Dolomite: Occurrence, Evolution and Economically Important Associations

Earth-Science Reviews 52Ž. 2000 1±81 www.elsevier.comrlocaterearscirev

Dolomite: occurrence, evolution and economically important associations

John Warren Department Petroleum Geosciences, UniÕersity Brunei Darussalam, Tungku Link, Bandar Seri Begawan, Brunei Received 14 October 1999; accepted 7 July 2000

Abstract

Dolomite is not a simple mineral; it can form as a primary precipitate, a diagenetic replacement, or as a hydrothermalrmetamorphic phase, all that it requires is permeability, a mechanism that facilitates fluid flow, and a sufficient supply of magnesium. Dolomite can form in lakes, on or beneath the shallow seafloor, in zones of brine reflux, and in early to late burial settings. It may form from seawater, from continental waters, from the mixing of basinal brines, the mixing of hypersaline brine with seawater, or the mixing of seawater with meteoric water, or via the cooling of basinal brines. Bacterial metabolism may aid the process of precipitation in settings where sulfate-reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings. Dolomite is a metastable mineral, early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism. Each new phase is formed by the partial or complete dissolution of an earlier dolomite. This continual re-equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures. Because subsurface dolomite evolves via dissolution and reprecipitation, a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart. Dolomitization also creates new crystals, with new rhomb growth following the dissolution of less stable precursors. Repetition of this process, without complete pore cementation, can generate intercrystalline porosity a number of times in the rock's burial history. Intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage. The fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings. The same association of evaporites Ž.sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals. This tends to occur at higher temperatures Ž.)608C±808C and so the resulting dolomites tend to be ferroan and consist of saddle-shaped crystals. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: dolomite; brine; models; porosity; permeability; oil and gas; base metal

E-mail address: [email protected]Ž. J. Warren .

1. The dolomite problem Ð an introduction than 50% of the unit's mineralogy and forms in patches across areas that are no more than tens of What is dolomite? For more than 150 years geolo- kilometers wide. In contrast, most ancient dolomite gists have addressed this question using both field entrains dolomiteŽ. the mineral that makes up more and laboratory based approachesŽ Von Morlot, 1847; than 90% of the rock volume, is secondary, exten- Skeats, 1905; Van Tuyl, 1918; Steidtmann, 1926; sive and often sparry. It has overprinted whole lime- Hewett, 1928; Young, 1933; Riviere, 1939; Landes, stone platforms as diagenetic units that are typically 1946; Behre, 1947; Chave, 1952; Graf and Gold- some hundreds of meters thick and extend across smith, 1956; Fairbridge, 1957; Strakhov, 1958; areas that may be hundreds of kilometers wide. Medlin, 1959; Baron, 1960; Wells, 1962; Alderman Possible reasons for the disparity between modern and von der Borch, 1963; Friedman and Sanders, and ancient modes of occurrence are many, and 1967; and the other numerous papers discussed in related to dolomite's unique mineralogy and chem- this review. . istry. The most important control on distribution is About 80% of the oil and gas reservoirs in North possibly kinetics; there has not been enough time in American carbonate rocks are in dolomites and up to the Holocene to form extensive areas of secondary 50% of the world's carbonate reservoirs are dolomites dolomiteŽ. Land 1985, 1998; Chai et al., 1995 . Ž.Zenger et al., 1980 . Most Mississippi Valley-type Then there is the problem of parity, do variations Ž.MVT deposits, with their economic accumulations in seawater chemistry influence dolomite volumes in of Pb and Zn, are hosted in dolomite, as are many carbonate platforms, or are changes in the volume of skarn ore bodiesŽ. Warren, 1999 . The imperatives for dolomite influencing seawater chemistry? Some understanding dolomite in its various forms are obvi- would argue that seawater chemistry has changed ous; why then, is its origin so poorly understood? from the Archean till nowŽ especially the proportions r Dolomite is an unusual carbonate mineral. It is of Mg Ca and pCO2 . . Were times of widespread common in ancient platform carbonates, yet it is rare platform growth also times of widespread dolomiti- in Holocene sediments and, without bacterial media- zationŽ. Given and Wilkinson, 1987 ? Spencer and tion, is near impossible to precipitate in the labora- HardieŽ. 1990 and Hardie Ž. 1996 have argued that tory at earth surface temperatures. Some argue that the level of Mg in the oceans has been relatively widespread dolomite forms mostly in association constant, but changes in the rate of seafloor spread- with hypersaline brines, others that it can form from ing have changed the levels of Ca in seawater and so schizohaline waters, or that the time required to form influenced the dominant mineralogies of marine car- large volumes of dolomite means that the process bonate and evaporite precipitates. In contrast, Hol- must be predominantly subsurface and perhaps tied land et al.Ž. 1996 proposed that the preponderance of to the long-term circulation of seawater through plat- MgSO4 -depleted evaporites in the Cretaceous might form sedimentsŽ. Land, 1985; Hardie, 1987 . reflect a substantial episode of dolomitization and Dolomitizing solutions include marine brinesŽ Be- carbonate platform growth. In this case, the forma- hrens and Land, 1972; Mackenzie, 1981. , continental tion of dolomite changed seawater chemistry, the brinesŽ Clayton et al., 1968; Von der Borch et al., opposite of the argument of HardieŽ. 1996 . Using 1975.Ž , essentially normal seawater Saller, 1984; micro-inclusion studies of halites of varying ages Carballo et al., 1987; Mazzullo et al., 1995. , seawa- ZimmermannŽ. 2000 has concluded that the evolving ter modified by extensive sulfate reductionŽ Baker chemistry of the Phanerozoic ocean is more indica- and Kastner, 1981; Kelts and Mackenzie, 1982. , tive of changing volumes of dolomite that it is of seawater mixed with meteoric waterŽ Land, 1973; changes in the rates of seafloor spreading. In con- Magaritz et al., 1980; Cander, 1994. , and seawater trast, Burns et al.Ž. 2000 argue that the relative level r mixed with hypersaline brinesŽ. Meyers et al., 1997 . of Mg Ca and pCO2 in the dolomite precipitating Most modern dolomite occurrences are penecon- fluid is far less important in precipitating the varying temporaneous, patchy and micritic. Holocene volumes of dolomite than the levels of oceanic oxy- dolomites comprises units that, at most, are a meter gen, which controlled the intensity of anaerobic bac- or so thick, dolomiteŽ. the mineral makes up less terial metabolism. J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 3

And so we can simply state of the dolomite Ž.Bourrouilh-Le Jan, 2000 . Dolomite, the rock, con- problem. At the current state of our understanding tains a large proportion of dolomite the mineral. we simply do not know:Ž. 1 why there is so little Ideal dolomite has a crystal lattice consisting of modern dolomite,Ž. 2 why there is so much ancient alternating layers of Ca and Mg, separated by layers dolomite, andŽ. 3 how the chemistry of ancient of CO3 and is typically represented by a stoichiomet- dolomite forming settings has evolved over time. In ric chemical composition of CaMgŽ. CO32 where cal- other words, we do not know the relative importance cium and magnesium are present in equal propor- of kinetics versus parity in dolomite precipitation, tionsŽ. Fig. 1A . and we do not yet have a good understanding of how the volume of dolomite precipitation has varied 2.1. Equilibrium and kinetics in dolomite precipita- through time. Is it controlled by seawater chemistry tion or does it control seawater chemistry, and what is the effect of bacterial mediation? Seawater is the only abundant source of Mg 2q capable of forming large and widespread volumes of sedimentary dolomite, at or near the earth's surface 2. Dolomite the mineral Ž.Land, 1980, 1985 . It contains 1290 ppm Mg Ž 0.052 mol ly1 .Ž , and 411 ppm Ca 0.01 mol ly1 . giving a The original mineral name dolomie was given by MgrCa ratio of 3.14 by weight or a molar ratio of N.T. Saussare, in 1792, in honor of the French 5.2. Meteoric water entrains lower and more variable geologist Deodat Guy de DolomieuŽ. 1750±1801 concentrations of both Mg and Ca; a typical river and was first applied to rocks in the Tyrolean Alps water has a MgrCa molar ratio of 0.44Ž 4 ppm Mg

Fig. 1. Dolomite lattice.Ž. A Ideal structure of stoichiometric dolomite consisting of layers of carbonate separated by alternating layers of calcium and magnesium ionsŽ.Ž. after Land, 1985; Warren, 1989 . B Schematic of non-ideal lattice structure showing how water molecules are preferentially bonded to cations on the surface of the growing crystallite. Because Ca ions are not as strongly hydrated as the Mg ions they tend to be incorporated in the magnesium layer creating a typical calcian dolomite. Carbonate ions are unhydrated but must have sufficient energy to displace water molecules adjacent to the cation layerŽ. after Lippman, 1973 . 4 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 and 15 ppm Ca. . Depending on the degree of for long periods without dolomite being precipitated. water±rock interaction, typical subsurface or forma- He argued that this reflects the relative strength of tion waters also show a wide range in Mg and Ca the electrostatic bond of the magnesium ion for concentrations, as well as in total ionic strength. The waterŽ some 20% greater than that for Ca and much r Mg Ca ratios of these are typically lower than that greater than for CO3. . Thus, although there is a of seawater, usually in the range 0.04 to 1.8Ž Folk theoretical saturation in seawater, in practice the and Land, 1975. . carbonate ions cannot overcome the hydration shell Precipitating primary dolomite from any of these to bond with the Mgq Ž. Fig. 1B . With magnesium waters requires the following reaction: effectively excluded from further reaction, the main q q y carbonate precipitate from modern seawater is arago- Ca2 qMg 2 q2CO2 sCaMgŽ. CO Ž.332 nite. The higher Mg concentrations of hypersaline This gives an equilibrium constant K defined by: waters means that the hydration barrier is more q q y easily overcome, but it is still difficult for calcium Ks wxCa2 Mg2 CO2 r CaMgŽ. CO Ž.332 2 and magnesium to segregate into the monolayers where the parentheses wxx indicate activities of the necessary to precipitate ideal or stoichiometric dissolved and solid species. Because of the difficul- dolomite. LippmannŽ. 1973 argues that this is why ties in synthesizing dolomite at low temperatures in highly disordered calcian-dolomite is the dominant the lab, the value of K is not known with any form in most Holocene hypersaline settings. In order precision. Arguing from several lines of evidence, to overcome the kinetic barrier, seawater may need HsuŽ. 1967 estimated Kf10y17 , while, based on to be concentrated, heated, cooled, diluted, have its modern metastable dolomites, HardieŽ. 1987 esti- levels of sulfate lowered, or its levels of activated f y16.5 2y mated it to be 10 . Knowing the approximate CO3 increasedŽ. see models . After running an ex- 2q 2q activities of Ca ,Mg and CO3 in seawater gives periment for 32 years that unsuccessfully attempted an ion activity product of the order of 10y15.01.It to precipitate dolomite at 258C, LandŽ. 1998 came to would appear that modern seawater is supersaturated the same conclusion, A . . . we allŽ. ? now agree that by one to two orders of magnitude with respect to `The Dolomite Problem' is one of kinetics.B dolomite, yet dolomite is a rare precipitate. This underlines the likelihood that reaction kinetics are a major control in the formation of dolomite from 2.2. Composition and lattice eÕolution modern seawater. A similar argument can be made for dolomitiza- Dolomite comprises a group of minerals with tion of a limestone by modern seawaterŽ. Hsu, 1967 similar but not identical MgrCa ratios, and with where the reaction is: differing ideal chemical and lattice compositions. When used to describe a mineral, the term dolomite 2CaCO qMg 2qsCaMg CO qCa2q 33Ž.2 indicates a compositional series in much the same Taking the activity of the solid phases as unity way that feldspar doesŽ. Land, 1985 . Just as there are reduces the equilibrium constant to: many kinds of feldspar, so dolomite exhibits varia- q q tion not only in chemical composition but also in Ks Mg 2 rwxCa2 s0.67 subtle atomic arrangementsŽ Reeder, 1981; Hardie, The reaction should go to the right when the 1987. . Very few, if any, sedimentary dolomites are w 2qxwr 2qx w x Mg Ca ratio is greater than 0.67. As mod- truly stoichiometric CaMgŽ. CO32 and are better r ern seawater has a Mg Ca molar value of around represented as: CaŽ1qx.Ž Mg1yx.Ž. CO32 . Documented 5.2, it should not only precipitate dolomite but should compositions range from Ca1.16 Mg 0.94Ž. CO 3 2 to also be capable of dolomitizing limestones. Again, Ca0.96 Mg 1.04Ž. CO 3 2 , encompassing the spectrum the reasons for this not happening over much of the from calcian to magnesian dolomites. Most ancient modern seafloor are probably in large part kinetic. dolomites are calcian-rich; they formed slowly and, LippmannŽ. 1973, 1982 showed that supersatura- once formed, are less soluble than their undolomi- tion in seawater with respect to dolomite can persist tized limestone equivalents. J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 5

Holocene dolomite precipitates or crystallizes as a disorderŽ. Hardie, 1987 . There is not one single metastable phase, but is a true dolomite in that it dolomite mineral, but many, each with different possesses at least partial ordering of the lattice struc- thermochemical properties depending on the degree ture. It differs from older sedimentary dolomites in of lattice ordering and nonstoichiometry. The free that much higher densities of defects, and greater energy differences due to order±disorder are size- variations in the degree of ordering andror calcium able: there may be more than 1.3 kcalrmol differ- A B enrichment, characterize the lattice across domains ence in DGf Ž.258C, 1 atm between ordered and tens to hundreds of angstroms wide. Transmission AdisorderedB dolomite. This is equivalent to a range electron microscopyŽ. TEM shows that ancient post- of one and a half orders of magnitude in solubility Paleozoic sedimentary dolomites tend to have much constants for the two forms of dolomite. Completely more regular lamellar or modulated structures than disordered phasesŽ. sometimes termed protodolomites Holocene dolomite. They typically possess a perva- are the most soluble and least stable phases in the sive modulated microstructureŽ wavelengths f200 dolomite spectrum; they are also the easiest to pre- AÊ . that is generally parallel toÄ4 1014Ž Reeder and cipitate at earth-surface temperatures. Variations in Sheppard, 1982; Wenk et al. 1983; Mitchell et al. free energy and solubility also mean that surface- 1987; Miser et al. 1987. . This modulated structure, formed dolomite has a built-in potential to recrystal- like the disordered structure of Holocene dolomite, is lize over time as it reorganizes structurally and com- metastable. positionally to more stable formsŽ Land, 1980, 1985; There is no reliable information as to the thermo- Hardie, 1987. . Thus, because all nonstoichiometric dynamic and kinetic properties of Ca-dolomites with dolomite is metastable, there must be a burial trend modulated structure, so their persistence in the diage- towards stoichiometry, so resetting isotopic values netic realm is poorly understood. Even Anearly stoi- and trace element levels during recrystallization chiometricB ancient Paleozoic dolomite still contains Ž.Morse and Mackenzie, 1990 . numerous defects, mostly fault-like phenomena. Al- Many workers now agree that sedimentary though defect density is typically less than that of dolomite ripens or ages as it follows a chemical other types of sedimentary dolomite and its crystals paradigm known as the Ostwald step rule or Ostwald are better ordered, it is still less stable than struc- ripening, first stated some 100 years agoŽ Gregg et turally and compositionally ideal dolomite. The latter al., 1992; Nordeng and Sibley, 1994. . Ostwald's rule is only commonplace as widespread stratiform units states that the transformation of an unstable to a in metamorphic carbonates and skarnsŽ. Land, 1985 . stable mineral phaseŽ. at earth surface conditions ReederŽ. 1981, 1983 interpreted the modulated requires one or more intermediate phases. The pro- microstructure of sedimentary dolomite in terms of cess is driven by the surface energy of the various compositional variations characterized by the incor- crystals and typically describes a recrystallization poration of excess calcium in the B sitesŽ which process whereby large crystals typically grow at the should be occupied by Mg. in the dolomite structure. expense of smaller onesŽ. Morse and Casey, 1988 . In Wenk et al.Ž. 1983 hypothesized that the modula- any supersaturated solution undergoing crystalliza- tions are introduced during crystal growth and possi- tion, crystallites are continuously forming. Those bly indicate stabilization through dissolution±repre- particles that attain critical radii become critical nu- cipitation. Reeder and ProskyŽ. 1986 and Miser et al. clei and continue to grow. Those with radii less than Ž.1987 showed that modulated microstructures re- the critical radii dissolve back into the solution. The flect growth defects and do not necessarily require size of the critical radius is controlled by the activa- replacement. tion energy for nucleation, which is a function of the Thus, the term dolomite describes a mineral series degree of supersaturation and the free surface energy that encompasses a range of chemical variation and of the nuclei. Ostwald ripening begins near the end lattice structures. It encapsulates an array of natural of the initial nucleation and growth processŽ Gregg Ca±Mg carbonates with chemical compositions close and Sibley, 1992. . Near equilibrium, the critical to ideal dolomite, but with weak or diffuse X-ray radius increases as the degree of supersaturation reflections that indicate varying degrees of cation decreases. Crystals with radii less than critical dis- 6 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 solve as the solute reprecipitates on the surface of During nucleation, the composition of the VHMC or crystals with radii larger than critical. During this nonstoichiometric dolomite is a function of MgrCa process there is no gain or loss of volume, but the of the solution. Other variables, such as dissolved number of individual crystals per unit volume de- Fe, can affect the composition of the nuclei. creases until the system reaches theoretical equilib- Ž.2 Induction period: Nucleation continues during rium with all crystals of the same size. induction but most of this period is taken up by the Ostwald ripening has been shown to operate dur- post-nucleation growth of VHMC, nonstoichiometric ing dolomite formation by Sibley et al.Ž. 1994 who and stoichiometric dolomite. Changes in the compo- found in laboratory-based high temperature experi- sition of the solution and substrate surface area affect ments that the rate of dolomitization increases with the duration of the period. temperature, reactant surface area, reactant solubil- Ž.3 Replacement period: Ž. a CaCO3 is replaced by ity, ionic strengthŽ. salinity and the MgrCa ratio of VHMC or nonstoichiometric dolomite. VHMC nu- the solution. In a related paper, Nordeng and Sibley cleates faster than stoichiometric dolomite and there-

Ž.1994 developed the kinetic theory that relates the fore begins to replace the substrate first.Ž. b CaCO3 Ostwald step rule to dolomite stoichiometry. They and VHMC andror nonstoichiometric dolomite are show that the surface free energy of high magnesian rapidly replaced by stoichiometric dolomite. This calcite is less than that of more stoichiometric phase of the reaction is sensitive to solution variables dolomiteŽ. see below and so the calcite to dolomite such as MgrCa and ionic strength. transformations observed in the laboratory are con- This three-phase model is consistent the Ostwald sistent with surface free energy controlled nucleation ripening and with many characteristics of natural kinetics whereby CaCO3 is replaced by dolomite or dolomites, including:Ž. 1 the fact that very Ca-rich very high Mg calcite. dolomite is common only in modern dolomites;Ž. 2 Sibley et al.Ž. 1994 showed that dolomite precipi- there is a direct relationship between the MgrCa in tated in the laboratory at elevated temperatures solution and the MgrCa ratio in the dolomite;Ž. 3 Ž.1508C±3008C is the result of a three-step reaction the stoichiometry of dolomites in some ancient rocks beginning with the precipitation of a very high-Mg is directly related to the percentage of dolomite in calcite crystallites followed by its rapid replacement. the rock;Ž. 4 the suppression of stoichiometric The stages are the followingŽ. Fig. 2 : dolomite nucleation allows the persistence of

Ž.1 Nucleation: Nucleation of very high-Mg cal- metastable Ca±Mg±CO3 phases;Ž. 5 dolomite± citeŽ. 35±40 mol% MgCO3 -VHMC or nonstoichio- limestone contacts are often sharp;Ž. 6 dolomite se- metric dolomite, followed by nucleation of more lectively replaces fine-grained CaCO3 ;Ž. 7 dolomite stoichiometric dolomite on the CaCO3 substrate. crystals often have cloudy centers and clear rims;Ž. 8 dolomite textures are mainly determined by the crystal size of the substrateŽ. Sibley et al., 1994 . Other key laboratory observations of Sibley et al.Ž. 1994 ,

suggesting that the presence of SO4 may slow the rate of dolomitization, that lithium may increases the rate, and that the induction period required for dolomitization is long, were not predicted from kinetic theory. In summary, laboratory work in the last two decades, since the publication of LandŽ. 1980 , has clearly shown that sedimentary dolomites are metastable. Older examples, and those associated with evaporites, tend to be better ordered and Amore B Fig. 2. Three-stage sigmoidal reaction curve for dolomitization of stoichiometric than more recent examples. Dolomite follows a ripening or aging paradigm first outlined in CaCO3 , based on hydrothermal bomb experimentsŽ after Sibley et al., 1994. . Precipitation of dolomite occurs via Ostwald ripening. the Ostwald step rule more than 100 years ago; J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 7 dolomite crystals are modified extensively during bonate phases usually dominated by terrigenous diagenesis as earlier phases are replaced by less- components or evaporitesŽ. Fig. 3A . When a carbon- soluble phases, in a AstepwiseB or interactive fash- ate is dominated by calcite, it is called limestone, ion. The intermediate stages are more disordered when it is dominated by dolomiteŽ. the mineral , it is than the ultimate product, a pure stoichiometric called dolomiteŽ. the rock . In North America, the dolomite mineral in equilibrium with its pore fluids. term dolostone is sometimes used to describe such a Most Phanerozoic sedimentary dolomites have not rock. Carbonate rocks tend to be composed either attained this ideal stoichiometric chemistry. We must mostly of calcite or mostly of dolomiteŽ Fig. 3B; conclude that the chemistry of dolomitization is an Blatt, 1992. . Whatever the conditions that favor iterative dissolution±reprecipitation process, as long dolomite or limestone formation the tendency is to as sedimentary dolomite retains some degree of per- form either one or the other end member, subequal meability, and there is a supply of Mg ions, new mixtures of calcite and dolomite in a carbonate rock dolomitization can occur. On a larger scale, are unusual. metastable phases can dissolve in one area and repre- In a refinement of this analysis and using other cipitate in another, locally causing porosity enhance- unpublished work as well as their own data, Sperber ment or porosity occlusion. et al.Ž. 1984 found a bimodal distribution in the This understanding that dolomite ripens with the percentage of dolomite in carbonate rocksŽ. Fig. 4A , passage of time adds another level of complication to with modes at 97%Ž. dolomiterdolostone and at the dolomite problem. It means that geochemical 20%Ž.Ž. dolomitic limestones . Sperber et al. 1984 dataŽ. trace element and isotopic are likely to be went on to suggest that dolomitic limestones, which reset numerous times during diagenetic evolution tend to contain rhombs of calcian dolomiteŽ hatchured Ž.Land, 1980 . When we look at such data, we are histogram in Fig. 4A. , originate in relatively closed looking at evidence of the passage of dolomitizing diagenetic systems whereby high Mg-calcite was fluids with chemistries that are likely to be domi- deposited and then dissolved to reprecipitate within nated by later AresettingB events. the same rock as calcite and dolomite. In contrast, the pure dolomitesŽ. dolostones , composed of more nearly stoichiometric dolomiteŽ open histogram in 3. Dolomite the rock Fig. 4A. , tend to form in more open systems charac- Ancient carbonate rocks are mostly composed of terized by greater volumes of fluid throughput and two minerals; calcite or dolomite, with any noncar- continuing dissolution±reprecipitation. DolomiteŽ the

Fig. 3. Dolomite classification.Ž. A General carbonate classification according to mineralogy of components Ž after Leighton and Pendexter, 1962.Ž. . B Computed percentages of calcite and dolomite for 1148 modal analyses of North American carbonatesŽ after Steidtmann, 1926; Blatt, 1992. . 8 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

diffused, plane-polarized light preserve depositional textures that reveal detailed microfacies. Even better results can be obtained using cathodoluminescence Ž.Braithwaite and Heath, 1996 , fluorescence mi- croscopyŽ. Dravis and Yurewicz, 1985 , or the much cheaper white-card technique of FolkŽ. 1987 . Advancing levels of replacement, ranging from mimetic to obliteratedŽ. mosaic or sucrosic texture can be present in a single formation and sometimes even in a single outcrop or thin sectionŽ Braithwaite, 1991. . An early attempt at classifying dolomite diversity based on texture was made by Friedman Ž.1965 . Sibley and Gregg Ž. 1987 later modified this system on the basis of crystal boundary relationships and crystal size distributionŽ. Table 1; Fig. 5 . Crystal boundaries are classified as either planar or nonplanar. Planar dolomite is characterized by planar compromise boundaries with many crystal- face junctions. Nonplanar dolomite is characterized by boundaries between crystals that are curved, lo- bate, serrated, or otherwise irregular, with few preserved crystal-face junctions. Planar dolomite develops when crystals undergo faceted growth and is characteristic of dolomite crystals formed early dur- Fig. 4. Phanerozoic dolomite chemistry.Ž. A Dolomites and ing diagenesis and, under certain conditions, at ele- dolomitic limestones of North America. Dolomites precipitated vated temperatures. Nonplanar boundaries develop within relatively closed hydrologies tend to be calcianŽ shaded when crystals undergo nonfaceted growth. According histogram region. , while those formed under more open hydrolo- to Gregg and SibleyŽ. 1984 , mosaics with planar giesŽ. white-filled histogram region tend toward more stoichiomet- Ž. ric chemistriesŽ.Ž.Ž. after Sperber et al., 1984 . B Evaporite black crystal boundaries idiotopic indicate growth tem- and nonevaporiteŽ. grey-shaded associated dolomites showing peratures below 508C, while those with nonplanar evaporitic dolomites tend to be more stoichiometricŽ after Lums- boundariesŽ. xenotopic result from elevated tempera- den and Chimahusky, 1980. . tures greater than 508C. Both planar and nonplanar dolomite can form as a cement, by replacement of limestone, or by neomorphic recrystallization of a precursor dolomiteŽ Gregg and Sibley, 1984; Sibley mineral. tends toward ideal stoichiometry when and Gregg, 1987. . Sibley and Gregg's simple classi- formed in evaporite settingsŽ. Fig. 4B, shaded black , fication is descriptive, but carries genetic implica- perhaps reflecting the higher MgrCa ratios of ma- tions because crystal size is controlled by both nucle- rine-derived hypersaline waters compared to many ation and growth kinetics, while crystal shape is other dolomite-precipitating waters. controlled by growth kinetics. At the hand-specimen scale, dolomites tend to be Many lower temperature sparry planar dolomites composed of either spar or micrite. Historically, are dominated by cloudy centered, clear-rimmed many workers have ignored the detailed petrography crystals. These cloudy centers may have developed of dolostones believing that most primary textures in precursors that originally consisted of low-Mg were obliterated by the growth of dolomite spar. calcite and may now contain inclusions of this type More recent work has shown this is not the case. As of calcite. Alternatively, the cloudy center can be CarozziŽ. 1993; p 134 ff. concludes, among all but dominated by voids created by Mg-calcite dissolu- the coarsest dolostones, many when examined with tion, or by fluid-filled micro-cavities. Chemical dif- J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 9

Table 1 Guide to observational steps used in classifying dolomite textures using classification of Sibley and GreggŽ. 1987 as shown in Fig. 5 Steps for classifying dolomite textures Step 1: What is the dolomite crystal size? UnimodalŽ. crystals mostly of the same size PolymodalŽ. at least two size populations

Step 2: What is the dominant dolomite crystal shape? PlanarŽ. euhedral, suhedral NonplanarŽ. anhedral

Step 3: Describe and name CaCO3 precursor Are the various components unreplaced, Allochems? replaced or moulds? If replaced, is it partial Matrix? or complete replacement? Is the replacement Cement? a mimic or nonmimic of the precursor texture

Step 4. Describe any void filling dolomite Limpid, rhombic, drusy, saddlerbaroque?

ferences between the inner and outer parts of these clear rims. These could be interpreted as indicating rhombs are mainly variations in iron and manganese the rims precipitated from solutions that were under- ŽChoquette and Steinen, 1980; Fairchild, 1983; saturated with respect to low-Mg calcite, thus ex- Gawthorpe, 1987.Ž. . Land et al. 1975 documented plaining the absence of inclusionsŽ. Sibley, 1980 . trace element differences between cloudy centers and Alternatively, the variations may reflect changes in

