1 the Origin of Dolomites in Tertiary Sediments From

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

PETER K. SWART1 AND LESLIE A. MELIM2

1-Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 2- Department of Geology, Western Illinois University, Macomb, IL 61455

Abstract Based on an integrated geochemical characterized by extremely high Sr and petrographic investigation of dolomites concentrations, which reflect high from two cores drilled on Great Bahama concentrations of Sr 2+ in the pore fluids. Bank, we have determined three different The high concentrations of Sr 2+ in the pore mechanisms of formation for the dolomites fluids arise through the continued which are common throughout the Pliocene recrystallization of meta-stable aragonite and and Miocene aged portions of these cores. high-Mg calcite to dolomite and LMC driven The first mechanism of dolomitization occurs by the oxidation of organic material by in association with development of non- sulfate. Sulfate reduction not only provides depositional surfaces. Dolomite typically the thermodynamic drive for recrystallization, forms below each of these surfaces, the but as the absolute concentration of strontium concentration and extent of which is governed in the pore fluids is governed by the solubility by the length of the period of non-deposition. of celestite, allows the Sr2+/Ca2+ ratio of the These dolomites are recognized by their interstitial fluid to become much higher than association with the non-depositional normally encountered. The final type of surfaces, characteristic heavy oxygen isotopes dolomite is a massive dolomite which occurs indicative of formation from cold bottom in coarse grained reefal sediments. The waters, and d18O and Sr profiles with depth pervasive nature of the dolomitization and the which suggest formation in the presence of relatively normal Sr concentrations, suggest diffusive temperature and Sr gradients. The the circulation of normal marine water in a second mechanism of dolomitization, occurs relatively open system. in pore fluids where the cation and anion profiles are governed by diffusive processes INTRODUCTION and forms what we term background It has long been known that a large dolomite. This is a microsucrosic dolomite proportion of the rocks in the subsurface of and forms both by the recrystallization of the Bahamas are pervasively dolomitized. The existing sediment and precipitation directly presence of dolomite was established through into void space. Dolomitization by this a series of cores, up to several hundred meters mechanism uses a local source of Mg2+ and in thickness, taken through Tertiary sediments consequently the dolomite never comprises (Beach and Ginsburg, 1980; Supko, 1977; more than between 5 and 10% of the Gidman, 1978; Pierson, 1982; Williams, sediment. This type of dolomite is c 1985). Although numerous modes of

1 Swart and Melim

BAHAMAS DRILLING PROJECT SUMMARY e in L rn Drilling Operations te s e W Cores Clino and Unda were obtained using a diamond coring system mounted

Unda Clino aboard a jackup barge. The two cores were located approximately 5 and 13.5 km respectively from the edge of Great Bahama Bank. They were drilled along a seismic

Cay Sal profile composed of Western Geophysical lines Bank GBB-82-03 and 82-03x previously interpreted by Eberli and Ginsburg (1989). Core Clino was drilled 677.71 m below the mud pit datum (7.3 m above sea level) and recovery averaged 80.8%. Core Unda was drilled 454.15 m below mud pit (5.2 m above sea level). Recovery in Unda averaged 82.9% In this paper all depths are reported in meters as Figure 1: Site location map, showing the depths below the mud pit. position of Clino and Unda near the western margin of Great Bahama Bank. Facies and Chronostratigraphy Clino, the more distal core, penetrated formation have been suggested for these inclined slope deposits overlain by a reef to dolomites including mixing-zone (Supko, platform sequence. The upper platform to 1977), normal seawater (Swart et al., 1987), reefal interval (197.4 to 21.6 m) consists of 7 reflux (Kaldi and Gidman, 1984), and Kohout sequences, each capped by sub-aerial exposure convection (Simms, 1984), the precise surfaces (Kievman, 1998). The reefal unit mechanism of formation remains uncertain. includes a deeper forereef facies that shallows to reef and eventually backreef facies. The This paper reports on the origin of remainder of the core is a 480 m thick dolomite 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-skeletal western 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 are of the Bahamas Drilling Project on a Western present (256-263, 367, and 536.3 m.), each of Geophysical seismic line (Eberli et al., 1997) in which represents a break in deposition, the order to date the seismic sequences identified longest of which (2 to 3 Myrs) occurs at 536.3 by Eberli and Ginsburg (1989) and to m. This surface represents the transition from investigate the nature of the carbonate the late Miocene to the early Pliocene. Based diagenesis in deeper water facies. on a combination of biostratigraphy (Lidz and McNeill, 1995a, 1995b), magnetostratigraphy (McNeill et al., in press) and strontium isotope

2 Dolomitization in Great Bahama Bank

Figure 2: Summary of the sedimentology, chronostratigraphy, mineralogy, and isotopic composition for Clino and Unda. Data are from Eberli et al. (1997), Kenter et al. (In press), 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 based Pleistocene 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 reef two, consists of three successions of shallow- with platy corals and rhodoliths (Budd and water platform sands and reefal deposits, that Kievman, in press) overlain by a sub-aerial alternate with sand and silt-sized deeper exposure surface that also is a phosphatic marginal 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 to platform sequences capped by sub-aerial 443.5 m) consists of shoaling-upward

