1

THE ORIGIN OF DOLOMITES IN TERTIARY SEDIMENTS FROM THE MARGINOF GREAT BAHAMA BANK

PETER K. SWART AND LESLIE A. MELIM1 2

1-Marine Geology and Geophysics, Rosenstiel School of Marine and AtmosphericScience, University of Miami, Miami, Florida 33149

2- Department of Geology, Western Illinois University, Macomb, IL 61455

AbstractBased on an integrated geochemical characterized by extremely high Sr

and petrographic investigation of dolomites concentrations, which reflect highfrom two cores drilled on Great Bahama concentrations of Sr in the pore fluids. Bank, we have determined three different The high concentrations of Sr in the poremechanisms of formation for the dolomites fluids arise through the continuedwhich are common throughout the Pliocene recrystallization of meta-stable aragonite andand Miocene aged portions of these cores. high-Mg calcite to dolomite and LMC drivenThe first mechanism of dolomitization occurs by the oxidation of organic material byin association with development of non- sulfate. Sulfate reduction not only providesdepositional surfaces. Dolomite typically the thermodynamic drive for recrystallization,forms below each of these surfaces, the but as the absolute concentration of strontiumconcentration and extent of which is governed in the pore fluids is governed by the solubilityby the length of the period of non-deposition. of celestite, allows the Sr /Ca ratio of theThese dolomites are recognized by their interstitial fluid to become much higher thanassociation with the non-depositional normally encountered. The final type ofsurfaces, characteristic heavy oxygen isotopes dolomite is a massive dolomite which occursindicative of formation from cold bottom in coarse grained reefal sediments. Thewaters, and δ O and Sr profiles with depth18

which suggest formation in the presence ofdiffusive temperature and Sr gradients. Thesecond mechanism of dolomitization, occursin pore fluids where the cation and anionprofiles are governed by diffusive processesand forms what we term backgrounddolomite. This is a microsucrosic dolomiteand forms both by the recrystallization of theexisting sediment and precipitation directlyinto void space. Dolomitization by thismechanism uses a local source of Mg and2+

consequently the dolomite never comprisesmore than between 5 and 10% of thesediment. This type of dolomite is c

2+

2+

2+ 2+

pervasive nature of the dolomitization and therelatively normal Sr concentrations, suggestthe circulation of normal marine water in arelatively open system.

INTRODUCTIONIt has long been known that a large

proportion of the rocks in the subsurface ofBahamas are pervasively dolomitized. Thepresence of dolomite was established througha series of cores, up to several hundred metersin thickness, taken through Tertiary sediments(Beach and Ginsburg, 1980; Supko, 1977;Gidman, 1978; Pierson, 1982; Williams,1985). Although numerous modes of

West

ern L

ine

Cay Sal Bank

UndaClino

Swart and Melim

2

Figure 1: Site location map, showing theposition of Clino and Unda near the westernmargin of Great Bahama Bank.

formation have been suggested for these inclined slope deposits overlain by a reef todolomites including mixing-zone (Supko, platform sequence. The upper platform to1977), normal seawater (Swart et al., 1987), reefal interval (197.4 to 21.6 m) consists of 7reflux (Kaldi and Gidman, 1984), and Kohout sequences, each capped by sub-aerial exposureconvection (Simms, 1984), the precise surfaces (Kievman, 1998). The reefal unitmechanism of formation remains uncertain. includes a deeper forereef facies that shallows

This paper reports on the origin of remainder of the core is a 480 m thickdolomite in Tertiary sediments retrieved from sequence of slope sediments composed of fine-two cores, Clino and Unda, drilled near the sand to silt-sized skeletal and non-skeletalwestern margin of Great Bahama Bank (GBB) grains interrupted by intervals of coarse-(Fig. 1). These two cores were drilled as part grained skeletal sands. Three hardgrounds areof the Bahamas Drilling Project on a Western present (256-263, 367, and 536.3 m.), each ofGeophysical seismic line (Eberli et al., 1997) in which represents a break in deposition, theorder to date the seismic sequences identified longest of which (2 to 3 Myrs) occurs at 536.3by Eberli and Ginsburg (1989) and to m. This surface represents the transition frominvestigate the nature of the carbonate the late Miocene to the early Pliocene. Baseddiagenesis in deeper water facies. on a combination of biostratigraphy (Lidz and

BAHAMAS DRILLING PROJECTSUMMARYDrilling Operations

Cores Clino and Unda were obtainedusing a diamond coring system mountedaboard a jackup barge. The two cores werelocated approximately 5 and 13.5 kmrespectively from the edge of Great BahamaBank. They were drilled along a seismicprofile composed of Western Geophysical linesGBB-82-03 and 82-03x previously interpretedby Eberli and Ginsburg (1989). Core Clinowas drilled 677.71 m below the mud pit datum(7.3 m above sea level) and recovery averaged80.8%. Core Unda was drilled 454.15 mbelow mud pit (5.2 m above sea level).Recovery in Unda averaged 82.9% In thispaper all depths are reported in meters asdepths below the mud pit.

Facies and ChronostratigraphyClino, the more distal core, penetrated

to reef and eventually backreef facies. The

McNeill, 1995a, 1995b), magnetostratigraphy(McNeill et al., in press) and strontium isotope

Dolomitization in Great Bahama Bank

3

Figure 2: Summary of the sedimentology, chronostratigraphy, mineralogy, and isotopiccomposition for Clino and Unda. Data are from Eberli et al. (1997), Kenter et al. (Inpress), Lidz et al. (1995a, b), Melim et al. (1995; In Press).

stratigraphy (Swart et al., in press) the Plio- exposure surfaces overlying a reefal unit basedPleistocene boundary can be placed at on a marine firmground (Kievman, 1998).approximately 110 m (Fig. 2). The middle shallow-water unit (354.7 to

Core Unda, the more proximal of the 292.82 m) is a somewhat deeper water reeftwo, consists of three successions of shallow- with platy corals and rhodoliths (Budd andwater platform sands and reefal deposits, that Kievman, in press) overlain by a sub-aerialalternate with sand and silt-sized deeper exposure surface that also is a phosphaticmarginal deposits. The Plio-Pleistocene marine hardground (Melim et al., in press).shallow-water interval (60 to 8.6 m) has 14 The deepest shallow-water unit (454.0 toplatform sequences capped by sub-aerial 443.5 m) consists of shoaling-upward

Swart and Melim

4

packages of coarse-grained skeletal to non- level caused development of a subaerialskeletal grainstones to rudstones (Kenter et al., exposure surface in Unda (at 292.82 m) andin press). The two deeper marginal deposits continued hardground development in Clino.sandwiched between the shallow-water units During the early Pliocene a major sea-level riseare fine-sand to silt-sized grainstones to forced eastward backstepping of the shallow-packstones that alternate with coarse-sand water platform and renewed deeper waterintervals. Hardgrounds occur at 270.36, facies in Unda and formed a condensed292.82, and 393.81 m and a firmground tops interval in Clino (sequence f). The Undathe deeper water facies at 108.1 m. The Plio- subaerial exposure surface (292.82 m) wasPleistocene boundary can be placed at overprinted by marine-hardground diagenesisapproximately 200 m and the Mio-Pliocene during the transgression. Before progradationboundary at 292.82 m (Fig. 2). could bring highstand deposits to the margin