Fig. 5. Common dolomite textures emphasizing the effect of temperature on the style of dolomite developmentŽ after Gregg and Sibley, 1984; Sibley and Gregg, 1987. . Used in combination with observation criteria listed in Table 1. See text for explanation. 10 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 crystal growth rate or in the dissolutionrreprecipita- burial history. The process only slows once matrix tion of the outer portions of precursor crystals. permeability is lost as strata enter the anchizone. Saddle dolomite, also called baroque dolomite, Rather than being seen as a single event, the dolomi- forms at the same elevated temperatures as nonpla- tization process now encompasses successive re- nar dolomiteŽ Fig. 5; Radke and Mathis, 1980; sponses to compositionally diverse and evolving pore Machel, 1987; Kretz, 1992. . Crystals may be clear or fluids that produce a suite of replacive and cement- turbid, with characteristic curved faces and cleavage ing phasesŽ Land, 1985; Warren, 1989, 1990; Gregg traces. They typically show a sweeping extinction and Shelton, 1990; Gregg et al., 1992; Qing and under crossed polars caused by a distorted crystal Mountjoy, 1994a,b; Suchy et al., 1996; Reinhold, lattice structure. Under cathodoluminescence, crys- 1998. . Thus, depositional or early diagenetic tals often display internal zonation due to varying dolomites can evolve or re-equilibrate into new forms iron and manganese content. Saddle dolomite, which as conditions change. may contain hydrocarbon and metal sulfide inclu- Hydrological constraints are central to the evolu- sions, is a useful geothermometer, indicating mini- tion of natural dolomites. For crystals to precipitate mum burial temperatures of 608 to 1508CŽ equivalent or re-equilibrate there must be a solution throughput to the oil window. . It is often a byproduct of chemi- that is capable of supplying and maintaining suffi- cal compaction and thermochemical sulfate reduc- cient volumes of magnesium to generate and sustain tion, and it is typically related to the throughflow of supersaturation. To dolomitize a typical limestone high-temperature hypersaline basinal brines that may containing 6% MgCO3 with 40% porosity requires also be carrying base metals. that 807 pore volumes of seawater flow through the Mosaics of anhedral to subhedral dolomite crys- rockŽ. Land, 1985 . With seawater diluted 10 times tals apparently represent the final product of dolomi- with meteoric waterŽ. mixing zone model , something tization, and a pervasive coarse-grained overprint can like 8100 pore volumes are needed, while with a destroy any original depositional fabrics. Typically, halite-saturated, seawater-derived brineŽ brine reflux some of the crystals in these mosaics retain cloudy model. , only 44 pore volumes are sufficient. If the cores and clear rims. The generation of dolomite brine has reached the bittern stage, only 10 to 12 mosaics indicates a lengthy diagenetic history often pore volumes are required. In these idealized scenar- involving fracture and rehealing of crystals, as well ios, no pore volume reduction has occurred during as overgrowths and replacements reflected in internal the throughflow. To reduce the porosity of our theo- dissolution surfaces and other crystallographicr retical model to values common in ancient dolomites mineralogic discontinuities. requires either chemical compaction or even greater volumes of solution to flow through the rock. These simple volume calculations clearly show 4. Dolomitization models the need for any model to be based on realistic physical mechanism relevant to the hydrological set- Our increasing understanding of the reactivity of ting. For example, it is difficult to move large vol- different types of dolomite in the laboratory has led umes of meteoric waters through rocks that are to an improvement in the way dolomites are inter- accumulating in a hypersaline settingŽ Chap. 2 in preted at the reservoir or outcrop scale and to a Warren, 1999. . Similarly, large volumes of basinal merging of dolomitization models that were previ- fluid cannot pass easily through deeply buried lime ously considered distinct but not well documented mudstones, unless some other earlier process has paradigms. Once the metastable nature of sedimen- generatedrretained porosity. The fact that much tary dolomite was recognized in the mid to late smaller pore volumes of hypersaline water are re- 1980s, it became apparent that the mineral evolves quired to dolomitize a given volume of limestone, Ž.AagesB as it moves through earth-space and time. and the allied observation that most modern dolomite Changing conditions of burial depth, pore brine is associated with evaporitic settings, has led Fried- chemistry, temperature and pressure mean that manŽ. 1980 to state Adolomite is an evaporite min- dolomites may re-equilibrate many times during their eral,B while SunŽ. 1994 suggests that most large-scale J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 11 dolomite has had an origin related to salinity-elevated Small volumes of micritic dolomite are common seawaters. Let us see if this is so by looking at the in modern intertidalrsupratidal sabkha sediments, current deposits and models that attempt to explain but also occur in subtidal sediments of the Arabian the various forms of dolomite. GulfŽ Illing et al., 1965; Illing and Taylor, 1993; Mackenzie, 1981; Wenk et al., 1993; Shinn, 1983; 4.1. Syndepositional dolomite Chafetz and Rush, 1994. . Dolomite is accumulating within the supratidal zone of the prograding sabkha, 4.1.1. Sabkha-style dolomite usually within aragonite muds that lie less than a Dolomite is currently forming as thin Ž-1±2 m meter beneath the sabkha surface. On sabkhas bor- thick. patchy stratiform beds in many evaporitic tidal dering the Qatar Peninsula, the dolomite is most flatsŽ. sabkhas on the southern and western margins abundant on the landward margins where it forms of the Arabian GulfŽ. Warren, 1991 . The Arabian 1±5 mm euhedral crystals in aragonitic muds beneath Gulf is probably the warmest sea in the world; the high intertidal zoneŽ Fig. 6; Illing and Taylor, surface temperatures in the open gulf range from 1993. . 208C in February to 348C in August. Water tempera- TEM study has shown that much of the dolomite tures on the open shelf in the vicinity of Abu Dhabi in the Abu Dhabi sabkha is actually a pore precipi- Island range from 238Cto348C, and those in the tate and not a replacement of precursor aragonite inner lagoons from 158 to 408C with an annual Ž.Wenk et al., 1993 . Only rarely does it directly average temperature of 298C. The shallow, inner replace an aragonite precursor at the scale of crystal- lagoon waters of Khor al Bazam have diurnal ranges lites. Whether one considers this dolomicrite, a re- of up to 108C. Air temperatures on the sabkha can be placement of aragonite mud, which sometimes ap- as low as 58C and as high as 508C with average pears to be dissolving, or a primary dolomite precipi- temperatures ranging from 238 to 338C. Surface tem- tate depends on how one defines primary versus peratures on the sabkha are even more variable, with secondary dolomiteŽ see Warren, 1989, Chap. 5 for values up to 608C±808C on the algal matsŽ Kinsman, discussion. . There are actually two forms of dolomite 1965. . Temperatures in shallow subsurface sedi- in the Abu Dhabi sabkhas:Ž. 1 an ordered dolomite, ments range from 228Cto408C. In the summer, the often showing a modulated microstructure, andŽ. 2 a sabkha can be extremely humid, especially at night mostly disordered calcium magnesium carbonate with when it is often above 95%, yet by midafternoon on submicrometer-sized ordered domainsŽ Wenk et al., the same day the humidity can fall to around 30% 1993. . Both the ordered and partially ordered Ž.Bush, 1973 . dolomite form by direct precipitation from pore flu-

Fig. 6. Dolomite in the Arabian Gulf. Cross-section of peritidal sediments on a sabkha on the Qatar peninsula, Arabian Gulf. Contours are of percentage dolomite in the fine fraction of the sediment. Note the strong tie to the spring tide high water markŽ. after Bush, 1973 . 12 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 ids. Both appear simultaneously during early diagen- 6; Bush, 1973.Ž. . Patterson and Kinsman 1982 argue esis. With time, the dolomite seems to AageB: there that the zone of maximum precipitation occurs in is an increased ordering in the crystal structure, an sediments located just above the high spring water- increase in crystal size, and continued isotopic equi- mark. This zone migrates seaward as the sabkha libration with brines at the lower temperatures be- progrades into the lagoon. neath the sabkha surfaceŽ. Mackenzie, 1981 . Storms and high spring tides drive seawater from Most dolomite in the Abu Dhabi sabkha is a the lagoon onto the sabkha flat, while continental cement, precipitating from hypersaline fluids into groundwaters and storm runoff move terrestrial wa- pores of varying sizes down to a few micronsŽ Wenk ters into the landward side of the sabkha. As surface et al., 1993. . It forms in isotopic equilibrium with waters sink into the sediments and begin to seep brines at temperatures between 348 and 498C. Cal- seaward, some pore water escapes to the atmosphere cium±magnesium-carbonate perhaps serves as an in- via capillary evaporation. As it does so, the remain- termediate in the formation of the dolomite, which ing pore water concentrates to the point where it then proceeds to a more ordered Ž.AagedB form via deposits aragonite and anhydrite or gypsum. Precipi- micro-scale dissolution±reprecipitation. Diagenetic tation of these minerals removes calcium from the changes and isotopic re-equilibration occur in the pore solution and so raises the MgrCa ratio favoring coexisting CaCO3 phases. dolomite precipitation or dolomitizationŽ. Fig. 7 . Currently, dolomite precipitates in areas of maxi- The relative proportions of continental, mixed or mum flux of hypersaline brines beneath the suprati- marine waters feeding the areas precipitating dal surfaceŽ Fig. 6; Bush, 1973; Patterson and Kins- dolomite are still not well understoodŽ. Hardie, 1987 . man, 1981, 1982. . It occurs sporadically to depths of Mackenzie et al.Ž. 1980 found that the MgrCa molar 2±3 m landward of the algal mat. The most intense ratios in areas of dolomite range from 2.5 to 7.5, precipitation typically occurs in buried Ž-1 m be- lower than seaward areas where flood recharge is low the surface.Ž algal mats and lagoonal muds Fig. more frequent and aragonite and gypsum are being

Fig. 7. Pore fluid chemistry across the Abu Dhabi sabkha showing variations in major ions and the MgrCa ratioŽ after Hardie, 1987 who replotted the data in Butler, 1969. . J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 13 precipitatedŽ. MgrCas7 to 27 . In contrast, Patter- unconfined aquifer; to the south of Abu Dhabi city, son and KinsmanŽ. 1982 found that in areas of the sabkha is composed of three aquifers separated dolomitization the MgrCa was )6, pHs6.3 to 6.9 by cemented crusts which form aquitards. and the sulfate content lower than seawater. These In Al-Khiran lagoon in Kuwait, there is another authors also concluded that dolomite was replacing modern sabkha dolomite, it also is interpreted as a an aragonite precursor, so supplying Ca to the pore diagenetic precipitate, but it is unlike the dolomite of solution that was subsequently removed by precipita- the Trucial Coast. The Kuwaiti dolomite is found in tion of more gypsum. Mackenzie et al.Ž. 1980 argued the upper 10 cm of isolated lagoon sediments and is that replacement of aragonite occurs via a high-Mg not associated with any gypsum or other evaporites calcite intermediate, while HardieŽ. 1987 argued for ŽGunatilaka and Mwango, 1987; Gunatilaka et al., direct microscale cementation in the pore waters, a 1987.Ž. . Gunatilaka and Mwango 1987 proposed that view later confirmed by TEM workŽ Wenk et al., bacterially induced sulfate reduction played a role in 1993. . Views on the ultimate genesis of the sabkha its formation. Nearby, in the sabkha behind the dolomite are still diverse and the origins of ground- lagoon, Mg-rich dolomite forms in thin layers alter- waters that precipitate the dolomite are not yet re- nating with more commonplace Ca-rich dolomites. solved. This Mg-rich variety occurs in parts of the sabkha Hsu and SchneiderŽ. 1973 proposed a hydrologi- that are today under the influence of continental cal alternative to the model of surface flooding fol- groundwaters and rarely covered by seawater lowed by seaward reflux of the dolomite precipitat- Ž.Gunatilaka et al., 1987 . In this region, the landward ing brine. They called it evaporative pumping. It hydrology appears more akin to that of the continen- involved continuous subsurface seepage of lagoonal tal Coorong system discussed below. seawater into the sabkha to replace groundwater lost Not all the dolomite found in the peritidal margins by evaporation at or near the surface. Under evapora- of the Arabian Gulf is modern. Chafetz et al.Ž. 1999 tive pumping, phreatic seawater flows landward be- document a 55-m-thick Pleistocene sabkha dolomite neath the sabkha flat in a direction opposite to section with abundant evaporite layers. This was seaward seepage in the surface flooding model. Field deposited as part of a series of vertically stacked evidence for evaporative pumping is contradictory sabkhas. Stable-isotope analyses of 385 samples de- and many authors feel that it does not explain the fined four correlatable horizons, each with marked distribution of dolomite in the sabkhaŽ Morrow, 1982; excursions to low d13C values; some shifts between Land, 1985. . If landward seepage of phreatic seawa- vertically adjacent samples exceed 6½. Values for ter was the sole mechanism of supply for sabkha d18 O show a similar, but less pronounced, downward brines, then marine evaporites would quickly precipi- shift in the same stratigraphic horizons. Samples tate and plug capillary poresŽ hydroseal of Kendall, with low d13C and d18 O values have greater admix- 1992. . Without subsequent dissolution beneath sur- tures of fine-grained siliciclastics, are light grayŽ im- face flood waters, this would reduce the permeability plying reducing conditions. , and have higher concen- of the sediments, cancel the head difference, and so trations of Fe, Mn, and Zn than other parts of the choke off landward flow before sufficient magne- deposit. Changes in d13C values may be due to sium had flushed through the region to form diagenesis reflecting the descent of meteoric water widespread dolomites landward of the evaporite plug. beneath a soil or to bacterially mediated sulfate-re- Perhaps the mechanisms supplying seawater to duction. Chafetz et al.Ž. 1999 favor sulfate reduction the Abu Dhabi sabkha are not as simple as these of laterally extensive microbial mats within the top models imply. Mackenzie et al.Ž. 1980 have shown of the sabkha sequences, very soon after the initial that the sabkha hydrology may be characterized by dolomitization. three sequential stages; flood recharge, capillary Not all surficial dolomite in the Arabian Gulf was evaporation and evaporative pumping. All three oc- deposited by the hydrological processes characteris- cur as the dolomitizing brine evolves and are re- tic of the current evaporitic setting. ShinnŽ. 1983 peated with each new flood event. In addition, the documents detrital seafloor dolomite deposited as hydrology does not operate within a homogeneous offshore detrital sediment, along with quartz-silt and 14 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 various clay minerals. PilkeyŽ. 1966 recorded 10% stricted marine to supratidal facies, coprecipitating detrital dolomite in fine-grained sediment from bot- with nodular evaporitesŽ Warren and Kendall, 1985; tom sediments in the central axis of the Gulf. Any- Warren, 1991. . Sabkha dolomitization is tied to ac- one who has stood on an offshore rig during a dust tive evaporative hydrologies, but in the sabkha these storm has its origin blasted across their person and are capillary scale mechanisms Ž.)1 m thick , which their equipment. Wind-carried silt-sized dolomite cannot alone explain the huge brine throughflow must be added to that found in the sabkha sediments. indicated by pervasive sparry dolomitization of ma- However, the contribution is probably minor. rine limestones within ancient carbonate±evaporite Whether or not detrital dolomite was also a catalyst platforms. for the nucleation of the initial calcian-rich diagenetic dolomites that formed the bulk of the modern 4.2. Coorong-style dolomite sabkha dolomite, has yet to be addressed. No matter what the ultimate origin of the sabkha The dolomite-forming schizohaline lakes of the dolomite, in terms of ancient counterparts, sabkha Coorong region are situated in seepage-fed inter- dolomites are syndepositional, thin Ž.-1±2 m thick , dunal corridors on the prograding coastal plain of stratiform, micritic, mimetic and confined to re- southeastern South AustraliaŽ. Fig. 8 . The Coorong

Fig. 8. Locality and map of the Coorong region. Geological plan of the Salt Creek regions shows how three of the four major lakes are joined by an interdurnal corridor and were connected to the Coorong lagoon earlier in the Holocene. Milne Lake, the best dolomite accumulator in the whole coastal plain, never had a surface connection with the Coorong Lagoon. Mineralogies: Lake 1sMilne Lake Ž.dolomiteqmagnesite , Lake 2sHalite Lake Ž gypsumqaragonite overlain by hydromagnesiteqaragonite in massive unit and currently covered by ephemeral halite crust.Ž , Lake 3sPellet Lake dolomiteqhydromagnesiteqaragonite . , Lake 4sNorth Stromatolite Lake Žhydromagnesiteqaragonite; minor dolomite in basal unit and about lake edge.Ž after Warren, 1990 . . J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 15

Region was first documented as an area of modern gested that south of the 700 mm isohyet, reduced dolomite precipitation by MawsonŽ. 1929 , but was evaporation rates and higher rainfall prevented the not studied in any detail until the work of Alderman concentration of lake waters to salinities where and SkinnerŽ. 1957 , Skinner Ž. 1963 , von der Borch dolomite could precipitate. Regional sampling of Ž.1965, 1976 , von der Borch and Lock Ž. 1979 , Rosen lakes across the area confirms his postulate and et al.Ž. 1988, 1989 , and Warren Ž. 1988, 1990 . The shows that the more magnesian-rich dolomites tend climate in the Coorong region is Mediterranean, with to occur in the more arid northwestern portion of the hot, dry summersŽ. Dec.±Mar. and cool wet winters coastal plain, centered on lakes in the vicinity of Salt Ž.June±Sept. . Maximum rainfall is 800 mm in the far CreekŽ. Warren, 1988, 1990 . southeast of South Australia, 600 mm around Coring has shown that many of the lakes in the Kingston, and 400 mm in the area about Salt Creek. Salt Creek region contain two geochemically and In the last region, evaporation exceeds precipitation isotopically distinct types of dolomite, typically in except for a few weeks in June and July. To the association with other precipitated carbonate miner- southeast, near Kingston, precipitation exceeds evap- als such as low Mg-calcite, high Mg-calcite, arago- oration for 3±4 months each yearŽ. May±August . nite, magnesite and hydromagnesiteŽ Table 2; Fig. Air temperatures range annually from y18Cto388C 9B,C. . Type A dolomite typically has a slightly with a mean of 13.58C. Water temperatures vary heavier oxygen isotope signature than type B, and is between 108C and 288C with a pH of 8±10. Summer 3%±6½ heavier in carbonŽ. Fig. 9B . Type A temperatures within subaerially exposed lake sedi- dolomite also has distinct unit cell dimensionsŽ Fig. ments are as high as 508C, while winter temperatures 9C. . It tends to be magnesium-rich with up to 3 r are as low as 58C. The Mg Ca ratios of waters from mol% excess MgCO3 while type B is near stoichio- different lakes varies from 1 to 20Ž Von der Borch, metric or calcian-rich. Type A dolomite commonly 1965. , sulfate concentrations in lakes in the coastal occurs in association with magnesite and hydromag- plain range from 6.1% to 8.2%Ž. Skinner, 1963 and nesite, type B with Mg-calcite. Transmission elec- are close to the expected value for seawater sulfate tron microscopyŽ. TEM shows that type A dolomites Ž.7.68% . have a heterogeneous microstructure due to closely Von der BorchŽ. 1976 observed that Holocene spaced random defects, while type B dolomites ex- dolomite occurs in areas of the coastal plain where hibit a more homogeneous microstructure implying rainfall is less than 700 mm, encompassing the area that excess calcium ions are evenly distributed between Kingston and Salt CreekŽ. Fig. 8 . He sug- throughout the lattice. TEM studies show that the

Table 2 Summary of properties of types A and B dolomite in Holocene lacustrine carbonates of the Coorong regionŽ after Rosen et al., 1989; Warren, 1990. Type A dolomite Type B dolomite Mg-rich Ca-rich

Unit cell: contracted in a00direction, variable c direction Unit cell: ideal to expanded in both a00and c directions

Isotopically heavy carbon Isotopically light carbon

Relatively small variation in isotopic signature related to Relatively large variation in isotopic signature, brine is perhaps brine concentration influenced by meteoric mixing

Heterogeneous microstructure under TEM Variable microstructure

Forms in evaporative centres of larger, more arid lakes. Forms about the edges of lakes in arid regions, also found in basal Uncommon in dolomitic lakes in more humid regions of units and interlake corridors near Salt Creek. Is more common the coastal plain than type A in dolomitic lakes in the more humid regions of the coastal plain 16 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 ures of type A and type Ž. Ž. Ž. Ž . Fig. 9. Sedimentology of Coorong dolomites. A Typical vertical sequence in Holocene evaporitic lakes of the Coorong coastal plain. B Isotopic signat B dolomite. C Unit cell parameters of type A and type B dolomite after Rosen et al., 1989; Warren, 1990 . J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 17 two types of dolomite are distinct, and are not inter- Like those of the sabkhas, Coorong-style models mixed with other mineral phases; they are primary of evaporative micritic dolomite precipitating in lo- precipitates, and not replacementsŽ Miser et al., calized groundwater fed depressions cannot explain 1987. . the bulk of ancient platform dolomites. These formed Regional sedimentological sequences in the by replacement of thick beds spread across laterally Coorong lakes are independent of their mineral as- extensive areas of platform carbonatesŽ Warren, semblagesŽ. Fig. 9A ; some are filling with dolomite, 1991. . Ancient equivalents of the Coorong in a others with low Mg-calcite still other with gypsum sea-marginal setting are most likely to be relatively and aragonite. Whatever the mineral assemblage, the thin, laminated subaqueous mudstone units showing dominant textural feature in the lacustrine sediment patchy facies mosaics in plan view rather than elon- columns is lamination passing up section into more gate belts. They tend to develop atop regional pale- massive units with evidence of seasonal subaerial oaquifers. Such dolomite typically occurs as a lacus- exposure of the sedimentation surfaces. Coorong trine capstone in regional paleotopographic lows. A dolomites are penecontemporaneous carbonates, once Coorong-style model can only be used to explain thought to be free of associated preserved evaporites local laminated micritic dolomites precipitated by Ž.von der Borch and Lock, 1979 . Yet, the best evaporation within outflow zones of an extensive dolomite-filled lake in the Coorong region, Milne carbonate paleoaquifer. Lake, lies less than a kilometer from Halite Lake a Probably the best ancient counterparts to the depression filled with laminated gypsum±aragonite Coorong Lakes are not in sea-marginal settings, but and capped by a massive hydromagnesiteraragonite in continental lacustrine deposits. The carbonate bedŽ. Fig. 8; Warren, 1990 . Milne Lake is fed by mudflats of the Eocene Green River Formation, Utah, seaward flowing continental groundwaters while contain all the structures and textures present in the Halite Lake is fed by landward-flowing marine Coorong Lakes, as well as many of the same vertical groundwaters during the gypsum-depositing draw- transitions from laminated calcium carbonate sedi- down stage. The close juxtaposition of these miner- ments to massive dolomites. The Green River For- alogies means that ancient counterparts of the mation was deposited in a large lake complex, part Coorong may also show a close association of evap- of a foreland basin that contained no marine waters. orative dolomite with salts such as gypsum and Green River carbonates consist of a series of stacked halite. shoaling lacustrine cycles of laminated to massive A model of Coorong-style dolomites must include dolomites and limestonesŽ Wolfbauer and Surdam, the following observations: 1974. . Individual units contain stromatolites, ripples, v There is a vertical sequence dominated by a flat pebble breccias, oolites, mudcracks and extru- laminated subaqueous unit, overlain by a massive sion tepees. unit capped by tepees, intraclast breccias and stroma- This does not imply that Holocene continental tolites. lacustrine dolomite only forms in a Coorong-like v Dolomite is a primary precipitate. There are two setting. While this is certainly true of the more distinct dolomites:Ž. i calcian dolomite associated widespread examples of Holocene lacustrine with Mg-calcite occurs throughout the coastal plain dolomite, there are numerous others. A comprehen- Ž.Ž.type B , ii dolomite associated with magnesite or sive and well-referenced review of these deposits is more rarely with hydromagnesiteŽ. type A . Type A provided by LastŽ. 1990 . However, like the Coorong, dolomites occur predominantly in the more arid most are localized seepage outflow deposits and are northern areas of the Coorong complex near Salt not analogues for ancient platform dolomites. Creek. v Dolomite precipitates in lakes and ponds and 4.3. Dolomite from normal seawater Ð a kinetic accumulates above an extensive Pleistocene aquifer problem? that supplies ions to the evaporating lake waters. In the Coorong region, the aquifer is a calcreted Quater- LandŽ. 1985 suggested that, if there is an efficient nary coastal dune system. pump mechanism to move large volumes of seawater 18 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Fig. 10. Various models of marine dolomite formation in which the dolomitizing fluid is unmodified seawaterŽ. connate that is pumped through porous limestones at depths ranging from at-surface to moderate burial Ž.f2km. through carbonate sediments, then dolomite may pre- Ž.1997 argues geometries and ages of most island cipitate as a marine cement directly from pores filled dolomites relative to overlying limestones indicate with completely unmodified seawater, all that is that seawater, derived laterally, is the parent fluid. needed is sufficient time to grow primary crystals. In He concludes that many Cenozoic island dolomites, a later paperŽ. Land, 1991 , he repudiated the mixing interpreted as mixing-zone or hypersaline reflux zone model in its type area, the Hope Gate Forma- products, are probably misinterpretations based on tion of Jamaica, in favor of dolomitization by normal unreasonable or extreme assumptions about D±Sr, seawater. In most pre-1990s dolomitization models, d18 O or the chemistry of hypothetical end-member the kinetic problems in precipitating dolomite di- dolomites. rectly from seawaterŽ or its subsurface equivalent- Mazzullo et al.Ž. 1995 referred to dolomite precip- connate1 water. are overcome by evaporation, dilu- itated from seawater as Asubtidal dolomite.B One of tion or cooling. Some early models of seawater-de- the first examples of Holocene subtidal dolomite rived dolomites invoked slight concentration of la- forming from unmodified seawater came from Ja- goonal seawaterŽ an increase of perhaps 5%±10½ maica, where dolomite precipitates in modern shal- above normal. to drive these slightly denser seawa- low water deposits and in underlying Pliocene± ters into the underlying platform sedimentsŽ. Fig. 10 . Pleistocene rocks bathed in normal seawaterŽ Mitchell Since the early 1990s, many widespread ancient et al., 1987; McCullough and Land, 1992. . Subtidal platform dolomites are increasingly being explained dolomite may also have precipitated in cool-water as the result of dolomitization by normal seawater or shelf deposits in AustraliaŽ. James et al., 1993 . How- connate pore fluidsŽ Vahrenkamp and Swart, 1994; ever, the Jamaican dolomites comprise -1% of the Whitaker et al., 1994. . In a similar fashion, Budd samples examined, and are only found within a local section of limited thickness. In contrast, an average of 5% subtidal dolomite 1 Connate water is used to describe a subsurface water that has Ž.range 1%±30% , with an average crystal size of 10 the ionic proportions of seawaterŽ. latin root connatuss with-born . mmŽ. range 10±50 mm , is found in the Cangrejo It has a specific geochemical implication and should not be used shoals mudbank in BelizeŽ. Mazzullo et al., 1995 . It to describe subsurface or basinal waters that have ionic proportions indicating extensive water±rock interaction. Such waters occurs throughout the upper 4.3 m of subtidal de- may have once been connate but this can no longer be determined posits, which are about the same age Ž.-5600 years from their ionic proportions. as those in Jamaica. Interestingly, dolomite has not J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 19 been found in or around the seasonally hypersaline precipitated after more than 610 m of burial. Frac- peritidal flats adjacent to the dolomitic Canjero tured grains first appear in the Enewetak core at this shoals, and so a hypersaline or a meteoric parent depth. This is too deep for the sediment to be fluid for these subtidal dolomites is unlikely. Mean experiencing active circulation by brine reflux or d18O Ž.q2½ compositions of these high-Sr meteoric infiltration. The only fluid currently circu- dolomitesŽ. mean 1000 ppm , together with near-nor- lating in pores at that depth is seawater. The d18 O mal marine salinities and MgrCa ratios of pore values of the dolomite Ž.q2.5½ are relatively heavy fluids, suggest their precipitation from near-normal and consistent with precipitation from normal seawa- seawater. Tidal and wind-driven circulation of sea- ter at lower temperaturesŽ. 158C±188C . The tempera- waterŽ. tidal pumping through the sediments supplies ture of marine waters at that depth is 108C±208C, most of the Mg for dolomitization. The process much cooler than normal Pacific surface water Žf appears to be promoted by elevated porewater alka- 288C. . The d13C of the dolomite Ž.q2.3½ also linity reflecting bacterially mediated oxidation of suggests a marine source for the carbonate. The organic matter and, locally, the early stages of calcite saturation depth in the adjacent ocean is methanogenesisŽ. Mazzullo et al., 1995 . A variation f1000 m. SallerŽ. 1984 noted that seawater is still on the marine subtidal dolomitization theme is docu- supersaturated with respect to dolomite at this depth. mented by Carballo et al.Ž. 1987 who describe He argued that the driving mechanism moving supratidal crusts formed by tidal pumping of normal dolomite-saturated cold seawater into the pores of seawater through elevated areas at Sugarloaf Key, the island slope is oceanic tidal pumping. Deep wells FLŽ. Fig. 10 . Similar seawater-derived humid-zone of the island record tidal fluctuations even when supratidal crust dolomites also occur on Ambergis cased to depths of more than 600 m and that temper- Cay in BelizeŽ Mazzullo et al., 1987; Gregg et al., ature profiles in them mimic those of the open 1992. . marine profileŽ. Fig. 11 . Both observations suggest LumsdenŽ. 1988 suggested that precipitation of small volumes of micritic dolomite Ž.f6 mm also takes place in cool marine pore waters within recent deep-water sediments. Such dolomite is interpreted as a direct precipitate from normal seawater and makes up, on average, 1% of deep-water marine carbonate worldwide. It occurs in all sampled ocean basins throughout post-Jurassic sediments and is typ- f ically nonstoichiometric Ž.56% CaCO3 and poorly ordered. This form of deep-water dolomite is distinct from that created by sulfate-reductionrmethanogene- sis within organic-rich marine hostsŽ. see later . Un- like other modern dolomites, it does not show down- hole increases in crystal size or improvements in its stoichiometry. That is, it does not appear to age by dissolution±reprecipitation. Mullins et al.Ž. 1988 reported the formation of similar early authigenic dolomiteŽ. 10±20 mm in the Neogene Florida± Bahamas Platform from marine pore waters where temperatures are as low as 1.88C±9.88C. SallerŽ. 1984 noted that cores from Enewetak Fig. 11. Temperature profiles of wells E-1 and F-1 compared to an atoll contain dolomitized Eocene algal-rotaline foram off-atoll ocean profile, Enewetak Atoll. Note the close parallelism grainstone at depths between 1250 and 1400 m of all profiles except near the base of the two wells where heating, related to nearby volcanic basement, caused the well profiles to below sea level. The dolomite postdates the brittle depart from the deep water portion of the oceanic profileŽ after fracture of the grains and so is thought to have Saller, 1984. . 20 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 that waters in the dolomitized intervals are in direct tion from cooler seawater and better explain the connection with deep ocean water. preferential formation of the dolomite in aquifers An alternative view is that a thermal gradient has rather than in muds. been set up within the atoll by the high geothermal In summary, based on the occurrence of Neogene gradient associated with the volcanic basement and dolomites, it appears that unaltered cold seawater, this may also be pumping cool dense seawater into seeping laterally into an island from the adjacent the atoll marginŽ Simms, 1984; Rougerie and Wau- deep ocean is capable of precipitating dolomite. The thy, 1988. . Evidence for geothermal heating can be main requirements are an effective pumping mecha- seen in increasing temperatures below 1000 m depth nism and sediments with suitable permeabilities to in wells E-1 and F-l close to the volcanic basement allow focused flushing of large volumes of seawater. Ž.Fig. 11 . This heating may drive convective circula- However, such systems appear to be currently pre- tion of seawater via a set of processes called Kohout cipitating no more than small volumes of dolomite circulation. Kohout circulation occurs when cold Ž.Fig. 10 . dense deep ocean waters are drawn into a platform margin to replace heatedŽ. less dense and so buoyant 4.4. Early burial dolomite pore waters. This occurs at depths on slopes where, as in oceanic pumping, pore waters are undersatu- 4.4.1. Brine reflux dolomite rated with respect to calcite but still saturated with Within ancient marine limestones, there are respect to dolomite. Dolomites in such circulatory widespread areas of sparry dolomite with distribu- systems tend to precipitate within permeable aquifers tions that can be tied to the presence of overlying or fed from deep forereef and rise sediments. Sediments adjacent platform and basinwide evaporitesŽ Fig. 12; further into the atoll interiorŽ. buried lagoonal muds Warren, 1999. . These evaporite-associated secondary are largely unaffected by this circulation. Kohout dolomites typically form in regimes of brine reflux: circulation is a long-term process that, although such hydrological systems, like their associated plat- largely used to explain deep dolomite in oceanic form and basinwide evaporites, have poor to nonex- atolls, probably has the potential to circulate large istent Holocene counterparts. volumes of cool dolomite-saturated ocean water Brine-reflux was proposed as a dolomitizing through continental platform-edge limestones mechanism by Adams and RhodesŽ. 1960 to explain Ž.Simms, 1984 . extensive lagoonal and reefal dolomites associated Aharon et al.Ž. 1987 in a study at Nieu and other with platform evaporites of Guadalupian age in the Pacific atolls confirm that many Tertiary-age island Permian Basin of West Texas. Reflux dolomites dolomites may have formed by geothermally driven form when, Ahypersaline brines eventually become seawater circulationŽ. Table 3 . They noted striking heavy enough to displace the connate waters and similarities between the dolomites from various seep slowly downward through the slightly perme- atolls:Ž. 1 all the dolomites are of late Tertiary age able carbonates at the lagoon floorB ŽAdams and with the exception of EnewetakŽ.Ž. Eocene ; 2 petrog- Rhodes, 1960. . It is thought that when CaCO3 and raphy shows that dolomitization is a late diagenetic gypsum precipitated in hypersaline areas on such event;Ž. 3 the dolomites contain an excess of Ca over platforms, the solution density was as high as 1.2 Mg and their Sr compositions are remarkably similar grcm3 and the MgrCa ratios of the remaining brines Ž.Ž.f150±200 ppm ; 418 O and 13 C values are con- had increased along with salinity. As gypsum- and sistently heavy, of the right order for a cold seawater halite-saturated solutions seeped basinward through parent, with little variation between atolls separated the platform limestones, they mixed with or dis- by large stretches of ocean. Earlier work on various placed subsurface fluids that were originally atolls had called upon a reflux of slightly concen- marine-derived waters with densities around 1.03 trated lagoonal seawater through the atoll interior to grcm3. In this way, large volumes of Mg-rich brine explain the heavy isotope signaturesŽ Fig. 10; Sibley, passed through previously deposited shelf limestones 1980; Ohde and Kitano, 1981. . But, as noted, these and precipitated dolomite, both as a pore fill and as a elevated isotope values are consistent with precipita- replacement. J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 21 3 Ca 10 r = Ca Sr Sr r age Mg B Sr to Pleistocene A to L. Miocene to Pleistocene to Pliocene to Pliocene to Pliocene 2 Miocene Pliocene 0.75 217 0.40 4 Miocene? Pliocene 0.75 165 0.33 " " Sr Biostrat. age r Ž. 0.3 0.70914 0.30.4 ± 0.70909 E. to L. Miocene ± ± 170 0.33 0.40.3 ± ± Miocene? L. Miocene ± ± ± ± 234 270 0.41 0.53 0.1 0.70882 Eocene L. Miocene 0.78 173 0.33 CSr " " " " " "