3 Swart and Melim

packages of coarse-grained skeletal to non- level caused development of a subaerial skeletal grainstones to rudstones (Kenter et al., exposure surface in Unda (at 292.82 m) and in 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 rise are fine-sand to silt-sized grainstones to forced eastward backstepping of the shallow- packstones that alternate with coarse-sand water platform and renewed deeper water intervals. Hardgrounds occur at 270.36, facies in Unda and formed a condensed 292.82, and 393.81 m and a firmground tops interval in Clino (sequence f). The Unda the deeper water facies at 108.1 m. The Plio- subaerial exposure surface (292.82 m) was Pleistocene boundary can be placed at overprinted by marine-hardground diagenesis approximately 200 m and the Mio-Pliocene during the transgression. Before progradation boundary at 292.82 m (Fig. 2). could bring highstand deposits to the margin locations of Clino and Unda, another sea level rise further backstepped the platform renewing Sequence stratigraphy transgressive deeper margin and slope facies. Facies successions document several The subsequent highstand (sequence e) hierarchies of changes in relative sea level in resulted in major progradation of the western cores Clino and Unda (Eberli et al., in press). margin of GBB. The late Pliocene began with These changes resulted in pulses of a relative sea-level fall then rapid rise, resulting progradation of the western margin of Great in a hardground (later partly eroded) in Clino Bahama Bank that are seen on seismic lines as (at 367 m) and a firmground in Unda (at 108.1 seismic sequences (Eberli et al., in press) and m). In Unda, the following sequence (d) is a in the cores as depositional sequences (Kenter reef, while in Clino a thick package of et al., in press; Kievman and Ginsburg, in proximal slope facies documents rapid press). The sequence boundaries are indicated progradation of the margin. An early by discontinuity horizons (subaerial exposure Pleistocene lowstand resulted in a lowstand on the platform, marine hardgrounds and reef in Clino and platform top facies and firmgrounds 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 the press). high frequency, high amplitude sea level Eight seismic sequences (a – i) record changes of the Pleistocene. The margin of the relative sea level changes of the middle GBB was located to the west of the cores by Miocene to Recent (Eberli et al., 1997) (Fig. this time resulting in only highstand platform 2). Platform facies of possible middle Miocene facies and numerous subaerial exposure age (sequence i) were deposited during a surfaces in both cores. relative lowstand. The following late Miocene highstand (sequence h) deposited a thick METHODS package of deeper margin facies in Unda and Samples were taken at 1.5 m intervals deeper slope facies in Clino. Sequence g throughout the two cores for X-ray diffraction deposited a late Miocene lowstand reef in (XRD) and stable carbon and oxygen isotopic Unda while a marine hardground was forming analysis. All materials were ground to finer in Clino (at 536.3 m). A further drop in sea than 63 µm. For XRD analysis, powdered

4 Dolomitization in Great Bahama Bank

to allow progressive removal of the CaCO3 components. After each leaching episode, the samples were re-analyzed by XRD and an aliquot was preserved for the determination of d13C and d18O. This process continued until only dolomite remained. The number of leaching episodes varied from 1 to 4, largely controlled by the amount of dolomite initially in the samples (more initial dolomite gave pure dolomite faster). The stoichiometry of the dolomite was determined by XRD analysis of the separates with calcium fluoride as an internal standard. A step scan was run from 24 to 60o 2 2 with a step size of 0.01o, count time of 2 seconds per step, source slits of 2 and 4, and receiving slits of 0.2 and 0.1. Scintag XRD software DMSNT version 1.1b was used to identify Figure 3: Comparison of 104 peaks for peaks and determine peak area, but peak samples with a single dolomite versus position corrections using the calcium fluoride samples with two dolomite peaks. peaks were determined manually. The profile fitting subroutine of the Scintag software fits samples were smear mounted on glass slides. a Pearson VII profile to a net intensity file Peak areas for aragonite, calcite and dolomite with background subtracted, but without the were determined using a Scintag XDS-2000 K-alpha-2 peaks subtracted. Eight samples of diffraction unit. Mineral concentrations were Paleozoic dolomite were run using the same calculated from peak area ratios assuming that operating conditions to determine the peak each sample was composed only of calcite, shape for a single stoichiometric dolomite. aragonite, and dolomite (the only other This provided a measure of the FWHM (the minerals present are clays (<<5%) and minor Full Width of the peak at Half of the celestite). Peak area ratios were calibrated and Maximum intensity) to use during peak fitting the concentrations calculated using calibration of multiple dolomite peaks. Figure 3 shows curves prepared from results using a series of examples of single, double, and triple pure mineral standards (verified by XRD dolomite peaks with the Pearson VII profile analysis). Duplicate analyses indicate fits. Mole percent MgCO3 in the dolomite reproducibility of ±3%. In order to isolate the was calculated using the corrected peak dolomite, sieved samples (> 63 mm) were positions and the formula N = 333.33 d - treated with buffered acetic acid for a period 911.99 where N is the mole percent Ca and d of 2 hours. This procedure selectively leaches is the observed d-spacing for the [104] the less stable minerals leaving the dolomite dolomite peak (Lumsden and Chimahusky, behind. This short leaching period was chosen 1980). For d13C and d18O analyses, all

5 Swart and Melim

Figure 4: X-ray mineralogy on samples at approximately 0.3 m interval from Clino and Unda. Also shown are stable isotopic data for the bulk rock (lines; Melim et al., 1995) and the 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 was o bath method at 90 C and the CO2 produced supplemented by scanning electron microscopy analyzed using a Finnigan-MAT 251. and cathodoluminescence. Dolomite was Standard isobaric corrections were applied, identified by staining slabs and/or thin sections but no correction has been applied for the using Alizarin red-S (after Dickson, 1965) or differences in the fractionation of oxygen as a Titan Yellow (after Miller, 1988). All result of the dissolution of dolomite and calcite petrographic descriptions were entered in a by phosphoric acid (Land, 1980; Vahrenkamp computer database to allow rapid retrieval and and Swart, 1994). Data are reported relative comparison between different sections in the to V-PDB using the conventional notation. cores. Thin sections were prepared at The strontium concentration of the approximately 3 meter intervals with closer dolomite separates was determined using sampling across selected intervals. Standard atomic absorption (Perkin-Elmer 4500). In