Sequence stratigraphyFacies successions document several The subsequent highstand (sequence e)

hierarchies of changes in relative sea level in resulted in major progradation of the westerncores Clino and Unda (Eberli et al., in press). margin of GBB. The late Pliocene began withThese changes resulted in pulses of a relative sea-level fall then rapid rise, resultingprogradation of the western margin of Great in a hardground (later partly eroded) in ClinoBahama Bank that are seen on seismic lines as (at 367 m) and a firmground in Unda (at 108.1seismic sequences (Eberli et al., in press) and m). In Unda, the following sequence (d) is ain the cores as depositional sequences (Kenter reef, while in Clino a thick package ofet al., in press; Kievman and Ginsburg, in proximal slope facies documents rapidpress). The sequence boundaries are indicated progradation of the margin. An earlyby discontinuity horizons (subaerial exposure Pleistocene lowstand resulted in a lowstandon the platform, marine hardgrounds and reef in Clino and platform top facies andfirmgrounds on the slope), changes in facies subaerial exposure in Unda (sequence c).and changes in diagenesis (Melim et al., in Sequences b and a were deposited during thepress). high frequency, high amplitude sea level

Eight seismic sequences (a – i) record changes of the Pleistocene. The margin ofthe relative sea level changes of the middle GBB was located to the west of the cores byMiocene to Recent (Eberli et al., 1997) (Fig. this time resulting in only highstand platform2). Platform facies of possible middle Miocene facies and numerous subaerial exposureage (sequence i) were deposited during a surfaces in both cores.relative lowstand. The following late Miocenehighstand (sequence h) deposited a thickpackage of deeper margin facies in Unda and Samples were taken at 1.5 m intervalsdeeper slope facies in Clino. Sequence g throughout the two cores for X-ray diffractiondeposited a late Miocene lowstand reef in (XRD) and stable carbon and oxygen isotopicUnda while a marine hardground was forming analysis. All materials were ground to finerin Clino (at 536.3 m). A further drop in sea than 63 µm. For XRD analysis, powdered

locations of Clino and Unda, another sea levelrise further backstepped the platform renewingtransgressive deeper margin and slope facies.

METHODS

Dolomitization in Great Bahama Bank

5

Figure 3: Comparison of 104 peaks forsamples with a single dolomite versussamples with two dolomite peaks.

samples were smear mounted on glass slides.Peak areas for aragonite, calcite and dolomitewere determined using a Scintag XDS-2000diffraction unit. Mineral concentrations werecalculated from peak area ratios assuming thateach sample was composed only of calcite,aragonite, and dolomite (the only otherminerals present are clays (<<5%) and minorcelestite). Peak area ratios were calibrated andthe concentrations calculated using calibrationcurves prepared from results using a series ofpure mineral standards (verified by XRDanalysis). Duplicate analyses indicatereproducibility of ±3%. In order to isolate thedolomite, sieved samples (> 63 µm) weretreated with buffered acetic acid for a periodof 2 hours. This procedure selectively leachesthe less stable minerals leaving the dolomitebehind. This short leaching period was chosen

to allow progressive removal of the CaCO3

components. After each leaching episode, thesamples were re-analyzed by XRD and analiquot was preserved for the determination ofδ C and δ O. This process continued until13 18

only dolomite remained. The number ofleaching episodes varied from 1 to 4, largelycontrolled by the amount of dolomite initiallyin the samples (more initial dolomite gave puredolomite faster).

The stoichiometry of the dolomite wasdetermined by XRD analysis of the separateswith calcium fluoride as an internal standard.A step scan was run from 24 to 60 2 2 with ao

step size of 0.01 , count time of 2 seconds pero

step, source slits of 2 and 4, and receiving slitsof 0.2 and 0.1. Scintag XRD softwareDMSNT version 1.1b was used to identifypeaks and determine peak area, but peakposition corrections using the calcium fluoridepeaks were determined manually. The profilefitting subroutine of the Scintag software fitsa Pearson VII profile to a net intensity filewith background subtracted, but without theK-alpha-2 peaks subtracted. Eight samples ofPaleozoic dolomite were run using the sameoperating conditions to determine the peakshape for a single stoichiometric dolomite.This provided a measure of the FWHM (theFull Width of the peak at Half of theMaximum intensity) to use during peak fittingof multiple dolomite peaks. Figure 3 showsexamples of single, double, and tripledolomite peaks with the Pearson VII profilefits. Mole percent MgCO in the dolomite3

was calculated using the corrected peakpositions and the formula N = 333.33 d -911.99 where N is the mole percent Ca and dis the observed d-spacing for the [104]dolomite peak (Lumsden and Chimahusky,1980).

For δ C and δ O analyses, all13 18

Swart and Melim

6

Figure 4: X-ray mineralogy on samples at approximately 0.3 m interval from Clino andUnda. Also shown are stable isotopic data for the bulk rock (lines; Melim et al., 1995) andthe dolomite separates(symbols, this paper), together with the sedimentology (Kenter et al.,in press), and the chronostratigraphy (McNeill et al., in press). Mineralogy: black =aragonite, white = low-Mg calcite(LMC), grey = dolomite.

samples were dissolved using the common acid transmitted light petrography wasbath method at 90 C and the CO produced supplemented by scanning electron microscopyo

2

analyzed using a Finnigan-MAT 251. and cathodoluminescence. Dolomite wasStandard isobaric corrections were applied,but no correction has been applied for thedifferences in the fractionation of oxygen as aresult of the dissolution of dolomite and calciteby phosphoric acid (Land, 1980; Vahrenkampand Swart, 1994). Data are reported relativeto V-PDB using the conventional notation.

Thin sections were prepared atapproximately 3 meter intervals with closersampling across selected intervals. Standard

identified by staining slabs and/or thin sectionsusing Alizarin red-S (after Dickson, 1965) orTitan Yellow (after Miller, 1988). Allpetrographic descriptions were entered in acomputer database to allow rapid retrieval andcomparison between different sections in thecores.

The strontium concentration of thedolomite separates was determined usingatomic absorption (Perkin-Elmer 4500). In

41 42 43 44 45 460

20

40

60

80

100

Mol % MgCO3

Per

cen

tag

e D

olo

mit

e

0 40 80% Aragonite

0

40

80

% D

olom

iteDolomitization in Great Bahama Bank

7

Figure 6: Cross plot of the relationshipbetween the percentage of dolomite andstoichiometry from Clino and Unda.

Figure 5: Plot of the relationship between theconcentration of aragonite and dolomite inClino and Unda.

this method approximately 100 mg of dolomiteseparate was dissolved in 10% nitric acidsolution, filtered, and the filtrate diluted to 25cm . Corrections were made for the3

percentage of insoluble residue. Standardswere made using specpure CaCO and MgCO3 3

(Johnson-Matthey) weighed out inapproximately the same concentrations ascontained in the samples. Standards were thenspiked with 1000 ppm Sr standard solution toprovide standards with similar intensities to theanalyzed samples. Reproducibility of thismethod is approximately +/-5%.