d 0.5 2.0 0.30.4 3.5 2.2 0.30.4 2.2 2.4 0.3 2.3 O " " " " " " 18 13 87 86

d Ž. 1318 3.4 147277 ± ± ± Pliocene to Pleistocene ± 0.79 202 0.38 147 3.6 340 4.9 3.1 ± L. Miocene ± ± ± ± 200? 2.8 20 ? 3.2 y 103 5.3 3.2 ± L. Miocene ± ± ± ± y y y y y y y 35 to 65 to ? 2.2 125 to 255 to 125 to 40 to 1316 to 194 to 5to q q y y y q y y q Ž. uplifting subsiding uplifting uplifting subsiding tectonic setting interval m sub sl ½ PDB ½ PDB molar ppm subsidingsubsiding uplifting to 0.70918 to Pleistocene Niue uplifting Ž. Ž. Nauru Minami-Daito-Jima Ž. Mururoa surface to ? 3.5 Ž. Midway Ž. Lifou Table 3 AtollŽEnewetak . Ž .Ž.Ž. Dolomitized Ž.Ž.Ž. Summary of stratigraphy and pertinent geochemistry of Pacific atoll dolomites after Aharon et al., 1987 Ž. Funafuti Ž. Kita-Daito-Jima Ž. 22 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Fig. 12. Brine reflux dolomitizes platform carbonates associated with widespread platform evaporitesŽ. after Warren, 1991 .

This model explained the dolomitization of the brines should be sinking and forming reflux dolomite Permian reef complex where backreef, shelf and in underlying sediments. They observed micritic lagoonal carbonates are intensely dolomitized, while dolomite in Holocene lagoon muds beneath gypsum shelf edge carbonates, such as the Capitan reef, are crusts but, with only a soft sediment coring device, not. It was thought that dolomitization took place via could not penetrate the cemented Pleistocene base. brine reflux through porous sub-saltern2 sediments to Outcropping on the opposite end of the island, well depths of several hundred meters. The hydrological away from the present lagoon, they located a dolomite volume modeling of KaufmanŽ. 1994 and Shields replacing what they interpreted as Pleistocene lime- and BradyŽ. 1995 mathematically confirmed the fea- stone and inferred that this had formed during an sibility of the modelŽ. Fig. 13 . In a study of the earlier episode of brine reflux. evaporitic carbonates of the Permian Salado Forma- LuciaŽ. 1968 subsequently drilled beneath the tion, Garber et al.Ž. 1990 also used a reflux model to Pekelmeer Lagoon and found no widespread explain magnesite in the same basin. They argued dolomites in the underlying Pleistocene carbonates. that when a brine has a MgrCa ratio of )40, it will He also found that porewaters were of normal ma- precipitate magnesite as a replacement rather than rine salinity in areas where brine was predicted by a dolomite. Similar reflux processes, driven by the seepage reflux model. A thin clayey ash layer of deposition of Zechstein evaporites, have precipitated generally low permeability was found forming a magnesite cements in siliciclastic dune sands of hydroseal that separated sediments below with nor- Rotliegende reservoirs in the North SeaŽ Purvis, mal salinity from lagoonal sediments with elevated 1989. . salinity. Thus, reflux was not occurring beneath the By the mid 1960s, seepage reflux had gained a lagoon. Springs supplying seawater to the salina large number of proponents working in ancient evap- were in areas where the ash bed was broken or orite-associated dolomitesŽ Hsu and Siegenthaler, missing and formed the terminations of flow path- 1969.Ž. . Deffeyes et al. 1965 reported modern Mg- ways in the underlying PleistocenerPlioceneŽ Mur- rich brines in a marine spring-fed gypsum salina ray, 1969. . For most of the year seawater seeped into Ž.Pekelmeer Lagoon on Bonaire in the Netherlands the springs driven by evaporative drawdown. How- Antilles in the Southern Caribbean. From the water ever, during a short period in the late summer, the chemistry, they predicted that concentrated lagoonal springs were inactive and the hydrostatic pressure on the landward side was greater than that on the seaward side. Murray concluded that return flow or reflux could occur at such times but did not appear 2 A saltern is an area of widespread subaqueous shoal water to do so with sufficient volumes of brine to form evaporite depositionŽ fromwx Warren, 1999, p. 69. . replacement dolomite. J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 23

Fig. 13. Dimensional analysis of seepage refluxŽ.Ž. after Shields and Brady, 1995 . A Table of brine properties and amount of Mg required to convert calcite to a dolomite with 7% porosity.Ž. B Dimension of slice model used in their calculations. Slices are similar except that flow length varies from 1 to 1000 km. Base level is 100 m in all cases and head drops are 9.3 m for anhydrite saturated brine 1 and 17.1 m for halite saturated brine 2.Ž. C Dimensional analysis of Darcy's Law for a regional brine reflux system with 100 m base level. Diagram illustrates the time necessary for the minimum volume of dolomitizing brine to flow through a rectangular slice. All slices are 100 m thick and 1 m wide with varying flow lengthsŽ. 1±4 . It is assumed 350 kgrm3 of Mg are required to dolomitize the block. The lower curve for each flow path represents brine at halite saturation with a Mg exchange efficiency of 47%. The upper curve represents brine at anhydrite saturation with a Mg exchange efficiency of 36%. Permeability conversions assume an average brine density of 1160 kgrm3 and a viscosity of 8.904=10y3 kgrŽ.mrs and a temperature of 408C. 24 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

The claim that the Pekelmeer was forming exten- precipitate salts within platform hydrologies that in sive seepage reflux dolomites beneath an evaporite many areas have yet to reach equilibrium with the lagoon was further challenged when SibleyŽ. 1980 last sealevel rise. They lack extensive evaporites, concluded that the solutions responsible for exten- sizeable brine volumes, and stable platform-wide sive dolomitization in Tertiary sediments on Bonaire groundwaterrdensity heads capable of circulating Ždolomites that were earlier used as evidence for large volumes of Mg-rich fluids into underlying reflux dolomite. were probably fresh to brackish limestones. meteoric waters mixed with seawater, and were not Hydrological modeling of ancient reflux systems derived by reflux of the modern Pekelmeer hydrol- by Shields and BradyŽ. 1995 , using very conserva- ogy. Later work by Fouke et al.Ž. 1996 on dolomite tive assumptions of brine head and drawdown, show in the Seroe DomiŽ. Miocene , a unit more than 20 m that reflux can explain platform dolomites within thick that extends 115 km across the Nederlands geologically realistic time frames of burialŽ. Fig. 13 . Antilles, has reinforced Sibley's conclusions. How- Their flow model used flow lengths varying from 1 ever, Lucia and MajorŽ. 1994 continue to argue for a to 1000 km and the relative densities of seawater and significant input of marinerhypersaline waters dur- brines of two end member compositionsŽ. Fig. 13A : ing Neogene dolomitization in the region. brine 1 is anhydrite-saturatedŽ density rs1120 There are, nevertheless, other islands in the trop- kgrm3.Ž and brine 2 is halite-saturated rs1200 ics where localized brine reflux, associated with kgrm3. . Brine 1 is approximately 9.3% denser than s r 3 hypersaline conditions, is apparently forming seawater Ž rseawater 1024.5 kg m. , and brine 2 is Holocene dolomites. MullerÈ and TeitzŽ. 1971 docu- 17.1% denser. Using a 100 m tall and 1 m wide flow mented brine-reflux dolomite replacing an earlier cellŽ. Fig. 13B , brine 1 has approximately 9.3 m carbonate cement in skeletal grainstones on the more hydraulic head than a comparable column of shoreline in Fuerteventura in the Canary Islands. seawater while brine 2 has a differential hydraulic KocurkoŽ. 1979 found brine-reflux dolomite a few head of 17.1 m. Shields and BradyŽ. 1995 assume a meters above the high tide line, in the spray-zone pure calcite precursor and a final product that is pools of the shoreline of San Andres, Columbia. 100% dolomite with 7% porosity; this requires 350 Aharon et al.Ž. 1977 described small volumes of kgrm3 of magnesium. The resultsŽ. Fig. 13C show reflux dolomite forming in Holocene sediments of that reflux flow could circulate the Mg necessary to ASolar LakeB a small coastal salina in the Gulf of completely dolomitize carbonate platforms with radii Aqaba. Thin, localized brine-reflux dolomites can of a few tens of kilometers in a few million years. also be found about the present-day edges of Ras Shelves with shoreline-to-basin distances of the same Mohammed on the Sinai PeninsulaŽ see Warren, order can be completely replaced in a similar period. 1991 for a summary discussion of these deposits. . Larger platforms require more time or that the evap- The hydrologies of modern brine-reflux dolomites orite recharge areas migrate with time. Carbonate never approach the scale of processes required to platforms tens to several hundred kilometers across dolomitize shelf carbonates adjacent to ancient evap- could be dolomitized by such hydrologies. orites. This lack of parity is more than just a reflec- The relative abilities of rocks to transmit fluids tion of reaction kinetics; there is also a problem of will control where the bulk of the fluid flux occurs parity of depositional setting. Many ancient evaporite within platform limestones. Kaufmann'sŽ. 1994 and deposits have no modern counterpartsŽ Warren, Shields and Brady'sŽ. 1995 models show that it is 1999. . In modern settings, where reflux dolomites unrealistic to think of reflux dolomitization as a have been noted beneath evaporite crusts, the areas homogenous process uniformly overprinting all plat- of evaporite precipitation are small scale and local- form limestones. The inherent permeabilities of the ized. Their inherently small fluid potentials are various precursors clearly play an important role in caused by drawdown of no more than a meter or two dolomite intensity. The most permeable lithologies through highly permeable sandrcoral barriers no beneath the evaporite lagoon will focus the bulk of more than a few hundreds of meters to a kilometer or flow as the brine seeps seaward. Whatever unit is two wide. Modern coastal-margin evaporite systems, acting as the aquifer at the time of reflux will also be J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 25 the subject of the most intense dolomitization. High there are no modern counterparts to brine-reflux permeability units within flow paths of tens to hun- dolomites. Holocene evaporites are accumulating dreds of kilometers can channel the volume of neces- within an AicehouseB climatic phase. The current sary brine in time frames f1 Ma. Such high perme- AicehouseB mode, typified by waxing and waning ability units also have the potential to be dolomitized polar icesheets, has dominated the earth's climate for in regions well removed from the brine sourceŽ dis- the last 12 Ma. AIcehouseB times, with their high cussed in the porosity and permeability section later frequency sealevel oscillations, are not conducive to in this review. . the formation of thick platform evaporites. In con- The validity of reflux as a mechanism in ancient trast, AgreenhouseB time, with their inherent lack of carbonates is reinforced by the depositional settings polar ice sheets, favor the formation of platform of ancient evaporites. Many authors have concluded evaporites and, hence, widespread reflux dolomitiza- that brine-reflux models can explain ancient thick tion of platform carbonates. Basinwide evaporites Ž.tens to hundreds of meters , laterally extensive and their associated dolomites form via tectonic con- Ž.thousands to millions of square kilometers , Asec- trols on basin isolation that are independent of ondaryB dolomites associated with evaporites or Aicehouse versus greenhouseB sealevel scenarios. evaporite±dissolution brecciasŽ Land, 1985; Machel An inherent problem with reflux hydrology, as and Mountjoy, 1986; Hardie, 1987; Warren, 1991. . inferred by Adams and RhodesŽ. 1960 , is that it The present may be the key to the past but it is requires reflux-associated evaporite deposition to oc- simply not a good time to document depositional or cur in a lagoon with a surface connection to the open diagenetic analogues for ancient platform and basin- ocean. This was the prevailing understanding of a wide evaporitesŽ. Fig. 14; Warren, 1999 . There are silled evaporite basin in the 1950s and was based on no modern counterparts to most types of ancient the models first postulated by Oschenius in the 1870s. marine-platform and basinwide evaporites, hence, We now know it requires near impossible internal

Fig. 14. Ancient evaporite settings that have no modern analoguesŽ.Ž. after Warren, 1999 . A Platform evaporites, a combination of saltern and evaporitic mudflat sediment.Ž. B Basinwide evaporites, a combination of evaporitic mudflat, saltern, slope and basinal sedimentŽ see text for explanation. . Scales are schematic. 26 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 hydrologies involving sloping haloclines and brines Andres Formation, USAŽ Leary and Vogt, 1986; of differing densities simultaneously depositing car- Ruppel and Cander, 1988; Elliott and Warren, 1989. ; bonate, anhydrite and halite within a single brine the Permian Tansill and Yates formations of the body connected at the surface to open marine waters. Central Basin Platform, USAŽ Andreason, 1992; Such models are no longer considered physically Garber et al., 1990. ; the Jurassic Smackover and viable for the deposition of widespread subaqueous Haynesville Formations of the Gulf of Mexico evaporitesŽ Lucia, 1972; Kendall, 1988, 1989; War- Ž.Moore et al., 1988 ; the Lower Cretaceous Edwards ren, 1999. . At times when widespread platform or Formation of TexasŽ. Fisher and Rodda, 1969 and basinwide evaporites were deposited, the accumulat- the Middle Palaeocene Beda Formation, of the Sirt ing evaporite area was not connected at the surface Basin, LibyaŽ. Garea and Braithwaite, 1996 . to the ocean. Much of the influx of seawater to the The hydrology of such platform-evaporite associ- depression was via phreatic connate seepage through ated systems responds to eustatic fluctuations that the margin of the brine lake or seaway. also control the geometries of the associated plat- Ancient marine-associated evaporites deposited form carbonates. It deposits interlayered platform either as platforms or as basinwide saltsŽ Warren, carbonates and evaporites, where beds are of the 1999. . Brines ranged from ephemeral to AdeepB order of tens of meters thick and salt beds are perennial, although most permanent brine bodies interlayered with restricted to normal marine plat- were less than a few tens of meters. Platform evapor- form carbonatesŽ. Fig. 14A . Reflux dolomitization in ites typically accumulated within carbonate plat- this setting can be easily recognized, even in core, as forms during minor falls in sea level following, or the intensity of dolomitization decreases the further during, third order transgressions in times of down core or laterally one moves from the evapor- AgreenhouseB earthŽ. Fig. 14A . These set up eustati- ite±carbonate contactŽ. Fig. 15 . If not completely cally isolated platforms behind carbonate rims or re-equilibrated during later burial dolomitization, the shoals. In contrast, the larger volumes of basinwide oxygen isotope signatures of platform dolomites evaporites accumulated in both AgreenhouseB and formed by reflux typically follow a lightening trend AicehouseB climatic modes but required tectonic iso- away from the evaporite unit; that is, the further one lation of basins within a suitably arid climate. In this moves from the contact, the less was the sediment's scenario, the center of the basin is the locus of thick isotopic signature influenced by the heavier reflux- subaqueous evaporite accumulation, while much of derived brines. Although they may also be remobi- the former basin margin or platform is a subaerially lized by later burial dolomitization, trace elements exposed seepage systemŽ. Fig. 14 . Under both plat- can give another clue to a reflux association. Sass form and basinwide conditions, evaporative draw- and BeinŽ. 1988 found Neogene to Permian dolomites down allows bedded salts to accumulate sub- associated with gypsumranhydrite tend to show aqueously in brine-filled depressions and seaways of 50%±57% CaCO3 and the highest levels of sodium varying depthŽ. Kendall, 1992; Warren, 1999 . Ž.up to 2700 ppm . Marine nonevaporitic dolomites Within platforms, drawdown moved phreatic sea- Žthe counterparts of subtidal dolomites discussed ear- water into the rocks beneath the exposed rim, under lier. have similar compositions, but sodium concen- head differences of no more than a few to tens of trations of only 150±350 ppm. meters. At the same time, solar concentration in the In contrast to well-documented platform evaporite isolated seaway created dense Mg-enriched that associations, models of basinwide evaporitesŽ saline seeped downward as brines plumes or curtains into giants. have only gained widespread acceptance in underlying limestones. Most published reflux models the geological community in the last decade. This utilize a platform evaporite setting. Examples of reflects the acceptance that the Mediterranean formed brine reflux dolomites within carbonate platforms such a drawdown salt giant in the Messinian some include: the Cambrian Ouldburra Formation in the 5.5 Ma. Models of dolomitizing hydrologies in saline Officer Basin, AustraliaŽ. Kamali et al., 1995 ; the giants are more complicated than those of reflux Devonian BirdbearŽ. Nisku Formation of Canada dolomites within platforms, and are still under study. Ž.Whittaker and Mountjoy, 1996 ; the Permian San When basinwide evaporite systems were actively J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 27

Fig. 15. Wireline signature and the interpreted core description of the Humble a1 Beltex Well, Bowie County, TX, show a close relationship between Buckner anhydrite and dolomitization in the underlying Smackover Formation. Note the changes in the proportion of dolomite and porosity in terms of proximity to evaporite contactŽ. after Moore et al., 1988 . Smaller scale porosity fluctuations within the dolomite indicate the effect of varying grain size, sorting and marine cementation in the limestone precursor. accumulating salts, the salt beds were tens to hun- covered by brine were continuously subject to reflux dreds of meters thick and deposited across brine- related to a permanent brine curtainŽ. Warren, 1999 . covered areas in the basin center that were hundreds The hydrological and diagenetic character of bas- to thousands of kilometers acrossŽ. Fig. 14B . The inwide systems summarized in this section is based system was driven by evaporative drawdown of the on published studies of the more accessible examples order of hundreds to thousands of meters. The strand at outcrop in the Messinian evaporiterdolomite asso- zone about the edges of such perennial brine lakes ciations of the circum-MediterraneanŽ Sun and Este- and seaways moved rapidly over lateral distances of ban, 1995; Meyers et al., 1997. , the Permian Zech- tens to hundreds of kilometers.3 In contrast, the steinkalk dolomites of NW EuropeŽ Clark, 1980; zones beneath more central regions that permanently Smith, 1981. and the Devonian evaporites and dolomites of the Keg River-Pine Point Presqu'ile of CanadaŽ Maiklem, 1971; Davies and Ludlam, 1973; Bebout and Maiklem, 1973; Skall, 1975; Kendall 1989. as well as my own work in these and other 3 Flood and ebb cycles of the strandzone in basinwide evaporite evaporite basinsŽ see Warren, 1999; Chaps. 2 and 3 settings are autocyclic and are driven by tectonism and climate for a discussion of the hydrology and sedimentology change. The brine levels in saline giants are not related to marine . tidal or sealevel influences. Hence, the use of the term peritidal or of ancient evaporite basins . supratidal is inappropriate in describing the evaporites of these Drawdown basins have the potential to flush huge regionswŽ. Warren, 1999; Chap. 3x . volumes of waters through the basin edge as waters 28 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 in the basin center are lost by evaporation. These is much more likely to be a combination of confined waters are continuously replaced via the seepage and unconfined aquifers feeding predominantly sea- influx of varying proportions of:Ž. a phreatic seawa- water into the basin margin, as well as supplying terŽ. connate water seeping in through the basin connate crossflows beneath the descending brine edge;Ž. b subsurface waters whose lateral movement plume of the basin center. to the basin floor is driven by the kilometer-scale Outflow areas of resurging connate briner drawdown of the water level in the basin; andŽ. c by meteoric waters develop characteristic sets of pisoli- much smaller and episodic influxes of meteoric wa- ticrtepee cementstone lithologies termed strand- terŽ. Figs. 14b and 16a . Potentiometric heads are not linerseepage facies by WarrenŽ. 1999 . Kendall measured in meters but hundreds of meters. The Ž.1992 shows how this carbonate seepage facies can position of seepage outflow zone is controlled by the be used to define seawater and mixed-brine outflow edge of the perennial brine pool or seawayŽ Warren, points in the Devonian Keg River basinŽ. Fig. 16B . 1999; Chap. 2. . Aprons of such spring-fed sediments are typically As well as the primary dolomite that formed thin located about exposed former shelf slopes and bio- beds intercalated with the evaporites in such systems, herms. These permeable regions acted as aquifer there are two styles of more widespread diagenetic outflow zones that were sometimes capable of dolomite formed in carbonate aquifers that underlie breaching the active reflux curtain well out onto the and surround the drawdown basin. Their precipita- basin floorŽ Fig. 16A,B; see Warren, 1999 for a tion is driven by a hydrology inherent to the huge more detailed discussion. . Slightly down dip of this isolated depressions that accumulated basinwide seepage facies, evaporitic mudflats are dominated by evaporites. Style 1 are Astrand-zone dolomitesB 4 cre- capillary anhydrite and halite fabrics, which pass into ated by the mixed water hydrology that is inherent to perennial surface brine bodiesŽ the halite seaways of regions subject to the rapid migration of the perma- the Keg River Basin. on the lowermost sections of nent edge of the perennial brine lake or seaway. the basin floor. Style 2 are classic reflux dolomites precipitated dur- In the past 5 years, more geologists have come to ing the continual sinking of brine beneath the regions accept that the strand-zone edges of drawdown evap- of a permanent brine curtain. Let us look at each in orite basins are subject to rapid climatic and water turn. level oscillations. Thus, modified mixing-zone mod- MaiklemŽ. 1971 and Kendall Ž. 1989 pointed out els have been increasingly used to explain dolomites the potential for widespread dolomitization via sea- precipitated from mixed waters in strand-zones typi- water seepage and mixing zone interactions beneath fied by marginal dolomitizedrpisolitic carbonates, subaerially exposed slopes about the drawndown which are interlayered with the edges of basinwide basin margin of a saline giantŽ. Fig. 16A . A ground- evaporite unitsŽ Gill et al., 1995; Magaritz and Peryt, water-mixing zone develops where meteoric water 1994; Meyers et al., 1997; Peryt and Scholle, 1996. . floats atop denser seawater or saline brines. As the Suitable mixing zones hydrologies beneath the draw- brine level in the basin responds to changes in down strand-zone form either between meteoric and tectonics and climate, this zone of mixed waters will hypersaline waters edge, or between displaced con- move back and forth over the exposed basin edge. nate waters and fresher phreatic waters. The Fig. 16A is drawn assuming an unconfined aquifer widespread formation of strand-zone dolomites in a separating the marine and seepage zones. The reality basinwide setting is a reflection of the interaction between climate and substantial drawdownŽ up to kilometers. . Such an oscillating mixing-zone system is not as well developed in platform evaporite set- 4 This is a new term, designed to emphasize the difference in tings where the adjacent carbonate shoal rim typi- hydrological setting of this style of dolomite formed where the Ato cally allows drawdown that is usually a few meters and froB of the edge of a perennial brine body across a laterally extensive strand-zone drives before it a mixing zone capable of and always less than 20±30 m. dolomitization. The brine passage that helps form this type of Some of the hydrological details of the mixed- dolomite lies up-dip relative to the classical reflux model. water strand-zone margins of drawdown basins can J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 29

Fig. 16. Evaporative drawdown dolomitization.Ž. A Hydrology of basin showing seepage facies and halite in basin centre. Note the potential for the mixing of connate seepage and meteoric waters with refluxing brines beneath the strand-zone about the margin of the drawdown basin and the influence of the reflux curtain across much of the interior of the basin.Ž. B Close-up of facies pattern and potential for at surface strand-zone dolomitization associated with seepage outflow zones, vertical exaggeration f10:1Ž after Maiklem, 1971; Kendall, 1989; Warren, 1999. . be inferred from dolomite distribution within the dolomitization occurred within 600,000 years and edges of the Neogene evaporitic sub-basins of the possibly in less than 200,000 years. Such a short Mediterranean. There, in various basinwide settings, time frame underlines the effects of high levels of the lateral movement of mixed-water zones were Mg throughput that typify concentrated seawater pore driven by fluctuations in the basin's brine level, brines in the fluctuating strand-zone of a drawdown which drove before it phreatic lenses of less dense basinŽ. Warren, 1999 . The high flow rates and Mg waters. Meyers et al.Ž. 1997 described pervasive concentrations of waters in such basins means the dolomites that cut across Miocene sequence bound- time required for pervasive dolomite formation is aries in the Nijar Platform and occur in the youngest much shorter than in regions of meteoricrseawater Messinian reefroolitic units. They do not cross the mixingŽ. see dorag discussion in Section 4.4.2 . Miocene±Pliocene unconformity, although dolomite The carbon and oxygen isotopes of the Nijar is present as clasts in overlying basal Pliocene grain- dolomites show marked positive covariation, with stones. This constrains the timing of the strand-zone most values ranging from q5.4½ to y1.2½ 18q y 13 dolomitization as latest Miocene and pre-Pliocene in d OPDB and 2.5 to 4.3½ d C PDB Ž Meyers et age, suggesting that widespread and pervasive al., 1997. . The highest d18 O values are heavier than 30 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 reasonable estimates of Messinian normal-salinity in the ACalcare di BaseB in Sicily, they have been seawater dolomites, suggesting the involvement of extensively dolomitized by refluxing brines evaporative brines in the dolomitization process. Cal- Ž.Schreiber, 1988; Warren, 1989 . Offshore, the sub- culations of covariations of O and C isotopes during strates of much larger 1±2 km thick saline giants, water±rock interaction and fluid-fluid mixing argue which now lie beneath the deeper parts of the against recrystallization of Nijar dolomites, an inter- Mediterranean, have yet to be drilled, yet it is these pretation supported by their Ca richnessŽ 49±57 more basin central regions where classic reflux mol%. . Mixing calculations show that four-fifths of dolomites should occur. These thick salt deposits Nijar data are consistent with dolomitization by mix- formed on the Messinian floors of huge saltern tures of freshwater and a few tens of percent of lakesrseaways during the period of maximum draw- marine-derived evaporite brine, while one-fifth are down when smaller marginward sub-basins were consistent with mixtures of freshwater and normal- largely inactiveŽ. Butler et al., 1995 . Hence, we must salinity seawater. Modeling of the covariations of go to older basinwide evaporite basins where the 87Srr86 Sr and SrrCa ratios that developed during substrates to the thick evaporites are exposed to mixing, further supports involvement of evaporite confirm the presence of reflux dolomite. brine, and suggests that any freshwaters involved in More basin-central reflux dolomitization pro- the dolomitization had low SrrCa ratios but rela- cesses have overprinted the Zechstein carbonates of tively high Sr and Ca concentrations. Na, Cl, and NW Europe where dolomitized algal-bryozoan reefs,

