Fiona Whitaker

... 105 Peter L. Smart, Patricia A. Beddows, Jim Coke, Stefan Doerr, Samantha Smith, and Fiona F. Whitaker 11. Karst of the Mariana Islands: The interaction of tectonics, glacio-eustasy and freshwater/seawater mixing in island carbonates..... ...

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.

... For *Correspondence to: Fiona F. Whitaker, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK. E-mail:Fiona.Whitaker@bristol.ac.uk instance, the classic study of Land et al. ...

Organic-rich mesohaline microbial mats occur in the intertidal zone of a lagoonal area developed to the lee of a coastal spit in Mesaieed, eastern Qatar. The mats grow on a substrate of seagrass-rich dolomitic lime mud with cerithid and monachid gastropods and other small bioclasts, reaching a thickness >20 cm. The mats are well laminated with different microbial communities, from cyanobacteria to sulphur bacteria, reflected in the distinct colour changes from green to pink to brown. The lipids reflect the biomass of the principal mat-building phototrophic and heterotrophic microorganisms. A variety of hydrocarbons, including n-alkanes, diploptene, and isoprenoids such as phytane, phytene, phytadiene and squalene were detected, in varying concentrations within the various mat layers. In particular, n-heptadecane, likely derived from cyanobacteria, dominated the nalkane distribution at a depth of 0-0.1 cm. The concentration and abundance of n-alkanes increase with depth through the mat, likely representing the early diagenetic initiation of hydrocarbon generation. The most representative compounds of sterols and stanols are 5α-cholestan-3β-ol or cholest-5-en-3β-ol but hopanol distribution with trimethyl-5α-cholest-22E-en-3β-ol (dinosterol) is predominant of all neutral fraction compounds. The significant contribution of dinoflagellates to the total biomass is also confirmed by the presence of spheroidal (2-10 μm in diameter) dinocysts. Abundance of C27-29 sterols and stanols decreases with depth whereas concentration of dinosterol (0.4-13 μg g-1TOC) is similar at all depths within the mat. It is very likely that marine organisms (algal phytoplankton) are the major contributor of sterols and stanols to the mats. High concentrations of dinosterol can be related to periodically increased productivity of marine algae during red tides which are common in the Arabian Gulf. Hopanoids are represented in the apolar fraction solely by diploptene and in the polar fraction by hopanols (C30-32) and C27 hopanoid ketone which is most abundant of all compounds eluting in the polar fraction. Hopanols have similar concentrations at almost all depths within the mat. As the C27 ketone is most abundant in dark layers of the mat it is very plausible that it is associated with anoxic conditions. Fatty acids are dominated by saturated, unsaturated and branched fatty acids and are most abundant in the top surface layer of the mat. The distribution of fatty acids provides evidence for the presence of different bacteria including sulphate-reducing bacteria. ‘Phospholipid’ and ‘glycolipid’ fractions reveal the presence of glycerol dialkyl glycerol tetraethers (GDGTs). They are solely dominated by methyl branched (Ia, IIa, IIIa) GDGTs (total 442 μg g-1TOC) and are most abundant in the middle part of the mat (0.9 cm depth). Isoprenoid GDGTs are mostly represented by archaeol and crenarchaeol whose concentrations increase with depth; this is consistent with dark anoxic layers of the mat. In summary, unravelling the early diagenetic alteration of organic matter and its preservation in marine carbonate-evaporite systems, as well as other associated processes such as CaCO3 and dolomite precipitation, could improve our understanding of the hydrocarbon potential of such systems. This will help considerably in the prediction of hydrocarbon occurrence in frontier, as well as mature, petroleum carbonate-evaporite basins.

The conventional model for saline groundwater circulation in coastal carbonate aquifers is that a shallow zone of saline outflow is entrained coastward by the discharge of the overlying fresh water lens, with a compensatory inflow of sea water at depth. However, this model is supported by only a limited number of field observations as in situ monitoring of groundwater circulation remains logistically challenging. Here we present an alternative model based on instrumental records (velocity, salinity, temperature) and dye tracing of ...

The coastal carbonate aquifers of the Yucatan Peninsula (Mexico) and Andros Island (Bahamas) are density stratified, with fresh to brackish water at the surface, and water of marine salinity encountered at depth. Cenotes or inland blue holes are common in both areas, and these are ground-water filled dissolution and collapse shafts which can be used to obtain temperature and salinity profiles to depths in excess of 100 m below the water table. Previous studies in the Yucatan (Stoessell et al. 2002, Geology, 40: 416-424) have ...

