Paleozoic rocks in Minnesota (Figures 1 and 2) record an interesting
history spanning some 200 million years when conditions were unlike
those anywhere on earth today. Although most people may not be
aware of their geologic history, the rocks are familiar to some
degree to anyone who has travelled in southeastern Minnesota.
The bluffs along the St Croix, Minnesota and Mississippi Rivers
and their tributaries are composed of layers of Paleozoic rock
such as the St Peter Sandstone and Oneota Dolomite (Figures 2
and 3). Paleozoic rock lie beneath glacial deposits across much
of southeastern Minnesota, from as far north as the Taylors Falls
area, southwest to the Mankato area. They extend southward into
Iowa and east across the Mississippi River into Wisconsin. Paleozoic
rocks also lie buried under thick glacial deposits in extreme
northwestern Minnesota. The focus of this report is on the Paleozoic
rocks of southeastern Minnesota.
Paleozoic rocks are sedimentary, formed of particles of pre-existing
rocks or minerals, or with a biogenic or chemical origin. To understand
the detailed history of Paleozoic rocks you need only be familiar
with the processes of erosion and deposition. Deposition is the
accumulation of particles into layers, or beds, as small grains
are dropped by wind or settle in water to form sandstone, or as
elements dissolved in water, such as calcium, magnesium, and iron,
precipitate to form limestone or dolomite. Different rocks reflect
the environmental conditions present at the time the rocks were
originally deposited as sediments. For example where sand was
scarce, carbonate minerals, chemically precipitated from seawater,
and carbonate shells of marine organisms accumulated to form limestone.
Erosion is the natural process whereby water and other agents
break down rocks and soil and shape the land. It can be chemical,
as when mildly acid water dissolves limestone, or mechanical,
as when wind blows away the soil or rainwater washes it away.
The origin of the Paleozoic rocks we see today can be explained
using those concepts in a two-part story. The first part of the
story explains how the Paleozoic bedrock was deposited and the
second part of the story concerns the erosional processes that
sculpted these rocks to form the bluffs and valleys of today.
DEPOSITION OF PALEOZOIC ROCKS
The General Setting: Shallow, Tropical Seas
In earliest Paleozoic time North America was positioned near the
equator, and Minnesota was a low lying, mostly flat area. Although
the climate was probably tropical-like, land plants had not yet
evolved so the land surface was barren of vegetation except perhaps
for some primitive algae and bacteria. Sea level, variable in
elevation throughout the history of the earth, began to rise about
540 million years ago causing the ocean to advance and eventually
cover most of North America. Southern Minnesota, Wisconsin, and
Iowa became a shallow sea by about 520 million years ago, with
islands at Baraboo, Wisconsin, in southwestern Minnesota, and
near Taylors Falls (Figure 4). The sea was bordered on the northeast
side by a highland called the Wisconsin Dome. Over the next 200
million years, sediments accumulated in more or less flat layers
in this sea. These were later buried and cemented eventually forming
the layers of bedrock in southeastern Minnesota. These Paleozoic
rocks are more than 1,500 feet thick in some places; they were
deposited during the three geologic time periods known as Cambrian,
Ordovician, and Devonian (Figures 2 and 3).
If you examine the bedrock exposed in quarry walls, road cuts
or steep hillsides in southeastern Minnesota, you can see that
it consists of more than one kind of rock-sandstone, shale, dolostone
and limestone (Figure 3). The coarsest sandstone bedrock layers
are very pure in composition, being composed almost entirely of
grains of quartz, a mineral rich in silica and similar in composition
to window glass. Other layers are composed mostly of limestone,
or dolostone, an altered limestone that is made up of calcium,
magnesium and carbon dioxide. Still others are mostly shale, or
a mixture of fine-grained sand, shale and carbonate rock. Geologists
have assigned names to such individual layers of rocks. The names
are from places where they are or were at one time well exposed.
For example the Jordan Sandstone is named for the city of Jordan,
Minnesota; the Shakopee Formation for the city of Shakopee, Minnesota;
and the St Peter Sandstone, for the St Peter (now Minnesota) River
near Fort Snelling.
Late Cambrian to Late Ordovician time (Mt Simon Sandstone up
through the Galena Group): Deposition in a texturally graded
shelf
To best envision what southeast Minnesota may have looked like
in Cambrian and most of Ordovician time picture the sandy coast
of the Gulf of Mexico, but with a barren, mostly lifeless land
surface. Sediments deposited at this time in Minnesota consisted
mostly of sand, silt and clay sized particles that were carried
by streams from the Wisconsin Dome to the shoreline. Shallow oceanic
currents subsequently sorted and deposited these sediments across
a "texturally graded shelf" (Figure 5). The coarsest
sand was laid down in a shallow marine environment known as the
shoreface where oceanic currents were relatively strong.
Finer sand, silt and and clay-sized particles were deposited in
deeper water away from the shoreface, on the offshore shelf.
In the deeper parts of the offshore shelf, hundreds of kilometers
from the shoreface, silt and clay sized particles and carbonate
grains slowly settled out of suspension. The most distal areas
were dominated by carbonate grains. Graded beds called tempestites
are common in all of these offshore settings. They were deposited
by storms.
