MIDDLE MISSISSIPPIAN (OSAGE) BEDDED CHERT IN THE REDWALL LIMESTONE OF ARIZONA AND DELLE PHOSPHATIC MEMBER OF UTAH: EVIDENCE OF COMMON ORIGIN?

Extensive bedded chert within the Thunder Springs member of the Middle Mississippian Redwall Limestone of Arizona is stratigraphically equivalent to an anomalous phosphate and chert interval in the Mississippian shelf sequence of central Utah, known as the Delle Phosphatic Member. Both units represent a significant change in the depositional conditions within the Devonian-Mississippian Antler foreland basin, and similarities between the two members may suggest a common origin. Located in the distal portion of the basin, both occupied a position to the east of the Antler forebulge axis, within the shallow backbulge basin, and were the final synorogenic strata deposited at the termination of Antler thrusting. Additionally, geochemical analysis of chert from the Thunder Springs and Delle members show both are strongly depleted in Cerium, evidence for formation under similar redox conditions. Chert and phosphate from within the Delle Phosphatic Member are generally believed to be the result of restricted circulation in the backbulge region of the Antler foreland basin during an episode of active flexural loading. Previous geochemical and petrologic studies of the Thunder Springs Member determined that chert formed when silica-rich fluid of hydrothermal affinity silicified the member’s thin-bedded carbonate wackstones and mudstones, either at the sediment-water interface or as the result of subsurface fluid flow. The geochemical and backbulge basin setting similarities between the stratigraphically equivalent Delle and Thunder Springs Members suggest a link in their origin, and indicate a silicification process active over large areas of the western carbonate shelf in the Middle Mississippian.