Article

Origin of atoll lagoons

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Abstract

A database of 301 atolls from the Pacific, Indian, and Atlantic Oceans has been analyzed with respect to factors governing maximum atoll lagoon depth. Statistically significant correlations between maximum atoll lagoon depth and both atoll area and present-day rainfall are viewed as the combined effect of paleorainfall precipitation and catchment area in contributing to overall atoll morphology. This interpretation is supported by the gross saucer-shaped morphology of several of the Lau group of the Fiji Islands, and the subsurface Cretaceous Golden Lane atoll of Mexico, where evidence of reef rim construction is lacking but evidence for significant solution relief is compelling. The contribution of reefs to atoll rim construction appears to be limited generally to ∼ 10 m, leaving more than 20 m of relief to be explained at most atolls. At a number of these, the last interglacial highstand surface is ∼ 15-20 m beneath Holocene rim sediments. Subsidence rates of even 5 cm/ k.y. do not suffice to explain the subsea depth of this unconformity, suggesting the dominating influence of solution on relief expression. Calculations of solution rates relative to the residence time of sea level below given depths during the past 700 k.y. suggest that the observed atoll relief is in part inherited from more than one Pleistocene, or perhaps earlier, glacial stage. Whatever the precise time of origin, the data available strongly suggest that atoll morphology is solution determined rather than growth predicated.

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... Meiji Atoll is located in the tropical climatic zone, and the climate is warm and humid. Seawater surface temperatures generally vary from 26°C to 33°C, and present-day mean annual precipitation is approximately 2,500 mm (Purdy & Winterer, 2001). Mainly influenced by the East Asian seasonal monsoon, the locally generated waves are driven by a bidirectional northeast-southwest pattern (Puchała, 2014). ...
... Because the area of Meiji Atoll is not very large (i.e., approximately 45 km 2 with the lagoon region), the variations in precipitation over the entire atoll region were not included in the best-case simulation. The current average annual rainfall is approximately 2.5 m/yr in the Spratly Islands in the southern South China Sea (Purdy & Winterer, 2001). Monsoon intensity varied during Quaternary glacial/interglacial cycles, with a maximum during the time of deglacial transition when solar insolation reached its maximum. ...
... Since the average precipitation was approximately 1.85 m/yr at Meiji Atoll since 2.65 Ma, in our simulations for Meiji Atoll, the dissolution rates were on average 6.94 × 10 −5 m∕yr and 12.5 × 10 −5 m∕yr at the margin and interior areas, respectively, which were close to those (i.e., 9.35 × 10 −5 m∕yr and 12.96 × 10 −5 m∕yr for the margin and interior areas, respectively) used for Enewetak Atoll in Toomey et al. (2016) and comparable to the values of 6 × 10 −5 − 27 × 10 −5 m∕yr on Grand Cayman Island by subsoil microerosion measurements (Spencer, 1985). Purdy and Winterer (2001) estimated that the dissolution rate could be 6.5 × 10 −5 − 13 × 10 −5 m∕yr at Enjebe Island, Enewetak Atoll. When considering the average rainfall at Enewetak Atoll to be 1.47 m/yr recorded for the 1974-1979 time period (Buddemeier & Oberdorfer, 1997), both the dissolution rate and dissolution factor used for Meiji Atoll in our simulations were close to those at Enewetak Atoll. ...
Article
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Several theories have been proposed to explain atoll formation. While karst dissolution during glacial periods and preferential coral reef accretion along raised bank margins during deglaciations and interglacials have been invoked to explain atoll formation, the respective roles of karst dissolution and reef margin construction in atoll formation have not been adequately evaluated by simulations. In this study, we conducted three‐dimensional numerical simulations of the Quaternary development of Meiji Atoll in the southern South China Sea based on interpreted data from a 2020‐m‐deep borehole drilled on its northeast rim in 2018. Our results suggest that the origin of atolls is more likely due to spatially differential dissolution across margin and interior areas (i.e., minimal along margins and maximal in bank centers) rather than preferential reef accretion along margins of flat‐topped banks. Preferential reef accretion along margins of flat‐topped banks that can result in central lagoons and atoll morphology can hardly result in the formation of lagoonal patch reefs that reach mean sea level. Preferential reef accretion along margins is mainly predicated on the karst‐induced morphology that has a central depression surrounded by raised rims, that is, using antecedent karst morphologies. If topographic highs in the lagoon are similar in elevation to the margins, reef accretion on these topographic highs can be similar to that observed on the margins, resulting in lagoonal patch reefs that reach mean sea level. Our simulation shows that spatially differential dissolution across margin and interior areas is a critical driver of worldwide central lagoons and atoll formation.
... These relations reflect the influence of physical oceanographic parameters on both the biology and physical sedimentology of these systems, although geologic history and local contingencies play a role in changes through geologic time (Lowenstam, 1957;Purdy, 1974;Hubbard, 1997;Montaggioni, 2005;Ting et al., 2021); 3) A common theme on reef flats is a platformward change in bottom type and sediment grade, from in-situ growth of corals and calcareous algae in the shelf-margin reef, to calcareous detritus of gravel and coarse sand, to coarse and medium sand, across the reef sand apron; that is, sediment fines lagoonward (Macintyre et al., 1987;Montaggioni, 2005;Harris et al., 2011;Rankey et al., 2011;Wasserman and Rankey, 2014;Hamylton et al., 2016) (Fig. 1D). 4) The transition from reef sand apron to lagoon commonly is a dip slope up to the angle of repose, passing into the deeper lagoon (Wiens, 1962;Purdy and Winterer, 2001;Purdy and Gischler, 2005) (Fig. 1B-F). Where well defined, this marked change represents the abrupt transition from the bed-load dominated transport of the reef sand apron to the suspended sediment and insitu production of the platform interior lagoon, where biological activity dominates. ...
... Most of the 1-m-deep sand apron scenarios included a 250m-wide 'reef rim' of 0.5 m water depth; deeper sand aprons included no slightly raised rim. Lagoon water depth for most models was 9.5 m, a shallow lagoon (Purdy and Winterer, 2001). One scenario considered a 20 m-deep lagoon, and another modeled a 5 m-deep lagoon, to test the impact of lagoon depth. ...
... [Again, note that this forereef and any peri-reefal or slope apron is not a focus here.] This platform size roughly represents the modal size of Miocene, Pliocene, and Pleistocene atolls and platforms (Rankey, 2020) and falls well within the range of modern atolls (Purdy and Winterer, 2001). The mesh included a highresolution (maximum element area of 15,000 m 2 ) bathymetric flexible mesh; mapped results illustrated herein represent data from the mesh gridded to 2500 m 2 pixels. ...
Article
In shallow water atop many isolated platforms and atolls, reef sand aprons largely consist of debris shed platformward from shelf-margin reefs towards the lagoon. Although their general geomorphology and sedimentology is broadly understood, quantitative details of the possible role of reef sand apron hydrodynamics on their geomorphic evolution remain less well constrained. To test the hypothesis that on-platform reef sand apron progradation is prone to completely infill adjacent lagoons, this study documents over 50 new hydrodynamic simulations that isolate and evaluate the relations among geomorphology, waves, and tides. The results show how deep-water waves hit the shelf margin and favor on-platform sediment transport, but highlight that tides modulate the influence of waves while generating currents on their own. Across the range of wave heights and tidal amplitudes, however, platform-directed bed shear stress on shallow reef flats decreases with increasing reef and reef sand apron width. Parts of broad reef sand aprons can even include off-platform-directed shear stress during incoming flood tide. These insights motivate a conceptual model for how the process of widening of a reef sand apron by lagoonward sediment transport decreases the magnitude of the very forces that drive the transport, inhibiting further progradation. Such an autogenic, self-limiting dynamic curbs the propensity of lagoons to fill with coarse sediment shed from the reef by reef sand apron expansion. Instead, many atolls are doomed to remain “half-empty buckets,” even in the absence of external change such as a relative change in sea level.
... A number of submerged reefs were described in the Pacific Ocean (Australia: Webster et al., 2009;Linklater et al., 2015-French Polynesia: Montaggioni et al., 1987Cabioch et al., 2008;Camoin et al., 2012;Seard et al., 2013-Hawaii: Faichney et al., 2009 and references herein -Papua New Guinea: Webster et al., 2004b;Ryukyus: Arai et al., 2016), in the Indian Ocean (Dullo et al., 1998;Rovere et al., 2018; and references herein) and the western Atlantic (Lighty et al., 1978;James and Ginsburg, 1979;Blanchon et al., 2002;Khanna et al., 2017;and references herein). Submerged reefs have proven to provide valuable information on relative sea-level and tectonic histories (Webster et al., 2004a(Webster et al., , 2004b(Webster et al., , 2006Cabioch et al., 2008;Faichney et al., 2010;Rovere et al., 2018) and sub-aerial erosional patterns as well (Purdy and Winterer, 2001). ...
... From estimates of the amount of reef rocks destroyed over the last 120 ky, the rates of carbonate ground lowering from some Pacific atolls and reliefs were thought to range between 0.011 mm·y −1 and 0.202 mm·y −1 , with mean rates of 0.11 mm·y −1 (Paulay and McEdward, 1990). These values are similar to those calculated (0.12 mm·y −1 on average) by Purdy and Winterer (2001) from Enewetak Atoll for the same time span in response to local rainfall regime. Since present-day annual rainfall at Takapoto is around 1500 mm·y −1 , quite similar to that recorded at Enewetak (Purdy and Winterer, 2001), a mean value of 0.12 mm·y −1 is taken as the maximum solution rate during Pleistocene low sea stands in the north-western Tuamotu. ...
... These values are similar to those calculated (0.12 mm·y −1 on average) by Purdy and Winterer (2001) from Enewetak Atoll for the same time span in response to local rainfall regime. Since present-day annual rainfall at Takapoto is around 1500 mm·y −1 , quite similar to that recorded at Enewetak (Purdy and Winterer, 2001), a mean value of 0.12 mm·y −1 is taken as the maximum solution rate during Pleistocene low sea stands in the north-western Tuamotu. ...
... Located far from active tectonic Maupiti, Society Islands, has a mature barrier reef. (c) Enewetak, Marshall Islands, is an example of 'empty-bucket' atoll morphology with a deep (max ≈ 62 m; Purdy and Winterer, 2001) unfilled lagoon. Cross and circle show locations of cores KAR-1 and XEN-3, respectively. ...
... In the model, lagoon sediment derives from in situ, primary production (P) and rim-transported material. A scaling factor is applied to the rim-derived sediment based on the area ratio of lagoon (a l ) to rim (a r ) (e.g., Daly, 1910;Purdy and Winterer, 2001): ...
... Charted depths were digitized in ArcGIS (Esri, Redlands, California) in order to calculate a spatial mean depth for each atoll's lagoon. Similar to previous efforts (Purdy and Winterer, 2001), Nukuoro was excluded because its lagoon depth represents a major outlier, particularly in comparison with nearby islands (e.g., Satawan, Luknor) that have likely experienced similar environmental conditions. ...
Article
Sea-level records from atolls, potentially spanning the Cenozoic, have been largely overlooked, in part because the processes that control atoll form (reef accretion, carbonate dissolution, sediment transport, vertical motion) are complex and, for many islands, unconstrained on million-year timescales. Here we combine existing observations of atoll morphology and corelog stratigraphy from Enewetak Atoll with a numerical model to (1) constrain the relative rates of subsidence, dissolution and sedimentation that have shaped modern Pacific atolls and (2) construct a record of sea level over the past 8.5 million years. Both the stratigraphy from Enewetak Atoll (constrained by a subsidence rate of ~20 m/Myr) and our numerical modeling results suggest that low sea levels (50-125 m below present), and presumably bi-polar glaciations, occurred throughout much of the late Miocene, preceding the warmer climate of the Pliocene, when sea level was higher than present. Carbonate dissolution through the subsequent sea-level fall that accompanied the onset of large glacial cycles in the late Pliocene, along with rapid highstand constructional reef growth, likely drove development of the rimmed atoll morphology we see today.
... In general, there is a positive relationship between atoll area and maximum lagoon depth (Purdy and Winterer, 2001;, although lagoon circulation and its renewal time is complicated by features of the rim and local hydrodynamic conditions. On large, relatively open atolls such as Enewetak (area > 1000 km 2 , Marshall Islands) where the lagoon extends to a depth of more than 60 m, renewal time may require 30-120 days (Atkinson et al., 1981), whereas on 35 m deep Rangiroa (Tuamotu Islands, area > 1700 km 2 ), estimates suggest that lagoon circulation may require 155 days . ...
... However, on Nukuoro (Caroline group), where the rim encloses an area of only 37 km 2 , the lagoon reaches a depth of 99 m. Likewise on Ngatik in the Caroline group, the rim surrounds an area of 55 km 2 (with the exception of a single, narrow, 11 m deep channel), but the lagoon is 159 m deep (Purdy and Winterer, 2001). Little is known about the circulation mechanics or renewal times of these unusual systems. ...
