Missouri Water Science Center
GEOHYDROLOGIC SETTING
Generalized geohydrologic section across exploration area
Three major geohydrologic units overlie the Precambrian basement rocks in the exploration area. The deepest unit, the St. Francois aquifer, is composed of the Lamotte Sandstone and overlying predominantly carbonate Bonneterre Formation and contains most of the lead-zinc mineralization. The aquifer yields sufficient water for domestic and municipal use where it crops out in southeastern Missouri; however, because the top of the unit is about 1,600 ft (feet) below land surface in the vicinity of the exploration area, it is not used as a water-supply source. Lead-zinc ore usually is concentrated within the Bonneterre Formation, and commonly is associated with ancient reef complexes near Precambrian igneous knobs that protrude upward into the aquifer(Leach, 1994; Goldhaber and others, 1995; Kleeschulte, 2000).
The St. Francois confining unit overlies the St. Francois aquifer. The confining unit consists of shales and carbonates of the Davis Formation and overlying Derby-Doe Run Dolomite. The approximately 300-ft thick confining unit impedes the flow of ground water between the St. Francois aquifer and the surficial Ozark aquifer. The Ozark aquifer is about 1,000 ft thick and primarily is composed of (from deepest to shallowest) the Potosi Dolomite, Eminence Dolomite, Gasconade Dolomite, Roubidoux Formation, and Jefferson City Dolomite in the vicinity of the exploration area. The Ozark aquifer is karstic and supplies large quantities of water to springs and to domestic and municipal wells throughout southern Missouri.
KARST INFORMATION
Falling Spring discharges from a cave in the bluff of Hurricane Creek.
The discharged water flows several hundred feet before sinking
underground through the Hurricane Creek streambed.
Karst is a type of terrain that is formed principally by the dissolution of carbonate rocks (mostly limestone and dolostone). Karst terrain is typified by unusual surface and subsurface features ranging from sinkholes, vertical shafts, losing streams, and springs, to complex underground drainage systems and caves.
The karstic Ozark aquifer supplies large quantities of water to springs and to domestic and municipal wells throughout southern Missouri. The term karst refers to a geologic terrain formed when carbonate rocks such as limestone and dolostone are dissolved by ground water. Karst terrain typically is characterized by the presence of sinkholes, caves, streams that lose water into the ground, springs, and complex underground drainage systems. Karst features are common throughout the exploration area and greatly increase the rate of ground-water flow through the aquifer and the potential for rapid transport of contaminants from upland areas to streams and springs and between surface-water basins.
Losing streams, a common feature in karst terrain of southern Missouri, occur where a combination of high streambed hydraulic conductivity and low ground-water levels cause a substantial part of the stream water to flow through the streambed into the bedrock. The presence of losing streams in the potential new mining area is of concern because contaminants released during mining activities or from stored mine waste (tailings piles) potentially can migrate through streambeds into the ground-water flow system.
For more information contact:
Michael J. Kleeschulte, Hydrologist
U.S. Geological Survey
Missouri Water Science Center
1400 Independence Rd., MS 100
Rolla, MO 65401
Telephone: (573) 308-3675
Fax:(573) 308-3645
