From: Bill Payne (bpayne15@juno.com)
Date: Thu Apr 03 2003 - 22:51:14 EST
Hi Joel,
On Wed, 2 Apr 2003 10:12:22 -0500 Joel Moore <jxm957@psu.edu> writes:
1) The Muav and Redwall limestones are not in direct contact in many or
most parts of the Grand Canyon. In the eastern part of the canyon, the
sequence (from oldest to youngest) is the Cambrian Muav limestone,
Devonian Temple Butte limestone, and the Mississippian Redwall limestone.
That being the case, then the paraconformity (time gap) isn't as large as
I'd thought. In the eastern part of the canyon the entire Ordovician
and Silurian are missing, which is only 100 million years.
For the western canyon, the sequence (from oldest to youngest) is the
Cambrian Muav limestone, unclassified dolomites, Devonian Temple Butte
Formation, and the Mississippian Redwall limestone.
Here again, the Ordovician and Silurian are missing. The only thing the
Devonian Temple Butte does, where it exists, is reduce the time gap by
about 1/3 - assuming that the Temple Butte represents the entire Devonian
of 50 million years.
2) In the places where the Redwall is in contact with the Muav, the
Redwall fills in surfaces that have been eroded through the Temple Butte
and into part of the Muav limestone.
First of all, you're assuming that the Temple Butte was actually
deposited in the areas where it is now missing. I understand the
correlation you're making of the Temple Butte from west to east, but
working with a bunch of engineers as I do, we are careful to separate
hard data from inference. It doesn't affect much in the world of
academia, but for consultants there is a huge liability issue there.
Second, whether the Temple Butte was ever continuous or not, what you've
presented above is not inconsistent with a short timeframe of deposition.
Given that the Ordovician and Silurian are missing, there is a
100-million-year time gap on top of the Muav, yet only limited erosion
into the Muav. Steve Austin, in his book _Grand Canyon_ (1994), says
"about 30 feet of channeling of Muav can be seen in excellent expoures in
Marble Canyon, but this disconformity has been interpreted to include
erosion of the Ordovician and Silurian Systems supposedly accumulated and
eroded over a period of 100 million years!" (p 70) Thirty feet of
channeling is nothing on a surface above which two entire systems were
eroded (if they were ever deposited in the first place). The weight of
these observations favors a short timeframe.
3) It's important to remember that the Muav limestone was not deposited
at the same time over the whole extent of the formation (i.e., a
cake-layer model is not correct). The lower Muav limestone is the same
age in the western part of the canyon as the Tapeats sandstone (an older
Cambrian sandstone) is in the eastern part of the formation.
Agreed. Austin diagrams this facies relationship in his GC book on page
69.
Thus there are several millions years difference between the age of the
bottom of the Muav limestone in the eastern and western parts of the
canyon because the sediments are part of a trangressive sequence
Your "several millions years" is inference, based upon something besides
"because the sediments are part of a trangressive sequence."
Transgression can happen slowly or quickly and there is certainly little
limit on the speed of deposition of sandstone and shale. Lime mud may
take a little longer, but not 'millions of years.'
Another thing to keep in mind about paraconformities is that they're
viewed with skepticism by some in the geologic community. Also from my
textbook, "Some authors doubt the need for the paraconformity category,
arguing that most reputed paraconformities, if traced laterally,
eventually exhibit some physical evidence of erosion and thus prove
actually to be disconformities (Davis, 1983)."
To turn the argument of Davis, 1983 (can you give the rest of the
reference?) around, we could also say that disconformities, if traced
laterally, eventually exhibit some lack of physical evidence of erosion
and thus prove actually to be paraconformities. I agree with Darryl
here, who said (post 2 Apr 2003): "Ok, I will admit some things are
indeterminant at a particular point and you have to look around a bit.
Maybe the geologists who hold this view are saying you should follow it a
few feet instead of the miles or so that I immediately thought of when I
read the comment." Davis is simply toying with sematics in order to
avoid the issue.
My sense is that the number of people who are in favor of the
paraconformity category has been declining over the last couple of
decades.
This sounds like the reason is because the concept of paraconformities is
inconsistent with millions of years. As I said before, one or the other
has to go; the two are mutually exculsive.
Regarding sheet flow erosion, I can't claim any strength in physics, but
my understanding of fluid flow is that flowing water cuts channels. Sheet
flow is definitely the exception rather than the rule.
Noah's Flood was the exception rather than the rule. We do see those
nice flat plateaus above the GC. Austin comments: "However, the King
James II Version of [Genesis] 8:3a, 5a, is: 'And the waters retreated
from the earth, going and retreating,...And the waters wre going and
falling until the tenth month.' The phrase 'going and retreating'
involves a Hebrew construction similar to that of the raven's motion
(8:7), indicating that the waters were rushing back and forth with an
action resembling tidal movement, as the overall level of water
progressively declined." (p 77) "The present erosion surface [plateau]
on top of the Kaibab Limestone, whiach forms many of the plateaus in the
Grand Canyon area, was caused by sheet-flood erosion, as northern Arizona
became exposed later in the Flood." (p 78)
If I remember correctly, I've seen channelized flow in some wave wash on
beaches in Cape Cod (so low slopes and small water supply). The more
water you have and the faster the water is flowing, the more likely the
water is to channelize.
I would think if you have a flooded condition and moving water,
especially on relatively plane, sloping land, it could not channelize. I
agree that with your "small water supply" we would see channeling, but on
the beaches I've seen, such as the Carolina and Gulf coasts, waves
retreat in sheet flow.
Bill
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