THE EFFECT OF FACIES VARIATIONS OF EOGENETIC CARBONATES IN THE DEVELOPMENT OF FLANK MARGIN CAVES (Invited Presentation)

Flank margin caves are a subset of hypogene caves, forming as a result of mixing in a fresh-water lens decoupled from direct surface hydrologic influence in coastal areas. A significant amount of the coastal carbonates globally are eogenetic. These eogenetic carbonates exhibit a variety of facies: reef, lagoonal, beach, eolian, tempestite, and epikarst/paleosol. Flank margin caves develop in all these facies and combinations of these facies, retaining their ramiform and spongework pattern. The definition of hypogene as decoupled from surface hydrology means that water flow is diffuse in nature and non-turbulent. In eogenetic carbonates, depositional porosity is retained as effective permeability and diffuse flow has a multitude of flow path opportunities regardless of most facies character, which reduces flow competition dynamics typical of epigene turbulent flow. The absence of mechanical forces due to turbulent flow preserves intricate dissolutional sculpture and eliminates abrasion as a cave genetic mechanism. Carbonate mud facies are deposited without primary porosity of the scale to enhance permeability, and flank margin caves in this facies mimic flank margin caves found in telogenetic rocks, where cave development occurs along joints, bedding planes and fractures. Where telogenetic rocks have been brecciated, deposited as a talus, or have closely spaced joints, their permeability is volumetrically similar to eogenetic carbonates and ramiform and spongework patterns can develop.