... Nomwin and Murilo (256 and 257, respectively) and Satawan, Lukunor and Etal (261-263) were listed previously as one group, but they are separated by depths of at least 300 m and are thus listed individually here. The area enclosed by Ngatik's (267) rim is ~55 km 2 , but despite its small size the lagoon is up to 159 m deep (Purdy and Winterer, 2001) and may be the deepest of any atoll. Two atolls are found in an isolated portion of the Pacific northeast of the Caroline Islands and north of the Marshall Islands (Table A.14). ...
Article
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There is only one published list of atolls of the world (Bryan, 1953) and it is the source of the often-quoted figure that there are 425 or “more than 400” of them. However, the original compendium included many banks and other reefs without lagoons. A re-examination of Bryan’s data, along with charts, satellite photographs and updated literature suggests that the number of atolls is indeed “more than 400,” despite the deletion of more than 100 of his entries. There are 439 atolls identified in the present summary, but the list is broadly constructed, inclusive, and not limited to those known to have formed on subsiding volcanic platforms. In addition, 171 of those listed (39%) are primarily subtidal atoll reefs with little or no island development. These particular atolls comprise 96% of those from Fiji, 94% of those in the South China Sea, and 62% of those in Indonesia. With few exceptions, all of these reef systems are specifically identified and verified using Google Earth, Landsat or other satellite imagery, making this group an important and under-appreciated element of atoll geomorphology. Eliminating atoll reefs from consideration reduces the list of atolls to 268. Of these, 104 are closed and lack a direct passage connecting the lagoon and the surrounding ocean. Closed lagoons are typical of atolls in French Polynesia (53 of 78 with lagoons), even though most of them are euhaline and are open to exchange of ocean water by indirect mechanisms. By contrast, many atolls in the central Pacific, including most of those in Tuvalu, the Phoenix Islands and the Line Islands, have developed isolated lagoons containing hypersaline, brackish, and even fresh water. The location and type of atoll (atoll reef, and atolls that are open, closed, or closed with altered lagoon salinity/oxygen) are specified on maps and tables appended to this work, and a photographic record of all but two of 439 atolls has been assembled as a supplement. This list is by no means complete. There are numerous atolls or atoll-like structures that do not have a satellite record or an adequate description on charts or in the literature. This is especially true of Indonesia, Fiji and islands east of Papua New Guinea where further exploration is likely to increase the number of entries. © 2016, Smithsonian National Museum of Natural History. All rights reserved.
... Two features of the studied lagoons stand out: reticulated basins and pinnacles. These structures are discussed here in the general context of atoll lagoon formation theories, which are considered to be dissolution-driven during Pleistocene or earlier low sea stands, with karstification explaining the current lagoon relief (Purdy and Winterer, 2001;Purdy and Gischler, 2005). From a geology standpoint, atoll lagoons have been less studied than atoll forereefs and rims. ...
... Formation of such karst requires an annual rainfall higher than 1500 mm (Sweeting, 1972) and a time lapse estimated to a several Ma by Fleurant et al. (2008). This timing is consistent with the order of magnitude of denudation rates ranging from 35 to 130 m/Ma found by Purdy and Winterer (2001) from observations and dissolution kinetic considerations. Tower karst consists of isolated limestone towers rising from alluvian plains (Huang et al., 2013). ...
... Despite several outliers, data from several well studied atolls confirmed with topographic evidences the role of dissolution during low sea-level stands to form the modern morphology of atoll lagoons. The topographic evidences for dissolution came from the absence of a thick, or significant, lagoonal Pleistocene sediment deposit that should have been produced during the -125ky sea-level high stands in several atolls for which drilling data are available (Purdy and Winterer, 2001). The narrow deposits can be explained only by dissolution during periods of emergence in the glacial periods. ...
Article
The lagoons of seven French Polynesia and Cook Islands pearl farming atolls (Raroia, Takume, Mopelia, Takapoto, Ahe, Takaroa and Manihiki) were surveyed using multibeam and mono-beam sounders. From the detailed bathymetry, morphometric variables (average and maximum depth, frequency-area of depth, lagoon area and volume) are computed and compared. Remarkable geomorphological structures highlighted by bathymetric variations include deep reticulated structures and pinnacles. The seven atolls appear very different in abundance, size and density of these entities. Considering them as markers of the geological, sedimentological and eustatic processes that shape atoll lagoons, they are discussed in the context of the general theory of atoll lagoon formations involving karstic dissolution during Pleistocene or earlier low sea-level stands. In terms of pearl farming management, accurate bathymetric maps help pearl oyster wild stock assessment, development of circulation and biogeochemical models, better lagoon zoning and strategy to remove pearl farming derelict gears.
... However, glacio-eustatic sea level fluctuations were an unknown concept in Darwin's time, although subsequent research has shown their importance on the modern geomorphology of atolls. A general correlation between atoll area and maximum atoll lagoon depth has been recognized for decades (e.g., Emery et al. 1954;Purdy 1974;Purdy and Winterer 2001;Umbgrove 1947). A positive correlation has also been reported for annual rainfall and the maximum depth of atoll lagoons (Purdy 1974;Purdy and Bertram 1993;Purdy and Winterer 2001). ...
... A general correlation between atoll area and maximum atoll lagoon depth has been recognized for decades (e.g., Emery et al. 1954;Purdy 1974;Purdy and Winterer 2001;Umbgrove 1947). A positive correlation has also been reported for annual rainfall and the maximum depth of atoll lagoons (Purdy 1974;Purdy and Bertram 1993;Purdy and Winterer 2001). Assuming that modern rainfall is a reasonable proxy for precipitation during glacial low stands, the correlations suggest that maximum lagoon depth is primarily a function of preferential subaerial dissolution of the surface of the central portion of an atoll. ...
... Assuming that modern rainfall is a reasonable proxy for precipitation during glacial low stands, the correlations suggest that maximum lagoon depth is primarily a function of preferential subaerial dissolution of the surface of the central portion of an atoll. However, Purdy and Winterer (2001) point out that subsequent interglacial sea levels high enough to flood atoll surfaces but not so high as to drown them would provide accommodation space for new reef growth. Atoll rims, elevated above the sediment-filled basins would preferentially accrete new reef growth. ...
Book
Citation: Rooney J, Wessel P, Hoeke R, Weiss J, Baker J, Parrish F, Fletcher CH, Chojnacki J, Garcia M, Brainard R, Vroom P (2008) Geology and geomorphology of coral reefs in the northwestern Hawaiian Islands. In: Riegl BM, Dodge RE (eds) Coral Reefs of the USA. Coral Reefs of the World, Vol 1, Springer, pp. 515-567
... Instead, various authors have suspected that there are additional, if not completely alternative explanations for the shape of atolls and lagoons. Several investigators have postulated that exposure to rainfall during glacially induced low stands of sea level is a means of wearing away limestone [16]. Rainfall will etch and dissolve carbonate through its relative acidity (compared with that of sea water), and exposure to it for tens of thousands of years at a time may be consequential for the stability of the limestone. ...
... In general, there is a positive correlation between atoll size and lagoon depth, but there are exceptions such as Nukuoro in the Caroline Islands. This small atoll is about 6 km across, but its lagoon is nearly a bottomless pit, more than 100 m deep [16]. Nukuoro should not be confused with Nukunonu in the Tokelau Islands, even though it too is a small atoll. ...
... The tops, now exposed to the atmosphere are likely to accumulate rainfall, especially near the center, and begin to dissolve limestone there. In fact, there is a good relationship between latitude and rainfall on one hand, and the area and maximum lagoon depth on the other, the average of which is a little over 30 m [16]. Lagoon depths may be better accounted for as solution features than by coral growth, even though the latter classical view is often taken as an article of faith for which there is little direct evidence. ...
Chapter
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The Pacific accounts for about 46% of the Earth’s water surface and about one-third of its total surface area, making it larger than all of Earth’s land area combined. The tropical portions are dotted with thousands of islands that despite their geographic position differ considerably climatically. A west-to-east decrease in rainfall is particularly notable during a typical year, amidst seasonal shifts in the trade wind-driven Intertropical Convergence Zones. Additional changes are superimposed by the El Niño Southern Oscillation, which has the effect of moving the rainfall and warm water from west to east, periodically imposing disruptive floods in relatively dry climates, and droughts in areas that are normally wet. Likewise, occasional but destructive cyclonic storm tracks are common north and south of the equator. Islands in the tropical northwestern Pacific are particularly vulnerable to typhoons. High volcanic islands exhibit a distinct climate compared with low coralline islands and this is reflected in the vegetation found on each. Coral reefs are typical of most tropical islands in the Pacific, but they differ in form that includes fringing, barrier and atoll reefs. A brief description of geological processes that control reef formation and reef islands is also given.
... The karstification of Pleistocene reef limestone has been identified as a controlling factor on variations in antecedent topography, which in turn is thought to influence the morphology of modern reefs (Purdy and Winterer, 2001). Rates of karstification are a function of exposure time, rainfall, porosity and original topography of exposed carbonate reefs. ...
... Applied to a real case, pyReef-Core can be used to test several scenarios with different rates of subsidence and karstification in order to explain for example the discrepancy in age-elevation data of LIG deposits observed in the GBR (Marshall and Davies, 1982;Dechnik et al., 2017). It can also be used to estimate the contribution of karst dissolution and subsidence (Hopley et al., 2007;Purdy and Winterer, 2001) with a more quantitive approach. By 13 ka, sea level re-floods the LIG reef, and Holocene reef growth initiates ∼ 10.5 ka in the experiment (Fig. 7). ...
Article
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Assemblages of corals characterise specific reef biozones and the environmental conditions that change spatially across a reef and with depth. Drill cores through fossil reefs record the time and depth distribution of assemblages, which captures a partial history of the vertical growth response of reefs to changing palaeoenvironmental conditions. The effects of environmental factors on reef growth are well understood on ecological timescales but are poorly constrained at centennial to geological timescales. pyReef-Core is a stratigraphic forward model designed to solve the problem of unobservable environmental processes controlling vertical reef development by simulating the physical, biological and sedimentological processes that determine vertical assemblage changes in drill cores. It models the stratigraphic development of coral reefs at centennial to millennial timescales under environmental forcing conditions including accommodation (relative sea-level upward growth), oceanic variability (flow speed, nutrients, pH and temperature), sediment input and tectonics. It also simulates competitive coral assemblage interactions using the generalised Lotka–Volterra system of equations (GLVEs) and can be used to infer the influence of environmental conditions on the zonation and vertical accretion and stratigraphic succession of coral assemblages over decadal timescales and greater. The tool can quantitatively test carbonate platform development under the influence of ecological and environmental processes and efficiently interpret vertical growth and karstification patterns observed in drill cores. We provide two realistic case studies illustrating the basic capabilities of the model and use it to reconstruct (1) the Holocene history (from 8500 years to present) of coral community responses to environmental changes and (2) the evolution of an idealised coral reef core since the last interglacial (from 140 000 years to present) under the influence of sea-level change, subsidence and karstification. We find that the model reproduces the details of the formation of existing coral reef stratigraphic sequences both in terms of assemblages succession, accretion rates and depositional thicknesses. It can be applied to estimate the impact of changing environmental conditions on growth rates and patterns under many different settings and initial conditions.
... Evolu&on of community popula&ons with depth Karstification of Pleistocene reef limestone has been identified as a controlling factor on variations of antecedent topography, which in turn is thought to influence the morphology of modern reefs (Purdy and Winterer, 2001). Rates of karstification are a function of exposure time, rainfall, porosity and original topography of exposed carbonate reefs. ...
... Applied to a real case, pyReef-Core can be used to test several scenarios with different rates of subsidence and karstification in order to explain for example the discrepancy in age/elevation data of LIG deposits observed in the GBR (Marshall and Davies, 1982;Dechnik et al., 2017). It can also be used to estimate the contribution of karst dissolution 5 and subsidence (Hopley et al., 2007;Purdy and Winterer, 2001) with a more quantitive approach. ...
Article
Full-text available
Assemblages of corals characterise specific reef biozones and the environmental conditions that change laterally across a reef and with depth. Drill cores through fossil reefs record the time- and depth-distribution of assemblages, which captures a partial history of the vertical growth response of reefs to changing palaeoenvironmental conditions. The effects of environmental factors on reef growth are well understood on ecological time-scales but are poorly constrained at centennial to millennial timescales. pyReef-Core is a stratigraphic forward model designed to solve the inverse problem of unobservable environmental processes controlling vertical reef development by simulating the physical, biological and sedimentological processes that determine vertical assemblage changes in drill cores. It models the stratigraphic development of coral reefs at centennial to millennial timescales under environmental forcing conditions including accommodation (relative sea level upward growth), oceanic variability (flow speed, nutrients, pH and temperature), sediment input and tectonics. It also simulates competitive coral assemblage interactions using the generalised Lotka-Volterra system of equations (GLVEs) and can be used to infer the influence of environmental conditions on the zonation and vertical accretion and stratigraphic succession of coral assemblages over decadal timescales and greater. The tool can quantitatively test carbonate platform development under the influence of ecological and environmental processes, and efficiently interpret vertical growth and karstification patterns observed in drill cores. We provide two realistic case studies illustrating the basic capabilities of the model and use it to reconstruct (1) the Holocene history (from 8500 years to present) of coral community responses to environmental changes, and (2) the evolution of an idealised coral-reef core since the Last Interglacial (from 140 000 years to present) under the influence of sea-level change, subsidence and karstification. We find that the model reproduces the details of the formation of existing coral-reef stratigraphic sequences both in terms of assemblages succession, accretion rates and depositional thicknesses. It can be applied to estimate the impact of changing environmental conditions on growth rates and patterns under many different settings and initial conditions.
