AIR-FILLED CAVES AS AN INDICATOR OF DIRECTIONAL TRANSMISSIVITY IN THE UNCONFINED UPPER FLORIDAN AQUIFER OF WEST CENTRAL FLORIDA

Significant conduit flow occurs in the triple-porosity Upper Floridan aquifer where it is unconfined in west-central Florida. The many large springs and the high transmissivities of the area are a manifestation of phreatic conduits. Air-filled caves occur at the boundary between confined and unconfined parts of the aquifer at the Cody Scarp, Ocala Uplift, and Brooksville Ridge. In many cases, these vadose caves are tiered, and commonly, they intersect and extend below the present-day water level. Conduit morphologies vary from amorphous chambers with low height-to-width ratios to linear fissures with high height-to-width ratios.

Length-weighted rose diagrams of the orientations of conduits within air-filled caves in the springsheds of Rainbow and Silver Springs reveal preferential orientations in sub-orthogonal azimuth sets. The sets vary regionally but generally trend NE-SW and NW-SE and conform to the directions of photolinears found in preceding studies. The likely association of these conduit orientations with fractures is confirmed by visual inspection and photography in several of the caves. Strikingly, many of the rose diagrams show that lengths are concentrated in one of the two directions, as if dissolution has selected one direction over the other, although the particular selection and its direction vary across the study area. Because the fracture systems that ultimately control the direction of conduits within the cave systems extend below the present water level, we hypothesize that similar patterns apply in the phreatic zone and account for directional transmissivities found in a recent modeling study of the Upper Floridan aquifer in the area.