St. Francis Mountains
2,500 to 544 million years ago
During the Proterozoic time, the area that was to become the Ozarks became a part of the future North American continent. The oldest rocks in Missouri are of mid- Proterozoic age, 1.6 to 1.8 billion years old. These ancient metamorphic and igneous rocks, referred to as rocks of the Central Plains Orogen, are buried deep beneath the more recent rocks of northern and central Missouri at depths of 2,000 feet or more. Some evidence suggests that the ancient basement rocks under the extreme northern part of the Bryant Watershed to the northeast of the Mansfield Fault zone are of this group.
The Precambrian rocks that are exposed at the surface in the St. Francis Mountains of southeast Missouri are younger, dating from 1.4 to 1.5 billion years old. These rocks, known as the St. Francis Terrane, are found in the subsurface beneath most of the rest of southeastern Missouri. The southwestern boundary of the St. Francis Terrane appears to be the Bolivar-Mansfield Tectonic Zone, which includes the Mansfield Fault Zone, which in turn parallels and underlies the Bryant Watershed.
To the southwest of the Bolivar-Mansfield Tectonic Zone, similar rocks that are 1.35 to 1.40 billion years old are found. These rocks are called the Spavinaw Terrane and extend in the subsurface into Oklahoma and Kansas. This transition zone of deeply buried rock sequences occurs about 2,000 feet below the surface of the Bryant Watershed.
The proterozoic rocks under the Bryant area are known only from a few deep drill holes in the area and from interpretation of other geophysical data. We know they are there about 2,000 feet down; we just don't have much detailed information about them.
The rocks of the St. Francis and the Spavinaw are igneous rocks consisting of intrusive granites and volcanic rhyolites. The area during the Proterozoic was a "hot" place; presumably along the margin of a growing continent. After the formation of these volcanic and intrusive rocks, the area was apparently above sea level and subject to erosion for the remainder of Precambrian time and well into the Cambrian. This long period of erosion lasted for 300 to 400 million years, time enough for vast mountains to erode away.
During the Proterozoic the earth's atmosphere changed drastically as oxygen became a major constituent. This change in the atmosphere was accompanied by a change in the type of life forms and fossil remains of these life forms are more common in the sedimentary rocks of the Proterozoic. Multicellular forms of life appear during the Proterozoic, some of which were quite exotic. However, the granites and volcanics of the Ozark region were not the places to preserve this record of changing life forms.
Reconstructions of the geography of the time (paleogeography)
show the migration of the developing North American continent from a position
near the south pole to an equatorial position at the end of Precambrian