... At a larger scale, these platforms are smaller than those evident in other systems. For example, the comparison of area of these Miocene platforms with mid to late Miocene platforms of Central Luconia, Malaysia [35], and modern atolls [36] reveals that these Northwest Shelf examples are smaller (Figure 17(i)). These Northwest Shelf platforms have a modal area of 0.08 km 2 ; Central Luconia Miocene platforms have a mode of 8 km 2 , and modern atolls have a modal extent of 316 km 2 . ...
... Note the general power-law correlation, broadly comparable to that of populations of patch reefs (e.g., [27]). (i) Size-frequency histograms for these middle to late Miocene platforms, middle to late Miocene platforms of Central Luconia (Malaysia) [35], and modern atolls [36]. Note the distinct populations; see text for discussion. ...
Article
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The Miocene represents an interval of marked global change, and this evolution is reflected in carbonate platforms from this epoch. Seismic stratigraphic characterization of high-resolution (ca 60 Hz) 3D seismic data from the Browse Basin, offshore Australia, reveals a middle to upper Miocene three-part seismic stratigraphic subdivision. Each unit consists of several seismic sequence sets and their component sequences. Seismic stratal geometries and seismic facies define a prograding shelf (Langhian and older), a barrier-reef complex with scattered platforms (upper Langhian–early Tortonian), and aggrading and prograding isolated platforms (early Tortonian–Messinian). The data permit description and interpretation of high-fidelity stratigraphic details of the initiation, expansion, termination, and geomorphology of over 100 platforms in this interval. The results reveal that the isolated platforms initiated following the Middle Miocene Climatic Optimum. The succession includes major seismic stratigraphic boundaries and overall patterns of platform growth and demise that correspond roughly with periods of pronounced eustatic change associated with initiation of eastern Antarctic ice sheets. Although invoking a eustatic control for coarse trends may be tempting, mismatch between the numbers and ages of sequences, as well as the variable stacking patterns among contemporaneous platforms regionally, precludes such an interpretation; conversely, some globally recognized eustatic changes do not have a pronounced manifestation in this area. Thus, it appears that the eustatic signal combined with dynamic physical regional processes such as waves, currents, and variable subsidence creates the complex architecture and geomorphology of platforms. These results illustrate how global changes can interact with local controls to create diverse patterns of birth, growth, and demise of carbonate platforms and drive local stratal heterogeneity.
... The karst hypothesis (Purdy, 1970), which posits erosion as the principal force during reef exposure in air, e.g., during the last glacial maximum; when the eustatic sea level dropped to 120 m below the current sea level (Woodroffe and Horton, 2005), may explain the mechanisms underlying patch reef formation in lagoons. The geomorphology of a lagoon and the patch reefs therein may be affected by the variable rates of erosion caused by freshwater (Purdy and Winterer, 2001). On the other hand, when the sea level rose during the Late Pleistocene, more vertical and horizontal spaces in the water column were available for reef development. ...
Article
Atolls are annular midocean reefs with various topographical features. The reef rim of an atoll encloses a central lagoon and lagoonal patch reefs therein. Two hypotheses explaining the origins of patch reefs in atoll lagoons have been developed: the karst hypothesis and the growth hypothesis, which emphasize the role of uneven erosion and uneven timing of reef accretion (especially horizontal expansion), respectively, in the formation and growth of patch reefs. In this study, we investigated trends in the locations, depths, ages, sizes, and other features of flat-top and pinnacle-top reefs in the Dongsha Atoll in the South China Sea by analyzing bathymetric LiDAR images and performing U-Th dating on samples we collected from various reefs in the atoll. Three of our main findings were consistent with the growth hypothesis: (1) the differences in ages of the centers of different flat-top patch reefs reflected the different times at which each patch reef reached the surface and started to expand horizontally; (2) the top surfaces of individual flat-top patch reefs had old centers and young peripheries, reflecting horizontal growth; and (3) all the pinnacle-top reefs we examined had more live corals on their tips than on the surrounding bases. We observed no positive correlation between the ages of the centers and the diameters of flat-top patch reefs; that is, larger reefs did not necessarily have older centers, indicating that various complex factors affect the expansion and age patterns of patch reefs. Overall, the findings of this study provide support for the growth hypothesis and for the use of flat-top patch reefs as sources of easily accessible long-term coral skeletal records. They may therefore serve as a reference for future studies on asymmetric reef expansion and other relevant topics.
... Despite being generally defined as mid-ocean annular reefs enclosing a lagoon (Barry et al., 2007), atolls have highly variable sizes and shapes (Shimazaki et al., 2006;Stoddart, 1965), with sizes ranging from > 10,000 km 2 to < 1 km 2 (Costa et al., 2017a). In general, large atolls exhibit very different morphological characteristics than small atolls, and the former are normally composed by deeper lagoons (Wiens, 1962) that can comprise numerous small reef platforms (or table reefs) (Purdy and Winterer, 2001;Woodroffe and Biribo, 2011). Rocas belongs to the small-sized atoll group, with a shallow lagoon and almost continuous reef rim interrupted only by two reef passages. ...
Article
The location and stability of low-lying carbonated reef islands are closely related to wave refraction over reef platforms, which create low energy wave convergence zones favorable for sediment deposition. Although there is great concern about the stability of reef islands in future decades, few studies have attempted to assess the effects of sea-level rise on wave refraction patterns and the migration of wave convergence zones, which may promote changes in island positions. To investigate the mechanisms of wave refraction over a shallow lagoon atoll (Rocas Atoll), we performed a detailed topo-bathymetric survey to simulate wave propagation for different water levels and wave conditions considering the complex atoll morphology. Our results show that the locations of convergence zones are not only influenced by wave direction and wave interactions with the elliptical reef shape but also controlled by topographic variations in the reef structure. In particular, the presence of a wide reef passage on the leeward margin of Rocas Atoll has an important role in the atoll wave refraction pattern. Model simulations show a displacement of the wave convergence zone and increase in wave energy under increased sea level. However, the direction of this displacement is more sensitive to the incident wave period than to the wave direction due to topographic control. Swell waves, either from the north or south, tend to move the convergence zone lagoonwards, whereas wind waves tend to move this zone seawards. Thus, the results suggest that, under sea-level rise scenarios, areas prone to sediment accumulation will become less stable. The relative frequency between swell and wind wave incidence will be an important driver of morphological change patterns in reef islands.
... There are about 400 atolls scattered throughout the world's oceans, a majority of which are found in the Pacific and Indian oceans, having developed in warmer waters that foster coral reef growth (Bryan 1953). The central lagoon on most atolls ranges from 20 to 100 m in depth, with its surface area typically several times larger than that of the surrounding islands (Purdy and Winterer 2001). Despite their size, atoll islands worldwide are home to around 700,000 people (Yamano et al. 2007). ...
Article
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The groundwater systems of small coral islands are highly dynamic, fluctuating in response to variable rainfall patterns, sea-level rise, and human management of water resources. Using a three-dimensional modeling approach, this study investigates the behavior of the freshwater lens in relation to major geohydrologic factors, including annual recharge rate, hydraulic conductivity (K) of the Holocene aquifer, depth to the contact between the Holocene and Pleistocene-age aquifers, and annual rate of coconut palm transpiration. Multiple islands within the Federated States of Micronesia are assessed, and therefore, the relations to island width, island surface area, and island orientation on the atoll (leeward versus windward) also are investigated. Results indicate that Holocene K and the rate of annual recharge are most influential on lens behavior. In general, a hydraulic conductivity of 25 m/day (0.03 cm/s) and between 100 and 200 m/day (0.12-0.23 cm/s) is found for leeward and windward islands, respectively. Results can be used to guide freshwater resource management by estimating extractable volume of a given atoll island aquifer if island surface area, atoll orientation, and rainfall are known.
... One notable feature since the ideas of Charles Darwin onwards is that the majority of theories proposed over the last two centuries to account for atoll formation (e.g. Darwin, 1839Darwin, , 1842Daly, 1910;MacNeil, 1954;Purdy and Winterer, 2001;Montaggioni et al., 2015) have tended to overlook the role of submarine landslides on atoll morphology. Early theories can of course be forgiven considering the then lack of bathymetric data that until recent decades were not available to reveal evidence (or otherwise) for submarine landslides on atoll flanks. ...
Article
Toomey et al. (2016) present a credible new model to explain variations in the formation of Pacific Ocean atolls through the Late Cenozoic, thus accounting for contemporary differences in atoll morphology. While we do not contradict the primary influences of dissolution and sedimentation processes, we suggest that the influence of submarine landsliding should not be ignored. Multiple slope failures exist on the flanks of many atolls. When large submarine landslides affected a significant portion of a palaeo-atoll rim, this had the potential to open up an atoll lagoon allowing much freer circulation between the lagoon and the ocean. Our atoll flank collapse model therefore provides an additional mechanism for an enclosed atoll lagoon to become a ‘leaking bucket’ system and an atoll more likely to evolve into the ‘empty bucket’ form with later sea-level rise. Submarine landsliding adds additional support for the new model of atoll formation proposed by Toomey et al.
... Deep coring through reefs subsequently took place on Kita-Daito-Jima Atoll (Ota, 1938, in Ladd et al., 1970, on the Great Barrier Reef of Australia (Richards and Hill, 1942), in the Marshall islands (Emery et al., 1954;Ladd and Schlanger, 1960), on Midway Atoll (Ladd et al., 1970), in New Caledonia (Avias and Coudray, 1967) and on Mururoa Atoll (Deneufbourg, 1969;Repellin, 1975). These works broadly supported Darwin's subsidence theory but also restated Daly's theory of glacial control on reef morphology in terms of karstic processes Szabo et al., 1985;Buigues et al., 1992;Purdy and Winterer, 2001). However, as emphasized by Steers and Stoddart (1977), while deep coring increased knowledge of the Tertiary history of a number of reefs, only rarely did it provide a thorough understanding of the geology and structure of Postglacial reef sections. ...
... One of the most important difficulties when studying karst forms in coastal environments lies in the fluctuating location of their base level, represented by the sea level. During the Quaternary, 70 % of the time the sea level was between −30 and −120 m with respect to the present sea level (Purdy & Winter 2001), which means that many forms presently submerged in the inner continental shelf were originally generated under subaerial conditions (Surić 2002;Van Hengstum et al. 2011). In karst environments typical submerged forms are marine caves, quite common in Mediterranean coasts, or valleys whose bottoms lie below the present sea level (Gams 2005). ...
Article
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The Vendicari coastal lake system (SE Sicily) presents several geomorphic characteristics, which suggest it was originated as a part of a 6 km long karst polje during the Late Pleistocene sea level lowstand. Exhumed cryptokarst karren and terraced concentric surfaces point to this working hypothesis. The generation of this depression could have been favoured by the low to moderate neotectonic activity in the zone, which consisted in slight uplifting and subsequent fracturing. Open joints in the Vendicari Pleistocene carbonates show a radial outline with the prevalence of NNW-SSE discontinuities. Once formed, polje evolution would have consisted in a progressive compartmentalisation and splitting into several polje bottoms, some of which form a part of the present Vendicari lake system and are surrounded by stepped corrosion surfaces. The postglacial sea level rise had drowned most part of the original polje, which can be still recognized in the inner continental shelf. Sea level stabilization after the Holocene eustatic maximum favoured the development of a beach barrier, which generated additional coastal lakes of lagoonal type.
... A modern coral reef, ~100 m wide, fringes Niue at sea level. Unlike other high carbonate islands with saucer shapes linked to antecedent karst (Purdy and Winterer, 2001), the one at Niue is likely caused by the preservation of its former coral atoll legacy (Schofield, 1959). ...
Chapter
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The high carbonate island of Niue has attracted the attention of scientists since the early European exploration of the South Pacific, but its extensive karst and numerous caves have so far received little attention. Our investigation recognizes two main types of caves on Niue: (1) steeply inclined, terraced, flank-margin caves that formed at the seaward edge of a migrating freshwater lens and that breach the vertical cliffs on the leeward side of the island, and (2) flat-roofed, water-table caves consisting of interconnected passages that developed at the paleo-water table and contain dissolution features caused by alternating vadose and phreatic conditions. The Niuean caves contain a large variety of both active and fossil speleothems, the forms and styles of which are controlled by the hydrodynamics of vadose and phreatic waters, the physical properties of the carbonate cap, and the tectono-eustatic history of the island. The stalagmites are composed exclusively of calcite and show pronounced laminations consisting of alternating light and dark couplets, likely representing austral summer and winter growth, respectively. The mean growth rate of stalagmites from coastal flank-margin caves (̃0.34 mm/yr) is faster by about a factor of 1.5 relative to stalagmites from inland water-table caves. In general, vadose and phreatic waters fall on the regional σ18O-σD meteoric water line, and their dissolved carbon is primarily derived from soil CO2. A material balance suggests that cave deposits act as sinks for Mg, Sr, Na, and CO2 during the recharge of the aquifer. The Niuean caves and their speleothems offer exceptional opportunities for investigating problems bearing on the regional tectono-eustatic history and paleoseismicity of the neighboring Tonga Trench, and answer questions concerning the prehistoric rhythms and irregularities of El Niño-Southern Oscillation.
