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

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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. 224 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/2/224/3434777/i0016-7606-98-2-224.pdf by guest on 01 October 2021 DOLOMITIZATION OF HOLOCENE Mg-CALCITE DEPOSITS, BELIZE 225 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 la- goons on and around the cay, and we have re- corded 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, foraminif- eral-molluscan sands and gravels. Our mapping CHETUMAL BAY indicates that dolomitized crusts (radiocarbon- dated at 845-2925 yr B.P.) and associated do- lomitic 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 sur- ficial 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) dolo- mite 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 com- pose 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/2/224/3434777/i0016-7606-98-2-224.pdf by guest on 01 October 2021 226 MAZZULLO AND OTHERS Figure 2.
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