6 Dolomitization in Great Bahama Bank

this method approximately 100 mg of dolomite 100 separate was dissolved in 10% nitric acid solution, filtered, and the filtrate diluted to 25

e 80

3 t cm . Corrections were made for the i m

percentage of insoluble residue. Standards o l

o 60

were made using specpure CaCO3 and MgCO3 D

(Johnson-Matthey) weighed out in g a approximately the same concentrations as t n 40 e

contained in the samples. Standards were then c r e

spiked with 1000 ppm Sr standard solution to P 20 provide standards with similar intensities to the analyzed samples. Reproducibility of this 0 method is approximately +/-5%. 41 42 43 44 45 46 Mol % MgCO3 Figure 6: Cross plot of the relationship between the percentage of dolomite and 80 stoichiometry from Clino and Unda. (Fig 4). Minor amounts of dolomite are

e ubiquitous below 108 m and increase in t i

m abundance beneath hardgrounds and o l

o firmgrounds (sensu Ekdale et al., 1984) at D 40 108.07 m, 270.36 m and 393.81 m. In % addition the sediments comprising the Miocene platform and overlying slope facies are 100% dolomitized (263-365 m). Aragonite is also a common minor component below 108 m. With the exception of the interval beneath the 0 firmground at 108.07 m, aragonite is <5% 0 40 80 (and usually absent) if the dolomite content % Aragonite exceeds 25% (Figs. 4 and 5). Trace amounts Figure 5: Plot of the relationship between the of celestite occur in the deeper water concentration of aragonite and dolomite in sediments. Clino and Unda. Clino bulk mineralogy

Core Clino is composed principally of RESULTS aragonite, LMC, and dolomite (Fig. 4). Small RESULTS concentrations of celestite are present below X-ray Diffraction Mineralogy 150 m. Minor dolomite is present everywhere Unda bulk mineralogy below 150 m and increases in abundance The mineralogy of core Unda consists beneath hardgrounds at 256.03 m, 263.65 m, primarily of aragonite, LMC, and dolomite 366.98 m and 536.3 m. A firmground at

7 Swart and Melim

197.20 m does not contain dolomite. In of different compositions. Peak fit was addition, the increase in dolomite below the calculated using a Pearson VII profile without surface at 536.3 m begins 1 m below the a K-alpha 2 correction to verify the secondary surface. In an interval with very little peaks were not artifacts of the correction dolomite, a single skeletal grainstone bed at method (hence the "double" look to the peaks 351.43 m is over 85% dolomite. Aragonite is in Figure 3). Samples with a single peak (Fig. much more common in Clino than in Unda 3A; N = 50) show smooth sides. Samples with with >40% aragonite common between 230 m two dolomites (N = 127) display a shoulder and 365 m. As in Unda, intervals with >25% on either the left (Fig. 3B) or right side (Fig. dolomite generally have <5% aragonite with 3C) of the 104 peak. Three samples show a the exception of the interval beneath the more complex 104 peak that was best hardground at 263.65 m (Figs. 4 and 5). In described by three separate peaks. In order addition, intervals with >20% aragonite for two peaks to be resolved, the distance seldom contain more than 5% dolomite (Figs. between them must be at least 0.03° 2-theta, 4 and 5). However, the concentrations of which translates into .1 mole % difference in aragonite and dolomite are not inversely the Mg composition. Because of this related, rather they are mutually exclusive at limitation on individual peak resolution, the higher concentrations. Of the 1231 samples multiple peaks should be seen as indicating a analyzed, <4% contain both >10% aragonite range of dolomite compositions for each and >10% dolomite, and most of these occur sample rather than two distinct end-member beneath two hardgrounds (Clino 263.65 m and compositions. Unda 108.07 m; open symbols Fig. 5). The average difference between the Dolomite Stoichiometry two calculated 104 peaks is 2.1 mole % Mg Based on the position of 104 peak, all (standard deviation, 0.5 mole % Mg; range, of the dolomite is calcian-rich with values 1.0 to 3.7 mole % Mg). The two dolomite ranging from between 41.8 to 45.8 mole % peaks are usually not of equal dimensions Mg (Fig. 6). The stoichiometry of the (Fig.3) and the position of the main peak shifts dolomites increase with increasing dolomite toward higher stoichiometry with increasing content (Fig. 6, r2 = 0.31, statistically dolomite content. In addition, the position of significant at the 99% confidence interval). At the secondary peak varies with dolomite any given dolomite content, the stoichiometry content. This is best seen by comparing those values have a range of 1 – 2 mole % Mg (only samples with <20% dolomite (N = 49) with 3 samples exceed this range), which is those sample having >80% dolomite (N = 21). reflected in the low r2 value. Although there Of the 49 samples with <20% dolomite, 41 are no overall trends of increasing or have a large peak between 41.5–43.5 mole % decreasing dolomite stoichiometry with depth, Mg with a secondary peak between 44–46 within the 100% dolomitized interval in Unda, mole % Mg (Fig. 3C). In contrast, 17 out of 21 samples with >80% dolomite have a large there is tendency for the mole% MgCO3 to increase with depth. peak around 44-45 mole % Mg with a Most of the dolomite samples show a secondary peak either at 42–43 mole % Mg or broad 104 peak (Fig. 3) that can be resolved at 46-48 mole % Mg (Fig. 3B). into 2 or more peaks representing dolomites

8 Dolomitization in Great Bahama Bank

Petrography Carbonate needle mud has recrystallized to At 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, dolomitization. Grains with originally very fine fabric (e.g., red algae) also have recrystallized to a coarser fabric (Fig. 7). Some dolomitized Halimeda grains retain some of the brown pleochroism typical of neomorphic Halimeda elsewhere in these cores (Melim et al., in press). The pleochroism in the dolomite is not as dark as in the calcitic neomorphic spar, but it is distinct from the clear dolomitized blocky spar cement Figure 7: Photomicrograph of infilling primary pores in the Halimeda. In completely dolomitized grainstone with addition to the dolomitized blocky spar, most fabric-preserving dolomite. Note the of the fabric-preserved rocks also contain well preserved Halimeda and red algae euhedral rhombs of dolomite spar partially grains. Sample Unda 961.92 m. Field of filling primary and secondary pores. The outer view is 3 mm. rims of this dolomite spar occasionally shows dull luminescence, while other dolomite is in the upper 100-150 m (Fig. 4). Two nonluminescent. principal textural types (fabric-preserving and A variety of the fabric-preserving microsucrosic) have been identified, but intermediate fabrics are also present. It is unlikely that all the dolomite formed at the same time and some of the variation represents individual stages in a process that begins with nucleation of dolomite and ends with 100% dolomite. The following discussion, therefore, focuses on describing each fabric and its distribution, leaving interpretation of dolomite timing to a later section. Fabric-preserving dolomite: Fabric- Figure 8: Photomicrograph of completely preserving dolomite is found in scattered dolomitized hardground with micritic locations in Clino and in the middle reef to dolomite. Light colored oval is a boring platform section (292.82-360.28 m) of Unda. coated with iron oxides and phosphate. This form of dolomite occurs exclusively in Sample Unda 887.04 m. Field of view is blocky spar-cemented skeletal grainstone to 3 mm. packstone lithologies (Fig. 7). Samples with fabric-preserving dolomite are always >80% dolomite is a micritic type which occurs both dolomite, even when found in intervals with as a replacement of micritic grains and minor dolomite (e.g. Clino 351.1 m).