... We can see these effects in the Geser-Gorom corridor. Darwin was unaware of Quaternary changes in sea level leaving coral exposed to chemical weathering (Purdy and Winterer 2001); neither could he appreciate pollution effects and ocean warming. We now know atoll formation to be a fluctuating rather than continual process. ...
Article
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Using field data for the 1980s and historic material, I show how the central places of networks crucial for regional and long-distance trades in the Moluccas between 1600 and the present were often environmentally vulnerable volcanic islands and low-lying reefs. After reviewing existing data on hazards, and evaluating the evidence for erosion and degradation, I suggest how resilience has been historically achieved through social and material exchanges between islands, accommodating the consequences of specific perturbations. Re-interpretation of published data shows how inter-island trade has re-organised patterns of biological interaction spatially and over the long term, helping us assess whether, in the face of climate-change effects, such areas are zones of robustness or of potential fragility.
... L'îlot M'Ba est géologiquement représentatif des nombreuses micro-îles du lagon et plus généralement des petites îles récifales de la région Indo-Pacifique. Il est constitué de sédiments carbonatés d'âge Holocène produits du démantèlement du socle récifal construit d'âge Pléistocène, sur lequel ils reposent en discordance [Buddemeier, 1981, à la modélisation des écoulements densitaires dans les aquifères côtiers Thèse de Doctorat -Université d'Avignon / Hydriad Wheatcraft & Buddemeier, 1981, Marshall & Jacobson, 1985, Ayers & Vacher, 1986, Woodroffe et a1., 1990, Castellaro, 1993, Purdy & Winterer, 2001. Les sédiments Holocène se caractérisent par des sables coralliens à grain fin à moyen contenant des débris de coraux et des niveaux discontinus consolidés (beach rocks). ...
Article
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The density equilibrium between freshwater and saltwater in coastal aquifers is an unsteady phenomenon difficult to characterize. On the basis of borehole observation data only, the validation of 2D/3D groundwater models remains precarious. Therefore, electrical resistivity tomography (ERT) constitutes a relevant geophysical technique of investigation for the characterization with high resolution of the 2D/3D salt distribution into the aquifer. A cross-validation methodology between geo-electrical and variable-density flow models is developed. First, the interpretation of ERT field data by inverse modeling provides relevant information for both the structuration and parameterization of groundwater models (reservoir geometry, flow velocities, etc.). Second, a qualitative validation is obtained by comparison between salinities interpreted from the geo-electrical inverse model and those simulated by the groundwater model. Third, a quantitative validation is obtained by the comparison between the calculated geo-electrical response of salinities simulated by the groundwater model (previously resistivity-transformed using a hydro-petrophysical model) and ERT field data. The reliability of the cross-validation method has been tested through sensitivity analyses carried out on the different models used (geo-electrical, hydrogeological and hydro-petrophysical), as well as its applicability has been tried on three different hydrogeological and climatic contexts. Thus, in the Magdalen Islands (Canada), seawater upconings under water wells have been characterized. On M'Ba islet (New Caledonia), spatial variations of groundwater recharge that control freshwater lens development have been evaluated. Finally, on Pikine peninsula (Senegal), intense evapotranspiration and concentration that affect low-lying inter-dunar sectors have been quantified.
... One notable feature since the ideas of Charles Darwin onwards is that the majority of theories proposed over the last two centuries to account for atoll formation (e.g. Darwin, 1839Darwin, , 1842Daly, 1910;MacNeil, 1954;Purdy and Winterer, 2001;Montaggioni et al., 2015) have tended to overlook the role of submarine landslides on atoll morphology. Early theories can of course be forgiven considering the then lack of bathymetric data that until recent decades were not available to reveal evidence (or otherwise) for submarine landslides on atoll flanks. ...
Conference Paper
It is often forgotten that Charles Darwin was an evolutionary geomorphologist as well as an evolutionary biologist. One of the most significant theories he introduced to geomorphology was his “Subsidence Theory of Atoll Formation” (STAF), conceived in 1836 during the Pacific voyage of HMS Beagle. In essence, STAF proposes that atolls develop from fringing and barrier reefs on sinking volcanic foundations as corals grow upwards to keep pace with rising sea level. Today the basic tenets of this paradigm remain widely accepted as they are largely compatible with modern ideas concerning plate tectonics. Yet, one atoll feature unmentioned by Darwin was the major arcuate ‘bight-like’ structures (ABLS) that are seen in the plan form of many atolls. These ABLS are important as they are the surficial expression of large submarine failures common on the slopes of volcanic edifices that support atoll reefs. ABLS therefore reveal the instability of volcanic foundations and provide a fundamental link with tsunami research in the context of oceanic islands. This is because large submarine landslides are known to be a source of localized tsunamis, although submarine failures are often ignored in tsunami hazard assessments that mostly focus on earthquake tsunami genesis. These ideas have recently been published as a modification to Darwin’s theory (Figure 1). This talk extends these ideas through preliminary empirical evidence in the South China Sea (SCS) (Figure 2) on the potential for atoll flank collapse. The underlying hypothesis is that any large atoll ABLS features may represent both past submarine failures and indicate possible tsunami risk (in the absence of an active fault margin to generate submarine earthquakes). The method uses a morphometric approach. Viable satellite imagery of selected atolls in the SCS is examined, followed by measurement of major ABLS shapes, sizes and geographical locations. Findings allow a reappraisal of societal vulnerability for populations inhabiting low-lying coasts bordering the SCS from a new perspective that goes beyond simple associations between submarine earthquakes and tsunami generation.
... However, the Carboniferous to Permian volcanic rocks are intercalated within the limestone suggesting that volcanism was episodically active throughout deposition of the carbonate platform (e.g., Monger, 1977). The Cache Creek carbonate platform is also anomalously large for atolls and carbonate banks in comparison to modern systems (Purdy and Winterer, 2001). The nature of the basement to the Horsefeed Formation, which is critical to assessing how exotic this platform is to adjacent terranes, has yet to be ascertained. ...
Technical Report
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The Geo-mapping for Energy and Minerals (GEM) program is laying the foundation for sustainable economic development in the North. The Program provides modern public geoscience that will set the stage for long-term decision making related to responsible land-use and resource development. Geoscience knowledge produced by GEM supports evidence-based exploration for new energy and mineral resources and enables northern communities to make informed decisions about their land, economy and society. Building upon the success of its first five-years, GEM has been renewed until 2020 to continue producing new, publically available, regional-scale geoscience knowledge in Canada’s North.During the 2018 field season, research scientists from the GEM program successfully carried out 18 research activities, 16 of which will produce an activity report and 14 of which included fieldwork. Activities applied a variety of geological, geochemical, and geophysical methods. These activities have been undertaken in collaboration with provincial and territorial governments, Northerners and their institutions, academia and the private sector. GEM will continue to work with these key partners as the program advances.
... As an alternative to the subsidence and planation theories, several authors, starting in the 1930s, proposed the antecedent karst model. This more realistic model, based on the thennew understanding of late Quaternary sea-level uctuations, was initially developed by Kuenen (1933Kuenen ( , 1954, improved by Hoffmeister & Ladd (1945, 1944, clearly and comprehensively explained (and nicely illustrated) by MacNeil (1954) (left side of Figure 3), and later improved once more by Purdy (1974) (right side of Figure 3), Purdy & Winterer (2001, 2006, Purdy & Gischler (2005), and Schlager & Purkis (2013). The antecedent karst model takes into account, in addition to subsidence, the repeated late Quaternary high-amplitude (>100-m) sea-level uctuations and relates the similarities of karst topography, formed during glacial periods of exposure when sea level had signi cantly fallen, to the development of the observed modern atoll morphology of a reef rim enclosing a central lagoon (Figure 3). ...
... There is a possible "edge effect of dissolution" where dissolution is more intensive in the center than at the edge. This is because the lower flow velocity and longer dissolution time in a homogeneous model led to an elevated margin and a lower interior of the platform (Purdy and Winterer, 2001). The differences in sedimentary facies almost exist and the stratum of the Z 2 dn 4 is actually heterogeneous. ...
Article
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The evaluation of reservoirs is of great importance in exploration practices, and reservoir distribution is dependent on multiple geological factors. The carbonate platform of the fourth member (Z2dn4) of the Dengying Formation (Z2dn) in the Gaoshiti-Moxi area (GMA) was uplifted above sea level during episode II of the Tongwan movement and underwent prolonged karstification by meteoric freshwater in the terminal Sinian, leading to a strong heterogeneity in reservoirs. Paleogeomorphology reflects hydraulic gradients that can affect the intensities of surface karstification and may be approximated by terrain slope gradients. Karst products (especially solution vugs and caverns) exist prevailingly in the Z2dn4; their development mirrors the extents of internal dissolution. Based on paleogeomorphological reconstruction using the elevation method, the slope angles of single boreholes were calculated through the digital elevation model (DEM). In accordance with single-borewell Fullbore Formation MicroImager (FMI) logging data, four electrofacies (host rock, vug, fracture, and cave) were identified in boreholes possessing FMI data. The identified electrofacies were matched with corresponding conventional logging curves to obtain the training data. A model generalized for other boreholes without FMI data was established with the random forest algorithm and the thicknesses of the four electrofacies in each borehole were determined to characterize the extent of karstification. Slope gradients and electrofacies thicknesses, together with multiple physical parameters of reservoirs in boreholes, were utilized for factor analysis. Four factors were obtained and fused into one comprehensive score based on their ratios of variance proportions. The favorability of reservoirs was described by the comprehensive scores of each borehole. These analyses showed that the most advantageous reservoirs are distributed in the vicinity of the platform margin in the west of the GMA, where they enjoy several benefits. Deposition of porous mound-shoal complexes occurs predominantly in the platform margin where large-scale caves are more developed in the mixing karst zone of seawater and freshwater. Meanwhile, the transmeridional lateral migration of hydrocarbons from the regional hydrocarbon-generating center abutting the west of the GMA contributes to earlier entrapment and accumulation in the mound-shoal complexes in the platform margin.
... Tectonic mechanism control the development of atoll, especially extensional force, was reported by Montaggioni et al. (2019) for Takapoto Atoll. Atoll development is also affected by surface processes such as surface lowering by karst erosion as part of the atoll lagoon dissolution process (Purdy and Winterer 2001). Another publication presents the surface process controlling the morphology of atoll such as submarine and volcanic flank landslides (Terry and Goff 2012), also summarized by Montaggioni et al. (2019). ...
Article
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This study aims at reconstructing the mechanism explaining the formation of the V-shaped open atoll of Maratua Island. The reconstruction was conducted by examining the landforms, lithology, and geological structure of the region. We first examined the island through ALOS, SRTM image and using an oblique aerial photograph taken using a drone. Then, a field survey was conducted to identify the detailed morphological features, lithology, and structure of the area. The results reveal that the island has developed into seven different landforms, namely, karst ridge, undulated karst hills, structural valley, marine terrace, beach, fringing reef, and sand cays. The sinistral fault system is the major factor that controlled the Maratua Island’s formation, wherein the island is a push-up morphology of the fault that formed a carbonate high. Accordingly, the atoll formation seems to be an inheritance of eroded rollover anticline of Pliocene carbonate or older and continuing to the present time. The open atoll is caused by the southern block’s downward movement resulting from an oblique subsidiary sinistral fault mechanism. The research findings unveil new mechanism of atoll formation in the strike-slip fault setting that enrich the previous models.
... On the other hand, sea-level fall episodes during the Pleistocene-Holocene (30-120 m below the present level, according to Purdy and Winterer, 2001) caused the relative elevation and emersion of most of the coastal karstic landforms, facilitating their development and extension. Thus, the cave speleothems recorded could be used for palaeoclimatological studies (Fairchild et al., 2000;Hellstrom and McCulloch, 2000) and seasonal climatic oscillations (Stoll et al., 2007). ...
Article
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This paper presents an analysis of the main morphologies observed in the coastal cliffs of northern Spain (Asturias and Cantabria regions). The objective of this contribution is to establish a hypothesis on the origin and evolution of this rocky coast, as well as to present a detailed inventory, to characterise quantitatively and qualitatively singular morphologies and to highlight the geological heritage of this protected coast. The evolution process starts with the formation of an ancient coastal planation surface characterized by a flat morphology caused by regional mainly uplift and to relative sea level falls. Afterwards, wave erosion processes would have started eroding the cliff foot and simultaneously, karst activity produced some exokarst morphologies (sinkholes, karren, etc.) through stratification and fracturing network, while the underground drainage systems produced some caves and chasms. In the following step, corresponding to the last glaciation from the paleoclimatic point of view, sea level fall together with a deepening of the fluvial network caused the preservation of the existing caves and chasms and the generation of new ones at a lower level. On the other hand, dissolution processes on limestones created sinkholes in those areas characterized by alternating layers of limestones and marls, generating collapses. When the sea level reached the maximum height during the Holocene a new erosion cycle of the coastal cliffs began. As a consequence, new landforms and processes were produced, like bays, caves fillings, and intrusion of new sediments in small confined estuaries. In these areas, other types of morphologies associated with the last sea level rise can be observed, such as closed beaches, uncommon closed estuaries developed inside a sinkhole, blowholes produced by mixed wave action and widening of prevailing vertical pipes inside the limestones (including the second largest in the world), total or partial sedimentary filling of small confined estuaries, as well as a tombolo deposit. It is important to point out, that some sites described are included in the Spanish Inventory of Sites of Geological Interest (IELIG). Due to the evolution model here proposed, a portion of the coastal sector described are included in the Global Geosites Project.
