"Let's get this train outa here. Those damn bees might be back any
minute." - General Slater, from the movie "The Swarm"
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