- Steve.
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Dear Professor Schimmrich:
I love a good challenge. Thank you for your e-mail. I'm sure that Al
Archer has responded to you by now, but I'll throw in my two-bits as
well. Professor Chadwick is quite right; there are vertical fronds
preserved in our tidal rhythmites. The preservation is absolutely
exquisite. We often refer to the preservation as "Solnhofen-like"
referring to the Bavarian lithographic limestone from which so many
beautiful fossils have been recovered. I disagree with Professor
Chadwick however in that the quality of the preservation and the
orientation of the fronds negates a tidal interpretation. To the
contrary, it supports our tidal model. First of all, the preservation
of the plant indicates rapid burial (mm/day rather than mm/year). This
was not a reducing environment therefore the plants did not settle
through a water column below an oxygen minimum. If these deposits were
lacustrine varves then the plants would have had to remain in a pristine
condition for decades in a tropical environment while they were being
buried - seems rather unlikely. Secondly, we know the setting was
periodically subaerially exposed because we have found raindrop
impression, insect crawling traces, flat-crested millimeter ripples and
subaerial siltstone volcanoes (dewatering structures that show a vent
and flank deposits). Regarding the laminae themselves, we know of no
mechanism other than tidal that would produce such sedimentation cycles.
Our correlation to modern tidal cycles is so exact that we can look at
these deposits and tell when the moon crossed the equator, when the moon
was at perigee or apogee, when the moon was either full or new and when
it was in quadrature. We also see the semiannual tidal cycles. I am
attaching a file with many references which document these ancient
cycles. Regarding the expected energy of the tidal currents, several
years ago we collected a series of vibracores from the fluvial tidal
transition at the head of the Bay of Fundy. As you may know, Bob
Dalrymple (Queens University) and his colleagues and students have
documented modern examples of tidal rhythmites in the Bay of Fundy which
resemble those that we have been studying in the rock record in the
Midwest. Our 10+ vibracores penetrated up to 15 feet of thinly
interbedded sandstone and mudstone (including successions organized into
tidal rhythmites). The sand was fine-grained and the largest structure
we encountered was a ripple. This was in an area where the tidal range
was 5 m and salinites varied from normal marine to freshwater on a twice
daily basis. The fact of the matter is that the water column was so
charged with sediment that the currents were no longer erosive (they had
achieved their sediment carrying capacity) and deposition rather than
erosion was dominant on the side bars. Regarding the whetstone
rhythmites, we know that deposition was primarily from sediment gravity
flows rather than suspension as we have examples of hollow lepidodendron
branches that lay on the tidal flat and filled with nicely laminated
siltstone. These currents must have been driven by the low-energy
ambient currents during high tide and absolutely saturated with
sediment. This is not the sort of setting that would flatten even a
delicate frond.
I hope I have supported my view sufficiently. If not, I'd be happy to
try again.
Sincerely,
Erik Kvale
PS. - Below is text that I have extracted from a booklet that will
accompany an SEPM Poster titled: "Modern and Ancient Tides" This
poster, put together by myself, K. Sowder, and B. Hill summarizes the 6
main short-term tidal cycles (<1 yr.) that can influence sedimentation.
The poster and booklet will be published this year. I have highlighted
the references that you might find to be the most interesting
Finally, the Indiana Geological Survey's Indiana Geology Today web page
at http://www.indiana.edu/~igs has animations of some of the tidal
cycles discussed in this booklet which could be used as more dynamic
examples of equilibrium tidal theory. To access the animations go to
"Time Warp" and then click on "Tidal Time."
Those of us studying the implications of tidal rhythmites are impressed
with the educational and scientific potential of these sedimentary
features. It is hoped that this poster and booklet will serve both to
enlighten and encourage others to investigate these fascinating
deposits.
REFERENCES CITED
Allen, J. R. L., 1981, Lower Cretaceous tides revealed by cross-bedded
sets with mud drapes: Nature, v. 289, p. 579-581.
Archer, A. W., 1996, Panthalassa: Paleotidal resonance and a global
paleocean seiche: Paleoceanography, v. 11, p. 625-632.
Archer, A. W., Kuecher, G. J., Kvale, E. P., 1995, The role of
tidal-velocity asymmetries in the deposition of silty tidal rhythmites
(Carboniferous, Eastern Interior Coal Basin, U.S.A.): Journal of
Sedimentary Research, v. A65, p. 408-416.
