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

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

Main Menu