Abstract Quaternary glacio-eustasy has traditionally been determined in part by the examination of fossil coral reefs on carbonate islands and coasts uplifted by tectonics. These studies do not properly account for dissolutional denudation, which is cumulative, making higher and therefore older terraces exist at elevations far below their assumed depositional elevation. Karst pedestals (karrentische) on Guam reveal the extent of the denudation ( c. 50 mm ka −1 ) and demonstrate that theoretical denudation models can be accurately applied to eogenetic carbonates in tropical settings. Aeolian calcarenite islands such as the Bahamas have been used as tectonically stable sea-level calibrations for other islands, which may not be correct. Flank margin caves, forming in the distal margin of the freshwater lens within a carbonate island, are excellent sea-level indicators. Analysis of flank margin cave elevations indicates that the Bahamas have had past sea-level highstands >6 m, perhaps up to 15 m or more, for which no fossil coral data exist. Denudational removal of these older corals has biased the record to younger events and only flank margin caves remain as viable terrestrial signatures of these older sea-level highstands.

Cave type and morphology are controlled by hydrogeological and geological factors; therefore, by inverse analogy, cave type and morphology could be used to determine the hydrogeological and geological conditions under which the caves developed. Euclidian metrics have traditionally been used to quantify and compare cave morphologies, even though caves have irregular and complex shapes. Caves have been shown to possess characteristics that identify them as fractals within certain ranges, so the use of Euclidean-based metrics alone to define and characterize them may be a limitation in morphometric analyses. Other factors that limit full morphometric analyses of caves include focus on two-dimensional cave data, as these are typically what are available, and exploration bias, as cave exploration and documentation are limited to spaces that are humanly passable or of immediate interest to the explorer, epitomizing the subjective nature of anthropogenic-based measurements. This research involves a proof-of-concept study that uses fractal indices as a means for identifying and classifying cave morphology and distinguishing genetic cave types. The fractal indices used are fractal dimension, which quantifies the complexity of a pattern, and lacunarity, which quantifies the texture of a pattern. Three-dimensional cave survey data were used to generate cave models that were converted to cave pattern image files and analyzed with image-processing software. Fractal indices were calculated for digital patterns of a limited subset of known cave types that included tafoni, littoral caves, stream caves, flank margin caves, and continental hypogene caves. The quantitative morphological distinctions in cave patterns as identified by fractal dimension and lacunarity proved to be statistically significant within the subset of cave types analyzed for this study. Similarities in geological and/or hydrogeological processes or overprinting by such processes can skew fractal indices, so geological and hydrogeological context is critical when interpreting fractal indices. The results of this study demonstrate that cave morphometry as defined by fractal indices can be used to augment the identification of cave type, which provides insight into the geological and hydrogeological controls on development of the cave type and its cavernous porosity and permeability.

Insights from previous karst studies of the relatively simple and stable carbonate islands of the Caribbean and Western Atlantic have been applied to develop the carbonate island karst model (CIKM), a general model with which we can interpret the karst of more complicated islands in the Western Pacific. This paper summarizes the karst of the five southernmost Mariana Islands in order of increasing complexity. All exhibit complicated histories of tectonic uplift and subsidence overprinted by glacio-eustasy. Each, however, is distinct and can be described in total or by subunit in terms of the four idealized carbonate island types defined in the CIKM: (1) simple carbonate island, (2) carbonate-cover island, (3) composite island, and (4) complex island. Aguijan is a simple carbonate island, but contains a probable phreatic-lift cave draining a confined aquifer. Tinian illustrates application of the CIKM to subunits: the northern lowland is a simple carbonate island area, while the southeastern ridge fits the carbonate-cover island category, and the central portion fits the composite island category. Rota is a composite island grading laterally from the volcanic core into a carbonate-cover island, thence to a simple carbonate island from the southwestern highland to the plains and terraces north and east. Northern Guam is a simple carbonate island ringing a carbonate-cover section, which contains a small composite island portion. Southern Guam exhibits composite and complex island features. Saipan is a complex island, where syndepositional volcaniclastic units interfingering with limestone are faulted to create isolated aquifers, including confined aquifers drained by phreatic-lift caves.

Abstract Quaternary carbonate eolianites on Babaroian islands form a complex constructional topography composed of deposits formed during different glacioeustatic sea-level highstands. These eolianites constitute spatially and temporally discontinuous sediment packages. Within deposits formed during individual sea-level highstand events, eolianites from transgressive, stillstand, and regressive phases can be differentiated, but only if exposure is good and detailed study is undertaken. The eolianites may be stacked atop one another, or they may be in contact with marine deposits. Eolianites may overlie older marine deposits, interfinger with coeval marine rocks, or be overlain by younger marine facies. Depositional packages including eolianites that are deposited during a single sea-level highstand are commonly bounded above and below by erosion surfaces characterized by terra rossa paleosols. However, the patchiness of eolian deposition, the occurrence of calcarenite protosols, possible erosion of a paleosol during an ensuing transgression, and the possibility of soil movement downward into karst features can create confusion about actual unit boundaries. Interpretation of carbonate eolianite sequences in the ancient rock record may be enhanced if the complexities of the Quaternary eolianite record are appreciated. The depth of study required to successfully interpret well-exposed Quaternary eolianites implies that ancient eolianites, with less exposure and greater diagenesis, are unlikely to be resolved to a similar degree.

Abstract Karst development on carbonate platforms occurs continuously on emergent portions of the platform. Surficial karst processes produce an irregular pitted and etched surface, or epikarst. The karst surface becomes mantled with soil, which may eventually result in the production of a resistant micritic paleosol. The epikarst transmits surface water into vadose pit caves, which in turn deliver water to a diffuse-flow aquifer. These pit caves form within a 100,000 yr time frame. On islands with a relatively thin carbonate cover over insoluble rock, vadose flow perched at the contact of carbonate rock with insoluble rock results in the lateral growth of vadose voids along the contact, creating large collapse chambers that may later stope to the surface. Carbonate islands record successive sequences of paleosols (platform emergence) and carbonate sedimentation (platform submergence). The appropriate interpretation of paleosols as past exposure surfaces is difficult, because carbonate deposition is not distributed uniformly, paleosol material is commonly transported into vadose and phreatic voids at depth, and micritized zones similar in appearance to paleosols can develop within existing carbonates. On carbonate islands, large dissolution voids called flank margin caves form preferentially in the discharging margin of the freshwater lens from the effects that result from freshwater/saltwater mixing. Similarly, smaller dissolution voids also develop at the top of the lens where vadose and phreatic freshwaters mix. Independent of fluid mixing, oxidation of organic carbon and oxidation/reduction reactions involving sulfur can produce acids that play an important role in phreatic dissolution. This enhanced dissolution can produce caves in freshwater lenses of very small size in less than 15,000 yr. Because dissolution voids develop at discrete horizons, they provide evidence of past sea level positions. The glacio-eustatic sea level changes of the Quaternary have overprinted the dissolutional record of many carbonate islands with multiple episodes of vadose, freshwater phreatic, mixing zone, and marine phreatic conditions. This record is further complicated by collapse of caves, which produces upwardly prograding voids whose current position does not correlate with past sea level positions. The location and type of porosity developed on emergent carbonate platforms depend on the degree of platform exposure, climate, carbonate lithology, and rate of sea level change. Slow, steady, partial transgression or regression will result in migration of the site of phreatic void production as the freshwater lens changes elevation and moves laterally in response to sea level change. The result can be a continuum of voids that may later lead to development of solution-collapse breccias over an extended area.

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Full article available in PDF version.

Full article available in PDF version.