... Only the accommodation space in small reefs and platforms occupying a few hundred square kilometres or less has been filled during the Holocene, largely by lateral sediment transport via sand aprons(Purdy & Gischler, 2005; and references therein). Over longer time scales during the Pleistocene, the saucer-shape of reefs and carbonate platforms is usually exacerbated by preferential karst dissolution in platform interiors during glacial sea-level lowstands as demonstrated byPurdy (1974) andPurdy and Winterer (2001) both in experiments and by the existence of Pleistocene reef island examples with central topographic depressions.Schlager and Purkis (2013) have shown that the bucket shape of carbonate platforms may also be a consequence of biotic self organization, based on the fact that carbonate producers at the margin potentially have easier access to food and nutrients and are less prone to be buried in sediment. ...
Article
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Accretion rates of Holocene tropical coral reefs in three areas in the Atlantic, Pacific, and Indian Oceans have been quantified in 79 dated core sections in 34 reef cores from Belize, the Maldives and French Polynesia. Holocene vertical reef accretion rate averages 5.05 m/kyr and has decreased during the past 10 kyrs. Accretion rates in branched and massive coral facies are statistically similar. Reef accretion rate is positively correlated with the rate of sea‐level rise, i.e., the degree of creation of accommodation space, and with climate as expressed in a Holocene sea surface temperature (SST) anomaly. Accommodation space is also created by subsidence, but at a rate one to two orders of magnitude lower than that created by glacio‐eustasy (0.04 to 0.16 m/kyr). Lagoonal background sedimentation in adjacent reef lagoons averages 0.89 m/kyr as measured in 72 dated core sections in 28 cores. Lagoonal carbonate sedimentation on top of underlying mangrove peat usually starts after a considerable hiatus of ca 3 kyr on average. The lagoonal background sedimentation rate increased during the Holocene, likely due to deepening. The differences between vertical reef accretion and lagoonal background sedimentation rates are a major factor in the production of the widely known saucer shapes typical of tropical reefs and carbonate platforms, i.e., the creation of unfilled accommodation space. Reef core recovery, used as a proxy for reef consolidation, and core depth exhibit a statistically negative correlation based on data from 326 core barrels. Recovery and marine cement abundance (average volume 8.6%) also decrease from windward to leeward core positions. These observations are presumably a result of both a decrease in the rate of sea‐level rise, i.e., the increase in time available for submarine cementation during the Holocene and the amount of flushing of reef interstices by marine waters. This article is protected by copyright. All rights reserved.
Article
The past one hundred years of ocean science have been distinguished by dramatic milestones, remarkable discoveries, and major revelations. This book is a clear and lively survey of many of these amazing findings. Beginning with a brief review of the elements that define what the ocean is and how it works-from plate tectonics to the thermocline and the life within it-Wolf H. Berger places current understanding in the context of history. Essays treat such topics as beach processes and coral reefs, the great ocean currents off the East and West Coasts, the productivity of the sea, and the geologic revolution that changed all knowledge of the earth in the twentieth century.
Article
Darwin's universally-known subsidence theory, based on Bora Bora as a model, was developed without information from the subsurface. To evaluate the influence of environmental factors on reef development, two traverses with three cores, each on the barrier and the fringing reefs of Bora Bora, were drilled and 34 uranium-series dates obtained and subsequently analyzed. Sea-level rise and, to a lesser degree, subsidence were crucial for Holocene reef development in that they have created accommodation space and controlled reef architecture. Antecedent topography played a role as well, because the Holocene barrier reef is located on a Pleistocene barrier reef forming a topographic high. The pedestal of the fringing reef was Pleistocene soil and basalt. Barrier and fringing reefs developed contemporaneously during the Holocene. The occurrence of five coralgal assemblages indicates an upcore increase in wave energy. Age–depth plots suggest that barrier and fringing reefs have prograded during the Holocene. The Holocene fringing reef is up to 20 m thick and comprises coralgal and microbial reef sections, and abundant unconsolidated sediment. Fringing reef growth started 8.780 ± 50 yr BP; accretion rates average 5.65 m/kyr. The barrier reef consists of >30 m thick Holocene coralgal and microbial successions. Holocene barrier reef growth began 10.030 ± 50 yr BP and accretion rates average 6.15 m/kyr. The underlying Pleistocene reef formed 116.900 ± 1.100 yr BP, i.e. during marine isotope stage 5e. Based on Pleistocene age, depth, and coralgal palaeobathymetry, the subsidence rate of Bora Bora was estimated to be 0.05 to 0.14 m/kyr. In addition to subsidence, reef development on shorter timescales like in the late Pleistocene and Holocene, has been driven by glacioeustatic sea-level changes causing alternations of periods of flooding and subaerial exposure. Comparisons with other oceanic barrier reef systems in Tahiti and Mayotte exhibit more differences than similarities. This article is protected by copyright. All rights reserved.
Chapter
From the morphological perspective, poljes are large depressions in karst terrain characterized by extremely flat floors. Corrosion plains have been reported in a wide range of climatic environments and geomorphic settings, such as tropical tower and cone karst landscapes, the floor of poljes, uplifted plateaus in mountain regions, alluvial plains, coastal areas, or the margins of sedimentary basins. The Chinese geomorphologists differentiate two types of landscapes with carbonate residual hills on the basis of morphological and hydrological criteria: fenglin karst and fengcong karst. However, most karst terrains display valleys carved by surface waters, indicating that fluvial activity and mechanical water erosion have played a role in the development of the landscape. Calcium carbonate can precipitate from supersaturated waters at springs, rivers (autogenic and allogenic), lakes and palustrine environments, generating deposits and constructional features with a wide diversity of facies and geometries known as calcareous tufas and travertines.
Article
Great Bahama Bank (GBB) is the modern example of a flat-topped, isolated carbonate platform. It is a major modern location of carbonate deposition that stands behind much of the understanding of modern processes of carbonate sedimentation, serves as a training venue for academia and industry, is the basis for numerous geological models, and is commonly used as a reservoir analog. GBB also provides valuable insight into the extent and patterns of sediment fill of accommodation space atop an isolated carbonate platform. Satellite imagery (Landsat TM and ETM+) and an extensive set of water-depth measurements (n=5,723) were used to map bathymetry across GBB and derive a digital terrain model (DTM). Analyzing the extent, patterns, and nature of sediment fill of accommodation space was facilitated by partitioning a depositional facies map on the basis of the DTM to show that 18% of accommodation on the GBB is "overfilled and filled" (emergent to-1.5 m water depth), 52% is "underfilled" (-1.5 to-6.0 m), and 30% is "unfilled" (<-6.0 m). Considering the bathymetric variation, the DTM shows that only 10% (10,800 km2) of the awash platform has aggraded to sea level in the form of sand shoals or mud flats (>-1.5 m water depth) and therefore the greater part of available accommodation (88,000 km2) remains incompletely filled with sediment. Areas of filled accommodation mostly extend platformward from the western coastlines of islands, which in turn are preferentially distributed along the eastern (windward) margin of the GBB. Seventy percent of sediment in the areas of filled accommodation is rudstone, high-energy grainstone, grainstone, and mud-poor packstone. Although dominated by grainstones (45%), since it occupies so much space (55,000 km2), the underfilled sector also contains the most heterogeneous facies mosaic and by definition, therefore, the greatest and most complex lateral facies variations. Although islands are numerous (n=1,430) they occupy only 8%, or 8,700 km2, of the platformtop. Despite their small proportional occupancy of the platform, islands play a direct role in the accumulation of muddy fabrics and exert a sphere of influence over the character of sedimentation formany tens of kilometers fromtheir coastlines. Further, since islands represent the portion of the bank where accommodation has been overfilled, their rarity emphasizes the challenges that such a large platform faces in filling accommodation space, even given the diverse grain factories producing carbonate sediment (mud precipitation throughwhitings, ooids and reefs along the platformmargins, skeletal sediments and nonskeletal grains such as fecal pellets, peloids, and pelletoids). Two factors can be evoked to explain the inability of GBB to fill accommodation space. First, falling sea level during the Pleistocene appears to have repeatedly aborted the filling process. By analogy, it will not be able to do so in the Holocene either, a situation exasperated by the second factor, which is that most of the Holocene GBB, particularly areas away from islands, presently lack any appreciable filling. Instead, facies analysis suggest reduced sedimentation and enhanced cementation. Through regional mapping, the work delivers a platform-wide assessment of how accommodation is filled and the facies responsible for the filling. Amongst other trends, by highlighting how the thickest accumulations of Holocene sediment are skewed to the windward margin of the GBB and related to the presence of islands, whereas the thinner accumulations, which dominate the platforminterior and are more grainy but alsomore heterogeneous than the thicker deposits, these datamight contain useful guidelines as to how depositional cyclesmight vary laterally in ancient systems.
Chapter
Many atolls and guyots in the Pacific are genetically connected by the processes of subsidence of volcanic islands, reef accretion, and reef drowning. Still, subsidence alone may not satisfactorily explain reef development and drowning. Sea-level is another crucial factor of reef development. Sea-level rise results in reef aggradation and retrogradation and/or backstepping. Sea-level stalling and fall may cause progradation and karstification during subaerial exposure. Possible causes of drowning include rapid pulses of sea-level rise, reef elevation and relief, and environmental stress such as low sea-surface temperatures as well as high sea-surface temperatures and upwelling of nutrient-rich and oxygen-depleted waters.
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French Polynesia atolls are spread on a vast 2300 by 1200 km Central Pacific Ocean area exposed to spatially and temporally dependent wave forcing. They also have a wide range of closed to open morphologies and several have been suitable to develop from black-lipped pearl oysters a substantial pearl farming activity in the past 30 years, representing nowadays the 2nd source of income for French Polynesia. Considering here only the component of lagoon renewal that is driven by waves, we investigate for 74 atolls different lagoon renewal metrics using 20 years of wave model data at 0.05° spatial resolution. Wavelet spectral analyses highlight that atolls, even in close vicinity, can be exposed to different and characteristic periodicities in wave-driven flows and water renewal. These characteristics are discussed in relation to pearl farming atolls, including atolls known to be efficient oyster spat producers, a critical activity for pearl farming sustainability.
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Because the shapes and forms of many coral reefs resemble karst (erosion landforms created by dissolution of limestone), it is widely believed that those reefs have grown on karst foundations, and that Holocene growth perpetuates the underlying topography. However, this concept has become difficult to reconcile with the growing amount of seismic and coring evidence demonstrating that several karst-like reef features are entirely constructional. Here I use cellular automata simulations to show that coral reefs resemble karst limestones not because they are built on karst foundations, but because reef growth and limestone erosion are fundamentally the same process, running in opposite directions. Coral reef landscapes are in fact inverse karst—the basic spectrum of reef growth forms mirrors the basic spectrum of limestone erosion forms. In both growth and erosion, the development of form is a self-organised phenomenon emerging from the cumulative action of small-scale processes. The essential morphological control in both cases is slope stability, which depends on the composition of each system: coral type in reefs and lithology (rock type) in limestones. Solid, well cemented reefs and limestones, which can maintain steep slopes without collapsing, produce nodular reefs and pinnacle karst respectively, whereas unconsolidated, friable reefs and limestones, which frequently collapse, produce cellular reefs and cone karst.The growth forms produced in the model should theoretically apply to all modular skeleton-building organisms growing in a fluid medium, and may therefore provide useful templates in the search for extraterrestrial life. While none of the model forms can be considered unequivocally diagnostic of life, because all could conceivably arise through inanimate crystallisation, the model’s seemingly accurate rendition of biogenic carbonate morphology on earth suggests that it may provide a useful foundation for evaluating and exploring the range of macroscale self-organised biogenic structures that could arise on other planets.
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Because the shapes and forms of many coral reefs resemble karst (erosion landforms created by dissolution of limestone), it is widely believed that those reefs have grown on karst foundations, and that Holocene growth perpetuates the underlying topography. However, this concept has become difficult to reconcile with the growing amount of seismic and coring evidence demonstrating that several karst-like reef features are entirely constructional. Here I use cellular automata simulations to show that coral reefs resemble karst limestones not because they are built on karst foundations, but because reef growth and limestone erosion are fundamentally the same process, running in opposite directions. Coral reef landscapes are in fact inverse karst—the basic spectrum of reef growth forms mirrors the basic spectrum of limestone erosion forms. In both growth and erosion, the development of form is a self-organised phenomenon emerging from the cumulative action of small-scale processes. The essential morphological control in both cases is slope stability, which depends on the composition of each system: coral type in reefs and lithology (rock type) in limestones. Solid, well cemented reefs and limestones, which can maintain steep slopes without collapsing, produce nodular reefs and pinnacle karst respectively, whereas unconsolidated, friable reefs and limestones, which frequently collapse, produce cellular reefs and cone karst.The growth forms produced in the model should theoretically apply to all modular skeleton-building organisms growing in a fluid medium, and may therefore provide useful templates in the search for extraterrestrial life. While none of the model forms can be considered unequivocally diagnostic of life, because all could conceivably arise through inanimate crystallisation, the model’s seemingly accurate rendition of biogenic carbonate morphology on earth suggests that it may provide a useful foundation for evaluating and exploring the range of macroscale self-organised biogenic structures that could arise on other planets.
