Dolomitization of the Pedro Castle Formation (Pliocene), Cayman Brac, British West Indies

Sedimentary Geology 162 (2003) 219 – 238 www.elsevier.com/locate/sedgeo

Dolomitization of the Pedro Castle Formation (Pliocene), Cayman Brac, British West Indies Alex MacNeil, Brian Jones * Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 Received 8 October 2002; accepted 25 April 2003

Abstract The Pedro Castle Formation (Pliocene) on Cayman Brac is variably dolomitized by texture preserving but non-mimetic and texture destructive replacive dolomite. Mimetic replacement of skeletal grains is limited to echinoderm plates, and with few rare exceptions, there is no mimetic replacement of red algae, foraminifera, green algae, or any other type of skeletal grain. The lack of mimetic dolomite is atypical of ‘‘island dolostones’’ found in the Caribbean Sea and the Pacific Ocean. Dolostones in the Pedro Castle Formation are formed entirely of high-Ca calcian dolomite (average of 57.4 mol% CaCO3). Oxygen isotopes (mean 1.25xPDB) from the dolomite indicate that dolomitization was mediated by seawater or modified seawater. Carbon isotopes in the dolomite, which range from 1.81xto 1.42xPDB, were probably inherited from the precursor limestone. The average Sr content in the dolomite (360 ppm) is higher than that found in most other island dolomites. The sediments that now form the Pedro Castle Formation were deposited in shallow water on an open bank during the early Pliocene. Pre-dolomitization diagenesis of those sediments included syntaxial overgrowths around echinoderm fragments, dissolution of aragonitic bioclasts, stabilization to low-magnesium calcite, and local precipitation of vadose cements. Thus, the limestones had been extensively stabilized by the time that dolomitization took place during the late Pliocene. The general paucity of mimetic replacement in these dolostones can probably be attributed to the calcite stabilization that took place before dolomitization. D 2003 Elsevier B.V. All rights reserved. Keywords: Dolomite; Mimetic; Pliocene; Cayman Brac; Diagenesis

1. Introduction Dolostones are the product of the reaction between the precursor sediments/rocks and the dolomitizing fluid. The porosity, permeability, mineralogy (e.g., aragonite versus calcite), and the availability of

* Corresponding author. Fax: +1-780-492-7598. E-mail address: [email protected] (B. Jones). 0037-0738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0037-0738(03)00153-2

nucleation sites in the limestones immediately before dolomitization were a reflection of the pre-dolomitization diagenesis. Discussions concerning post-depositional dolomitization, however, typically focus on the dolomitizing fluids rather than the properties of the precursor sediments/rocks. In part, this reflects the fact that dolomitization typically destroys and/or significantly alters the fabrics and geochemistry of the original limestones. Determining the exact cause of post-depositional dolomitization is also severely

220

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

hindered by the difficulties associated with interpreting the plethora of data that have been used to characterize dolostones. As a result, attention has commonly been focused on Cenozoic dolostones found on isolated oceanic islands ( = ‘‘island dolomites’’ of Budd, 1997) in the Caribbean Sea and the Pacific Ocean because (1) they are geologically young and therefore should not have undergone significant post-dolomitization alteration, (2) they are geographically isolated by deep oceanic waters, (3) the Mg needed for dolomitization must have come from the surrounding seawater, and (4) the rocks have never been buried to significant depths (Budd, 1997). Collectively, these constraints limit the factors that may be used to explain dolomitization of the original limestones. This paper examines dolostones found in the Pedro Castle Formation (Pliocene) on Cayman Brac (Fig. 1). The dolostones, which are formed mainly of nonmimetic dolomite, are atypical of most ‘‘island dolostones’’ due to a lack of mimetic dolomite (cf. Sibley,

1991; Budd, 1997). The finely crystalline dolostones are closely associated with limestones and partly dolomitized limestones, which collectively provide valuable insights into the nature of the original rocks (e.g., Jones, 1994; MacNeil, 2001). The petrography of the dolostones and their geochemical characteristics indicate that the precursor limestones underwent significant diagenesis prior to dolomitization. Furthermore, our interpretations suggest that many features of the dolostones were influenced and/or inherited from the precursor limestones. As such, this study highlights the importance of considering pre-dolomitization diagenesis in the development of any dolomitization model.

2. Methods Nine outcrop sections measured for this study were combined with seven outcrop sections measured by Jones and Hunter (1994a) in order to delineate facies

Fig. 1. Location, stratigraphy, and cross-section of Cayman Brac (CB). BF = Brac Formation; CF = Cayman Formation; PCF = Pedro Castle Formation; IF = Ironshore Formation.

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

architecture and dolostone distribution in the Pedro Castle Formation on Cayman Brac. Thin sections (n = 57) were made from samples collected from nine of these sections for detailed petrographic examination. Herein, mimetic dolomite is defined as a texturepreserving dolomite in which the skeletal allochems are replaced so that original skeletal micro-architecture and crystallographic orientations of the precursor phase are preserved (cf. Bullen and Sibley, 1984; Budd, 1997). X-ray diffraction (XRD) analysis was used to determine carbonate mineralogy. The mol% CaCO3 content of dolomite was determined using electron microprobe analyses and the peakfitting-XRD (PFXRD) method of Jones et al. (2001). Carbon and oxygen stable isotopes of calcite and dolomite were measured in the Stable Isotope Laboratory, University of Alberta, directed by Dr. K. Muehlenbachs. All analyzed samples contained at least 20% dolomite (based on relative calcite – dolomite XRD peak intensities) and a modified method of the procedure by Walters et al. (1972) was used (cf. Pleydell et al., 1990). Powders (38 – 45 Am) were reacted with anhydrous phosphoric acid at 25.2 jC for 4 h before the reaction tubes were vented. Gases were analyzed using a Finnigan-MAT 251 mass spectrometer. All results are reported relative to PDB normalized to NBS-18, and the dolomite values have not been corrected for any fractionation factor with phosphoric acid. Selected samples were analyzed for 87Sr/86Sr ratios in the Radiogenic Isotope Laboratory, University of Alberta, directed by Dr. R. Creaser. Strontium in dolomite concentrates was separated from a chloride form on a 10-cm column containing f 4 ml of 200– 400 mesh AG50W-X8 resin. Purified Sr was loaded in the chloride form onto a single rhenium filament together with a tantalum activator gel. Isotopic compositions were measured on a VG 354 instrument using dynamic multi-collector routines, corrected for variable mass discrimination to 86Sr/88Sr = 0.1194. Multiple runs of the SRM 987 standard gave a value of 0.7102716, so all data was normalized by 0.999963, to agree with the international value for SRM 987. Electron microprobe analysis (EMPA), with a Jeol JXA-8900 R, was used to determine Mg/Ca zoning in individual dolomite crystals, verify XRD analyses, determine the elemental concentration of Sr, and

221

measure the Mg composition of calcite inclusions in the dolomite. Spot analyses (3 Am beam diameter) for measurement of Mg and Ca in dolomite were randomly selected and individually adjusted to ensure analysis of dolomite rather than calcite. The probe was operated with an acceleration voltage of 15 kV and sample current of 15 nA. Selected samples were later analyzed for Sr content (Ca and Mg were also re-measured with the Sr) using a larger beam diameter of 15 Am, to avoid sample damage. These samples (and standards) were coated with silver (cf. Smith, 1986). The La-peak of Sr was measured from the TAP crystal spectrometer using count times of 150 s and 80 s for background counts. These operating conditions produced statistically valid results with detection limits of f 75 ppm and no visible damage to the samples.

3. Geology of Cayman Brac Cayman Brac (f 39 km2) is the summit of a pinnacle located on the Cayman Ridge (Perfit and Heezen, 1978; Emery and Milliman, 1980). The core of Cayman Brac, which has been uplifted to f 45 m above sea level at the eastern end, is formed of the Bluff Group (Fig. 1) which encompasses the Brac Formation (Late Oligocene), the Cayman Formation (Middle Miocene), and the Pedro Castle Formation (Pliocene) (Jones, 1994; Jones and Hunter, 1994a). The Ironshore Formation (Pleistocene) forms a lowlying apron around the uplifted core (Fig. 1). The Cayman Unconformity, which forms the boundary between the Cayman Formation and the overlying Pedro Castle Formation, is a paleokarst surface that developed during the Messinian (Jones and Hunter, 1994b). Although only minor topographic relief has been found on this unconformity on Cayman Brac (MacNeil, 2001), there is f 40 m of relief on that unconformity on Grand Cayman (Jones and Hunter, 1994b). The Pedro Castle Formation on Cayman Brac, 6 – 10 m thick, is formed of carbonate sediments (Fig. 2A) that were deposited in shallow ( < 20 m), low to moderate energy, normal marine conditions. Wackestones with a variable biota of algae, foraminifera, molluscs, and echinoids dominate. Distribution of the various sandy facies probably reflects abrupt spatial

222

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

Fig. 2. Sections measured through the Pedro Castle Formation on Cayman Brac. (A) Representative section through the lower and middle parts of the Pedro Castle Formation. Facies 1 = bioclastic – rhodolite wackestone – packstone; Facies 2 = bioclastic wackestone; Facies 3 = Stylophora – Porites thicket. Transitions between dolostone, dolomitic limestone, and limestone are approximate, and based on sample intervals every 30 cm. CF = Cayman Formation; CU = Cayman Unconformity; PCF = Pedro Castle Formation. (B) Distribution of dolostone, dolomitic limestone, and limestone in measured sections through Pedro Castle Formation on west end of Cayman Brac. Sections GAM, STS, SQA-2, SQA-4, and HPS are cited in text.

changes in bank morphology (MacNeil, 2001). Thickets of Stylophora and Porites grew in slightly deeper water towards the western margin of the bank. There are no evaporitic, supratidal, or intertidal facies in the formation. Early marine diagenesis in the Pedro Castle Formation included micritization of skeletal grains and local cementation (MacNeil, 2001). Prior to dolomi-

tization, extensive fossil-moldic porosity evolved as bioclasts formed of aragonite (e.g., bivalves, gastropods, green algae, and coral) were dissolved. Subsequent dolomitization included precipitation of dolomite cement in those pores. Bivalve fragments, in rare exceptions, were partially preserved and may or may not be replaced by dolomite. With the possible exception of syntaxial overgrowths on echi-

Fig. 3. Dolomite in Pedro Castle Formation, Cayman Brac. (A, B) SEM photomicrographs of RI dolomite formed of small euhedral crystals embedded in matrix of smaller sub-micron crystals. (C) Thin section photomicrograph of RII dolomite (white arrows) and pore filled with coarse calcite cement (cc). (D) SEM photomicrograph of large, euhedral RII dolomite crystals (white arrows) held in matrix of microcrystalline calcite (mc). (E) Thin section photomicrograph showing pore lined with C1 dolomite cement. White arrows point to clear rims. Pore later filled with coarse calcite cement (CC). (F) SEM photomicrograph of C1 dolomite cement growing from matrix of RI and RII dolomite. (G) Thin section photomicrograph of C2 dolomite (black arrows) lining wall of pore in RI dolomite. Late coarse calcite cement (CC) filled pore. (H) SEM photomicrograph of C2 dolomite cement. Sample locations: A – C, E – H-section SQA-2; D-section GAM.

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

noderm fragments, there are no obvious meteoric phreatic cements that pre-date dolomitization of the formation. Pre-dolomitization vadose features and cements are limited to rare micro-speleothemic struc-

223

tures that were later replaced by dolomite. Intense post-dolomitization diagenesis included development of phytokarst and subsurface karst features (e.g., caves), rhizoconcretions, deposition of terra rossa

224

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

and terrestrial oncoids, and meteoric cementation (MacNeil, 2001). Dolostone distribution in the Pedro Castle Formation is laterally and vertically variable (MacNeil, 2001). In most sections, there is a vertical transition from basal dolostone (>75% dolomite) into dolomitic limestone (20 – 75% dolomite), and limestone (Fig. 2A,B). This pattern is most pronounced along the north coast and northern part of the Active Scott Quarry (SQA). Sections along the south coast have less extensive dolomitization, and two sections in the southern half of the SQA are formed entirely of limestone (Fig. 2B). There is no readily discernable correlation between the depositional facies and the amount of dolomite.

4. Dolomite of the Pedro Castle Formation 4.1. Petrography Replacive dolomite in the Pedro Castle Formation is divided into Types RI and RII according to crystal size and morphology. Type RI dolomite (Fig. 3A,B) is a microcrystalline (subhedral rhombs, < 2 Am long with rounded edges and corners), texture-preserving, but non-mimetic dolomite that is similar to the crystalline, non-mimetic dolomite described from the Bahamas (Dawans and Swart, 1988) and Nuie Atoll (Wheeler et al., 1999). Neighboring crystals do not interlock significantly and good intercrystalline porosity exists. RII dolomite (Fig. 3C,D) consists of euhedral cloudy rhombs, 15 –35 Am long that have sharp edges and corners. This dolomite, which poorly preserves the pre-existent texture, is similar to the microsucrosic dolomite described by Dawans and Swart (1988) and Wheeler et al. (1999). Type RI is found by itself or with RII dolomite. Type RII dolomite, however, is never found by itself. Two types of dolomite cement (C1, C2) are present in the Pedro Castle Formation. Clear dolomite cement (C1) locally overgrew the RII dolomite, forming clear rims around cloudy-centered crystals (Fig. 3E,F). Similar dolomite cements have been described from the Bahamas (Dawans and Swart, 1988), Bonaire (Sibley, 1980, 1982), Isla de Mona (Gonzalez et al., 1997), and Mururoa Atoll (Aissaoui et al., 1986).

Limpid dolomite (C2), 5 –35 Am long (Fig. 3G,H), is found lining moldic cavities and intergranular pores, and is similar to that described by Jones et al. (1984). 4.2. Dolomite composition Dolomite in the Pedro Castle Formation on Cayman Brac has an average of 57.4 F 1.2 mol% CaCO3 (Fig. 4). As such it corresponds to the high-Ca calcian dolomite (HCD) defined by Jones and Luth (2002). In contrast, the dolostones in the underlying Cayman Formation are formed entirely of low-Ca calcian dolomite (LCD) or mixtures of LCD and HCD. Electron microprobe analysis and backscatter imaging shows that the dolomite cements have the same composition as the dolomite that replaced the matrix. Oscillatory zoning of the cements with alternating bands of LCD and HCD, such as has been found in the dolostones on Grand Cayman (e.g., Jones et al., 2001; Jones and Luth, 2002) is rarely seen in the cements from the Pedro Castle Formation on Cayman Brac. If present, the zoning is limited to one band f 1– 2 Am thick around the rim of the crystal. Replacive dolomite and dolomite cements associated with micro-speleothemic structures are an exception to the compositional homogeneity of the Pedro Castle Formation. The mol% CaCO3 in this replacive dolomite is highly variable and zones enriched in Sr are present. The textural and compositional heterogeneity complexity was probably inherited from the preexisting structures. 4.3. Mode of replacement dolomite Limestones in the Pedro Castle Formation were replaced to varying degrees by RI dolomite and lesser amounts of RII dolomite. Pervasive dolomitization is generally limited to the basal meter of the formation. Elsewhere, scattered dolomite rhombs or patches of dolomite are held in microcrystalline low-magnesium calcite (LMC) matrices. Echinoderm plates, red algae, foraminifera, and other skeletal grains in the Pedro Castle Formation have been dolomitized in different ways. Mimetic dolomitization is limited to echinoderm plates (Fig. 5A,B). They are not considered any further because

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

225

echinoderm plates may be mimetically replaced by a single dolomite crystal, irrespective of precursor mineralogy (Sibley, 1991). With few exceptions, dolomitization of red algae fragments was non-mimetic. Initially, dolomite nucleated on the calcitic cell walls and then grew into the open intracellular spaces (Fig. 5C,D), or RI dolomite replaced the calcitic fragment, or portions of the calcitic fragment, in a non-mimetic manner (Fig. 5E,F). In some red algae fragments the calcitic walls were retained despite the fact that their cells were filled with dolomite cement. In some specimens, subsequent, or possibly coeval, leaching of the calcite walls produced fossil-moldic porosity. These styles of replacement are not consistent with the definition of mimetic dolomitization (cf. Bullen and Sibley, 1984; Budd, 1997). Foraminifera (Fig. 5G,H) display highly variable preservation. Some were replaced by texture-preserving but non-mimetic RI dolomite, whereas others were preserved as microcrystalline calcite, or leached. 4.4. Dolomitized speleothemic structures

Fig. 4. Frequency histograms showing uniformity of mol% CaCO3 in dolomite (electron microprobe analyses) from section SQA-2 (locations measured relative to land surface).

In section STS (Fig. 2), speleothemic structures including popcorn-like features, pendant cements, and coarse bands of calcite cements with sharp crystal terminations were replaced by RI dolomite (Fig. 6). These dolomitized vadose cements hang from cavity roofs formed of RI dolomite with abundant clay inclusions (Fig. 6A,B). Pockets and bands of clays, and vugs lined with clays (Fig. 6H) are found close to the cavity walls. There is no detectable calcite in the matrix dolomite around the cavity. Zoned dolomite– calcite (ZDC), which forms the pendant and popcorn-like structures that hang from the cavity roofs (Fig. 6B –G), resulted from partial dolomitization of the original zoned calcite cement (Fig. 6E – G). This interpretation explains (1) why some of the dolomite bands are laterally continuous, whereas others are laterally discontinuous, (2) the lack of euhedral or inclusion-free dolomite crystals in the speleothems, (3) the ‘‘grungy’’ nature and irregular textures of the dolomite (Fig. 6D), and (4) why C2 dolomite is only found coating the outside of the speleothemic structures (Fig. 6B,C). The high micro-porosity, which is restricted to the dolomite bands, probably formed through dissolution of cal-

226

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

cite inclusions (Fig. 6C). Texturally, the ZDC is similar to that found in many calcitic speleothems (Fig. 7).

The vadose structures in the Pedro Castle Formation on Cayman Brac are similar to alternating calcite – dolomite cements described by Jones et al. (1984),

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

Ward and Halley (1985), and Humphrey (1988, 2000). Comparisons between these examples, however, reveal the following differences. 











The dolomitized vadose cements in the Pedro Castle Formation lack the first generation dolomite cements that are found in the calcite – dolomite cements of Jones et al. (1984) and the limpid dolomite of Humphrey (1988, 2000). In the Pedro Castle Formation cements, C2 dolomite is the last phase of cement. Alternating zones of C2 dolomite and calcite are not found in the Pedro Castle Formation. Red clays in the Pedro Castle Formation are found throughout the replacive dolomite around the cavities, as inclusions in the dolomite crystals, and as bands and pockets intercalated with the dolomite. These fabrics are not found in the cements of Jones et al. (1984) or Humphrey (1988, 2000). In the Pedro Castle Formation cements, the dust layers of Jones et al. (1984) are not found at the boundary between the ZDC and the late, postdolomitization coarse calcite cement that fills in the remaining cavity space. None of the vadose cements in the Pedro Castle Formation are rooted on coarse calcite cements as they are in the Cayman Formation on Grand Cayman (cf. Jones et al., 1984). The euhedral crystal terminations found in the zoned dolomite – calcite cements described by Humphrey (2000, his Fig. 2) are not present in the cements in the Pedro Castle Formation. The structures described by Humphrey (1988, 2000) do not include the pendant and popcornlike morphologies found in the Pedro Castle Formation. Similarly, Jones et al. (1984) did not describe any popcorn-like structures.

227

Collectively, these differences support the interpretation that the pendant cements and popcorn-like cements in the Pedro Castle Formation on Cayman Brac formed by partial dolomitization of pre-existing calcitic vadose cements.

5. Implications of dolomite petrography The mode of dolomite replacement and the susceptibility of different grains to replacement is a function of various intrinsic factors such as the precursor mineralogy, the number of nucleation sites, and the saturation state of the bulk fluid with respect to the mineral being replaced (Sibley, 1982). Mimetic replacement has generally been attributed to dolomitization of aragonite or high-magnesium calcite (HMC) (e.g., Sibley, 1980, 1982; Dawans and Swart, 1988; Pleydell et al., 1990; Land, 1991; Meyers et al., 1997; Budd, 1997). The lack of mimetic dolomite in the Pedro Castle Formation may therefore reflect dolomitization of a limestone that had undergone significant diagenesis and stabilization before the onset of dolomitization. The possibility that the availability of nucleation sites or kinetics inhibited mimetic dolomitization in the Pedro Castle Formation on Cayman Brac is difficult to assess from petrographic evidence. Nevertheless, replacement by RI dolomite and nucleation of dolomite on the walls of red algae skeletons indicate that nucleation sites were common. Replacement of echinoderm fragments and non-mimetic, but texture-preserving replacement of other skeletal grains also argue against any kinetic factors. Unless some unknown factor in the chemistry of the dolomitizing fluid inhibited mimetic replacement, precursor mineralogy must have been the dominant control on the mode of the replacive

Fig. 5. Dolomitization of skeletal grains in Pedro Castle Formation, Cayman Brac. (A, B) Thin section photomicrographs showing echinoderm plates mimetically replaced by dolomite and enlarged by syntaxial dolomite overgrowths. (C) Thin section photomicrograph showing calcitic red algae fragment held in matrix of RI dolomite. (D) Electron microprobe backscatter image of fossil-moldic porosity (black) in a red algae fragment. Dolomite (grey) partially filled intracellular spaces, but did not replace cell walls. White = calcite. (E) Thin section photomicrograph showing partial, non-mimetic replacement of a calcitic red algae fragment by RI dolomite. (F) Thin section photomicrograph of calcitic (C) red algae fragments and fragment replaced by RI dolomite. Center fragment shows calcitic structure with partial replacement by RI dolomite. Matrix is pervasively dolomitized by RI dolomite. (G) Thin section photomicrograph showing calcitic foraminifera (C), partially leached foraminifera with fossil-moldic porosity (P), and foraminifera partially replaced by RI dolomite. (H) Thin section photomicrograph of internal structure of an amphisteginid foraminifera partly replaced by RI dolomite (outlined in black). Remaining microstructure is calcitic (C).

228

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

dolomite. This establishes the influence of the precursor limestone on the nature of the replacive dolomite in the Pedro Castle Formation. The situa-

tion with the dolostones from Cayman Brac contrasts sharply with many other island dolostones that are characterized by mimetic dolomitization, despite

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

229

Formation is impossible. Nevertheless, the level and nature of alteration (e.g., degree of stabilization towards LMC) may be inferred from the following features. 







Fig. 7. Comparison of a speleothem with cavity-filling pisoids (P) from Grand Cayman with dolomitized vadose structures in Pedro Castle Formation on Cayman Brac. (A) Thin section photomicrograph of speleothem (calcite) showing clay layers (CLY) and irregular zones of calcite cement (white). (B) Electron backscatter image of dolomitized vadose deposits. Note similarities in clay layers (CLY) and band-like zones of irregular porosity filled with late calcite cement. Note vug lined with clays (dark grey) and C2 dolomite (white arrow). Calcite cement fills remaining porosity (white).

having also undergone partial stabilization of the precursor limestone. Quantification of calcite stability in the limestone at the time of dolomitization in the Pedro Castle



The hollow micrite envelopes and fossil-moldic pores lined with dolomite cements (Fig. 8A – C) are indicative of extensive, pre-dolomitization dissolution of the less stable phases. The lack of replaced, or partially replaced, aragonitic bioclasts indicates that aragonitic dissolution took place prior to dolomitization. Some calcitic grains, including those later replaced by dolomite, show evidence of leaching prior to the dolomitization event. In some cases, the grain has not been replaced, but intragranular moldic porosity lined or filled with dolomite cements (Fig. 8D). These indicate that partial dissolution of the more stable phases had commenced prior to dolomitization. LMC (1 – 2 mol% MgCO3) inclusions in the replacement dolomite (Fig. 8E,F) indicate that some of the precursor limestone was formed of LMC at the time of dolomitization. Partly dolomitized vadose cements (Fig. 6) indicate that significant diagenesis had taken place prior to dolomitization.

These observations and interpretations indicate that the original limestones had undergone significant diagenesis prior to dolomitization. The evidence of vadose diagenesis, even if only local in distribution, implies that meteoric phreatic and mixing-zone diagenesis must have affected the succession prior to dolomitization. Although the extent of pre-dolomitization stabilization is not clear, the non-mimetic but

Fig. 6. Speleothemic structures replaced by dolomite. (A) Thin section photomicrograph of cavity roof formed of RI dolomite, lined with zoned calcite cements that have been partly replaced by dolomite. Cavity filled by coarse calcite cements (CC), terrestrial pisoids (P) and clays. (B) Thin section photomicrograph showing enlarged view of (A). Center part of cavity roof is replaced by RI dolomite and includes irregular clayrich bands (CLY). A region of zoned dolomite and calcite (ZDC) separates cavity roof from cavity. Dark bands in the ZDC are clay-rich zones. CC = coarse calcite cement. (C) Electron backscatter image of ZDC. Note bands of micro-porosity and irregular pores (white arrows). Coarse calcite cement (CC) separates bands of dolomite. (D) SEM photomicrograph of band of replacive dolomite (RD) between two bands of calcite cement (CC). Note irregular shapes of dolomite crystals and well-developed micro-porosity. (E) Thin section photomicrograph of dolomitized popcorn-like structure. (F) Thin section photomicrograph of popcorn-like structures replaced by dolomite. Note pocket of clays (CLY) in dolomitized roof structure. White = calcite. (G) Electron microprobe backscatter image of pendant structures (grey) replaced by dolomite (RD). Cavity filled by coarse calcite cement (CC). (H) Thin section photomicrograph of vug lined with C2 dolomite cement and filled with coarse calcite cement (CC). Surrounding matrix of RI dolomite is clay-rich and separated from C2 cement by a banded dolomite – clay zone (CLY). A – H from section STS.

230

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

Fig. 8. Pre-dolomitization diagenesis. (A) Fossil-moldic porosity of bivalve shell in matrix of RI dolomite. Pore lined with C2 dolomite cement (black arrows) and later filled with coarse calcite cement (CC). (B, C) Micritic envelopes lined on both sides with C2 dolomite cements. Black arrows in (C) point to envelope. Later coarse calcite cement (CC) filled remaining porosity. (D) Electron backscatter image of calcitic echinoid grain (Ech) with intragranular moldic porosity filled by C1 dolomite cement. Surrounding matrix replaced by RI dolomite. White = calcite, grey = dolomite. (E) Electron backscatter image of micritic envelopes (subsequently leached out) and dolomite cements that grew from the envelopes. Black arrows point to micro-inclusions of calcite in dolomite. Later coarse calcite cement filled remaining porosity (CC). Black = porosity. (F) Electron backscatter image of C2 dolomite cement lining and growing from matrix calcite (MC) substrate into pore space. EMPA of Mg and Ca content in the MC indicated a LMC composition. Pore space later filled by coarse calcite cement (CC). A, D, and E from SQA-2; B, C, and F from GAM.

texture-preserving dolomitization of some skeletal grains indicates that complete stabilization to LMC had not taken place (cf. Sibley, 1982; Pleydell et al., 1990; Budd, 1997). Conversely, the paucity of mi-

metic dolomite in the Pedro Castle Formation on Cayman Brac suggests that the degree of calcite stabilization in the precursor limestone must have been significant.

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

231

variable oxygen signature, and a scattered to positive co-variant trend (Fig. 9).

6. Oxygen and carbon isotopes Dolostones from the Pedro Castle Formation yielded d18O values of 0.08xto 2.16xand d13C values of 1.81xto 1.42x(Table 1). The mean d18O and d13C values (n = 40) are 1.25 F 0.51x and 0.27 F 0.67x, respectively. These values are characterized by a variable carbon signature, a less

6.1. Oxygen isotopes The oxygen isotopic signature of dolomite in the Pedro Castle Formation is consistent with the oxygen isotopic signature of dolomite formed from seawater,

Table 1 Summary of geochemical data for dolostones from the Pedro Castle Formation, Cayman Brac Section GAM GAM GAM GAM GAM GAM GAM GAM GAM GAM STS STS STS SQA-2 SQA-2 SQA-2 SQA-2 SQA-2 SQA-2 SQA-2 SQA-2 SQA-2 SQA-2 SQA-4 SQA-4 SQA-4 SQA-4 SQA-4 SQA-4 SQA-4 SQA-4 SQA-4 SQA-4 SQA-4 SQA-4 SQA-4 HPS HPS HPS HPS

Depth below surface (m) 0.3 0.7 1.0 1.3 1.7 2.0 2.3 2.7 3.0 3.2 0.0 0.3 0.7 0.0 0.1 0.7 1.3 2.0 2.7 3.0 3.3 3.7 4.0 0.0 0.2 0.5 0.8 1.2 1.5 1.8 2.1 2.5 2.7 2.9 3.1 3.8 0.1 0.7 1.3 2.0

See Fig. 2B for location of sections.

Oxygen

Carbon

1.52 1.27 0.08 1.55 0.99 1.46 1.72 1.05 1.63 1.75 1.57 1.51 1.37 2.11 2.16 0.38 1.52 1.51 0.48 1.07 1.34 1.39 1.58 0.60 0.08 0.42 1.07 1.51 1.79 1.12 1.21 1.16 1.07 1.48 1.51 0.45 1.10 1.37 1.57 1.63

0.55 0.50 1.34 0.45 0.63 0.88 1.07 0.78 0.76 1.16 0.92 1.06 0.96 0.21 0.17 0.95 0.07 0.75 1.81 0.63 0.70 0.52 1.42 0.41 0.69 0.35 0.21 0.14 0.21 0.37 0.49 0.17 0.34 0.39 0.48 0.45 0.17 0.20 0.21 0.22

Sr content (ppm)

Ca content (ppm)

87

Sr/86Sr

0.709073 F 0.000023

0.709057 F 0.000014 0.709032 F 0.000017

323

256,471

355 359 363

255,069 253,942 251,254

0.709042 F 0.000014

400 327

255,362 256,467

0.709064 F 0.000015 0.709051 F 0.000014

351

249,138

0.709046 F 0.000015

0.709108 F 0.000014

0.709085 F 0.000020

0.709073 F 0.000015 0.709090 F 0.000017

0.709102 F 0.000014 0.709064 F 0.000018 0.709069 F 0.000018

232

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

for dolomite precipitated from saline water is the only range that is in agreement with the modern temperatures measured by Ng (1990). Although the calculated temperature of seawater overlaps with the observed surface temperature of Ng (1990), these temperatures are probably too high for the shallow subsurface of the Cayman Islands. On this basis, the oxygen isotopic evidence indicates that saline water mediated dolomitization of the Pedro Castle Formation. 6.2. Carbon isotopes

Fig. 9. Cross-plot (n = 40) of oxygen and carbon isotopes in dolomite from Pedro Castle Formation on Cayman Brac (see Table 1 for data).

seawater modified through water – rock interaction (hereafter termed saline water), or a seawater –meteoric water mixture (cf. Land, 1991; Budd, 1997). In the ‘‘mixing zones’’ of Cayman Brac, meteoric waters mix with saline waters rather than normal seawater (Ng, 1990). The range in oxygen isotopes is inconsistent with dolomitization by either evaporated seawater (cf. Budd, 1997) or brines mixed with meteoric water (cf. Gill et al., 1995; Meyers et al., 1997). The possibility that seawater, saline water, or mixed water mediated dolomitization can be evaluated by calculating the formation temperatures of the dolomite from the oxygen isotopes and comparing those temperatures with the temperatures of present-day waters found in the different groundwater zones in the Cayman Islands (Table 2). Herein, the Friedman and O’Neil (1977) equation is used with a Ddolomite – calcite value of 3.84x(cf. Land, 1985, 1991; Machel and Burton, 1994; Vahrenkamp and Swart, 1994; Swart and Melim, 2000). On Grand Cayman, water in the shallow subsurface mixing zone has a temperature of 27– 30 jC, whereas the deep saline zone is 25 – 27 jC (Ng, 1990). Shallow lagoonal waters have a temperature of 27 – 34 jC, which is believed to be equivalent to Pliocene temperatures (Krantz, 1991; Cronin and Dowsett, 1993). The temperature calculated (Table 2)

The d13C of dolomite in the Pedro Castle Formation on Cayman Brac was probably inherited from the precursor limestone rather than the dolomitizing fluid (cf. Lohmann, 1988). Petrographic evidence indicates that the precursor limestones had been partly stabilized and subjected to meteoric diagenesis before dolomitization took place. It is reasonable that those processes also led to pre-dolomitization depletion of the carbon isotopic ratios (cf. James and Choquette, 1990). This conclusion is similar to that reached for dolostones on San Salvador and the Little Bahama Bank (Vahrenkamp and Swart, 1994; Budd, 1997). The proposed relationship between the carbon isotopes of the replacive dolomite and pre-existing calcite may be further examined by considering the carbon isotope signatures obtained from meteoric calcite that post-dated dolomitization, and modern marine sediments in the Cayman Islands. The d13C of post-dolomitization meteoric calcite in the Pedro

Table 2 Dolomite formation in different types and temperatures of water, as calculated from data collected by Ng (1990) Water type

Average isotopic composition (xSMOW)

Salinity

Temperature of dolomite formation (jC)

Fresh groundwater Lightly brackish

4.54

< 1.01 ppt

5.5 – 9.6

4.16

7.0 – 11.2

Highly brackish

1.82

Saline water Normal seawater

0.63 1.34

< 15% seawater >15% seawater f 35 ppt f 35 ppt

16.8 – 21.4 28.1 – 33.3 31.7 – 37.1

All data are from the Cayman Islands. Saline water is derived from seawater and altered through increasing water – rock interaction (Ng, 1990).

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

Castle Formation ranges from 8.4xto 2.25x (MacNeil, 2001). The progressive change from waterbuffered alteration near the surface to rock-buffered alteration at depth, with regards to carbon in the meteoric calcite, explains the range from highly negative to more positive isotopic values, respectively (cf. Lohmann, 1988). Neomorphic calcite in the rockbuffered portion of the system should have carbon isotope values progressively closer to the original marine composition, which in modern sediments from the Cayman Islands is 2.75 F 0.83x(Pleydell, 1987). With respect to the replacement dolomite, the carbon values are interpreted to reflect inheritance from calcite in this more distal, rock-buffered portion of the pre-existing diagenetic environment. This explains the 3.2x spread of values, between 1.81xand + 1.42x, for the carbon isotopic signature.

7. Sr content The average strontium content of the dolostones from the Pedro Castle Formation is 354 F 26 ppm and

233

the average Sr/Ca is 0.00139 F 0.00009 (based on 607 spot analyses from seven samples; Table 1). The Sr content is higher than that usually found in island dolomite (Fig. 10; Budd, 1997). This may be due to modification of the Sr2 +/Ca2 + ratio of the dolomitizing fluid, a kinetic factor related to dolomite stoichiometry, or the addition of Sr2 + from an unknown diagenetic process. Using a distribution coefficient value (DSrDolomite) of 0.0407 for Sr2 + (calculated using the method of Vahrenkamp and Swart, 1990), the (Sr2 +/Ca2 +)fluid is 0.03415. Assuming that the DSrDolomite value is accurate, the (Sr2 +/Ca2 +)fluid value is enriched relative to end-member values for modern seawater (0.01947) and meteoric waters (0.00667) from Grand Cayman. Residual Sr2 + from earlier aragonite dissolution or some other diagenetic process may have caused elevated Sr2 +/Ca2 + ratios of the dolomitizing fluid. Processes that may elevate the Sr2 +/Ca2 + in ambient fluids and thereby produce high Sr in dolomite include aragonite dissolution with concurrent calcite precipitation (Budd, 1988; Humphrey, 2000; Swart and Melim, 2000) and calcite stabilization (Mucci and Morse, 1983). The lack of obvious pre-dolomitization meteoric

Fig. 10. Comparison of Sr (ppm) in dolomite from the Pedro Castle Formation on Cayman Brac with Sr found in other island dolomites (modified from Budd, 1997, his Fig. 13). The average Sr content does not include the Barbados examples because of their anomalously high Sr contents.

234

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

phreatic cements, the lack of preserved or dolomitized aragonitic components, and the only slight enrichment in Sr content (e.g., compare to values of Humphrey, 1988, 2000) suggests that aragonite dissolution with concurrent calcite precipitation was not a significant factor. Migration of fluids from other areas on the island, or from overlying strata that have since been removed by erosion could have contributed extraneous Sr2 + to the fluid that mediated dolomitization. The possibility that the diagenetic process of calcite stabilization resulted in the elevated Sr2 +/Ca2 + cannot be reasonably evaluated in this study. Without precise information on the degree of stabilization and the amount of extraneous Sr2 +, the extent of rock – water interaction, or the amount and rate of fluid flow through the system, this possibility remains enigmatic. Kinetic factors may have also caused slight enrichment in the Sr content. Banner (1995) noted that mole for mole replacement of calcite by dolomite, which may characterize RII dolomite, can increase the fluid Sr2 +/Ca2 + ratios of seawater and saline water 2 –5 times as dolomitization proceeds. The extraneous Sr2 + may have also been derived from clays (e.g., terra rossa) in the underlying Cayman Formation (cf. Jones and Hunter, 1994a), or from the upper parts of the Pedro Castle Formation that have since been lost to erosion. Dolomite replacing the speleothemic cements has elevated Sr2 + concentrations (f 800 –

Fig. 11.

87

1000 ppm) that are consistent with contamination from the associated reddish clays. At this time, the cause of Sr2 + enrichment is unclear. Nevertheless, the fluid that mediated dolomitization was modified with respect to its Sr chemistry.

8.

87

Sr/86Sr values

Dolostones from the Pedro Castle Formation on Cayman Brac have 87Sr/86Sr values that range from 0.709032 to 0.709108 (n = 14) with an average of 0.709068 F 0.000022 (Table 1). Comparison of the values with their associated error margins show that 12 of the 14 analyses are the same (Fig. 11). Given that the dolomitizing fluid was altered with respect to its Sr2 + content, the strontium ratio values must be considered with some caution. The range in values, for example, may reflect ‘‘older’’ strontium in the system, caused by rock – water interaction in the underlying Cayman Formation. Machel (2000) suggested that large ranges in the 87Sr/86Sr ratios of Cayman dolostones, as reported by Pleydell et al. (1990) might be caused by enrichment in 86Sr from magmatic hydrothermal solutions circulated by hydrothermal convection. For the dolomite in the Pedro Castle Formation on Cayman Brac, there is no evidence to support the type of model described by

Sr/86Sr values (n = 14) from dolomite in the Pedro Castle Formation plotted on the

87

Sr/86Sr curve of Farrell et al. (1995).

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

Machel (2000). Furthermore, the Cayman dolostones discussed by Machel (2000) have since been shown to be composite dolostones produced by multiple phases of dolomitization (Jones et al., 2001; Jones and Luth, 2003). Dolomitization must have post-dated deposition of the Pedro Castle Formation and pre-dated deposition of the Ironshore Formation. If the 87Sr/86Sr curve of Farrell et al. (1995) is used, 87Sr/86Sr values of the Cayman Brac dolostones indicate that dolomitization took place 4.4 to 1.2 Ma ago (Fig. 11). Most of the 87 Sr/86Sr values, however, coincide with the late Pliocene plateau (cf. Vahrenkamp et al., 1991) of the 87 Sr/86Sr curve (Fig. 11), which lasted from 4.5 to 2.4 Ma ago. Thus, it is difficult to determine a more precise age for the dolomitization.

9. Discussion The distribution and fabric of replacive dolomite in island dolostone is partly related to the degree of calcification of the precursor limestone (Sibley, 1982, 1991; Bullen and Sibley, 1984; Dawans and Swart, 1988; Pleydell et al., 1990; Vahrenkamp and Swart, 1994), and in some cases, the permeability pathways that were present in that precursor limestone (Dawans and Swart, 1988; Vahrenkamp and Swart, 1994; Wheeler et al., 1999). Mimetic dolomite has been attributed to replacement of HMC and aragonite during the early diagenetic history of the rocks (Sibley, 1982, 1991; Bullen and Sibley, 1984; Dawans and Swart, 1988; Pleydell et al., 1990; Vahrenkamp and Swart, 1994; Budd, 1997). Non-mimetic but texture preserving fabrics, and texture destructive fabrics, are generally attributed to dolomitization of a precursor limestone that had been partly or fully stabilized to LMC, or dolomite neomorphism (Budd, 1997; Wheeler et al., 1999). Sediments that now form the Pedro Castle Formation were originally deposited in shallow marine waters during the early Pliocene (MacNeil, 2001). Early diagenesis produced limestones that had, at least in part, been stabilized towards LMC. Associated with this process was dissolution of the aragonitic constituents and minor dissolution of the HMC constituents. Syntaxial cements on echinoderm fragments probably developed prior to dolomitization. Speleothemic

235

cements, later replaced by dolomite, also attest to vadose diagenesis prior to dolomitization. LMC inclusions in the dolomite are also a record of the diagenetic stability of the precursor limestone. Collectively, predolomitization diagenesis produced limestones that were less prone to mimetic dolomitization or more pervasive dolomitization. The nature of the fluids that mediated dolomitization in the Pedro Castle Formation may be best understood from the stable isotopic geochemistry combined with petrographic observations, and to a lesser degree, from the strontium data. This approach is feasible given that the dolostones are formed entirely of HCD; thus, the geochemical analyses do not reflect averages of mixed populations (cf. Banner, 1995; Budd, 1997; Wheeler et al., 1999; Jones et al., 2001). The enriched strontium content in the dolomite indicates modification of the Sr2 +/Ca2 + ratio in the dolomitizing fluid, which precludes normal seawater as the fluid that mediated dolomitization. This is consistent with the oxygen isotopic data which indicate that dolomitization was mediated by saline water. Calculations using the oxygen isotopic data for waters from the mixing zone indicate temperatures of dolomitization between 7jC and 21.4jC, which are too cold for near-surface dolomitization in the Cayman Islands. Additional petrographic features that argue against a mixing zone origin of the dolomite include: 

The lack of alternating dolomite– calcite –dolomite cements (cf. Jones et al., 1984; Ward and Halley, 1985; Humphrey, 1988).  The lack of compositional zoning, with respect to mol% CaCO3, in the replacive dolomite and dolomite cements, that is expected in dolomite that develops from fluctuating water chemistry in a mixing zone (cf. Ward and Halley, 1985; Humphrey and Radjef, 1991).  The calcite inclusions in the dolomite would have probably been dissolved concurrently with dolomitization had the waters been undersaturated with respect to calcite but supersaturated with respect to dolomite, which is the nature of mixed fluids that favour dolomitization (Badiozamani, 1973; Plummer, 1975; Hardie, 1987). Carbon isotopic signatures were probably inherited from the calcite in the pre-existing limestone. The

236

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

scattered to positive co-variant relationship between the oxygen and carbon isotope values (Fig. 9) has commonly been attributed to dolomitization in the mixing zone (e.g., Budd, 1997). Such a trend, however, can also be produced by inheritance of the carbon isotopic signature from the precursor limestone. This agrees with the evidence that seawater, modified through rock – water interaction, mediated dolomitization. Land (1992), Vahrenkamp and Swart (1994), and Budd (1997) have all noted that the carbon isotopic signature of the precursor (which in this case was depleted) is usually preserved when dolomitization is mediated by seawater or modified seawater.

10. Conclusions Petrographic and geochemical analyses of dolomite from the Pedro Castle Formation on Cayman Brac have yielded the following important conclusions. 









The sediments that now form the Pedro Castle Formation were originally deposited in a shallow marine setting. Pre-dolomitization diagenesis promoted dissolution of aragonitic bioclasts and mineralogical stabilization of the limestone. Mimetic dolomitization of skeletal grains, and possibly more pervasive dolomitization, was inhibited because of the pre-dolomitization stabilization of the limestones. Oxygen isotopes indicate that dolomitization was probably mediated by seawater that had been modified through water – rock interaction (saline water). This is supported by the lack of petrographic features including oscillatory zonation in the dolomite or alternating calcite – dolomite cements. Carbon isotopes of the replacive dolomite were inherited from the precursor limestone.

These conclusions show that pre-dolomitization diagenesis has a significant impact on the petrography and geochemistry of the dolostones that are produced during dolomitization. Thus, dolostones are products of a dynamic interplay between precursor limestones and fluids that mediate dolomitization. On this basis,

it is important that future studies of dolomite and island dolostones examine the pre-dolomitization diagenesis and consider how those processes conditioned the strata for later dolomitization. Acknowledgements We are grateful to the Natural Sciences and Engineering Research Council of Canada, which funded this research (grants A6090 to Jones), Mr. Paul Scott who gave us permission to collect samples from his quarry, Mr. M. Wojcik who assisted in the field, Dr. K. Muehlenbachs, University of Alberta, who provided the laboratory needed for the stable isotope analyses, Dr. Robert Creaser, University of Alberta, who provided the 87Sr/86Sr analyses used in this study, George Braybrook, University of Alberta, who took most of the SEM photomicrographs used herein, and Drs. J. Humphrey and L. Melim who critically reviewed an earlier version of this manuscript. References Aissaoui, D.M., Buiges, D., Purser, B.H., 1986. Model of reef diagenesis: Mururoa Atoll, French Polynesia. In: Schroeder, J.H., Purser, B.H. (Eds.), Reef Diagenesis. Springer-Verlag, Berlin, pp. 27 – 52. Badiozamani, K., 1973. The Dorag dolomitization model application to the Middle Ordovician of Wisconsin. Journal of Sedimentary Petrology 43, 965 – 984. Banner, J.L., 1995. Application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis. Sedimentology 42, 805 – 824. Budd, D.A., 1988. Aragonite-to-calcite transformation during freshwater diagenesis of carbonates: insights from pore-water chemistry. Geological Society of America Bulletin 100, 1260 – 1270. Budd, D.A., 1997. Cenozoic dolomites of carbonate islands: their attributes and origin. Earth Science Reviews 42, 1 – 47. Bullen, S.B., Sibley, D.F., 1984. Dolomite selectivity and mimic replacement. Geology 12, 655 – 658. Cronin, T.M., Dowsett, H.J., 1993. PRISM: warm climates of the Pliocene. Geotimes 11, 17 – 19. Dawans, J.M., Swart, P.K., 1988. Textural and geochemical alternations in Late Cenozoic Bahamian dolomites. Sedimentology 35, 385 – 403. Emery, K.O., Milliman, J.D., 1980. Shallow water limestones from slope off Grand Cayman Island. Journal of Geology 88, 483 – 488. Farrell, J.W., Clemens, S.C., Gromet, L.P., 1995. Improved chronostratigraphic reference curve of late Neogene seawater 87Sr/86Sr. Geology 23, 403 – 406.

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238 Friedman, I., O’Neil, J.P., 1977. Compilation of stable isotope fractionation factors of geochemical interest. United States Geological Survey Professional Paper, 440KK (12 pp.). Gill, I.P., Moore, C.H., Aharon, P., 1995. Evaporitic mixed-water dolomitization on St. Croix, U.S.V.I. Journal of Sedimentary Research, A 65, 591 – 604. Gonzalez, L.A., Ruiz, H.M., Taggart, B.E., Budd, A.F., Monell, V., 1997. Geology of Isla de Mona, Puerto Rico. In: Vacher, H.L., Quinn, T.M. (Eds.), Geology and Hydrogeology of Carbonate Islands. Developments in Sedimentology, vol. 54. Elsevier, Amsterdam, pp. 327 – 358. Hardie, L.A., 1987. Dolomitization: a critical view of some current views. Journal of Sedimentary Petrology 57, 166 – 183. Humphrey, J.D., 1988. Late Pleistocene mixing zone dolomitization, southeastern Barbados, West Indies. Sedimentology 35, 327 – 348. Humphrey, J.D., 2000. New geochemical support for mixing-zone dolomitization at Golden Grove, Barbados. Journal of Sedimentary Research 70, 1160 – 1170. Humphrey, J.D., Radjef, E.M., 1991. Dolomite stoichiometric variability resulting from changing aquifer conditions, Barbados, West Indies. Sedimentary Geology 71, 129 – 136. James, N.P., Choquette, P.W., 1990. Limestones—The meteoric diagenetic environment. In: McIlreath, I.A., Morrow, D.W. (Eds.), Diagenesis. Geoscience Canada Reprint Series. Geological Association of Canada, vol. 4, pp. 35 – 74. Jones, B., 1994. Geology of the Cayman Islands. In: Brunt, M.A., Davies, J.E. (Eds.), The Cayman Islands: Natural History and Biogeography. Kluwer Academic Publishing, Dordrecht, The Netherlands, pp. 13 – 49. Jones, B., Hunter, I.G., 1994a. Evolution of an isolated carbonate bank during oligocene, miocene, and pliocene times, Cayman Brac, British West Indies. Facies 30, 25 – 50. Jones, B., Hunter, I.G., 1994b. Messinian (late miocene) karst on Grand Cayman, British West Indies: an example of an erosional sequence boundary. Journal of Sedimentary Research, B 64, 531 – 541. Jones, B., Luth, R.W., 2002. Dolostones from Grand Cayman, British West Indies. Journal of Sedimentary Research 72, 559 – 569. Jones, B., Luth, R.W., 2003. Temporal evolution of Tertiary dolostones on Grand Cayman as determined by 87Sr/86Sr. Journal of Sedimentary Research 73, 186 – 203. Jones, B., Lockhart, E.B., Squair, C.A., 1984. Phreatic and vadose cements in the Tertiary Bluff Formation of Grand Cayman Island, British West Indies. Bulletin of Canadian Petroleum Geology 32, 382 – 397. Jones, B., Luth, R.W., MacNeil, A., 2001. Powder X-ray diffraction analysis of homogeneous and heterogeneous sedimentary dolostones. Journal of Sedimentary Research 71, 790 – 799. Krantz, D.E., 1991. A chronology of Pliocene sea level fluctuations: the U.S. middle Atlantic coastal plain record. Quaternary Science Reviews 10, 163 – 174. Land, L.S., 1985. The origin of massive dolomite. Journal of Geological Education 33, 112 – 125. Land, L.S., 1991. Dolomitization of the Hope Gate Formation (north Jamaica) by seawater: reassessment of mixing-zone do-

237

lomite. In: Taylor, H.P., O’Neil, J.R., Kaplan, I.R. (Eds.), Stable Isotope Geochemistry: A Tribute to Samuel Epstein. The Geochemical Society, Special Publication, vol. 3, pp. 121 – 133. Land, L.S., 1992. The dolomite problem: stable and radiogenic isotope clues. In: Clauer, N., Chaudhuri, S. (Eds.), Isotopic Signatures and Sedimentary Records. Lecture Notes in Earth Science, vol. 43, pp. 49 – 68. Lohmann, K.C., 1988. Geochemical patterns of meteoric diagenetic systems and their applications to studies of paleokarst. In: Choquette, P.W., James, N.P. (Eds.), Paleokarst. Springer-Verlag, New York, pp. 55 – 80. Machel, H.G., 2000. Dolomite formation in Caribbean Islands— driven by plate tectonics?! Journal of Sedimentary Research 70, 977 – 984. Machel, H.G., Burton, E.A., 1994. Golden grove dolomite, Barbados: origin from modified seawater. Journal of Sedimentary Research, A 64, 741 – 751. MacNeil, A.J., 2001. Sedimentology, diagenesis, and dolomitization of the Pedro Castle Formation on Cayman Brac, British West Indies. Unpublished MSc Thesis, University of Alberta. 128 pp. Meyers, W.J., Feng, H.L., Zachariah, J.K., 1997. Dolomitization by mixed evaporative brines and freshwater, upper miocene carbonates, Nijar, Spain. Journal of Sedimentary Research 67, 898 – 912. Mucci, A., Morse, J.W., 1983. The incorporation of Mg2 + and Sr2 + into calcite overgrowths: influences of growth rate and solution composition. Geochimica et Cosmochimica Acta 47, 217 – 233. Ng, K.-C., 1990. Diagenesis of the Oligocene – Miocene Bluff Formation of the Cayman Islands. Unpublished PhD Thesis, University of Alberta. 344 pp. Perfit, M.R., Heezen, B.C., 1978. The geology and evolution of the Cayman Trench. Geological Society of America Bulletin 89, 1155 – 1174. Pleydell, S.M., 1987. Aspects of diagenesis and ichnology in the Oligocene – Miocene Bluff Formation, Grand Cayman Island, British West Indies. Unpublished MSc Thesis, University of Alberta. 209 pp. Pleydell, S.M., Jones, B., Longstaffe, F.J., Baadsgaard, H., 1990. Dolomitization of the oligocene – miocene bluff formation on Grand Cayman, British West Indies. Canadian Journal of Earth Sciences 27, 1098 – 1110. Plummer, L.N., 1975. Mixing of sea water with calcium carbonate ground water. In: Whitten, E.H.T. (Ed.), Quantitative Studies in Geological Sciences. Geological Society of America Memoir, vol. 142, pp. 219 – 236. Sibley, D.F., 1980. Climatic control on dolomitization, Seroe Domi Formation (Pliocene), Bonaire, N.A. In: Zenger, D.H., Dunham, J.B., Ethington, R.L. (Eds.), Concepts and Models of Dolomitization. Society of Economic Paleontologists and Mineralogists Special Publication, vol. 28, pp. 247 – 258. Sibley, D.F., 1982. The origin of common dolomite fabrics: clues from the Pliocene. Journal of Sedimentary Petrology 52, 1087 – 1100. Sibley, D.F., 1991. Secular changes in the amount and texture of dolomite. Geology 19, 151 – 154. Smith, M.P., 1986. Silver coating inhibits electron microprobe beam

238

A. MacNeil, B. Jones / Sedimentary Geology 162 (2003) 219–238

damage of carbonates. Journal of Sedimentary Petrology 56, 560 – 561. Swart, P.K., Melim, L.A., 2000. The origin of dolomites in Tertiary sediments from the margin of Great Bahama Bank. Journal of Sedimentary Research 70, 738 – 748. Vahrenkamp, V.C., Swart, P.K., 1990. New distribution coefficient for the incorporation of strontium into dolomite and its implications for the formation of ancient dolomites. Geology 18, 387 – 391. Vahrenkamp, V.C., Swart, P.K., 1994. Late Cenozoic dolomites of the Bahamas: metastable analogues for the generation of ancient platform dolomites. In: Purser, B.H., Tucker, M.E., Zenger, D. (Eds.), Dolomites: A Volume in Honor of Dolomieu. International Association of Sedimentology Special Publications, vol. 21, pp. 133 – 153.

Vahrenkamp, V.C., Swart, P.K., Ruiz, J., 1991. Episodic dolomitization of Late Cenozoic carbonates in the Bahamas: evidence from strontium isotopes. Journal of Sedimentary Petrology 61, 1002 – 1014. Walters, L.J., Claypool, G.E., Choquette, P.W., 1972. Reaction rates and d18O variation for the carbonate-phosphoric acid preparation method. Geochimica et Cosmochimica Acta 36, 129 – 140. Ward, W.C., Halley, R.B., 1985. Dolomitization in a mixing zone of near-seawater composition, Late Pleistocene, Northeastern Yucatan Peninsula. Journal of Sedimentary Petrology 55, 407 – 420. Wheeler, C., Aharon, P., Ferrell, R.E., 1999. Successions of Late Cenozoic platform dolomites distinguished by texture, geochemistry, and crystal chemistry: Niue, South Pacific. Journal of Sedimentary Research 69, 239 – 255.