Kansas State University Libraries
New Prairie Press
Symphony in the Flint Hills Field Journal
2012 – The Prairie: Its Seasons and Rhythms
(Laurie J. Hamilton, Editor)
Geological Cycles of the Flint Hills: Ancient Ice
Ages, Sea Levels, and Climate Change
Keith B. Miller
Follow this and additional works at: http://newprairiepress.org/sfh
Recommended Citation
Miller, Keith B. (2012). "Geological Cycles of the Flint Hills: Ancient Ice Ages, Sea Levels, and Climate Change," Symphony in the Flint
Hills Field Journal.
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Geological Cycles of the Flint Hills:
A ncient I ce A ges , S ea L evels ,
and C li m ate C hange
florence
LIMESTONE
MUDSTONEs
florence
LIMESTONE
blue springs
SHALE
The history of the Earth is dynamic. Though it
may not be apparent to us during our short span
of life, the surface of the Earth and the life it
supports are in a constant state of change---both
progressive (directional) and cyclic. Earth history
is also vast, with time scales that are hard to
grasp. Geologic time is often referred to as “deep
time” to emphasize its distinction from time as
seen from a human perspective.
LIMESTONE
Rocks provide our window into the
a typical K ansas ‘ cyclothe m ’
Along K-113 in Manhattan, Kansas
Keith Miller
interpreting the rock record is referred to
deep time of Earth history. In Kansas,
as the principle of “superposition”---the
the rocks we see consist almost entirely
rock layers or “strata” at the bottom of a
of sedimentary rocks stacked in nearly
sequence are older than those at the top.
horizontal layers. These sedimentary
In addition, sedimentary rocks preserve
layers are like the pages of a book, and
evidence that enable us to reconstruct
to read the story you must know where
the environments in which the sediments
to start. If we are reading rocks, we must
were originally deposited. A vertical
read them from the bottom upward---
sequence of diverse sedimentary rock
from those sediments deposited earlier
layers thus indicates a changing series of
to those deposited later. This key to
environments over geologic time.
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S edi m entary R ocks of
the F lint H ills
The sedimentary rock strata of the
Flint Hills of Kansas record a period
of time extending from the latest
Pennsylvanian Period through the end
of the early Permian. This encompasses a
time interval of roughly 20 million years
the slopes. The most predominant
limestones commonly culminate in
characteristic of the Flint Hills geology
layers recording intertidal environments
The sedimentary rock
is this cyclic repetition of rock types.
and eventual exposure to the air.
strata of the Flint Hills of
These cycles are known as “cyclothems,”
Variously colored mudstones lie above
Kansas record a period of
and they have been widely traced
these shallowing marine rocks, and
time extending from the
throughout the Kansas Flint Hills region
show evidence of repeated extensive
soil development. This interval of
latest Pennsylvanian Period
from Nebraska to Oklahoma.
Many of the carbonate units contain
from somewhat before 290 million years
ago, to about 270 million years ago.
The sedimentary rock strata of the
Flint Hills can be broadly classified
into two types. The resistant layers
are carbonate rocks (limestones and
dolomites) that are commonly associated
with thin, finely-layered dark gray shales.
These carbonates and gray shales are
typically quite fossil-rich. The much less
resistant, reddish and greenish-colored
through the end of the
vertically stacked ancient fossil soils,
early Permian.
abundant flint (or chert) nodules,
or “paleosols,” is then sharply overlain
which in some cases merge to form
by another marine limestone, and the
thin layers. It is these hard, weather-
cycle begins again. In the early Permian,
environments show evidence of arid
resistant nodules that give their name
these cycles are commonly on the order
climate conditions such as replaced
to the area. As the enclosing carbonates
of 6 to 10 meters thick. Dozens of
gypsum nodules (often present as quartz
are slowly removed by weathering, the
such alternations occur within the late
or calcite-filled geodes), salt crystal
flint nodules are left behind as a layer
Pennsylvanian through early Permian
molds, and finely-layered dolomites.
of rubble on the surface. These hard
time period. The mid-continent was
Overlying these arid coastal sediments
nodules prevented the early settlers from
thus subjected to repeated flooding by
are reddish fossil soils that typically
tilling the uplands---thus also preserving
shallow seas followed by exposure as
contain caliche nodules (some originally
the native prairie for later generations.
dry land.
formed around plant roots). The
intervals are mudstones and siltstones.
The individual cyclothems of
calcium carbonate that forms caliche
the Flint Hills also show consistent
nodules accumulates in modern soils
internal patterns that record repeated
where the climate is relatively dry.
The early Permian cyclothems in
cycles of climate change occurring in
Above the reddish paleosols are found
landscape of the Flint Hills area, with
Kansas typically begin with limestones
step with changes in sea level. Many
the very different looking greenish-
the carbonates forming the tops of
and gray fossil-rich shales that represent
of the carbonate beds that formed
colored paleosols that record yet
terraces and the mudstones forming
deposition in a shallow sea. These
in very shallow marine and coastal
another ancient landscape. The most
The repeated alternation between the
W hat can
C yclothe m s T ell U s ?
carbonates and colorful soft mudstones
has produced the characteristic terraced
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characteristic features of these paleosols
oceans be rising and falling like a yo-yo?
are undulating or wavy layers. Such soil
To answer this question we must look
structures form today in highly seasonal
elsewhere than the rocks in the Flint
wet-dry, or monsoonal, climates.
