The Precambrian is an age of mystery: beyond all but the deepest borings, off the charts, in the realm of sea-monsters. Earlier posts explored the myth and mystery of the Precambrian Eon--the longest of Earth's fitful history--and the awe and ire inspired by this early epoch as humans began to grasp their own finitude in the face of what must have felt like forever. The Precambrian Eon is both utterly foreign and utterly intimate, an alien world that is yet wrapped up in our DNA. This post explores the simultaneous near-and-far of the Precambrian, probing almost blindly into the depths of Illinois bedrock.
Illinois Precambrian: Shrouded in Mystery
The remote nature of the Precambrian is perhaps no more apparent than in Illinois. The pre-Cambrian history of Illinois is hidden beneath waves of glacial sloshing and thousands of feet of bedrock. That is to say, it is shrouded by almost 20,000 feet of mystery. Nowhere in Illinois does Precambrian rock come up for air. Out of sight, in another realm, and older than time, we often only remember the foundational role the Precambrian plays in our lives when its wrath explodes in earth-quaking shudders. It is in some sense, as Jules Verne wrote of the sea, “The physical embodiment of a supernatural existence.”
However, there is very little by way of holy writings for this Atlasian deity. In the 250+ page tome the Handbook of Illinois Stratigraphy (the “Bible of Illinois Geology” as many Illinois geologists call it), the Precambrian Eon is afforded less than one page of text even though it accounts for nearly 90% of Earth’s ~4.5-billion-year history. Published in 1975, the Handbook mentions 20 drill borings that manage to venture into the Precambrian eonothem–the only physical contact humans have had with these rocks under Illinois. Twenty small pinholes through which to glimpse the abyss. To give perspective, the year following the publication of the Handbook, NASA published their Lunar core catalog which also listed exactly 20 drill borings and a dozen or so other samples from Earth’s only natural satellite: there were more samples from the Moon than the Illinois Precambrian.
Illinois Precambrian: What “We” Know
In the years since 1975, another 20 or so holes have been drilled into the Illinois Precambrian, but this oldest eonothem still remains aloof. What we do know is beautiful:
But what we know about Illinois’ Precambrian eon is difficult to define. Not only because of the lack of data, but because of definitions. Specifically, the word “we.”
For example, in 1999 “we” got a new set of data that helped tear through the shroud. This new dataset was a seismic reflection profile, which shows the reflection of seismic waves produced by dynamite as they bounce off of subsurface strata. This seismic reflection profile was able to “see” down somewhere in the neighborhood of 7 miles (12 km or 37,000 feet). In short, it was the most valuable information about the Precambrian rocks in Illinois “we” had at the time (though a few other seismic profiles were also available).
However, even though “we” got this data in 1999, the data had been collected in “the early to mid-1970s with either a Vibroseis or dynamite source.”. In other words, the scientists didn’t know exactly when or how the data had been collected. That’s because it had been collected (probably) by an oil company: it was “industry” data.
In a very inclusive sense of the word, we know a lot about the Illinois Precambrian, especially in the area of Illinois where the pressure and temperature underground were just right for the creation of oil (a region know as the the hydrocarbon kitchen). Oil companies drilling there likely have mapped down into the complex Precambrian layers in relatively high detail. But “we” know little about it, because the data is proprietary. And data is often more valuable than hydrocarbons for oil companies.
Illinois Precambrian: What We “Know”
The scientific literature about the Precambrian rocks is littered with uncertainty. One of the leading geologists on the topic dubs the region, “the cryptic craton of the U.S. midcontinent.” Another leading geologist, attempting to excuse the torrent of uncertainties in his paper, confesses: “We have presented our best assessment of the data available.” “We have clearly had to grope for adequate plate-tectonic mechanisms,” a third apologizes.
