The St. Francois Mountains in southeast Missouri are a mountain range of Precambrianigneous mountains rising over the Ozark Plateau. This range is one of the oldest exposures of igneous rock in North America.
The name of the range is spelled out as Saint Francois Mountains in official GNIS sources,[1] but it is sometimes misspelled in use as St. Francis Mountains to match the anglicized pronunciation of both the range and St. Francois County.[2][3][4]
Name
Detail map of the St. Francois Mountain region
The name of the range derives from the St. Francis River, which originates in the St. Francois Mountains. The origin of the river's name, which also was originally spelled "François" in the French manner, is unclear. The area, as part of the Louisiana district of New France, is near some of the earliest French settlements in Missouri, where many French place names survive. Some sources conjecture that the name honors St. Francis of Assisi (1181/1182–1226), the patron saint of the Franciscan order, but none of the region's early explorers were Franciscans.[5] Others propose that Jacques Marquette, a FrenchJesuit, named the river when he explored its mouth in present-day Arkansas in 1673. Before his voyage down the Mississippi River, Marquette had spent some time at the mission of St. François Xavier, named for the Jesuit missionary Francis Xavier (1506–1552).[5] The spelling of the river's name shifted from "Francois" to "Francis" in the early 20th century.[5]
Geology
Geologic map of the St. Francois Mountain region
The St. Francois Mountains were formed by volcanic and intrusive activity 1.485 billion (1.485 x 109) years ago.[6] By comparison, the Appalachians started forming about 460 million years ago, and the Rockies a mere 140 million years ago. When the Appalachians started forming, the St. Francois range was already twice as old as the Appalachians are today.
The subvolcanic intrusives are similar in geochemistry to the associated rhyolite volcanics, which they intrude into. They are granite with granophyricquartz, perthiticpotassium feldspar, biotite and magnetite. They are intrusive into the rhyolites with development of fine grained granophyre at the contact. At depth they exhibit a coarse-grained rapakivi texture.
The subvolcanic granites are the most widespread igneous rocks and were thought to have been covered with extensive volcanics that have been removed by erosion.[7]
The exposed igneous rocks of the St. Francois are surrounded at depth by the slightly younger (~100 my) widely distributed igneous Spavinaw terrane. The Spavinaw rocks are intersected in drill core across southern Missouri, southern Illinois, northern Arkansas, southern Kansas and northeastern Oklahoma. The Spavinaw rocks occur in outcrop only near Spavinaw, Oklahoma. The rhyolites and ash flowtuffs of the Spavinaw are essentially identical to the volcanics of the St. Francois mountains (the Washington County volcanics).[7]
These ancient mountains may be the only area in the Midwestern United States never to have been submerged, (as evidenced by the lack of marine fossils) existing as an island archipelago in the Paleozoic seas. Fossilizedcoral, the remains of ancient reefs, can be found among the rocks around the flanks of the mountains. These ancient reef complexes formed the localizing structures for the mineralizing fluids that resulted in the rich ore deposits of the area.
The igneous rocks of the Saint Francois Mts. are interpreted to be a series of caldera complexes,[8] similar in scale to the Yellowstone Caldera complex. However, it is debated whether the igneous activity was related to a hotspot, like Yellowstone, or whether it was related to an ancient subduction zone.[citation needed]
Mineral resources
Boulders in Elephant Rocks State Park are residual boulders of weathered Graniteville Granite, a pluton formed 1.4 billion years ago in the Proterozoic by the cooling of magma.
Granite has been commercially quarried since 1869 in the vicinity of Elephant Rocks State Park, a tor with huge weathered granite boulders. The red architectural granite quarried in the area has been used in buildings in St. Louis and other areas in the country. It is currently marketed as Missouri Red monument stone.
Taum Sauk Mountain is the highest point in Missouri.
Mountains in this range include; Taum Sauk Mountain, Bell Mountain, Buford Mountain, Proffit Mountain, Pilot Knob, Hughes Mountain, Goggin Mountain and Lead Hill Mountain. The elevations range from 500 to 1,772 feet (152 to 540 meters). Taum Sauk Mountain is the highest peak in the range, and the highest point in the state, with an elevation of 1,772 ft (540 m). In the southernmost reaches of the range is Clark's Mountain, elevation 1,450 ft (440 m), with the distinction of being the highest isolated point in the range and in the state; it reaches its top from surrounding terrain of about 900 ft (270 m) in less than two miles (3.2 km). Black Mountain, in Madison County, has the highest rise in elevation from its base, along the St. Francis River (540 feet above sea level) to its summit (1,502 feet above sea level), a rise of just under 1,000 feet in elevation.
Recreation
Ozark Trail on Goggins Mountain in Johnson's Shut-Ins State Park
^Tilton, G. R.; Wetherill, G. W.; Davis, G. L. (1962). "Mineral Ages from the Wichita and Arbuckle Mountains, Oklahoma, and the St. Francis Mountains, Missouri". Journal of Geophysical Research. 67 (10): 4011. Bibcode:1962JGR....67.4011T. doi:10.1029/jz067i010p04011.
^Denison, Rodger E., et al., Geology and Geochemistry of the Precambrian Rocks in the Central Interior Region of the United States, Geological Survey Professional Paper 1241-C, 1984
^ abcdeSims, P. K., Eva B. Kisvarsanyi and G. B. Morey, 1987, Geology and Metallogeny of Archean and Proterozoic Basement Terranes in the Northern Midcontinent, U.S.A., USGS Bulletin 1819
Unklesbay, A.G; & Vineyard, Jerry D. (1992). Missouri Geology — Three Billion Years of Volcanoes, Seas, Sediments, and Erosion. University of Missouri Press. ISBN0-8262-0836-3.