RESULTSRESULTSX-ray Diffraction MineralogyUnda bulk mineralogy

The mineralogy of core Unda consistsprimarily of aragonite, LMC, and dolomite

(Fig 4). Minor amounts of dolomite areubiquitous below 108 m and increase inabundance beneath hardgrounds andfirmgrounds (sensu Ekdale et al., 1984) at108.07 m, 270.36 m and 393.81 m. Inaddition the sediments comprising the Mioceneplatform and overlying slope facies are 100%dolomitized (263-365 m). Aragonite is also acommon minor component below 108 m.With the exception of the interval beneath thefirmground at 108.07 m, aragonite is <5%(and usually absent) if the dolomite contentexceeds 25% (Figs. 4 and 5). Trace amountsof celestite occur in the deeper watersediments.Clino bulk mineralogy

Core Clino is composed principally ofaragonite, LMC, and dolomite (Fig. 4). Smallconcentrations of celestite are present below150 m. Minor dolomite is present everywherebelow 150 m and increases in abundancebeneath hardgrounds at 256.03 m, 263.65 m,366.98 m and 536.3 m. A firmground at

Swart and Melim

8

197.20 m does not contain dolomite. In of different compositions. Peak fit wasaddition, the increase in dolomite below thesurface at 536.3 m begins 1 m below thesurface. In an interval with very littledolomite, a single skeletal grainstone bed at351.43 m is over 85% dolomite. Aragonite ismuch more common in Clino than in Undawith >40% aragonite common between 230 mand 365 m. As in Unda, intervals with >25%dolomite generally have <5% aragonite withthe exception of the interval beneath thehardground at 263.65 m (Figs. 4 and 5). Inaddition, intervals with >20% aragoniteseldom contain more than 5% dolomite (Figs.4 and 5). However, the concentrations ofaragonite and dolomite are not inverselyrelated, rather they are mutually exclusive athigher concentrations. Of the 1231 samplesanalyzed, <4% contain both >10% aragoniteand >10% dolomite, and most of these occurbeneath two hardgrounds (Clino 263.65 m andUnda 108.07 m; open symbols Fig. 5).Dolomite Stoichiometry

Based on the position of 104 peak, allof the dolomite is calcian-rich with valuesranging from between 41.8 to 45.8 mole %Mg (Fig. 6). The stoichiometry of thedolomites increase with increasing dolomitecontent (Fig. 6, r = 0.31, statistically2

significant at the 99% confidence interval). Atany given dolomite content, the stoichiometryvalues have a range of 1 – 2 mole % Mg (only3 samples exceed this range), which isreflected in the low r value. Although there2

are no overall trends of increasing ordecreasing dolomite stoichiometry with depth,within the 100% dolomitized interval in Unda,there is tendency for the mole% MgCO to3

increase with depth.Most of the dolomite samples show a

broad 104 peak (Fig. 3) that can be resolvedinto 2 or more peaks representing dolomites

calculated using a Pearson VII profile withouta K-alpha 2 correction to verify the secondarypeaks were not artifacts of the correctionmethod (hence the "double" look to the peaksin Figure 3). Samples with a single peak (Fig.3A; N = 50) show smooth sides. Samples withtwo dolomites (N = 127) display a shoulderon either the left (Fig. 3B) or right side (Fig.3C) of the 104 peak. Three samples show amore complex 104 peak that was bestdescribed by three separate peaks. In orderfor two peaks to be resolved, the distancebetween them must be at least 0.03° 2-theta,which translates into .1 mole % difference inthe Mg composition. Because of thislimitation on individual peak resolution, themultiple peaks should be seen as indicating arange of dolomite compositions for eachsample rather than two distinct end-membercompositions.

The average difference between thetwo calculated 104 peaks is 2.1 mole % Mg(standard deviation, 0.5 mole % Mg; range,1.0 to 3.7 mole % Mg). The two dolomitepeaks are usually not of equal dimensions(Fig.3) and the position of the main peak shiftstoward higher stoichiometry with increasingdolomite content. In addition, the position ofthe secondary peak varies with dolomitecontent. This is best seen by comparing thosesamples with <20% dolomite (N = 49) withthose sample having >80% dolomite (N = 21).Of the 49 samples with <20% dolomite, 41have a large peak between 41.5–43.5 mole %Mg with a secondary peak between 44–46mole % Mg (Fig. 3C). In contrast, 17 out of21 samples with >80% dolomite have a largepeak around 44-45 mole % Mg with asecondary peak either at 42–43 mole % Mg orat 46-48 mole % Mg (Fig. 3B).

Dolomitization in Great Bahama Bank

9

Figure 7: Photomicrograph ofcompletely dolomitized grainstone withfabric-preserving dolomite. Note thewell preserved Halimeda and red algaegrains. Sample Unda 961.92 m. Field ofview is 3 mm.

Figure 8: Photomicrograph of completelydolomitized hardground with micriticdolomite. Light colored oval is a boringcoated with iron oxides and phosphate. Sample Unda 887.04 m. Field of view is3 mm.

PetrographyAt least trace amounts of dolomite are micrite or fine microspar but this could have

present throughout most of both cores except occurred prior to, rather than during,

in the upper 100-150 m (Fig. 4). Two nonluminescent.principal textural types (fabric-preserving and A variety of the fabric-preservingmicrosucrosic) have been identified, butintermediate fabrics are also present. It isunlikely that all the dolomite formed at thesame time and some of the variation representsindividual stages in a process that begins withnucleation of dolomite and ends with 100%dolomite. The following discussion, therefore,focuses on describing each fabric and itsdistribution, leaving interpretation of dolomitetiming to a later section. Fabric-preserving dolomite: Fabric-preserving dolomite is found in scatteredlocations in Clino and in the middle reef toplatform section (292.82-360.28 m) of Unda.This form of dolomite occurs exclusively inblocky spar-cemented skeletal grainstone topackstone lithologies (Fig. 7). Samples withfabric-preserving dolomite are always >80% dolomite is a micritic type which occurs bothdolomite, even when found in intervals with as a replacement of micritic grains andminor dolomite (e.g. Clino 351.1 m).

Carbonate needle mud has recrystallized to

dolomitization. Grains with originally veryfine fabric (e.g., red algae) also haverecrystallized to a coarser fabric (Fig. 7).Some dolomitized Halimeda grains retainsome of the brown pleochroism typical ofneomorphic Halimeda elsewhere in thesecores (Melim et al., in press). Thepleochroism in the dolomite is not as dark as inthe calcitic neomorphic spar, but it is distinctfrom the clear dolomitized blocky spar cementinfilling primary pores in the Halimeda. Inaddition to the dolomitized blocky spar, mostof the fabric-preserved rocks also containeuhedral rhombs of dolomite spar partiallyfilling primary and secondary pores. The outerrims of this dolomite spar occasionally showsdull luminescence, while other dolomite is

500

550

600

650

700

1 2 3 4

3

18Oδ o/oo

DolomiteCarbonate

36 C/1000mo

Dep

th (

m)

Swart and Melim

10

Figure 9: Photomicrograph of completelydolomitized slope sediment withmicrosucrosic dolomite. Large pores areprobably molds of bioclasts. Sample Unda924.58 m. Field of view is 1.3 mm.