Hills, or even in the midcontinent of
the U.S. We must go to the continents
To summarize, what we see within
each of the many stacked cyclothems
of South America, Africa, Australia,
is first a change from a shallow sea
India and Antarctica, all of which were
to intertidal conditions under rather
gathered around the south pole during
arid climates. This was followed by the
this time. Here we find evidence for a
continued lowering of sealevel and
great period of continental glaciations,
the exposure of the area as land---first
an ice age, during the time represented
under a relatively dry semi-arid climate,
by our rocks. Such glacial periods are
and then under a seasonally wet/dry
characterized by repeated advances
monsoonal climate. The story of each of
and retreats of the continental ice
these cycles is ended with the dramatic
sheets. Each time the great glaciers
flooding of a vegetated landscape by
advanced, the sealevel would fall. Water
rising sealevel. We have come full circle,
evaporated from the seas would fall
returning back to the shallow sea where
as snow and be trapped in the glacial
the story began.
ice. Conversely, each time the glaciers
retreated and melted, water would flow
C auses of the
C yclicity
back into the seas causing the sealevel
to rise. Thus the cycles observed in
You may be wondering what
caused these dramatic and repeated
the rocks of Kansas record glacial and
alternations between marine and
interglacial periods on continents half
terrestrial conditions. Why would the
a world away.
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T he T hree - Mile li m estone
showing the irregular flint nodules and layers that give the region its name
Keith Miller
Climate change was the cause of
Earth’s surface, and others. However,
comparisons with modern soils, some
years. The Pleistocene sediments are
the glacial advances and retreats that
one important part of the answer can
individual fossil soils preserved within
fine windblown deposits (loess), glacial
generated the fluctuations in global
be found by looking at the record of the
these cycles could themselves represent
lake deposits, and sediments deposited
sealevel. The consistent vertical pattern
much more recent Pleistocene “ice age.”
several tens of thousands of years. It is
by glacial meltwaters. Fossil bones and
thus hard to estimate the duration of
teeth of ice age mammals are preserved
of climatically-sensitive sedimentary
Ice volume changes during the
features within early Permian cyclothems
Pleistocene (about 2.5 million to
Pennsylvanian and Permian cyclothems,
within these loose sediments. Lastly,
indicates that climate changed from arid
10,000 years ago) have been attributed
but they were likely between about
on the surface, one can find the record
to progressively wetter, and provided
to variations in the solar radiation
250,000 and 400,000 years long. To
of North America’s native peoples
more seasonal conditions during the
received at the Earth’s surface. These
put this in perspective, 200,000 years is
who immigrated during the closing
formation of each cyclothem. Such a
changes result from periodic changes
about as long as anatomically modern
millennia of the Pleistocene.
pattern appears to be consistent with
in the Earth’s orbit, axial tilt, and
humans have been on the Earth!
computer climate models developed for
“wobble.” Several changes in these
this period of Earth history.
orbital variables combine to generate
cycles, or “Milankovitch periodicities,”
C auses of
C li m ate C hange
R eturning to
the P resent
Due to a major erosional period
that range from 20,000 to 400,000
following the Permian, the younger
years long. Detailed records of
rocks of the Triassic and Jurassic
what is causing the climate cycles that
climate change and sealevel from the
are missing over much of Kansas.
resulted in the advance and retreat of
Pleistocene show cycles that fall within
Cretaceous rocks in western Kansas
continental ice sheets and consequently
this range.
consist mostly of marine chalks and
A further question can be asked:
the rise and fall of global sealevel? This
Because the sedimentary cycles
is a very difficult and complex question
preserved in the Flint Hills of Kansas
ironic coincidence, in the Flint Hills
and involves many interrelated factors
are so much older, the dating is much
the rocks of the Permian cyclothems
including atmospheric carbon dioxide
less precise. For rocks of Permian age, it
are overlain by loose sediments of the
concentration, ocean circulation
is very difficult to resolve time intervals
Pleistocene. We thus have sediments
patterns, continental positions and
even as long as tens or hundreds
recording two global ice age periods
topography, the reflectivity of the
of thousands of years. Based on
separated by a time gap of 270 million
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Keith Miller is currently a research assistant
professor in geology at Kansas State
University. Collecting rocks, shells, and
insects, Keith had an early interest in geology.
He attended Franklin and Marshall College
where he became fascinated with paleontology
pursuing that subject as a graduate student
at SUNY Binghamton and the University
of Rochester. Keith’s research interests have
included the preservation and paleoecology of
marine invertebrate communities, the record
of short-term cyclicity within the geologic
record, and models for sea level and climate
change in the US mid-continent during the
Pennsylvanian and Permian Periods.
some shoreline sands. In a rather
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