Geology of Southern Illinois

The landscape of southern Illinois stands in stark contrast to most of the state. Here, a chain of hills emerge from the thick blanket of glacial debris that make up the bulk of the state's farmland. This rugged terrain preserves the character of the midwestern landscape prior to the onset of the great Ice Ages. The sharp cliffs and small canyons in southern Illinois have been turned into a number of state parks and wilderness areas. In addition to the natural beauty and opportunities for recreation, the rocks exposed at the surface provide an accessible way for geologists to study the deep history of the region.

Piecing together the geologic history of a location often relies on an understanding on the movement of and changes to fixed locations relative to one another. For geology on the North American continent (also known as Laurentia in geologic literature, and which I'll be using here and elsewhere), these changes are tracked relative to the Canadian Shield. The Canadian Shield has remained largely maintained its geologic configuration for nearly 2 billion years, avoiding later large-scale geologic processes like mountain building and rifting. As a result, it provides a convenient reference frame, as subsequent geologic events can be described as the addition or removal of material around this stable core region. After the formation of the Canadian Shield, Laurentia steadily grew in size as plate tectonics delivered small island arcs and uncollected fragments of continental crust to its shores.

For southern Illinois, geologic history begins with a suite of volcanic rocks that are around 1.4 billion years old, which are apparently plastered onto what was once the southeastern rim of Laurentia. The way in which these rocks came to be is still not particularly well understood, primarily because they are hard to access and study. In southern Illinois, they are buried under up to 10 km (6 mi) of sedimentary rock, only a few deep searches for oil have sampled these rocks. They are better exposed across the Mississippi River in the St. Francois Mountains of southeastern Missouri (which I have written about here), but even these exposures are fragmentary. Other outcrops, extending as far west as southeastern Kansas and northeastern Oklahoma, give a sense of the scale of the event, but provide few details. These volcanic rocks are unusual in that they don't appear to be typical island arc rocks or the usual types of volcanic rock that form in the cores of collisional mountain ranges, which are typically how continents grow in size. They might be the result oceanic crust that subducted beneath Laurentia at a very shallow angle, or mark the passage of an ancient hotspot through the area. Regardless, these rocks are pockmarked by large caldera complexes and small domes of very sticky silica-rich lava flows.

After the formation of these rocks, there is another very large gap in the geologic record lasting more than 700 million years. This interruption in the rock record, known as the Great Unconformity, can be found in sites across most of North America and in several other continents as well. The old view of the Great Uncomformity, which developed prior to the recognition of plate tectonics and persisted well into the 1980s, described the Great Unconformity as a 'long nothing', with Earth's surface topography developed more from erosion than tectonics. During this time period, the rock record was gradually erased as erosion sanded ever deeper into the continents, eventually forming flat plains. As more field sites have been analyzed and the consequences of plate tectonics more fully appreciated, geologists have realized this is probably an oversimplification of the situation. Instead, the Great Unconformity probably represents a single intense period of erosion associated with the formation and disintegration of a pre-Pangaea supercontinent roughly 700 million years ago.

This view was shaped by the recognition that many continents also record a gap in the rock record associated with the formation of Pangaea. Additionally, paleomagnetic measurements and interpretations of the parts of the rock record not impacted by the Great Unconformity suggest that most of the continents were located very close to one another around 700 million years ago, probably forming a supercontinent which has been named Rodinia. The impact of forming a supercontinent is that it substantially reduces the number of locations that can record geologic history, typically low areas within continents and continental platforms. Much of the continental platform area is lost (since large parts of the perimeter are tacked onto another continent), and internal basins are pushed high above sea level by the mountains. As a result, relatively few areas escape a (geologically) short period of more intense erosion. In this newer view, Earth is an active world, and the formation of Rodinia was the geological equivalent of torching Library of Alexandria.

The disintegration of Rodinia marks the resumption of the geologic record in southern Illinois. Rifting penetrated deeply into the heart of the supercontinent, more or less pulling it apart along its sutures. Deep fissures formed a series of interconnected rift basins, some of which opened up to form new oceans. Other fissures opened as the continents pushed and rotated away from one another, sometimes forming a new rift, other times failing as the forces pulling it open lessened. The Reelfoot Rift in southeastern Missouri was one of the latter. Although probably a minor side branch of a larger rift valley, its formation still heavily influences the geology of the area today. Most infamously, its faults are associated with the New Madrid earthquake sequence of 1811-1812, which severely damaged white settlements in the region. The devestation of these earthquakes also convinced many Native American groups, already targeted by an ethnic cleansing campaign by colonists moving into the area, were convinced that an apocalypse was underway and joined Tecumseh in his war against the Americans.