The different layers of Paleozoic rocks that stretch across southeastern
Minnesota were formed when sea level, and therefore the depth
of ocean changed dramatically through time (Figures 6, 7 and 8).
Large magnitude changes in sea level led to drastic changes in
the position of the sandy shoreface, moving it off of the Wisconsin
Dome, and back and forth across southeastern Minnesota. Each time
the shoreface passed across southeastern Minnesota, it left behind
sandy shoreface deposits. The Mt Simon Sandstone, the oldest Paleozoic
formation, was deposited during the initial Cambrian flooding
of Minnesota during which the shoreface migrated northward as
southeastern Minnesota was covered with water. The younger Ironton/
Galesville and Jordan sandstones were deposited during subsequent
major changes in sea level. When sea level fell, the sandy shoreface
retreated southward out of Minnesota and into Iowa leaving behind
a "trail" of quartz sand (Figures 6 and 7). When sea
level rose the shoreface moved northward across Minnesota, also,
leaving behind a trail of sand. The last major shoreface sandstone
accumulation is represented by the Ordovician St Peter Sandstone.
The St Peter was deposited during a slow rise in sea level that
followed an extended period of low sea level and erosion across
much of Minnesota.
When sea level was relatively high, the sandy shoreface was positioned
outside of Minnesota, on higher ground of the Wisconsin Dome to
the northeast (e.g. Figure 7). At these times, most of southeastern
Minnesota was a large offshore shelf environment under relatively
deep water where clay, silt and fine sand accumulated. Such clastic
offshore shelf deposits form the layers now called the Eau Claire
and Franconia Formations. Even higher sea level and deeper water
led to offshore conditions where only silt, clay and carbonate
particles accumulated. The layers called the St Lawrence, Glenwood
and Decorah formations are composed of variable proportions of
shale, siltstone and carbonate sediments deposited in such a setting.
When the sea was at its highest levels, nearly all of Minnesota
and surrounding areas was flooded, and carbonate deposition occurred
in the deep water that covered southeastern Minnesota. The Platteville
Formation, Galena Group, and part of the Prairie du Chien Group
were deposited under such very deep water conditions. At about
the same time, the small patch of Paleozoic rocks in northwestern
Minnesota were also deposited. The Galena Group represents the
last deposit of the "texturally graded shelf" system.
Late Ordovician to Devonian time (Dubuque Fm. to Cedar Valley Group): Deposition in carbonate dominated systems
Latest Ordovician and Devonian depositional conditions differed
from that in earlier periods in that carbonate sediments were
dominant in all depositional settings. Sandstone is largely absent
even in the shallowest water deposits laid down at this time,
indicating that the Minnesota may have been very low-lying, and
perhaps much of its surface was covered with vegetation. Distinct
beaches were not present and sand was not abundant. Instead, warm,
clastic sediment-poor conditions led to the development of communities
of animals that precipitate lime. The latest part of the Ordovician
period came to a close with the deposition of the Dubuque and
Maquokada formations (Figure 3) which together record a change
from deep water deposition, to shallow tidal flat deposition
as sea level fell and the shoreline retreated out of Minnesota.
The tidal flat is a broad shoreline area where limy muds accumulated
as water advanced and retreated during high and low tide cycles.
After a long period of erosion during Silurian and Early Devonian
time the seas returned to Minnesota in Middle Devonian time. Sea
level was relatively low, and the shoreline remained at or near
extreme south-central Minnesota, rarely if ever extending as far
north as the Twin Cities area. The Cedar Valley and Wapsipinicon
groups were deposited at this time (Figures 1 and 2). Conditions
were similar to those in latest Ordovician time; shallow water
deposition dominated by carbonate sediments.
POST-DEVONIAN HISTORY
There are no rocks in Minnesota representing the remainder of
the Paleozoic era and much of the early Mesozoic (from about 350
to 100 million years ago). For most of this time the region was
above sea level allowing the land surface to be eroded by wind
and water. The seas never became high enough for the shoreline
to advance further north than Iowa. The sea returned to Minnesota
for the last time about 100 million years ago during the last
period of the Mesozoic, the Cretaceous Period, a time when dinosaurs
were common. Deposits laid down at this time are common beneath
the surface of south-western Minnesota, but in southeastern Minnesota
only thin, patchy remnants of Cretaceous strata are present.
EROSION OF PALEOZOIC ROCKS
Paleozoic rock formations are no longer the continous layers they were when they were first laid down. Instead they have been eroded in places by relatively recent geologic activities, particularly during glacial activity that began about 2 million years ago. At times, glaciers covered much of the the state, but large parts of southeastern Minnesota remained ice-free. The Paleozoic bedrock of southeastern Minnesota was deeply eroded as large amounts of meltwater from glaciers to the north caused the ancient Mississippi, St Croix, and Minnesota Rivers and their tributaries to erode more deeply into their valleys. Thus, the bluffs today along the major rivers in southeastern Minnesota were not mountains (as early explorers thought) but instead are more or less horizontal layers of Paleozoic rock