Flank Margin Caves Research Articles

Hypogenic caves of Syracuse area, Sicily (Italy): geomorphological evidence of CO2 degassing, fresh-salt water mixing, and late condensation corrosion

Many caves in Sicily have been shown to have a sulfuric acid or other hypogenic origin. We studied three caves (Palombara, Scrivilleri, Monello) near Syracuse (eastern Sicily), in an area that was strongly uplifted and faulted, creating multiple Pleistocene marine terraces. Mineralogy, stable isotopes and dating methods (paleomagnetism, U/Th) were used to characterize cave sediments, some of which were related to the initial hypogenic phase (Fe and Mn oxides, calcite spar), others were introduced by surface runoff later. Many other sediments are the result of in situ weathering, such as lime sands produced by condensation-corrosion processes on the calcarenite walls. Phosphates, kaolinite and montmorillonite are related to bat guano decay. Stable isotopes show that the speleothems derive from surface seepage with temperatures similar to the present, with no evidence of a hydrothermal origin. Other deep sources of aggressivity are also excluded. We obtained an age of 603 ka for a marine notch deposit near Palombara, as well as a possible paleomagnetic inversion (>780 ka) for clastic allogenic sediments. These ages are discussed, raising the question of the reliability of calculations extrapolated from marine terrace dating and the possibility that the caves may be older than expected. Cave morphologies clearly indicate a hypogenic phase, with aggressive ascending flows creating the typical Morphologic Suites of Rising Flow (MSRF). The bubble trails and acid notches are formed by carbonic degassing and subsequent acidification in more or less closed aerated environments at the water table. Carbon dioxide probably derived from both the bedrock and the oxidation of surface-derived organic carbon at the density boundaries of the freshwater lens. We propose a mixed Flank Margin Cave and hypogenic speleogenesis model, where dissolution was favored in areas of greater CO2 concentration, producing phreatic maze patterns recording past sea-level positions. We suggest that aggressiveness of the rising fluids could have partly originated at a shallow depth, in the mixing zone between fresh and salt water.

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

AbstractFlank 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.

Seismic expression of carbonate build-up karstification: karst modelling strategies and insights from synthetic seismic

Abstract Many ancient carbonate build-ups are impacted by meteoric diagenetic processes, including karstification. However, very little is known about the acoustic properties and seismic expression of karstified reservoirs. This paper investigates potential seismic expressions of karstified build-ups by means of synthetic seismic modelling. The overall stratigraphic architecture and physical properties of rock are derived from published examples of SE Asian carbonate build-ups. Various karst systems, corresponding to distinct stages of karstification of varying intensity, have been superimposed to the background facies-related acoustic model. Three-dimensional synthetic seismic cubes have been computed from the karst-bearing acoustic model and by testing various wavelet frequencies. The proposed approach allowed several types of palaeokarst to be reproduced, from dendritic karst and flank-margin caves to cave networks, in amplitude sections. However, geometrical seismic attributes, like coherency, cannot be accurately reproduced with modelling based on the available literature. In addition, the exploration of this synthetic seismic data shows that non-stratiform diagenetic bodies (i.e. leached carbonate wedges in a mixing zone) appear as stratiform even in very-high-resolution seismic. Thus, their detection and characterization require advanced techniques or prior knowledge, such as borehole data.

Coastal caves on the Interview Island of Andaman Islands, India

By the presence of well-developed terraces on the Interview Island of Middle Andaman, we conducted an exploratory study on the Interview Island of Andaman and Nicobar Islands to understand the development of coastal caves here. During the investigation, we identified seven coastal caves on the island. Morphometric data of these study caves were used for further quantitative analysis by following the area/perimeter (A/P) and entrance width/maximum width (E/M) ratios to classify the cave types. We recognized from the results that the coastal caves could not be easily differentiated as flank margin caves based on A/P and E/M ratios. Nevertheless, based on their morphometry, we could confirm the occurrence of sea caves on the East and West coast of the Interview Island. The present study, as part of the more extensive ongoing research to conserve a cave-dwelling bird, allowed us to report and discuss the occurrence of the coastal caves, and their importance to understanding the speleological significance of the Interview Island.