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The Rocas Atoll is in the Fernando de Noronha Fracture Zone South Atlantic Ocean and was developed on top of an ancient oceanic volcano, mainly during the Holocene. Quaternary sea-level fluctuations, especially during the last transgression, left geomorphic marks on the atoll and its surrounding shelf. This study investigated the shelf of Rocas Atoll based on single-beam, sonography, grabbed bottom sediments, and multi-temporal satellite imagery. The Rocas Atoll has a surrounding shelf with general E-W orientation, predominantly flat with a water depth of 25 m. The shelf break occurs between 45 and 50 m water depth, reaching up to 80 m locally on northwestern. Inner and outer shelf zones are marked by step-terrace features. The deepest break in relief (T1) occurs at 70 m water depth, a terrace (T2) at 40 m, and another terrace (T3) occurs at 30 m. Most of the hard-bottoms are reefs. Close to the atoll, in shallow depths, reefs occur aggregated, forming a tridimensional structure of low relief mounds and paths. As the water depth increases towards the outer shelf, the reefs occur isolated. The geomorphologic patterns of this shelf are analogous to the northeast Brazilian shelf, reflecting the influence of Pleistocene/Holocene sea-level oscillations. Furthermore, multi-temporal satellite images between 2001 and 2019 reveal that modern processes of reef erosion, as a response to decadal climate phenomena, intensify the shelf sedimentation, and the South Equatorial Currents shape the modern shelf and affect seasonally the sediment transport and sand body dimensions.
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On atoll islands, fresh groundwater occurs as a buoyant lens-shaped body surrounded by saltwater derived from the sea, forming the main freshwater source for many island communities. A review of the state of knowledge of atoll island groundwater is overdue given their susceptibility to adverse impacts, and the task to address water access and sanitation issues within the United Nations’ Sustainable Development Goals framework before the year 2030. In this article, we review available literature to summarise the key processes, investigation techniques and management approaches of atoll island groundwater systems. Over fifty years of investigation has led to important advancements in the understanding of atoll hydrogeology, but a paucity of hydrogeological data persists on all but a small number of atoll islands. We find that the combined effects of buoyancy forces, complex geology, tides, episodic ocean events, strong climatic variability and human impacts create highly dynamic fresh groundwater lenses. Methods used to quantify freshwater availability range from simple empirical relationships to three-dimensional density-dependent models. Generic atoll island numerical models have proven popular in trying to unravel the individual factors controlling fresh groundwater lens behaviour. Major challenges face the inhabitants and custodians of atoll island aquifers, with rising anthropogenic stresses compounded by the threats of climate variability and change, sea-level rise, and some atolls already extracting freshwater at or above sustainability limits. We find that the study of atoll groundwater systems remains a critical area for further research effort to address persistent knowledge gaps, which lead to high uncertainties in water security issues for both island residents and surrounding environs.
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Plain Language Summary Fossil coral reefs include the skeletal remains of corals and are rarely preserved in good condition above modern sea level. Such reefs are rich archives that hold information about past environmental and ecological conditions at the time of reef development. A rare and expansive example of fossil reefs grew during the period 7,000 to 1,000 years ago at Kiritimati Island, Kiribati. Surveys and radiometric dating were used to reveal, for the first time, the age, elevation, and coral ecology of two of the most well‐preserved fossil reefs on Kiritimati Island (between 4,000 and 2,000 years ago). Results show that the reefs flourished during different time periods and were dominated by different coral taxa, before dying off. We argue that reef die‐off is attributed to changing lagoon seawater conditions that became intolerable for coral growth. These changing water conditions were the result of storm‐induced changes at the atoll rim that enclosed the lagoon and reduced flushing. As tropical cyclones are projected to increase in frequency and severity under global climate change, our results are critical for understanding the future responses of mid‐oceanic atolls and their supporting ecologies.
Article
Tropical coral reef lagoons that are characterized by high deposition rates are potentially useful for high‐resolution palaeoenvironmental reconstructions. We present the geochemical and accelerator mass spectrometry (AMS) ¹⁴C‐dating data of a 113‐cm gravity core obtained from the Yongle atoll lagoon in the South China Sea. The core record preserves a continuous 1,500‐year depositional history of the back‐reef lagoon environment, with sedimentation rates varying from 0.55 to 1.55 mm yr⁻¹ (mean of 0.74 mm yr⁻¹). Calcium carbonate (CaCO3) production was calculated based on the sediment accumulation rates and sediment density to be 807 g m⁻² yr⁻¹. The geochemical records of the studied core displayed a progressively increasing trend of storms over the past 1,500 years. The stable carbon and oxygen isotopes of the lagoon sediment have the potential to record monsoon intensity. The lagoon system of the Yongle atoll appears to have been influenced by minor terrestrial input from 1,500 calendar years before present (cal yr BP) to 500 cal yr BP, and by a strong anthropogenic influence over the past 200 years.
Article
In 1842, Darwin identified three types of reefs: fringing reefs, which are directly attached to volcanic islands; barrier reefs, which are separated from volcanic islands by lagoons; and ring reefs, which enclose only a lagoon and are defined as atolls. Moreover, he linked these reef types through an evolutionary model in which an atoll is the logical end point of a subsiding volcanic edifice, as he was unaware of Quaternary glaciations. As an alternative, starting in the 1930s, several authors proposed the antecedent karst model; in this model, atolls formed as a direct interaction between subsidence and karst dissolution that occurred preferentially in the bank interiors rather than on their margins through exposure during glacial lowstands of sea level. Atolls then developed during deglacial reflooding of the glacial karstic morphologies by preferential stacked coral-reef growth along their margins. Here, a comprehensive new model is proposed, based on the antecedent karst model and well-established sea-level fluctuations during the last 5 million years, by demonstrating that most modern atolls from the Maldives Archipelago and from the tropical Pacific and southwest Indian Oceans are rooted on top of late Pliocene flat-topped banks. The volcanic basement, therefore, has had no influence on the late Quaternary development of these flat-topped banks into modern atolls. During the multiple glacial sea-level lowstands that intensified throughout the Quaternary, the tops of these banks were karstified; then, during each of the five mid-to-late Brunhes deglaciations, coral reoccupied their raised margins and grew vertically, keeping up with sea-level rise and creating the modern atolls. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 4, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
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Preservation of original mineralogy of cements and grains may be attributed to a minor influence of fresh water, resulting from the semi-arid climate of the southeastern Bahamas, and the unlikely formation of a freshwater lens during periods of emergence.-from Authors
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The primary factors controlling the occurrence and flow of ground water in the leeward reef islet of Laura are (1) the depositional history of the upper hydrologic unit, which has resulted in a greater accumulation of low-permeability (fine-grained) sediments beneath the lagoon side of the island and a high- to low-permeability (coarse- to fine-grained sediment) gradation between the ocean and lagoon; and (2) the diagenetic history of the lower hydrologic unit, which has resulted in a highly permeable basement. -from Authors
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Laboratory dissolution of Middle Ordovician rock samples from central Pennsylvania was studied at 23°C and 1 atm carbon dioxide pressure. Carbonate dissolution rates were compared at 22% bicarbonate saturation with respect to both calcite and dolomite. The results show that carbonate lithology exerts a strong influence on the dissolution rate and hence on the degree of cavity development in karst aquifers. The dissolution rate is most significantly affected by dolomite and impurity content. The rate decreases as percentages of dolomite and disseminated insolubles increase. Maximum dissolution rates occur for carbonate rocks with 1.0-2.5% MgO content and having abundant silty streaks. The sparite content is inversely related to cave development but is independent of dissolution rates measured under the laboratory conditions adopted in this study.
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In less than 10 years, subaerially-exposed Holocene oolitic sand on Eleuthera Island, Bahamas was completely case-hardened by freshwater vadose calcite cementation. This phenomenon demonstrates the rapidity of freshwater carbonate diagenesis and explains, in part, why carbonate platforms preferentially shed unconsolidated sediment during highstands and not during lowstands when they are subaerially exposed. It further proves, contrary to what the Exxon conceptual sequence stratigraphic models imply, that carbonate deposits do, in fact, respond differently to subaerial exposure than siliciclastic deposits.Case hardening of unconsolidated carbonate sediment in response to subaerial exposure and early, near-surface freshwater diagenesis helps explain why shallow-water platform sediments are preferentially shed into adjacent basins during highstands of sea level and not during lowstands. During lowstands, rapid subaerial lithification of carbonate sediments retards their reworking by wind or water. Once lithified, these deposits are further stabilized by vegetation and karstification, inhibiting their transport as unconsolidated sediment into adjacent deeper-water basins.Observations from Eleuthera Island also document the reciprocal processes associated with freshwater carbonate diagenesis. Calcium carbonate released by partial dissolution of aragonitic ooids was locally reprecipitated as pore-filling, low-magnesian calcite cement. This reciprocity is confirmed by the confinement of freshwater dissolution and cementation to the upper 10–20 cm of the subaerially-exposed body of oolitic sand. Below this case-hardened surface, oolitic sands remained unconsolidated. Only very minor amounts of calcite cement were required for effective casae hardening.
Article
Because postevent processes obscure depth, diameter, and ejecta debris of submarine nuclear craters, high-resolution multichannel seismic reflection data are essential to define their subsurface geometry. The primary goal of the multichannel seismic survey was to acquire data on the distribution, continuity, and structure of subsurface reflectors to a depth of at least 500 m (1640 ft) below bottom. A secondary goal was to provide data for the selection of drilling sites. Shallow (0-30m, or 0-100ft) subsurface reflectors were mapped with the single-channel seismic system (chap. C, this volume), and deeper reflectors were mapped with the multichannel seismic system.-from Authors
Chapter
The upper surface of the Albian platform limestone on many Mid-Pacific Mountains (MPM) guyots, as shown by multibeam-sonar, bathymetric data, and seismic-reflection data from where pelagic sediments have buried the Albian platform limestone, shows that the top of the limestone sequence has been sculptured by streams, by waves, and by dissolution during an episode of emergence to heights of as much as 180 m above sea level. Petrographic and stable-isotopic studies of samples recovered in drill holes in the upper parts of the platform limestone on Allison and Resolution guyots show dissolution cavities tens of meters below the top of the limestone, some filled with pelagic sediments and others still open and lined with stalagmite-like calcite structures and cements showing evidence of accumulation in vadose environments. -from Authors
Article
The petroleum producing Albian-Cenomanian limestone of the Tampico embayment consists of three regional facies belts: the Tamaulipas (basinal), El Abra (reef, backreef, lagoon), and Tamabra (basin margin). The principle lithologies of the Tamabra Limestone, in order of abundance, are skeletal-fragment grainstone/packstone, breccia, rudist-fragment wackestone, and pelagic-microfossil wackestone. Regional trends are to finer-grained lithologies upward and into the basin. Grainstone/packstone predominates in the greater Poza Rica field; elsewhere breccia is more common to dominant. The skeletal grainstone/packstone was deposited grain-for-grain by sediment gravity flow. The breccia, interpreted as debris flow deposits, contains clasts of basinal pelagic wackestone and cemented, shelf-derived skeletal-fragment limestone. No rudists in growth position were identified in Tamabra rocks although some fossils were mechanically deposited with long axes vertical. The hypothesis of Coogan and others (1972), that the Poza Rica trend is the true shelf edge and the Golden Lane a faulted backreef-lagoon sequence, is unacceptable because of stratigraphic thickening and evident lack of structure at the Golden Lane escarpment. The hypothesis that the Poza Rica trend is a subsidiary low sea-level shelf margin seaward of the major Golden Lane shelf margin is disfavored, because of the requisite repeated major sea level fluctuations and the facies contrasts between the Golden Lane and the Poza Rica trend. The geometry and sedimentology of the deposits indicate that the Golden Lane is the true shelf edge and the Tamabra facies is reef-derived debris deposited in the basin. Shelf-edge slope and relief are considered the best guides to basin-margin debris deposits in early stages of exploration for Poza Rica-type reservoirs.
