EXTENSIVE MISSISSIPPIAN BEDDED CHERT IN THE ANTLER FORELAND BASIN: SILICIFICATION DUE TO TECTONICALLY DRIVEN SUBSURFACE FLOW?

Exposed throughout the Grand Canyon and elsewhere in western Arizona, southeastern Nevada, and southwestern Utah, extensive bedded chert deposits within the Thunder Springs Member of the Redwall Limestone are typical of the so-called ”Mississippian chert problem.” Like other bedded chert bodies within stratigraphically equivalent Mississippian carbonates of the Antler foreland basin, the origin of the Redwall bedded chert has remained enigmatic because the source of silica and mode of emplacement are unclear. Prior analysis has shown that silicification of the shallow-water carbonate was due to interaction with a fluid of hydrothermal affinity before major loss of primary porosity. The hypothesis that gravitationally driven hydrothermal fluids from the Antler orogenic belt pervasively silicified the Thunder Springs Member can be tested by determining the fluid flux rates required to silicify the carbonate host rock within known time constraints. Necessary fluid flux is defined as the minimum amount of fluid that must pass through the member to deliver a mass of silica equal to that of bedded chert, and is dependent on the silica concentration of the fluid. A set of simple, uncoupled equations describing fluid flow through a lithology of specified permeability and simple geometry are to used to estimate the range of flux values. Results indicate that both relatively rapid (<100,000y) and slow (>100,000y) silicification of the member is possible. Fluid migration through the distal regions of the Antler foreland basin may also explain the pattern of pervasive dolomitization along the western margin of the Thunder Springs member, corresponding to the zone of greatest silicification. Additionally, maximum uplift of the Antler orogenic belt is coincident with deposition and silicification of the Thunder Springs Member. If validated by further study, the concept of tectonically driven silicification may improve our understanding of bedded chert genesis along the western margin of the Cordillera in Middle Paleozoic time.