Flank Margin Caves and the Position of Mid- to Late Pleistocene Sea Level in the Bahamas

Mylroie, J.; Lace, M.; Albury, N., and Mylroie, J., 2020. Flank margin caves and the position of mid- to late Pleistocene sea level in the Bahamas. Journal of Coastal Research, 36(2), 249–260. Coconut Creek (Florida), ISSN 0749-0208.The Bahamian Archipelago has abundant fossil coral reefs and related subtidal deposits as evidence of the last interglacial (marine isotope stage [MIS] 5e) sea-level highstand; evidence of earlier highstands from the mid-Pleistocene is limited and controversial (excepting Mayaguana Island). Data from flank margin cave elevations, used as a sea-level proxy, were initially interpreted to demonstrate an MIS 5e origin derived from ∼60 mapped caves across the archipelago. A new analysis of cave morphologies and distribution has now produced 363 maps of flank margin caves, demonstrating for the first time that 26 caves with associated elevations between +8 and +24 m above modern sea level are found spanning the northwest to the southeast boundaries of the archipelago. Flank margin caves are the primary remaining evidence of past sea-level position, because almost all mid-Pleistocene subtidal deposits, and related sea-level indicators such as subtidal facies, sea caves, and bioerosion notches, have been removed by karst denudation. Cave elevations up to 24 m (above sea level) indicate that prior assumptions as to the rate of subsidence of the Bahama Banks at 1–2 m per 100 ka may not be correct. The activity of MIS 5e was recent enough, and its subtidal deposits voluminous enough, to survive to the present, along with the majority of flank margin caves formed at that time. Karst denudation may have created sufficient mass loss that isostatic subsidence stopped, or was possibly reversed, as has been demonstrated for the Florida peninsula. If true, then mid-Pleistocene flank margin caves may not represent eustatic sea-level position at the time of speleogenesis.

Light attenuation as a control for microbiogeomorphic features: Implications for coastal cave speleogenesis

San Salvador (Bahamas) is a carbonate island with dozens of flank margin caves formed in the phreatic zone by fresh seawater mixing within the freshwater lens. These caves have no direct connection with the sea, and form at or close to the tidally influenced fluctuating water table. After sea-level fall, in their subaerial parts caves are enlarged mainly by rock dissolution and by erosion close to the water level, condensation-corrosion and breakdown processes. For understanding the geomorphological features observed in these caves and how they are related to light attenuation, we investigated three sampling sites in the tidally influenced zone of Lighthouse Cave, which has been re-invaded by seawater during the Holocene sea-level highstand. A freshwater lens no longer exists within or adjacent to the cave. Rock samples were collected above and below the internal lake shores close to the entrance, and in the twilight and dark zones of this cave. Light and electron microscopy examinations were conducted for detecting microbial cells, as well as bioconstruction and bioweathering features. In addition, a high precision laser scanner was used for characterising sample microtopography. Our data showed that the microtopography and geomorphology of the lake shore samples (cave entrance) are dominated by bioweathering, whereas the samples of the twilight and dark zones are controlled by a combination of both bioweathering and bioconstructive processes depending on light availability. Bioconstructive structures, such as semi-planar lamination, at the fluctuating water level of the Lighthouse Cave show that dissolution due to water mixing of sea and freshwater in the Holocene is no longer the most important speleogenetic process. We propose that the geomorphological evolution is strongly influenced by the degree of rock diagenesis more than the initial mechanism of speleogenesis.

