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MISSOURI Taum Sauk Mountain 7 ˝’ X 15’ quad: Ironton, MO Iron County Private property near Taum Sauk State Park and St. Francois Natural Area. St. Francois Mountains, Ozark uplift
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Metamorphic (metamorphosed volcanic) 1,772 ft (540 m) |
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| Mina Sauk Falls. | Mina Sauk Falls from above. |
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Bedrock: Rhyolite |
Middle Proterozoic |
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Dark red to pink rhyolitic ignimbrite, poorly exposed at the summit, but well exposed at Mina Sauk Falls, about a mile southwest of the summit; also along an old pioneer wagon road at the Devil’s Tollgate and at Johnson Shut-Ins. Rhyolitic ignimbrite and lava was erupted in three episodes at 1480, 1380 and 1330 Ma. The volcanics are intruded by rapakivi granite, hybrid ring plutons and basaltic dikes. Some geologists have proposed that the rhyolite was ejected from a giant volcanic caldera, the Taum Sauk caldera, which was later intruded by granite from the same magma chamber, much like the Katahdin Granite in Maine. The Precambrian rocks were eventually covered by a thick sequence of Paleozoic carbonates, later exposed by erosion as the Ozark Plateau was uplifted in the Paleozoic. An excellent exposure of the unconformity can be seen at Taum Sauk Hydroelectric Plant, five miles southwest of the highpoint, where Paleozoic dolostone lies above columnar-jointed rhyolite. There are important iron and lead deposits near Pilot Knob and Des Arc.
For a discussion of all the volcanic highpoints, see http://www.unh.edu/esci/volcanic_highpoints03.pdf. Surficial Geology: Beyond the reach of Pleistocene glaciers, but covered by a mantle of wind-blown loess. Soil Series: Knobtop silt loam: Moderately deep, gently sloping, moderately well drained grayish-brown silt loam, developed from Late Pleistocene loess and the bedrock at average depths of three feet.
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Porphyritic rhyolite at Johnson Shut-Ins. (Photo by Fred Ramstad)
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