SO4 concentrationsŽ 200±1700 ppm, 300±600 ppm, oolites and intertidal to shallow subtidal skeletal and 600±6500 ppm, respectively. also argue for grainstones and mudstones underlie widespread thick brine involvement and for mixings of brine with evaporites. The same dolomites are also interlayered freshwater andror seawater. with the platform portions of basinwide evaporites Meyers et al.Ž. 1997 argues that most dolomitiza- composed of anhydrite and halite deposited during tion occurred after Messinian reef formation, during the complete isolation and drawdown of the Zech- multiple brine level changes associated with basin- stein basin. It was at such time of drawdown that wide evaporite precipitation. Carbonate deposition reflux dolomites replaced the underlying Zechstein on the Nijar shelf occurred during higher sealevels limestonesŽ. Smith, 1981; Clark, 1980 . At the same mostly prior to widespread evaporite deposition, time, the regression of facies associated with the while evaporite deposition took place nearer the drawdown of the brine base level caused the en- sub-basin center during times of lowered water level croachment and displacement of formerly marine and basin isolationŽ. drawdown episodes . Brines with water by hypersaline and meteoric waters about the up to 5x-to6x-seawater concentration developed on basin marginsŽ e.g. dolomites of the Magnesian the floor of the basin during drawdown. At their Limestone; Kaldi and Gidman, 1982. . most concentrated these brine precipitated widespread Away from the strand-zone of any saline giant, subaqueous gypsum. Fluctuating brine base levels, the downward reflux of Mg-rich brine into underly- associated with the onset of intrabasinal transgres- ing sediments will ultimately mix with underlying sive episodes, drove before them zones of fresh- connate waters and other sub-evaporite watersŽ Fig. enedrmeteoric water that when mixed with the ris- 16B. . This continuously replenished hydrology of ing evaporitic pore brines were capable of dolomiti- descending evaporitic creates a brine curtain with a zation. subsurface chemistry capable of leaching calcite and The problem with dolomite distribution in the precipitating dolomiteŽ. Bein and Land, 1983 . Be- various Messinian sub-basins of the Mediterranean is neath a saline giant, this seawaterrhypersaline brine that studies are concentrated in the exposed margins mixing zone may extend under much of the evapor- of relatively small circum-Mediterranean sub-basins iteŽ. Fig. 16A . Its extent and potential to dolomitize in Spain, Italy, Cyprus and Sicily. The rocks beneath is possibly far greater than that of zones of mete- the thick basin-center evaporite fills of the various oricrseawater mixing in the strand-zones of the sub-basins, where classic reflux dolomites are more same basins and it operates in the same way as likely, are only poorly exposed. Where exposed, as reflux dolomites form in platform settings. J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 31

Brine entering an evaporite basin may be connate sodium salts are deposited. In this situation, the Ž.phreatic seawater or continental water mixed with MgrCa ratio increases only slowly, so creating a seawater or meteoric waterŽ. see next section . So far brine chemistry that is less likely to precipitate in our discussion of evaporite-associated dolomite, dolomite. we have assumed seawater-like ionic proportions. The fundamental chemical requirement to form a dolomite from brine is an increase in MgrCa ratio at 4.4.2. Meteoric mixing zone dolomite elevated salinities. In this context, it is important to All the mixing zone model requires is the mixing note that not all continental waters will achieve of waters of two saturation states with respect to the elevated MgrCa ratios at elevated salinities. Rather, minerals of interestŽ. Fig. 18A,B . In the preceding it happens only to those waters that have parents section, we developed notions of dolomite formation - with Ca SO4 . This is the same relationship as around the margins of a drawdown basin by the seawater-derived brines where sulfate is always pre- mixing of phreatic seawater with hypersaline brines sent in much higher concentrations than CaŽ Fig. or meteoric waters. At the same time, more extensive 17A. . In contrast, a sulfate-depleted parent water reflux dolomites were forming in zones of mixing ) Ž.Ca SO4 will have residual calcium, even after all between phreatic seawater and refluxing brines be- the CaSO4 is depositedŽ. Fig. 17B . This concentrates low the central portions of a saline giant. along with brine components, including Mg, until However, for more than three decades, a mixing zone model has been invoked to explain ancient subtidal shelf dolomites that were not associated with contemporaneous evaporites. The dolomitization event was typically a relatively early diagenetic episodeŽ. prior to mechanical grain compaction . In this nonevaporitic scenario, the mixing was envi- sioned as involving freshwater and seawater in a phreatic marine coastal zone. This is the Amixing zoneB model used by Hanshow et al.Ž. 1971 to explain dolomite in the Tertiary limestone aquifers in Florida and by LandŽ. 1973 to explain dolomites in the Hope Gate Formation, Jamaica. BadiozamaniŽ. 1973 in a keynote paper used a meteoric-seawater mixing-zone model to explain extensive Middle Ordovician shelf dolomites that were again not associated with evaporites. He called this Adorag dolomitizationB; dorag is Persian or Farsi for mixed blood. His model involved creating a dolomitizing solution by the subsurface mixing of seawater with meteoric water. Other examples of ancient meteoric-seawater mixing zone dolomites include the Palaeoproterozoic Malmani Dolomite of the Transvaal, South AfricaŽ. Eriksson and Warren, 1983 , the Silurian reefs of MichiganŽ Sears and Lucia, 1980.Ž ; the Devonian of Guangxi, south China Xun and Fairchild, 1987. ; the lower Visean of Belgium Ž.Muchez and Viaene, 1994 ; and the Jurassic reefs in r Fig. 17. Effect of ionic proportions in inflow water on Mg Ca Ž. ratio in concentrating brineŽ.Ž. after Hardie, 1987 . A Concentra- the Jura of eastern France Fookes, 1995 . ) Chemically, the mixing zone theory is straightfor- tion of seawater or continental water with SO4 Ca.Ž. B Conti- ) nental water or brine with SO4 Ca. ward and follows a very simple geochemical law. 32 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Fig. 18. Mixing-zone solubility curve chemistry.Ž. A Solubility of calcite as a function of partial pressure of CO2 . When solution A mixes with solution B the resultŽ. lies on dashed line AB is undersaturated with respect to calcite Ž.Ž. after Runnells, 1969 . B Solubility of calcite with respect to salinityŽ.Ž. after Runnells, 1969 . C Solubility curves for dolomite and calcite in a seawaterrfreshwater mixture. Dolomitization is thought to occur in the zone where the mix is undersaturated with respect to calcite and supersaturated with respect to A B s y17 dolomiteŽ. this is the dorag zone . This curve is calculated using a Ksp 10 , the value of ideal dolomite as used by Badiozamani s y16.5 Ž.Ž.1973 . D Solubility curve using dolomite Ksp 10 , the value for typical disordered dolomite precipitates as estimated by Hardie Ž.1987 . Note the much narrower dorag zone Ž C and D after Hardie, 1987 . .

When two waters that are saturated with a particular interval of mixingŽ 10%±35% seawater using the phase are mixed, the resulting solution may be un- data of Badiozamani, 1973. where the solution is dersaturated with respect to that particular phase, undersaturated with respect to calcite but supersatu- while at the same time may remain saturated with rated with respect to dolomite. This is the interval respect to another mineral phase. The saturation state where mixing zone dolomitization is theoretically during mixing depends on the solubility curve of the possible. The resultant texture should show evidence mineral phase of interestŽ. Runnells, 1969 . The only of calcite dissolution and dolomite precipitation. requirement is that the solubility curve for that par- Hence, when CO2 -saturated meteoric groundwaters ticular component is nonlinear. Consider the solubil- are diluted by seawater they will maintain the same ity curve of calcite as a function of the partial MgrCa ratio but, because of lowered ionic strength, pressure of CO2 Ž. Fig. 18A or as a function of the lowered concentrations of sulfate, and the in- salinityŽ. Fig. 18B When two waters are mixed Ž e.g. creased proportion of carbonate over bicarbonate in A and B in Fig. 18A. then the resultant solution is the solution, are more liable to precipitate dolomite undersaturated with respect to calcite, which is thus Ž.Fig. 19; Lippmann, 1973; Folk and Land, 1975 . liable to dissolutionŽ. the dashed line in Fig. 18A . The magnesium ions that maintain the high MgrCa Fig. 18C shows theoretical saturation curves for ratio of the diluted solution are derived mainly from both calcite and dolomite in varying mixtures of the seawater, but some can also come from the seawater and freshwater. It indicates that there is an carbonates dissolved in the mixing-zone. Many J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 33

been shown by Machel and BurtonŽ. 1994 not to be a meteoric-influenced precipitate. Their isotopic and chemical data show the dolomitizing fluid most likely was seawater of slightly variable temperature andror slight degrees of evaporation, with enriched levels of bicarbonate from the local oxidation of methane. Even LandŽ. 1991 , one of the leading proponents of mixing zone models in the early 1970s, has now come to the conclusion that the Neogene dolomites in the Hope Gate Formation of North Jamaica, which he once documented as type examples of a meteoric influence, are not mixing zone dolomites but are seawater derived. There are very few examples of dolomite forming in an active mixing zone in recent sedimentsŽ Ward and Halley, 1985. . One lies in the coastal plain of Israel, where Magaritz et al.Ž. 1980 reported dolomite cements composed of subhedral rhombs up to 50 mm Fig. 19. Stability of major carbonate minerals in relation to across occurring in two distinct horizons in the Qua- MgrCa ratio and salinity of natural waters. Note how dilution in ternary calcareous sands of the coastal aquifer. The seawater or a brine by fresh groundwaters can move the solution into the dolomite stability fieldŽ. after Folk and Land, 1975 . shallower horizon is in the present mixing zone, the deeper is interpreted as an earlier mixing zone. How- ever, the dolomites are found only as void-filling cements, and never appear to replace or develop dolomite crystals from the Plio-Pleistocene mixing pseudomorphs of a CaCO3 phase as the ideal mixing zones of the Caribbean region display distinctive zone model requires. Another possible example oc- cloudy centers surrounded by clear limpid rims. Sib- curs in Eocene dolomites in central western Florida leyŽ. 1980 suggested this was a result of pore fluid Ž.Cander, 1994 . Here, in a 20-m-thick interval, local- evolution from a state of near saturation with calcite ized modern mixing zone dolomite overgrows pre- in the early stages of crystal growth to a state of cursor crystals of pervasive reflux dolomite. Interest- undersaturation as growth continues in the mixing ingly, the mixing zone dolomite does not form in zone. limestone equivalents of the same aquifer where Recently, some workers have come to question calcite and not dolomite forms the substrate. the validity of a mixing zone between meteoric and The formation of extensive shelf dolomites by seawater providing a viable mechanism for diluted waters of a meteoric-seawater mixing zone widespread platform dolomitizationŽ Hardie, 1987; probably requires a more stable hydrology in a shelf Machel and Mountjoy, 1986; Machel and Burton, than has prevailed throughout the Quaternary. This 1994.Ž. . Hardie 1987 showed that if the solubility has been a time of rapid, glacially induced sealevel products for the more soluble disordered dolomites change Ž.AicehouseB earth . Few if any stable hydro- that are the modern day precipitates Ž Kf10y16.5. logical systems have yet formed along the coastlines are used instead of the values that Badiozamani of the world. The depositional systems, dynamics, Ž.1973 used, based on ancient stoichiometric and subsurface hydrologies of most present-day dolomites Ž Ks10y17. , then the range of fluid com- coasts are still adjusting to a sealevel rise of 120 to position suitable for mixing-zone dolomitization is 140 m in the last 18,000 years. Such rapid changes greatly reducedŽ. Fig. 18D . in sealevel and the associated climatic changes have The type example of Quaternary mixing zone led to frequent and rapid changes in the position of dolomite is the Golden Grove dolomite in Barbados, shelf mixing zones over the last 1 to 2 million years. as documented by Humphrey and QuinnŽ. 1989 , has The mixing-zone model may have application in 34 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 ancient sequences, yet, like the examples of reflux dian; Ye and Mazzullo, 1993. . This is best preserved dolomite in today's AicehouseB world, it cannot be in the supratidal±intertidal dolostones. More coarsely fully tested within the short hydrological histories of crystalline Ž.)40 mm, means78 mm dolomites are present carbonate shelves. more common in the subtidal and subtidal±intertidal Ye and MazzulloŽ. 1993 suggested that meteoric sediments. These have d18 O compositions that are dilution of seawaterrevaporitic brines drove precipi- slightly lighterŽ. meansy0.25½ than the finely tation of the bulk of the platform dolostones in the crystalline dolomites, but are still enriched by a PermianŽ. Lower Leonardian Wichita Formation in mean of 1.6½ relative to Leonardian sea waterŽ Fig. the Midland Basin, Texas. There are three dolostones 20A. . All these dolomites are near-stoichiometric, in these regionally widespread platform dolomites: with 87Srr86 Sr within the range of values of lower Ž.1 nonporous dolostones comprising supratidal-in- Permian seawater. The coarse crystalline dolomites tertidal faciesŽ. 2 porous dolostones comprising sub- are more depleted in Sr, Mn, and Fe than the finely tidal±intertidal faciesŽ. 3 porous subtidal dolostones. crystalline forms although all dolomites, regardless These consist of two types of dolomite, finely crys- of crystal size, contain less than 125 ppm Sr and less tallineŽ. 5±40 mm dolomite occurs in all three dolo- than 100 ppm MnŽ. Fig. 20B,C . stones and has a mean d18 O composition of q0.6½ The petrographic and geochemical data of Ye and PDB, but is enriched in d18 O by as much as 3.3½ MazzulloŽ. 1993 , in conjunction with their modeling relative to estimated lower Leonardian sea water of isotope data, suggest:Ž. a finely crystalline 18 sy y Žd Oseawater 1.7½ to 1.9½ during Leonar- dolomites formed and evolved penecontemporane-

Fig. 20. Isotope and trace element signatures of Permian Wichita Formation, TXŽ.Ž. after Ye and Mazzullo, 1993 . A Oxygen isotope composition versus crystal size.Ž. B Oxygen isotope composition versus Sr concentration. Ž. C Sr versus Mn. J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 35 ously to 40 mm crystals in supratidal±intertidal set- occur beneath the deep sea floor may not be justi- tings. These were flushed by contemporaneous hy- fied. Their conclusions are disputed by many persaline fluids. Oxygen isotope values and trace dolomite workers who have argued that the presence element data suggest that these waters were not of sulfate encourages dolomitizationŽ Eugster and much more saline than seawater and had not attained Hardie, 1978; Land 1985; Hardie, 1987. . the levels required to precipitate evaporite salts.Ž. b The common association of gypsumranhydrite Enhanced fluid flow continued through more porous, with mimetic dolomite in ancient evaporite succes- subtidal and subtidal±intertidal dolostones syndepo- sionsŽ. e.g. Warren, 1991, 1999 argues against the sitionally or following very shallow burial. This inhibition of dolomite formation by high sulfate lev- resulted in further transformation of the micritic els in ambient waters. HardieŽ. 1987 suggested that dolomite into mosaics of )40 mm crystals. Despite the precipitation of dolomite in areas of sulfate re- the slightly evaporitic setting, these later fluids were moval may be more related to the local enrichment y most likely of near-normal marine salinity and were in HCO3 Ž. alkalinity rather than the dissolved sul- probably derived by periodic meteoric dilution of fate acting as an inhibitor to precipitation. The other hypersaline Leonardian marine pore waters. The significant factor in the formation of organogenic oxygen and Sr isotope data, along with the lowered dolomite is the ubiquitous presence of bacteria in the Mn composition, suggest these fluids were similar zone of sulfate reduction and the underlying zone of to those that precipitated the initial fine-grained methanogenesis. The influence of bacteria in generat- dolomite. Most alteration occurred during relative- ing sulfate-depleted microenvironments, which are ly shallow burial Ž.-1.5 km by flushing by near conducive to dolomite precipitation, is largely un- normal marine waters with only minor meteoric known, but such bacteria may act as important facili- mixing. tatorsŽ. see Section 4.5 Bacterial mediation . In the Gulf of California dolomite is first encoun- tered in an ooze 60 m below the seafloor in water 4.4.3. Organogenic r methanogenic dolomite in more than 700 m deepŽ Fig. 21A; Kelts and Macken- hemipelagic marine sediments zie, 1982. . At still deeper levels, there are firm to Precipitation from normal seawater today forms lithified and impervious beds of micritic dolomite. dolomites that are less than 1% of the total sediment Most appear to be cements within generally CaCO - volumeŽ. see section Dolomite from normal seawater . 3 poor muds where some pelagic microfossils have However, much higher proportions of dolomiteŽ up dissolved, while others are replaced. Invariably, the to 10% of a 100-m-thick section. occur locally be- dolomitic mudstones retain the fine structure of the neath organic-rich deep-marine sediments in the Gulf original deep-water sediment and dolomitic horizons of CaliforniaŽ Baker and Kastner, 1981; Morrow and typically make up no more than 3%±5% of the Ricketts, 1988.Ž. . Baker and Kastner 1981 demon- sediment column. The dolomites range from Ca-rich strated experimentally that the rate of dolomitization to near stoichiometricŽ. 42%±51% MgCO with more is increased when the level of sulfate is decreased. 3 stoichiometric forms occurring at greater depths and They concluded that high levels of sulfate, such as in in the more lithified layers. Since the work of Kelts normal seawater, inhibit dolomite formation. By con- and MackenzieŽ. 1982 , many other authigenic trast, the low sulfate levels, found in marine pore dolomite layers and concretions have been docu- fluids that have undergone microbial reduction in mented in bacterially altered organic-rich deepwater organic-rich sediments, may create suitable environ- sedimentsŽ Garrison and Graham, 1984; Baker and ments for dolomite precipitation. Such reactions can Burns, 1985; Compton and Siever, 1986; Compton, be expressed chemically as: 1988; Suess et al., 1988; Middleburg et al., 1990. . q 2y™ q y Dolomite precipitating in the sulfate reduction 2CH2423 OŽ. organic material SO HS 2HCO zone shows negativeŽ. depleted d13C signatures of However, Baker and Kastner'sŽ. 1981 experi- y20½, while that formed slightly deeper in the ments were conducted at temperatures of 2008C and zone of methane oxidation have even more depleted their extrapolation to the lower temperatures that Ž.-ve d13C values as low as y70½Ž. Fig. 21B . 36 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Fig. 21. Quaternary deep-sea dolomite, Gulf of CaliforniaŽ.Ž. after Kelts and Mackenzie, 1982 : A Locality map of DSDP sites 474 to 481, rectangles indicate areas with documented diagenetic dolomite;Ž. B Summary model for depth of nucleation and the slow accretion of dolomite layers with burial, it is based on site 479. The vertical scale on the isotopic trends is schematic. It is not directly tied to the listed depth scale of the various dolomite occurrences as the extent of the zones varies according to sedimentation rate.

Dolomite forming deeper in the zone CO2 reduction continuity and local intrabed swells up to 2 to 3 m and methanogenesis shows somewhat more positive thick within the Miocene Monterey Formation of the values but these values too trend toward isotopically Santa Barbara Basin, CaliforniaŽ Garrison and Gra- lighter carbon at greater depths. When sedimentation ham, 1984. . There they are sandwiched between rates are low, dolomites may form in or just below dark organic-rich laminated diatomaceous opaline the zone of sulfate reduction and have negative d13C cherts. In total, the dolomitized horizons make up no valuesŽ. Malone et al., 1994 . When sedimentation more than 10% of the chert section. rates are high, dolomite will precipitate within the These dolomites demonstrate continuing dolomite zone of methanogenesis and well below the zone of aging through recrystallizationŽ. Malone et al., 1994 . sulfate reduction and so have positive d13C values. They show decreasing d18 O and d13C values, de- This style of methanogenic dolomite has positive creasing Sr and increasing Mg contents with increas- values up to 13.8½ in the Gulf of CaliforniaŽ av. ing burial depths and temperatures from east to west 9.8½; Kelts and Mackenzie, 1982. . With further in the basin. Maximum temperature probably in- burial newly formed dolomite once again takes on creased from -458C in the east to )808C in the d13C values that are increasingly negative. west. The d18 O values vary from 5.3½ in the east to Roehl and WeinbrandtŽ. 1985 propose that this y5.5½ PDB in the west and are interpreted to increasingly negative trend continues beyond the 400 reflect the greater extent and higher temperature of q-m depths penetrated by the DSDP wells in the dolomite recrystallization in the west. The d13C val- Gulf of California. They argue that growing crystals ues vary in parallel with d18 O and decrease from acquire increasingly lighter carbon from newly gen- 13.6½ in the east to y8.7½ PDB in the west. Sr erated petroleum. Hence, dolomites in reservoirs of concentrations also correlate positively with d18 O the Miocene Monterey Formation have much more values and decrease from a mean of 750 ppm in the negative carbon values than those from the Plio- east to a mean of 250 ppm in the west. Mol% 18 Pleistocene of the Gulf of California. Deepwater MgCO3 values inversely correlate with d O values organic-rich dolomites comprise thin stratiform, and increase from a minimum of 41.0½ in the east pinch and swell beds with kilometer-scale lateral to a maximum of 51.4½ in the west. This region J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 37 shows undeniable evidence that dolomite isotopes bacterial processes in facilitating precipitation in and trace elements are reset during diagenesis. other settings. Dolomite does not easily precipitate in Organogenic dolomite is also forming in organic- the laboratory at earth-surface temperatures, or in rich sediments beneath lagoonal waters up to 400 m natural modern day settings. This barrier is related to deep in Kau Bay, Indonesia. It occurs as a 2-cm-thick kinetic factorsŽ Land, 1985; Machel and Mountjoy, layer some 3.5 m below the present sediment surface 1986; Sibley and Gregg, 1987. . Recently, Vasconce- where sulfate levels are bacterially depleted and los et al.Ž. 1995 have suggested that direct mediation there is a marked increase in the alkalinity of the of bacteria can overcome this kinetic barrier and may interstitial watersŽ. Middleburg et al., 1990 . Similar play an active role in the formation of dolomite. This dolomite is found in an older 8-cm-thick layer some can occur wherever conditions are suitable for bacte- 7.45 m below the sedimentation surface and defines rial growth and so can range from the depositional the interface between marine and freshwater sedi- surface to depths of 1±3 km. ment. The isotope signature of this dolomite indi- Petrographic work is showing that particular cates that carbon was derived from degrading or- dolomite crystal habits and textures are related to ganic matterŽ. Fig. 27 . It is interpreted as forming specific microbial physicochemical environments and under highly alkalinity conditions initiated in the processes. GunatilakaŽ. 1989 correlated spheroidal zone of anoxic methane oxidation below the zone of dolomite occurrences with hydrocarbon seepages and sulfate reduction. The reaction can be summarized suggested that bacteria feeding on hydrocarbons me- as: diated the growth of the dolomite. Host sediments y y y included Miocene±Pleistocene continental strata and CH qSO2 ™HCO qHS qHO 44 3 2 extensively karstified, silicified and dedolomitized Pore fluids in the vicinity of the upper layer show Eocene lithologies. Ca depletion suggesting that the supply of calcium Dumbbell and spindle-shaped dolomite crystals rather than of magnesium is the rate-limiting factor have also been recorded from other microbially in- in this region. fluenced settings and include groundwater dolocretes Few would doubt that organogenic dolomite pre- of QuaternaryŽ. Khalaf, 1990; El-Sayed et al., 1991 cipitation is a widespread early diagenetic mecha- and Late Triassic ageŽ. Spotl and Wright, 1992 as nism of marine dolomite precipitation. But the well as cavities in vadose and phreatic environments amount of dolomite that can form by such a mecha- of outcropping Miocene reefsŽ. Monty et al., 1987 . nism is small Ž.-5% , and is limited by the rate of The presence of former AbacterialB microspheres in diffusive resupply of calcium or magnesium, always ancient rocks has also been inferred from SEM an ineffective pump beneath the deep seafloorŽ Land, examinationŽ e.g. Camoin and Maurin, 1988; Camoin 1985; Kelts and Mackenzie, 1982; Baker and Burns, et al. 1989; Folk, 1993, 1994; Nielsen et al., 1997. . 1985. . Such Ca or Mg as is present is supplied by These ApaleoB bacteria are generally preserved as advection or diffusion from the overlying seawater spherical andror deflated carbonate bodies. Spheri- or is derived from dewatering and diagenetic alter- cal bacteria, filaments and organic tissues from other ation of clays, biogenic silica, organic matter, inter- primitive organisms readily decay, leaving moldic stitial water, and the solution of Mg calcite or cal- microporosityŽ. Chafetz and Folk, 1984 . This could cite. Additional carbon is supplied by methane de- explain the abundance of micropores within rived from below the zone of high alkalinity or the spherulitic and dumbbell crystals in many ancient upward passage of thermocatalytic methane into dolomites. shallow seeps on the seafloor. Many authors have suggested that bacteria can concentrate Ca and Mg on their cell surfaces and so 4.5. Bacterial mediation create microenvironments where the ion activity product of calcite or dolomite exceeds saturation, so The recognition of the importance of bacterial favoring carbonate precipitation around the bacterial metabolism in dolomite formation in organic-rich bodyŽ e.g. Folk, 1993, 1994; Krumbein, 1979; hemipelagic sediments has led to an appreciation of Gebelein and Hoffman, 1973. . Many noticed that 38 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 magnesium can be preferentially retained on an or- gether in space like a single particle. When sulfate- ganic matter substrate and that these Mg-enriched reducing bacteria, metabolize SO4 ions, they also use sites may act as dolomite nucleation and crystalliza- the accompanying Mg 2q ions inside their cells. tion centers. Sulfate-reducing bacteria are often cited Magnesium is essential for many vital physiological as the taxa most involved in carbonate precipitation. functions in the cell, and makes up 0.5% of the cell's The discovery of bacterially mediated dolomite dry weight. During bacterial metabolism, excess Mg accumulations in Lagoa Vermelha, a shallow-water is released together with other byproducts of sulfate isolated hypersaline coastal salina east of Rio de reduction, such as bicarbonate ions and hydrogen Janeiro, Brazil, provides the best evidence to date of sulfide. Saturation with magnesium in microenviron- the participation of particular species of sulfate-re- ments around the bacterial cell probably creates con- ducing bacteria in dolomite precipitationŽ Vasconce- ditions favorable for preferential precipitation of los and Mackenzie, 1997. . There Ca-dolomite crys- dolomite, as was observed in lab-based bacterial tallizes under anoxic hypersaline conditions within a culture experiments. WrightŽ. 1999 presents a similar black sludge layer at the sediment surfaceŽ. Fig. 22 . set of arguments to explain dolomite precipitation in Following precipitation, the dolomite undergoes an the Coorong lakes. AagingB process, in which increased ordering of the Using the Lagoa Vermelha system as their start- crystal structure occurs. Both the initial precipitation ing point, Vasconcelos and MackenzieŽ. 1997 pro- and early diagenetic evolution are strongly mediated pose a new actualistic model for dolomite formation by microbial activity. In fact, sulfate-reducing bacte- that they call the microbial dolomite modelŽ. Fig. 22 . ria cultured in the laboratory at low temperatures In this model, the anoxic turbid layer at the sediment from Lagoa Vermelha samples produced a highly surface creates a special environment where photo- ordered dolomiteŽ Vasconcelos and Mackenzie, synthetically produced organic matter can be micro- 1997. . bially recycled by sulfate-reducing bacteria on a Vasconcelos et al.Ž. 1995 suggest that sulfate-re- submicron scale, with concomitant precipitation of ducing bacteria promote the essential conditions high-Mg calcite and Ca-dolomite. In contrast to the needed for dolomite precipitation. Sulfate ions form sulfate-reduction model of KastnerŽ. 1984 , this model strong ion pairs with Mg 2q ions and are held to- requires a continuing supply of sulfate to precipitate

Fig. 22. Schematic of microbial dolomite formation in Lagoa Vermelha. Evaporation in the dry season lowers the lagoon water level and drawdown then allows seawater to enter via seepage through the dune barrier. Elevated productivity in the lagoon leads to anoxia at the sediment±water interface and the formation of a black organic-rich sludge on the lagoon floor. The highly saline waters have elevated sulphate levels that provide a sulphur source for bacterial sulphate reduction as well as elevated magnesium that is precipitated as Mg-calcite and calcian dolomite. Microbial activity in the sludge mediates the precipitation of these primary carbonates that then AageB with burial. Eventually, the microbial activity ceases, but the diagenetic precipitation of dolomite continues atop the microbially precipitated crystal substrateŽ. after Vasconcelos and Mackenzie, 1997 . J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 39 the dolomiteŽ. Fig. 22 . The bacteria are required to The microspheres described by Nielsen et al. overcome the kinetic problems of dolomite forma- Ž.1997 were mainly composed of pyrite or carbonate. tion. They control the microenvironment and provide Some carbonate microspheres were not dissolved a surface on which the dolomite nanocrystals nucle- during surface etching of the dolomite crystals, sug- ate. The bacteria require sulfate ions for their gesting that a thin organic coating may still exist. metabolism, whereas dolomite precipitation requires Calcite spots observed within polished dolomites excess Mg along with the increased alkalinity. under SEM-backscatter mode might relate to Once nucleation has occurred, the Ca-dolomite calcite-filled micropores after bacteria but may also undergoes an AagingB process, as seen by concre- represent relics of a precursor limestone or exsolu- tions still growing some 14 cm below the Lagoa tion of calcite during dolomite recrystallization Vermelha sediment surface. Growth and aging are The presence of framboidal pyrite in association also seen in the trend towards more negative d13C with spheroidal dolomites testifies to a period of values and increased ordering in the crystals with bacterial sulfate reduction during early diagenesis depth. Bacteria apparently remain important, as seen Ž.Raiswell, 1982; Berner and Westrich, 1985 . If iron by the continued presence of nanobacteria on the is present, the H2 S generated during sulfate reduc- surfaces of the AagingB crystals. With time, and tion usually precipitates at or near the site as iron deeper in the sedimentary sectionŽ. 70±90 cm , the monosulfide or pyrite, although it may only form in crystals grow by a layer-by-layer mechanism that minor amounts. creates smooth euhedral forms. Apparently, once the Dolomite can also form by sulfate reduction at dolomite is nucleated, the crystals can continue to oil±water or gas±water contacts in zones of oilrgas grow by inorganic processes, but each new layers washing and bacterially induced hydrocarbon degra- still appears to be composed of coalesced dation. In the subsurface, oil±water or gas±water submicron-size bacterial ballsŽ. spheroids . The mi- contacts define aerobicranaerobicŽ. redox transition crobial dolomite model requires an ongoing supply zones between reduced paraffinic crude oils or gases of sulfate to maintain the microbial activity promot- and oxygenated groundwaters. Locally anoxic condi- ing dolomite precipitation. In contrast, it may be that tions provided by the buildup of hydrocarbons at the presence of sulfate in a sterileŽ. bacteria-free these interfaces mean that aerobic bacteria can pro- environment may act to inhibit dolomite precipita- vide nutrients to the anaerobes, while the through- tion as shown in the experiments of Baker and flowing basinal waters supply the required Ca, Mg

KastnerŽ. 1981 and Morrow and Ricketts Ž. 1988 . and SO4 . Such oil±water or gas±water contact In eastern Belgium, bacterially induced spheroidal dolomites can even form cements in otherwise car- fabrics have been discovered within an ancient, kars- bonate free sandstones. Carbon isotope signatures in tified, and Pb±Zn mineralized, Lower Carboniferous the dolomites tend to be very negative as the carbon dolomiteŽ. Nielsen et al., 1997 . Arguments support- in the dolomite is derived from methane and other ing a bacterial interpretation in this and similar an- 13C-depleted hydrocarbons. ThickŽ. 2±3 m dolomite cient dolomites include: spar cements occur at present-day and former gas± v the textural similarities to documented bacterio- water contacts in some oil and gas fields hosted in genic precipitates, such as dumbbell-shaped crystal quartz sands in offshore BruneiŽ. pers. obs. . There aggregates; the carbon isotope signatures of the dolomites are in 13 v the presence of microspheres embedded in the range d Cfy20½ to y50½Ž. Fig. 27 . This is spheroidal dolomite crystals; in marked contrast to the isotopically heavy signa- 13 v the negative d C of the dolomites; tures in carbon of organogenic dolomites that formed v the omnipresence of framboidal pyrite embedded in the much shallower zone of methanogenesis. in the spheroidal dolomite crystals and their ab- Carbon-13 depleted siderite and ankerite are com- sence in the cement; mon coprecipitates with these subsurface-formed 34 v the very negative d S of later framboidal pyrites; dolomites. The varying mineralogies reflect the vary- v the general geological setting in which bacterial ing availability of Fe, Mg and Ca in the pore processes are likely to have occurred. fluids. 40 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

4.6. Burial dolomite

Burial dolomites are subsurface cements and replacements that form below the active phreatic zone Ž.reflux and mixing zones in permeable intervals flushed by warm to hot magnesium-enriched basinal and hydrothermal waters. Prior to the 1960s, burial processes were considered important to dolomite formation; they explained the paucity of modern dolomites and their increased abundance in Paleo- zoic and Precambrian rocks compared to those of the Mesozoic and the Paleozoic. The idea lost its appeal as more and more ancient dolomites were interpreted using Quaternary analogues. Today, most geologists recognize the kinetic limitations inherent in the strict application of Quaternary analog models, and the implications of dissolution±reprecipitation processes. The notion of burial dolomitization is again gaining in popularity. Unlike earlier models, however, recent concepts of burial dolomitization typically suggest that it is part of a continuing series of dolomitization overprints. Many dolomitization events begin early in the depositionalrdiagenetic cycle and are overprinted or cannibalized during burialŽ Gao et al., Fig. 23. Subsurface waters. Lower diagram is a plot of the expected stability field of dolomite as a function of temperature 1992, 1995; Amthor et al., 1993; Wojcik et al., 1994; and CarMg ratio in 1 M and 2 M chloride brines. The upper Montanez,Ä 1994; Ayalon and Longstaffe, 1995. . diagram is a histogram of the CarMg ratios of natural groundwa- Dolomitization under burial conditions probably ters in contact with a variety of sedimentary rocks. The diagram suffers least from the kinetic inhibition associated clearly shows that most natural waters are capable of becoming 8 8 Ž with lower temperature syntheses of dolomite precip- dolomitizing fluids at temperatures above 60 C±70 C after Hardie, 1987. . itates. At its simplest, it requires a throughflow of warm to hot, magnesium-rich basinal fluids. By a depth of a few kilometers temperatures are in excess of 608C±708C. Dolomite that is precipitatingrre- rillonite-to-illite transformation, and from the deep crystallizing in such an environment exceeds the subsurface dissolution of bittern salts such as carnal- P critical roughening temperature and so resets its tex- lite Ð K23 MgCl 6H2O, polyhalite Ð K 2 MgCa 2 P P tural-chemical parametersŽ. Gregg and Sibley, 1984 . Ž.SO442H 2 O, and kieserite Ð MgSO 4HO. 2 . Fig. 23 is a plot of waters in such a system, it shows Historically, clay mineral transformations in shales that most basinal waters are capable of becoming were the most commonly cited source of magnesium dolomitizing fluids at temperatures above 608C±708C Ž.Kahle, 1965 whereby the illitization of smectite Ž.Hardie, 1987 . released magnesium into basinal brines to dolomitize Rather than being limited by lower temperatures, adjacent platform carbonates. In a similar fashion, a burial dolomite model must explain how substan- Boles and FranksŽ. 1979 argued that the illitization tial volumes of magnesium are supplied and how of clay minerals in interbedded shales at depths of fluids are pumped through deeply buried low-per- more than 2500 m was a likely source of the iron meability strata at viable rates. Magnesium can come and magnesium precipitated in ferroan dolomite ce- from a number of sources: from the chemical re- ments in Tertiary sands of the Gulf of Mexico. But working of nearby and perhaps earlier dolomites, the amount of dolomite formed by the escape of from structural Mg 2q expelled during the montmo- basinal waters in the Gulf Coast is small in relation J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 41 to the volume of fluid flow and only small amounts of dolomite cement has formedŽ Milliken et al., 1981; Land, 1984. . In fact, dolomite is much less common than calcite as an authigenic cement in the sandstones of the Texas Gulf coast. Where magnesian-rich cements are locally abundant, they are ankeritic rather than dolomiticŽ. Boles, 1978 . Diage- netic formation of chlorite in a sedimentary basin commonly consumes magnesium, not supplies it, so that in the compacting shales of the Gulf of Mexico, the formation of chlorite and illite from smectite provides an efficient internal sink for magnesium, little is released into escaping basinal brinesŽ Land, 1985. . No massive release of magnesium by compacting shales has yet been observed in any compacting sedimentary basin. There are also flow rate problems inherent in moving large volumes of Mg-entraining subsurface waters through a deeply buried shelf limestone. Dolomitization must take place at depths where compaction has reduced matrix porosity to less than 5% to 10% and permeabilities to -0.1 md. These limitations imply that little new dolomite can be generated from compactional flow of modified-marine pore fluids. At best, burial fluids can introduce relatively small volumes of new magnesium into the Ž carbonate platform of a subsiding and compacting Fig. 24. Hydrologic flow patterns for burial dolomitization after Heydari, 1997.Ž. . A Passive margin circulation with burial water shale basinŽ. Morrow, 1982; Land, 1985 . Burial flow driven by sediment compaction and catabaricrthermohaline dolomitization within such a setting is either a fo- flow.Ž. B Collision margin circulation where thrust sheet emplace- cused flow of hot brine along permeable beds and ment drives burial water from sediments caught up in the orogenic faults at the basin edge, or it is largely the result of belt into the craton interior.Ž. C Post-tectonic circulation where cannibalization and re-equilibration of earlier deep burial flow in the basin sediments is driven by gravity head supplied by adjacent weathering mountain belt. Note the varying dolomites. The potential for moving larger volumes vertical and horizontal scales. of Mg-rich basinal fluids through a platform margin is greater in a basin undergoing collision compression, especially if the basin also contains thick dis- subsidence and, in some basins, by salt tectonics solving salt sequences. Hydrologic mechanisms that Ž.Fig. 24A . The Gulf of Mexico and Atlantic-margin drive and focus fluids creating regional burial basins during the Mesozoic are typical examples of dolomites are still not fully understood, our 3D this realm. The shallowest part of this realm, down models are largely qualitative rather than quantitative to f1 km depth, is typified by active phreatic Ž.Garven et al., 1999 . circulationŽ. mixing zone and brine reflux that was Hydro-tectonic processes driving burial dolomiti- discussed earlier. Below this Ž.)1 km depth is a zation have been divided by HeydariŽ. 1997 into zone of compactional flow which entrains both hori- three hydrologic realms:Ž. 1 passive margin burial zontal flow regimesŽ. average rates of 6 cmryear diagenesis,Ž. 2 collision margin burial diagenesis, and vertical flow regimesŽ average rates of 0.15 andŽ. 3 post-orogenic burial diagenesis. Passive mar- cmryear. . Deeper still, or in zones of thermal gin burial diagenesis is characterized by extensional anomalies around salt structures and mud diapirs, tectonics, growth faulting, relatively slow and steady there is the thermohaline flow regime. Salt beds and 42 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 allochthons act as foci for the escaping basinal fluids ogy is more variable and so less understood than that Ž.Warren, 1999 . Away from local thermal anomalies, of collision or passive margins, and some authors temperatures within this realm increase steadily with have claimed it tends to dedolomitize rather than depth according to regional geothermal gradients, dolomitize the craton carbonatesŽ Back et al., 1983; which range from 158C±308Crkm. Sediments shal- Morrow, 1998; Ramsey and Onasch, 1999. . lower than 3 km are typically hydrostatically pres- Qing and MountjoyŽ. 1994a,b,c found that late- sured, while deeper strata are geopressured. A transi- stage, coarsely crystalline replacement dolomite and tion to overpressuring can occur at much shallower associated saddle dolomite cements, along with late- depths where salt, not shale, is the aquitard Žf1 km; stage dissolution vugs and caverns, constitute a Warren, 1999. . Burial dolomites formed in passive widespread collision margin burial diagenetic facies margin platforms are often intimately associated with in the Middle Devonian Presqu'ile barrier. The hy- the movement of compactional fluids along the ex- drologic realm extends southwestward from Pine tensional faults or active growth faultsŽ McManus Point to the subsurface of the Foothills in northeast- and Wallace, 1992. . ern British Columbia. The resulting burial dolomites Collision margin burial diagenesis is characterized create hydrocarbon reservoirs in otherwise tight by compressional tectonics, thrust faulting, extensive limestones in the subsurface of the Northwest Terri- fracturing of carbonate strata and episodic fluid flow tories and northeastern British Columbia, as well as in response to tectonic loadingŽ Fig. 24B; Heydari, acting as ore hosts to MVT Pb±Zn deposits at Pine 1997.Ž . Hot 1008Cto)2508C . , saline and highly Point. TheyŽ. a replace blocky sparry calcite cement, pressured fluids are moved laterally out of the colli- Ž.b occur continuously across the sub-Watt Mountain sion zone, typically toward the craton or continental unconformity,Ž. c postdate stylolites and earlier re- interior through porous and permeable horizons and placement dolomites, andŽ. d overlap sulfide miner- vertically through faults and fracturesŽ. Oliver, 1986 . alization. Along the Presqu'ile barrier, the d18 O val- Such systems are typified by burial and hydrother- ues of both replacement and saddle dolomites in- mal dolomites formed in the Appalachian±Ouachita crease eastward and updipŽ i.e. away from the region collision beltŽ. Montanez,Ä 1994 , the Devonian± of most intense orogenesis. . Values range from Carboniferous and Tertiary orogenies of Alberta y16½ PDB in the deeper areas of northeastern Ž.Qing and Mountjoy, 1994a , the Variscan orogeny British Columbia to y7½ PDB at Pine Point. Corre- of SardiniaŽ. Boni et al., 2000 and the Hercynian sponding homogenization temperatures of saddle orogeny of southeastern IrelandŽ Hitzman et al., dolomite fluid inclusions decrease from 1788Cto 1998. . All these compression driven hydrologies are 928C. The 87Srr86 Sr ratios of coarsely crystalline capable of forming widespread burial dolomites and and saddle dolomites also decrease eastward along Pb±Zn deposits both within the orogenic belts and the Presqu'ile barrier from about 0.7106 in northeast- the continental interiorŽ see Dolomite as ore hosts ern British Columbia to 0.7081 at Pine Point. All of section. . these geochemical trends imply basin-scale migra- Post-orogenic burial diagenesis is characterized tion of hot, dolomitizing fluids updip along the by a lack of tectonic activity, dominance of topo- Presqu'ile barrier. These large-scale fluid movements graphically driven flow, and high fluid flow rates are interpreted to be related to tectonic compression ranging from 100 to 3000 cmryearŽ Fig. 24C; Hey- and sedimentary loading of the Western Canada dari, 1997. . This hydrology begins once the main Basin. Such compression occurred at least twice in phase of tectonic collision ends and is driven by the the Phanerozoic history of the basin: first during elevation gradient created during the preceding oro- early burial between the Late Devonian and Early genic phase. The composition of the deeply circulat- Carboniferous and then during deep burial between ing basinal and meteoric waters is controlled by the Late Jurassic and early TertiaryŽ Qing and Moun- water±rock interaction with the aquifers through tjoy, 1994a,b,c. . which they flow. This hydrologic style is thought to MontanezÄ Ž. 1994 concluded that the late diage- exemplify late stage dolomites of the US mid-conti- netic burial dolomites of the Lower Ordovician, Up- nent. In terms of dolomitization, this style of hydrol- per Knox Group in the southern Appalachian basin J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 43 were also associated with tectonic compression, viable reservoirs during hydrocarbon migration in widespread secondary porosity development, hydro- the late Paleozoic. carbon migration, and local MVT mineralization. MorrowŽ. 1998 argues that both Atectonic com- This regionally extensiveŽ approximately 70,000 pressionB and Atopographic rechargeB models for km2 . unit is made up of replacement dolomites and subsurface fluid flow may have been overapplied zoned dolomite cements. Replacement dolomite when explaining regional burial dolomite bodies. He comprises 15% to 50% of all the Knox matrix argues tectonic compression requires an unreason- dolomites. The d18 O Ž.y11.9½ to y5.3½ , d13C able degree of fluid focusing to achieve precipitation Ž.y3.8½ to q0.9½ , and87 Srr 86SrŽ 0.70895 to temperatures equal to observed fluid inclusion tem- 0.70918. values of replacement dolomites overlap peratures, which are typically much hotter than those with those of earlier cements and Lower Ordovician of their surrounding carbonate hosts. This argument marine calcites. This overlap is interpreted as indicat- is based on a limited sampling set, some burial ing rock buffering of initial dolomitizing fluids, as dolomites are hotter, some are the same and others well as extensive neomorphism of replacement are cooler than their surrounds. The variability in dolomites by late diagenetic fluids. The later types of focused burial dolomite will be discussed in dolomites have porosities of 1%±16% and perme- a later section dealing with dolomite as an ore host. abilities of 0±1030 md, values significantly greater He goes on to argue that topographic recharge has than those of either earlier replacements or their host the limitation of flushing solutes out of the system, limestonesŽ. 1%±6% and 0.0±0.04 md . rendering it incapable of further dolomitization. In MontanezÄ Ž. 1994 was able to regionally correlate addition, he feels both topographic recharge and five generations of dolomite cements that recorded tectonic compaction are unlikely flow mechanisms to regionally extensive diagenetic events. The d18 O explain the origin of extensive open space dolomite Ž.y12.4 to y3.0½ and87 Srr 86SrŽ 0.70885 to cement because of their limited supply of solute. He 0.71000.Ž. values, along with Sr 18 to 147 ppm , Mn argues that thermal convectionŽ a variation on the Ž.Ž.63 to 1069 ppm , and Fe 109 to 8452 ppm con- Kohout circulation discussed earlier. is a more viable tents of zoned dolomites, in conjunction with fluid hydrologic drive as it can support long-lived flow inclusion data, indicate that late dolomites precipi- systems in the subsurface, which are also capable of tated from hotŽ.Ž 808Cto)1658C , saline 13 to 22 recycling solutions many times through the deeply wt.% NaCl equivalent. basinal brines that had under- buried rock mass. This enhances the opportunity for gone extensive fluid±rock interaction with clastics. ongoing cementation and cannibalization. Because Precipitation temperatures estimated from fluid in- thermal convection can occur in confined aquifers clusion geothermometry and systematic trends in beneath the seabed, seawater-derivedŽ. connate solu- d18 O values record a regionally developed, prograde- tions enhancing the dolomitization potential may be to-retrograde thermal history. MontanezÄ Ž. 1994 inter- continually added to the convection system. The prets these dolomites as recording the spatial and documentation of crustal scale thermal convection temporal evolution of large-scale fluid flow systems systems within subaerially exposed orogenic belts, that had developed in response to differing burial and the outcrop evidence of both upward and down- and tectonic stages of the southern Appalachian basin. ward extending bodies of burial dolomite add The occurrence of zoned dolomite cements in tec- credence to the hypothesis that thermally driven tonic fractures and breccias, and their close associa- convective flow occurred within ancient platform tion with noncarbonate diagenetic minerals of Penn- carbonates, and may have induced regional burial sylvanian to Early Permian ages, suggest that most dolomitizationŽ. Morrow, 1998 . Knox Group dolomites record deep subsurfaceŽ 2 to Whatever process drives the circulation, burial )5 km. fluid migration in response to late Paleo- dolomites are typically characterized by coarsely zoic Alleghenian tectonismŽ. 330 to 265 Ma . They crystalline nonplanar and sparry crystals, often of served as long-lived conduits that focused and chan- saddle dolomite within residual pores. Saddle neled fluids. The occurrence of bitumen in secondary dolomite is turbid, coarsely crystalline and typically porosity indicates that these dolomites were the most has a curved crystal lattice showing sweeping extinc- 44 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 tion in cross-polarized lightŽ Radke and Mathis, pable of dolomite precipitationrreplacement, when 1980. . Burial dolomites tend have negative d18 O they are transmitted upward into cooler, shallower values indicating precipitation from fluids at some- parts of the basinŽ. Hardie, 1991 . They reflect a what higher temperatures than those of earlier plat- focused outflow of hydrothermal waters that some- form dolomites. Trace elements such as Sr and Na times also fed base metal mineralizing systems are low as the limestone precursor has long since ŽBraithwaite and Rizzi, 1997; Warren and Kempton, converted to low Mg±Calcite and successive episodes 1997. . They are, in effect, more focused variants of of dolomitization have reduced the levels of these the burial dolomitization mechanisms discussed in elements even further. Some burial dolomites may the preceding section and can form in all three burial have slightly elevated levels of Fe and other metals realms. They are discussed as a separate grouping indicating a throughflux of metalliferous catabaric because they are more obvious examples of focused pore waters. Growing crystals may also entrain ma- hydrologic outflow and cooling typically along faults ture hydrocarbons in their fluid inclusions for the and fractures. They indicate the effect of fluid flows same reason. The modification of earlier formed with high Rayleigh numbers, their inclusion and dolomites by basinal fluids can generate crystals isotopic values typically indicate precipitation from with low d18 O values, modified 87Srr86 Sr ratios, and fluids that were much warmer than those effecting more saline higher temperature inclusionsŽ Banner et immediately adjacent carbonates. al., 1988. . As basinal waters become heated during hy- As primary dolomitization of a limestone by basi- drothermal circulation their buoyancy increases and nal fluids generates similar characteristics to those of they rise upwards and outwards along bedding, faults a replacive burial dolomiteŽ Machel and Burton, or other permeability pathsŽ Freeze and Cherry, 1991. , there is often controversy when two groups of 1979. . The shape of these flow paths is governed by workers study the same dolomite body. For example, the Rayleigh numberŽ the ratio of buoyant to viscous Lee and FriedmanŽ. 1987, 1988 concluded that most forces. . For relatively cool systems, fluids will rise of the Ordovician Ellenberger dolomite in Texas and and geometry will be determined by the ratio of New Mexico was the result of burial dolomitization. vertical to horizontal permeability. For high Raleigh In contrast, Kupecz and LandŽ. 1994 concluded that numbers, buoyant forces are stronger, and the rising it formed during early diagenesis and was modified fluid forms a concentrated, dominantly vertical, during burial by discharge of basinal fluids along plume. Within this plume, temperatures, flow rates, dolomite aquifers. and chemical potential may be expected to decrease from the center towards the margins. Maximum tem- 4.7. Hydrothermal dolomite peratures and the temperature differentials needed to drive such a circulatory system may be quite low Not all late stage or burial dolomites are pervasive Ž.208C±258C; Aharon et al., 1987 . or widespread replacements of shelf dolomites. Some One of the best-documented occurrences of such a are much more focused in their geometry and form late-diagenetic fault-focused dolomite is hosted in within higher temperature haloes about what were the Ordovician Trenton-Black River limestones of relatively permeable conduits, such as faults, thrust Michigan and southwestern OntarioŽ Fig. 25A; Tay- planes, or zones beneath impermeable, often evapor- lor and Sibley, 1986; Hurley and Budros, 1990; itic, seals. Deeply circulating basinal waters only Middleton et al., 1993. . There the dolomite defines become hydrothermal,5 locally mineralizing and ca- zones of faulting and fracturing in the surrounding limestones. It is of two types, a widespread AcapB dolomite and a more localized AtrendB dolomite associated with fault planes. The latter forms highly 5 Hydrothermal water is a subsurface water whose temperature localized and linear hydrocarbon plays such as the is high enough to make it geologically or hydrologically signifi- Ž. cant, whether or not it is hotter than the rock containing it. It does Scipio-Albion field Fig. 25B . not have to be generated by magmatic processesŽ see AGI Glos- The cap dolomite replaces the upper few meters sary of Geology. . of the Trenton Group, while the later AtrendB J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 45

Fig. 25. Fault-focused late dolomitization in the Scipio-Albion trend, MichiganŽ.Ž. after Hurley and Budros, 1990 . A Structural stratigraphic interpretation of the Albion-Scipio trend. The faultrfracture set controlling dolomitization is pre-Devonian. The dashed lines indicate bedding planes beneath the unconformity at the base of the Trenton-Black River.Ž. B Structural cross-section of the Albion-Scipio field. Note that the field is located in a synclinal trendŽ. not usual for a hydrocarbon trap and there is a rapid lateral transition from a regional nonreservoir limestone into a permeable reservoir dolomite. Lost circulation and fractures zones are common in this dolomite.

dolomite is developed in the vicinity of fractures ing shales of the Blue MountainrUtica Formation. Ž.Middleton et al., 1993 . Late stage calcite and anhy- Subsequently, the fracture-related 'trend' dolomite drite cement, as well as minor sulfides, fluorite and appears to have resulted from fluid flow along reacti- barite, postdate dolomitization. The cap dolomite is vated fractures. Various possible Mg and fluid petrographically and geochemically distinct from sources for dolomitization exist:Ž. 1 compactional fracture-related dolomite. Where fracture-related dewatering of basinal shales and evaporites and fo- dolomite occurs in the upper few meters of the cusing of fluids updip towards the basin margins and Trenton Group; however, the cap dolomite is no Ž.2 flushing of Silurian or younger brines down into longer obvious, suggesting that it was overprinted. the Ordovician sequence. The cause of the elevated Fluid inclusions in fracture-related saddle dolomite temperatures associated with the dolomitization is cements in the AtrendB dolomite are salineŽ 24 to 41 still not known. wt.% NaCl eq.. and homogenization temperatures range between 1008C and 2208CŽ Middleton et al., 4.8. Applying the models 1993. . These temperatures are considerably higher than those likely to have been generated during peak To decipher the depositional and diagenetic his- burial of the adjacent limestones Ž.f708C . Late-stage tory of a dolomite requires the investigator to call calcite cements have fluid inclusion homogenization upon as many chemical and physical techniques as temperatures ranging from 708C to 1708C and were he or she can economically justify. The four most precipitated by less saline brinesŽ 16 to 28 wt.% common groups of tools in use are:Ž. a stable iso- NaCl eq.. . topes,Ž. b trace elements, Ž. c fluid inclusions, and Ž. d Middleton et al.Ž. 1993 interpret the cap dolomite geometryrpetrography of a deposit. The first three as a result of compactional dewatering of the overly- are best conducted on fresh core or cuttings, while a 46 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 study of geometryrpetrography requires a combina- most important of which are:Ž. a our inability to tion of seismic analysis, detailed sedimentological synthesize dolomite in the laboratory and so measure logging of core or outcrop andror a suite of detailed partition coefficients under controlled conditions at wireline logs, cuttings and sidewall cores from repre- room temperatures, andŽ. b difficulties in quantifying sentative wells.Ž The reader interested in more infor- the volumes of various diagenetic fluids and their mation on chemical techniques is referred to: Roed- chemical overprints, which alter crystal chemistry der, 1984; Aulstead et al., 1988; Tucker and Wright, from the time of initial dolomite precipitation. The 1990; Banner, 1995.. Despite more than three lumping problems inherent to older analytical tech- decades of intense study, unequivocal genetic inter- niques using whole rock sampling are no longer a pretations tracing the complete depositional and problem with the rise of microsamplingŽ microprobe burial evolution of a dolomite are still problematic and micropipette. techniques. Ž.Land, 1980 vs. Veizer, 1983 . Difficulties in synthesizing dolomite have led to a At the core of this problem is the notion of, how poor knowledge of distribution coefficients of ele- does an ancient dolomite form? What is the point of ments in the mineral, and so large degrees of varia- studying the trace element and isotopic signatures of tion must be acceptedŽ. Banner, 1995 . For example, a mineral we know has undergone multiple episodes because seawater has such a high ionic strength in of dissolution and reprecipitationŽ. Land, 1980 ? The comparison to meteoric water, mixtures with more trace element and isotopic character will be reset than 5% seawater will have SrrCa molar ratios each time the dolomite re-equilibrates with a new similar to that of seawater. Dolomites precipitated water. Neither trace element nor isotopic studies will from a meteoricrseawater mixture with more than determine unequivocally the ultimate origin of an 20% seawater may have a typical marine Sr content ancient dolomite. Fluid inclusions tend to preserve Žwhatever that is, as we still have a poor understand- the chemistry of the resetting fluid and are most ing of effective distribution coefficients. . A com- useful in determining the character of the burial monly used value is f550 ppmŽ Tucker and Wright, environment. What approachesŽ. a through Ž. c can 1990. . Ancient dolomites with Sr values higher than indicate are factors like: the evolution of fluid chem- this are thought to be associated with hypersaline istry with time, fluid sources and pathways, and the waters. degree of fluid±rock interaction. The geometry of a In relatively closed burial systems, where the dolomite body is a far better indicator of its origin level of water±rock interaction is low, the mineral- than its geochemistry. ogy of the precursor carbonate is also important in An evolutionary history of dolomitization can be controlling the trace element signature. This further outlined only when all these methodologies are com- complicates interpretations based on trace elements. bined under the umbrella of detailed petrographic Marine aragonite, with its high Sr content of 8000 to study, which allows a tie of the various geochemical 10,000 ppm will form dolomites with Sr contents of values to appropriate crystal stratigraphyŽ e.g. Drivet 500±600 ppmŽ. e.g. Table 3 . Marine high- and low- and Mountjoy, 1997; Yoo et al., 2000. . Mg calcitesŽ. Srf1000±2000 ppm should form dolomites with Srf200±300 ppm, but a diagenetic 4.8.1. Trace elements low-Mg calcite precursor, with its much lower Sr The trace elements usually used to constrain content will form Sr-depleted dolomites. Generally, dolomite evolution are Sr, Na, Fe and Na. Their fine-grained early dolomites have higher Sr contents concentrations in ancient dolomites are determined than later diagenetic coarsely crystalline dolomites. by:Ž. 1 the concentrations of the element in the Sodium levels are relatively high in marine parent fluids,Ž. 2 the level of water±rock interaction dolomites. Modern marine dolomites in the Ž.Ž.how open was the system? and 3 what is the Caribbean, the Arabian Gulf, and Texas entrain effective distribution coefficient between the 1000±3000 ppm NaŽ. Land and Hoops, 1973 . Mix- dolomite and its parent water. ing-zoneŽ. ? dolomites have Na levels of several Ambiguity in interpreting trace element values in hundred ppm while Pacific atollŽ. seawater flushed dolomites revolve around a number of factors, the dolomites have 500±800 ppm of NaŽ Rodgers et al., J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 47

1982. . Most ancient dolomites have only a few hundred ppm Na. As with Sr values, the levels of Na in a dolomite can be used to distinguish dolomite types and perhaps give an indication of possible parent waters but all values are relative, not absolute, indicators. Fe and Mn tend to increase during successively later episodes of diagenetic recrystallizationŽ Tucker and Wright, 1990. . There are two reasons for this: Ž.1 iron and manganese are typically present only at very low levels in seawater but can be present in significant amounts in diagenetic pore fluids and,Ž. 2 the distribution coefficients for both elements are greater than unity so that Fe and Mn are preferentially taken into the dolomite lattice during diagenesis. Apart from cation concentration, the redox potential of the pore fluids also controls the levels of these trace elements in the dolomite lattice. The presence of both elements is favored by reducing conditions in the burial fluids. Thus, early nearsur- face dolomites tend to have low Fe and Mn levels owing to oxidizing conditions, while later burial dolomites may have much higher levels due to the reducing conditions that typify most deeper basinal watersŽ. also higher levels of Fe and Mn in solution . This variation in trace element content is also used to explain why later burial calcites and dolomites are nonluminescent phases under cathodoluminescence Ž.e.g. Dix, 1993 .

