St. Francois Mountains Groundwater Province
There are few places in Missouri where it is more difficult to develop a groundwater supply than the St. Francois Mountains groundwater province. The St. Francois Mountains are the structural high point of a Precambrian-age mountain range where the oldest rocks that outcrop in Missouri are found (Fig.1). The Precambrian-age rocks are igneous in origin, and consist mostly of rhyolites (Fig. 5) and granites (Fig. 3), though other types of igneous rocks do occur. This is a beautiful part of Missouri, with steep, rounded mountains and narrow valleys. Several of Missouri's premier state parks such as Elephant Rocks and Johnson's Shut-ins show off these natural features.
Although the igneous rock formations in this area are very scenic, they are poor water producers. These rocks are nearly impermeable except where they are fractured. Thus, yields of wells drilled into the Precambrian igneous rock are generally only a few gallons per minute or less. The most permeable zones are usually within 100 to 200 feet from land surface.
The groundwater possibilities are better in areas where Cambrian-age sedimentary rocks overlie the Precambrian. The St. Francois aquifer, which is comprised of the Bonneterre Formation and the Lamotte Sandstone, is the best yielding aquifer in this region. The Bonneterre Formation is a dolomite or limestone in this region and can range in thickness from zero to more than 200 feet. It normally has a moderately low production, providing modest but adequate yields to private water-supply wells (Fig. 4).
Figure 2 - Map showing thickness and location of the Lamotte sandstone in the St. Francois Mountains area
Generally, the only high-yielding aquifer zone in this province is the Lamotte Sandstone. The Lamotte is the oldest Cambrian rock unit in the state. Where it is present it consists of up to 500 feet of sandstone. The lowermost few feet of the Lamotte commonly contains many igneous-rock pebbles and fragments that have been cemented together to form a clastic rock called arkose. Where the Lamotte is relatively thick it is capable of yielding up to about 300 gallons per minute.
Unfortunately, the Lamotte is not present in all areas in the St. Francois Mountains groundwater province. In a large area in the southwestern part of the province the Bonneterre Formation directly rests upon the Precambrian rocks (Fig. 2). Low-yield wells producing a few gallons per minute to a few tens of gallons per minute are possible where the Lamotte is absent, but high-yield wells are generally not feasible.