Although destructive in human terms, the Reelfoot Rift is responsible for the creation of a geologic record. After the rift ceased opening, the crust began to cool and sag, creating a broad bowl in the Laurentian crust. East-to-west, it extends from eastern Missouri to central Indiana, north-to-south from Wisconsin to southern Illinois. This bowl was asymetric, shallower in the north, much deeper in the south. At its southern edge, it dropped into deep Reelfoot Rift valleys, which coincide with the locations of the Mississippi and Ohio Rivers. For the next 200 million years or so, the sediments collecting within this bowl would provide a detailed geologic record of the midcontinent. Most of this record consists of limestones deposited within a shallow inland sea, which gave way more frequently to large pulses of river sediment originating from the growing Appalachian Mountains to the east. Towards the end of this record, the region was covered in large coal swamps, which in modern times fueled the economic development of the region.

The mountain-building events that filled the tropical seas were formed during the assembly stages of Pangaea. Like the formation of Rodinia, this marked a major disruption in the geological record. From this point onwards, the geologic record of events in southern Illinois is sparse. Evidence from surrounding regions indicate that after our geologic record ends, the region probably developed into a vast inland desert far from any oceans. To the west stretched desert across the entire continent of Laurentia; to the north one needed to travel more than 1000 km (600 mi) to reach temperate forest. To the east and southeast, the mighty Appalachians, towering as high as the Himalaya today) formed a massive barrier. Beyond lie the modern continents of Europe, Africa and South America, sprawling thousands of kilometers in each direction. The gradual erosion of the Appalachians through the Age of the Dinosaurs probably first blanketed the area in a sizeable amount of loose material, which was then removed as Pangaea rifted apart as river networks reestablished themselves and drained to the new ocean basins to the south.

The disintegration of Pangaea was accompanied by volcanic activity. A series of intrusions originating within the mantle pressed upwards into the crust. As this material moved upwards, it caused the overlying sedimentary rocks to bulge upwards, forming several domes in far southeastern Illinois. The largest of these is Hicks Dome, where rocks have been pressed upwards nearly 1600 m (5000 ft). These intrusions were associated with large amounts of fluorine, which reacted with the limestone sedimentary rocks to form large fluorite deposits. Many of the large, beautiful fluorite crystals were used for industrial steel refinement, but specimens quickly found their way to natural history museums around the world. Check your local one - there's probably a crystal of Illinois/Kentucky fluorite there.

Towards the end of the Cretaceous Period, the last period in the Age of Dinosaurs, the Laurentian midcontinent was covered by a shallow inland sea. Small deposits of sand and mud - barely consolidated into rock at all - are found in scattered locations in western and southwestern Illinois. These probably mark the last remnants of large tidal flats along the eastern shore of this inland sea. As sea level fell towards the end of the Cretaceous, this sediment immediately started being flushed into the Mississippi Embayment, a shallow arm of the Gulf of Mexico reaching as far north as modern Cairo, IL. The movement of sediment slowly filled in the embayment, pushing the shoreline southwards. An outcrop of tsunami deposits near Sikeston, Missouri records the asteroid impact marking the end of the Cretaceous. This impact, which occurred just offshore of the modern Yucatan Peninsula in Mexico, created a large tsunami which was funneled into the narrow embayment, creating waves up to 100 m (300 feet) tall.

The recovery period is not recorded within the geologic record, nor is most of the Age of Mammals. Prior to the start of the large Ice Ages roughly 120,000 years ago, the region probably resembled the deeply eroded bedrock in the Ozark Plateau. The advent of the large continental ice sheets planed the terrain, alternately grinding down high points while dumping thick blankets of glacial sediment in the valleys. The rocks of southern Illinois remained just beyond the reach of the ice sheets. This may have been helped slightly by their orientation. A subtle rise in the sedimentary rocks (the Pascola Arch) developed in southwestern Kentucky sometime after the Cretaceous, causing the rocks in southern Illinois to tilt slightly northwards. Although the ice sheets were near their maximum extent, this shallow rise might have provided an additional topographic barrier that stopped their advance a few tens of km short.

The slight tilt to the rocks also contributes to the ruggedness of the terrain in southern Illinois. It exposes a thick series of rocks to view, which have eroded to form a series of long cliff faces and buttes which span between the Mississippi and Ohio Rivers. These escarpments, also called cuestas, were deeply cut by the large water discharges from the retreating glaciers to form small canyons. The exposures of rock along the escarpments will be our base for exploring the geology of southern Illinois in more detail on this website.