The flank margin model for dissolution cave development in carbonate platforms

Abstract

The Bahama Islands contain many abandoned dissolution caves at elevations between two and seven metres above current sea level. The development of dissolution caves in tropical carbonate islands is dependent on the position and nature of the freshwater lens. Lens position is controlled by sea level, which in stable carbonate platforms like the Bahamas is a function of glacioeustatic sea level still stands. Caves in the Bahamas that are currently subaerial must have developed during past higher sea levels. During the Late Quaternary, sea levels higher than present have been relatively short-lived, and that limits the amount of time that a freshwater lens could be situated at the elevation required for the cave formation. The Bahama Islands are low-lying landforms where only aeolian ridges extend to elevations higher than six metres above current sea level. Past high sea level events greatly reduced the exposed land area of the Bahama Islands, thus also limiting both the catchment for and size of freshwater lenses. Caves must be younger than the rock in which they are developed; most subaerial Bahamian caves are found in limestones that are less than 150000 years old. Development of large dissolution caves under these limitations of time and lens size requires a powerful dissolutional mechanism. The mixing of discharging freshwater with tide-pulsed incoming marine water under the flanks of emergent dune ridges may have produced the conditions necessary. Bahamian caves formed by this process are phreatic chambers with complex interconnections and blind tubes. Their presence demonstrates that significant dissolution can occur rapidly as a result of the mixing of fresh and marine waters beneath small carbonate islands.

Late Quaternary speleogenesis and landscape evolution in a tropical carbonate island: Pango la Kuumbi (Kuumbi Cave), Zanzibar

“…50 cm unfilled burrows, associated with former sea level stillstands (Mylroie & Carew, 1990;Mylroie at al., 2001;Mylroie, 2013 and references therein).…”

“…Such a link is becoming increasingly recognised in carbonate islands and coastal regions throughout the world. The model of 'flank margin speleogenesis', originally proposed to interpret cave formation in relatively young (Late Cainozoic), eogenetic limestones, and its subsequent evolution into the more generic and nuanced Carbonate Island Karst model emphasise the critical speleogenetic role of the freshwater lens -a feature highly responsive to sea level change (Mylroie & Carew, 1990;Frank et al, 1998;Mylroie et al, 2001;Fratesi, 2013;Mylroie, 2013). Various processes operating at the seaward (halocline: fresh and seawater mixing zone) and upper (groundwater table) boundaries of this lens promote undersaturation of circulating aqueous solutions in CaCO 3 , limestone dissolution and the removal of dissolution products.…”

“…Mylroie & Carew, 1990;Vacher & Mylroie, 2002). These pores provided early foci for karstic dissolution: progressively enlarging porosity from which larger cavities were to evolve.…”

“…The concept of flank margin speleogenesis and its more comprehensive derivative, the Carbonate Island Karst Model (Mylroie & Carew, 1990;Mylroie et al, 1995;2008;Mylroie & Vacher, 1999;Mylroie & Mylroie, 2007;Mylroie, 2013), have further highlighted this distinctiveness by emphasising the crucial role of carbonate dissolution in the distal, seaward part of the freshwater lens underneath carbonate islands (and, also, continental littorals). In this speleogenetic realm, discrete from those of epigenic and hypogenic speleogenesis, critical domains of carbonate dissolution (groundwater table; fresh/saltwater mixing zone) are highly responsive to relative sea level change, in turn driven by eustatic sea level change (at several timescales), and isostatic or tectonic crustal uplift or subsidence (Ginés & Ginés, 2007;Mylroie & Mylroie 2007;Fratesi, 2013;Ginés et al, 2014).…”

Linking mineral deposits to speleogenetic processes in Cova des Pas de Vallgornera (Mallorca, Spain)

“…The primary mechanism in control of forming these caves is the dissolution caused by mixing of two water bodies (fresh and salt) with contrasting temperature, chemistry, salinity, or CO 2 content (Herman et al, 1985;Mylroie & Carew, 1990). This particular type of caves form in shallow or deep coastal mixing zones and are characterized by spongework or crude ramifying passages.…”