Dolomitization of Holocene Mg-Calcite Supratidal Deposits, Ambergris Cay, Belize

Dolomitization of Holocene Mg-calcite supratidal deposits, Ambergris Cay, Belize

^ Geological Consultants, Midland, Texas 79701 A. M. REID J J. M. GREGG St Joe Minerals Corp., Viburnum, Missouri 65566

ABSTRACT Skinner, 1957; Von der Borch and Lock, 1979; slow rates and over-all limited amounts of do- Von der Borch and others, 1964; Muir and oth- lomite found in most recent supratidal deposits Dolomitized crusts and associated sedi- ers, 1980), and the arid Persian Gulf sabkhas (Machel and Mountjoy, 1986) seemingly are in- ments 845-2925 yr old on Ambergris Cay, (Curtis and others, 1963; Illing and others, 1965; compatible with the formation of many thick Belize, Central America, compose a signifi- McKenzie and others, 1980). These dolomites sequences of ancient bedded peritidal dolomites. cant portion of the Holocene sediment section characteristically are microcrystalline, poorly An addition to the inventory of Holocene do- on many supratidal flats. The dolomitic sec- ordered and calcium-rich, and compose from a lomites described above are those crusts and as- tions are as thick as 0.7 m and contain an trace to, rarely, as much as 95%-100% of some sociated dolomitic sediments found on the average of 70% calcic dolomite. Dolomite oc- indurated supratidal crusts (Shinn and others, humid, supratidal carbonate flats on Ambergris curs as a replacement of high-Mg calcite mic- 1965; Deffeyes and others, 1965; Illing and oth- Cay in Belize, Central America (Fig. 1). Recent rite matrix and allochems, and as pas- ers, 1965), or they occur as poorly consolidated mapping has shown that supratidal dolomitiza- sively precipitated cements. Isotopic analyses sediments (Von der Borch and Lock, 1979; tion here is much more extensive than pre- suggest that dolomitization appears to be pro- Gebelein and others, 1980). In general, how- viously reported (Ebanks, 1975) and may in fact moted primarily by reactions of the magne- ever, amounts typically average only 20%-30% be more: pervasive and widespread than at most sian calcite supratidal sediments with essen- of sediment volume at most sites other than the other Holocene sites in the world (Mazzullo and tially near-normal marine waters. The effects Coorong Lagoon, which locally contains metre- Reid, 1985). Average dolomite content is 70%, of seasonal influxes of meteoric and hybrid thick beds of 100% dolomite (Von der Borch and the dolomitic crusts and sediments com- fluids, however, ¡ire also clearly evident in the and Lock, 1979). Predominantly aragonitic sed- monly compose more than one-half of the total crusts as indicated by isotopes, dolomite- iments are being dolomitized in these areas, the Holocene vertical section over wide areas of crystal etching, the selective leaching of arag- exception being the magnesian calcite-rich many supratidal flats; the dolomite is less than onite, and the local loss of Mg2+ from the host muds in the Coorong lakes (Von der Borch and 3,000 yr old. Furthermore, the supratidal sedi- micrite and included skeletal fragments. The Lock, 1979). Dolomitization in all of these areas ments here, as in the Coorong, are composed amounts, rapid rates of dolomitization, and appears to involve passive cementation, re- mainly of high-magnesian calcite rather than initial Mg-calcite mineralogy of the sediments placement of micrite matrix and allochems aragonite and mixed aragonite-calcite sediments make the Ambergris Cay supratidal flats ex- likely via dissolution-reprecipitation (Zenger that typically host the dolomites in most other ceptional in the Holocene, and this area may and Dunham, 1980), and, possibly, primary nu- areas. Hence, Ambergris Cay represents a major be a model for i:he genesis of some ancient cleation as well (Von der Borch and Lock, occurrence of supratidal dolomites that contrasts peritidal dolomites. 1979). Individual dolomitized crusts in the with other modern sea-marginal provinces in Florida-Bahamas-Antilles-Persian Gulf regions terms of host mineralogy, and amount and rapid typically are thin (several centimetres to locally rates of dolomite formation. This paper docu- INTRODUCTION 0.5 m), discontinuous, and compose only a rel- ments this most significant Holocene dolomite atively small portion of the total Holocene sec- occurrence in terms of describing basic aspects tion present. In the Coorong, however, as much of the sedimentology and petrographic attributes Holocene dolomites of penecontemporaneous as 1 m of Holocene dolomite mud occurs, over- of these supratidal deposits. Whereas detailed origin are known from several transitional- lying undolomitized aragonitic and high Mg- studies of the supratidal ground water system marine carbonate: provinces. The most publi- calcitic lagoonal sediments, which in turn and isotope and trace-element geochemistry of cized of these occ urrences are from the humid- overlie 4 m of microcrystalline dolomite in Pleis- the dolomites are currently in progress, the pre- zone supratidal flats of the Bahamas-South tocene beds beneath the modern relict coastal- liminary data presented herein provide some Florida region (Shinn and Ginsburg, 1964; plain deposits. Although these occurrences constraints on the chemistry of dolomitization Shinn and others, 1965; Atwood and Bubb, provide useful paleoenvironmental and diage- on Ambergris Cay. The nature and extent of 1970; Gebelein and others, 1980; Carballo and netic models of dolomitization, several discrep- dolomite formation here are somewhat unique Land, 1984) and Bonaire, Netherlands Antilles ancies are evident upon the uniformitarianistic in the Holocene, and may serve as a model for (Deffeyes and others, 1965; Lucia, 1968), the comparison of certain aspects of Holocene and the genesis of some thick supratidal dolomites in coastal lakes of the seasonally humid-arid Coo- ancient dolomites. Specifically, the relatively ancient rock sequences. rong region, South Australia (Alderman and

Geological Society of America Bulletin, v. 98, p. 224-231, 5 figs., 1 table, February 1987.

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of summer season (Ebanks, 1975). Average air temperatures in this easterly tradewinds belt vary seasonally from 24-27 °C. Surface-water temperatures are 30-34 °C in intra-island lagoons on and around the cay, and we have recorded temperatures of from 29 to 40 °C in ponded and interstitial waters in supratidal areas Reef Pt on which the dolomite is found. The Holocene sedimentology of Ambergris Cay was studied initially by Ebanks (1975), who mapped it from aerial photographs and re- connaissance ground surveys. The cay is a mo- saic of shallow intra-island lagoons and locally, \ heavily vegetated, broad supratidal flats (Fig. 1) that developed during slow sea-level rise during \ the past 6,100 yr (Pusey, 1975) on an irregular surface of karstic Pleistocene limestone (Purdy, 1974). Surficial Holocene sediments of the inte- km rior and leeward margin of the cay are predomi- CARIBBEAN nantly high-Mg calcite peloidal micrites, soritid SEA foraminiferal-molluscan sandy micrites and, along narrow beach or storm ridges, foraminiferal-molluscan sands and gravels. Our mapping CHETUMAL BAY indicates that dolomitized crusts (radiocarbon- dated at 845-2925 yr B.P.) and associated dolomitic sediments are abundant and widespread on many of the supratidal flats of northern and Little Tomas Savanna central Ambergris Cay (Fig. 1 and Table 1). Lesser occurrences of crusts occur on the leeside of the southern end of Ambergris Cay (Ebanks, 1975). Most of the dolomitized supratidal flats on the cay stand less than 23 cm above mean sea level, although the actual dolomite flats and surficial or near-surface crusts are essentially at or Bulkhead within a few centimetres of sea level, locally in Q Shoal topographic depressions on the underlying Pleis- tocene limestone (Figs. 2, 3). Accordingly, the areas of dolomite formation are influenced by meteoric as well as lagoon-derived marine fluids. Single and multiple crust horizons and, locally, composite (vertically coalesced) dolomite crusts are found on the flats (Figs. 2, 3). Individual crusts vary in thickness from 1-15 cm (avg 10 cm), although composite crusts are as much as 23 cm thick; inter-crust dolomitic Figure 1. Location of Belize, Central America, and Ambergris Cay (inset map). Detailed map sediments average 5-7 cm in thickness. In of Ambergris Cay shows distribution of exposed Pleistocene limestone (black), supratidal flats places, the total thickness of dolomitic crust and and intra-island lagoons, the windward beach ridge (stippled), and locations of dolomitic inter-crust sediments is nearly 0.7 m within a supratidal flats ("savannahs") discussed herein. Areas sampled by Ebanks (1975) that report- total Holocene section of 1-1.5 m thickness. The edly contain Holocene supratidal dolomite are indicated by the letter "E." dolomitic crusts and inter-crust sediments compose from 50%-75% of the Holocene vertical sediment section on most of the supratidal flats OCCURRENCE AND PETROLOGY Central America, along the southeastern Yuca- on Ambergris Cay that have been examined to OF DOLOMITE CRUSTS tan Platform (Fig. 1). The cay is a major hydro- date; dolomitized flats compose nearly one-third 2 graphic barrier that separates the narrow, of the estimated total 30-km area of Ambergris high-energy outer-shelf-facies and reef-facies Cay that contains any appreciable thickness of Geologic Setting tracts and the protected Chetumal Bay lagoon to Holocene supratidal sediment (Fig. 1). The its lee. The region is located entirely in the widths of the dolomitized portions of supratidal Ambergris Cay is a low-lying peninsula lo- humid tropical zone, with annual rainfall of flats generally averages 60-100 m across 150- cated on the northern carbonate shelf of Belize, 127-152 cm/yr distributed during 7-9 months to 300-m-wide flats.

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Figure 2. (A) Geared bedding-plane exposure of upper dolomite crust (D) benealh 1.0 cm of desiccated, doiomitic muds (M); note mangrove pneuinatophores (arrows); Tomas Savannah, penknife (circled) for scale. (B) Pit dug into supratidal flat on Tomas Savannah; pencil for scale (arrow). Water level during this neap tidal period coincided with the top of the Pleistocene limestone (dashed) and was 30 cm below the surface (S). At this locality, there are 3 superposed crusts (UC, MC, LC), the upper two separated by thinner layers of doiomitic sediment, the lower crust resting directly on the Pleistocene bedrock. (C) Detail of upper dolomite crust from Tomas Savannah showing lack of megascopic features diagnostic of supratidal environments, excepi: for possible rare incipient sheetcracks (arrow); note casts of aragonitic pelecypods and extensive porosity (53%, 7 dairies permeability: daila courtesy of Core Laboratories). Scale, 3.0 cm in length.

Crusts and Associated Supratidal Sediments vealed the presence of no cements in the crusts commun.) and annelids. The crusts, whether other than dolomite. Hence, lithification of the surficial or buried, vary from homogeneous to The crusts and associated sediments consist of crusts and inter-crust sediments is due entirely to those in which some textural laminations, com- dolomitized micrites and some sandy micrites dolomitization, although the reasons for the only ponent allochems and skeletal casts, and biotur- and micritic sands of original high-Mg calcite limited induration of inter-crust sediments are bation features are readily apparent. Similar (14-17 mole%) composition, with only minor not readily apparent. By contrast, some Holo- crusts have been described from Holocene de- amounts of cerithid gastropod and molluscan- cene doiomitic crusts reported from elsewhere in posits by Shinn and others (1965), Shinn derived aragonit;, the low aragonite content the world are partly cemented by Mg-calcite and (1968),, Hardie (1977), and Muir and others being the result of initially low shell contribu- aragonite (for example, Shinn and others, 1965; (1980), as well as from many ancient supratidal tions to the supratidal sediments, as well as their Gebelein and others, 1980). dolomites (Zenger, 1972). Desiccation cracks, wholescale dissolution; the aragonitic shells are Those "pavement-like" crusts (Ebanks, 1975) unequivocal algal laminations, and fenestrae found mainly in the surficial muds and some found immediately beneath the surface, which were not observed, however, in any of the crusts inter-crust sediment layers and are virtually ab- can be traced for most of the length of individual or inter-crust sediments examined (Fig. 2C), al- sent in the dolomite crusts. Locally, however, supratidal flats, are overlain by several centime- though supposed algal mat crenulations were some of the youngest crusts contain as much as tres of bare, moist to desiccated sediment or noted in some crust samples by Ebanks (1975). 20% skeletal aragonite (Table 1). Traces of detri- mantles of Scytonema-matted (cyanobacterial) The upper surfaces of some of the (youngest) tal quartz, clays, variable amounts of volcanic muds (Fig. 2A) with minor aragonite and as surficial crusts are blackened by algal and/or ash, and low-Mg calcite of particulate skeletal much as 17% authigenic dolomite replacing mic- fungal micritization, although buried crusts are and diagenetic origin were detected in most rite and high-Mg calcite allochems (Table 1). not so discolored and do not exhibit any physi- crust and inter-crust samples. The degree of in- Surficial and similarly widespread, more deeply cal or diagenetic indications of having been ex- duration decreases downward within individual buried crusts and inter-crust sediments are heav- posed subaerially. Such features, however, typi- crusts into the nodular (incipiently cemented) to ily penetrated locally by pneumatophores of the cally are rapidly lost upon shallow burial (E. A. unconsolidated inter-crust sediments (Fig. 2B), mangrove Avicennia nitida (Fig. 2A), the fine Shinn, 1985, personal commun.). Field relations although this transition is not always related to a rootlets of Rhizophora mangle and other sa- and the above-mentioned features suggest that decrease in the total amount of dolomite present vannah halophytes and, in places, by small each crust initially was formed at the sediment- (Table 1). Scanning electron microscopy has re- arthropods (W. J. Ebanks, 1985, personal air and/or sediment-shallow-water (2-3 cm) in-

TABLE 1. GEOCHEMICAL DATA FOR AMBERGRIS CAY DOLOMITES

Locality c'V Wt % dolo. Dolomite Wt% Mole % MgCOj Max. % arag. Dolomite (yr) chemistry Mg-calcite 6»C ä'8° PDB

Tomas Savannah

Surficial sediment (3) 15-17 Ca57 83-85 15.4 tr-15

Upper crust (15) Ca 905-1075 60-90 53-58 10-40 14-17* tr 1.5-1.7(2) -0.6 to -0.9(2) Inter-crust (4) 35 C*58 65 15.1 tr

Middle crust (14) a 1690 62-82 57-58 18-38 14.7-16.8 0 1.2-1.9(2) -1.7 to-2.4(2)

Inter-crust (4) 97 Ca57 3 17.7 tr Lower crust (12) 2925 79-97 <^57-58 3-21 1.5-31.4^ 0 2.21(1) -5.2(1) So. surficial crust (8) 60-90 Ca57 1040 8-13.8 tr-20

Little Tomas Savannah (3) 1035 46 Ca54 54 14-17 tr San Pedro Savannah (4) 52-77 "^55-57 2348 14-15 tr

Isolated flat (I) 845 100 Ca55 0 0 0 1.91(1) -2.5(1)

San Juan Savannah (3) 72 Ca57 28 15.6 tr

Unaltered soritids (2) Modem 13-15 0

Dolomitic soritids (2) 905-2925 14-17 0

Note: refer to Appendix I for discussions of analytical methods; (N = number of samples analyzed; isotope analyses indicated separately).

•Whole-rock analysis; several foraminiferids (for example. Fig. 4B) contained 2.0 mole% MgC03 or less. 'Whole-rock analysis.

terface at sea level, in a regime of vertically present in inter-crust sediments ranged from San Pedro Savannah to the immediate east fluctuating interstitial and standing lagoonal wa- 35%-97% (Table 1). By contrast, Ebanks (1975, (Fig. 1), widespread shallow-buried crusts con- ters. Those originally surficial crusts on the su- Appendix B) reported an average of only 25% tain a range from 52%-77% dolomite. A near- pratidal flats subsequently have been covered dolomite in the crusts he examined (25 sam- surficial crust on an isolated flat here, dated at beneath thin layers of storm-deposited muds ples), although the amounts ranged from traces 845 yr B.P., was found to be completely dolo- (Fig. 2A). The occurrence of as much as 17% to as much as 97% of individual surface samples. mitized (11 XRD runs). Hence, there does not dolomite in surficial mud layers (higher in bur- The apparent discrepancy in these data arises appear to be a positive correlation between age ied inter-crust layers: Table 1), and the very re- from the fact that Ebanks analyzed only bulk and dolomite content within the suite of crust cent age of the most surficial crusts (845-1,075 surface sediment samples, some of which in- and inter-crust sediment samples examined yr) suggest that incipient crust formation cur- cluded crust fragments, rather than specifically (Table 1). Where multiple crusts are present, rently is operative on and within many of the sampling vertical profiles of the crusts and inter- such as on Tomas Savannah (Fig. 3), each supratidal flats on Ambergris Cay. The succes- crust sediments, which in most places are buried grades downward into nodular, partly indurated sion of surficial and buried crusts found on some beneath a veneer of more recent deposits. The to unconsolidated dolomitized sediment com- flats (Figs. 2B, 3) accordingly preserves a record oldest crust collected on Ambergris Cay, at posed of soritid foraminiferal micrites with of Holocene sea-level rise and gradual transgres- nearly 0.7 m beneath the surface on Tomas Sa- traces of skeletal aragonite (Table 1 and sion of the Ambergris Cay supratidal flats (for vannah (Figs. 1, 3), was whole-rock radiocar- Fig. 2B). In places, multiple crusts and inter- example, Shinn and others, 1965). bon dated at 2925 yr B.P. and contains from crust sediment horizons merge into thick (23 The amount of dolomite in 60 Ambergris Cay 79%-97% dolomite (Table 1). The youngest cm) units of massive dolomite. crust samples examined to date averages 70%, crusts on this and an adjoining flat (Little Tomas with no less than 46% dolomite found in any of Savannah) contain from 46%-90% dolomite, Dolomite Chemistry and Petrography the indurated crusts; the amount of dolomite and are dated at 905-1075 yr B.P. (Table 1). On The dolomites of the crusts and inter-crust sediments are poorly ordered, non-stoichiometric calcic dolomites that are similar compositionally to other Holocene supratidal dolomites, ranging

from Ca53Mg47 to Ca5gMg42- Ebanks (1975) reported compositions of the crusts he exam-

ined of Ca54Mg46 to Ca^oMg4Q. Petrographic examination indicates that the dolomite occurs in three modes in the Ambergris Cay crusts and partly indurated inter-crust sediments, and that crystal textures vary with regard to mode of dolomitization and lithology of the host sedi- Figure 3. Schematic of superposed dolomite crusts ("upper," "middle," and "lower" of text ments (Fig. 4). The most common mode is and Table 1) and inter-crust dolomitic sediments on northern Tomas Savannah, illustrating matrix-replacive and/or selective dolomitization relations to adjoining supratidal fades and Pleistocene limestone. The dolomite "ponds" here of peloids, intraclasts, and soritid foraminifera in are topographic lows essentially at sea level, although seasonally they may be inundated or dry. the Mg-calcite micrite-dominated deposits Meteoric waters discharge seasonally into the supratidal sediments from the Pleistocene (Fig. 4A). Replacement of the foraminiferal tests aquifer (curved arrows). may be texture-obliterative (Fig. 4A) or, locally,

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Figure 4. (A) Scanning electron micrograph of upper crust on Tomas Savannah (84% dolomite): extensive dolomitization of micrite matrix and nearly complete obliteration of the test of the soritid Archaias angulatus (open arrow); dolomite cements in aragonite-solution vugs (bold arrow); bar scale, 100 y.m. (B) Backscattered electron image of a foraminiferid composed of low-Mg calcite (LMC: Ca9g), with intraskeletal

dolomite cement (D: Cass). The matrix (M) is heavily dolomitized (Ca56). Both the matrix and included unaltered soritids (not shown) in this rock are high-Mg calcite (12.8-14.5 mole% MgC03). Electron microprobe analyses and photograph courtesy of D. R. Brosnahan. Bar scale, 50 ^im. (C) Electron micrograph of dolomite rhombs and clusters of rhombs forming as intraparticle cements in chambers of Archaias angulatus. Naniiocrystal dolomite has replaced the walls of the foraminiferid (top of photo). Note the solution-rounding of the dolomite crystals (arrows). Uppeir crust, Tomas Savannah; bar scale, 100 jum. (D) Electron micrograph of interparticlle dolomite cement (arrows) in coarse foraminiferal-pelecypod sands, upper crust on southern Tomas Savannah; bar scale, 1.0 mm.

mimetic of compositionally altered tests ment, which is most abundant within the tests of the smallest of these crystals are similar to those (Fig. 4B). Mosaics of 0.5- to 2.5-jum subhedral originally high-Mg calcite soritid foraminifera described by Carballo and Land (1984) from crystals predominate in these rocks; the finest (Fig. 4A), as well as in vugs that resulted from dolomite crusts on Sugarloaf Key in Florida. In crystal sizes occur in the replaced allochems. the complete dissolution of aragonitic skeletal the less commonly encountered crusts formed in Here, dolomitization is strongly preferential to fragments (Figs. 2C, 4A; Deffeyes and others, coarse, mixed aragonitic and Mg-calcitic skeletal high-Mg calcite allochems, whereas it is not in 1965). This dolomite occurs as 1- to 3-jum, well- sands (relict beach or storm ridges), the third other Holocene occurrences, except during the formed rhombs and larger clusters of rhombs mode of dolomite occurs as inter- and intra- advanced stages of replacement (for example, (Fig. 4C). Solution-etching and rounding of particle cements (Fig. 4D) with habits and tex- Illing and others, 1965). The second mode also some of the rhombs are evident in many of the tures essentially identical to those found in the occurs in the micritic deposits as dolomite ce- samples (Fig. 4C), and it is possible that some of micritic crusts. Although similar dolomite habits

are documented from other Holocene occur- seasonally on the flats, with attendant ephemeral however, in contrast to their presence in well- rences, nowhere are replacement and cementa- halite crusts, we nevertheless have yet to find documented meteoric and mixed-fluid diage- tion so extensive as to produce indurated crusts any textural, microfabric, isotopic or trace- netic environments (for example, Sibley, 1980). and inter-crust sediments with such high average element evidence in the supratidal sediments or dolomite contents as in the Ambergris Cay su- dolomite samples examined that could be re- SIGNIFICANCE AND ANALOGY TO pratidal flats (Table 1). lated to diagenetic interactions with any hyper- ANCIENT ROCKS saline fluids. Dolomitization appears to be occurring by reactions with essentially unmodi- HYDROLOGY AND DIAGENETIC fied sea water, as postulated for other areas by The dolomite occurrences on Ambergris Cay INFERENCES Atwood and Bubb (1970), Sailer (1984), and are anomalous relative to other Holocene peri- Machel and Mountjoy (1986). tidal sites in terms of their abundance, amounts, Preliminary data on dolomite and host- Several indications of the diagenetic signifi- and rapid rates of accumulation. Although the sediment compositions, oxygen and carbon sta- cance of the seasonal influx of meteoric and/or reasons for such pervasive dolomitization are ble isotopes and water chemistry, in conjunction mixed meteoric-marine fluids are noted, how- not readily apparent, it may be related in large with field and textural observations, provide ever, in the dolomites examined. For example, part to the initial mineralogy of the supratidal some insight into the nature of the fluids respon- the samples of lower crust with a <513C of sediments here or to the high alkalinity of inter- 18 sible for dolomitization and attendant diagenetic -5.2°/oo and a slightly enriched S 0 of 2.2°/00, stitial solutions (Lippmann, 1973; Machel and processes on the Ambergris Cay supratidal flats. and similar samples from the upper crust (both Mountjoy, 1986). According to Land (1980, Presently available analyses of interstitial waters from Tomas Savannah), locally contain zones of 1985), greater amounts of dolomite can theoret- collected on the Tomas Savannah supratidal flat Mg-calcite matrix and included soritid forami- ically be formed with increasing temperatures, indicate strongly seasonally fluctuating salinities nifera tests that are depleted in Mg2+, being content of particulate high-Mg calcite in the host of from 25°/oo or less to periodically greater than 1.5-1.7 mole% in contrast to typical concentra- sediments (not so, however, according to

75°/oo (adjoining lagoonal waters 30°/oo to tions of 13-15 mole% MgC03 (Table 1). The Gaines, 1980), and total mole% MgC03 in the 43°/oo). The Mg/Ca ratio of these waters varies isotope data from these crusts may indicate the reactant calcite. Accordingly, the dominance of from 3:1 to 5:1. Field observations have indi- influence of meteoric waters transporting soil- magnesian calcite (14-17 mole%) and only cated that a lateral bidirectional flow through derived carbon through the flats, or the oxida- minor amounts of aragonite on the Ambergris the supratidal flats of marine-derived waters is tion of organic matter accompanying sulfate Cay supratidal flats, in concert with high am- maintained by weak semi-diurnal tides (maxi- reduction (Baker and Kastner, 1981) within the bient temperatures, high alkalinity, and possibly mum 20-cm tidal range). Superimposed on this hot supratidal sediments. Gaines (1980) likewise sulfate reduction (Baker and Kastner, 1981; background marine-fluid input system, there are reported that the oxidation of certain organic Kastner, 1983; but see Morrow and Ricketts, hydrochemical perturbations induced by sea- material, particularly aspartic acids which are 1986), inherently may promote such rapid do- sonal periods of (short-term) hypersalinity and common in aragonite but presumably less so in lomitization, perhaps coupled with the low to meteoric influx and dilution of interstitial and the Ambergris Cay high-Mg calcite supratidal average Mg/Ca ratios found here (Folk and ponded waters on the supratidal flats and in ad- deposits, may favor increased rates of dolomiti- Land, 1975; Gaines, 1980; Lippmann, 1973). joining lagoons. Hence, the supratidal flats rep- zation. As noted by Zenger and Dunham Enhanced rates of dolomitization may also be resent environments of schizohalinity and mix- (1980), however, the presence of other organic related to the complex schizohaline hydrochem- ing. Yet, analysis of dolomite separates from compounds also are known to promote dolo- ical system of the supratidal flats (Folk and crusts that contain greater than 60% dolomite mitization. The low sodium content of the 100% Land, 1975), wherein a continual flux of Mg2+ 18 yielded S 0 isotope values of 1.2 to 2.2°/00 and dolomite crust from San Pedro Savannah (660 is supplied from sea water and also a large 13 6 C values of-0.6 to -2.5°/oo (PDB), and one ppm) may also indicate some meteoric altera- amount being liberated from the host sediments 13 0 sample gave a <5 C of -5.2 /00 (Table 1). The tion of the dolomites (Land and Hoops, 1973). by partial dissolution via reactions with alkaline oxygen-isotope data are suggestive of the in- Likewise, the local presence in these and other waters of meteoric and mixed meteoric-marine volvement, during dolomitization, of marine-de- crust samples of etched dolomite rhombs and derivation. rived fluids of near-normal to only slightly observed pervasive dissolution of aragonitic An obvious corollary of the suggestion of in- elevated salinities rather than to reactions with skeletal fragments (Figs. 2C, 4), in addition to itial mineralogical control on dolomitization is hypersaline or brackish waters. Analysis of the the observed local depletion of Mg2+ in the mic- the reaffirmation that many platform dolomites 100% dolomite crust from San Pedro Savannah rite and allochems, are additional evidence of in the rock record may have originally been (one sample) indicated concentrations of Sr2+ of dolomitization interrupted by periods of mete- composed of such susceptible high-Mg calcite 600 ppm, a value similarly within the range of oric influx. Vertical and lateral discharge of rather than aragonite (Wilkinson, 1982; Land, other normal-marine dolomites (Land, 1980). meteoric and brackish waters from the Pleisto- 1985). By contrast, the relatively limited The suggestion of slightly excessive salinity pro- cene limestone aquifer into the supratidal flats amounts of dolomite found in other Holocene vided by some of the oxygen-isotope data likely occurs during rainy seasons as observed by us on areas, with the exception of the Coorong region, results from periodic evaporative concentration Tomas Savannah (Fig. 3). Accordingly, the al- may be related to, among other kinetic factors, on the flats of lagoon-derived fluids. In fact, teration and removal of metastable mineralogies certain constraints imposed by the predominant Ebanks (1975) recorded salinities as high as in the dolomite crusts and associated sediments aragonitic mineralogy of those supratidal sedi- 76%o to 123%o in supratidal ponds; he sug- may instead be related to reactions with mixed ments (although Sibley, 1980, and others have gested that dolomitization may be related to a marine-meteoric fluids (for example, Back and also shown the susceptibility of aragonite to re- lagoonward reflux and/or tidal pumping-capil- others, 1986) rather than to dissolution by me- placement). With rates of dolomitization of 14 lary concentration mechanism. Although evap- teoric waters alone. Calcite cements were decid- cm/1,000 yr as calculated for one supratidal flat orative concentration undoubtedly occurs edly absent from the dolomitic sediments, alone (Mazzullo and Reid, 1985), and extrapo-

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Figure 5. Detail of Upper Permian island fades in Tansill For- mation, Guadalupe Mountains, New Mexico, showing subtidal biograinstones (SG) overlying a dolomite crust (C) similar tex- turally to those on Ambergris Cay. The porous crust has sharp upper and gradational lower contacts, respectively, and included molds of formerly aragonitic bivalves (bold arrow). The crust overlies porous biograinstones deposited initially in a subtidal environment followed by a peritidal environment. This unit (I-S) has abundant laminar fenestrae (small arrows), locally with internal silt and pendulous cements; length of scale on right, 3.0 cm.

raneous dolomitization of Mg-calcite sediments apparently by marine fluids in environments not associated with evaporites, may be applicable to the origin of some supratidal dolomites in the geologic record.

lating from the estimated amounts and areal ex- Ambergris Cay, apparently more so than in ACKNOWLEDGMENTS tent of dolomite found in the Holocene on most other areas of the world. Single, multiple, Ambergris Cay, a considerable thickness of do- and coalesced dolomite crusts interbedded with K. C. Lohmann and L. S. Land kindly pro- lomite conceivably could be formed under the dolomitic sediments here compose a major por- vided carbon- and oxygen-isotope data for sev- requisite conditions of subsidence, Mg supply, tion of the total Holocene supratidal section and eral dolomite samples from Ambergris Cay. and so on, in similar settings over relatively short contain an average of 70% calcic dolomite. The Don Buelter provided atomic absorption anal- periods of geologic time. Hence, the pervasive crusts are nearly entirely dolomite, in contrast to yses for Na and Sr for some crust samples, dolomitization of humid-zone supratidal flats as the only partly dolomitic (with aragonite and/or and D. R. Brosnahan performed several electron on Ambergris Cay, possibly involving essentially calcite cements) crusts found in other peritidal microprobe runs on selected samples. R. W. unmodified sea water, may be a reasonable occurrences. This recent (less than 3,000 yr), Smith assisted J. M. Gregg in developing the analog for the origin of some thick sequences of relatively abundant dolomitization may be in- quantitative X-ray diffraction techniques used in peritidal dolomites in the stratigraphic record. herently related, at least in part, to the dominant this study. Radiocarbon dates of crust samples Zenger (1972) discussed the genesis and rec- high-Mg calcite (14-17 mole% MgC03) com- were from Geochron Laboratories. SEM facili- ognition of peritidal and shallow subtidal do- position of the host sediments, which contrasts ties at Texas A&M University were made avail- lomitization in ancient rocks. He argued that it is with the aragonite mineralogy of other Holo- able by Jim Mazzullo, and Pam Leshack assisted the sum of all available textural and petrograph- cene dolomitic supratidal deposits (Coorong as technician. The cooperation and assistance of ic criteria, and not merely a Holocene analogy, lakes being the only exception). The dolomite Seferino Paz, Lydia Gonzalez, and Parker Lat- that allows an interpretation of the likely pene- occurs as a replacement of allochems and mic- shaw in field logistics, and Señor Tomas Paz for contemporaneous supratidal origin of some an- rite and as passively precipitated cements in the navigation around Ambergris Cay are gratefully cient dolomites. Within this context, those crusts and inter-crust sediments, the former of acknowledged. Ambergris Cay dolomite crusts with homoge- which formed surficially on the supratidal flats. Original versions of this manuscript were re- neous or relict component textures (Fig. 2C), if Presently available geochemical data appear to viewed by W. J. Ebanks, D. H. Zenger, E. A. preserved in the geologic record, would not be suggest that dolomitization is effected by the Shinn, and L. S. Land. We thank reviewers J. readily recognized as being of supratidal origin passage through the sediment of essentially un- Kaufman, J. Kaldi, and E. Mountjoy for their because of the paucity of textures indicative of modified marine waters. The diagenetic effects comments and suggestions on manuscript such depositional environments (Fig. 5). The ab- of seasonal influxes of meteoric and hybrid improvement. sence or rarity of such diagnostic features in fluids, however, are also indicated by the selec- these crusts is related to their formation in prox- tive removal by dissolution of particulate arago- 2+ imity to sea level, on "wet" (for example, low- nite, the Mg depletion of micrite matrix and APPENDIX 1: ANALYTICAL METHODS relief, geomorptiically youthful or micro-tidal) allochems within the dolomitized sediments, and supratidal flats lhat are only rarely and briefly the presently available oxygen-isotope data. Stoichiometry of dolomites and calcites was deter- desiccated below the upper few centimetres of mined using a computer-automated X-ray diffrac- Field examples of Holocene supratidal do- tometer, by measuring [104] peak shifts (Lumsden and section. Many similar, apparently nondescript lomitization have provided basic models for un- Chimahusky, 1980). Accuracy of this method was dolomites in ancient platform carbonate se- derstanding the depositional environments and verified using electron microprobe. Ordering in do- quences, particularly if interbedded with subtid- timing of diagenesis of ancient dolomites lomite samples was qualitatively determined by exam- ining the intensities of the [101], [015], and [021] al lithologies (Fig. 5), may be directly analo- (Zenger, 1972). Yet, the limited amounts and gous to the Aml>ergris Cay dolomite crusts. reflections (Graf and Goldsmith, 1956). thicknesses of Holocene supratidal dolomites Weight % dolomite in the samples was also deter- (Machel and Mountjoy, 1986) are seemingly in- mined by XRD methods. The areas under the [113] CONCLUSIONS compatible with the origin of many widespread dolomite peaks were measured and compared to those and thick, ancient dolomites that appear to have under the [111] peak of a weighted fluorite internal standard. The ratios obtained were compared to Dolomitization of Holocene sediments is formed in short periods of time. Hence, the Am- standards prepared using a sample from one of the widespread on many of the supratidal flats on bergris Cay example of rapid, penecontempo- Ambergris Cay crusts that contains 100% dolomite.

The [113] dolomite peak was used for weight % do- Folk, R. L., and Land, L. S., 1975, Mg/Ca ratio and salinity: Two controls over Muir, M., Lock, D., and Von der Borch, C., 1980, The Coorong model for crystallization of dolomite: American Association of Petroleum Geolo- penecontemporaneous dolomite formation in the Middle Proterozoic lomite analysis, rather than the [104] peak, in order to gists Bulletin, v. 59, p. 60-68. McArthur Group, Northern Territory, Australia, in Zenger, D. H., avoid error due to inherent variability in dolomite Gaines, A. M., 1980, Dolomitization kinetics: Recent experimental studies, in Dunham, J. B., and Ethington, R. L., eds., Concepts and models of Zenger, D. H., Dunham, J. B., and Ethington, R. L., eds., Concepts and dolomitization: Society of Economic Paleontologists and Mineralogists stoichiometry (Lumsden, 1979). models of dolomitization: Society of Economic Paleontologists and Special Publication No. 28, p, 51-67. Samples for XRD analysis were ground to less than Mineralogists Special Publication No. 28, p. 81-86, Purdy, E. G„ 1974, Reef configurations: Cause and effect, in Laporte, L. F., ed., 200 mesh and side-loaded into X-ray mounts to assure Gebelein, C. D., Steinen, R. P., Garrett, P., Hoffman, E. J., Queen, J. M., and Reefs in time and space: Society of Economic Paleontologists and Min- Plummer, L. N., 1980, Subsurface dolomitization beneath the tidal flats eralogists Special Publication No. 18, p. 9-76. random orientation of crystals. All samples were re- of central west Andros Island, Bahamas, IBID, p. 31-49. Pusey, W. G, 1975, Holocene carbonate sedimentation on northern Belize mounted and X-rayed a minimum of three times to Graf, D. L., and Goldsmith, J. R , 1956, Some hydrothermal syntheses of shelf, in Wantland, K. F., and Pusey, W. C., eds., Belize shelf- assure reproducibility of results. dolomite and protodolomite: Journal of Geology, v. 64, p. 173-186. Carbonate sediments, clastic sediments, and ecology: American Associ- Hardie, L. A., 1977, Sedimentation on the modern carbonate tidal flats of ation of Petroleum Geologists, Studies in Geology No. 2, Tulsa, Oxygen- and carbon-isotope data were obtained northwest Andros Island, Bahamas: Baltimore, Maryland, Johns p. 131-233. (from K. C. Lohmann and L. S. Land) from bulk Hopkins University Press, Studies in Geology No. 22, 202 p. Sailer, A. H., 1984, Petrologic and geochemical constraints on the origin of IUing, L. V., Wells, A. J., and Taylor, J.C.M., 1965, Penecontemporary subsurface dolomite, Enewetak Atoll: An example of dolomitization by samples that were treated with 0.25 M tetrasodium dolomite in the Persian Gulf, in Pray, L. C., and Murray, R. C., eds., normal sea water: Geology, v. 12, p. 217-220. EDTA to remove calcite. The samples were analyzed Dolomitization and limestone diagenesis: Society of Economic Paleon- Shinn, E. A., 1968, Selective dolomitization of some Recent sedimentary struc- by XRD to verify that all of the calcite was in fact tologists and Mineralogists Special Publication No. 13, p. 89-111. tures: Journal of Sedimentary Petrology, v. 38, p. 612-616. Kastner, M., 1983, Origin of dolomite and its spatial and chronological Shinn, E. A., and Ginsburg, R. N., 1964, Formation of Recent dolomite in removed during this procedure. Dolomites were distribution—A new insight: American Association of Petroleum Geol- Florida and the Bahamas: American Association of Petroleum Geolo- reacted at 50 °C with phosphoric acid, and isotope ogists Bulletin, v. 67, p. 2156. gists Bulletin, v. 48, p. 547. enrichments were corrected for <5I70 contributions. Land, L. S., 1980, The isotopic and trace element geochemistry of dolomite: Shinn, E. A., Ginsburg, R. N„ and Uoyd, R. M., 1965, Recent supratidal The state of the art, in Zenger, D. H., Dunham, J. 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