Figure 10: Oxygen isotopic composition ofdolomites and co-existing calcites belowthe 536.3 m hardground in Clino. Notethe steady increase in the oxygen isotopiccomposition towards the non-depositionalsurface.

apparently as a primary cement associated with dolomite is non-luminescent undermarine hardgrounds (Fig. 8). The most cathodoluminescence.common type of grains replaced are red algae, In Clino, microsucrosic dolomitebut trace amounts of dolomitized micrite rims forms up to 50% of the lower slope faciesand micritized skeletal grains are also presentin the deeper water facies. Micritic dolomitecement is apparently the primary lithificationelement in several hardgrounds. Micriticdolomite formed during hardground formationas it is sometimes directly overlies phosphatecrusts. This dolomite is nonluminescentunder cathodoluminescence.Microsucrosic dolomite: The most pervasivetype of dolomite corresponds to themicrosucrosic variety as described by Dawansand Swart (1988). This dolomite consists ofsmall euhedral rhombs (1 to 40 Fm in size). Itis similar to dolomites commonly found indeep sea cores (Swart and Guzikowski, 1988;Dix and Mullins, 1988). In the lowpermeability, aragonite-rich interval in Clinoaround 300 m, dolomite is <5% of thesediment and occurs only as very fine (<1µm)crystals. More commonly, 10-20% dolomiteis present (Fig. 4) and has a variety ofdolomite crystal sizes (1-30 µm). This

between 536 and 600 m. In Unda, 30-100%microsucrosic dolomite occurs in the upperslope facies (at 240-292.82 m) andthroughout the middle reef to platform section(292.82-360.28 m). Samples with fabricsintermediate between fabric-preserving andmicrosucrosic are also present in the middlereef to platform section, usually in packstones.

The microsucrosic dolomite forms as

-0.5 0 0.5 1 1.5 2 2.5 32.5

3

3.5

4

4.5

5

Coexisiting Carbonate

18Oδ

o/oo

Dol

omite

18O

δo

/oo

1 1.5 2 2.5 3 3.5 42

2.5

3

3.5

4

4.5

5

Dol

omite

Coexisiting Carbonate

13Cδ

o/oo

13C

δo

/oo

Dolomitization in Great Bahama Bank

11

Figure 11: Relationship between the oxygenisotopic composition of the precursor andthe δδ O of the dolomite. There are two18

trends in the data. Above the 536.3 mhardground there appears to be no trend,but below 536.3 (circled data points) thereis a positive correlation suggesting that theprecursor was already diagenetically alteredalong the geothermal gradient prior todolomitization.

Figure 12: Relationship between the carbonisotopic composition of the precursor andthe carbon isotopic composition of thedolomite. The intercept of approximately+1 ‰ corresponds to the expectedequilibrium difference in the δδ C between13

dolomite and calcite (Sheppard andSchwarcz 1970).

both a primary void filling cement and byreplacing fine micritic sediments, red algae andechinoderm grains. Using partiallydolomitized samples, the following sequencehas been identified. The first dolomite formsas very fine (1-10µm) euhedral rhombs withinthe matrix of packstones to wackestones. Thisearly stage also includes replacement of HMCgrains such as echinoderms (usually by a single sucrosic dolomite is not found as the dolomitedolomite crystal) and red algae (as micritic retains a range of crystal sizes rather than thedolomite). Aragonitic skeletal grains are uniform crystal size of true sucrosic dolomite.dissolved to produce molds before 10% This dolomite is nonluminescent underdolomite forms; aragonitic peloids last longer cathodoluminescence.but are dissolved by the time dolomite reachesabout 20%. As the percentage of dolomiteincreases, the size of the rhombs becomeslarger and they impinge on surrounding varies between -6.8 and +5.2‰ (Fig. 4). Themicritic grains. At a composition of 50-70%dolomite, dolomite rhombs 1 to 50µm in sizeoccupy nearly all of the matrix and grow into diagenetically altered by meteoric waters

pores. As dolomitization approaches 100%,most remaining LMC skeletal grains (mainlyForaminifera and molluscs) are dissolvedleaving an open network of 10 to 50µm,subhedral to euhedral rhombic crystals (Fig.9). This dissolution appears to coincide withthe final stage of dolomitization as partiallydissolved LMC grains are common whendolomite content is 90 to 95%. Classic

Stable Oxygen and Carbon IsotopesOxygen: The δ O of the bulk sediments18

lowest δ O values occur in the upper section18

of both cores which have been shown to be

0 500 1000 1500 2000 2500Strontium (ppm)

0

200

400

600

Dep

th (

m)

A B

108

270

367

536

Swart and Melim

12

Figure 13: Concentration of strontium in the dolomites from Clino and Unda. Note thatnear the non-depositional surfaces at 536.3 m in Clino and at 108.07 and 270.36 m inUnda, the concentration of strontium approaches that which we would expect in dolomitesformed from normal seawater. The massively dolomitized interval in Unda (263-365 m)also has values indicative of normal seawater. The eroded hardground at 367 m in Clinohas higher values as does the hardground at 263 m Clino.

(Melim et al., 1995). Zones rich in dolomite (Fig. 10). This corresponds to the knownhave more positive δ O values,18

corresponding to the fact that the dolomites and calcite (Land, 1980). The mean δ Ofrom Clino and Unda are approximately +3‰ values of dolomite separates in Clino andenriched in δ O relative to the limestone18

oxygen isotopic difference between dolomite18

Unda are +3.82 ‰ (+/- 0.38‰) and +3.67‰

40 42 44 46 48 500

500

1000

1500

2000

2500

Dolomite Mol% MgCO

Str

on

tiu

m (

pp

m)

3

Stoichiometry

Fluid Sr/Ca

0 500 1000 1500 2000 25000

10

20

30

40

50

60

70

80

Strontium

% D

olo

mit

eDolomitization in Great Bahama Bank

13

Figure 14: a) Relationship between Sr(determined by AA) and MgCO . The lower line3

represents the relationship between Sr andMgCO identified by Vahrenkamp (1988) and3

Vahrenkamp and Swart (1990). The verticalextent of the data represent changes in the Sr/Caratio of the pore fluids.

Figure 14b) The relationship between thepercentage of dolomite in the sediment andthe Sr concentration of the dolomite.

(+/-0.62‰) respectively. The highest δ O18

values of dolomites are those associated withnon-depositional surfaces. Above the 536.3m hardground in Clino there is norelationship between the δ O of the18

dolomite and the co-existing dolomite (Fig.11). However, below 536.3 m there is apositive relationship between the limestoneand the dolomite. There appears to be nostatistically significant difference betweenthe δ O values of the microsucrosic and18

fabric-preserving dolomites.Carbon: The δ C of the bulk carbonate lies13

between -6.45 and +3.8 ‰ (Fig. 4). Themean δ C of the dolomite from Clino is13

+2.97 ‰(+/-0.49 ‰) and Unda +2.37 ‰(+/-0.61‰). As in the case of the δ O18

values, the lowest δ C values are associated13

with the upper portion of the core affectedby meteoric diagenesis (Fig. 10). Dolomites at the non-depositional surfacestend to have lower δ C values relative to13

sediments above and below the surface. Theδ C of dolomite is positively correlated with13

the δ C of the co-mingled carbonate with an13

intercept of +1‰, approximately equivalent tothe estimated equilibrium difference betweencalcite and dolomite (Sheppard and Schwarcz,1970) (Fig. 12).