Article
Data from a shallow cored borehole through 105,000-year-old reef-tract sediments on the south coast of Barbados indicate that the carbonate sediments have been altered more rapidly and more extensively in freshwater phreatic diagenetic environments than in the vadose or marine phreatic diagenetic environments. The upper part of the borehole section has been exposed only to subaerial and shallow vadose processes since emergence from the marine depositional environment about 105,000 years ago. These sediments are recrystallized only partly. Total amounts of cement, mainly needle-fiber cement and dense micrite coatings, are high. Porosity is low. Dissolution, a minor feature in this part of the section, is associated with localized development of needle-fiber fabrics. Those parts of the borehole section which have been occupied by a freshwater phreatic lens have been cemented by calcite microspar. Aragonite grains have been dissolved on a massive scale, resulting in a very well-developed moldic porosity. Neomorphic grain alteration has been volumetrically important. Packstones and grainstones in the lowest part of the borehole are largely unaltered; metastable carbonate mineralogy dominates. This part of the sedimentary column has been subjected alternately to vadose conditions during low stands of sea level (> 70,000 years total) and to marine phreatic conditions during high stands of the sea ( If the recrystallized lime muds contain greater than 25 percent cement (micrite), then mass-balance calculations suggest large amounts of calcium carbonate have been transported to that part of the sedimentary section exposed to freshwater phreatic diagenetic processes. The source of the additional carbonate may be updip in the aquifer, from mixing of underlying marine pore fluids with the freshwater pore fluids of the coastal water lens, from the overlying shallow vadose zone, or from marine water soon after deposition. Processes which affect early diagenetic modification of carbonate sediments on the south coast of Barbados operate more rapidly and more effectively in the freshwater phreatic environment than in vadose or marine phreatic environments.
Article
The Cayman Unconformity, which separates the Pedro Castle Formation (Pliocene) from the underlying Cayman Formation (Miocene), is a sequence boundary that developed during the Messinian, when sea level was at a lowstand due to glaciation in the Southern Hemisphere. By the end of the Messinian, Grand Cayman was an atoll-like island that had an elevated peripheral rim that was up to 41 m above the central depression. The Cayman Formation contains paleocaves and paleosinkholes that were linked to the Cayman Unconformity. The topography on the Cayman Unconformity is attributed to erosional processes, because: 1) there is no evidence of carbonates that formed by constructional processes (i.e., reefs, dunes) in the elevated peripheral rim, and 2) there is ample evidence of dissolutional features in the Cayman Formation. -from Authors
Article
We show on geomorphological grounds, supported by data on water chemistry of streams and lakes, that the limestone topography is of erosional origin. On the outer slopes of the limestone we identify a prominent terrace at 26-27.5 m and a notch at 23-25 m, together with a much younger surface of last interglacial reef deposits rising from the present sea-level (where old fore-reef structures are found) to 14.5 m. All these features are related to the tectonic and eustatic history of the island. -from Authors
Article
Summarizes results from a laboratory investigation of the dissolution rates of various biogenic carbonates and evaluates the roles mineralogy, grain microstructure, and solution saturation state play in determining the relative stability of aragonite and magnesian calcite during the dissolution phase of diagenesis.-from Author
Article
Locally operating factors such as topography of the reef basement and exposure to waves and currents rather than regionally effective factors such as the post-glacial sea level rise in the western Atlantic explain the different Holocene developments of the three isolated carbonate platforms Glovers Reef, Lighthouse Reef, and Turneffe Islands offshore Belize. A series of NNE-striking tilted fault-blocks at the passive continental margin forms the deep basement of the Belize reefs. Glovers and Lighthouse Reefs are located on the same fault-block, while Turneffe Islands is situated west of Lighthouse Reef on an adjacent fault-block. The three platforms are surrounded by deep water and have surfacebreaking reef rims. Significant differences exist between platform interiors. Glovers Reef has only 0.2% of land and an 18 m deep, well-circulated lagoon with over 800 patch reefs. Lighthouse Reef has 3% of land and a well-circulated lagoon area. Patch reefs are aligned along a NNE-striking trend that separates a shallow western (3 m) and a deeper eastern (8 m) lagoon. Turneffe Islands has 22% of land that is mainly red mangrove. Interior lagoons are up to 8 m deep and most have restricted circulation and no patch reefs. Surface sediments are rich in organic matter. In contrast, the northernmost part of Turneffe Islands has no extensive mangrove development and the well-circulated lagoon area has abundant patch reefs. Holocene reef development was investigated by means of 9 rotary core holes that all reached Pleistocene reef limestones, and by radiometric dating of corals. Maximal Holocene reef thickness reaches 11.7 m on Glovers Reef, 7.9 m on Lighthouse Reef, and 3.8 m on Turneffe Islands. Factors that controlled Holocene reef development include the following. (1) Holocene sea level. The margin of Glovers Reef was flooded by the rising Holocene sea ca. 7500 YBP, that of Lighthouse Reef ca. 6500 YBP, and that of Turneffe Islands between 5400 and 4750 YBP. All investigated Holocene reefs belong to the keep-up type, even though the three platforms were flooded successively and, hence, the reefs had to keep pace with different rates of sea level rise. (2) Pre-Holocene topography. Pleistocene elevation and relief are different on the three platforms. This is the consequence of both tectonics and karst. Different elevations caused successive reef initiation and they also resulted in differences in lagoon depths. Variations in Pleistocene topography also explain the different facies distribution patterns on the windward platforms that are located on the same fault-block. On Lighthouse Reef tectonic structures are clearly visible such as the linear patch reef trend that is aligned along a Pleistocene fault. On Glovers Reef only short linear trends of patch reefs can be detected because the Pleistocene tectonic structures are presumably masked by the higher Holocene thickness. The lower Pleistocene elevation on Glovers Reef is probably a consequence of both a southward tectonic tilt, and stronger karstification towards the south related to higher rainfall. (3) Exposure to waves and currents. Glovers Reef, Lighthouse Reef, and the northernmost part of Turneffe Islands receive the maximum wave force as they are open to the Caribbean Sea. Adjacent lagoons are well-circulated and have luxuriant patch reef growth and no extensive mangrove development. By contrast, most of Turneffe Islands is protected from the open Caribbean Sea by Lighthouse Reef to the east and is only exposed to reduced wave forces, allowing extensive mangrove growth in these protected areas.
Article
We report the first radiometric dates (thermal-ionization mass spectrometry) from late Pleistocene reef deposits from offshore Belize, the location of the largest modern reef complex in the Atlantic Ocean. The results presented here can be used to explain significant differences in bathymetry, sedimentary facies, and reef development of this major reef area, and the results are significant because they contribute to the knowledge of the regional geology of the eastern Yucatán. The previously held concept of a neotectonically stable eastern Yucatán is challenged. The dates indicate that Pleistocene reefs and shallow-water limestones, which form the basement of modern reefs in the area, accumulated ca. 125 130 ka. Significant differences in elevation of the samples relative to present sea level (>10 m) have several possible causes. Differential subsidence along a series of continental margin fault blocks in combination with variation in karstification are probably the prime causes. Differential subsidence is presumably related to initial extension and later left-lateral movements along the adjacent active boundary between the North American and Caribbean plates. Increasing dissolution toward the south during Pleistocene sea-level lowstands is probably a consequence of higher precipitation rates in mountainous southern Belize.
Article
Movement of ground water in maturely karsted limestone aquifers is commonly through solution conduits of considerable size. The object of this study was to determine the lithologic controls on the distribution of conduits within a heterogeneous sequence of carbonate rocks. The volume of all accessible caves more than 100 feet in length in the Nittany Valley area of central Pennsylvania was measured. Most caves are entirely within limestones, cave development in dolomite is extremely rare. Within the limestone sequence the bulk of the cave volume is concentrated in a few members. Chemical and petrographic analyses of the carbonate rocks were associated with cave volume by bar graphs and components analyses. The more cavernous limestones seem to be those with low amounts of dolomite, clay, and other impurities, and a high micrite grain fraction. Coarse limestones and dolomites are less cavernous.
Article
Data from three Bahamian and South Floridan cores with deeper water facies show that, contrary to current models, there is no phreatic meteoric diagenesis that can be attributed to large-scale sea-level lowstands. Models of meteoric diagenesis in carbonate sediments commonly assume major phreatic alteration during glacio-eustatic sea-level lowstands. However, the diagenesis previously attributed to large-scale (>80 m) lowstands is found in shallow-water facies that have been repetitively exposed. In order to document diagenesis from large-scale lowstands, an interval is needed that was deposited in deeper water and not immediately exposed to meteoric fluids. Three cores with such deeper-water facies indicate that the majority of phreatic meteoric diagenesis in Florida and the Bahamas happened during glacio-eustatic highstands, not during lowstands. The data also suggest that diagenetically active meteoric lenses in Florida and the Bahamas are restricted to within 60 m, and perhaps less, of the land surface. The most likely reasons deeper lenses are chemically inactive are (1) the greater percolation distance allows the water to reach saturation prior to entering the lens, and (2) the large distance exceeds the reach of soil-derived organic matter, known to drive diagenesis in meteoric lenses. Models that currently assume a constant rate for alteration in meteoric fluids need to accommodate this variation with thickness of the vadose zone. This study also questions the use of paleophreatic lenses as records of eustatic sea level, as the large-scale falls may not leave any record.
Article
Many guyots in the north Pacific are built of drowned Cretaceous shallow-water carbonates that rest on edifice basalt. Dating of these limestones, using strontium- and carbon-isotope stratigraphy, illustrates a number of events in the evolution of these carbonate platforms: local deposition of marine black shales during the early Aptian oceanic anoxic event; synchronous development of oolitic deposits during the Aptian; and drowning at different times during the Cretaceous (and Tertiary). Dating the youngest levels of these platform carbonate shows that the shallow-water systems drowned sequentially in the order in which plate-tectonic movement transported them into low latitudes south of the Equator (paleolati- tude 0°-10° south). The chemistry of peri-equatorial waters, rich in upwelled nutrients and carbon dioxide, may have been a contributory factor to the suppression of carbonate precipitation on these platforms. However, oceanic anoxic events, thought to reflect high nutrient availability and increased produc- tivity of planktonic organic-walled and siliceous microfossils, did not occasion platform drowning. Neither is there any evidence that relative sealevel changes were the primary cause of platform drowning, which is consistent with the established resilience of shallow-water carbonate systems when influenced by such phenomena. Comparisons with paleotemperature data show that platform drowning took place closer to the Equator during cooler intervals, such as the early Albian and Maastrichtian, and farther south of the Equator during warmer periods such as Albian-Cenomanian boundary time and the mid-Eocene. Initiation of one carbon- ate platform relatively close to the Equator, at paleolatitudes more northerly than those where others drowned, took place during the cool early mid-Aptian. These correlations are in accord with an interpretation that excess warmth in shallow peri-equatorial waters proved inimical for many carbonate-secreting organisms living on the platforms, allowing subsidence or eustatic sealevel rise to outpace sedimentation and guyots to form. A parallel may be drawn with the recent phenomenon of coral and foraminiferal bleaching, whereby photosynthetic sym- bionts succumb to prolonged high temperatures (G30°C) and the host organism dies. The fact that most Cretaceous guyots reside in the north Pacific may not be solely related to the age-distribution pattern of ocean floor but to their having run the gauntlet of a difficult and dangerous passage across the Equator. North Pacific guyots are relics from the Cretaceous (and Eocene) ''greenhouse'' Earth.
Article
The Loyalty Islands (SW Pacific) are uplifted as they are progressively affected by the lithospheric flexure of the Australian plate, before its subduction under the New Hebrides Arc. These geodynamic changes are constrained by magnetostratigraphically dating two sections from Mar6 Island, where mineral extractions coupled with rock-magnetic experiments suggest that the magnetic remanence is mostly carried by a mixture of single-domain to multidomain magnetite/ maghemite. With the help of faunal determinations and radiometric dating, the sequences of polarity reversals, correlated to the geomagnetic polarity timescale, range from the top of Chron C4n (late Miocene) to the top of the Gauss Chron (late Pliocene). This new chronostratigraphy re- fines the timing of two distinctive carbonate units (rhodolith platform/coral reefs) separated by a hardground whose transition is known to coincide approximately with a regional event around the Miocene/Pliocene boundary. The magnetostratigraphic indicates that the hardground repre- sents about a 1.9 m.y. hiatus and suggests variable sedimentation rates ranging from 4.7 to 65.4 m/m.y. during the atoll construction. The lithospheric bulge seems to have influenced the evolu- tion of Mar6 Island some 3.1 m.y. ago, leading to a diachronous emersion of the northeast and southwest rim of the atoll with a mean uplift rate of the order of 4 cm/kyr.
Article
Gridded fields (analyses) of global monthly precipitation have been constructed on a 2.5° latitude-longitude grid for the 17-yr period from 1979 to 1995 by merging several kinds of information sources with different characteristics, including gauge observations, estimates inferred from a variety of satellite observations, and the NCEP-NCAR reanalysis. This new dataset, which the authors have named the CPC Merged Analysis of Precipitation (CMAP), contains precipitation distributions with full global coverage and improved quality compared to the individual data sources. Examinations showed no discontinuity during the 17-yr period, despite the different data sources used for the different subperiods. Comparisons of the CMAP with the merged analysis of Huffman et al. revealed remarkable agreements over the global land areas and over tropical and subtropical oceanic areas, with differences observed over extratropical oceanic areas. The 17-yr CMAP dataset is used to investigate the annual and interannual variability in large-scale precipitation. The mean distribution and the annual cycle in the 17-yr dataset exhibit reasonable agreement with existing long-term means except over the eastern tropical Pacific. The interannual variability associated with the El Niño-Southern Oscillation phenomenon resembles that found in previous studies, but with substantial additional details, particularly over the oceans. With complete global coverage, extended period and improved quality, the 17-yr dataset of the CMAP provides very useful information for climate analysis, numerical model validation, hydrological research, and many other applications. Further work is under way to improve the quality, extend the temporal coverage, and to refine the resolution of the merged analysis.
