John E. Mylroie

Eogenetic karst from the perspective of an equivalent porous medium

The porosity of young limestones experiencing meteoric diagenesis in the vicinity of their deposition (eogenetic karst) is mainly a double porosity consisting of touching-vug channels and preferred passageways lacing through a matrix of interparticle porosity. In contrast, the porosity of limestones experiencing subaerial erosion following burial diagenesis and uplift (telogenetic karst) is mainly a double porosity consisting of conduits within a network of fractures. The stark contrast between these two kinds of karst is illustrated by their position on a graph showing the hydraulic characteristics of an equivalent porous medium consisting of straight, cylindrical tubes (n-D space, where n is porosity,D is the diameter of the tubes, and logn is plotted against logD).Studies of the hydrology of small carbonate islands show that large-scale, horizontal hydraulic conductivity (K) increases by orders of magnitude during the evolution of eogenetic karst. Earlier petrologic studies have shown there is little if any change in the total porosity of the limestone during eogenetic diagenesis. The limestone of eogenetic karst, therefore, tracks horizontally inn-D space. In contrast, the path from initial sedimentary material to telogenetic karst comprises a descent on the graph with reduction ofn during burial diagenesis, then a sideways shift with increasingD due to opening of fractures during uplift and exposure, and finally an increase inD andn during development of the conduits along the fractures.Eogenetic caves are mainly limited to boundaries between geologic units and hydrologic zones: stream caves at the contact between carbonates and underlying impermeable rocks (and collapse-origin caves derived therefrom); vertical caves along platform-margin fractures; epikarst; phreatic pockets (banana holes) along the water table; and flank margin caves that form as mixing chambers at the coastal freshwater-saltwater “interface”. In contrast, the caverns of telogenetic karst are part of a system of interconnected conduits that drain an entire region. The eogenetic caves of small carbonate islands are, for the most part, not significantly involved in the drainage of the island.

Quaternary tectonic stability of the Bahamian archipelago: evidence from fossil coral reefs and flank margin caves

Abstract Throughout the islands of the Bahamian archipelago fossil coral reefs are found from current sea level up to a maximum elevation of +4 m. 234 U 230 Th radiometric dates obtained from in situ corals from these reefs, by both alpha-count and mass-spectrometric techniques, indicates that they were all formed during Oxygen Isotope Substage 5e (ca. 125,000 years ago). Those data are consistent with a maximum sea-level highstand of +6 m during Substage 5e, and either no vertical motion of the Bahamas, or possible isostatic subsidence of up to 2 m during the past 120,000 years. No older in situ fossil corals, or other subtidal deposits, have been found subaerially exposed anywhere in the Bahamas. That finding suggests that late Quaternary (at least the past 300,000 years) isostatic subsidence has occurred at a rate of 1–2 m per hundred thousand years, and/or no pre-5e highstands were above modern sea level. An independent corroboration of the conclusions drawn about sea level amplitude and tectonic stability of the Bahamas from the coral reef data is available from examination of abundant flank margin caves (horizontal, phreatic dissolution caves) found above modern sea level throughout the Bahamas. These horizontally extensive air-filled caves have dissolutional ceilings with elevations that are restricted to +1 to +7 m, which is consistent with formation at the margin of a thin freshwater lens elevated by a past +6 m sea-level highstand. The restricted cave elevations, and the lack of stalagmites in these caves that are older than 100,000 years, are also consistent with cave formation during Substage 5e, and possible subsequent isostatic subsidence of a few metres. The subsurface geology of the southeastern Bahamas contains a long-term record (millions of years) that has been attributed to past tectonic activity along the North American/Caribbean plate boundary. While that record suggests differential subsidence across the Bahamas in the Tertiary Period, the data from fossil coral reefs (and subtidal deposits) and flank margin caves indicate that all Bahamian banks on which there are islands have been tectonically stable, and behaving similarly, for at least the past several hundred thousand years.

Blue holes: Definition and genesis

Blue holes are karst features that were initially described from Bahamian islands and banks, which have been documented for over 100 years. They are water-filled vertical openings in the carbonate rock that exhibit complex morphologies, ecologies, and water chemistries. Their deep blue color, for which they are named, is the result of their great depth, and they may lead to cave systems below sea level. Blue holes are polygenetic in origin, having formed: by drowning of dissolutional sinkholes and shafts developed in the vadose zone; by phreatic dissolution along an ascending halocline; by progradational collapse upward from deep dissolution voids produced in the phreatic zone; or by fracture of the bank margin. Blue holes are the cumulative result of carbonate deposition and dissolution cycles which have been controlled by Quaternary glacioeustatic fluctuations of sea-level.Blue holes have been widely studied during the past 30 years, and they have provided information regarding karst processes, global climate change, marine ecology, and carbonate geochemistry. The literature contains a wealth of references regarding blue holes that are at times misleading, and often confusing. To standardize use of the term blue hole, and to familiarize the scientific community with their nature, we herein define them as follows: “Blue holes are subsurface voids that are developed in carbonate banks and islands; are open to the earths surface; contain tidally-influenced waters of fresh, marine, or mixed chemistry; extend below sea level for a majority of their depth; and may provide access to submerged cave passages.” Blue holes are found in two settings: ocean holes open directly into the present marine environment and usually contain marine water with tidal flow; inland blue holes are isolated by present topography from surface marine conditions, and open directly onto the land surface or into an isolated pond or lake, and contain tidally-influenced water of a variety of chemistries from fresh to marine.