StrontiumThe strontium concentration of the

dolomites ranges from 70 to over 2000 ppm(Fig. 13) with the concentration of strontiumbeing inversely related to both thestoichiometry and concentration of dolomite inthe rock (Fig. 14a and b). The lowestconcentrations of Sr occur in dolomites nearnon-depositional surfaces or associated withthe massively dolomitized interval in Unda.The eroded hardgrounds at 367 m in Clino andhardground at 263 m in Unda have slighterelevated values compared to the non-eroded

200 400 600 800 1000 1200Strontium (uM)

500

550

600

650

700

Dep

th (

mbs

f)

Celestite Solubility

Non-depositional surface

Swart and Melim

14

Figure 15: Changes in the estimated strontiumconcentration of the pore fluids below thehardground at 586.3m. Concentrations are based onan estimated increase of 5 mM in the concentrationof Ca over the same interval.2+

surfaces. Trending away from non-depositional surfaces there is a tendency forthe concentration of strontium to increase withdepth (Fig. 13).

DISCUSSION

Although the interpretation of thegeochemistry of dolomite is still equivocal(Land, 1980 and others), the concentration ofstrontium, δ C, and δ O of the dolomites13 18

from Clino and Unda can be used to placeconstraints on the mechanism, source ofmagnesium, and location of dolomiteformation in both Clino and Unda.

Strontium: The observed variation in theconcentration of Sr measured in thesedolomites ranges between 70 and 2378ppm. High concentrations of Sr areparticularly unusual in dolomites(Guzikowski, 1987) and the inverserelationship between the Srconcentration in the dolomite and theoriginal dolomite concentration of thesediment might suggest the presence ofa residual contamination of aragonite orHMC derived from the accompanyingsediments. However, contamination canbe discounted for several reasons. First,the nature of the treatment to whichthese samples were subjected (Seemethods section) meant that the sievedcrushed samples were leached withacetic acid for continually until X-raydiffraction showed them to consistentirely of dolomite. Second, there wasno correlation between the amount ofdolomite and aragonite (Fig. 5). In factmost of the samples with dolomite

contained no aragonite, suggesting that eitherthese samples never contained aragonite, ormore likely that the aragonite was largelydissolved before dolomitization occurred.

The concentration of strontium indolomites formed from normal marine waterslies between 70 to 250 ppm and has beensuggested to be related to the calcian nature ofthe dolomite, (Vahrenkamp and Swart, 1990;Malone et al , 1996). Vahrenkamp and Swart(1990) suggested that the distributioncoefficient (D ) for the incorporation of SrSr

into dolomite varied with the MgCO content3

of the dolomite according to equation 1.

DSr'

Srs%20(x

Cadolomite

Sr

Cafluid

Dolomitization in Great Bahama Bank

15

(1)

In this equation, Sr = the strontium s

concentration of dolomite with an ideal 50:50stoichiometry and x= number of moles ofexcess CaCO . Using a seawater Sr/Ca ratio3

of 8.67 x 10 then a value for D of 0.0165-3Sr

can be calculated assuming a Sr of 70 ppms

(Vahrenkamp and Swart, 1990). Applying this ratios, while with increasing depth the higherequation to the Sr concentration of dolomitesmeasured in this study we are able to estimatedthe Sr/Ca ratios of the pore fluids from whichthe dolomites formed and hence constrain theenvironment of dolomitization.

The massive dolomites in Unda have D based on equation 1 (Vahrenkamp anda mean Sr concentration of 230 ppm. Usingequation 1 and the mean stoichiometry ofdolomites in this interval an equilibrium Srconcentration of 190 ppm would be expectedin these dolomites assuming formation fromnormal seawater. Although it is possible thatthe fluids which dolomitized Unda hadslightly elevated Sr concentrations, we2+

consider that these data suggest that Unda was

dolomitized by seawater with near normalcomposition.

The hard ground dolomites in bothClino and Unda typically show lowerconcentrations of Sr near the hardgroundsurface, but increasing Sr concentrations withdepth (Fig. 13). Using the hardground at536.3 m in Clino as an example, the Srconcentration increases from between 200 and250 ppm in the 30 meters below thehardground to over 1500 ppm at a depth of650 m. The low concentrations of Sr near thehardground surface are consistent withformation from fluids with marine Sr /Ca2+ 2+

Sr concentrations suggest formation fromfluids with increasing Sr /Ca ratios. An2+ 2+

estimate of the Sr concentration in the fluids2+

below the 536.3 m hardground are shown infigure 15. In this calculation we have used a

Sr

Swart, 1990) and assumed an increase in theconcentration of Ca over the thickness of the2+

sequence of 5 mM. The increase of Ca is2+

only an approximation and it is possible thatthe change may be greater than 5 mM asincreases of over 10 mM were noted inporewater retrieved from these sites (Swart etal. In Press). The nature of the estimatedprofile is very similar to the types of Sr2+

profiles observed in deep-sea and peri-platformsediments (Baker et al., 1982; Baker et al.,1982; Swart and Guzikowski, 1988; Swart andBurns, 1990; and others ) and arise from therecrystallization of meta-stable forms ofcalcium carbonate such as high-Mg calcite andaragonite which contain relatively high Srconcentrations compared to low-Mg calciteand dolomite. The limit of the maximumamount of Sr in the pore-fluids has been2+

shown to be dictated by the solubility productof celestite (SrSO ) (Baker and Bloomer,4

38

4244

4648

40

Mol% MgCO 3

100300

500700

900Sr/Ca (x 1000)

400

800

1200

1600

2000

Str

ontiu

m (

ppm

)Swart and Melim

16

Figure 16: Three dimensional plot showing the relationship between mole% MgCO , fluid3

Sr /Ca ratio, and dolomite Sr concentration.2+ 2+

1988). This relationship can be readily seen by the pore waters at this location. In thethe presence of celestite in many cores and in absence of sulfate reduction there is an upperthe ion molar product of Sr and SO in many limit on the estimated Sr concentration of4

2-

ODP sites (Swart and Burns, 1990). Celestitewas detected in both Clino and Unda andtherefore it is likely that similar relationshipsalso control the concentration of strontium in

approximately 600 µM. In the absence ofchanges in the concentration of Ca , such as2+

concentration of Sr in the pore fluids could2+

Dolomitization in Great Bahama Bank

17

form a dolomite with approximately 1500 ppm downward and the local-dissolution of HMCSr.