This is in part because the Precambrian rocks are locked beneath (in some places) miles of younger rock and sediment. Like the eponymous apostle at the pearly gates, the Mt. Simon sandstone is the final guardian before entry to the unknown. And it is a robust guardian. Half a mile thick in places, the sandstone formation underlies all of Illinois except the few areas where a Precambrian hill breaches the top.
The Mt. Simon Sandstone is named after a crop of rock at the top of a large hill called Mt. Simon that rises above a curve of the Chippewa River in northwestern Wisconsin. Not all geologic type sections are so scenic:
Though the Mt. Simon Sandstone blankets the Precambrian rocks, even it would not claim intimacy with this foreign eon. One thing we do know about the Precambrian rocks in Illinois (and much of the United States) is that they are even older than their stratigraphy initially suggests. In geologic terms, they are bounded at the top by a large “unconformity.” An unconformity is missing history; rocks are not always the faithful scribes we wish they would be. In fact, between the Precambrian rocks (mostly granite or granite-like, from what we can tell) and the overlying Cambrian rocks (mostly sandstone at the base), there is about 500,000,000 years of dark ages, when Illinois was illiterate: half a billion years where the Earth-that-would-become-Illinois was silent and deposited nothing for us to find. A “Great Unconformity“, as it is now known. To uncover the details of this history requires context beyond Illinois.
How to Build a Continent
A craton is the stable interior of a continent, and by almost any standard, Illinois is in the heart of the North American Craton. However, this was not always the case. The oldest part of the North American Craton is the area of Canada north of the Great Lakes called the Canadian Shield. The rocks of the Canadian Shield were some of the first in the early Earth to cool and solidify. These rocks formed the first and oldest known craton: the Superior Craton. The Superior Craton is the seed that blossomed first into Laurentia, and then into the North American continent.
As the Earth further cooled, relatively small pieces of continental crust began to form, and rammed into the older cratons, like continent-sized ice floes. The supercontinents were extreme examples of this, with even the cratons plunging chaotically into one another. The Columbia Supercontinent was one of the first, and held the Superior Craton and its additions (called Laurentia by that time) against the other cratons and their additions.
While the cratons were held in a vice-grip together, they continued to add pieces of continental crust along their margins, including on the beaches of the Superior Craton. Penokean, Mazatzal, Grenville: from present-day North to South, these describe the successive pieces of continent cemented onto the Superior Craton. Somewhere amidst the Mazatzal and Grenville is where Illinois was pasted into the growing continent, part of the Granite-Rhyolite Province. There, it remains today, a seemingly stable part of the North American Craton in the ever-changing North American Continent.
Just because a craton becomes “stable” does not mean that geologic processes stop. On top of the basement rock that forms the foundation of the craton, “platforms” often begin to form. Illinois is no exception (The depth of this “platform” is where this post gets its title: nearly 20,000 feet of mostly sedimentary rock overly the igneous basement at the nadir of the Illinois Basin). These depths lie beneath Shawnee National Forest, the largest public tract of land in Illinois. Here, it would take somewhere between 100-150 of the mature trees of the park stacked one below the other to reach the Precambrian of the Illinois Basin.
A “basin” in geology does not necessarily mean there is a depression on the land surface. If there is, it will almost certainly not be as dramatic. The Illinois Basin, for its part, does have small relief on the surface, but it is nothing compared to the large trough traced with Precambrian and Cambrian rock in the subsurface (see image above). The exact origin of the Illinois Basin is uncertain, though we have pretty good guesses. Probably, the initial stages of the Basin’s formation began sometime in the late Precambrian. As the Rodinia supercontinent was opening up into the ancient Iapetus Ocean, a rift began to form running southwest from the current southern tip of Illinois (the “Reelfoot Rift”). These first cracks in the crust were probably caused by the other continents pulling away from Laurentia: “it is easier to…break a craton under tension than under compression,” one geologist notes. But this was only the beginning.