9 Swart and Melim apparently as a primary cement associated with dolomite is non-luminescent under marine hardgrounds (Fig. 8). The most cathodoluminescence. common type of grains replaced are red algae, In Clino, microsucrosic dolomite but trace amounts of dolomitized micrite rims forms up to 50% of the lower slope facies and micritized skeletal grains are also present 18 o in the deeper water facies. Micritic dolomite d O /oo cement is apparently the primary lithification 1 2 3 4 element in several hardgrounds. Micritic 500 dolomite formed during hardground formation as it is sometimes directly overlies phosphate crusts. This dolomite is nonluminescent under cathodoluminescence. 550 3 Microsucrosic dolomite: The most pervasive type of dolomite corresponds to the microsucrosic variety as described by Dawans ) m ( and Swart (1988). This dolomite consists of h t small euhedral rhombs (1 to 40 Fm in size). It p 600 e

is similar to dolomites commonly found in D deep sea cores (Swart and Guzikowski, 1988; Dix and Mullins, 1988). In the low permeability, aragonite-rich interval in Clino 650 around 300 m, dolomite is <5% of the sediment and occurs only as very fine (<1µm) Carbonate Dolomite

crystals. More commonly, 10-20% dolomite o is present (Fig. 4) and has a variety of 36 C/1000m dolomite crystal sizes (1-30 µm). This 700 Figure 10: Oxygen isotopic composition of dolomites and co-existing calcites below the 536.3 m hardground in Clino. Note the steady increase in the oxygen isotopic composition towards the non-depositional surface.

between 536 and 600 m. In Unda, 30-100% microsucrosic dolomite occurs in the upper slope facies (at 240-292.82 m) and throughout the middle reef to platform section Figure 9: Photomicrograph of completely (292.82-360.28 m). Samples with fabrics dolomitized slope sediment with intermediate between fabric-preserving and microsucrosic dolomite. Large pores are microsucrosic are also present in the middle probably molds of bioclasts. Sample Unda reef to platform section, usually in packstones. 924.58 m. Field of view is 1.3 mm. The microsucrosic dolomite forms as

10 Dolomitization in Great Bahama Bank

5 pores. As dolomitization approaches 100%, most remaining LMC skeletal grains (mainly 4.5 Foraminifera and molluscs) are dissolved

4 leaving an open network of 10 to 50µm, o o e / t i o m subhedral to euhedral rhombic crystals (Fig. o l O o 8 D

1 3.5

d 9). This dissolution appears to coincide with the final stage of dolomitization as partially 3 dissolved LMC grains are common when dolomite content is 90 to 95%. Classic 2.5 -0.5 0 0.5 1 1.5 2 2.5 3 18 o d O /oo 5 Coexisiting Carbonate

Figure 11: Relationship between the oxygen 4.5 isotopic composition of the precursor and 18 4 o

the d O of the dolomite. There are two o / e t o i 3.5 m C

trends in the data. Above the 536.3 m o l 3 o 1 D hardground there appears to be no trend, d 3 but below 536.3 (circled data points) there is a positive correlation suggesting that the 2.5 precursor was already diagenetically altered 2 1 1.5 2 2.5 3 3.5 4 along the geothermal gradient prior to 13 o d C /oo dolomitization. Coexisiting Carbonate Figure 12: Relationship between the carbon both a primary void filling cement and by isotopic composition of the precursor and replacing fine micritic sediments, red algae and the carbon isotopic composition of the echinoderm grains. Using partially dolomite. The intercept of approximately dolomitized samples, the following sequence +1 ‰ corresponds to the expected has been identified. The first dolomite forms equilibrium difference in the d13C between as very fine (1-10µm) euhedral rhombs within dolomite and calcite (Sheppard and the matrix of packstones to wackestones. This Schwarcz 1970). early stage also includes replacement of HMC grains such as echinoderms (usually by a single sucrosic dolomite is not found as the dolomite dolomite crystal) and red algae (as micritic retains a range of crystal sizes rather than the dolomite). Aragonitic skeletal grains are uniform crystal size of true sucrosic dolomite. dissolved to produce molds before 10% This dolomite is nonluminescent under dolomite forms; aragonitic peloids last longer cathodoluminescence. but are dissolved by the time dolomite reaches about 20%. As the percentage of dolomite Stable Oxygen and Carbon Isotopes increases, the size of the rhombs becomes Oxygen: The d18O of the bulk sediments larger and they impinge on surrounding varies between -6.8 and +5.2‰ (Fig. 4). The micritic grains. At a composition of 50-70% lowest d18O values occur in the upper section dolomite, dolomite rhombs 1 to 50µm in size of both cores which have been shown to be occupy nearly all of the matrix and grow into diagenetically altered by meteoric waters

11 Swart and Melim

0 A B

108

200

270 ) m (

h t 367 p e

D 400

536

600

0 500 1000 1500 2000 2500 Strontium (ppm) Figure 13: Concentration of strontium in the dolomites from Clino and Unda. Note that near the non-depositional surfaces at 536.3 m in Clino and at 108.07 and 270.36 m in Unda, the concentration of strontium approaches that which we would expect in dolomites formed 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 Clino has higher values as does the hardground at 263 m Clino.

