Geology of Southern Illinois

The landscape of Southern Illinois shows a marked contrast from most of the state. The lower few tiers of counties avoided the glaciation that planed much of the landscape to the north. Instead of the endless fields of corn and soybean, the area preserves the rugged terrain that covered much of the Midwest prior to the onset of the ice ages. Sharp cliffs and small canyons are common in the region, giving rise to a number of state parks, wilderness areas, and National Forest land. The amount of rock exposed at the surface provides a good tool for geologists attempting to reconstruct the deep history of the region.

Here's a brief overview of the region's history. The oldest preserved rocks formed in this region roughly 1.4 billion years ago. These rocks exist in a belt running from the Texas Panhandle northeast through southern Ontario. The nearest exposure of these rocks at the surface is in the St. Francois Mountains in Southeastern Missouri, which I discuss in more detail here. After that is a long gap in the geological record spanning more than 700 million years. This interruption in the rock record covers most of North America (and can be found in several other continents!) and is known as the Great Unconformity. What exactly happened during this period of time is unclear, but we do know that it was a period of deep erosion, shaving off 1-2 km of material at the surface. However, in broad strokes, this time period corresponds to the formation of a supercontinent named Rodinia, which formed starting about 1.2 billion years ago. This area was probably far from the ocean, and had probably been uplifted by the formation of a mountain belt somewhere to the south. About 700 million years ago, Rodinia began to disintegrate.

The disintegration of Rodinia probably sparked the next recorded event in the region's history - the Reelfoot Rift. Infamous for its association with the New Madrid earthquakes in the winter of 1812-1813, this rift likely started life as a spur of the rift systems tearing Rodinia apart.

Southern Illinois is home to a number of wonderful state parks. Most of these state parks are located along the Pounds Escarpment, a ~100 km long south-facing cliff extending from the Ohio River near Shawneeville, IL in the east to near Grand Tower, IL in the west. The escarpment marks the contact between limestones and lime-rich shales deposited near the end of the Mississippian Period and sandstones deposited at the beginning of the Pennsylvanian Period. The Pennsylvanian sandstones form a resistant caprock that is slowly being undermined by the erosion of the softer rocks beneath. In places, the cliffs are over 30 m high, and the terrain commonly drops by 100-150 m over a few km moving south off the escarpment.

The geological story of the escarpment is one of environmental transition. The rocks along the Pounds Escarpment are a part of the Illinois Basin, a broad bowl in the Laurentian continent (now North America) covering all of Illinois, the western half of Indiana, and small parts of Kentucky and Missouri. For much of the Paleozoic Era, the basin was filled with a shallow tropical sea. The warm, clear waters of this sea proved to be a paradise for sea life, with coral reefs common for much of its existence. This age began drawing to the end during the late Mississippian era. Beginning ~330 milion years ago, continental ice sheets began to grow on the supercontinent of Gondwana (a landmass composed of South America, Africa, India, Australia, and Antarctica). These ice sheets impounded water from the oceans, causing sea level to steadily drop as the ice sheets grew to the size and volume of the modern Antarctic ice sheet. Correspondingly the sea slowly receded from the Illinois basin, beginning with the basin's shallow northeastern end and slowly working towards the deepest waters in the south. Despite the changing conditions, they were still ideal for the formation of limestone: over this 10 million year period nearly 500 m of limestones and lime-rich mud were deposited on the floor of this draining sea.

Around the end of the Mississippian Period (318 million years ago), the ice sheets suddenly surged northwards to cover nearly half of Gondwana in only a few tens of thousands of years. To give an idea of the scale, these ice sheets covered all of India, Australia and Antarctica, along with much of sub-Saharan Africa and the southern third of Argentina. The sudden growth of the ice sheets left much of the Illinois Basin high and dry. Deep valleys (up to 100 m deep in places) carved into what had been a vast sea floor in the span of a few thousand years. The ice sheets eventually stabilized, keeping sea level relatively stable The sharp change in rock types at the escarpment is due to a brief pause in sedimentary deposition (or unconformity) that occurred around the end of the Mississippian Period. This short but substantial The escarpment marks the contact between sandstones and shalesThese sandstones are part of the Caseyville fm., which marks the resumption of sedimentary deposition after a short but substantial period of erosion at the end of the Mississippian Period. This period of erosion marks an environmental transition of the region from one dominated by a shallow tropical sea to one increasingly dominated by coal swamps and sandy river deltas. Rocks deposited in the former environment were dominated by limestones and lime-rich shales, those in the latter mostly sandstones and siltstones. The cause of this transition was the growth of the Appalachian Mountains to the west. These towering mountains began to shed vast amounts of sediment that flushed west across the midcontinent.

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