Joan R. Mylroie

Caves and Karst of the Bahama Islands

The Bahama Islands provide the simplest setting for expression of island karst as described by the Carbonate Island Karst Model (CIKM). The rocks are carbonates of mid to late Quaternary age, relief is low, climate is stable, and tectonics non-existent. As a result, there are significant constraints in time (a few thousand years) and space (a few hectares) for cave and karst development. Despite these constraints, cave development is prolific, and caves of large size are common across the entire archipelago. The Bahamas demonstrate the complex interaction between deposition and dissolution, with syndepositional caves forming as the carbonates they are in are still being deposited in immediately adjacent areas. Despite all the time and spatial constraints, the cave variety and morphology is complex and well represents the special ground-water flow conditions and geochemistry that exist within the fresh-water lens. The Bahamas were the site of origin for both the flank margin cave model, and the subsequent CIKM, as the conditions present allowed establishment of the fundamental theoretical controls of cave and karst development in coastal settings.

Pseudokarst Caves in the Littoral Environment

Rocky coastal regions can host caves produced by karst (dissolutional) processes, and caves produced by pseudokarst (non-dissolutional) processes. On limestone coasts, which are common world wide, both processes can be active and a complex interplay can result. Lava tubes, calcaerous tufa deposition, and reef growth all produce constructional caves, voids formed as the rock itself is formed. Only reef growth is an obligatory result of the marine coastal environment. Tafoni result from subaerial weathering of a variety of lithologies exposed on a cliff or steep slope, and can mimic other types of pseudokarst caves and karst caves. Talus and fissure caves result from failure of steep slopes and cliffs, themselves a result of coastal erosion which can quickly remove these pseudokarst cave types. Sea arches and sea caves are abundant on rocky coasts, as the interaction of wave dynamics and rock properties create a variety of erosional voids. Sea cave processes can overprint other cave types to produce a hybrid cave. Sea caves are likely the most common cave type in the world, but on limestone coasts, dissolutional mixing zone caves also form in great numbers, and are commonly overprinted to make abundant hybrid caves.

Flank margin cave development and tectonic uplift, Cape Range, Australia

Cape Range, Australia, on the northwest coast of the continent at 218 S, 1138 E, is a north-northeast-striking anticlinal ridge 315 m high, 130 km long, and 32 km wide extending into the sea and consisting of Miocene carbonate rocks with a series of coastal terraces of Pliocene and Quaternary carbonates and siliciclastic dunes. Inland escarpments, representing former sea cliffs, and deep valleys cutting the limbs of the anticlinal ridge host many cave entrances at a variety of elevations. The lowest unit, the Mandu Formation, a chalky and marly limestone, contains many tafoni (pseudokarst) caves with simple, singlechamber plans and widths up to 15 m or more and height up to 10 m. The higher, purer Miocene limestones and the younger Pliocene and Pleistocene coastal terrace limestones host numerous flank margin caves from 300 m elevation in the Miocene rocks to sea level in the Quaternary rocks. These caves have entrances up to 30 m wide and heights of 6 m, with single-chamber caves being common, but complex caves are also present. Some caves are entered by small entrances that lead to large phreatic chambers, which eliminates both sea cave and tafoni as possible explanations. The close association of these caves with sea cliffs and incised valleys argues against a deep hypogene origin, which would leave a cave pattern unrelated to the surface configuration. Miocene uplift tapered off into the Pliocene. The flank margin caves in the paleo sea cliffs represent the outcome of the interplay of tectonic activity and glacioeustasy over a 300 m vertical range, with lowstands causing valley incision, while highstands raised the fresh-water lens and allowed cave development in the valley walls. Cave development began with the first tectonic-driven subaerial exposure in the Miocene and continued through to the last Pleistocene interglacial.

Telogenetic Limestones and Island Karst

The development of flank margin caves in telogenetic carbonates is in most cases restricted to flow and mixing along fractures, joints and bedding planes, and the caves in this setting tend to have planar, two dimensional configuration. Under special circumstances, as when the telogenetic carbonates are highly fractured, as in New Zealand, or the telogenetic carbonates have been altered into a breccia faces, as in paleotalus in Croatia, a three dimensional series of phreatic chambers can develop. In both cases of fracturing and paleotalus, the three dimensional configuration of the void space and potential flow paths mimic eogenetic carbonates, and flank margin caves similar to those found in eogenetic carbonates have developed. These examples demonstrate that no single characteristic determines the morphological nature of dissolution in the coastal environment. While the coastal geochemical environment of mixing is unique, rock diagenetic maturity and structure also play an important role.