There are four non-depositionalsurfaces in Clino (256.03 m, 263.65 m,366.98 m and 544 m) and three Unda ( 108.07m, 270.36 m and 393.81 m) where theconcentration of dolomite is at a maximum ator slightly below the surfaces and tends todecrease downward away from the surface(Fig.4). For the three surfaces associated withthe highest concentration of dolomite (Clino367 and 536.3 m and Unda 108.07 m), the Srconcentration of the dolomites below thesurfaces increases downward away from thenon-depositional surfaces (Fig. 13). The bestdeveloped of these trends is at 536.3 m inClino which represents the longest time ofnon-deposition (2-3 My). We suggest that theincrease in the Sr concentration of thedolomite with depth represents formation ofdolomite along a gradient in which Sr is2+

diffusing upward out of the sediments andMg is diffusing downwards from the2+

overlying seawater. Several studies haveshown that concentrations of dolomite similar where the dolomite was found wasto those measured in this study can be formedby Mg diffusing in to the sediments from2+

overlying seawater (Baker and Burns, 1986;Compton and Siever, 1986). Increases in Sr,such as seen in the dolomites closely resemblethe Sr profiles seen in pore water from deep2+

sea and peri-platform sediments (Baker et al.,1982; Swart and Guzikowski, 1988; Swart andBurns, 1990; Swart et al., 1994 and others). Deep within the sediments, Sr is being added2+

to the pore waters through recrystallization ofaragonite and HMC and precipitation of LMC.The Sr subsequently diffuses upwards2+

towards the relatively low concentration ofSr in the overlying seawater. Dolomite2+

forming along this gradient with Mg being2+

supplied both by the diffusion of Mg2+

captures this gradient, thereby constrainingthe timing of dolomite formation . Theslightly higher Sr concentrations found in

dolomite from eroded non-depositionalsurfaces are consistent with this hypothesis aserosion would have removed the dolomitesformed near the interface between theseawater and the underlying sediment withseawater Sr concentrations.2+

Dolomites which are situated wellaway from the non-depositional surfaces inClino and Unda, have the highest Srconcentrations, sometimes in excess of 2000ppm. Based on the previous discussions thesedolomites must have formed from porewaterswith elevated Sr /Ca ratios and in which2+ 2+

there was a significant depletion in sulfateand/or an enrichment in Ca . Using the2+

highest concentration of Sr measured in thedolomites from Clino (2397 ppm), a seawaterCa concentration of 15 mM, then the2+

estimated concentration of Sr in the pore2+

fluids at the time of formation in the interval

approximately 1600 µM. A model of therelationship between stoichiometry, fluidSr /Ca ratio and the Sr concentration in2+ 2+

dolomites is shown in figure 16. In order forthe pore fluids to have such a high Sr2+

concentration, the concentration of SO in42-

the pore fluids needs to be below 10 mMcompared to normal seawater concentrationsof 28 mM. These data suggest that the porefluids, from which these dolomites formed,have experienced significant amounts ofsulfate reduction.

Notwithstanding any potentialinfluence that the removal of sulfate may haveon the kinetics of the dolomitization reaction(Baker and Kastner, 1981), the fact thatsulfate reduction is taking place provides an

Swart and Melim

18

additional mechanism whereby pore waters as the dolomite, that is it is isotopicallyundersaturated with respect to HMC andaragonite can be produced, causing dissolutionand precipitation of LMC and dolomite. Asthere are two calcium carbonate minerals(aragonite and LMC) present with differentsolubilities, once the pore waters becomeundersaturated with respect to aragonite,continued sulfate reduction by the oxidationof organic material is not necessary to drivethe system as the dissolution and precipitationis controlled by the solubility differencebetween the two minerals.

Oxygen Isotopes: The δ O of carbonates18

normally responds to the temperature offormation and the δ O of the water. The18

δ O of the dolomites confirm the methanogenesis (Irwin et al., 1977). The δ C18

interpretation of dolomite formation beneathhardgrounds based on the strontiumconcentrations discussed previously.Dolomites near the non-depositional surfacesare all enriched in O suggesting formation18

from relatively cold bottom waters (Fig. 17).With increasing depth from the non-depositional surface, the δ O of the dolomites18

becomes isotopically more negative reflectingthe normal increase in temperature with depth.Eventually at some distance beneath the non-depositional surface the δ O of the dolomites18

approaches the δ O found in the dolomites18

which contain high concentrations of Sr. Infact the approximate magnitude of thegeothermal gradient can be estimated by usingthe change in δ O with increasing depth (Fig.18

17). This calculation estimates an increase intemperature of approximately 5°C over adepth of 100m, equivalent to a typicalgeothermal gradient over continental crust .

It is interesting to note that thecalculated δ O of the sediment without the18

dolomite, follows the approximate same trend

positive near the hard ground and thendecreases with increasing depth. Thiscovariance of the dolomite and the precursorsuggests that the sediment was altered to LMCprior to dolomitization and supports theconclusion that aragonite alteration waslargely complete prior to dolomitization (Fig.5).

Carbon Isotopes: The δ C of diagenetic13

carbonates principally changes in relationshipto the amount of organic carbon beingoxidized. Lower δ C values therefore are13

usually interpreted as reflecting the input ofoxidized organic carbon, while higher δ C13

values might reflect CO associated with213

of the sediments and dolomites from Clinoand Unda show lower δ C values associated13

with hardground surfaces (Fig. 16). Theselower values do not extend very far below thehardground surface and are probably causedby the oxidation of organic material near thesurface of the hardground. The occurrence ofa depletion in the δ C at hardground surfaces13

is contrary to the current dogma whichsuggests that only subaerial exposure surfacesand not hardgrounds exhibit depletions in theδ C (Allan and Matthews, 1982). A more13

pronounced change in carbon isotopiccomposition occurs above the hardgrounds.This change typically manifests itself as anenrichment (Fig. 17) and is probably a resultof a change in the carbon isotopiccomposition of the sediment as sea level risesand carbonate sediment production on theadjacent carbonate platform is turned onbringing material which is higher in δ C13

compared to the pelagic material which

200

300

400

500

600

Dep

th (

m) 367

536

CO

A

B

Dolomitization in Great Bahama Bank

19

Figure 17: Changes in the carbon and oxygen isotopic compositionof the bulk sediment relative to the hardground surfaces in Clinoand Unda. Note the depletions in carbon isotopic compositionclose to the non-depositional surfaces

dominated the sedimentbelow the non-depositionalsurface (Shinn et al.,1989).

Stoichiometry: Mostdolomites isolated fromClino and Unda containdolomites with severaldifferent Mg/Ca ratios(Fig. 3). The exception tothis are the dolomitesfound immediately belowthe hardgrounds at 367and 536.3 m in Clino and108.8 m in Unda which aremore uniform in theircomposition. Thedolomite in the reefalsec t ion, al thoughcontaining dolomites withmore than onecomposition, containslightly more Mg thanthose in Clino.

Although, it hasbeen suggested thatcalcian dolomites form inassociation with lowersalinity fluids (Lumsdenand Chimahusky, 1980),the origin of thedifferences in dolomite stoichiometry is still amatter of speculation (Morrow, 1982). It isgenerally believed that when first formed,most dolomites are calcian in composition andapproach an ideal composition with increasingage and depth. In the dolomites investigatedin this study, there is no trend with increasingage or burial. However, the trend between thestoichiometry and the percentage of dolomite(Fig. 6) suggests that early formed dolomite

are calcian in composition and that as moredolomite forms, the bulk compositionbecomes more Mg rich. As there is also anincrease in crystal size as the amount ofdolomite increases, there may be an elementof Oswald rippening similar to thatdocumented by Gregg et al. (1992). Althoughnear-surface samples in the study of Gregg etal (1992) did not show a change in

Swart and Melim

20

stoichiometry with crystal size, these workers dolomite, with a concentration of dolomitewere looking at a change from 0.4 Fm to decreasing downward and one in which the Sr1Fm, while in this study there is a change and O concentration of the dolomite mimicsfrom <1Fm to .50Fm. Recrystallization that in the steady state pore water profileduring dolomitization is also supported by the which developed over this time period.shift in the dominant dolomite peak towardmore stoichiometric values in samples withgreater amounts of dolomite (Fig. 3). Thedolomites associated with the hardgroundstend to have more uniform compositionscompared with the rest of the dolomite andalso tend to have a more uniform crystal size(although not necessarily a larger crystal size)perhaps indicating less recrystallization.