4.8.2. Stable isotopes As in trace element studies, the use of stable isotopes in defining the conditions of dolomite formation is only as good as the detailed petrography Fig. 26. Oxygen isotopes in dolomites.Ž. A Dolomite-water frac- that must accompany such analysis. Many isotopic 18 fields overlap and, due to continuing recrystalliza- tionation equations as a function of temperature plotted for a d O of 0½ SMOW from a number of published sources listed in Land tion, isotope proportions are overprinted and reset by Ž.1985 . The calcite-water equation is also shown Ž after Tucker and 18 later diagenetic episodes, especially in open Wright, 1990.Ž. . B d Odolomite versus temperature for various 18 3 fluidrrock systems. An important problem to over- d Owater values derived from the fractionation equation; 10 d s = 62y Ž. come when interpreting the isotopic composition of ln dolomite water 3.2 10 T 3.3 after Land, 1985 . the various types of ancient dolomite is to define the 18 18 relationship between d Owater and d O dolomite . Fig. 18 26A shows a plot of temperature against d Odolomite as determined by various workers, giving a variation dolomite in the laboratory leads to problems in the in temperature of around 158C for any given isotopic interpretation of isotope values in natural dolomites composition or a variation of 4½ for any given Ž.see Tucker and Wright, 1990 for discussion . Also temperature. Once again, an inability to synthesize shown in Fig. 26A is the much better constrained 48 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

3 calcite±water fractionation curveŽ 10 lnacalcite water tailed petrography. It also emphasizes the importance s2.78=1062T y2.89. . Fig. 26B shows the tem- of microsampling rather than whole-rock analysis. 18 perature against d Owater based on a fractionation; 10 3 lna s3.2=106T 2 y3.3 dolomite water 4.8.3. Fluid inclusions Ž.as calculated from Fig. 26A; Land, 1985 . Fluid inclusion studies of dolomite basically track The evolution of oxygen isotopes in a dolomite the movement and thermal history of the of aqueous undergoing burialralteration reflects the successive and petroleumrmineralising fluids that were present temperatures of precipitation and the isotopic com- as the crystal precipitated. Fluids are trapped when- positions of the dolomitizing fluidsŽ. Fig. 26B . Fluid ever the normal crystal growth is interrupted. This composition can be influenced both by temperature occurs when stresses or foreign objects dislocate the and the composition of the CaCO3 precursor. Be- plane of growth. Subsequent disruption, after the cause most pore fluids contain abundant oxygen, the crystal has stopped growing, can also cause cracking, isotopic compositions of precursor minerals are only and any surrounding fluids will then enter the cracks. preserved in relatively closed systems characterized In either case, the crystal grows around the fluid, by low water±rock interactions. In most cases in the sealing in the exact conditions of fluid entrapment, diagenetic realm dolomitization takes place in a hy- i.e. the fluid composition and pressure±temperature drologically open system and is accompanied by conditions at the time of entrapment. Sizes of fluid throughflushing pore waters. Hence, the isotopic inclusions range from less than 1 mm to several compositions of many subsurface dolomites reflect centimeters. When a dolomite cools after precipita- evolving pore fluid compositions and temperatures. tion, the fluid in the inclusion can shrink more than In contrast, the d13C value of a dolomite strongly the surrounding mineral, and so create a vapour reflects the value of the precursor carbonate, or, less bubble. Heating the inclusion to the temperature at often the influence of abundant hydrocarbons or which the bubble is reabsorbed and daughter crystals organic matter. Typical pore fluids are initially low dissolve gives an estimate of the minimum tempera- in carbon so that the d13C value of the dolomite ture at the moment of dolomite formation. Such reflects that of its precursor. In addition, there is studies should only be conducted on inclusion groups little isotopic fractionation of 13Cr12 C due to tem- in crystals that show no evidence of inclusion perature. Hence, the d13C signature gives valuable stretching or leakage. information on the source of carbon in the carbonate. A heating-cooling stage is used to bring the sam- Values between 0½ and 4½ are typically marine. ple through a range of temperatures. Liquid nitrogen When diagenesis of organic matter is involvedŽ as in is typically pumped through the microscope stage, organogenic dolomites. then extreme carbon signa- allowing a minimum temperature between y1808C tures are possible. Very negative or depleted carbon and -1908C. A variable voltage source connected to a values indicate that the carbon is derived from or- thermal coil is the heat source, and a thermocouple is ganic, rather than inorganic, sources via bacterially used to monitor temperature to 0.18C. As the sample mediated sulfate reduction or methane oxidation. is brought through temperature ranges, groups of Typical d13C values lie in the range y22½ to fluid inclusions in the dolomite are observed for y30½. Very positive carbon values, up to q15½, phase changes. The type of phase change is depen- also indicate an organic carbon source and result dent on the fluid system and conditions of entrap- from organic fermentation and methanogenesis. ment. Water and NaCl systems trapped at typical A cross plot of carbon and oxygen values is far pressures for dolomite precipitation will show melt- more effective in distinguishing different styles of ing of ice below 08C and a homogenization of the dolomite than a simple uni-axial determinationŽ Fig. liquid and vapour phases to the liquid phase typically 27. . Even so, the various fields show a strong degree above 708C. There are many other inclusion fluid of overlap. This underlines the importance of using systems that are possible in dolomite crystals, includ- stable isotope values, along with trace elements and ing hydrocarbons. A H22 O±CO system may show inclusion studies, as tools in combination with de- the formation and disappearance of clathrateŽ a cage J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 49 Ž s are forming compiled . from various sources listed in text . Fig. 27. Carbon±oxygen isotopic compositions of modern and ancient dolomites subdivided according to dominant pore water fluids where the dolomite 50 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Table 4 Characteristics of dolomitization modelsŽ. see text for discussion Syndepositional Shallow burialŽ early diagenetic, LacustrineŽ. Coorong-style Normal marine Arid Peritidal Ž. Sabkha-style Brine reflux dolomite

Laminar to massive lacus- Subtidal to supratidal dolomites Fine-grained Ž.-10 mm , strati- Medium-grained Ž 10±100 mm . trine basin centre precipitates form micritic dolomitesŽ mud- stratabound replacement dolomi- Fine-grained Ž.-10 mm , strati- stones . . Typically stratiform with te typically developed in re- Playa margin mudflats and form syngenetic dolomiteŽ mud- sharp upper erosioal contact and gional palaeo-topographic de- dolocretes stones. and cement horizons. more diffuse lower contact. pressions as units that may be Crystal size may increase during Dolomite forms bed )1±2 m tens to hundreds of metres thick Fine-grained Ž.- 10 mm , later burial. Beds are typically thick near the contact of the and extend over large areas. The stratiform micritic dolomites stratiform and thin with sharp supratidal and intertidal facies. It top of the dolomite lies beneath Ž.mudstones . Typically, strat- upper and lower contact. Ancient is part of a shoaling peritidal a widespread, mostly subaqueous iform with sharp upper and units often are part of a stacked sequence by algal laminites as evaporite horizon or a dissolu- lower contacts in beds usu- peritidal platform succession. predominant intertidal facies, tion breccia associated with a ally 1 m, but up to 5 m, Dolomite tends to precipitate in open-restricted marine subtidal former evaporite horizon. Inten- thick. Often show good a micritic host. Unless recrystal- facies and the supratidal facies sity of dolomitization decreases preservation of primary la- lized by later burial re-equilibra- characterized by nodulat and dis- downward or away from the for- custrine sedimentary features tion, the unit preserves fine grain placive evaporites capped by an mer evaporite layer of source of indicative of shoaling deposi- size and typical sedimentary erosion surface. Sabkha dolomi- the refluxing brine. Commonly tional settingŽ laminites over- structures that indicate a peritidal tes can be either marine or conti- reflux dolomites are associated lain by tepees, intraclast mode of deposition. Geochem- nental. If the sequence is buried with platform and basinwide breccias, rhizoconcretions, istry reflects marine pore fluids as part of a relatively closed evaporite bedsŽ evaporitic salt- mudcracks, domal stromato- if the sequence is buried and system, its geochemistry reflects erns and mudflats. . If not reset lites and dolocretes. Geo- remains as a relatively closed evaporitic pore fluidsŽ positive by later burial overprint, d18 O chemistry reflects evaporitic system. Otherwise, ongoing d18 O, elevated Sr and Na. . Oth- values tend to decrease away pore fluids if sequence is burial dissolutionrreprecipita- erwise, ongoing burial dissolu- from the evaporite dolomite con- buried as a relatively closed tion can reset stable isotopes and tionrreprecipitation can reset tact. Trace elements such as Na systemŽ positived18 O, ele- trace element values. stable isotopes and trace element and Sr also tend to decrease vated Sr and Na. . Otherwise, values. away from the source of the dissolution reprecipitation refluxing brine can reset stable isotopes and trace element values

Bacterial mediation can occur and may drive precipitation of initial crystallites that then age via Ostwald ripeningŽ bacterial mediation

of CO22 around H O. and separate melting of CO 2often difficult and until recently was typically a and H2 O. The post-entrapment deformation of inclu- two-phase process. Once the total amount of dis- sion walls, the necking-down of single inclusions solved solids is determined by observing the freez- into multiple inclusions, or leakage of contents prior ingrmelting points of the inclusions, the sample is to observation, can all lead to changes in the homog- then crushed and rinsed with water. This water is enization temperatures. recovered and analyzed by using a sensitive analyti- Because of the extremely small size of fluid cal technique to determine the ratios of the elements inclusions in most sedimentary dolomite, determin- contributed by the trapped fluid. These ratios are ing the ionic composition of the trapped fluids is used to calculate the composition of the fluid. Com- J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 51

active phreatic. Deep burialŽ. later diagenetic Mixing zone dolomite Hemipelagic organogenic Regional and focused hydrothermal Marine-meteoricŽ. humid Hypersaline-marine-meteoric Dolomite tend to be micritic to Dolomite tend to be sparry and Žoscillating arid-humid and be- microsparry with beds up to a nonplanar with saddle morpholo- Medium-grainedŽ. 10±100 neath strand-zone metre or two thick and hosted in gies indicating its formation at mm. stratabound replacement laminated organic-rich deep wa- temperatures)508C. Tends to dolomite typically developed Medium-grainedŽ. 10±100 mm ter muds Ž precursor was typi- show geometries controlled by in platform carbonate units stratabound replacement dolomi- cally a limestone bed within a geometry of conduits in deep as units that are tens to hun- te typically developed as replace- siliceousrchert host. . Character- subsurface; faults and or residual dreds of metres thick and ments in marine carbonates about istic association with deep water aquifer porosity. Forms in pas- extend over large areas. The the margins of saline giants dur- host and forms beds that make sive margin, collision margin and top of the dolomite lies being times of evaporative draw- up not more than 10% of the post-orogenic burial diagenetic neath a regional karstic un- down. It forms beneath strand- total sediment interval. Carbon settings. Bodies may show evi- conformity and formed be- zones as units that are tens to isotope values depend on sedi- dence of bathyphreatic karstifica- neath paleo-shorelines. Tend hundreds of metres thick and mentation rate. When sedimentation or may follow earlier formed to be best developed about extend over large areas. Lies tion rates are high dolomite tends permeability trends. Vugs may the edge of the sedimentary below and unconformity that to precipitate in the zone of contain base metals andror de- basin. Dolomite distribution marks the main basinwide draw- methanogenesis so that carbon graded hydrocarbon films. Oxy- is not tied to evaporites but down episode. Connate outflow values tend to be positiveŽ. 10½ . gen isotopes values tend to be to petrographic features that zones are typically associated When sedimentation rates are depleted, indicating precipitation indicate meteoric diagenesis: with pisoliticrtepee strandline low, dolomite forms in or just at elevated temperatures. Carbon moldic porosity, early pre- facies and an apron of mudflat below the zone of sulphate re- isotopes may indicate a possible compaction isopachous cal- anhydrite. Tend to be best devel- duction and carbon values tend association with hydrocarbons cite spar cement, vadose ce- oped about the edge of the sedi- to be negative. Isotope values sourced from thermochemical ments. Unless reset by later mentary basin, but may even tend to be reset by increasing sulphate reduction. Na and Sr burial overprint, the isotopic form in areas located well into temperatures and the presence of tend to be low while Fe and Mn values parallel the meteoric the evaporite facies of the draw- hydrocarbons may be elevated due to reducing calcite line but are slightly down basin where connate sea- conditions that typify the deep offset to more positive values water is seeping back to the subsurface due to dolomitization. Di- surface through a reflux curtain. luted trace element values for If not reset by later burial over- Sr and Na relative to brines print, d18 O values tend to be formed from seawater or hy- more positive than the meteoric persaline brines mixing zone dolomites. Trace elements such as Na and Sr also tend to be elevated. indicated by characteristic textures: spheroidal dolomite, dumb-bell and spindle outlines in cores of dolomite crystals.

positions in dolomites range from aqueous solutions detailed petrographic analysis, and so allow the de- with salt content similar to meteoric water to hyper- velopment of an inclusion history in a single coarse- saline brines. The problem with this method, like the grained crystal. older techniques of whole-rock analyses for stable Inclusion studies in the last decade, in combina- isotopes, is that it tends to give results that average tion with isotope and trace element studies and de- multiple generations of inclusion fluids. In the last tailed petrographic control on sample position, have decade, micro-sampling techniques have become in- illustrated the complex, multistage history of most creasingly important. These techniques allow the ancient dolomitesŽ e.g. Lopez-Gomez et al., 1993; direct sampling of inclusions, typically based on Mriheel and Anketell, 1995; Nicolaides, 1995; 52 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Zenger, 1996; Mountjoy et al., 2000; Yoo et al., other early burial dolomites. The most important 2000. . feature is their strong tie to evaporites or evaporite indicators and the fact that their geometries are 4.8.4. Geometry and petrographic character generally stratiform and follow the distribution of the As HardieŽ. 1987 , Warren Ž. 1989 , Wilson et al. evaporites whose depositional system created them. Ž.1990 , Braithwaite Ž. 1991 and others have noted, Less common are dolomite beds tied to the em- the most important practical guides in determining placement of salt allochthons andror the dissolution the origin of a dolomite are the geometry of the of deeply burial salt units. Dolomites formed in this dolomite body and its lithologicalrpetrographic as- way are similar to burial or hydrothermal dolomites sociationsŽ. Fig. 28; Table 4 . The position of a and may form coarse sparry ferroan haloes about salt dolomite within a sedimentary basin is a direct indi- and fault welds that acted as escape conduits for hot cation of its position within the basin hydrology. metal-rich brinesŽ. Warren, 1999 . Unlike limestones or siliciclastics, its position does Mixing zone dolomites occur near the edges of not necessarily reflect biology or wavercurrent en- the former lenses of meteoric water. They are de- ergy. However, one must always keep in mind that pleted in Sr and Na and generally more depleted once formed, a dolomite body can evolve and alter with respect to oxygen and carbon isotopes than and so may modify its geometry throughout its burial either marine or evaporitic dolomites. There may history. With this in mind, let us look at some of the also be a positive correlation of d18 O with d13C more important geometric and petrographic charac- resulting from the mixing of waters with differing teristics of the various styles of dolomite formation. compositions. The formation of mixing zones associ- Relevant literature examples were discussed in detail ated with marine seepage or the reopening of basin- in appropriate sections earlier in this review. wide evaporite basin to marine waters is more com- Sabkha- and Coorong-style dolomites are charac- plicated than that of the flow paths associated with terized by their fine-grained crystal sizes Ž-10 to 20 marine platforms that still dominate the literature mm. , their stratiform occurrence, and the characteris- Žsee Warren, 1999 for a discussion of the hydrology tic sedimentary features associated with sabkhas or of drawdown basins. . Coorong-style settingsŽ. Table 4 . Sulfate-reduction, When a dolomite formed by the mixing of seawa- methanogenic and deepwater dolomites are also fine ter and meteoric water, the most reliable tool for grainedŽ. typically mimetic in ancient counterparts predicting occurrence is to map either intrabasin and show a characteristic sedimentary association of paleohighs or shorelinerstrandline trends. Hence, lagoonal or deepwater sediments, perhaps with asso- widespread mixing-zone dolomites that formed in ciated chert depositionrdiagenesis. The finer-grained unconfined aquifers commonly occur below regional crystals of these three settings reflect relatively rapid unconformities recognizable as sequence boundaries nucleation as dolomite precipitates as multiple bacte- in seismic sections. rially mediated crystallites or replaces aragonitic mi- Confined aquifer systems are not so easily charac- crites within meters of the sediment±water interface. terized. Meteoric groundwater circulation is most Reflux dolomites can be tied to overlying or active in humid environments. A suitable hydrology adjacent evaporites or their dissolution breccias. Early for the formation of a seawaterrmeteoric mixing dolomites located proximal to brine sources tend to zone dolomite in an open marine basin is best devel- be fine grained and mimetic, while those located oped during regional regression. This creates pro- distally tend to be replacive and more coarsely crys- grading hydrological fronts that displace near-surface tallineŽ see discussion in Dolomite porosity and per- marine waters. Dolomites formed by mixing of sea- meability section. . The proportion of dolomite in a water with hypersaline water in central parts of reflux system generally increases towards the source drawdown basins have trends similar to reflux of the Mg-rich evaporite solutionsŽ. Fig. 15 . Oxygen dolomites and it can be very difficult to separate the isotopes are progressively enriched in the same di- two. Toward the edges of the drawdown basins rection together with levels of Sr and Na. Reflux mixing zone dolomites follow the subaerial uncon- dolomites tend more rapidly to stoichiometry than formities that define the drawdown stages of the J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 53 Ž. Fig. 28. Dolomitization models see text for discussion . Note that the position and geometry of the dolomite is a result of its hydrological setting. 54 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 basin's evolution. Basinwide evaporite systems do sucrosic textures with high intercrystal porosityŽ Fig. not fit the current marine paradigms of sequence 29.. ; Murray, 1960; Weyl, 1960 . This model as- stratigraphy, while platform evaporites doŽ see War- sumes dolomitization occurs within a relatively 2y ren, 1999; Chap. 3. . Rapid sealevel or paleo- closed system, with a local source for Mg and CO3 waterlevel changes during and soon after dolomite ions. In such a system, the replacement of limestone precipitation can greatly complicate either trend. Ž.calcite or aragonite by dolomite reduces mineral Evaporitic reflux and strand-zone dolomites away volumeŽ. and so increases porosity as dolomite has a from the brine source are coarser than penecontem- smaller molar volume than either calcite or arago- poraneous dolomites; crystals are often in excess of nite. The observation that increasing proportions of 20 mm. Coarser crystals probably reflect more stable dolomite lead to an improvement in porosity was the pore fluid chemistries. Ancient coarse-grained basis for this model. A second factor recognized in dolomites are often termed AsucrosicB and are usu- these early models was the ability of the growing ally composed of euhedral rhombs with crystals be- rhombs to create a rigid cemented framework. As tween 20 and 120 mm. Marinermeteoric mixing calcite between the growing rhombs dissolves and zone dolomites show negative oxygen isotope signa- collapses it leaves behind a rock framework that is tures, while evaporite-associated mixing zone and made up of a porous grid of dolomite rhombs in reflux dolomites show more positive values. crystal-to-crystal contactŽ the porosity crossover at Burial dolomites show negative oxygen isotope 50% dolomite in Fig. 29. signatures, reflecting their elevated temperatures of Later work showed that if there is an external formation. They form at temperatures that exceed the source of carbonate and magnesium ions then CRT and crystals have nonplanar faces. They are dolomitization of limestone typically results in a often associated with saddle dolomites. They can be volume increase and a loss of total porosityŽ Lucia either regional dolomitizing circulatory systems and Major, 1994. . Dolomitization does not necessar- where whole aquifers are effectedŽ Hitzman et al., ily improve porosity or permeability. Assumptions of 1998. , or more focused upwelling systems where permeability, using carbonate classifications based faults and fractures tend to limit the dolomitization on depositional textures, can result in oversimplified processŽ Braithwaite and Rizzi, 1997; Warren and notions of porosityrpermeability distribution within Kempton, 1997. . The two dolomitization styles, re- dolomitic reservoirs. Early formed dolomite porosity gional burial and more localized hydrothermal up- evolves with burial so that later diagenetic alteration welling, are typically part of the same deeply circu- Ž.recrystallization and cementation in the deeper parts lating aquifer system. of a basin can dramatically change the character of the porosity and permeability of dolomite formed earlier in the diagenetic historyŽ e.g. Gregg et al., 1993. . 5. Dolomite porosity and permeability WardlawŽ. 1976 divided the total pore system, or total porosity, of a dolomite into two different re- Interpreting dolomites as fluid conduits, or fluid gions: the relatively large pores, representing the reservoirs, requires fundamental understanding of the larger volume of porosity; and the pore throats, complex interrelationships between petrophysical which comprise a relatively smaller volume. Perme- propertiesŽ porosity, permeability, and capillary pres- ability in many dolomites is not directly related to sure. and crystal textures in various styles of dolomite total porosity or crystal size, but is dependent on the Že.g. Wardlaw and Taylor, 1976; Wardlaw and Cas- connectivity of the pores via the pore throats. Con- san, 1978; Wardlaw et al., 1988; Ghosh and Fried- nectivity concepts must include both the number of man, 1989; Kopaska-Merkel and Friedman, 1989; pore throats and their size. That is, two dolomites Choquette et al., 1992; Woody et al., 1996; Durocher can have the same porosity but widely different and Alaasm, 1997. . permeabilities due to different pore throat properties. Early models of porosity development in dolomite Those with intercrystalline porosity developed be- used the notions of volume reduction to explain tween large crystals generally have higher permeabil- J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 55

should be divided either into interparticleŽ inter- grainr intercrystalline.Ž. or vuggy pores Table 5 . In nonvuggy carbonatesŽ. including some dolomites , the permeability and capillary properties can be described in terms of particlercrystal size, sorting, and interparticle porosity. They respond to fluid flow in the same way as siliciclastic sands. With vuggy dolomites, the degree of vug isolation is of paramount importance in controlling the rate of fluid flow through the rock. Lucia'sŽ. 1995 classification catalogues porosity and lithology as it currently exists in the reservoir rock and is not based on depositional texture or fabric. For this reason, when Lucia applies Dunham's Ž.1962 classification to a carbonate reservoir, he talks about grain-dominated or mud-dominated lithologies in terms of current fabric or crystal sizes. These may or may not be a direct indication of the grainrmud fabrics of the primary sediment. Lucia's Ž.1995 porosity classification is a refinement of the widely used genetic porosity classification of Cho- quette and PrayŽ. 1970 . The advantage of Lucia's

Fig. 29. Relationship of porosity to percentage of dolomite in the Midale Beds, Charles Formation, Midale Field, Saskatchewan Ž.Murray, 1960 . ities than those with porosity developed between rhombs of microspar or micrite. Using studies of resin pore casts, WardlawŽ. 1976 described carbonate pore systems as polyhedral or tetrahedral pores connected by sheet-like pore throats formed at compromise boundariesŽ. Fig. 30 . As crystal size increases, pore geometries changeŽ Wardlaw, 1976. and ultimately pore throat networks are dis- rupted, leading to a reduction in permeability. Lucia Ž.1983 calls this pore occlusion overdolomitization. As crystals grow, pores may be reduced from polyhedral to tetrahedral forms and ultimately to interboundary sheet poresŽ. Fig. 30; Wardlaw, 1976 . Using the observations of Wardlaw, LuciaŽ. 1983 noted that the geometry of interparticle pore space in any carbonate is related to the size and shape of the Fig. 30. Shape and evolution of dolomite pores. Note how ongo- particles, and to the amount and distribution of ce- ing growth finally occludes the porosity as the pores undergo a ment, postdepositional compaction, pore-filling min- transition from polyhedral to tetrahedral to interboundary pores. Rocks dominated by this last stage where growth and compaction eralization, and crystal leaching. In a later paper, has occluded almost all effective porosity are sometimes called LuciaŽ. 1995 concluded that all carbonate pore space overdolomitizedŽ. after Wardlaw, 1976 . 56 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Table 5 equivalents, possibly reflecting overdolomitization. Terminology of pore types comparing LuciaŽ. 1983 with Cho- If dolomitization had occurred in a closed system, quette and PrayŽ. 1970 then the dolomites should have porosities f35%. In Porosity type Abbreviations contrast to this relationship, LuciaŽ. 1999 found that Lucia Choquette and Ž.1983 Pray Ž. 1970 Interparticle IP BP Intergrain IG ± Intercrystal IX BC

Vug VUG VUG Separate Vug SV ± Ž.vug to matrix to vug Moldic MO MO Intraparticle WP WP Intragrain WG ± Intracrystal WX ± Intrafossil WF ± Intragrain microporosity mG± Shelter SH SH

Touching Vug TV ± Ž.vug to vug Fracture FR FR Solution-enlarged SF CH fractureŽ. channel Cavernous CV CV Breccia BR BR Fenestral FE FE approach is that it does not try to interpret the depositional texture and grain-size. It looks only at the rocks current character and so allows a direct comparison of porosity with the likely permeability and petrophysical properties in a lithology. This is more useful when constructing a model of fluid flow. He suggests that when describing particle size and sorting in dolostones, dolomite crystal size terms should be added to a modified Dunham terminology Žfine crystal size: -20 mm; medium crystal size: 20±100 mm; and coarse crystal size: )100 mm.. This gives a partial measure of the potential size of intercrystalline pore throats. LuciaŽ. 1999 argues that Holocene carbonates show little change in pore volume with increasing proportions of dolomiteŽ. Fig. 31A . He also found Fig. 31. Dolomite porosity.Ž. A Relationships between composi- that Plio-Pleistocene dolomites are typically less tion, porosity and timeŽ. after Lucia, 1999 . Dolomite is less porous than their limestone counterparts. For exam- porous than limestone counterparts in younger carbonates and Ž. ple, dolomitized Plio-Pleistocene limestones from more porous in older carbonates. B Relationship of porosity to depths in limestones and dolomites in South FloridaŽ after Halley Bonaire have an average porosity of 25%, while and Schmoker, 1983. . Dolomite crossover, below which dolomite average porosity is only 11% for their dolomite becomes more porous than limestone, occurs at 1800 mŽ. 6000 ft . J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 57

Paleozoic dolomites are usually more porous than They showed that as porosity increases in a dolomite their associated limestones. Halley and Schmoker reservoir, permeability increases at a greater rate in Ž.1983 demonstrated a possible reason for this: they planar-e dolomite than in planar-s dolomiteŽ. Fig. 32 . showed that dolomite porosity does not decrease as Also, the porosity to permeability relationship in rapidly as that of limestones during burial. At shal- both categories of planar dolomite is more obvious lower depths, the limestones have higher porosities, than that in nonplanar dolomite. The relative loss of while at greater depths their porosities are less than intercrystalline porosity volume and pore throat size, that of the dolomitesŽ. Fig. 31B . In South Florida, that is inherent in a transition from polyhedral to the crossover depth is around 1800 mŽ Schmoker et interboundary pores, encompasses the transition from al., 1985; Amthor et al., 1994.Ž. . Lucia 1999 also planar-e to planar-s texturesŽ. Fig. 30 . Woody et al. concluded that, for samples at the same depth, went on to conclude that, as planar or nonplanar dolomitized muds are much more likely to generate textures are variously imparted to dolomite as the and retain porosity than their undolomitized grain- result of different burial histories, the diagenetic stone equivalents. history of a dolomite exerts a major effect on its Woody et al.Ž. 1996 used a somewhat different petrophysical properties. approach, based on Sibley and Gregg'sŽ. 1987 classi- To help explain this, and other similar poro- fication but once again concentrating on the rocks sityrpermeability observations, LuciaŽ. 1999 sepa- current texture and not on their depositional texture. rates ancient dolomites into two end members:Ž. a

Fig. 32. Plots of porosity versus permeability for nonplanar, planar-s and planar-e dolomitesŽ replotted from data in Table 1 in Woody et al., 1996. . 58 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 dolomites where the precursor was mud-dominated crystals which may be several hundred microns andŽ. b dolomites where the precursor was a grain- across. When a coarse dolomite spar replaces mud, stone. He goes on to argue that there is an increase in any porosity residing between the euhedral rhombs pore size during dolomitization of mud-dominated will have better flow properties than its finer-grained limestone precursorsŽ. Fig. 31A . No such change mud counterpart. seems to occur during the dolomitization of grain- The concept of vuggy pore space must be added dominated limestone precursors. He argues grain- to that of intercrystalline pore throat size to complete stone precursors are usually composed of grains with the description of pore space in a dolomiteŽ Table 5; diameters that are much larger than the dolomite Lucia, 1995, 1999. . Vuggy pores are divided into crystal size. Hence, the growth of dolomite rhombs two groups depending on how they are intercon- does not have a significant effect on pore-size char- nected:Ž. 1 separate vugs, Ž. 2 touching vugs. Sepa- acter. Grain-dominated packstones also have large rate vugs are defined as interconnected only through grains that are in grain-to-grain contact and so during interparticleŽ. intercrystalline pore space, while dolomitization behave much in the same way as a touching vugs themselves form an interconnected grainstone. In contrast, replacement of any interparti- pore system often with much higher permeabilities. cle mud by 100-mm-diameter dolomite crystals tends We are now ready to consider how the various styles to enhance the intergrain flow character in such a of porosity and models of dolomitization can control rock. When mud-dominated limestones are dolomi- the quality of dolomitic oil and gas reservoirs. tized, there is often an enhancement of the flow capacity of the total rock. Newly formed dolomite 6. Dolomite as a hydrocarbon reservoir crystals in the dissolving mud matrix show a range in crystal sizes from particles with diameters equiva- Dolomite reservoirs account for approximately lent to the muds they are replacing, up to sparry 80% of recoverable oil and gas in carbonate rocks of

