Fiona F Whitaker

Active circulation of saline ground waters in carbonate platforms: Evidence from the Great Bahama Bank

Measurements of salinity, temperature, and ground-water discharges within the Great Bahama Bank provide evidence of active circulation of near-normal ocean water beneath North Andros Island. Elevated-salinity waters (38‰-42‰) derived by density refulx from the Great Bahama Bank flow eastward beneath the island and mix with normal-salinity cold ground waters (19-20 °C) from deeper than 250 m in the adjacent oceans. East to west flow of water from the Straits of Florida may be driven by head differences generated by the Florida Current, which impinges on the western margins of the platform. Alternatively, density differences between the reflux and Tongue of the Ocean sea waters may cause more local circulation on the eastern flanks. These flows have important diagenetic implications, particularly in the explanation of pervasive secondary dolomitization widely reported at shallow depths in the Bahamas.

Geochemistry and isotope systematics of sulphur in the mixing zone of Bahamian blue holes

Abstraet--A profile of aqueous S species concentrations and stable isotope compositions is presented for the mixing zone of Cousteaus Blue Hole, North Andros, Bahamas, together with similar, but less detailed results from two other blue holes. These data show that S o and S 2- are produced by bacterially mediated sulphate reduction near the base of the mixing zone and that these species are reoxidized at shallower levels. Acidity generated by oxidation can contribute to corrosiOn of limestone walbrock. Estimated rates of such corrosion range up to 1200 mm wall-rock recession per 10 ka at Cousteaus Blue Hole (equivalent to 16% porosity generation per l0 ka) and are comparable with rates of dissolution caused by inorganic mixing corrosion.

Fate of reflux brines in carbonate platforms

Active reflux may occur during periods of platform-top brine generation, but the role and fate of these brines after reflux events are uncertain. We have used a numerical flow model to investigate and quantify the response of reflux brines to changes in platform-top salinity. Simulations suggest that reflux brines, originally concentrated to gypsum saturation (150‰), have a relatively long platform residence time, on the order of 100 times the duration of the reflux event. When brine-generating conditions cease, brines will continue to sink through the platform, entraining seawater, a variant of reflux circulation we term latent reflux. Mesosaline brines intercepted by drilling of carbonate margins by the Ocean Drilling Program may have originated from Pleistocene reflux event(s) on the adjacent platform top and be currently moving by latent reflux. Latent-reflux circulation could deliver a significant quantity of dissolved reactants to platform carbonates, including Mg for dolomitization.

Numerical modeling of reflux dolomitization in the Grosmont platform complex (Upper Devonian), Western Canada sedimentary basin

The Upper Devonian Grosmont platform in the Western Canada sedimentary basin is a pervasively dolomitized giant heavy-oil reservoir with reserves of 317 billion bbl of bitumen. The principal type of Grosmont platform dolomite formed early and on the basis of stratigraphic and geochemical evidence is interpreted as early diagenetic reflux dolomite. We use a numerical ground-water flow model to investigate the viability of reflux to dolomitize the Grosmont platform. We simulate reflux at four key stages of platform evolution, incorporating the transient effects of changes in platform architecture, rock properties, and the salinity of platform-top waters. The pattern and magnitude of reflux is critically controlled by permeability and the distribution of platform-top brines, which are concentrated up to gypsum saturation. Reflux flow is focused in the relatively permeable carbonates of the Grosmont Formation and is from the platform interior toward the platform margin. The 120-m-thick shales of the Ireton Formation that separate the Grosmont and Cooking Lake formations restrict cross-formational flow and brine transport. During a 100-k.y. period of relative sea level rise and platform-top drowning, brines of reflux origin continue to sink and entrain platform-top waters (latent reflux). Where the intervening aquitards are thin or absent, reefs of the Leduc Formation capture reflux brines from the overlying Grosmont platform and focus cross-formational brine transport. Lateral contrasts in salinity are sufficient to drive a series of free convection cells in the relatively permeable reefs of the Leduc Formation. Computed distributions of fluid flux in conjunction with magnesium mass-balance calculations that incorporate the range of uncertainty, particularly in permeability, support the suggestion that the reflux of gypsum-saturated brines could have formed much if not most of the dolomite in the Grosmont Formation in the 1.6 m.y. available.

Long-term Quaternary uplift rates inferred from limestone caves in Sarawak, Malaysia

ABSTRACTTherateoflong-term(2m.y.)base-levelloweringestimatedinanextensivesequenceoflimestonecavesinSarawak,Malaysia,fromuraniumseries,electronspinresonance,andpaleomagneticdatingis0.19 1 0.03/ 2 0.04m/ka.Thisratehasremainedconstantoveratleastthelast700ka,asshownbycomparisonofthenumberandspacingofwallnotchesformedduringphasesofinterstadialandinterglacialaggradationwithpeaksinthedeep-seaoxygenisotopecurve.Itisarguedthatbase-levelloweringoccursinresponsetoepi-rogenicupliftofthemoreresistantlimestonesduetoregionaldenudationofthesoftershales,andtoflexuralisostacyassociatedwithhighratesofoffshoresedimentation.INTRODUCTION Ratesofupliftareneededtoconstrainmodelsofreliefgeneration(PazzagliaandGardiner,1994)andtectonicprocesses(En-glandandMolnar,1990)overgeologictimescales.Mostestimatesofuplifthavecomefromtectonicallyactivecoasts,whereemer-gentmarineterracescanbedatedusingura-nium series and other techniques (e.g.,Bloometal.,1974),andstudiesofriverter-racesusingradiocarbondating(e.g.,Mer-rittsetal.,1994).However,althoughlong-termsequencesofbothcoastalandriverterraceshavebeenobtained(Pirazzolietal.,1993;PazzagliaandGardiner,1994),thechronologicalcontrolontheseisoftenpoor.Furthermore,fluvialterracesarenotubiq-uitous;theirdevelopmentdependsoncrit-icalinterrelationsbetweenupliftrate,basinarea,andtheeffectsofeustaticsealevel(Merrittsetal.,1994).Thusfewwell-con-strainedestimatesofratesofupliftoverthelongertimescaleareavailable.Herewepro-videestimatesofthelong-termrateofbase-levellowering(Merrittsetal.,1994)derivedfromuraniumseriesandpaleomagneticdat-ingofdepositsfromextensivecavesystemsin the Gunung Mulu National Park,Sarawak,Malaysia.

Hydrology and hydrogeology of the Bahamian archipeligo

This chapter describes the hydrogeology of the Bahamian Archipelago. The Bahamian archipelago includes the separate political units of the Bahamas and the Turks and Caicos Islands. It stretches some 1,000 km from southern Florida to Haiti and covers a total area of 260,000 km 2 . The islands of the Bahamian archipelago and the surrounding banks have been a keystone in the development of depositional models of carbonate sedimentology. There is now increasing awareness of the pivotal role that the hydrology of fresh, mixed, and saline groundwaters may play in controlling the distribution and extent of carbonate diagenesis. The wide range of environments across the archipelago allow examination of a range of extrinsic controls (e.g., climate and island physiography) and intrinsic controls (e.g., sedimentology and mineralogy of depositional facies) on the various groundwater flow systems and the associated diagenesis. Thus the islands of the Bahamian archipelago may also prove to be a keystone of models of carbonate diagenesis.

Reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection

Reactive transport models (RTMs) permit quantitative investigation of diagenesis and its effects on reservoir quality. The RTM TOUGHREACT is used to investigate diagenesis in an isolated platform driven by geothermal (Kohout) convection of seawater, which has been invoked to explain dolomitization during early burial. Previous short (0.1 m.y.) RTM simulations suggested that convection can drive dolomitization, mostly at greater than 50C, and anhydritization, but complete dolomitization requires greater than 30–60 m.y. Our more extended RTM simulations (30 m.y.) indicate significant nonlinearities in the system, consistent with high-temperature experiments, with parts of the platform completely dolomitized within 10–15 m.y. As dolomitization proceeds, the process becomes predominantly flux controlled, with development of a wedge-shaped dolomite body, which thins from the margin to the interior, at considerably shallower depth and cooler temperatures (20–30C) than suggested by short simulations. Dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms. Sediment permeability and reactive surface area, commonly inversely related, are key controls. Dolomitization is limited to the margin of low-permeability muddy platforms despite a high reactive surface area. Dolomitization of more permeable grainy platforms is limited by a lower reactive surface area, occurring only in the platform core due to widespread cooling. Sedimentary layering produces a complex diagenetic stratigraphy, dolomitization favoring more reactive beds at shallow depth where permeability is not limiting, but switching to more permeable beds at depth. Bank-marginal fracturing limits dolomitization of the platform interior, whether the fractures are baffles or conduits for flow.

Dolomitization: from conceptual to numerical models

Abstract Dolomitization requires not only favourable thermodynamic and kinetic conditions, but also a fluid-flow mechanism to transport reactants to and products from the site of dolomitization. This paper reviews work that seeks to provide a quantitative framework for conceptual models of dolomitization, using analytical and, particularly, numerical simulation models of fluid flow and rock-water interaction. This approach is starting to yield new insights into the major controls on the rate and pattern of fluid flux, and the resultant dolomitization. Three sets of forces can drive the fluid flow required for dolomitization: elevation (topographic) head of meteoric water and/or seawater; gradients in fluid density due to variation in salinity and/or temperature; and pressure due to sedimentological and/or tectonic compaction. However, in many situations individual flow mechanisms may not operate in isolation. Rather fluid flow will commonly be a product of a number of different drives acting simultaneously. The balance between drives will change over time with variations in relative sea-level, climate, platform geometry and palaeogeography (which collectively comprise the critical boundary conditions). The simplistic prediction of dolomite body geometry from a single driving force may be misleading, as fluid flow will critically depend both on the boundary conditions and the distribution of permeability. Indeed, even for single driving forces, model predictions change significantly as simplistic assumptions are relaxed and these key parameters are specified with increasing realism. The coupled modelling of dolomitization reactions within the flow field is less tractable than that of groundwater circulation because the kinetics of dolomitization are less well understood, particularly at lower temperatures. Dolomitization is likely to occur along a reaction front, where a favourable balance is struck between mass transport and reaction kinetics. For instance, in simulations of geothermal convection dolomitization focuses along the 50–60 °C isotherm. Dolomitization reactions are favoured by higher temperatures in deeper zones, but rates are limited by low flow because of lower permeability. Although flow rates are higher in shallow more permeable carbonates, lower temperatures limit reactions. High flow rates during reflux of platform-top brines give rapid dolomitization. This is associated with porosity occlusion in front of and behind the broad zone of replacement dolomitization driven by anhydrite cementation and overdolomitization, respectively. Lithological heterogeneities strongly affect the pattern of dolomitization, which is highly focused within more permeable beds and those with a higher reactive surface area. While we focus here on dolomitization, models can also provide insights into diagenetic processes such as marine calcite cementation and aragonite, calcite and evaporite dissolution by refluxing brines, and by seawater circulation below the aragonite and calcite compensation depths. However, it is important to be aware of the assumptions and limitations of the numerical model(s) used. Particular attention must be paid to specification of boundary conditions, permeability and reactive surface area. The uncritical application of numerical techniques to particular cases of dolomitization is at best uninformative and at worst misleading. Careful application of these techniques offers great promise for well-constrained field problems, with greater inclusion of natural heterogeneity and time-variant boundary conditions. We also need to model feedbacks between diagenesis and porosity-permeability, and to include platform growth in simulations of slower diagenetic processes.

COUPLED TWO-DIMENSIONAL DIAGENETIC AND SEDIMENTOLOGICAL MODELING OF CARBONATE PLATFORM EVOLUTION

We have used a new coupled two-dimensional diagenetic and sedimentological model to investigate the evolution of a simple aggrading carbonate platform subject to glacio-eustatic sea-level fluctuations. The model employs variable hydrological zones, internally defined from platform exposure and climate, to provide a framework within which diagenetic processes are simulated at specified rates. Simulations show a clear stacked sequence of diagenetic zones, which are readily recognizable because of their lateral continuity and distinct trends in diagenetic evolution from platform interior to margin. However, spatial associations between unconformity surfaces and underlying diagenetic zones that suggest causality are misleading. The interaction between high amplitude sea-level fluctuations and subsidence results in substantial overprinting and a complex diagenetic history that could not be unraveled by traditional stratigraphic and sedimentological methods.

Numerical modelling of geothermal and reflux circulation in Enewetak Atoll: Implications for dolomitization

Abstract Two types of regional-scale seawater circulation have been proposed to explain the formation of Enewetak Atoll dolomites: geothermal and reflux circulation. We have used a finite element groundwater flow model to examine the pattern, magnitude and dynamic interaction of these two different circulation mechanisms in Enewetak Atoll. Geothermal circulation is concentrated around the atoll-margin whereas refluxing mesosaline brines flow from the atoll interior towards the margin to restrict and eventually shut off geothermal circulation. Refluxing brines of 36–80‰ can account for the salinity signature recorded in dolomite fluid inclusions. Distributions of fluid flux and Mg mass-balance calculations suggest that both geothermal and reflux circulation mechanisms could account for the observed distribution of dolomite in Enewetak Atoll. Furthermore, the atoll interior may be extensively dolomitized as observed in other atolls.