SUMMARY

The dolomites which are found in Clino andUnda formed by three different mechanismswhich we will term (i) hardgrounddolomitization, (ii) backgrounddolomitization, and (iii) massivedolomitization.

Hardground DolomitesThese dolomites formed in response to

the presence of a non-depositional surface.The time represented by the period of non-deposition allows Mg from the overlying 2+

seawater to diffuse into the sediments andtherefore the concentration of dolomite isgreatest nearest the non-depositional surfaceand decreases downward. The dolomiteclosest to the surface has the heaviest oxygenisotopic composition, reflecting formation atlow bottom water temperatures, and thelowest concentration of strontium, indicatingfluids with normal seawater Sr /Ca ratios.2+ 2+

The longer the period of non-deposition thegreater the concentration and thickness of thedolomite rich zone. A mature hardgroundwould therefore contain a thick zone of

18

Background DolomitesThe background dolomites comprise

less than 10% of the sediment and possessvery high Sr concentrations, typically in excessof 1000 ppm. In such locations the Mg2+

necessary for dolomite formation is suppliedby that present in the pore fluids and by localdiffusion. The high Sr content of the dolomiteidentifies the region of formation as being anarea characterized by high a Sr /Ca ratio2+ 2+

in the pore fluids and depletion in the SO42-

concentration of the pore fluids, probably as aresult of the oxidation of organic material.

Massive DolomitesThe massive dolomites found in the

middle reefal and overlying deeper marginsections of Unda clearly formed by a differentmechanism and from a different fluid than thedolomites found in the deeper water facies ofClino and Unda. The two principal clues inconstraining their formation are the pervasivedolomitization, the sediments are 100%dolomitized in this interval compared toClino, and the relatively low Sr concentrationscompared to the hardground and backgrounddolomites. The low Sr concentrations may beexplained by the fact that most of thedolomite followed extensive diagenesis ofaragonite to LMC in an open system thatactually removed substantial carbonateforming secondary porosity (Melim et al.,1995). Any elevated Sr concentrationsformed during this earlier aragonite diagenesishad apparently been flushed prior to

Dolomitization in Great Bahama Bank

21

dolomitization. Clearly, a mechanism had to p. 71-82.exist to allow circulation of large quantities ofseawater with normal Sr /Ca ratios to2+ 2+

supply the needed Mg for the extensivedolomitization. Since this massive dolomiteextends up to .260 m, the underlying reef(354.7 to 292.8) was dolomitized at aminimum of 50 to 100 m burial depth. Ourdata do not allow distinguishing between thevarious models for circulating seawater incarbonate platforms.

ACKNOWLEDGEMENTS

The authors would like to thank Dr.R.N. Ginsburg whose ideas provided theinspiration for many of the ideas developed inthis project. Drilling of the BDP holes wassupported by NSF grants OCE-8917295 and9204294. We are also indebted to G. Eberlifor discussion and friendship in this project.This project was supported by a grant fromDOE grant DE-FG05-92ER14253 to G. Eberliand P.K. Swart and the Industrial Associatesof the Comparative SedimentologyLaboratory.

REFERENCES

Allan, J.R. and Matthews, R.K. 1982.Isotope signatures associated with earlymeteoric diagenesis. Sedimentology, v.29, p. 797-818.

Baker, P., and Kastner, M., 1981, Constraintson the formation of sedimentarydolomite: Science, v. 213, p. 214-216.

Baker, P.A., Gieskes, J.M., and Elderfield, H.,1982, Diagenesis of carbonates in deep-sea sediments; evidence from Sr/Caratios and interstitial dissolved Sr data:2+

Journal of Sedimentary Petrology, v. 52,

Baker, P.A. and Burns, S.A. 1985.Occurrence and formation of dolomite inorganic-rich continental marginsediments, Bull. Am. Assoc. Petrol.Geol., v. 69, p. 1917-1930.

Baker, P.A. and Bloomer, 1988 The origin ofcelestite in deep-sea carbonates,Geochim. Cosmochim. Acta, v. 52, p.335-340.

Beach, D. K., and Ginsburg, R. N., 1980,Facies succession, Plio-Pleistocenecarbonates, Northwestern Great BahamaBank. Amer. Ass. Petr. Geol. Bull., v.64, p.1634-1642.

Beach, D.K., 1993, Submarine cementation ofsubsurface Pliocene carbonates from theinterior of Great Bahama Bank. J. Sed.Pet., v. 63, p. 1059-1069.

Budd, A.F. and Kievman, C.M., In Press,Coral assemblages and reef environmentsin the Bahamas Drilling Project Cores.In:SEPM Contributions inSedimentology (ed Ginsburg, R. N.),SEPM.

Compton, J., and Siever, R., 1986, Diffusionand mass balance of Mg during earlydolomite formation MontereyFormation: Geochimica CosmochimicaActa, v. 50, p. 125-136.

Dawans, J. M. & Swart, P. K., 1988. Texturaland geochemical alternations in lateCenozoic Bahamian dolomites,Sedimentology, v. 35, p. 385-403.

Dix, G., and Mullins, H., 1992, Shallow-burialdiagenesis of deep-water carbonates,northern Bahamas:Results from deep-ocean drilling transects: Geol. Soc.America Bull., v. 104, p. 393-415.

Eberli, G.P. and Ginsburg, R.N., 1987.Segmentation and coalescence ofplatforms, Tertiary, NW Great Bahama

Swart and Melim

22

Bank. Geology, v. 15, p. 75-79. 1977, Isotopic evidence for source ofEberli, G.P. and Ginsburg, R.N., 1989. diagenetic carbonates formed during

Cenozoic progradation of NW Great burial of organic-rich sediments: Nature,Bahama Bank - a record of lateral v. 269, p. 209-213.platform growth and sea fluctuations. In: Kaldi, J. and Gidman, J. 1982. EarlyP.D. Crevello et al. (Editors), SEPM diagenetic dolomite cements: ExamplesSpec. Public., v. 44, p. 339-355. from the Permian Lower Magnesian

Eberli, G.P., Swart, P.K. McNeill, D.F., Limestone of England and theKenter, J.A.M., Anselmetti, F.S., Melim, Pleistocene carbonates of the Bahamas,L.A. and Ginsburg, R.N., 1997. A Jour. Sedimen. Petrol., v. 52, p. 1073-synopsis of the Bahamas Drilling 1085.Project: results from two deep core Kenter ,J.A.M., Ginsburg, R.N.,and Treolstra,borings drilled on Great Bahama Bank, S.R., In Press. The western GreatIn: Eberli, G.P., Swart, P.K., Malone, Bahama Bank: sea-level-drivenM. et al. Proc. ODP Init. Repts., v. 166, sedimentation patterns on the slope andp. 23- 41. College Station, TX (Ocean margin In: SEPM Contributions inDrilling Program). Sedimentology (ed Ginsburg, R. N.),

Ekdale, A. A., Bromley, R. G. and Pemberton, SEPM.S. G., 1984, Ichnology. The use of trace Kievman, C. 1998. Match between latefossils in sedimentology and Pleistocene Great Bahama Bank andstratigraphy: Tulsa, OK, SEPM Short deep-sea oxygen isotope records of seaCourse #15, 317 p. level, Geology, v. 26, p. 577-672.

