Viewer Comments and Discussion:
The Lake Neosho shale subunits are very comparable with
the transgressive,
cycle center and regressive shale deposits found in the center of some
northTexas Pennsylvanian cyclothems, especially the Finis cyclothem and the
Bunger-Necessity-Gunsight cyclothem, as seen at well studied locations. Even
the disjunct occurrence of scattered phosphate nodules in a lower zone (your
calcareous shale subunit) and concentration of phosphate nodules in an upper
zone (your phosphatic shale) is comparable to occurrence in the above named
Texas cycles. Also, the sparse fossil content of a middle zone (your blocky
claystone) is typical. In Kansas, such cycle center units are often fissile
black
developments of a core shale. Your blocky claystone subunit is likely to be
a
lateral variant of a core shale. The greater concentration of phosphate nodules
just below the regressive limestone is a feature of similarity with the Texas
cycles. The apparent lack of a distinct transgressive limestone is not
troublesome, since they frequently are very reduced. The scattered limestone
pieces found just below the calcareous shale subunit in your section could be
the
equivalent of a transgressive limestone.
Dr. Tom Yancey
Texas A&M University
8-16-03
Question: Several people have asked me why I did not
correlate the strata at I-170 with
those in the Illinois basin, and compare depositional events between I-170 and
the
Illinois basin, especially since the Illinois basin is fairly close on the east
while the
nearest exposures in the western outcrop belt are about 150 miles away.
Answer: The nearest exposures to the east of beds that
are time-equivalent to those at
I-170 are barely 25 miles to the NNE, near Piasa, Illinois. In fact, the Piasa
Limestone
has been correlated with the Worland Limestone. The schemes of stratigraphic
nomenclature for Midcontinent and Illinois basin are completely different, and
I thought
that trying to make sense of these different systems would complicate my story,
which is
about the beds exposed at I-170. It was rather easy, however, to bring in discussion
of
correlative strata in the western outcrop belt because the stratigraphic development
is
similar, and previous studies on Midcontinent cyclothems provide a reference
for
understanding the depositional environments at I-170. The fact that Pennsylvanian
strata in the St. Louis area resemble those in the western outcrop belt was
noted
decades ago, and justifies using Midcontinent stratigraphic nomenclature throughout
Missouri. I have examined several cores (exposures are very poor) of correlative
beds
in western Illinois, and found that the interval exposed at I-170 is quite different
there,
even in the westernmost cores I have seen, obtained in Macoupin, Madison, and
Clinton Counties. For example, in some cores from western Illinois there are
two major
limestones a short distance above the Danville (=Mulberry) Coal, and neither
of them
displays the three-tiered development seen at I-170 that is generally typical
for
Midcontinent regressive limestones. Where only a single limestone bed occurs
above
the Danville/Mulberry, it also lacks the vertical zonation. I have not observed
a shaly
conglomeratic zone between the Danville/Mulberry and Worland/Piasa level in
Illinois. In
addition, there is very little claystone paleosol development below the Danville/Mulberry
level in western Illinois, so that heavily pedogenized limestones of the underlying
Pawnee Formation (this is Midcontinent term; the upper Pawnee limestone is called
Bankston Fork Limestone in the Illinois basin) lie only a few feet below the
coal. Strata
farther to the east in the Illinois basin are different yet. For example, the
interval that
correlates with the Worland Limestone appears to be in the lower part of the
West
Franklin Limestone Member of the Shelburn Formation. Beds between the Danville
and
West Franklin increase to as much as 200 feet in thickness including a prominent
coarsening upward, apparently deltaic, interval that is not present at all in
the St. Louis
area or western outcrop belt. These differences are certainly worthy of study,
and a
regional synthesis is needed. But the situation becomes quite complicated, and
bringing
in these details would not add anything to the interpretations for the I-170
succession
while needlessly confusing the issue. The present report is long enough already.
Let’s
do one thing at a time!