Fig. 33. Major dolomite reservoirs subdivided by age and geographyŽ. after Sun, 1995 . J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 59 Kerans, 1988 Ž. Ž . fracturingburial corrosion Horn, 1990 fracturing Bentley, 1984 modification % md OrdovicianOrdovician burial corrosion Ž.Ž. province Formation Geologic age Postdolomitization Porosity Perm. Reference r Table 6 Peritidal syndepositional-mimetic reservoirsBasin after Sun, 1995 Bohai Gulf Wumishan Late Precambrian Karstification and 5 0.1±10.0 Zhai and Zha, 1982; TarimTarim Qigebulake Qiulitage Late Precambrian Late Cambrian±Early Karstification and Karstification and 2.0±5.0 3.0±6.0 NA NA Yie and Liu, 1991 Yie, 1991; Hu, 1992 AnadarkoPermian Arbuckle Ellenburger Late Cambrian±Early Early Ordovician Karstification Karstification NA 3.5 NA 10.0±50.0 Gao and Land, 1991 Loucks and Anderson, 1985; CanningWillistonPoPoN Arabian PlatformN Nita Arabian Platform Interlake Kurrachine Butmah Late Triassic Dolomia Middle Ordovician Silurian Zandobbio Early Jurassic Late Karstification Triassic Early and Jurassic Fracturing 2.0±18.0 Fracturing Karstification Fracturing Fracturing 35±3310 3.5±6.3 15 NA Karajas and Kernick, 1984; 40 3 3 5 NA unpublished data 50 50 unpublished data Roehl, 1985 Mattavelli Mattavelli and and Margarucci, Margarucci, 1990 1990 60 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 Ž. Ž. Craig, 1988 Teisserene and Villemin, 1989 1985; Fischer et al., 1990; Longman et al., 1992 Ž. Ž . e.g., Yates field Chuber and Pusey, 1985; modification and fracturing Ž. lagoonal settings on carbonate platforms±reflux dolomites after Sun, 1995 r group Geologic age Postdolomitization Porosity % Perm. md Reference r Ž. province Formation r PermianArabian PlatformN Gulf of Khuff MexicoSE Mexico SmackoverE Arabia GrayburgE ArabiaSE Mexico NACentral Mexico Late Jurassic Late PermianN Gulf Arab-C Late of Permian MexicoN NA Iraq Arab-D NA EdwardsCongo Late Jurassic NA LateCuanza Jurassic Fracturing Angola NA LateGabes-Tripolitana Qamchuqa Middle Jurassic CretaceousSirte Middle Zebbag Binga Cretaceous Early±Middle Sendji CretaceousSirteSirte 8.0±25.0 Fracturing Middle NA CretaceousArabian Fracturing Platform NA Karstification 16±25Mesopotamian 2±120 Umm NA er Middle 9 RudhumaMesopotamian Cretaceous Middle Lidam Cretaceous Middle 3.0±9.0 Cretaceous Eocene Asmari Sabil Karstification NA 3.0±6.0 Euphrates-Jeribe 63±245 Alsharhan, Gir 1993; Bos, 1989 NA NA 14±21 Early Miocene Fracturing 10.0±30.0 Feazel, NA 12.0±25.0 2.5 1985; NA Petta and Rapp, Middle 15±26 1990 NA Cretaceous Oligocene±Early Miocene 50±250 12±100 Paleocene 2±200 Fracturing Peterson, 1983; Santiago 15 and Baro, NA Peterson, Eocene Harris 1992 Unpublished 1983; and data 50±500 Santiago Walker, Late source Wilson, and 1990 corrosion NA 1985 Wilson, Baro, 1975 1992 10.0±25.0 Cook, 1979; Fisher and 20±30 Rodda, 1969 9.0±14.0 Powers, 1962; 100±1000 Wilson, 20±30 1985; QGPC, 1991 50 Unpublished NA data 10 source 700±1000 20±30 10±100 12 Baudouy and NA LeGorjus, 1991; Brognon and Verrier, 1966 Sun and Hull NA Esteban, and 1994 Warman, 1979; McQuillan, 1985 10.0±15.0 NA Danielli, NA 1988 15 Unpublished data source Unpublished data source NA Unpublished data source FossilWillistonParadoxAnadarkoSichuan CharlesParadox Mission CanyonPermian Leadville Warsaw Early Carboniferous NA Hermosa Early Carboniferous San NA Early Andres Carboniferous Early Carboniferous NA Late Karstification Carboniferous Late Permian Early Carboniferous Karstification 6.0±8.0 5.5 NA 15 Karstification 14 0.7±1.5 local karstification 6.0±12.0 22 10.0±12.0 Sieverding 35 and Royse 1990 2.2±11 1.1±18.5 10 1.0±10.0 Miller, Yang, 1985 1986 Longman Cowan and and Ebanks, Schnidtman, Harris, 1991; 1985 1986; Johnson and Budd, 1994 15 Peterson, 1992 N MichiganPermianWillistonWilliston NiagaraSE AlbertaWilliston Thirty-one Duperow Birdbear Warbamun Silurian Mission Early-Middle Devonian Canyon Late Devonian Early Late NA Carboniferous Late Devonian Devonian NA NA NA NA NA 6.8±12.6 6.2±11.7 14±16 NA NA Saller et al., 1991 6 NA 25±30 NA NA Lindsay NA and Kendall, 1985; NA DeMis, 1992 Sears Ehrets and and Lucia, Kissling, 1980 1985 Ehrets and Kissling, Metherell 1985 and Workman, 1969 Table 7 Williston Red River Late Ordovician NA 11.0±17.8 5.0±11.0 Clement, 1985; Ruzla and Friedman, Subtidal dolomite reservoirs associated with evaporiticBasin tidal flat J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 61

Table 8 Dolomite reservoirs associated with basinwide evaporitesŽ. after Sun, 1995 Basinrprovince Formation Geologic age Postdolomitization PorosityŽ. % Perm. Ž md . Reference modification South Oman Huqf Late Precambrian± NA 6.0±12.0 1.0±100.0 Mattes and Conway, 1990 Early Cambrian SE Michigan Niagara Silurian NA 10 7.9 Gill, 1985; Jodry, 1969 Alberta Keg River Middle Devonian Late corrosion 10.1 184 Schmidt et al., 1985 Zechstein Zechstein Late Permian Karstification and 5.0±20.0 10±100 Clark, 1980 late corrosion Gulf of Suez Rudeis Middle Miocene Late corrosion 17±22 50 Sun and Esteban, 1994 Gulf of Suez Belayim Middle Miocene Late corrosion 15±16 32 Sun and Esteban, 1994

North AmericaŽ. Zenger et al., 1980 . Significant ume of fluids passing through carbonate sediments, proportions of the hydrocarbons in the former Soviet dolomitization can destroy, maintain, or enhance Union, northwestern and southern Europe, north and porosity. During the course of exploration or produc- west Africa, the Middle East, and the Far FastŽ Fig. tion, geologists need to predict not only where 33; Sun, 1995. are also found in dolomite reservoirs. dolomite forms within a stratigraphic or structural Once formed, dolomites preserve porosity and per- framework, but more importantly where it contains meability much better during burial than limestones and maintains the porosity of reservoir qualityŽ Sun, Ž.Fig. 31B , which is in part a reflection of their 1995; Lucia 1999. . greater ability to resist pressure dissolutionŽ e.g. In a worldwide study of dolomite reservoirs, Sun Amthor et al., 1994. . Ž.1995 found that the majority of hydrocarbon-pro- Nevertheless, although they generally provide bet- ducing reservoirs occur in four situations:Ž. 1 periti- ter reservoirs than limestones at depth, not all dal-dominated carbonate,Ž. 2 subtidal carbonate asso- dolomites are good reservoirs. Depending upon their ciated with evaporitic tidal flatrlagoon,Ž. 3 subtidal original depositional fabric and nature and the vol- carbonate associated with basinwide evaporites, and

Table 9 Dolomite reservoirs in nonevaporitic carbonate settingsŽ. after Sun, 1995 Basinrprovince Formation Geologic age Postdolo- PorosityŽ. % Perm. Ž md . Reference mitization modification Michigan Trenton and Middle Ordovician NA 4 10 Hurley and Budros, 1990 Black River Anadarko Clarita Silurian NA 8 93 Morgan, 1985 Anadarko Henryhouse Silurian NA 7 0.1±30 Morgan, 1985 Alberta Sulphur Point Middle Devonian NA NA NA Qing and Mountjoy, 1994a Alberta Slave Point Late Devonian NA NA NA Phipps, 1989 Alberta Swan Hills Late Devonian NA NA NA Kaufman et al., 1991 Alberta Leduc Late Devonian NA 7 25 Mountjoy and Amthor, 1994 Alberta Nisku Late Devonian NA 9 340 Machel and Anderson, 1989; Watts et al., 1994 N Alberta Wabamun Late Devonian NA NA NA Mountjoy and Halim-Dihardja, 1991 Canning Nullara Late Devonian NA NA NA McManus and Wallace, 1992; Wallace, 1990 Illinois St. Genevieve Early Carboniferous NA 27 12 Choquette and Steinen, 1980, 1985 Ragusa Siracusa Early Jurassic NA 12.0±16.0 NA Schramm and Livraga, 1986 Gulf of Valencia NA Late Jurassic NA 7.0±12.0 1.0±2000 Watson, 1982; Esteban, 1988 Central Luconia NA Middle±Late Miocene NA 15±30 10±500 Epting, 1980 62 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Ž.4 nonevaporitic carbonate sequences associated mudflat carbonates and evaporites. Reservoir quality with topographic highrunconformity: platform- in the subtidal dolomite is related to depositional and margin buildup or faultrfractureŽ. Tables 6±9 . diagenetic fabric. The best reservoirs are in high-en- Peritidal dolomite reservoirs are often evaporitic ergy, grain-dominated sedimentsŽ. ooliticrpisolitic and typically have low matrix porosity and perme- or low-energy lagoonal dolomudstonerwackestone ability adjacent to any platform evaporite due to with a coarsely crystalline overprint. Finely crys- overdolomitization of the relatively fine-grained sed- talline, mud-dominated mimetic dolomites nearer the iment host. The porosity occlusion associated with brine source are commonly poor reservoirs. In most overdolomitization reflects the extended times for cases, primary intergranular porosity in grain- tidal-flat development under relatively stable dominated rocks is preserved due to the relatively AgreenhouseB platform conditions. This allowed re- arid climate and limited recharge of fresh water, peated pore-water flooding of magnesium and cal- minimizing meteoric leaching and calcite cementa- cium bearing marine fluids during storm-driven re- tion. The extensive impermeable mudflat dolomites plenishment or seepage into the saltern and evapor- and anhydrites, which cap upward-shoaling cycles itic mudflats. Such fine-grained dolomites, interca- effectively, seal off the underlying carbonates from lated with platform evaporites, form few economic any later influx of freshwater or sulfate-bearing flu- reservoirs unless they are modified by karst or frac- ids. turedŽ. Table 6 . Porous dolomites are generated via brine reflux as Peritidal dolomites are often associated down dip dense saline brines, generated by evaporative con- with dolomite reservoirs formed by brine reflux from centration in salterns and mudflats, percolate down- the mudflats into marine subtidal platform carbon- ward and basinward through the sedimentsŽ Adams atesŽ. Fig. 34 . Porosity in such subtidal dolomites and Rhodes, 1960; Elliott and Warren, 1989; Saller typically occurs in multiple zones within stacked and Henderson, 1998. . As they first pass out from shallowing-upward cyclesŽ Table 7; Sun, 1995; War- their source, these refluxing brines are highly super- ren, 1999. . The base of each zone is formed by saturated with respect to dolomite and so precipitate porous, partially to completely dolomitized subtidal large quantities in the proximal parts of the refluxing carbonate that terminates upward in relatively tight system. This pervasive precipitation lowers the

Fig. 34. Model of mudflat and subtidal reflux dolomite reservoirs in a platform evaporite setting exemplified by the various reservoirs of the Central Basin Platform in the Permian Basin of west TexasŽ. in part after Saller and Henderson, 1998 . Primaryrmimetic dolomite that formed in and around brine poolrseaway is overdolomitized and so does not retain economic levels of porosity unless enhanced by later exposure or fracturing. Refluxrsecondary dolomite formed as the dense brines sank through platform sediment pile and replaced permeable carbonates. This form of secondary dolomite tends to retain its permeability in more distal areas. J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 63 dolomite saturation in pore waters flushing the more ever, in such sediments the hypersaline flushing distal portions of the reflux system. Lower saturation commonly leached calcium ions, which then reacted results in fewer nuclei and larger crystals. Hence, with sulfate-rich brines to form anhydrite cements. In porosity and permeability are greater in the more some regions, both depositional and diagenetic hy- distal parts of the systemŽ. Fig. 34 . As reflux brines persaline leaching porosity was almost occluded by continued to circulate, they precipitate more this cement. In these cases, karst formation, fractur- dolomite, further decreasing porosity, especially in ing, or later corrosion were required to restore proximal platform-interior sediments. Burial and re- porosities and permeabilities beneath the evaporite flux salts also form pervasive cements in the more seal. Unlike the dolomites associated with platform proximal areasŽ see Warren, 1999; Chap. 4 for a evaporites, this group of reservoirs cuts across both discussion of burial stage evaporites. . icehouse and greenhouse periods because basinwide LuciaŽ. 1999; p. 147 pointed out that this down- evaporites require tectonic settings that are indepen- ward cyclic transition from a relatively impervious dent of sealevel characterŽ. Warren, 1999 . peritidal dolomite capŽ with intercalated evaporite Most dolomite reservoirs in nonevaporitic carbon- seals. into dolomitized subtidal carbonate reservoirs ates are associated with topographic highs or uncon- corresponds to an downward increase in dolomite formities, platform-margin buildups, or fault and crystal size. He relates this to a downward decrease fracture systemsŽ. Table 9; Sun, 1995 . Formation of in dolomite saturation as the refluxing brines sink these dolomites is related either to reflux of slightly deeper and further from the evaporite source. Such evaporated seawater, long-term flushing by connate mudflat and reflux dolomites are not forming today water, mixing of seawater with fresher water or under climatic conditions characterized by major resurging burial-derived dolomitizing fluids. In the continental glaciation and high frequency high am- formerŽ e.g., Early Carboniferous Saint Genevieve plitude sealevel fluctuationŽ. icehouse climatic mode . dolomite in the Illinois basin, USA, and middle-up- Formation of dolomite reservoirs associated with per Miocene dolomite in the central Luconian plat- platform evaporites is more likely during times char- form, offshore Borneo. , pore space is mainly of acterized by lower frequency, lower amplitude micro-intercrystalline type with subordinate moldic sealevel changesŽ. greenhouse mode . pores. Permeability resulted directly from early Dolomite reservoirs associated with basinwide dolomitization of lime-muds that were preserved in evaporites are often preserved beneath the seal of the subsequent burial diagenesis. Porosity occlusion, due overlying salts. Such thick massive evaporites trap to overdolomitization, is generally not a problem for and preserve some of the oldest oil fields in the reservoir development because of the limited dura- worldŽ.Ž. Table 8 . Sun 1995 found that porosity tion of dolomitization. distribution in subtidal dolomite reservoirs associated Owing to the relatively low MgrCa ratio and with basinal evaporites typically shows no direct limited volumes of burial-derived dolomitizing flu- relationship to stratigraphic position or depositional ids, burial processes typically led to the formation of trends. This reflects a strong diagenetic overprint of burial dolomite with a relatively coarse, sucrosic primary porosity and patchy cementation of early fabric. Usually, this evolves into nonplanar dolomite generated secondary porosity by anhydriterhalite. It typified by poor to nonexistent porosity and perme- also indicates the problems of porosity prediction in abilityŽ. Gregg et al., 1993 . Sometimes, the burial a hydrological system characterized by large auto- evolution process was arrested before porosity was cyclic fluctuations in hydrologic base level Ž.f1km completely lost, often by the emplacement of hydro- over thousands of years and the lack of differentia- carbons that shut down further cementation. In this tion of strand-zone versus basin-center reflux case, burial dolomitization improves reservoir prop- dolomites. Both dolomite styles typify basinwide erties compared with adjacent limestones that would evaporite settingsŽ. Warren, 1999 . otherwise make poor reservoirs. Solution cavities, During early dolomitization and limestone leach- vugs, and intercrystalline porosity are commonly ing, significant porosity was developed in sediments developed in local often structurally focused bathy- containing metastable skeletal components. How- phreatic sites, where there was an excess of dissolu- 64 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

tion over precipitation during replacement. Dissolu- isotopically light H2 S as they metabolize organic tion processes associated with burial dolomitization matterŽ. or hydrocarbons while utilizing sulfate as can be particularly intense in zones where hot, the oxidizing agent: basin-derived, organic-entraining fluids moved updip 2qq 2qq yq q q along faults, fractures or unconformities. Ca Mg 4SO444CH 4H ™ q q q 4H2322 S CaMgŽ. CO 2 6H O 2CO

7. Dolomite: the ore host Sulfate-reducing bacteria commonly thrive within or near hydrocarbon±water contacts in situations Dolomite formation has often played an integral varying from near surface methane seeps to depths part in the mineralization of ore deposits, many of a few kilometersŽ. Warren, 1999 . mineralized dolomites are more than passive hosts Bacterial sulfate reduction can occur at tempera- Ž.Table 10 . Certainly, some have acted as passive tures up to 1108C in some pressured hydrothermal porous vessels allowing passage and mixing of met- environments, but at higher temperatures TSR comes alliferous solutions, they did little more than hold the to dominate. For example, in MVT deposits, a resulting mineralization. More often, the formation metal-sulfide phase typically coprecipitates with hy- of late stage burial and hydrothermal dolomites was drothermal or saddle dolomite at temperatures of intimately related to basinal solutions that soon after 608C±2008CŽ. Hill, 1995 : were actively involved in the ore precipitation pro- HSqCO qMeClqqMg 2qq2CaCO qHO cess. Because late-stage dolomitization extends be- 22 32 ™ q 2qq yond the zone of sulfide mineralization, these MeS Ca CaMgŽ. CO3 2 coarse-grained dolomites can sometimes be used as y y q qHCO qCl q3H pointers to structurally focused sites of potential 3 mineralizationŽ. Warren and Kempton, 1997 . Al- Where Fe is abundant in the throughflushing wa- though other, often diagenetically earlier, textures ters, pyrite forms by reacting with the H2 S; where are occasionally present, most ore hosting dolomites Pb and Zn are available, galena and sphalerite form. are dominated by nonplanarŽ. xenotopic mosaics with The acidic nature of both the BSR and TSR reaction crystals that range from 100 mm to more than 1 cm. environments means that carbonate dissolution is These late stage associations typically include sad- often associated. Hence, the ore is often hosted in dle-dolomite cements and show negative burial-re- vugs, secondary pores or interclast porosity in car- lated oxygen isotope signaturesŽ. Fig. 27 . bonate breccias. Sometimes, the required sulfate is Lower temperatureŽ. 608C±2008C late-stage dia- carried by throughflushing basinal waters; at other genetic or burial dolomites typically host MVT min- times, it is derived from nearby dissolving evaporite eralization. Ore minerals are dominated by sphalerite beds, which may also act as hydrocarbon trapsŽ War- and galena, with minor barite, pyrite and marcasite. ren, 1999. . The ability of dissolving anhydrites to act Texturally, most ores are dominated by colloform- as sulfur sources explains the intimate relationship of banded sphalerite with isolated crystals of galena some MVT deposits to evaporitesŽ Table 10; Gays dispersed within the sphalerite bands. Late-stage River, Cadjebut, some Knox Group deposits. . How- dolomites may form a substrate to mineralization, or ever, others are associated with intervals of hydro- an intermixed layering especially in the early stages carbons within traps that are not evaporitic or they of mineralizationŽ e.g. Cadjebut, Gays River; Warren are precipitated in zones where metalliferous sulfur- and Kempton, 1997; Kontak, 1998. . depleted brines mix when sulfate-entraining waters Nonmagmatic base metal mineralization typically and the organics come from hydrocarbons in the occurs at redox interfaces where sulfate reduction is pore fluidsŽ. Table 10 . either by bacterial sulfate reductionŽ. BSR or ther- Some, but not all, MVT-associated late stage mochemical sulfate reductionŽ TSR; see Warren, dolomite hosts are laterally limited masses of coarsely 1999, Chaps. 8 and 9 for discussion. . Anaerobic crystalline dolomite whose positions are controlled sulfate reducing bacteria can produce dolomite and by major regional faultsŽ. Table 10 . Such faults tend J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 65 to be steep and so cut across diverse thick sedimen- a deeper circulating system that rose along and seeped tary successionsŽ e.g. Navan and Cadjebut Pb±Zn from these faults, extending laterally where perme- deposits; Fig. 35; Warren and Kempton, 1997. . The ability was greater. These deep regional aquifers relationship between major faults and dolomite dis- feeding the faults can also contain burial dolomites tribution suggests the hot basinal fluids were part of hosting Pb±Zn ores.

Table 10 Late stage dolomite acting as an ore host in MVT deposits. Mineralization typically emplaced at low temperatures and associated with hydrocarbons and dissolving evaporites Pb±Zn ores, western Basque± Ore host dolomites are massive homogeneous burial dolomites that overprint primary structures. Cantabrian region, northern Spain Most display textures consisting of geometric, xenotopic mosaics with crystals that range in size Ž.Lower Cretaceous from 100 mm to 1 mm. They are fault-focused dolomitized masses in the partially dolomitized Ž.Bustillo and Ordonez, 1995 Urgonian Complex. Ores are related to the regional migration of Pb- and Zn-enriched brines that were expelled from adjacent basinal mudstones. Hydrocarbons were important in fixing sulphides and oil- and gas-fields of similar age are located near the Pb±Zn deposits.

Zn±Pb±Ba Iranuh District, Esfahan Ore hosts are coarse-grained discordant, late diagenetic dolomites emplaced along the Irankuh fault. Area, Iran The ore-stage dolomites formed from warmŽ. 1008C saline,87 Sr-enriched fluids similar to oilfield Ž.Lower Cretaceous brines. Host dolostones show extensive depletion d18 O Žy4.9½ to y12.7½ . indicating increased Ž.Ghazban et al., 1994 temperatures. A progressive depletion in stable carbon isotope ratios of the host dolostone occurs as mineralization is approached. Overlap in d18O values and 87 Srr 86S ratios between coarse-grained ore hosing dolomite and adjacent sucrosic and saddle dolomites implies that these rocks were isotopically modified by similar fluids.

Gays River Pb±Zn, Nova Scotia Paragenetically, constant volume dolomitization of the host rock is followed by euhedral, Ž.ŽVisean Kontak, 1998; Savard, manganiferous dolomite cement, then sphalerite and galena. Syn- to postore calcite with trace 1996. amounts of barite, fluorite, and quartz, occlude the remaining porosity. Dolomitization by hot basinal brines, in a burial setting typified by red bed clastics overlain by thick halokinetic rift evaporites and marginward carbonate platforms. Overlying and capping evaporites, prior to their dissolution, focused and trapped escaping metalliferous basinal fluids into the platform. Evaporites probably supplied sulphate that helped fix the metals via thermochemical sulphate reduction.

Navan Zn±Pb, IrelandŽŽ.Ž. Lower Car- Ore hosted principally 97% by the Meath Formation Navan Group . Most dolomitization at boniferous.Ž Rizzi and Braithwaite, Navanis confined to the Meath Formation. Dolomite is linear trending NE±SW, with a flattened, 1997; Braithwaite and Rizzi, 1997; laterally limited, tabular geometry in cross-section. Dolomitizing fluids initially rose vertically, Hitzman et al., 1998. cross-cutting stratigraphic and sedimentological boundaries, although fluid flow was subsequently controlled by these features. Metalliferous brines were fed from a deeper regional aquifer that was also forming widespread burial dolomite. Ores follows the dolomite trend and were precipitated during ongoing dolomite cementation. Three stages of dolomitization are indicated. Isotopic and fluid inclusion data suggest growth of successive stages from waters which became progressively hotterŽ. 608C±1608C.

Cadjebut Pb±Zn, Canning Basin, Cadjebut MVT mineralization is hosted by carbonate-evaporite units in the Givetian lower dolomite AustraliaŽ.Ž Devonian Warren and sequence of the Pillara platform, occurs as two ore types: Ž. 1 an early Zn-rich, stratiform and Kempton, 1997; Tompkins et al., rhythmically banded ore and its laterally equivalent halo of banded marcasite, barite"calcite; and 1994.Ž. 2 a cross-cutting, breccia-fill, Pb-rich ore. Both ore types are intimately related to the bathyphreatic dissolution of stratiform anhydrite units which the ore replacesŽ. sulphur source . There are two forms of dolomite in the region. An earlier brine reflux dolomite, with its lateral extent controlled by the distribution of platform ramp evaporites. A later coarser-grained burialrsaddle dolomite that hosts the ore bodies. The distribution of this later dolomite is tied to basin-defining extensionalrgrowth faults that acted as conduits and foci for the upward flow of metalliferous basinal fluidsŽ. Fig. 35 . Isotopically, the oxygen isotope signature of this later burial dolomite is lighter than the reflux dolomite, reflecting focused upwelling of higher temperature basinal fluidsŽ. Fig. 27 .

()continued on next page 66 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Table 2 Ž.continued Polaris Pb±Zn, Canadian Arctic Coarsely recrystallized ore hosting dolomite, white dolospar, internal sediments, clayminerals and ArchipelagoŽ. Upper Ordovician brown dolomitized limestone constitute the alteration halo at Polaris. Host-rock dissolution was Ž.Randell, 1994; Heroux et al., 1996 synchronous with pre-folding ore precipitation. Synsedimentary faults and gravity-slump breccias in the host limestones probably afforded preferential permeability. Halokinesis driving much of the structural focusing of the mineralising fluids. Polaris lies in shelf carbonates of the Thumb Mountain Formation. Ore was deposited from tectonically driven bittern-derived 1058 to 1258C basinal brines, while burial temperatures in the adjacent unaltered carbonates were around 608C. The mineralized zone is characterized by a zone of pure illite above a kaolinite envelope surrounding the deposit. Faults and folds marking the culmination at Polaris of the late Devonian±Ellesmerian orogeny that ruptured the shale seal, terminating the mineralization.

Pb±Zn deposits, Knox Group and Mineralization, consisting of sphalerite and sparry dolomite, fills interstices in the breccias that Shady Dolomite Formation carbon- formed in limestone. These MVT deposits are found largely in two major paleoaquifers in the ates, southern Appalachia, USA region, the Lower Cambrian Shady Dolomite and the Lower Ordovician Knox Group karst zone. Ž.Ž.Cambrian and Lower Ordovican Regionally extensive approximately 70,000 km2 , late diagenetic dolomites that also hosts the ore ŽMontanez,Ä 1994; Haynes and consist of replacement dolomites and zoned dolomite cements. As much as 50% of the limestone in Kesler, 1994; Jones et al., 1996. mineralized breccia sections is lost over enormous areas of paleoaquifer that extend far beyond significant mineralization. Late diagenetic dolomitization of the Upper Knox Group in the southern Appalachian basin was closely associated with widespread secondary porosity development, and hydrocarbon migration. Late diagenetic matrix dolomites served as long-lived aquifers that focused and channeled diagenetic fluids in the deep subsurfaceŽ. 2 to )5 km driven by fluid migration that was a response to late Paleozoic Alleghenian tectonismŽ. 330 to 265 Ma . Thermochemical sulfate reduction is the main metal fixing mechanism with sulfate derived from dissolving Cambrian and Ordovician evaporates.

Pb±Zn deposits, southern Canadian Ore hosting dolomite, made up of white, sparry, hydrothermal, replacement, and open-space filling RockiesŽ.Ž Cambro-Ordovician Yao dolomite form strata-bound sheets over broad platforms, except near the margin, where they also and Demicco, 1995; Nesbitt and form dikes that crosscut sedimentary bedding planes. Contained within the dolomites are Muehlenbachs, 1994; Nesbitt and occurrences of talc and MVT Pb±ZnŽ. the former Kicking Horse and Monarch mines Prochaska, 1998; Qing and Moun- mineralization and economic concentrations of magnesiteŽ. Mount Brussilof mine . Fluid inclusion tjoy, 1994a,b,c. data indicated that the primary dolomitizing fluids were warm to hotŽ. 1008C to 2008C saline brines Ž.13±25 wt.% . The high salinity brines were derived by the subsurface dissolution of thick salt sequences. Field evidence indicates that the two kinds of dolomite bodies were formed by pervasive, formation-parallel flow and fracture-channeled, cross-formational flow, respectively. The timing of dolomitizationŽ. Silurian to Late Devonian coincided with the timing of the early Paleozoic contractional deformation in the west. Finite element modeling showed that the observed dolomite geometry and geochemistry are consistent with miogeocline-scale ground-water flow driven by west-to-east topographic relief.

