Quartz geodes from the dolostones of the Fort Payne and "Warsaw" formations near Woodbury, Tennessee have proved to be pseudomorphs after early diagenetic anhydrite nodules. Their lithological association suggests that the anhydrite developed in an arid tidal-flat environment by a process similar to that currently operating in the sabkhas of the Persian Gulf. Silicification took place prior to compaction and lithification of the sediments, the most likely source of silica solutions being the abundant sponge spicules which characterize the peritidal dolostones. Similar geodes from other Mississippian localities share the same lithological association and mode of origin. They help to define a recurrent sabkha facies and serve as important shoreline indicators which may be used in the reconstruction of regional patterns of marine transgression and regression.