Illinois Geological History
Today, much of Illinois’
surface geology shows only geologically recent, glacier-related features.
However, the geologic record in Illinois is much deeper than that. About
1.5 billion years ago, during the Precambrian, volcanism was much more active
than now. The type of volcanic rock type found in Illinois consists of
felsites and felsite porphyries. This occurrence suggests that
superheated ash, dust, and gas erupted in powerful, booming blasts similar to
the Mt. St. Helen’s eruption in 1980 (see Photo, Schuberth, 1986).
Following the first major phase of volcanism, there was an extended period of
erosion followed by two more large volcanic eruptions.
Several hundreds of thousand years later, granitic
intrusions cut through the volcanics but did not penetrate all the way through
the strata. After a long period of erosion, some of the intrusions became
exposed, leaving a granite and felsite terrain with up to 1,000 feet of
relief. The terrain in much of Illinois at this time was rather barren,
but mineralizing events were occurring in the subsurface. By the
Cambrian, many life forms evolved in the sea. Ancient volcanoes had been eroded
below the Cambrian sea level and sedimentary rocks were being deposited in
Illinois.
Today, sedimentary rocks underlie thick glacial deposits in
DeKalb and Lee counties. During the late Cambrian, Illinois was part of a
broad epicontinental sea. During the Ordovician, sediment derived from
rocks in the Appalachian orogeny, were swept into Illinois. In the middle
Ordovician, there was a regression of the sea, followed by a short period of
erosion and a re-advance of the sea. erosion and a re-advance of the sea.
During this time, the well sorted, St. Peters Sandstone was deposited in
Illinois along the Ordovician shoreline. Throughout the Silurian,
Illinois was below sea level though many of the deposits have been eroded since
then. During the Devonian sedimentation continued and there was
repetition of sea advances and regression. In the late Devonian, thinly
laminated black shales were deposited over a broad area. The
Mississippian Period is represented in Illinois by fossiliferous marine
limestone, indicative of a shallow, clear, warm tropical sea.
Mississippian strata are up to 3,200 
feet thick in the Illinois Basin.
During the late Mississippian, a small delta, fed by the ancient Michigan
River, formed near the present Illinois Basin (see Figure, Schuberth, 1986).
Pennsylvanian strata consist mostly of clastic deposits,
with sandstone being the dominant rock type. However, there is repetition
of strata sequences, called cyclothems (see Figure, Schuberth, 1986), not seen
before Pennsylvanian. This sequence consists of thick sandstone followed
by shale, coal layers, and another shale layer. After this second shale
deposit, there is a thin layer of limestone, followed by another shale and a 
sandstone deposit. After the sea
withdrew for the last time near the end of the Mississippian, a gradual epeirogeny occurred within the continent began and
continued for the next several hundred thousand years. Higher topography
in northeast Illinois caused rivers to flow to the west and south, including
into the Illinois Basin, which had been established by this time from 250 to
300 meters deep.
There is not any sediment record from the Permian, Triassic
and Jurassic Periods in Illinois. The Triassic and Jurassic may
have been have been eroded but it is more likely that there was a depositional
hiatus. Cretaceous sediments contain a high content of heavy trace
minerals (kyanite, staurolite, tourmaline, sillimanite, zircon, rutile), all of
which are found in metamorphic rocks, indicating a metamorphic source region,
possibly the North Carolina Piedmonts. During the Tertiary until about 15
million years ago, the topography in parts of Illinois was becoming more
uniform due to subsidence of the Illinois basin. The global climate began
to cool towards the end of the Pliocene, leading to increased precipitation.
These changes caused the onset of Pleistocene Epoch ice age. During the
Pleistocene, glaciers rearranged the landscape and changed river flow
paths. Glacial meltwater floods created new channels, deepened others and
may have forced the Mississippi River changes its course at Thebes.
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