Hypogene Karst Influences in the Upper Floridan Aquifer

Abstract

Dissolution of eogenetic carbonates in the Upper Floridan aquifer has produced the world’s densest assemblage of first-magnitude cave springs. Conceptual and numerical models of cave origin in the aquifer have emphasized epigenic and mixing-dissolution processes. We draw upon recent research concerning phreatic caves in the Suwannee River Basin, and dry caves in the west and central Florida, to suggest instead that many caves in the aquifer formed by hypogenic processes. Formerly, undersaturation generated in the subsurface has usually been ascribed to the mixing of, or temperature changes in, subsurface fluids. Here, we describe an alternate process. Cave formation at modern water tables in the aquifer has been linked to respiration of CO2 in the deep vadose zone and at water tables. Respired CO2 generates carbonate mineral undersaturation when it hydrates to carbonic acid at water tables. Because undersaturated waters are created at the water table, caves form as isolated macropores. Caves at modern water tables in the aquifer lack initial connections to the surface (e.g., entrances) and have morphologies that are unrelated to surface drainage, making them similar in many respects to flank margin caves (see Mylroie and Mylroie, Chap. 51). Since many caves that are below the modern water table have similar morphologies to caves that are at the modern water table, it is likely that they formed by similar processes operating at water tables associated with lower sea levels. These caves became sources of springs and flooded sinkholes when Holocene sea-level rise elevated water tables close to, and above, the land surface.