MINERALOGY AND FORMATION OF GEODES IN THE MISSISSIPPIAN SALEM AND WARSAW FORMATION, SOUTH-CENTRAL KENTUCKY AND NORTHERN TENNESSEE

Although geodes are a common occurrence in Mississippian dolostones of Kentucky and Tennessee, their origins are a mystery. Five locations within the Salem and Warsaw Formation were examined along a north-south trending line from south-central Kentucky into northern Tennessee. Detailed hand sample and thin section analyses indicate that three types of geodes occur in the Salem and Warsaw Formation: geodes with completely siliceous interiors, geodes with entirely calcitic interiors, and geodes with a combination of calcitic and siliceous interiors. All geodes examined displayed an outer shell of chalcedony with inclusions of pyrite and interiors with inclusions of anhydrite. Pyrite and anhydrite are indicative of precipitation from anoxic marine waters and evaporitic brines, respectively.

Geodes initially formed in a shallow-burial environment as nodules of anhydrite, which later altered to calcite and/or silica. Geochemical conditions necessary for the formation of anhydrite appear to be linked to evaporite formation in the overlying St. Louis Limestone and dolomitization of the host Salem and Warsaw Formation. Such conditions have been proposed by Maliva (1987) to account for geode development in other Mississippian rocks. Although the source(s) of silica in the Salem and Warsaw geodes is unknown, it is likely that the silica was derived from the dissolution of sponge spicules. Other evidence supporting a shallow-burial environment of geode formation includes soft-sediment deformation of strata surrounding geodes and the spherical shape of the geodes themselves. These characteristics both suggest that initial formation of geodes took place before the surrounding sediments were lithified and that formation was influenced by hydrostatic pressure.