Diffuse Versus Conduit Flow in Coastal Karst Aquifers: The Consequences of Island Area and Perimeter Relationships

The majority of limestone islands are made of eogenetic carbonate rock, with intrinsic high porosity and permeability. The freshwater lenses of small islands are dominated by diffuse flow regimes as the island perimeter is everywhere close to the meteoric catchment of the island interior. This flow regime produces flank margin caves at the lens margin, where dissolution is enhanced by mixing corrosion, superposition of organic decay horizons and higher flow velocities as the lens thins. The lens interior develops touching-vug flow systems that result in enhanced permeability and lens thinning over time. As islands become larger, the area (meteoric catchment) goes up by the square, but the island perimeter (discharge zone) goes up linearly; diffuse flow becomes inefficient; conduit flow develops to produce traditional epigenic cave systems that discharge the freshwater lens by specific turbulent flow routes, which in turn are fed by diffuse flow in the island interior. Locally, diffuse flow to the island perimeter continues in coastal proximal areas between major conduit flow routes to produce flank margin caves. The Bahamian Archipelago represents a case history in which tectonics is limited, the rocks are entirely eogenetic and the diffuse to conduit flow transition is demonstrated.

Role of karst denudation on the accurate assessment of glacio-eustasy and tectonic uplift on carbonate coasts

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.

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

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

Rodrigues Island: carbonate deposition and karst processes as indicators of platform stability

Rodrigues Island is located in the western Indian Ocean ~600 km east of Mauritius, at 19°42′S and 63°24′E, a volcanic island with a small overlay of Quaternary eolian limestones, an area of 108 km2, elongated ENE to WSW, 18 km long and 6.5 km wide, maximum elevation of 396 m. The last volcanic activity was 1.3 ma, and only one lava tube is known. The eolian calcarenites are restricted to a narrow segmented band on the east coast, and two larger patches, Plaine Caverne and Plaine Corail, on the south coast. No subtidal carbonate facies are known, and no flank margin caves are known, indicating that platform subsidence has been in excess of 5 m/100 ka to bring such features from the MIS 5e +6 m sea-level highstand below modern sea level. The eastern eolianite outcrops contain rectangular incisions similar to the bokas found in the Netherlands Antilles, as a large volcanic rock watershed traverses a thin coastal limestone outcrop. The southern calcarenite exposures each contain large epigene cave passages developed by surface catchment from volcanic rocks. The Plaine Corail caves are abandoned by streams today, are segmented and modified by collapse and contain massive calcite speleothems, evidence of an eolianite age of perhaps 1 ma or more. Plaine Caverne contains two long, linear stream caves, mostly intact. Mauritius has flank margin caves, subtidal facies above modern sea level, and abundant lava tubes (volcanic activity as recent as 26 ka), consistent with thermal inflation now long ended on subsiding Rodrigues.

Sistema Faro, Isla de Mona, Puerto Rico: speleogenesis of the world’s largest flank margin cave

Isla de Mona, a small, uplifted carbonate plateau jutting out of the waters of the Mona Passage, is an incredibly fragile and densely karstic environment. Expedition work was conducted by the Isla de Mona Project in cooperation with the Departamento Recursos Naturales y Ambientales de Puerto Rico (DRNA), including contributions from many researchers and cavers volunteering from across the U.S and Puerto Rico in the course of 12 separate expeditions, spanning a 14-year period (1998 to 2013). Over 200 caves have been documented on the island to date, the majority of this inventory is composed of flank margin caves but also includes sea caves, pit caves and talus caves. The most extensive example of cave development on the island is Sistema Faro – a sprawling maze-like series of chambers formed within the eastern point of the island with over 40 cliffside entrances overlooking the Caribbean Sea. Detailed cartography and analysis of the geomorphology and development of the Sistema Faro has helped form a complex model of carbonate island cave development as a function of tectonic uplift, lithology, sea level changes, karst hydrogeology and cliff retreat. This communication examines the roles these controls have played in the genesis of the world’s largest flank margin cave.