(Melim et al., 1995). Zones rich in dolomite (Fig. 10). This corresponds to the known have more positive d18O values, oxygen isotopic difference between dolomite corresponding to the fact that the dolomites and calcite (Land, 1980). The mean d18O from Clino and Unda are approximately +3‰ values of dolomite separates in Clino and enriched in d18O relative to the limestone Unda are +3.82 ‰ (+/- 0.38‰) and +3.67‰

12 Dolomitization in Great Bahama Bank

2500 values of dolomites are those associated with non-depositional surfaces. Above the 536.3 m hardground in Clino there is no 2000 relationship between the d18O of the

) Fluid Sr/Ca

m dolomite and the co-existing dolomite (Fig. p p

( 1500

11). However, below 536.3 m there is a

m positive relationship between the limestone u i t

n 1000 and the dolomite. There appears to be no o r

t statistically significant difference between S the d18O values of the microsucrosic and 500 fabric-preserving dolomites. 13 Stoichiometry Carbon: The d C of the bulk carbonate lies 0 between -6.45 and +3.8 ‰ (Fig. 4). The 40 42 44 46 48 50 mean d13C of the dolomite from Clino is Dolomite Mol% MgCO 3 +2.97 ‰(+/-0.49 ‰) and Unda +2.37 ‰ Figure 14: a) Relationship between Sr (+/-0.61‰). As in the case of the d18O 13 (determined by AA) and MgCO3. The lower line values, the lowest d C values are associated represents the relationship between Sr and with the upper portion of the core affected MgCO3 identified by Vahrenkamp (1988) and by meteoric diagenesis (Fig. 10). Vahrenkamp and Swart (1990). The vertical Dolomites at the non-depositional surfaces extent of the data represent changes in the Sr/Ca tend to have lower d13C values relative to ratio of the pore fluids. sediments above and below the surface. The d13C of dolomite is positively correlated with 80 the d13C of the co-mingled carbonate with an 70 intercept of +1‰, approximately equivalent to 60 the estimated equilibrium difference between calcite and dolomite (Sheppard and Schwarcz, e t i 50 1970) (Fig. 12). m o l 40 o

D Strontium 30 % The strontium concentration of the 20 dolomites ranges from 70 to over 2000 ppm (Fig. 13) with the concentration of strontium 10 being inversely related to both the 0 stoichiometry and concentration of dolomite in 0 500 1000 1500 2000 2500 the rock (Fig. 14a and b). The lowest Strontium concentrations of Sr occur in dolomites near Figure 14b) The relationship between the non-depositional surfaces or associated with percentage of dolomite in the sediment and the massively dolomitized interval in Unda. the Sr concentration of the dolomite. The eroded hardgrounds at 367 m in Clino and (+/-0.62‰) respectively. The highest d18O hardground at 263 m in Unda have slighter elevated values compared to the non-eroded

13 Swart and Melim

500 Celestite Solubility Strontium: The observed variation in the concentration of Sr measured in these dolomites ranges between 70 and 2378 Non-depositional surface ppm. High concentrations of Sr are 550 particularly unusual in dolomites (Guzikowski, 1987) and the inverse ) f

s relationship between the Sr b

m concentration in the dolomite and the (

600 h

t original dolomite concentration of the p

e sediment might suggest the presence of D a residual contamination of aragonite or 650 HMC derived from the accompanying sediments. However, contamination can be discounted for several reasons. First, the nature of the treatment to which 700 these samples were subjected (See 200 400 600 800 1000 1200 methods section) meant that the sieved Strontium (uM) crushed samples were leached with Figure 15: Changes in the estimated strontium acetic acid for continually until X-ray concentration of the pore fluids below the diffraction showed them to consist hardground at 586.3m. Concentrations are based on entirely of dolomite. Second, there was an estimated increase of 5 mM in the concentration no correlation between the amount of of Ca2+ over the same interval. dolomite and aragonite (Fig. 5). In fact most of the samples with dolomite surfaces. Trending away from non- contained no aragonite, suggesting that either depositional surfaces there is a tendency for these samples never contained aragonite, or the concentration of strontium to increase with more likely that the aragonite was largely depth (Fig. 13). dissolved before dolomitization occurred. The concentration of strontium in dolomites formed from normal marine waters lies between 70 to 250 ppm and has been DISCUSSION suggested to be related to the calcian nature of the dolomite, (Vahrenkamp and Swart, 1990; Although the interpretation of the Malone et al , 1996). Vahrenkamp and Swart geochemistry of dolomite is still equivocal (1990) suggested that the distribution (Land, 1980 and others), the concentration of coefficient (DSr) for the incorporation of Sr 13 18 strontium, d C, and d O of the dolomites into dolomite varied with the MgCO3 content from Clino and Unda can be used to place of the dolomite according to equation 1. constraints on the mechanism, source of magnesium, and location of dolomite formation in both Clino and Unda.

14 Dolomitization in Great Bahama Bank

Sr %20(x dolomitized by seawater with near normal s composition. Ca The hard ground dolomites in both D ' dolomite (1) Sr Sr Clino and Unda typically show lower Ca concentrations of Sr near the hardground fluid surface, but increasing Sr concentrations with depth (Fig. 13). Using the hardground at 536.3 m in Clino as an example, the Sr

In this equation, Srs = the strontium concentration increases from between 200 and concentration of dolomite with an ideal 50:50 250 ppm in the 30 meters below the stoichiometry and x= number of moles of hardground to over 1500 ppm at a depth of excess CaCO3. Using a seawater Sr/Ca ratio 650 m. The low concentrations of Sr near the -3 of 8.67 x 10 then a value for DSr of 0.0165 hardground surface are consistent with 2+ 2+ can be calculated assuming a Srs of 70 ppm formation from fluids with marine Sr /Ca (Vahrenkamp and Swart, 1990). Applying this ratios, while with increasing depth the higher equation to the Sr concentration of dolomites Sr concentrations suggest formation from measured in this study we are able to estimated fluids with increasing Sr2+/Ca2+ ratios. An the Sr/Ca ratios of the pore fluids from which estimate of the Sr2+ concentration in the fluids the dolomites formed and hence constrain the below the 536.3 m hardground are shown in environment of dolomitization. figure 15. In this calculation we have used a

