THE TAMIAMI FORMATION—HAWTHORN FORMATION CONTACT IN SOUTHWEST FLORIDA

Stratigraphic placement of the Tamiami Formation—Hawthorn Formation contact has been arbitrary in subsurface investigations in southwest Florida. This contact is properly positioned beneath a dark colored carbonate mud unit, which contains large concentrations of detrital phosphorite, quartz sand, and various clay minerals. The mud unit, known informally as the Fort Myers clay member of the Tamiami Formation, covers the Hawthorn Formation in nearly all of southwest Florida. The formational contact is distinctive in western Lee and Charlotte Counties, where it is a stratigraphic disconformity, and becomes less distinctive to the south and east, where deposition has been continuous and no break in the stratigraphic record is evident.