H. L. Vacher

U-series and amino-acid racemization geochronology of Bermuda: implications for eustatic sea-level fluctuation over the past 250,000 years

Abstract Bermuda is a stable, mid-oceanic carbonate platform for which a particularly complete record of Late Pleistocene eustatic sea-level fluctuation has been reconstructed from a detailed study of geological field relationships combined with an extensive programme of U-series and amino-acid racemization geochronology. Only twice in the past 250,000 yr. has sea level in Bermuda been above its present level, once at approximately 200 k.y. when it stood at about + 2 m and most recently at 125 ± 4 k.y. when it stood at 5 ± 1 m. These times of interglacial high sea level are characterized by the development of patch reefs and marine calcarenites at elevations above present sea level. Episodes of lower sea stand onto the Bermuda platform at elevations higher than −20 m are observed within the two interglacial periods and are characterized by the deposition of eolianites. By contrast glacial periods are times of residual soil formation and deposition of speleothems in caves at elevations below present sea level. Excellent correlation is observed between the Bermuda glacio-eustatic sea-level record and other marine and terrestrial paleoclimate records.

Eogenetic karst from the perspective of an equivalent porous medium

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

History of stage 5 sea level in Bermuda: Review with new evidence of a brief rise to present sea level during Substage 5a

Abstract Oxygen-isotope Substage 5e is represented by the Devonshire Formation in Bermuda. The occurrence of these coastal-marine deposits to about 5 m indicates Pleistocene sea-level data in Bermuda do not need tectonic corrections. A few published U-series coral dates suggest that a conglomerate (Spencers Point Formation) may have formed during Substage 5c. Geologic mapping, however, indicates that the eolianite beneath the conglomerate is pre- rather than post-Devonshire Formation in age as was previously thought, and the aminostratigraphy of marine sheels also confirms the correlation of the Spencers Point and Devonshire Formations. Substages 5c and 5a are represented by the Southampton Formation. Large eolianites comprise the bulk of this formation. They are onlapped by a few, small, younger marine (+1 m) and eolian deposits. Aminostratigraphy confirms that these late Southampton age deposits are significantly younger than the Devonshire Formation, and U-series dates indicate a Substage 5a age. The setting of some of these deposits along a protected shoreline precludes discounting their position as anomalous due to large storms. This evidence that sea level in Bermuda was at about present datum late in Substage 5a agrees with interpretations of Neotyrrhenian deposits of Mediterranean shorelines, but not with inferences from the deep-sea oxygen-isotope record, or with data from uplifted coral reefs.

Hydrogeochemistry of Bermuda: A case history of ground-water diagenesis of biocalcarenites

Bermuda is composed of relatively young skeletal limestones currently undergoing diagenesis by the ground water passing through them. The saturated zone consists of separate fresh-water bodies laterally surrounded and underlain by extensive brackish aureoles, in which the meteoric water is mixed with sea water. The meteoric water enters the aquifer after passing through the soil or through marshes (outcrops of the ground-water bodies), in each case causing an influx of CO 2 to the saturated zone. Examination of the ground-water chemistry enables mapping of (a) the extent of mixing of meteoric ground water and sea water; (b) P CO 2 ; (c) the extent of saturation with calcite and aragonite; (d) concentration of Sr; and (e) the amount of calcium and magnesium derived from the limestones. It is concluded that three processes control the chemistry of Bermudian ground water: (a) generation of elevated CO 2 partial pressures in soils and marshes; (b) dissolution of metastable carbonate minerals (principally aragonite); and (c) mixing with sea water. Bermuda ground water apparently approaches a steady state of aragonite dissolution (at slight subsaturation) and concurrent precipitation of calcite cement. Large Sr/Ca ratios in the ground water indicate that the dissolution of aragonite is incongruent. Dissolution is most pronounced near the marshes where CO 2 content is highest. Mixing with sea water is not significant in controlling calcite saturation. Only small amounts of magnesium enter the ground water by incongruent dissolution of magnesium calcite, an apparently slow process on the time scale of passage of the ground water through the saturated zone. All of the waters are well undersaturated with respect to dolomite. It is estimated that the present rate of recrystallization of aragonite to calcite is about 0.32 cm 3 of aragonite to calcite per m 3 of the saturated zone per year. At the present rate of chemical weathering, 360 m 3 of the saturated zone is lost each year through solution and transported to the sea by ground water.

Hydrogeology of Bermuda — Significance of an across-the-island variation in permeability

The distribution of fresh groundwater in Bermuda reflects the lateral partitioning of the saturated zone into two sectors: a band of low-permeability limestones (Paget Formation) along one shoreline, and a band of older, more highly permeable limestones (Belmont Formation) along the opposite shoreline. The fresh groundwater occurs preferentially closer to the shoreline composed of Paget Formation. The an-axisymmetric distribution of fresh groundwater results from the effects of the across-the-island variation of permeability on the interface-bounded lens (the theoretical Ghyben-Herzberg lens) and the transition zone, which, in Bermuda, makes up a sizable fraction of the interface-bounded lens. The variation in size of the interface-bounded lens can be modeled closely by application of Ghyben-Herzberg-Dupuit methodologies to an infinite-strip vertically stratified island — that is, an island partitioned into two strips, one of which is some 14 times as permeable as the other. The areal variation in thickness of the transition zone is similar to that of the amplitude of short-period water-table fluctuations that are generated by changes in ocean level (e.g., from astronomical tides and changes in atmospheric pressure). The amplitude of these water-table fluctuations diminish inland, more so in the Paget sector than in the more permeable, Belmont sector. Thus for a given distance inland of the shoreline, water-table fluctuations are greater and the transition zone is thicker in the Belmont sector than in the Paget sector. In a larger perspective, the occurrence of fresh groundwater in Bermuda reflects the depositional and diagenetic history of the Pleistocene marginal-marine limestones that compose Bermuda. The depositional history involves a seaward accretion of bioclastic eolian ridges (eolianites) formed during successive sea-level highstands of Pleistocene interglacial stages. The diagenetic history includes large-scale erosion by CO2-enriched water; this chemical erosion involves solutional lowering of the landscape and phreatic solution peripheral to marshes. As a result of these processes operating during a period of large-scale sea-level fluctuations, older (intra-Belmont) inter-eolianite depressions have evolved through a marsh stage and are now interior sounds. The result is the across-the-island variation in permeability — solution-altered Belmont rocks adjoining the interior sounds and less permeable younger calcarenites of the Paget Formation along the external sea-facing shoreline.

Hydrology of meteoric diagenesis: Residence time of meteoric ground water in island fresh-water lenses with application to aragonite-calcite stabilization rate in Bermuda

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, τ 0 is easily calculated by Dupuit-Ghyben-Herzberg analysis (DGH). From estimates of the controlling variables (recharge, hydraulic conductivity, and porosity), it is estimated that τ 0 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 τ 0 /2, and it equals τ 0 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.

Chapter 2 – Geology and Hydrogeology of Bermuda

This chapter provides information on the geology and hydrogeology of Bermuda. Bermuda is an isolated group of limestone islands in the western North Atlantic Ocean. The nearest land is Cape Hatteras, North Carolina, USA. The island group consists of more than 150 islands that lie together in an elongate cluster near the southern margin of the shallow, 650 km 2 Bermuda Platform. The islands are visited by some 600,000 tourists per year who come to enjoy the many hotels and cottage colonies, the taxi tours and motorbikes, the friendly environment, and the world-famous carbonate beaches. Bermuda is high on the list of areas in the world in terms of total number of words in geological articles per km 2 of area. The chapter discusses one of the long-standing topics of geologic inquiry in Bermuda—the history of late Pleistocene sea level—and illustrates why one can expect many more words per km 2 to come, even on such much-studied problems.

Predicting the thickness of fresh-water lenses in carbonate paleo-islands

ABSTRACT Fresh-water lenses in carbonate sand and grainstone islands can be modeled using Dupuit-Ghyben-Herzberg methods. The results indicate that the thickness of the lens (H) in a homogeneous, infinite-strip island will be a function of the islands width (a), hydraulic conductivity (K), and the amount of recharge (R). The ratio of R/K in most modern carbonate islands of Holocene sands and Pleistocene rocks is in the range of 10-4 to 10-6. This means H should be 1% to 3% the magnitude of a. However, vertical and lateral variations in K, variations in island shape, the presence of intra-island evaporitic ponds and marshes, and the presence of a mixing zone result in actual H/a ratios that range from 0.2% to about 2%, with 1% being the most representative value. The thic ness of a paleo-lens in an ancient carbonate shoal or reef complex can, therefore, be estimated if the width of the interpreted island can be approximated from facies or isopach maps. An example from the Late Jurassic Smackover Formation in Oaks Field, Louisiana, indicates that the estimated thickness of paleo-lenses is identical to the thickness of the porosity zones in a series of off-lapping oolite shoal complexes. A second example from the Late Permian San Andres Formation in Yates Field, west Texas, reveals that the estimated thickness of paleo-lenses is essentially equivalent to the thickness of a phreatic cave system formed on paleo-islands developed along the San Andres shelf margin. The H/a method for estimating the thickness of paleo-lenses and the possible extent of island-stage fresh-water diagenesis in ancient carbonates is more useful than the Ghyben-Herzberg rule (H = 40 times the elevation of the water table), because the GH rule requires estimation of paleo water-table and paleo-sea level positions to within a couple of centimeters. Such estimates are typically impossible in ancient rocks.

Hydrology of small oceanic islands — Utility of an estimate of recharge inferred from the chloride concentration of the freshwater lenses

Abstract In Bermuda, as in other coastal and oceanic-island environments, rainfall has a significant chloride concentration. As a result, there is a relatively straightforward way of estimating groundwater recharge by considering the Cl− ion as a tracer which is concentrated by evapotranspiration. The Cl− concentration of rainfall in Bermuda is about 15 ppm. That of the freshest part of the largest Ghyben-Herzberg lens is about 60 ppm. Taking the 60 ppm value as an indicator of the Cl− concentration of recharge, the average recharge rate is estimated at 0.25 of the 147-cm/yr. average rainfall, or about 37 cm/yr. This estimate is similar to two other estimates of recharge in Bermuda, each derived from hydrogeologic field data: 1. (1) A 33-cm/yr. estimate inferred from a 2·106-m2 area of diversion in which: (a) outflows (extractions) are 2870 m3/day; and (b) the change in storage is estimated at 1100 m3/day, from the rate of lowering of the water table. 2. (2) An estimate of 35 to 45 cm/yr. resulting from combination of: (a) the ratio of recharge to hydraulic conductivity of the Paget Formation (12·10−6), determined from the configuration of the Ghyben-Herzberg lenses; and (b) the hydraulic conductivity of the Paget Formation (85–100 m/day), estimated from the behavior of the water table. The agreement between the three estimates of recharge indicates that the rather simple and inexpensive technique of calculating recharge from Cl− content of rainfall and fresh groundwater can be a useful addition to the arsenal of techniques by which recharge of small oceanic islands can be evaluated.

Hydrogeology of freshwater lens beneath a Holocene strandplain, Great Exuma, Bahamas

Abstract The beach-ridge strandplain at Ocean Bight, Great Exuma, partially fills a large embayment bordered by the Lucayan Formation (Pleistocene limestone), the bedrock of Great Exuma Island. Seismic refraction profiles indicate that the Holocene sand body rests in an amphitheater-like bedrock bowl beneath the strandplain. Direct current resistivity and electromagnetic surveys show that the lens extends downward into the Lucayan bedrock below much of the strandplain. The lens is thickest along the central, shore-parallel axis of the strandplain; therefore, flow is not only shoreward but also inland. The reason for the inland groundwater flow is a large net water loss (− 0.5 m year −1 according to earlier studies) at inland ponds near the Holocene/Pleistocene contact bounding the strandplain. Dupuit-Ghyben-Herzberg (DGH) modeling using negative recharge at the ponds confirms the explanation for the landward flow and the corollary isolation of the Ocean Bight lens. For the purpose of DGH modeling, the aquifer at the Ocean Bight strandplain can be considered a two-layer (Holocene on Pleistocene) island that conforms to the outline of the strandplain. The buried high-conductivity layer (the Lucayan) reduces the thickness of the lens.