Region 2: Rocks of the Central Lowland
While most of the Central Lowland’s bedrock is Paleozoic in age, and some is Mesozoic, there is a single exposure that is as much as 1.6 billion years old. Both rhyolite and the pink to purple quartzite (an extremely hard metamorphic rock) (Figure 2.6) found at Baraboo, Wisconsin stand out above the flatter and much younger landscape around it. At just 40 kilometers (25 miles) long and 16 kilometers (10 miles) wide, it is not an extensive range, yet, as it is more than a billion years older than any other rock in the region, it is extremely significant. Based on the ripple marks that persist in many of the quartzite layers, it has been determined that this rock was deposited during the Proterozoic eons on the shores of an ancient sea.
Figure 2.6: The Baraboo Quartzite is the remains of a 1.6-billion-year-old shoreline that, in its incredibly long history, has been tilted nearly vertical.
The vast majority of bedrock in the Central Lowland is sedimentary and was formed during the Paleozoic. These rocks were primarily formed in shallow seas that covered great portions of North America at different times during its history.
See Chapter 6: Glaciers for more on the Driftless Area.
Cambrian age sandstones and shales are exposed around the Mississippi River Valley through the Driftless Area, or Paleozoic Plateau, near where the borders of Iowa, Illinois, Minnesota, and Wisconsin converge, and they extend in a band northeast across central Wisconsin to Michigan’s Upper Peninsula. These rocks are often fossiliferous, but they are covered by glacial material in many areas. Since the sea level was quite high at this time, and most of the Midwest was underwater, much of the rock formed is fine-grained.
By the Ordovician, reef ecosystems had become more extensive, forming expanses of limestone, dolostone, and limey (i.e., calcium carbonate-rich) shales. Occasional bentonite deposits may be found in Minnesota and Iowa, the result of volcanic ash blown from other parts of the continent. In the western part of the region, the Ordovician deposits overlie, and may be found generally southward of, the Cambrian deposits mentioned above. Ordovician rocks are also found around the southern part of Indiana and Ohio’s shared border. While the rocks here are similar in composition, the sediment came primarily from the Taconic Mountains to the northeast.
Sedimentary Structures
Sedimentary rocks often reveal the type of environment in which they formed by the presence of structures within the rock. Sedimentary structures include ripple marks, cross-beds, mud cracks, and even raindrop impressions. Consider the type of environments in which you see these sedimentary structures today in the world around you.
Ripple marks suggest the presence of moving water (though wind can also create ripples and even dunes). Mud cracks indicate that the sediment was wet but exposed to the air so that it dried and cracked.
Cross-beds form as flowing water or wind pushes sediment downcurrent, creating thin beds that slope gently in the direction of the flow as migrating ripples. The downstream slope of the ripple may be preserved as a thin layer dipping in the direction of the current, across the natural flat-lying repose of the beds. Another migrating ripple will form an additional layer on top of the previous one.
Why are there different sedimentary rocks in different environments?
Most sedimentary rock deposited in underwater settings originated from material eroded on land and washed down streams or rivers before settling to the bottom of a body of water. Intuitively, the faster the water is moving, the larger the sediments it may carry. As the water slows down, the size of sediments it can carry decreases. Furthermore, the farther the grains of sediment are carried, the more rounded they become as they are tumbled against each other. In this way, rivers emptying into a sea are effectively able to sort sediment. Near the mouth of the river, the water is still relatively high-energy, dropping only the largest pieces; farther from the shore, the dropped particles get smaller. Therefore, conglomerates and sandstones are interpreted to have been deposited on or near the shore, siltstone farther from the shore, and shale in deep water quite far from shore where currents are slow enough that even very tiny particles may settle.
Increased distance from shore and water depth can also reduce the presence of oxygen in the water, causing organic material to decompose less completely. This causes more carbon-rich, darker rocks, including some that contain exploitable fossil fuels, to form in these areas. Limestone is made primarily of calcium carbonate, the components of which are dissolved in the water. Living creatures, like coral and foraminifera, take those components out of the water to make calcium carbonate shells, which, after the creatures die, accumulate to become limestone. This process happens over much of the seafloor, yet if more than 50% of the sediment being deposited is from another source, the rock that forms is, by definition, not limestone. Furthermore, these shelled creatures tend to fare better in clear water, so limestone tends to form far from other sources of sediment.
Similar processes continued through the Silurian and helped form many of the dominant ridges around the northwest of Lake Michigan, an arc through Lake Huron, and, most famously, the foundation of Niagara Falls in the east. The top of the escarpment is a hard dolostone, which protects the softer shales beneath it from erosion. This resistant capstone helps define the shapes of the basins of the Great Lakes, but these rock formations are also found beneath the surface in parts of Illinois, Michigan, and Ohio (Figure 2.7).
Figure 2.7: The Niagara Escarpment, shown in blue, creates such a distinct topographic and geologic boundary that the Great Lakes Michigan, Huron, and Ontario owe large portions of their shorelines to its influence.
In the east, Ohio and Indiana were sediment-starved as tens of millions of years of weathering made the Taconic Mountains low and mature, allowing reefs to cover the area and produce thick beds of limestone. West of Indiana, limestones were also deposited extensively, and often subsequently converted to dolostone. Relatively deep basins in the Upper Peninsula were surrounded by reefs that reduced circulation. This resulted in increased salinity in the basin, leading to the deposition of salt and gypsum on the basin floor. Evaporation in shallow basins in Michigan and northern Ohio caused a similar process to occur, evidenced by the salt and gypsum deposits also present there.
The Devonian saw the formation of the Acadian Mountains in the east, which provided a new source of sediment to Ohio, Indiana, and Michigan. Because of this, rocks found in these states that date from this time are mostly shales and mudstones. Limestone and dolostone dominate the swath of Devonian rock in eastern Iowa, spilling over into Minnesota, Illinois, and Wisconsin.
Changes in sea level during the Mississippian and Pennsylvanian periods turned much of the Midwest into dry land at various times. Sequences of familiar marine rocks began to give way to sediments deposited near shore, in extensive deltas, and in freshwater settings. Sandstones and mudstones dominate the bedrock from these periods, as well as coal formed from vast volumes of vegetation from the swampy forests covering the region.
For most of the Central Lowland, the rock record stops here—if much rock formed afterward, it was subsequently eroded away. In western Minnesota and Iowa, following a 240-million-year gap, rocks of the Cretaceous period are found. The oldest of these are sandstones deposited by rivers running towards the Western Interior Seaway. As time went on, the shores of the seaway moved eastward, laying down muddier sediment and finally limey shales, which sometimes contain marine fossils. Two isolated outcrops in Illinois contain sandstone and mudstone deposited by rivers flowing into an arm of the ancient Gulf of Mexico, called the Mississippi Embayment. One of these outcrops is in the Central Lowland, near Quincy, Illinois, while the other is at the southern tip of the state and is part of the Inland Basin.