The crust had now been weakened, and as the South American continent (called “Amazonia” at that time) later crushed into Laurentia, the effects rippled across the craton. In later eons, as more continent was added and Illinois was further and further away from the edges of the continent, the forces that helped the Illinois Basin form became less intense, and gave way to gently undulating subsidence and uplift in the craton. One of those areas of subsidence that developed was the Illinois Basin.
The rocks that make up the Illinois Basin are both unique and not terribly unique. They are part of a large geologic province that runs from Ohio to the Texas Panhandle, and in that sense, they are not unique. This whole area was part of a large, underground “bath” of magma, some of which was released through volcanoes, but most of which cooled while underground. Unimaginatively named the “Eastern Granite-Rhyolite Province” (see continental map earlier in post), this subsurface sea of magma is essentially the same type of structure that formed the bluffs of Yosemite and powers the geysers in Yellowstone.
The reason these rocks are unique, however, is that they don’t match with what geologists would expect of them. They expect Illinois to be like California. At the time these rocks were cooling, Illinois was the California of the North American/Laurentian Continent: on the cutting edge, pushing another tectonic plate beneath it (the melting of this bottom plate is where the underground magma sea comes from). The process looks a lot like what we know from California, except for one major difference: there are no mountains in Illinois. Not only are there not mountains, there doesn’t appear to ever have been any. There is no deformation of any kind that is characteristic of the continental convergence that is suspected. Between that, and an unexpected composition (known in part through the use of magnetic anomaly remote sensing, pictured below), the Illinois Precambrian remains more secretive than even initially anticipated.
Mobilis in mobili
20,000 feet under the present-day trees of the Shawnee National Forest, an ancient sea of magma frothed and bubbled, but eventually settled into an uneasy rest. As the eons passed, the tumultuous history of Illinois was buried beneath a sea of sediment, exiled to the depths, quietly changing among the changes. As North America develops geologically, the Precambrian in Illinois is often forgotten, discarded as dull craton. Even the few attempting to read the writing in its rocks cannot glimpse as much as they would like. But beneath the layered disinterest, the Precambrian in Illinois is very much alive and shaping the surface, supporting geologic reservoirs and sometimes participating in a terrible dance of trembling fury. Submerged in a sea of stone and sediment, it is quiet, but by no means has it been stilled. In geology, the past seldom stays in the past.
[This is the third and final installment of a three-part series. The first was The Precambrian: An Origin Story and tells the story of the feud between the two geologists who discovered and named the Precambrian Eon. The second post, The Precambrian: Mystery and Myth, explores the big history of Earth’s infancy.]
References for quotes:
 McBride, J. H. and Kolata, D. R. (1999) ‘Upper crust beneath the central Illinois basin, United States’, GSA Bulletin, 111(3). https://doi.org/10.1130/0016-7606(1999)111<0375:UCBTCI>2.3.CO;2
 Hinze, W. (1996) ‘The crust of the northern U.S. craton: A search for beginnings’, Geology Society of America Special Paper, (308), pp. 187–201. Link
 Van Schmus, W. R., Bickford, M. E., and Turek, A., 1996, Proterozoic geology of the east-central Midcontinent basement, in van der Pluijm, B. A., and Catacosinos, P. A., eds., Basement and Basins of Eastern North America: Boulder, Colorado, Geological Society of America Special Paper 308. Link
 Van Schums, W. R. and Bickford, M. E. (no date) ‘Proterozoic Chronology and Evolution of the Midcontinent Region, North America’, in Kröner, A. (ed.) Developments in Precambrian Geology. Elsevier, p. 1981. Available at: http://www.sciencedirect.com/science/article/pii/S0166263508700161.
 Marshak, S. and Paulsen, T. (1996) ‘Midcontinent U.S. fault and fold zones: A legacy of Proterozoic intracratonic extensional tectonism?’, Geology, 24(2), pp. 151–154. https://doi.org/10.1130/0091-7613(1996)024<0151:MUSFAF>2.3.CO;2