Solution conduits as indicators of late Quaternary sea level position

Abstract Solution conduits or caves can provide a measure of glacio-eustatic sea level position for the Quaternary. In high latitudes, cave deposits contain artifacts, fossils and sediments that reflect surface conditions with respect to climate. Secondary calcite deposits, most notably stalagmites, record growth episodes during ice minima and non-growth during ice maxima. They provide an indirect estimate of areal ice cover at high latitudes, and are an indirect measure of sea level. In low latitudes, stable carbonate platforms contain caves that provide a direct measure of past sea level position. Solution conduits developing in a fresh water lens will record the position of the lens and therefore sea level. As sea level fluctuates, fresh water lens horizons and conduit development horizons will shift, preserving a record of past sea level position. Cave wall rock and cave deposits can produce a chronology for given conduit positions. Secondary calcite deposits, notably stalagmites, record growth periods during sea level low stands (or ice maxima) and record growth hiatuses during flooding by sea level high stands (or ice minima); the inverse of high latitude stalagmites. Lighthouse Cave, San Salvador Island, Bahamas, records a sea level high stand at +1 to +6 m for some time period between 85,000 and 70,000 years ago, based on position and timing of conduit development; and records sea level at present elevation for some time period between 49,000 and 37,000 years ago, based on a stalagmite with a marine overgrowth.

Banana holes: Unique karst features of the Bahamas

Banana holes are circular to oval voids with diameters ranging from 2 meters to more than 10 meters, and with depths up to 5 meters, which are found throughout the Bahamas. They are named for a specialty crop sometimes grown in the thick moist soils that accumulate in them. They commonly have vertical or overhung walls, and exhibit phreatic dissolutional morphology. Occasionally, banana holes are found with complete or nearly complete roofs.Banana holes are the result of shallow-phreatic dissolution in the top of a fresh-water lens supported by the last interglacial sea-level highstand (ca. 125,000 years ago). Their current surface expression is the result of the partial or total collapse of their thin roofs. They did not originate by progradational collapse from depth, or by vadose processes. Once expressed on the surface by roof collapse, however, banana hole floors are modified by vadose waters with elevated CO2 concentrations derived from the organic material that collects within them.Banana holes pose a significant land use hazard in the Bahamas, especially those with intact roofs. Geophysical techniques such as ground-penetrating radar are necessary to locate these cryptic banana holes.

Geology and karst geomorphology of San Salvador Island, Bahamas

The exposed carbonates of the Bahamas consist of late Quaternary limestones that were deposited during glacio-eustatic highstands of sea level. Each highstand event produced transgressive-phase, stillstand-phase, and regressive-phase units. Because of slow platform subsidence, Pleistocene carbonates deposited on highstands prior to the last interglacial (oxygen isotope substage 5e, circa 125,000 years ago) are represented solely by eolianites. The Owls Hole Formation comprises these eolianites, which are generally fossiliferous pelsparites. The deposits of the last interglacial form the Grotto Beach Formation, and contain a complete sequence of subtidal, intertidal, and eolian carbonates. These deposits are predominantly oolitic. Holocene deposits are represented by the Rice Bay Formation, which consists of intertidal and eolian pelsparites deposited during the transgressive-phase and stillstand-phase of the current sea-level highstand. The three formations are separated from one another by well-developed terra-rossa paleosols or other erosion surfaces that formed predominantly during intervening sea-level lowstands.The karst landforms of San Salvador consist of karren, depressions, caves, and blue holes. Karren are small-scale dissolutional etchings on exposed and soil-covered bedrock that grade downward into the epikarst, the system of tubes and holes that drain the bedrock surface. Depressions are constructional features, such as swales between eolian ridges, but they have been dissolutionally maintained. Pit caves are vertical voids in the vadose zone that link the epikarst to the water table. Flank margin caves are horizontal voids that formed in the distal margin of a past fresh-water lens; whereas banana holes are horizontal voids that developed at the top of a past fresh-water lens, landward of the lens margin. Lake drains are conduits that connect some flooded depressions to the sea. Blue holes are flooded vertical shafts, of polygenetic origin, that may lead into caves systems at depth.The paleokarst of San Salvador is represented by flank margin caves and banana holes formed in a past fresh-water lens elevated by the last interglacial sea-level highstand, and by epikarst buried under paleosols formed during sea-level lowstands. Both carbonate deposition and its subsequent karstification is controlled by glacio-eustatic sea-level position. On San Salvador, the geographic isolation of the island, its small size, and the rapidity of past sea level changes have placed major constraints on the production of the paleokarst.

Early diagenetic origin and persistence of gamma-ray and magnetosusceptibility patterns in platform carbonates: comparison of Devonian and Quaternary sections

Abstract Gamma-ray logs from boreholes in the Middle–Upper Devonian carbonate platform of Moravia display tripartite anomalies at locations, where lithological and biostratigraphic data suggest the occurrence of 4th order sedimentary cycle boundaries. Further, where sedimentary boundaries have been changed by later development of caves (usually phreatic caves changed to unroofed caves––erosion), the carbonate infillings in these corroded cycle boundaries are marked by another pattern that consists of a smooth symmetrical peak on gamma-ray activity in vertical section. The details procured using gamma-ray spectrometric and magnetosusceptibility methods suggest that the upper peak of the tripartite pattern corresponds solely to uranium concentration (flooding surface). The middle peak is marked by a thorium signal and a magnetosusceptibility response from paramagnetic minerals (paleosols). The lower peak corresponds to trapped uranium and microbial magnetite in cemented rock pores (originally dysoxic microenvironments in calcite). The boundaries marked with filled caves display only one broad and symmetrical uranium-related peak, and the thorium peak that is roughly similar to that seen at normal boundaries, but it is shifted slightly downward. At boundaries with caves the magnetosusceptibility peaks are shifted downward considerably, and may even occur within the underlying host rock. The question of whether these patterns are a primary imprint of early diagenetic influences or a much later redistribution that originated during pressure solution and cementation, was answered by study of Late Quaternary sections on San Salvador Island, Bahamas. This pragmatic test on young carbonate sediments confirmed the early origin and fixation of these geophysical patterns.

Mineralogy of Cave Deposits on San Salvador Island, Bahamas

Speleothem samples from ten caves located in the northeastern and southwestern corners of San Salvador Island (Bahamas) were analyzed by means of X-ray diffraction, scanning electron microscopy, and the electron microprobe. In addition to the prominent calcite, aragonite, and gypsum, already known to occur in San Salvador caves, eleven other minerals were identified. The minerals are celestite, SrSO4; cesanite, Na3Ca2(SO4)3OH; ardealite, Ca2(HPO4)(SO4)·4H2O; brushite, CaHPO4·2H2O; hydroxylapatite, Ca5(PO4)3OH; fluorapatite, Ca5(PO4)3F; chlorapatite, Ca5(PO4)3Cl; collinsite, Ca2(Mg,Fe)(PO4)2·2H2O; witlockite, β-Ca3(PO4)2; niter, KNO3, and nitratine, NaNO3. Cesanite has not been previously reported from a cave. This is the second reported occurrence of collinsite.San Salvador Island, on the eastern edge of the Bahamian Platform, is the location of a large number of relatively small flank margin caves. In adition TO the more obvious speleothems — stalactites, stalagmites, and flowstone — the San Salvador caves contain a variety of crusts and soils of unknown mineralogy. This paper is an account of an investigation of samples collected from ten of these caves. Prior to the result reported here, only calcite, aragonite, and gypsum had been identified in the various speleothems from caves on San Salvador Island (Vogelet al. 1990; Schwabeet al. 1993).

Hydrodynamic behavior of caves formed in the fresh-water lens of carbonate islands

Flank margin caves are developed in carbonate islands at the edge of the fresh-water lens during sea-level highstands. The development of these caves in the Bahamas was constrained by the short period of glacioeustatic sea-level highstand available during the last interglacial (10,000–15,000 years) and by the small size of the fresh-water lens at the time of the sea-level highstand. The mixing of fresh and saline water, and bacterial oxidation and reduction of organic matter are the chemical processes that drive the origin of the flank margin caves. In the fresh-water lens, the average specific discharge (discharge per unit volume of aquifer) drastically increases at distances less than 100 m from the shoreline, as the lens thins. This area is named theactive edge of the fresh-water lens. The higher specific discharge results in enhanced dissolution, which, coupled with the chemistry of the distal margin of the lens, increases porosity and hydraulic conductivity. As a consequence the thickness of the fresh-water lens decreases with time and the average specific discharge continues to increase at the active edge of the lens. Mapped flank margin caves in the Bahamas show short penetration distances into the eolian ridges containing them (averging 29 m), perpendicular to the paleoshoreline, indicating development within the active edge.

Some pitfalls in paleosol interpretation in carbonate sequences

In Quaternary carbonate units composed mostly of eolianites, paleosols are important stratigraphic markers that help differentiate episodes of carbonate deposition tied to glacio-eustatic sea level fluctuations. Paleosols used in this manner can be misinterpreted, and thus lead to errors in interpretation of the geologic record. Some possible pitfalls include: failure to differentiate between terra-rossa paleosols and calcarenite protosols; failure to recognize that separate paleosols may merge laterally into composite paleosols; failure to recognize that single paleosols may bifurcate in highly weathered bedrock; and failure to recognize soil-derived material that infills karst features. The Quaternary carbonates of the Bahamas are used to illustrate these pitfalls, which may occur in carbonates of any age.