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

Kuumbi Cave is one of a group of caves that underlie a flight of marine terraces in Pleistocene limestone in eastern Zanzibar (Indian Ocean). Drawing on the findings of geoarchaeological field survey and archaeological excavation, we discuss the formation and evolution of Kuumbi Cave and its wider littoral landscape. In the later part of the Quaternary (last ca. 250,000 years?), speleogenesis and terrace formation were driven by the interplay between glacioeustatic sea level change and crustal uplift at rates of ca. 0.10-0.20 mm/yr. Two units of backreef/reef limestone were deposited during 'ptimal' (highest) highstands, tentatively correlated with MIS 7 and 5; (mainly) erosive marine terraces formed in these limestones in 'suboptimal' highstands. Kuumbi and other sub-terrace caves developed as flank margin caves, in the seaward portion of freshwater lenses during such 'suboptimal' highstands. Glacioeustacy-induced fluctuations of the groundwater table may have resulted in shifts from vadose (with deposition of well-developed speleothems) to phreatic/epiphreatic conditions in these caves. At Kuumbi, Late Pleistocene (pre-20,000 cal. BP) ceiling collapse initiated colluvial deposition near-entrance and opened the cave to large plants and animals, including humans. A phase of terminal Pleistocene human occupation ca. 18,500-17,000 cal. BP resulted in the deposition of a dense assemblage of Achatina spp. landsnails, alongside marine molluscs and mammal remains (including zebra, buffalo and other taxa now extinct on Zanzibar). The Holocene part of the cave stratigraphy near-entrance records phases of abandonment and intensified late Holocene human use.

Cave development within a paleosol breccia in the dolomitized Seroe Domi Formation, Aruba, Lesser Antilles

Payoff Cave, found within dolomitized sections of the Seroe Domi Formation on Aruba, provides a set of diagenetic observations unique to island karst. Paleosol breccia found as wall and ceiling rock and as bedrock pillars within Payoff Cave was determined to be low-Mg calcite in mineralogy with clasts of dolomitized host limestone. Payoff Cave is interpreted to be a flank margin (FM) cave based on morphologic features. The relevant diagenetic history of this section of the Seroe Domi is: (1) dolomitization and initial cave formation, (2) collapse and paleosol breccia formation within the initial cave, (3) reoccupation of the mixing zone to form a new FM cave within the more soluble calcitic paleosol breccia, and (4) cliff retreat and exposure of Payoff Cave as found today. Dolomitization, karstification, and the presence of a paleosol breccia combine in a complex diagenetic history on Aruba.

Heterogeneous distributions of CO2 may be more important for dissolution and karstification in coastal eogenetic limestone than mixing dissolution

AbstractMixing dissolution, a process whereby mixtures of two waters with different chemical compositions drive undersaturation with respect to carbonate minerals, is commonly considered to form cavernous macroporosity (e.g. flank margin caves and banana holes) in eogenetic karst aquifers. On small islands, macroporosity commonly originates when focused dissolution forms globular chambers lacking entrances to the surface, suggesting that dissolution processes are decoupled from surface hydrology. Mixing dissolution has been thought to be the primary dissolution process because meteoric water would equilibrate rapidly with calcium carbonate as it infiltrates through matrix porosity and because pCO2 was assumed to be homogeneously distributed within the phreatic zone. Here, we report data from two abandoned well fields in an eogenetic karst aquifer on San Salvador Island, Bahamas, that demonstrate pCO2 in the phreatic zone is distributed heterogeneously. The pCO2 varied from less than log −2.0 to more than log −1.0 atm over distances of less than 30 m, generating dissolution in the subsurface where water flows from regions of low to high pCO2 and cementation where water flows from regions of high to low pCO2. Using simple geochemical models, we show dissolution caused by heterogeneously distributed pCO2 can dissolve 2.5 to 10 times more calcite than the maximum amount possible by mixing of freshwater and seawater. Dissolution resulting from spatial variability in pCO2 forms isolated, globular chambers lacking initial entrances to the surface, a morphology that is characteristic of flank margin caves and banana holes, both of which have entrances that form by erosion or collapse after cave formation. Our results indicate that heterogeneous pCO2, rather than mixing dissolution, may be the dominant mechanism for observed spatial distribution of dissolution, cementation and macroporosity generation in eogenetic karst aquifers and for landscape development in these settings. Copyright © 2015 John Wiley & Sons, Ltd.