Question (paraphrased): Pennsylvanian black shales have
been interpreted as deep
water deposits indicative of quiet bottom water, and the lower part of regressive
limestones are also believed to be relatively deep-water. You have stated that
there
was an erosional event between the phosphatic shale and bioclastic shale in
the Lake
Neosho. How is this possible in a deep-water deposit? Could the “splintery”
erosion
surface you mention actually show the presence of burrows that went down into
the
phosphatic shale rather than effects of exposure to submarine erosion?
Answer: Burrows retain the same diameter from top to
bottom, because normally each
burrow is constructed by a single animal—not by a larger animal at the
top and a
smaller animal at the bottom. That the indentations into the Lake Neosho phosphatic
shale become thinner and gradually die out with depth shows they are not burrows.
I am
familiar with the “standard” interpretation for Pennsylvanian cyclothem
“core” black
shales, and an erosion surface between the Lake Neosho phosphatic shale and
bioclastic shale indeed seems to be inconsistent with that interpretation. However,
the
erosion surface is obvious and I will not deny its presence just to make the
overall
interpretation fit an existing model. The lower part of the Worland also has
indications of
strong current action, with limestone suggestive of quieter conditions higher
up (the
yellow weathering clayey limestone). This also seems contrary to the existing
model.
The I-170 outcrop is on the north flank of the Ozark dome, directly astride
a saddle that
separates the Midcontinent area from the Illinois basin. The sea may have been
shallower here, and this area may therefore have been emergent more often and
for
longer times than in areas to the west or east. Therefore the Lake Neosho (and
perhaps
the Worland Limestone as well) has features that are not typical of adjacent
areas.
These features will help us improve our knowledge of the Pennsylvanian in areas
that
have not been studied as much as the western outcrop belt or the Illinois basin.
Question: How do you know the numerous transgressions
and regressions of the
Midcontinent sea were caused by fluctuating sea level? Wouldn’t subsidence
and uplift
do the same thing?
Answer: Yes, subsidence and uplift would produce the
same effect. There was quite a
rivalry in the 1930's and 1940's between Harold Wanless (University of Illinois)
and
Marvin Weller (Illinois State Geological Survey) over the origin of Pennsylvanian
cyclothems, in particular those in the Illinois basin. Wanless favored climatic
changes
that affected sea level, and Weller favored tectonic origin—rapid subsidence
and
uplift—with sea level staying much the same. The tectonic explanation lost
favor in the
1970’s because there had never (and still hasn’t) been a satisfactory
geophysical/
tectonic mechanism found to account for such vertical “vibrations.”
Why would the crust
regularly shift from subsidence to uplift and back again, always after subsiding
just a
certain amount, and then reversing and causing uplift of the same amount? That
would
be about like having multiple episodes of North America splitting from Europe/Africa
and
drifting westward for a small amount, and then reversing and going back, always
the
same amount each time. Convection currents in the mantle, which operate persistently
on vast scales to drive continental drift, would have to reverse themselves
each time. In
fact, we don’t think convection currents are so easily established, stopped,
or reversed.
For subsidence to occur, material in the mantle must also move laterally to
make space
for the crust to sink. Then, for the crust to rise, mantle material would have
to flow in
from the edges to push up the crust. No one has conjured up a cause for mantle
currents to reverse so quickly. On the other hand, we have lots of evidence
for frequent
changes (frequent on a geologic time scale, at least) of climate, and we know
the
astronomical cycles that cause climatic fluctuations. The frequency of climatic
changes
that significantly affect sea level is on the order of 100,000 years, more or
less. These
climatic changes have been especially effective at changing sea level during
times of
regional (“global”) glaciation, as happened on Gondwana during the
Pennsylvanian. I
won’t try to explain the whole story of Milankovich cycles in this answer,
because a web
search on “Milankovich” will produce several very straightforward
and well-illustrated
explanations. Mantle convection currents can indeed change and even reverse,
but the
time scale is at least one order, and more likely two orders, of magnitude greater
than
for climatic reversals. Repetition of precisely the same mantle current reversals,
time
and time again in the same location, is not geologically reasonable, and cannot
be
documented by any independent geological data.
Send comments to the author, Norm
King
altamont@insightbb.com