The massive dolomites in Unda have DSr based on equation 1 (Vahrenkamp and a mean Sr concentration of 230 ppm. Using Swart, 1990) and assumed an increase in the equation 1 and the mean stoichiometry of concentration of Ca2+ over the thickness of the dolomites in this interval an equilibrium Sr sequence of 5 mM. The increase of Ca 2+ is concentration of 190 ppm would be expected only an approximation and it is possible that in these dolomites assuming formation from the change may be greater than 5 mM as normal seawater. Although it is possible that increases of over 10 mM were noted in the fluids which dolomitized Unda had porewater retrieved from these sites (Swart et slightly elevated Sr2+ concentrations, we al. In Press). The nature of the estimated consider that these data suggest that Unda was profile is very similar to the types of Sr2+ profiles observed in deep-sea and peri-platform sediments (Baker et al., 1982; Baker et al., 1982; Swart and Guzikowski, 1988; Swart and Burns, 1990; and others ) and arise from the recrystallization of meta-stable forms of calcium carbonate such as high-Mg calcite and aragonite which contain relatively high Sr concentrations compared to low-Mg calcite and dolomite. The limit of the maximum amount of Sr 2+ in the pore-fluids has been shown to be dictated by the solubility product

of celestite (SrSO4) (Baker and Bloomer,

15 Swart and Melim

2000 ) m p

p 1600 (

m u i

t 1200 n o r t

S 800

400

900 48 46 S 700 r/ 44 Ca 500 42 (x CO 3 1 300 40 Mg 00 38 l% 0) 100 Mo

Figure 16: Three dimensional plot showing the relationship between mole% MgCO3, fluid Sr2+/Ca2+ ratio, and dolomite Sr concentration.

1988). This relationship can be readily seen by the pore waters at this location. In the the presence of celestite in many cores and in absence of sulfate reduction there is an upper 2- the ion molar product of Sr and SO4 in many limit on the estimated Sr concentration of ODP sites (Swart and Burns, 1990). Celestite approximately 600 mM. In the absence of was detected in both Clino and Unda and changes in the concentration of Ca2+, such as therefore it is likely that similar relationships concentration of Sr2+ in the pore fluids could also control the concentration of strontium in

16 Dolomitization in Great Bahama Bank

form a dolomite with approximately 1500 ppm downward and the local-dissolution of HMC Sr. captures this gradient, thereby constraining There are four non-depositional the timing of dolomite formation . The surfaces in Clino (256.03 m, 263.65 m, slightly higher Sr concentrations found in 366.98 m and 544 m) and three Unda ( 108.07 dolomite from eroded non-depositional m, 270.36 m and 393.81 m) where the surfaces are consistent with this hypothesis as concentration of dolomite is at a maximum at erosion would have removed the dolomites or slightly below the surfaces and tends to formed near the interface between the decrease downward away from the surface seawater and the underlying sediment with (Fig.4). For the three surfaces associated with seawater Sr2+ concentrations. the highest concentration of dolomite (Clino Dolomites which are situated well 367 and 536.3 m and Unda 108.07 m), the Sr away from the non-depositional surfaces in concentration of the dolomites below the Clino and Unda, have the highest Sr surfaces increases downward away from the concentrations, sometimes in excess of 2000 non-depositional surfaces (Fig. 13). The best ppm. Based on the previous discussions these developed of these trends is at 536.3 m in dolomites must have formed from porewaters Clino which represents the longest time of with elevated Sr2+/Ca2+ ratios and in which non-deposition (2-3 My). We suggest that the there was a significant depletion in sulfate increase in the Sr concentration of the and/or an enrichment in Ca2+. Using the dolomite with depth represents formation of highest concentration of Sr measured in the dolomite along a gradient in which Sr2+ is dolomites from Clino (2397 ppm), a seawater diffusing upward out of the sediments and Ca2+ concentration of 15 mM, then the Mg2+ is diffusing downwards from the estimated concentration of Sr2+ in the pore overlying seawater. Several studies have fluids at the time of formation in the interval shown that concentrations of dolomite similar where the dolomite was found was to those measured in this study can be formed approximately 1600 mM. A model of the by Mg2+ diffusing in to the sediments from relationship between stoichiometry, fluid overlying seawater (Baker and Burns, 1986; Sr2+/Ca2+ ratio and the Sr concentration in Compton and Siever, 1986). Increases in Sr, dolomites is shown in figure 16. In order for such as seen in the dolomites closely resemble the pore fluids to have such a high Sr2+ 2+ 2- the Sr profiles seen in pore water from deep concentration, the concentration of SO4 in sea and peri-platform sediments (Baker et al., the pore fluids needs to be below 10 mM 1982; Swart and Guzikowski, 1988; Swart and compared to normal seawater concentrations Burns, 1990; Swart et al., 1994 and others). of 28 mM. These data suggest that the pore Deep within the sediments, Sr2+ is being added fluids, from which these dolomites formed, to the pore waters through recrystallization of have experienced significant amounts of aragonite and HMC and precipitation of LMC. sulfate reduction. The Sr 2+ subsequently diffuses upwards Notwithstanding any potential towards the relatively low concentration of influence that the removal of sulfate may have Sr2+ in the overlying seawater. Dolomite on the kinetics of the dolomitization reaction forming along this gradient with Mg2+ being (Baker and Kastner, 1981), the fact that supplied both by the diffusion of Mg2+ sulfate reduction is taking place provides an