Gidman, J., 1978. Diagenesis of cored Land, L.S., 1980, The isotopic and tracePleistocene carbonates, Great Abaco element geochemistry of dolomite: theIsland, Little Bahama Bank. Unpubl. state of the art: In: SEPM Spec. Publ.Ph.D Thesis, Liverpool. (eds Zenger, D. H., Dunham, J. B. &

Gregg, J. M., Howard, S. A. and Mazzullo, S. Ethington, R. L.) v. 28, p. 87-110.J., 1992, Early diagenetic Lidz, B.H., and McNeill, D.F., 1995a, Deep-recrystallization of Holocene (<3000 sea biostratigraphy of progradingyears old) peritidal dolomites, Ambergris platform margins (Neogene, Bahamas):Cay, Belize: Sedimentology, v. 39, p. Key evidence linked to depositional143-160. rhythm. Mar. Micropal., v. 25, p. 87-

Guzikowski, M., 1987. Evolution of pore fluid 125.chemistry during the recrystallization of Lidz, B.H., and McNeill, D.F., 1995b.periplatform carbonates Bahamas, Reworked Paleogene to early NeogeneUnpublished M.S. thesis University of planktic foraminifera: implications of anMiami, 215p. intriguing distribution at a late Neogene

Haq, B.U., J. Hardenbol, and P.R. Vail, P., prograding margin, Bahamas: Mar.1987, Chronology of fluctuating sea Micropal., v. 25, p. 221-268.levels since the Triassic. Science, Lidz, B.H. and Brawlower, T. J., 1994.235:1156-1167. Microfossil biostratigraphy of

Irwin, H., Curtis, C., and Coleman, M.L., prograding Neogene platform-margin

Dolomitization in Great Bahama Bank

23

carbonates, Bahamas: Age constraints Thesis, University of Miami,and alternatives: Mar. Micropal., v. 23, Sheppard, S.M.F. and Schwarcz, H.P., 1970.p. 265-344. Fractionation of carbon and oxygen

Lumsden, D.N. and Chimahusky, J.S., 1980. isotopes and magnesium betweenRelationship between dolomite non metamorphic calcite and dolomite,stoichiometry and carbonate facies Contrib.Mineralogy Petrology, v. 26, p.parameters. In: Concepts and Models of 161-198.Dolomitization (Ed. Zenger, D.H., Shinn, E.A., Steinen, R.P., Lidz, B.H., Swart,Dunham, J.B., & Ethington, R.L.) Spec. P.K., 1989. Whitings a sedimentologicalPubls. Econ. Paleont. Mineral., Tulsa v. dilemma, Jour. Sedim. Petrol., v. 59, p.28, p. 111-123. 147-161.

Malone, M. J., Baker P.A. and Burns, S.J., Simms, M., 1984. Dolomitization by1996. Recrystallization of dolomite: An groundwater flow systems in carbonateexperimental study from 50-200 C, platforms. Gulf Coast Assoc. Geol.o

Geochimica Cosmochimica Acta, v. 60, Socs., v. 24, p. 411-420.p. 2189-2207. Supko, P.R., 1977, Subsurface dolomites, San

McNeill, D. F., Eberli, G.P., Lidz, B.H., Salvador, Bahamas: Jour. Sedimen.Swart, P.K. and Kenter, J.A.M. In Press. Petrol, v. 47, p. 1063-77.Chronostratigraphy of progradingcarbonate platform margins: A record of Swart, P.K., 1993. The Formation ofdynamic slope sedimentation, Western Dolomite in Sediments from theGreat Bahama Bank, In:SEPM Continental Margin of N.E. Queensland,Contributions in Sedimentology (ed In: McKenzie, J.A., Davies, P.J., Palmer-Ginsburg, R.N.), SEPM. Julson, A. et al., Proc. ODP Sci. Res.,

Melim, L.A., Swart, P.K., and Maliva, R.G., 133: College Station, TX (Ocean1995, Meteoric-like fabrics forming in Drilling Program), 513-524.marine waters: implications for the use Swart, P. K., Ruiz, J., and Holmes, C., 1987,of petrography to identify diagenetic The use of strontium isotopes toenvironments. Geology, v. 23, p. 755- constrain the timing and mode of758. dolomitization of upper Cenozoic

Melim, L. A., Swart, P. K. & Maliva, R. G., In sediments in a core from San Salvador,Press. Meteoric and marine burial Bahamas, Geology, v. 15, p. 262-265.diagenesis in the subsurface of great Swart, P.K. and Guzikowski, M., 1988.Bahama Bank. In: SEPM Contributions Interstitial water chemistry andin Sedimentology (ed Ginsburg, R. N.), diagenesis of periplatform sedimentsSEPM. from the Bahamas, Ocean Drilling

Morrow, D.W. 1982. Diagenesis 2. Dolomite- Program Leg 101. In Austin, J.A. Jr.,part 2 dolomitization models and ancient Schlager, W. Palmer, A.A. et al., Proc.dolostones, Geoscience Canada, v. 9, p. ODP Sci. Results, 101: College Station,95-107. TX, 363-380.

Pierson, P. J., 1982. Cyclic sedimentation, Swart, P.K. and Burns, S.,1990. Pore-waterlimestone diagenesis and. Unpub. Ph.D chemistry and carbonate diagenesis in

Swart and Melim

24

sediments from Leg 115: Indian Ocean, [Unpublished Ph.D. dissertation],Proc. ODP Sci. Results, 115: College University of Miami, Coral Gables, 217Station, TX, 629-645. p.

Swart, P.K., Elderfield, H. and Beets, K., InPress, a. The Sr/ Sr ratios of87 86

carbonates, phosphorites, and fluidscollected during the Bahama DrillingProject cores Clino and Unda:Implications for dating and diagenesis.,In: SEPM Contributions inSedimentology (ed Ginsburg, R. N.),SEPM.

Swart, P.K., Elderfield, H. and Ostlund, G., InPress, b. The geochemistry of pore fluidsfrom bore holes in the Great BahamaBank. In: SEPM Contributions inSedimentology (ed Ginsburg, R. N.),SEPM

Vahrenkamp, V., 1988, Constraints on theformation of platform dolomites: Ageochemical study of late Tertiarydolomite from Little Bahama Bank,Bahamas [unpublished Ph.D. thesis]:University of Miami, Coral Gables, 434p.

Vahrenkamp, V. C., and Swart, P. K., 1990.New distribution coefficient for theincorporation of strontium into dolomiteand its implication for the formation ofancient dolomites, Geology, v. 18, p.387-391.

Vahrenkamp, V.C. and Swart P.K., 1994.Late Cenozoic dolomites of theBahamas: metastable analogues for thegenesis of ancient platform dolomites,Spec. Publs. Int. Ass. Sediment, 21:133-153.

Williams, S. C., 1985, Stratigraphy, faciesevolution, and diagenesis of lateCenozoic limestones and dolomites,Little Bahama Bank, Bahamas: