TRACKING THE MYSTERIOUS MISSISSIPPIAN CHERT BELT: WESTERN, CENTRAL, AND SOUTHERN UNITED STATES

Tremendous volumes of bedded chert are found within Mississippian carbonate strata throughout the western, central and southeastern portions of the United States. Within the Osagean series, a continuous belt of carbonate-hosted bedded chert stretches from Tennessee and Illinois in the east to Arizona and Nevada in the southwest, and into the western states of Utah, Wyoming and Colorado. The origin of these persistent chert beds has remained unclear, and the source of silica problematic. Studies by several workers have shown that these cherts formed primarily in shallow-water nearshore and shelf environments through replacement of carbonate host rock early in the diagenetic process, often within a few meters of the sediment-water interface. Although silica in some Osagean bedded chert deposits has been attributed to a biologic source, primarily sponge spicules, most workers have rejected a principally biogenic origin based on either petrographic or geochemical evidence. Other possible sources of dissolved silica for bedded chert genesis during the Mississippian include 1) chemical weathering of silicates generated during orogenesis, 2) hydrothermal alteration of basalts along mid-ocean spreading ridges, 3) increased weathering of silicates due to development of land plant features such as expanded root systems and broad leaves, 4) vigorous ocean circulation during glaciation, and 5) input of volcanic ash. Evidence suggests that geochemical analysis of Osagean bedded cherts (particularly REE and trace elements) hold the best promise for determining the relative contributions of possible silica sources.