Australian Examples of Coastal Caves

Rottnest Island and Kangaroo Island, eolianite-containing islands off Australia’s west and southern coasts, respectively, display extensive coastal caves and karst that contain valuable geologic information. On Rottnest Island, Late Quaternary eolianites contain flank margin caves formed during MIS 5e, and may contain flank margin caves developed during the mid-Holocene sea-level highstand. Pit cave development, with subsequent cementation of infilling deposits, results in inversion of topography following surficial denudation. Pseudokarst forms such as sea caves and tafoni, and surficial polygonal structures, complicate karst interpretations. On Kangaroo Island, flank margin caves in Quaternary eolianites are preferentially preserved in protected embayments. Flank margin caves at ∼30 m elevation indicate much greater uplift rates and magnitudes than previously believed. As with Rottnest Island, sea caves and tafoni complicate karst interpretations and provide a cautionary note to those working on coastal caves and karst.

REPLY TO COMMENTS ON: COASTAL CAVES IN BAHAMIAN EOLIAN CALCARENITES: DIFFERENTIATING BETWEEN SEA CAVES AND FLANK MARGIN CAVES USING QUANTITATIVE MORPHOLOGY

We agree that the data are not normally distributed, which puts the results of Waterstrat et al. (2010) into question as use of the Student T-test would be then inappropriate. We agree with Curl’s attempt to normalize the data with a log transformation, and the data certainly are closer to normal than they were before the natural log transformation. However, we do not agree that the data are now normal. The log transformed data fail the Anderson-Darling test for normality (p50.026), and the Shapiro-Wilk test for normality (p50.018). This still invalidates the use of the Student T-test as it assumes normality. However, when we conduct the Wilcoxon-Rank Sum Test (the non-parametric equivalent of the Student Ttest) with the natural log transformed A/P ratios, we find that the Flank Margin Caves and San Salvador Sea Caves are not significantly different (p50.457), as Curl noted using the Student T-test. Further work needs to be done to confirm or reject the use of the various other ratios discussed in Waterstrat et al. (2010). To clarify Curl’s statement about the use of speleothems to differentiate cave types, the controls for this work were sea caves developed in Holocene eolianites, which by definition, could not contain flank margin caves of Pleistocene age, and flank margin caves near higher paleo-shorelines entered by ceiling collapse or vadose shafts, with no lateral opening to a past sea-level highstand. These end-member caves allowed qualitative comparison of diagnostic features unique to each cave type. The various measurements subsequently used were an attempt to quantify these observations. In regard to Bill Mixon’s statements, comparison of caves from differing geologic environments would certainly be expected to give different results; how they differ was the important question. In carbonate islands, one cannot tell at a glance of a cave map if a cave is a sea cave or a flank margin cave; that is precisely why the quantitative attempt was undertaken. These San Salvador caves are developed in geologically identical Quaternary eolianites, and the caves formed in geologically recent time, and can be overprinted. As a result, criteria for differentiation are subtle. The issue of dimensionless ratios is irrelevant; hydraulic radius is an area to perimeter ratio of well known effectiveness in stream hydrology. The authors thank Mr. Erik Larson for his application and evaluation of the statistics discussed above.

Bahamian Flank Margin Caves as Hypogene Caves

Fieldwork in the Bahamian Archipelago in the 1970s and 1980s identified a new cave type, the flank margin cave, as macroscopic dissolutional voids developed in the margin of a freshwater lens, under the flank of the enclosing landmass. These voids are produced by three conditions that exist at the lens margin: mixing dissolution, organic decay horizons, and the increase in freshwater flow rate. The water flow enters flank margin caves as diffuse flow and exits as diffuse flow, a flow regime that produces dissolutional sculpture lacking turbulent flow features, such as asymmetric scallops. The caves are tied to sea level, which controls the freshwater lens position, and as such are excellent indicators of past sea-level position. The caves form without entrances and become accessible only after subaerial erosion has breached their ceilings or walls. Flank margin caves initiate as individual globular dissolutional voids that then intersect as the voids enlarge, increasing cave size in a sudden stepwise manner. As cave development is restricted to the lens margin, the largest flank margin caves acquire a linear shape as voids interconnect parallel to the lens margin. Flank margin caves are hypogene caves based on their diffuse, slow-flow regimes, because the dissolutional aggressiveness is generated below the surface by mixing, and the ascending marine water following the base of the lens to the site of dissolution at the lens margin. Because these caves form rapidly at shallow depths, there is a debate as to their hypogene classification, but they meet all criteria for hypogene speleogenesis.