Article
The average age of ground water that discharges at the shoreline from an island fresh-water lens is equal to the volume of ground water in the lens divided by the total recharge to the island. This island average residence time, τ0 is easily calculated by Dupuit-Ghyben-Herzberg analysis (DGH). From estimates of the controlling variables (recharge, hydraulic conductivity, and porosity), it is estimated that τ0 is usually on the order of 1 to a few tens of years in fresh-water lenses of small (width on the order of 100 to a few thousand meters), strip islands where calcarenite is undergoing early diagenesis. Lateral variation in interstitial velocity and distribution of ground-water age within the lens can be calculated from an approximate theory using DGH potentials and an assumption that discharge is uniformly distributed with depth. Results are within a few percent of those from rigorous flow-net construction. Velocities range laterally through two orders of magnitude. Contours of ground-water age are nearly horizontal over most of the lens; at a depth of about 40% of the depth to the interface, the ground-water age is τ0/2, and it equals τ0 at about 60% of the depth to the interface. Flow-net construction shows that velocities are greatest at the water table and decrease rapidly downward to the value given by DGH. Representative velocities halfway between the ground-water flow divide and the shoreline are 10 to 100 m/yr in these islands. Application of these calculation procedures to Bermuda leads to a revised estimate of ground-water age for water samples that have been used to estimate the rate of aragonite-to-calcite transformation in the fresh-water lenses of that island. This rate is an order of magnitude less than that in lenses in Holocene oolitic cays of the Bahamas. The ratio of stabilization rate to amount of aragonite appears to be about the same in the two settings. The value of the ratio implies a half-life of 6,000-7,000 yr for aragonite-to-calcite transformation in these lenses.
Article
Rennell Island is a coral atoll in the South Solomon Sea whose entire coastline is formed by an uplifted reef complex. This complex consists of a succession of superimposed fore reefs and their associated reef flats, all of which have been completely dolomitized. During subsequent uplift, undolomitized fringing reef limestone was deposited upon the atoll reef complex, and lagoonal deposits accumulated in the enclosed central region. The atoll reef complex grew on a subsiding basement platform, the instability of which resulted in the drowning of successive stages of fore-reef development.
Article
The writer reviews investigations on coral reefs in the East Indies carried out especially during the last 15 years before the war. Every atoll and barrier reef studied in some detail shows conclusive evidence of subsidence. The extreme thickness of some reefs, as demonstrated by their steep submarine slopes, cannot be explained by Glacial Control only. The influence of shifting sea level during the ice ages has been of minor importance on the East Indian reefs. In this region, as a rule, narrow lagoons are shallower than wide lagoons; the phenomenon is obviously related to aggradation of the bottom. It is claimed that solvent action of sea water in the tropics is limited to action above the surface of the sea. The paper bears on the important influence of prevailing wind and wave action on the upper structure of reefs. Additional examples are given of sea currents as factors of morphological importance. The great influence of sedimentation and of a recent world-wide sinking of sea level is stressed. Elevated atolls, reef terraces, and tilting movements are considered. Results are mentioned of investigations on such subjects as the depth penetration of light and oxygen consumption.
Article
The post-Miocene diagenetic and eustatic history of Enewetak Atoll was investigated using a one-dimensional forward model. Comparison of model and data suggests that the post-Miocene history of Enewetak Atoll was dominated by multiple episodes of meteoric phreatic diagenesis attendant with high-frequency (10⁴ to 10⁵ yr) fluctuations in sea level and a subsidence rate of 39.0 m/m.y. Sensitivity testing indicates that subaerial erosion results in the preservation of additional subaerial unconformities because stratigraphic shortening permits a succeeding sea-level rise to flood the exposure surface and deposit sediment, whereas without subaerial erosion this sea-level rise would be recorded as a paleophreatic lens. Model results indicate that less then 10% of lapsed time is recorded by sediment deposition during periods of high-frequency changes in sea level. Incompleteness of the stratigraphic record suggest that magnetostratigraphy may give erroneous ages for shallow-marine carbonate sequences deposited during times of high-frequency changes in sea level and frequent magnetic polarity reversals.
Article
A period of atoll emergence results in the development of a karst surface on the atoll. Upon resubmergence, this surface is preserved as a subsurface solution unconformity that can be identified by petrographic characteristics and a distinctive stable isotope signature imprinted by subaerial or phreatic diagenesis. Determination of the magnitude of Miocene sea-level falls by considering subsidence rates and depths to solution unconformities must take into account the lowering of the emergent atoll surface by dissolution. Previously, the depth to a Miocene solution unconformity below Midway Atoll was assumed to record a maximum highstand of sea level that preceded a period of island emergence. However, because of surface dissolution, the solution unconformity must represent some elevation between a past highstand and a subsequent lowstand. The subsurface stratigraphy of Midway Atoll shows that a late Miocene sea-level fall of 75 125 m below present sea level took place. The magnitude of this fall is greater than that previously determined, on the basis of stable isotope data, from Midway Atoll but approximates Miocene sea-level falls determined by seismic stratigraphic methods.
Article
Bioclastic grainstone, packstone, and wackestone are interbedded with polymictic lime breccia and dolomitized debris; all were deposited by sediment gravity flow. Indigenous sediment was pelagic lime mud. Typical reservoir porosities are about 10%; permeabilities averaged 2 md and rarely exceed 100 md. Porosity is largely the result of selective dissolution of rudist fragments, which were originally aragonite. The agent for both early cementation and development of secondary porosity appears to have been meteoric water. -from Author
Article
Stratigraphic hiatuses and solution unconformities in the subsurface of Enewetak Atoll, northern Marshall Islands, record periods of atoll emergence during low stands of sea level. Changes in sea level are also recorded in the atoll subsurface by variations in the rate of sediment accumulation relative to the subsidence rate of the underlying volcanic edifice. Past sea levels can be derived from atoll stratigraphy by correcting the present depth of dated subsurface horizons for thermal subsidence and lithospheric flexure since the time of deposition, but past sea levels derived from atoll stratigraphy can only be estimated to within ±50 m relative to present sea level. Based on the biostratrigraphic ages of subsurface horizons at Enewetak, we can discern very little long-term change in sea level from late Eocene through late Oligocene, a rise to ~110 m above present sea level in the early Miocene, a long-term fall of ~170 m through middle and late Miocene time, and a long-term rise of ~60 m from the end of the Miocene to present. Interpretation of past sea levels based on 87Sr/86Sr chronostratigraphy from Enewetak confirms the long-term sea level trends inferred from biostratigraphic subsurface ages. In addition, we interpret three Oligocene sea level falls with minimum magnitudes of 30-50 m at ~35, 33-30, and ~25 Ma and four Micoene sea levels greater than 30-95 m at 16-14, ~12, ~10, and ~5 Ma. -from Authors
Article
Data from recent surveys and drilling suggest that carbonate platforms over a wide region of the oceanic western Pacific became emergent by amounts of 100 200 m during late Albian time, ca. 100 Ma. The resulting erosional landscapes, now modified by differential compaction over the buried volcanic basement hills, included central basins surrounded by perimeter rims, sinkhole-like depressions, and perimeter benches interpreted as wave-cut terraces. Resubmergence resulted in the sealing of atoll-like erosional topography by pelagic sediments. These Cretaceous proto-atolls, now guyots, provide evidence that ancient as well as modern atolls inherit their form mainly from subaerial landscapes.
Chapter
This chapter describes the geology and hydrology of Cook Islands that are a part of Polynesia. The Cook Islands are located in the central South Pacific between the Society Islands to the east and the Tonga and Samoa Islands to the west. The Islands consist of 15 islands divided into a northern group of six islands and a southern group of nine islands. The population of the Cook Islands has been steadily declining because of dual citizenship with New Zealand and the consequent migration of many to that country. The two groups have distinct climates and oceanographic settings, tectonic origins, geomorphologies, hydrologic resources, and geologic histories. The southern group of islands form two linear volcanic chains; crustal loading and flexure associated with Quaternary volcanism on Rarotonga uplifted the makatea islands. In contrast, most islands of the northern group are located on the tectonically stable Manihiki Plateau, which was formed from an outpouring of lava at a triple junction during the Early Cretaceous.
Article
The mineralogy, chemistry, distribution of grain size, diagenesis, and ages of modern carbonates accumulating on the New Providence Platform, Bahamas, have been studied. Lithification by aragonite cement is taking place beneath sixteen feet of sea water. These marine sediments have never been exposed to fresh water as determined by virtual absence of low-magnesium calcite and lack of recrystallization of aragonite and high-magnesium calcite. Depth of water and abundance of plant life appear to localize lithification at sites we have termed bioherms. Significant vertical variations in facies have been found in the modern carbonates that overlie the recrystallized, biosparite, chemically dissolved Pleistocene bedrock. Silt- and clay-sized particles of unconsolidated modern carbonates rest unconformably on the biosparite and increase progressively in size toward the surface. Where modern lithified material is present, rarely is it found deeper than three feet below the water-sediment interface.
Article
Drill cores of Enewetak Atoll, Marshall Islands, reveal six stratigraphic intervals, numbered in downward sequence, which represent vertical coral growth during Quaternary interglaciations. Radiocarbon dates indicate that the Holocene sea transgressed the emergent reef platform by about 8000 yr B.P. The reef grew rapidly upward (about 5 to 10 mm/yr) until about 6500 yr B.P. Afterward vertical growth slowed to about 0.5 mm/yr, then lateral development became dominant during the last several thousand years. The second interval is dated at 131,000 ± 3000 yr B.P. by uranium series. This unit correlates with oxygen-isotope substage 5e and with terrace VIIa of Huon Peninsula, New Guinea, and of Main Reef-2 terrace at Atauro Island. The third interval is not dated because corals were recrystallized and it is tentatively correlated with either oxygen-isotope stages 7 or 9. The age of the fourth interval is estimated at 454,000 ± 100,000 yr B.P. from measured activity ratios. This unit is correlated with either oxygen-isotope stage 9, 11, or 13.
Article
Upper Cretaceous shallow-water limestones rich in rudists have been studied in the Matese Mountains. They show a preponderance of skeletal components (molluscs and benthic foraminifers) and a lack of non-skeletal grains. The most prominent sediments of the studied stratigraphic interval are rudist-dominated fine- to coarse-grained lithologies. Most of the Senonian successions are characterized by massive neritic limestones rich in molluscs (mostly rudists and subordinate gastropods) and a few corals. The sediments were generated in situ on shelves where rudist bivalves were the primary sediment producers. These sediments were actively moved by storms, waves and swells. The finer fractions were probably winnowed out and deposited in deeper water. Well-bedded dolomitized mudstones/wackestones, microbial laminites and benthic foraminifer wackestones characterize some successions where they rhythmically alternate and testify to deposition in tidal-flat environments.
Article
Groundwater flow in karst aquifers is extremely difficult to predict because of the presence of solution-enlarged fractures and conduits in the bedrock. The location and volume of this solution development is the result of interrelated geologic, hydrologic and climatic factors. This paper examines lithologic properties which influence apparent solution rates in carbonate aquifers as indicated by weathered profiles in caverns. Rock samples collected from cavern entrances and walls in southwestern Missouri, U.S.A., were analyzed for grain size, sorting, percent quartz, percent magnesium carbonate, percent opaque minerals and percent acid-insoluble residue. Discriminant function analyses of these measurements show that grain size is the most important parameter in distinguishing the grouping of apparently resistant and non-resistant samples. Small grain size favors dissolution and strongly influences the relief of the weathered profile regardless of rock composition. Significant variability in magnesium carbonate content may also affect the apparent solution rate but to a lesser degree than grain size.
Article
A careful comparison is made between the most detailed records of sea level over the last glacial cycle, and two high-quality oxygen isotope records. One is a high-resolution benthonic record that contains superb detail but proves to record temperature change as well as ice volume; the other is a planktonic record from the west equatorial Pacific where the temperature effect may be minimal but where high resolution is not available. A combined record is generated which may be a better approximation to ice volume than was previously available.This approach cannot yet be applied to the whole Pleistocene. However, comparison of glacial extremes suggests that glacial extremes of stages 12 and 16 significantly exceeded the last glacial maximum as regards ice volume and hence as regards sea level lowering. Interglacial stages 7, 13, 15, 17 and 19 did not attain Holocene oxygen isotope values; possibly the sea did not reach its present level. It is unlikely that sea level was glacio-eustatically higher than present by more than a few metres during any interglacial of the past 2.5 million years.
Article
Thesis (M.S.)--University of Calgary, 1971. Includes bibliographical references (leaves 77-81). Photocopy.
Article
Detailed seismic stratigraphy, lithostratigraphy, and chemostratigraphy indicate that atoll-wide subaerial exposure surfaces (major disconformities) developed during major sea-level lowstands form prominent seismic reflectors and are coincident with biostratigraphic breaks in the Plio-Pleistocene on Enewetak Atoll. Sea-level models based on the stratigraphic position and age of major disconformities suggest a maximum sea-level highstand elevation of 36 m above present sea level and a maximum sea-level lowstand elevation of 63 m below present sea level for the Pliocene. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/29585/1/0000674.pdf