Flank margin caves can connect to regionally extensive touching vug networks before burial: Implications for cave formation and fluid flow

Abstract

AbstractFlank margin caves are extreme endmembers of vuggy porosity that form as diagenesis drives the progressive coalescence of smaller solutional pore spaces. Due to their morphological isolation during formation, the prevailing hypothesis has been that fluid flow in and out of flank margin caves occurs via the matrix permeability and that adjacent chambers only become hydraulically connected through nonmatrix porosity during burial, collapse, and fracturing. To our knowledge, however, no studies have evaluated how flank margin caves are connected to regional flow systems in modern carbonate platforms. In this study, we evaluate the connectivity of wells, boreholes, blue holes, and flank margin caves in increasingly older bedrock on San Salvador Island, Bahamas, using tidal attenuation analysis. Phreatic karst features are not reported in Holocene bedrock, and permeability magnitudes from wells suggest Holocene deposits connect to the ocean along matrix‐dominated flow paths. Permeability magnitudes in bedrock surrounding wells, boreholes, and karst features deposited during Marine Isotope Substage (MIS) 5e suggest connection to the ocean through matrix and touching vug porosity. Boreholes, blue holes, and flank margin caves in pre‐MIS5 bedrock connect to the ocean via touching vugs. We suggest that increasing bedrock permeability, cave number, and cave size observed within progressively older bedrock on San Salvador is a function of the cumulative number of freshwater lenses emplaced over successive sea‐level stillstands. We suggest that the morphologies of the two largest caves are consistent with dissolution in multiple lenses occupying lower elevations, collapse resulting in breakout domes, and overprinting of collapse chambers during subsequent highstands. As a result, some caves may not reflect connectivity of the bedrock surrounding the main chambers but may reflect connectivity of more diagenetically mature bedrock at lower elevations where their antecedent chambers formed.