Flank margin caves form at sea level through a combination of enhanced dissolution by greater groundwater flux in the distal part of the freshwater lens and undersaturation associated with mixing of saline and freshwater discharging at the island margin (Smart et al., 1988). Measurement of the elevation of distinctive horizontally extensive and vertically restricted phreatic passages relative to the present

ABSTRACT Our understanding of burial diagenesis within carbonates is often limited by poor constraints on available fluid volumes and geochemistry. However, regional stratigraphic and burial history data are often readily available. Using these data to develop numerical models which couple sedimentological and hydrological basin evolution, we estimate the volumes and geochemistry of fluids that were available to drive dolomitisation and Pb-ore genesis within the Carboniferous, Derbyshire Platform of northern England. Current conceptual models of these processes invoke tectonic release of burial-induced overpressure developed within adjacent Dinantian basins as a drive for reactive fluid flow to the platform. Our simulations show that compaction-driven flow may lead to a supply of fluids that is more complex in its temporal evolution than may be expected. Spatial variations in the rate of fluid expulsion from different sediments lead to a staggered delivery of fluids from different sources. Rapid fluid expulsion within deeper sediments leads to a downwards-decreasing pressure gradient that subsequently draws down fluid from within overlying sediments. Thus, early fluid supply to the platform is sourced from the deep basins while later fluid supply descends from above the platform as well as from the sides. We suggest that such a flow development may have important implications for the relative timing and distribution of a sequence of diagenetic products within the platform. This hypothesis is tested using volume estimates from our simulations. We conclude that although this staggered fluid supply model may be applicable generically, it appears that it is only valid for explaining mineralisation in the Derbyshire Platform. Fluid volumes supplied to the platform are insufficient to explain dolomitisation. Our simulations are supported by a sensitivity analysis that identifies that compaction-driven flow in this system is strongly controlled by the rate of burial and sediment permeabilities within the compacting basins.

Abstract: Conch Bar Cave comprises a series of large phreatic chambers open to the surface along the southern margin of an eolianite ridg e.From these, phreatic passages run towards and terminate abruptly in the interior of the ridge, continuing in most cases only as centimeter-sized tubes. The simplest passages are elliptical tubes with relatively horizontal floors and ceilings punctuated by domes. Mos t passagesections are more complex, with remnants of former roof and floor levels. Some of these are bedding-concordant and lithologicallycontrolled. But overall, there are strong modes present in the passage ceiling-elevation data, which suggest that the ceilings indicate pastpositions of the formative mixing zone or water table. The highest level present in the cave is more than 17 m above present se a level, andsuggests either that there has been uplift of the platform or that past sea levels have been above this elevation. The eastern parts of the caveshow submergence, while the western parts are dry. This suggests that there may have been tilting of the platform to the east t oward thedirection of the open-platform margin.Despite the size (>3 km surveyed passages) and antiquity of the cave (U-series age >195 ka U obtained for a speleothem at sea l evel), itspreservation potential in the geological record is low. The cave has been intercepted by surface lowering and back-wasting of t he ridge,and the low mechanical strength of poorly cemented carbonates limits roof stability. Such caves may therefore be only rarely pr eservedas paleokarst cavities, more commonly being represented by residual cliffed embayments and breccia bodies, if at all. Under ice houseconditions with frequent sea level changes, it is unlikely that any clear association between past sea-level still-stands and c ave or enhancedporosity zones could be established from the rock record.

