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

Abstract

Flank 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.

A transient, perched aquifer model for banana hole formation: Evidence from San Salvador Island, Bahamas

“…Correcting for the average daily evaporation (3 mm/d, Crump & Gamble, 2006), and neglecting the 5–7 cm change falling head, we approximate the 1D hydraulic conductivity of the perching surface to be about 10 –1.5 m/d, or on the order of 10 –15.5 m 2 (Equation ). This value is consistent with the upper quartile of permeabilities collected from exposure zones (Figure 5) and is well below permeability values reported for MIS 5 bedrock (Breithaupt et al , 2021).…”

“…Phreatic models for banana hole formation have largely assumed that a freshwater lens must be present to dissolve bedrock and form their pre‐collapse voids (Gulley et al , 2015; Mylroie et al , 2016; Mylroie & Carew, 1995). Because the position of the freshwater lens tracks with sea levels, opportunities for dissolution to occur at, and above, modern sea levels are limited to short lived sea level high stands (Harris et al, 1995: Florea et al , 2007; Mylroie et al , 2016; Breithaupt et al , 2021). While the biological and geochemical processes proposed earlier may contribute to cave formation in environments where water tables and/or mixing zones are stable over >10 ka timescales, the dissolution rates required for banana hole formation during the MIS 5e highstand in the Bahamas are orders of magnitude greater than those calculated from the most optimistic mixing and pCO 2 models (Figure 16).…”

“…Recharge occurs during two wet seasons from May to June, and September to November, when monthly rainfall can exceed monthly evapotranspiration. Freshwater lenses are discontinuous across the island and are concentrated within ridges in Holocene and MIS 5 bedrock which have lower permeability relative to older pre‐MIS 5 deposits (Breithaupt et al, 2021). Older bedrock on the island typically contains saltwater because permeabilities are too high to support lens formation in San Salvador Island's arid climate (Davis & Johnson, 1989; Vacher, 1988; Vacher & Wallis, 1992).…”

Three‐dimensional stratigraphic architecture, secondary pore system development and the Middle Pleistocene transition, Sandy Point area, San Salvador Island, Bahamas

“…Void apertures were estimated in the field from bit drops, lost core recovery, and observations of vuggy and recrystallized core (e.g. Breithaupt et al, 2021;Fig. 5).…”

“…Further, it is likely that the increased sea-level amplitude and longer cycle time of the 100 kyr forcing drove greater meteoric dissolution during sea-level lowstands, and thus the increased development of caliche deposits and cave systems (James, 1972). Breithaupt et al (2021) have presented compelling evidence that these laminar caliches are central to patterns and localization of meteoric dissolution; the authors add to the discussion below.…”