17 Swart and Melim additional mechanism whereby pore waters as the dolomite, that is it is isotopically undersaturated with respect to HMC and positive near the hard ground and then aragonite can be produced, causing dissolution decreases with increasing depth. This and precipitation of LMC and dolomite. As covariance of the dolomite and the precursor there are two calcium carbonate minerals suggests that the sediment was altered to LMC (aragonite and LMC) present with different prior to dolomitization and supports the solubilities, once the pore waters become conclusion that aragonite alteration was undersaturated with respect to aragonite, largely complete prior to dolomitization (Fig. continued sulfate reduction by the oxidation 5). of organic material is not necessary to drive the system as the dissolution and precipitation Carbon Isotopes: The d13C of diagenetic is controlled by the solubility difference carbonates principally changes in relationship between the two minerals. to the amount of organic carbon being oxidized. Lower d13C values therefore are Oxygen Isotopes: The d18O of carbonates usually interpreted as reflecting the input of normally responds to the temperature of oxidized organic carbon, while higher d13C 18 formation and the d O of the water. The values might reflect CO2 associated with d18O of the dolomites confirm the methanogenesis (Irwin et al., 1977). The d13C interpretation of dolomite formation beneath of the sediments and dolomites from Clino hardgrounds based on the strontium and Unda show lower d13C values associated concentrations discussed previously. with hardground surfaces (Fig. 16). These Dolomites near the non-depositional surfaces lower values do not extend very far below the are all enriched in 18O suggesting formation hardground surface and are probably caused from relatively cold bottom waters (Fig. 17). by the oxidation of organic material near the With increasing depth from the non- surface of the hardground. The occurrence of depositional surface, the d18O of the dolomites a depletion in the d13C at hardground surfaces becomes isotopically more negative reflecting is contrary to the current dogma which the normal increase in temperature with depth. suggests that only subaerial exposure surfaces Eventually at some distance beneath the non- and not hardgrounds exhibit depletions in the depositional surface the d18O of the dolomites d13C (Allan and Matthews, 1982). A more approaches the d18O found in the dolomites pronounced change in carbon isotopic which contain high concentrations of Sr. In composition occurs above the hardgrounds. fact the approximate magnitude of the This change typically manifests itself as an geothermal gradient can be estimated by using enrichment (Fig. 17) and is probably a result the change in d18O with increasing depth (Fig. of a change in the carbon isotopic 17). This calculation estimates an increase in composition of the sediment as sea level rises temperature of approximately 5°C over a and carbonate sediment production on the depth of 100m, equivalent to a typical adjacent carbonate platform is turned on geothermal gradient over continental crust . bringing material which is higher in d13C It is interesting to note that the compared to the pelagic material which calculated d18O of the sediment without the dolomite, follows the approximate same trend

18 Dolomitization in Great Bahama Bank dominated the sediment below the non-depositional surface (Shinn et al., 1989).

Stoichiometry: Most dolomites isolated from Clino and Unda contain dolomites with several different Mg/Ca ratios O C (Fig. 3). The exception to 200 this are the dolomites found immediately below the hardgrounds at 367 and 536.3 m in Clino and 300 108.8 m in Unda which are more uniform in their composition. The ) 367 m B (

dolomite in the reefal h

t 400 p section, although e containing dolomites with D more than one composition, contain 500 slightly more Mg than those in Clino. 536 Although, it has A been suggested that 600 calcian dolomites form in Figure 17: Changes in the carbon and oxygen isotopic composition association with lower of the bulk sediment relative to the hardground surfaces in Clino salinity fluids (Lumsden and Unda. Note the depletions in carbon isotopic composition and Chimahusky, 1980), close to the non-depositional surfaces the origin of the differences in dolomite stoichiometry is still a matter of speculation (Morrow, 1982). It is are calcian in composition and that as more generally believed that when first formed, dolomite forms, the bulk composition most dolomites are calcian in composition and becomes more Mg rich. As there is also an approach an ideal composition with increasing increase in crystal size as the amount of age and depth. In the dolomites investigated dolomite increases, there may be an element in this study, there is no trend with increasing of Oswald rippening similar to that age or burial. However, the trend between the documented by Gregg et al. (1992). Although stoichiometry and the percentage of dolomite near-surface samples in the study of Gregg et (Fig. 6) suggests that early formed dolomite al (1992) did not show a change in

19 Swart and Melim

stoichiometry with crystal size, these workers dolomite, with a concentration of dolomite were looking at a change from 0.4 Fm to decreasing downward and one in which the Sr 1Fm, while in this study there is a change and 18O concentration of the dolomite mimics from <1Fm to .50Fm. Recrystallization that in the steady state pore water profile during dolomitization is also supported by the which developed over this time period. shift in the dominant dolomite peak toward more stoichiometric values in samples with Background Dolomites greater amounts of dolomite (Fig. 3). The The background dolomites comprise dolomites associated with the hardgrounds less than 10% of the sediment and possess tend to have more uniform compositions very high Sr concentrations, typically in excess compared with the rest of the dolomite and of 1000 ppm. In such locations the Mg2+ also tend to have a more uniform crystal size necessary for dolomite formation is supplied (although not necessarily a larger crystal size) by that present in the pore fluids and by local perhaps indicating less recrystallization. diffusion. The high Sr content of the dolomite identifies the region of formation as being an SUMMARY area characterized by high a Sr2+/Ca2+ ratio 2- in the pore fluids and depletion in the SO4 The dolomites which are found in Clino and concentration of the pore fluids, probably as a Unda formed by three different mechanisms result of the oxidation of organic material. which we will term (i) hardground dolomitization, (ii) background dolomitization, and (iii) massive Massive Dolomites dolomitization. The massive dolomites found in the middle reefal and overlying deeper margin Hardground Dolomites sections of Unda clearly formed by a different These dolomites formed in response to mechanism and from a different fluid than the the presence of a non-depositional surface. dolomites found in the deeper water facies of The time represented by the period of non- 2+ Clino and Unda. The two principal clues in deposition allows Mg from the overlying constraining their formation are the pervasive seawater to diffuse into the sediments and dolomitization, the sediments are 100% therefore the concentration of dolomite is dolomitized in this interval compared to greatest nearest the non-depositional surface Clino, and the relatively low Sr concentrations and decreases downward. The dolomite compared to the hardground and background closest to the surface has the heaviest oxygen dolomites. The low Sr concentrations may be isotopic composition, reflecting formation at explained by the fact that most of the low bottom water temperatures, and the dolomite followed extensive diagenesis of lowest concentration of strontium, indicating 2+ 2+ aragonite to LMC in an open system that fluids with normal seawater Sr /Ca ratios. actually removed substantial carbonate The longer the period of non-deposition the forming secondary porosity (Melim et al., greater the concentration and thickness of the 1995). Any elevated Sr concentrations dolomite rich zone. A mature hardground formed during this earlier aragonite diagenesis would therefore contain a thick zone of had apparently been flushed prior to