Reactive transport modelling is increasingly deployed to quantitatively evaluate conceptual models of diagenetic processes. However, construction of models of complex systems involves trade‐offs between accuracy and simplification. This tension is explored for models of fault‐associated dolomitisation by sea water convection in a syn‐rift carbonate platform, evaluating the contribution of incorporating stratigraphic growth and fault propagation. Simulations of the high heat flux southern margin of the Derbyshire Platform (Northern England), with heterogeneous matrix permeability that reflects the evolving stratal architecture and burial compaction focusses dolomitisation in more permeable units at all depths. A permeable platform margin fault zone enhances dolomitisation in a broad area on the upper slope and margin, and to a lesser but significant extent, across the interior as platform top waters are entrained and discharge via the fault. Stepwise simulation of flow and reactions during stratigraphic growth suggests that static models over‐predict dolomite abundance in younger sediments and show how regions optimally supplied with reactants and heat to drive dolomite formation migrate vertically and laterally during platform growth. Dolomitisation intensity increases with depth due to greater time for reactions and kinetically favourable temperatures. Adding the fault zone to this model focusses and accelerates flow, giving a more spatially restricted dolostone body and reducing dolomitisation temperature. Changes in fault connectivity with the surface of the evolving platform shift fluid flow pathways and change the rate and temperature of dolomite formation. Results concur with petrographic, isotopic and geochemical observations of the early dolomite on the Derbyshire Platform. This work demonstrates the importance of understanding diagenesis as the product of an evolving set of processes that respond to geological and palaeoenvironmental changes rather than as a sequence of individual diagenetic events. This is particularly critical for reactions, such as dolomitisation by geothermal convection of sea water, which occur over timescales synchronous with platform development.

ABSTRACT Whilst sophisticated multiphase fluid flow models are routinely employed to understand behaviour of oil and gas reservoirs, high-resolution data describing the three-dimensional (3D) distribution of rock characteristics is rarely available to populate models. We present a new approach to developing a quantitative understanding of the effect of individual controls on the distribution of petrophysical properties and their impact on fluid flow. This involves simulating flow through high-detail permeability architectures generated by forward modelling of the coupled depositional-diagenetic evolution of isolated platforms using CARB3D(+). This workflow is exemplified by an investigation of interactions between subsidence and climate, and their expression in spatial variations in reservoir quality in an isolated carbonate platform of similar size and subsidence history to the Triassic Latemar Platform. Dissolutional lowering during subaerial exposure controls platform-top graininess via platform top hydrodynamics during the subsequent transgression. Dissolved carbonate is reprecipitated as cements by percolating meteoric waters. However, associated subsurface meteoric dissolution generates significant secondary porosity under a more humid climate. Slower subsidence enhances diagenetic overprinting during repeated exposure events. Single-phase streamline simulations show how early diagenesis develops more permeable fairways within the finer-grained condensed units that can act as thief zones for flow from the grainier but less diagenetically altered cyclic units.

An understanding of hydrological processes on volcanic islands is vital for both resource and hazard management. The hydrological system can modify the volcanic hazard and react to volcanic perturbations. Understanding this interaction is essential for the development of a truly multi-parameter hazard-monitoring dataset. The Caribbean island of Montserrat provides a unique environment to study such interactions, since it has both active volcanic and hydrological systems. We aim to gain a more complete understanding of the fundamental hydrology in active volcanic island arc settings by using TOUGH2 models to explore the natural productive springs on the flanks of the extinct volcanic center, adjacent to the active Soufriere Hills Volcano.

Reflux of a penesaline (186‰) brine and resulting water-rock interactions were simulated using a modification of TOUGHREACT that incorporates the Pitzer ion-interaction theory. The brine is sourced from a 5 km wide brine pool in the interior of a 25 km wide platform and flows basinward through a 3 km thick sequence of grain-dominated packstone sediments. Ion interactions reduce dolomite supersaturation and anhydrite undersaturation predicted using the Pitzer approach compared with the DebyeHuckel approach, but increase calcite undersaturation. This increased undersaturation, combined with enhanced fluid flow above a shallower zone of anhydrite precipitation, results in more rapid dolomitization despite the lower initial dolomite saturation. The penesaline brines reflux at up to 7 my -1 , three times the maximum flow rate in identical simulations involving a mesohaline (85‰) brine, in direct proportion to the difference in the density gradient. However, dolomitization is an order of magnitude faster for the penesaline brine, with replacement of all limestone to a depth of 43 m within 50 ky. Even after 1 My of penesaline-brine reflux, alteration is largely limited to sediments underlying the brine pool. In contrast, the penesaline brine forms a tabular dolomite body extending some 20 km to the platform margin. The upper 200 m of porous dolomite, lacking significant primary dolomite cements, overlies a thick (>700 m) zone where anhydrite cements plug up to 25% of the porosity. Penesaline brine reflux forms less anhydrite cement, due both to slower anhydrite precipitation (driven by slower upstream dolomitization), and to more .rapid anhydrite dissolution once dolomitization is complete.