Uplifted flank margin caves in telogenetic limestones in the Gulf of Orosei (Central-East Sardinia—Italy) and their palaeogeographic significance

This work reports the results of geomorphological observations carried out in the coastal Fico Cave and surrounding areas (Baunei, Central East Sardinia) in the Gulf of Orosei. A tidal notch, generally believed to be of Eemian (MIS 5e) age, is barely visible at 8.5 above present sea level (asl), some metres below the main entrance of the cave. Old cave passages, now partially opened by cliff retreat and parallel to the coastline, are clearly visible at around 14m asl and correspond to the main level of Fico Cave. Two more notches are located higher, at 22 and 50m asl. Fico Cave itself is composed of at least 6 clearly distinguished more or less horizontal levels (−10m below present sea level (bsl), and +14, +22, +40, +50, and +63m asl), independent of the stratal dip, arguing for a sea-level, and hence, fresh-water lens control. Cave passages develop along main fractures more or less parallel to the coastline and never extend landward for more than 150m, mostly ending blindly, or diminishing in their dimensions progressively landward. Most passages only contain clay deposits, lacking fluvial or marine sediments or typical fluvial erosion morphologies (i.e. scallops).It is suggested from this body of evidence that Fico Cave was formed in the coastal mixing zone along major discontinuities during several Quaternary interglacial periods, when sea level was high and relatively stable for enough time to develop large dissolutional voids. The geomorphological observations indicate the main +14m asl level of the cave to have formed during MIS 9, and was heavily reworked during MIS 5, while the higher levels are relative to older interglacial highstands that occurred between 1Ma and 325ka. The small active branch developed below present sea level has formed during MIS 7 (225ka). These observations shed new light on the position of the MIS 5e highstand markers in this area of the coast, much higher than previously thought.

The influence of tectonics on flank margin cave formation on a passive continental margin: Naracoorte, Southeastern Australia

Intensive cave development within the highly porous and permeable Eocene–Middle Miocene Gambier Limestone in southeastern South Australia is restricted to a 1×11km area at Naracoorte. The caves are overwhelmingly horizontal, consisting of large solutional domes connected by smaller passages, with bell holes, small pendants and large non-directional scallops on the walls and ceilings. Orientation is strongly controlled by NW/SE joints. Cave entrances have been opened by subsequent collapse, and breakdown is common.The caves are located on the Kanawinka Fault escarpment, which was uplifted in the Late Miocene–Early Pliocene and then overlain by a series of Pleistocene carbonate strandline dunes, deposited as the sea retreated following a Late Miocene transgression. The coastline lay along the Kanawinka escarpment at ~0.9–1.1Ma, when the caves formed just inland of the shoreline within the zone of enhanced dissolution at the seaward margin of the freshwater lens. They have the typical flank margin cave morphology, except that joint development adjacent to the fault caused the strong linear orientation parallel to the coastline.The beach dune deposited to the west of the Kanawinka escarpment shows that sea-level had dropped sufficiently to completely drain the caves at 780–880ka. Cave development was therefore confined to a period spanning ~0.8–1.1Ma. Continuing gradual uplift through the Pleistocene means that the caves are now >100km inland, obscuring the essentially coastal nature of the Naracoorte karst.Speleogenesis at Naracoorte reflects the interaction of neotectonics with coastal dissolution, and emphasizes the role that tectonism can play in Pleistocene karst development even on passive continental margins, where glacio/eustatic sea-level fluctuations are generally ascribed as the dominant role.

Lower- to Middle Pleistocene flank margin caves at Custonaci (Trapani, NW Sicily) and their relation with past sea levels

The peninsula of San Vito Lo Capo, 50 km West of Palermo (Sicily), is characterised by the presence of a wide set of evidences of past sea level changes, such as marine terraces, notches, marine and coastal caves with phreatic overgrowths on speleothems, and continental and marine deposits. The exceptional good preservation of these landforms and deposits has been used by different authors for the reconstruction of sea level changes and neotectonic movements. Among the many caves discussed by previous authors, most are of marine origin and can preserve signs of old sea level highstands such as notches and marine or continental sediments. However, two caves in particular, Fantasma Cave and Falesia Rocca Rumena I cave, show evidences to be flank margin caves. Both caves are records of rising and falling sea level, and their position and the correlation with marine terraces suggest them to be around 0.8 and 1.1–1.2 Ma old respectively. This study shows that not all sea level high stands are preserved in the stratigraphical and geomorphological record.Keywords: telogenetic limestones, coastal karst, mixing zone speleogenesis, sea level changes, Quaternary.

Paleokarstic macroporosity development at platform margins: Lessons from the Paleocene of north Spain

Platform margins have been targets for carbonate exploration because they are commonly seismically resolvable and reservoir prone for several critical reasons including karstic porosity. Platform margin karst models, mainly based on the Quaternary of the Caribbean (tropical humid, including “flank margin caves”) are well documented, but analogs remain rarely identified in the stratigraphic record. Analysis of a remarkably well-exposed macroporosity paleokarst system from the Paleocene of north Spain, formed under an arid to semiarid climate, provided a model that differs from the Caribbean template. Implications for exploration and appraisal strategies, include provisos regarding (1) how early fracture intensity can be estimated at platform margins, (2) the absence of flank margin caves at the margin, (3) the absence of karstic features at or immediately below the main lowstand surface, (4) the presence of cave-free corridors such that the porosity zones could be missed completely, and (5) the stratigraphic inheritance of caves into successive levels adjacent to the cave-free zones. Quantitative data were evaluated on the main macroporous intervals. We also compared this model with other paleokarst models in the literature to emphasize the diversity of approaches that can be used to evaluate paleokarst targets: “one size does not fit all.”

Paleoenvironmental investigations, chemical analysis and characterization of underwater strata of Marigondon Cave

Abstract The submarine Marigondon Cave (MC) is not only one of the most spectacular diving caves but also an unstudied, submerged rock-shelter with potential for intact stratigraphy. The Marigondon Cave yielded two rock samples, one coral limestone fossil with a travertine mantle, and a sclerosponge specimen that provided evidence to understand the external and environmental conditions to which the cave might have been exposed. The study's aimed to determine the links between the cave's underwater conditions and the samples studied. Analytical methodologies such as IR spectroscopy, X-ray diffraction, Optical Microscopy, SEM/EDX and U-series isotope analyses were carried out to determine the chemical composition of two rock samples, and their crystal structures and formation ages. Results concluded that the two samples were composed mainly of the two calcium carbonate polymorphs calcite and aragonite; however, the sclerosponges contained a low percentage of Mg- calcite, illustrating a period of submergence. At other periods, Marigondon Cave would have been above water and favorable for Pleistocene fauna as well as possibly human settlement, as demonstrated by the travertine deposit on the coral limestone. This suggests that MC provides prospects for discovery of fossil assemblages that may be preserved in terrestrial deposits in the caves during periods of emergence. These periodic submersions conform to eustatic sea level changes during the past 130,000 years during the late Pleistocene and demonstrate that in that period a marine coral reef platform was the substrate within which the Marigondon Cave formed as a flank margin cave during nearly 80,000 years of emergence when sea level was lower than the present elevation of the cave.

Microbial mixing zone dolomitization and karst development within Isla de Mona Dolomite, Isla de Mona, Puerto Rico

Modification of the mixing zone model to include microbial processes that promote dolomitization explains many of the petrographic, isotopic, and outcrop observations found on Isla de Mona. Large flank margin caves at the contact between the Isla de Mona Dolomite and the capping Lirio Limestone formed during prolonged, likely episodic, periods with a stable lens position. These periods would produce a stable halocline. The halocline is hypothesized to be the zone of dolomitization in the microbial mixing zone model. Collection of organics at the density interface of the halocline coupled with sulfate reduction by sulfate-reducing bacteria would establish a geochemical environment that promotes dolomitization of the precursor limestone. This microbial community and geochemical environment would overcome the kinetic and thermodynamic barriers associated with the previous mixing zone model.