20 Dolomitization in Great Bahama Bank

dolomitization. Clearly, a mechanism had to p. 71-82. exist to allow circulation of large quantities of Baker, P.A. and Burns, S.A. 1985. seawater with normal Sr2+/Ca2+ ratios to Occurrence and formation of dolomite in supply the needed Mg for the extensive organic-rich continental margin dolomitization. Since this massive dolomite sediments, Bull. Am. Assoc. Petrol. extends up to .260 m, the underlying reef Geol., v. 69, p. 1917-1930. (354.7 to 292.8) was dolomitized at a Baker, P.A. and Bloomer, 1988 The origin of minimum of 50 to 100 m burial depth. Our celestite in deep-sea carbonates, data do not allow distinguishing between the Geochim. Cosmochim. Acta, v. 52, p. various models for circulating seawater in 335-340. carbonate platforms. Beach, D. K., and Ginsburg, R. N., 1980, Facies succession, Plio-Pleistocene carbonates, Northwestern Great Bahama ACKNOWLEDGEMENTS Bank. Amer. Ass. Petr. Geol. Bull., v. 64, p.1634-1642. The authors would like to thank Dr. Beach, D.K., 1993, Submarine cementation of R.N. Ginsburg whose ideas provided the subsurface Pliocene carbonates from the inspiration for many of the ideas developed in interior of Great Bahama Bank. J. Sed. this project. Drilling of the BDP holes was Pet., v. 63, p. 1059-1069. supported by NSF grants OCE-8917295 and Budd, A.F. and Kievman, C.M., In Press, 9204294. We are also indebted to G. Eberli Coral assemblages and reef environments for discussion and friendship in this project. in the Bahamas Drilling Project Cores. This project was supported by a grant from In:SEPM Contributions in DOE grant DE-FG05-92ER14253 to G. Eberli Sedimentology (ed Ginsburg, R. N.), and P.K. Swart and the Industrial Associates SEPM. of the Comparative Sedimentology Compton, J., and Siever, R., 1986, Diffusion Laboratory. and mass balance of Mg during early dolomite formation Monterey REFERENCES Formation: Geochimica Cosmochimica Acta, v. 50, p. 125-136. Allan, J.R. and Matthews, R.K. 1982. Dawans, J. M. & Swart, P. K., 1988. Textural Isotope signatures associated with early and geochemical alternations in late meteoric diagenesis. Sedimentology, v. Cenozoic Bahamian dolomites, 29, p. 797-818. Sedimentology, v. 35, p. 385-403. Baker, P., and Kastner, M., 1981, Constraints Dix, G., and Mullins, H., 1992, Shallow-burial on the formation of sedimentary diagenesis of deep-water carbonates, dolomite: Science, v. 213, p. 214-216. northern Bahamas:Results from deep- Baker, P.A., Gieskes, J.M., and Elderfield, H., ocean drilling transects: Geol. Soc. 1982, Diagenesis of carbonates in deep- America Bull., v. 104, p. 393-415. sea sediments; evidence from Sr/Ca Eberli, G.P. and Ginsburg, R.N., 1987. 2+ ratios and interstitial dissolved Sr data: Segmentation and coalescence of Journal of Sedimentary Petrology, v. 52, platforms, Tertiary, NW Great Bahama

21 Swart and Melim

Bank. Geology, v. 15, p. 75-79. 1977, Isotopic evidence for source of Eberli, 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. Early P.D. Crevello et al. (Editors), SEPM diagenetic dolomite cements: Examples Spec. Public., v. 44, p. 339-355. from the Permian Lower Magnesian Eberli, G.P., Swart, P.K. McNeill, D.F., Limestone of England and the Kenter, 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 Great In: Eberli, G.P., Swart, P.K., Malone, Bahama Bank: sea-level-driven M. et al. Proc. ODP Init. Repts., v. 166, sedimentation patterns on the slope and p. 23- 41. College Station, TX (Ocean margin In: SEPM Contributions in Drilling 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 late fossils in sedimentology and Pleistocene Great Bahama Bank and stratigraphy: Tulsa, OK, SEPM Short deep-sea oxygen isotope records of sea Course #15, 317 p. level, Geology, v. 26, p. 577-672. Gidman, J., 1978. Diagenesis of cored Land, L.S., 1980, The isotopic and trace Pleistocene carbonates, Great Abaco element geochemistry of dolomite: the Island, 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 prograding years old) peritidal dolomites, Ambergris platform margins (Neogene, Bahamas): Cay, Belize: Sedimentology, v. 39, p. Key evidence linked to depositional 143-160. 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 Neogene Unpublished M.S. thesis University of planktic foraminifera: implications of an Miami, 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

22 Dolomitization in Great Bahama Bank

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Vahrenkamp, V. C., and Swart, P. K., 1990. New distribution coefficient for the incorporation of strontium into dolomite and its implication for the formation of ancient dolomites, Geology, v. 18, p. 387-391. Vahrenkamp, V.C. and Swart P.K., 1994. Late Cenozoic dolomites of the Bahamas: metastable analogues for the genesis of ancient platform dolomites, Spec. Publs. Int. Ass. Sediment, 21:133- 153. Williams, S. C., 1985, Stratigraphy, facies evolution, and diagenesis of late Cenozoic limestones and dolomites, Little Bahama Bank, Bahamas: