Proterozoic or Precambrian

Geologic Map of Missouri, 1990, Missouri Department of Natural Resources, Division of Geology and Land Survey (now Geological Survey and Resource Assessment Division).

Proterozoic or Precambrian (1.5 billion to 544 million Years Ago)

The Proterozoic is one of the most studied time units in Missouri. According to current geological theory, the North American continent first formed as an accretion of smaller sub-continents, beginning about 3.9 billion years ago. The oldest subsurface rocks which have been drilled into beneath Missouri are ones approximately 1.8 billion years old under northern Missouri. These rocks are the igneous and metamorphic base left by the Central Plains Orogen. An orogen is an episode of mountain building. It is believed that the Central Plains Orogen was the episode which stuck the ancient sub-continent which now underlies much of the north central US plains onto already existing proto-North America. These rocks are at great depth, and very little is known about them.

What makes Missouri fascinating is our surface exposure of only slightly younger (1.48 to 1.38 billion years) igneous rocks (granites, rhyolites, ignimbrites, and felsites) in the St. Francois (say Francis) Mountain area of Iron, Reynolds, Shannon and Wayne counties. The most visited examples of these are two state parks: Elephant Rocks (red granite) near Graniteville, and Johnsons Shut-Ins (rhyolite and ignimbrite) near Lesterville.

In this region, two more igneous rocks, basalt and diabase, form as dikes and sills--very narrow rock bodies which fill cracks and add intrigue to the story of Missouri geology. These rocks were put in place about 1.1 billion years ago. Both of these rocks are durable, but the amount of weathering one sees gives testimony to their ancient age.

Reconstructing geologic events of the distant past is difficult. Often, geologists look for similar rock types and structures, but of a much younger age, or for events which are occurring right now, and compare the two. Or, they look at dissimilar structures of the same age in order to rule out other possible events.

In the case of the St. Francois Mountains, a similar sort of igneous rock (called a terrane) underlies a wide part of the Midwest from Oklahoma to Ohio, but it is only exposed in the St. Francois Mountains and a few square miles of Oklahoma. Geologists call this an anorogenic igneous terrane, because it no longer shows the great blocks of deformed, faulted and otherwise disturbed rock which is common in orogenic zones, like the US West Coast, or the Himalaya. They still aren't entirely sure what happened here so long ago, and scientists, like the rest of us, love a puzzle.

The dominant hypothesis for the formation of the Ozarks (and hence much of Missouri) is that the St. Francois Mountains were a volcanic area powered from below by a mantle plume or hotspot. The modern example they use to explain it is Yellowstone National Park. Although the volcanoes are dormant there, they are far from extinguished. Huge "calderas" or circular volcanic basins dot both Yellowstone and the St. Francois Mountains. The volcanoes of Yellowstone (now eroding) were never tens of thousands of feet high, though the area built up from lava flows is 7000 to 8000 feet above sea level. Yellowstone is still explosive; less than a million years have passed since its last violent release. The "yellow stone" of Yellowstone is a sort of rhyolite--extrusive volcanic lava similar to the ash flow and ash fall tuffs found in the Ozarks. And finally, the reasonably flat lava flows of Yellowstone extend many hundreds of square miles, similar to the rhyolite which underlies the southern mid-continent.

A second hypothesis notes the closeness of the Ozarks to the Proterozoic continental margin, and claims that the Ozarks rose somewhat like the Cascade volcanoes of Oregon and Washington, as a result of volcanism related to subduction. Not enough is known about plate movement 1.5 billion years ago to confirm or deny this proposal. The deep subsurface rocks of southern Missouri and northern Arkansas are little studied, but there is little structural evidence to support this idea.

The third hypothesis draws an analogy with the volcanoes of the African Rift Zone, and connects the New Madrid Fault Zone (thought by many to be a failed continental rift dating from this time) with the mountain building to its northwest. In this case, the Ozarks in their prime would more resemble Mt. Kilimanjaro than the West Coast or uplands of Wyoming.

Whatever scenario turns out to be most likely, the development of the St. Francois landscape is the story of fire, pressure, explosion, subsidence and erosion. By whatever mechanism, hot magma rose to a swelling surface, erupted, (probably violently because of the composition of the lava) and built up a lava plateau most probably with some peaks. Lava on the surface cooled as fine grained or porphyritic rhyolite, felsite, and ignimbrite. Lava which cooled below ground between eruptions formed shallow granite "plutons". Like Mt. St. Helens, eruptions blew away entire mountaintops, and the caldera subsided, leaving the subsurface plutons closer to the surface than before. After about 100 million years of intense activity, weathering proceeded for a few hundred million years, until at 1.1 billion years ago, something (perhaps rift development) stoked the fires again. This time, deep, dark basaltic mantle magmas came up, "caulking" fissures formed previously, while probably splitting some new ones of its own. After this brief period of dike emplacement, the earth once again cooled. Though still wracked by earthquakes at depth, it seems unlikely that this region will once again spurt fire and molten rock.

Economic rocks of importance from the Proterozoic/Precambrian have included magnetite and hematite (high grade iron ore), granite for building and cobble stone, and to a less extent, rhyolite, which has been mined, crushed, and sold as roofing granules for shingles, railroad ballast, and decorative stone. The only bonafide silver mine ever worked primarily for that precious metal in the state was also in the Precambrian Silver Mines granite.

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