Archer, A. W., and Kvale, E. P., 1993, Origin of gray-shale lithofacies
("clastic wedges") in U.S. midcontinental coal measures (Pennsylvanian):
An alternative explanation, in Cobb, J.C., and Cecil, C.B., eds., Modern
and Ancient Coal Forming Environments: Geological Society of America,
Special Paper 286, p. 181-192.
Kvale, E. P. and Archer, A. W., 1991, Characteristics of two
Pennsylvanian-age semidiurnal tidal deposits in the Illinois Basin,
U.S.A., in Smith, D.G., Reinson, G. E., Zaitlin, B. A., and Rahami, R.
A., eds., Clastic Tidal Sedimentology, Canadian Society Petroleum
Geologists: Memoir 16, p. 179-188.
Kvale, E. P., Fraser, G.S., Archer, A. W., Zawistoski, A., Kemp, N.,
and McGough, P., 1994, Evidence of seasonal precipitation in
Pennsylvanian sediments of the Illinois Basin: Geology, v. 22, p.
331-334.
SUGGESTED READINGS AND RESOURCES
Tidal Theory and Applications to Tidal Rhythmites
Archer, A. W., 1996, Panthalassa: Paleotidal resonance and a global
paleocean seiche: Paleoceanography, v. 11, p. 625-632.
Archer, A. W., 1994, Modeling of cyclic tidal rhythmites based on a
range of diurnal to semidiurnal tidal station data: Marine Geology, v.
123, p. 1-10.
Pugh, D. T., 1987, Tides, surges and mean sea level: New York, John
Wiley and Sons, 472 p.
Rosenberg, G. D., 1997, How long was the day of the dinosaur? And why
does it matter? in Wolberg, D. L., Stump E., and Rosenberg, G. D., eds.,
Dinofest International: Proceedings of a Symposium Sponsored by Arizona
State University, The Academy of Sciences, Philadelphia, p. 493-512.
Wood, F. J., 1978, The strategic role of perigean spring tides: U. S.
Department of Commerce, National Oceanic and Atmospheric Administration,
538 p.
Annual (Nontidal) Cycles Preserved in Tidal Rhythmite Successionse
Kvale, E. P., Fraser, G.S., Archer, A. W., Zawistoski, A., Kemp, N.,
and McGough, P., 1994, Evidence of seasonal precipitation in
Pennsylvanian sediments of the Illinois Basin: Geology, v. 22, p.
331-334.
Cowan, E. A., Cai, J., Powell, R. D., Seramur, K.C., Spurgeonow, V. L.,
1998, Modern tidal rhythmites deposited in a deep-water estuary:
Geo-Marine Letters, v. 18, p. 40-48.
Multiyearly Tidal Cycles Preserved in the Rock Record
Chan, M. A., Kvale, E. P., Archer, A. W., and Sonett, C. P., 1994,
Oldest direct evidence of lunar-solar tidal forcing encoded in
sedimentary rhythmites, Proterozoic Big Cottonwood Formation, central
Utah: Geology, v. 22, p. 791-794.
Miller, D.J. and Erikssen, K. A., 1997, Late Mississippian prodeltaic
rhythmites in the Appalachian Basin: a hierarchical record of tidal and
climatic periodicities: Journal of Sedimentary Research, v. 67, p.
653-660.
Oost A.P., de Haas, H., IJnsen, F., van den Boogert, J. M. and de Boer,
P. L., 1993, The 18.6 year nodal cycle and its impact on tidal
sedimentation: Sedimentary Geology, v. 87, p. 1-11.
Wells, J. T., and Coleman, J. M., 1981, Periodic mudflat progradation,
northeastern coast of South America: a hypothesis: Journal OF
Sedimentary Petrology, v. 51, p. 1069-1075.
Williams, G. E., 1989, Late Proterozoic tidal rhythmites in South
Australia and the history of the Earth's rotation: Journal Geological
Society of London, v. 146, p. 97-111.
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-- Steven H. Schimmrich, Assistant Professor of Geology Department of Geology, Geography, and Environmental Studies Calvin College, 3201 Burton Street SE, Grand Rapids, Michigan 49546 sschimmr@calvin.edu (office), schimmri@earthlink.net (home) 616-957-7053 (voice mail), 616-957-6501 (fax) http://home.earthlink.net/~schimmrich/