The varying styles of burial and hydrothermal rocks and encompasses the majority of the studied dolomite ore hosts can be clearly seen when temper- MVT depositsŽ Pine Point, Newfoundland Zinc, atures of the deposit and the surrounding host rock Mascot-Jefferson City, Copper Ridge, Sweetwater, are compared, as was done in a comparison study of Central Missouri, Northern Arkansas, and Tri-State. . 14 major MVT districts of Canada and the USA. Group 2 deposits have ore zones that are signifi- There thermal alteration in deposit surrounds were cantly hotter than surrounding host rocks and, as compared to inclusion-derived temperatures in the such, define a positive thermal anomalyŽ Upper Mis- ore zone interiorŽ. Sangster et al., 1994 . This temper- sissippi Valley, Polaris, and central Tennessee. . ature comparison divided the various dolomite-hosted Group 3 deposits revealed the highest color alter- deposits into three groups. Group 1 deposits have ation index values of the studied North American ores that are in thermal equilibrium with their host MVT districts, with host-rock temperatures greatly J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 67

Fig. 35. Fault-focused burial dolomitization controls the distribution of coarse-grained dolomite in Devonian carbonates of the northern Canning Basin Margin, Australia. Dolomitizing fluids were expelled from the neighbouring Fitzroy trough and migrated up extensional fault conduits to penetrate further into the more permeable intervals in the carbonate. Evaporitic intervals and dissolution breccias facilitated greater volumes of fluid incursionŽ. after Warren and Kempton, 1997 . exceeding ore mineral fluid inclusion temperatures, tures away from the orogenic front. The correspon- that is they show a negative thermal anomaly. They dence between host-rock and ore temperatures sug- are the least common styleŽ Austinville-Ivanhoe, gests that group 1 deposits were deposited essentially northern Newfoundland, and Robb Lake deposits. . within their own aquifers. Group 2 deposits were Group 2 depositsŽ. hot ore in cool rocks all lie within formed when the same, or genetically similar, fluids the continental interior, some group 1 depositsŽ ore rose from the regional aquiferŽ. s into structurally temperatureshost-rock temperature. are well within controlled conduits in the continental interior, possi- the continental interior but others are adjacent to bly developed as a result of synorogenic doming and orogenic belts on the continental side, while group 3 arching. This upward channeling of the fluids re- depositsŽ. cool ore in hot rocks are all positioned sulted in rapid and localized fluid flow, which pre- within orogenic belts but on the continental side. cluded or at least inhibited widespread heating of the This is consistent with current hypotheses regard- host rocks by ore fluids. Group 3 deposits formed ing the formation of many MVT deposits and their when cooler ore carrying fluids are squeezed into dolomite hosts via fluids squeezed cratonward from hotter orogenic zones. But not all Pb±Zn deposits active orogenic beltsŽ Fig. 24B; Sangster et al., formed via compression-driven hydrologies, some 1994. . Orogenic uplift at the craton edge induces provincesŽ e.g. Pb±Zn deposits along the Devonian heating of host rocks via the migration of hot brines margin of the Canning basin, Australia. formed via driven cratonward by the hydraulic gradient. Once fault-focused escape of basinal fluids in a passive the ore-bearing fluids entered the foreland carbonate margin settingŽ McManus and Wallace, 1992; War- platforms, they traveled through sedimentary aquifers ren and Kempton, 1997. . of the continental interior raising the temperature of Permeability, which allowed penetration of the thinly covered carbonate rocks. The fluids left, as dolomitizing solutions, and then sulfide mineraliza- evidence of their passage, group 1 MVT deposits tion, varies in origin. It may be a reactivated mete- distributed with decreasing homogenization tempera- oric karst network, a synmineralization bathyphreatic 68 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 karst, an earlier permeable dolomite horizon, or an In the Niccioleta ore deposit, Tuscany, Italy, evaporite dissolution breccia. The dissolution breccia metasomatic reactions have transformed anhydrites may be formed by the processes that promote the to garnet skarns, dolomites to pyroxene skarns and mineralizationŽ. as at Cadjebut , or may be an inher- mica schists to epidote skarnsŽ. Dechomets, 1985 . ited dissolution horizon indicating earlier and shal- All transformations have involved pyrite formation lower meteoric dissolution. via thermochemical sulfate reduction where acidic

Ore hosting dolomites may also form at higher Fe-rich solutions have leached Na, K, CO2 , and parts temperatures generated by igneous or metamorphic of the Ca and Mg in the protolith and so created processes. These include mineralized contact haloes space for mineralization. As part of the transforma- of dolomite skarn and dolomitic marbles in which tion process, dolomites adjacent to the deposits were the mineralization is associated with structural focus- altered to coarsely crystalline high temperature xeno- ing within regional metamorphic settings. At these topic forms. higher temperature Ž.)2008C±2508C , and in regions Recrystallization and reprecipitation of coarse of suitable source rock, the formation of coarsely dolomites also formed a halo around parts of the crystalline dolomites can be associated with the pre- contact metamorphic magnesian skarns that formed cipitation of copper sulfides in addition to those of during the magmatic and postmagmatic stages in the Pb and Zn. Some MVT areas exposed to warmer intracontinental paleorift zones of the Siberian Plat- fluids can also contain copper sulfides in regions of formŽ. Mazurov and Titov, 1999 . Spinel-forsterite hotter through flowing brinesŽ e.g. southeast Mis- magnesian skarns of the magmatic stage are confined souri region. . to the overdome parts of doleritic bodies and are the In most large mineralized skarn systems the skarn result of the interaction between massive sedimen- and the ore come from the same hydraulic system, tary dolomites and fluids released from liquid magma. the association between hydrothermal dolomite and The magnesian skarns of the postmagmatic stage are mineralization is not surprising. Skarns can form in typically localized in the margins and along the both contact and regional metamorphic settings and frontsŽ. outwedges of doleritic sills, apophyses, and typically, but not exclusively involve a carbonate the branches of intrusive bodies that crosscut the protolithŽ. Meinert, 1993 . Some minerals, such as adjacent carbonate-evaporite series. These skarns calcite and quartz are to be found in almost all skarn have a banded structure and resemble gravel con- deposits with a carbonate protolith. Others, such as glomerates with carbonateŽ. often dolomitic cement. humite, periclase, phlogopite, talc, serpentine and The AclastsB are globules of disintegrated doleritic brucite characterize magnesian skarnsŽ often with a porphyrites that are completely or partially substi- dolomiterevaporite protolith. and are absent from tuted by zonal magnesian skarns. Intrusion and solid- other types. ification of basic magma within a highly reactive Dolomitic skarns form under the influence of a carbonate±evaporite sequence formed these unusual variety of metasomatic processes and involve fluids dolomitic rocks. of magmatic, metamorphic, basinal, meteoric and The Mount Bischoff tin deposit, Tasmania, is a marine origin. Skarns form over a wide range of more typical dolomite-hosted style of skarn mineral- temperatures that overlap with those of other miner- ization. It occurs within a locally recrystallized alized deposits. It is only at the higher temperature dolomite unit in structurally complex, late-Pre- end of the dolomite recrystallization spectrum that cambrian rocks that were intruded by a swarm of they are easily separatedŽ. Meinert, 1993 . Dolomitic narrow, vertical-to-steeply dipping, quartz±feldspar skarns can form adjacent to plutons, along major porphyry dikes of Devonian ageŽ Halley and Walshe, faults and shear zones, in seafloor moundsŽ black 1995. . The dikes are distributed in a crudely radial smokers. , and in deeply buried metamorphic ter- pattern and have undergone an early stage of potas- ranes. This group of settings is too broad and well sic alteration, overprinted to a variable degree by documented to be adequately covered in this review, greisening. The alteration is zoned along and across so we will restrict our discussion to a few examples the dikes consistent with their acting as major fluid illustrating this diversity. foci. Within the dolomite associated with this alter- J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 69 ation, replacement and recrystallization occurred in 8. Summary two distinct stages. During stage 1, serpentine and chondrodite assemblages formed at 4008C to 4608C We started off this review by asking the question, from hydrothermal fluids with 30±36 wt.% NaCl. A AWhat is Dolomite?B Let us see if we can answer zonation from serpentine to chondrodite to magnesite that question with a summary of our most important reflects the lowering of silica activity in the fluid as observations and conclusions: it reacted with the dolomite. During stage 2, quartz, v Both dolomite the rock and dolomite the min- talc, phlogopite, and carbonate formed at 3208Cto eral encompass a wide range of Ca±Mg carbonates 3608C from hydrothermal fluids with about 2 m with similar, but not identical, MgrCa ratios.

v NaCl, 1.5 m CO24 , and 0.3 m CH . Sedimentary dolomite is a metastable In the Kokomo mining district of Colorado, mineralrrock that precipitates or replaces earlier dolomite formed some 40 Ma as hydrothermal haloes more soluble phases a number of times as it ap- or skarns about volcanic plugs, associated with manto proaches a more ideal stoichiometry. It undergoes ores in sedimentary carbonate hostsŽ Ag±Pb±Zn Ostwald ripening or aging. massive sulfide replacement deposits.Ž Thompson, v As it moves through earth-space and time, a 1998. . The mantos are surrounded by halos of sedimentary dolomite evolves in response to chang- siderite-manganosiderite, coarsely crystalline hy- ing conditions of burial, pore brine chemistry, tem- drothermal dolomite, andror jasperoid. The massive perature and pressure. As dolomite passes through sulfide replacement ores show banded and tabular various diagenetic settings its chemical and porosity textures similar to those of ores at Leadville and characteristics are reset. Gilman, CO. Ore deposition occurred, in association v A dolomite body may form for the first time at with hydrothermal dolomitization, in the general se- the surface or in the subsurface and it may generate quence: jasperoid, iron sulfidesŽ pyrrhotite, marca- or lose porosity a number of times in its burial site, and pyrite.qsiderite, quartz, high iron spha- history. lerite, ferroan dolomite, galena, chalcopyrite and sul- v Depositional settings include; sabkhas, salinas, fosalts, barite, ferroan calcite, and calcite. Fluid in- salterns, humid subtidal mud shoals and evaporite- clusion analyses of quartz and carbonaterdolomite free schizohaline lakes. The resulting dolomites are minerals indicate that temperatures averaged 3508C typically fine-grained or mimetic with sedimentary during the early stages of ore deposition and de- structures indicative of their depositional setting. creased to about 1608C during the late stages. Ore Bacterial mediation is significant in precipitating fluid salinities also decreased from 4.6 to -2 wt.% dolomite in most earth surface and some shallow NaCl equiv. subsurface settings. Coarse-grained dolomite, magnesite and siderite v Diagenetic settings include the shallow seafloor, are characteristic hydrothermal alteration minerals in shallow burial marineŽ. connate settings, the interiors Miocene hanging wall dacites about Kuroko orebod- of atolls, mixing zones between waters of differing iesŽ.Ž. Shikazono et al., 1998 . The Mgr MgqFe salinities, zones of brine reflux, and deep burial ratios of the carbonates decrease from the central settings. Later formed diagenetic dolomites are chlorite alteration zone to marginal zone and the coarser-grained than depositional dolomites and the MgrŽ.MgqFe ratios of carbonates and chlorite pos- geometries of the various bodies reflect the forma- itively correlate. The d18 O and d13C values also tive subsurface hydrologies. positively correlate with each other and lie between v Dolomites with well-developed intercrystalline the igneous and marine carbonate values. The sparry porosity make excellent hydrocarbon reservoirs. dolomite and the magnesite formed in the central Many such reservoirs have an intimate association zone close to the orebodies via interaction of hy- with evaporites. This reflects both the likelihood of drothermal solutions with biogenic marine carbon- the dolomite forming by brine refluxŽ platform, ates. Calcite formed further from the orebodies from strand-zone and basin-center evaporite hydrologies. hydrothermal fluids that did not entrain a biogenic and the ability of evaporites to form effective seals marine carbon component. to permeable intervals. Nonevaporitic dolomite reser- 70 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 voirs typically formed via fracturing or karstification. evaporative environments of the Permian Yates Formation, Many dolomite reservoirs can be related to regional North Ward-Estes field, Ward County, Texas. Am. Assoc. Ž. unconformities. Petrol. Geol. Bull. 76 11 , 1735±1759. Aulstead, K.L., Spencer, R.J., Krouse, H.R., 1988. Fluid inclu- v Late diagenetic dolomiteŽ burial and hydrother- sions and isotopic evidence on dolomitization, Devonian of mal. can act as an ore host. Many MVT deposits are Western Canada. Geochim. Cosmochim. Acta 52, 1027±1035. hosted in such dolomites. Typically, the same solu- Ayalon, A., Longstaffe, F.J., 1995. Stable isotope evidence for the tions that formed the burial dolomite host were also origin of diagenetic carbonate minerals from the Lower Juras- Ž. capable of carrying metals. Where a suitable struc- sic Inmar Formation, southern Israel. Sedimentology 42 1 , 147±160. tural focus and redox system coincide, ore deposits Back, W., Hanshaw, B.B., Plummer, L.N., Rahn, P.H., Rightmire, form in a burial dolomite host. Dolomite hosts also C.T., Rubin, M., 1983. Process and rate of dedolomitization; form as skarns, mantos and during metamorphism. mass transfer and 14C dating in a regional carbonate aquifer. In summary, dolomite is a complex mineral that Geol. Soc. Am. Bull. 94Ž. 12 , 1415±1429. evolves throughout its burial history. It does so by a Badiozamani, K., 1973. The dorag dolomitization model Ð application to the middle Ordovician of Wisconsin. J. Sediment. series of dissolution±reprecipitation reactions that Petrol. 43, 965±984. favor the retention of intergranular porosity. Hence, Baker, P.A., Kastner, M., 1981. Constraints on the formation of it is often an excellent reservoir for hydrocarbons or sedimentary dolomite. Science 213, 214±216. a host rock to base metal ores. Baker, P.A., Burns, S.J., 1985. Occurrence and formation of dolomite in organic-rich continental margin sediments. Am. Assoc. Petrol. Geol. Bull. 69, 1917±1930. Acknowledgements Banner, J.L., 1995. Applications of the trace element and isotope geochemisty of strontium to studies of carbonate diagenesis. Ž. This paper benefited greatly from the helpful Sedimentology 45 2 , 805±824. Banner, J.L., Hanson, G.N., Meyers, W.J., 1988. Water±rock suggestions of the two reviewers, Professors C.J.R. interaction history of regionally extensive dolomites of the Braithwaite and G.M. Friedman. Burlington-Keokuk FormationŽ. Mississippian ; isotopic evidence. In: Shukla, V., Baker, P.A.Ž. Eds. , Sedimentology and Geochemistry of Dolostones, Based on a Symposium. Spec. References Publ.-SEPM vol. 43, pp. 97±113. Baron, G., 1960. Sur la synthese de la dolomie. Application au Adams, J.F., Rhodes, M.L., 1960. Dolomitization by seepage phenomene de dolomitization. Rev. Inst. Fr. Pet. Ann. Com- refluxion. Am. Assoc. Petrol. Geol. Bull. 44, 1912±1920. bust. Liq. 15Ž. 1 , 3±68. Aharon, P., Kolodny, Y., Sass, E., 1977. Recent hot brine dolomi- Baudouy, S., LeGorjus, C., 1991. Sendji field Ð People's Repub- tization in the ASolar LakeB, Gulf of Elat; isotopic, chemical, lic of Congo, Congo Basin. In: Beaumont, E.A., Foster, N.H. and mineralogical study. J. Geol. 85Ž. 1 , 27±48. Ž.Eds. , Structural Traps V: AAPG Treatise of Petroleum Geol- Aharon, P., Socki, R.A., Chan, L., 1987. Dolomitization of atolls ogy, Atlas of Oil and Gas Fields. pp. 121±149. by sea water convective flow: test of a hypothesis at Niue, Bebout, D.G., Maiklem, W.R., 1973. Ancient anhydrite facies and South Pacific. J. Geol. 95, 187±203. environments; their role in reconstructing geologic history of Alderman, A.R., Skinner, H.C., 1957. Dolomite sedimentation in middle Devonian Elk Point Basin, Alberta. Am. Assoc. Petrol. the southeast of South Australia. Am. J. Sci. 255, 561±567. Geol. Bull. 57Ž. 4 , 769±770. Alderman, A.R., von der Borch, C.C., 1963. Dolomite reaction Behre, C.H. Jr., 1947. Geochemistry and localization of dolomiti- series. Nature 198Ž. 4879 , 465±466. zation. J. Geol. 42, 540±542. Alsharhan, A.S., 1993. Facies and sedimentary environment of the Behrens, E.W., Land, L.S., 1972. Subtidal Holocene dolomite, Permian carbonateŽ. Khuff Formation in the United Arab Baffin Bay, Texas. J. Sediment. Petrol. 42, 155±161. Emirates. Sediment. Geol. 84Ž. 1±4 , 89±99. Bein, A., Land, L.S., 1983. Carbonate sedimentation and diagene- Amthor, J.E., Mountjoy, E.W., Machel, H.G., 1993. Subsurface sis associated with Mg±Ca-chloride brines; the Permian San dolomites in Upper Devonian Leduc Formation buildups, cen- Andres Formation in the Texas Panhandle. J. Sediment. Petrol. tral part of Rimbey-Meadowbrook reef trend, Alberta, Canada. 53, 243±260. Bull. Can. Petrol. Geol. 41Ž. 2 , 164±185. Bentley, J., 1984. Petroleum geology of the central Broome Amthor, J.E., Mountjoy, E.W., Machel, H.G., 1994. Regional-scale Platform. In: Purcell, P.G.Ž. Ed. , The Canning Basin, Western porosity and permeability variations in Upper Devonian Leduc Australia. Proceedings Geological Society of Australia and buildups; implications for reservoir development and predic- Petroleum Exploration Society of Australia, Canning Basin tion in carbonates. Am. Assoc. Petrol. Geol. Bull. 78Ž. 10 , Symposium, Perth, WA, pp. 157±168. 1541±1559. Berner, R.A., Westrich, J.T., 1985. Bioturbation and the early Andreason, M.W., 1992. Coastal siliciclastic sabkhas and related diagenesis of carbon and sulfur. Am. J. Sci. 285, 193±206. J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 71

Blatt, H., 1992. Sedimentary Petrology. W.H. Freeman, New ŽRoles of micro-organisms, bacteria, cyanobacteria, in the York, 514 pp. formation of mud mounds, examples from the Turonian strata Boles, J.R., 1978. Active ankeritic cementation in the subsurface of jebels Bireno and Mrhila, Tunisia. C. R. Acad. Sci., Ser. II: Eocene of southwest Texas. Contrib. Mineral. Petrol. 68, Mec., Phys., Chim., Sci. Terre Univers 307Ž. 4 , 401±407. 13±22. Camoin, G., Debrenne, F., Gandin, A., 1989. Premieres images Boles, J.R., Franks, S.G., 1979. Clay diagenesis in Wilcox Sand- des communautes microbiennes dans les ecosystemes cam- stones of southwest Texas: implications of smectite diagenesis briensŽ first images of microbial communities in Cambrian on sandstone cementation. J. Sediment. Petrol. 49, 59±70. ecosystems. . C. R. Acad. Sci., Ser. II: Mec., Phys., Chim., Boni, M., Parente, G., Bechstadt, T., De Vivo, B., Iannace, A., Sci. Terre Univers 308Ž. 16 , 1451±1458. 2000. Hydrothermal dolomites in SW SardiniaŽ. Italy : evi- Cander, H.S., 1994. An example of mixing-zone dolomite, middle dence for a widespread late-Variscan fluid flow event. Sedi- Eocene Avon Park Formation, Floridan Aquifer system. J. ment. Geol. 131Ž. 3±4 , 181±200. Sediment. Res., Sect. A 64Ž. 3 , 615±629. Bos, C.F.M., 1989. Planning of an appraisalrdevelopment pro- Carballo, J.B., Land, L.S., Miser, D.E., 1987. Holocene dolomiti- gram for the complex Khuff carbonate reservoir in the Yibal zation of supratidal sediments by active tidal pumping, Sugar- field, north Oman. Proceedings 6th Society of Petroleum loaf Key, Florida. J. Sediment. Petrol. 57Ž. 1 , 153±165. Engineers Middle East Oil Show, Bahrain, SPE 17988. pp. Carozzi, A.V., 1993. Sedimentary Petrography. Prentice-Hall, En- 631±640. glewood Cliffs, NJ, 263 pp. Bourrouilh-Le Jan, F.G., 2000. Deodat Gratet de Dolomieu. Life Chafetz, H.S., Folk, R.L., 1984. Travertines: depositional mor- and works of a European geologist, naturalist and lithologist. phology and the bacterially constructed constituents. J. Sedi- C. R. Acad. Sci., Ser. IIa: Sci. Terre Planetes 330Ž. 1 , 83±95, ment. Petrol. 54Ž. 1 , 289±316. French. Chafetz, H.S., Rush, P.F., 1994. Diagenetically altered sabkha-type Braithwaite, C.J.R., 1991. Dolomites; a review of origins, geome- Pleistocene dolomite from the Arabian Gulf. Sedimentology try and textures. Trans. R. Soc. Edinburgh: Earth Sci. 82Ž. 2 , 41Ž. 3 , 409±421. 99±112. Chafetz, H.S., Imerito-Tetzlaff, A.A., Zhang, J.L., 1999. Stable- Braithwaite, C.J.R., Heath, R.A., 1996. Regional geometry, petro- isotope and elemental trends in Pleistocene sabkha dolomites: graphic variation, and origins of Upper Ordovician dolomites Descending meteoric water vs. sulfate reduction. J. Sediment. in Hadeland, Norway. Nor. Geol. Tidsskr. 76, 63±74. Res., Sect. A 69Ž. 1A , 256±266. Braithwaite, C.J.R., Rizzi, G., 1997. The geometry and petrogene- Chave, K.E., 1952. A solid solution between calcite and dolomite. sis of hydrothermal dolomites at Navan, Ireland. Sedimentol- J. Geol. 60, 190±192. ogy 44Ž. 3 , 421±440. Chai, L., Navrotsky, A., Reeder, R.J., 1995. Energetics of cal- Brognon, G.P., Verrier, G.R., 1966. Oil and geology in Cuanza cium-rich dolomite. Geochim. Cosmochim. Acta 59Ž. 5 , 939± basin of Angola. Am. Assoc. Petrol. Geol. Bull. 50, 108±158. 944. Budd, D.A., 1997. Cenozoic dolomites of carbonate islands; their Choquette, P.W., Pray, L.C., 1970. Geologic nomenclature and attributes and origin. Earth Sci. Rev. 42Ž. 1±2 , 1±47. classification of porosity in sedimentary carbonates. Am. As- Burns, S.J., McKenzie, J.A., Vasconcelos, C., 2000. Dolomite soc. Petrol. Geol. Bull. 54, 207±250. formation and biogeochemical cycles in the Phanerozoic. Sedi- Choquette, P.W., Steinen, R.P., 1980. Mississippian nonsupratidal mentology 1, 49±61, Suppl. dolomite, Ste. Genevieve Limestone, Illinois Basin: evidence Bush, P., 1973. Some aspects of the diagenetic history of the for mixed water dolomititzation. In: Zenger, D.H., Dunham, sabkha in Abu Dhabi, Persian Gulf. In: Purser, B.H.Ž. Ed. , The J.B., Ethington, R.L.Ž. Eds. , Concepts and Models of Dolomi- Persian Gulf, Holocene Carbonate Sedimentation and Diagen- tization. Spec. Publ.-SEPM 28, pp. 168±196. esis in a Shallow Epicontinental Sea. Springer, New York, pp. Choquette, P.W., Cox, A., Meyers, W.J., 1992. Characteristics, 395±407. distribution and origin of porosity in shelf dolostones; Burling- Bustillo, M., Ordonez, S., 1995. Lower Cretaceous Pb±Zn ores of ton-Keokuk FormationŽ. Mississippian , U.S. Mid-Continent. J. Cantabria, Northern Spain Ð new considerations based on Sediment. Petrol. 62Ž. 2 , 167±189. petrological and geochemical evidence. Trans.-Inst. Min. Met- Choquette, P.W., Steinen, R.P., 1985. Mississippian oolite and all., Sect. B 104, B55±B65. non-supratidal dolomite reservoirs in the Ste. Genevieve For- Butler, G.P., 1969. Modern evaporite deposition and geochemistry mation, North Bridgeport Field, Illinois basin. In: Roehl, P.O., of coexisting brines, the Sabkha, Trucial coast, Arabian Gulf. Choquette, P.W.Ž. Eds. , Carbonate Petroleum Reservoirs. J. Sediment. Petrol. 39, 70±89. Springer, New York, pp. 207±225. Butler, R.W.H., Lickorish, W.H., Grasso, M., Pedley, H.M., Chuber, S., Pusey, W.C., 1985. Productive Permian carbonate Ramberti, L., 1995. Tectonics and sequence stratigraphy in cycles, San Andres Formation, Reeves field, West Texas. In: Messinian basins, Sicily; constraints on the initiation and Roehl, P.O., Choquette, P.W.Ž. Eds. , Carbonate Petroleum termination of the Mediterranean salinity crisis. Geol. Soc. Reservoirs. Springer, New York, pp. 289±307. Am. Bull. 107Ž. 4 , 425±439. Clark, D.N., 1980. The diagenesis of Zechstein carbonate sedi- Camoin, G., Maurin, A.F., 1988. Roles des micro-organismes ments. In: Fuchtbauer, H., Peryt, T.Ž. Eds. , The Zechstein Ž.bacteries, cyanobacteries dans la genese des AMud MoundsB, Basin with Emphasis on Carbonate Sequences, E. Schweizer- exemples du Turonien des Jebels Bireno et MrhilaŽ. Tunisie bart'sche VbH. Contrib. Sediment. vol. 9, pp. 167±203. 72 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Clement, J.H., 1985. Depositional sequences and characteristics of Kansas. In: Beaumont, E.A., Foster, N.H.Ž. Eds. , Stratigraphic Ordovician Red River reservoirs, Pennel field, Williston basin, Traps II: AAPG Treatise of Petroleum Geology, Atlas of Oil Montana. In: Roehl, P.O., Choquette, P.W.Ž. Eds. , Carbonate and Gas Fields. pp. 117±133. Petroleum Reservoirs. Springer, New York, pp. 71±84. Ehrets, J.R., Kissling, D.L., 1985. Deposition, diagenesis and Clayton, R.N., Skinner, H.C.W., Berner, R.A., Rubinson, M., paleostructural control of Duperow and BirdbearŽ. Nisku 1968. Isotopic compositions of recent South Australian la- reservoirs, Williston Basin. In: Longman, M.W., Shanley, goonal carbonates. Geochim. Cosmochim. Acta 32, 983±988. K.W., Lindsay, R.F., Eby, D.E.Ž. Eds. , Rocky Mountain Car- Compton, J.S., Siever, R., 1986. Diffusion and mass balance of bonate Reservoirs. SEPM Core Workshop, pp. 183±216. Mg during early dolomite formation, Monterey Formation. El-Sayed, M.I., Fairchild, I.J., Spiro, B., 1991. Kuwaiti dolocrete; Geochim. Cosmochim. Acta 50Ž. 1 , 125±135. petrology, geochemistry and groundwater origin. Sediment. Compton, J.S., 1988. Degree of supersaturation and precipitation Geol. 73Ž. 1±2 , 59±75. of organogenic dolomite. Geology 16Ž. 4 , 318±321. Elliott, L.A., Warren, J.K., 1989. Stratigraphy and depositional Cook, T.D., 1979. Exploration history of South Texas Lower environment of lower San Andres Formation in subsurface and Cretaceous carbonate platform. Am. Assoc. Petrol. Geol. Bull. equivalent outcrops; Chaves, Lincoln, and Roosevelt counties, 70, 32±49. New Mexico. Am. Assoc. Petrol. Geol. Bull. 73Ž. 11 , 1307± Cowan, P.E., Harris, P.M., 1986. Porosity in the San Andres 1325. FormationŽ. Permian , Cochran and Hockley counties, Texas. Epting, M., 1980. Sedimentology of Miocene carbonate buildups, Am. Assoc. Petrol. Geol. Bull. 70, 888±897. central Luconia, offshore Sarawak. Geol. Soc. Malays. Bull. Craig, D.H., 1988. Caves and other features of Permian karst in 12, 17±30. San Andres dolomite, Yates field reservoirs, west Texas. In: Eriksson, K.A., Warren, J.K., 1983. A paleohydrologic model for James, N.P., Choquette, P.W.Ž. Eds. , Paleokarst. Springer, Early Proterozoic dolomitization and silicification. Precam- New York, pp. 342±363. brian Res. 21Ž. 3±4 , 299±321. Danielli, H.M.C., 1988. The Eocene reservoirs of Wafra field, Eugster, H.P., Hardie, L.A., 1978. Saline Lakes. In: Lerman, A. KuwaitrSaudi Arabia partitioned neutral zone. In: Lomando, Ž.Ed. , Lakes; Chemistry, Geology, Physics. Springer, New A.J., Harris, P.M.Ž. Eds. , Giant Oil and Gas Fields. SEPM York, pp. 237±293. Core Workshop, pp. 119±154. Fairbridge, R.W., 1957. The dolomite question. In: LeBlanc, R.J., Davies, G.R., Ludlam, S.D., 1973. Origin of Laminated and Breeding, J.G.Ž. Eds. , Regional Aspects of Carbonate Deposi- Graded Sediments, Middle Devonian of Western Canada. Geol. tion. Spec. Publ.-SEPM vol. 5, pp. 125±178. Soc. Am. Bull. 84Ž. 11 , 3527±3546. Fairchild, I.J., 1983. Chemical controls of cathodoluminescence of Dechomets, R., 1985. Sur l'origine de la pyrite et des skarns du natural dolomites and calcite: new data and review. Sedimen- gisement, en contexte evaporitique, de NiccioletaŽ Toscane, tology 30, 579±583. Italie.Ž. . Miner. Deposita 20 3 , 201±210. Feazel, C.T., 1985. Diagenesis of Jurassic grainstone reservoirs in Deffeyes, K.S., Lucia, F.J., Weyl, P.K., 1965. Dolomitization of Smackover Formation, Chatom field, Alabama. In: Roehl, recent and Plio-Pleistocene sediments by marine evaporite P.O., Choquette, P.W.Ž. Eds. , Carbonate Petroleum Reser- waters on Bonaire, Netherlands Antilles. In: Pray, L.C., Mur- voirs. Springer, New York, pp. 357±367. ray, R.C.Ž. Eds. , Dolomitization and Limestone Diagenesis. Fischer, D., Heck, T., Gerhard, L.C., 1990. Medicine Pole Hills Spec. Publ.-SEPM vol. 13, pp. 71±88. field Ð USA Williston Basin. In: Beaumont, E.A., Foster, DeMis, W.D., 1992. Elkhorn Ranch field Ð USA Williston N.H.Ž. Eds. , Stratigraphic Traps I: AAPG Treatise of Petroleum Basin. In: Foster, N.H., Beaumont, E.A.Ž. Eds. , Stratigraphic Geology, Atlas of Oil and Gas Fields, Tulsa, OK. pp. 229±251. Traps II: AAPG Treatise of Petroleum Geology, Atlas of Oil Fisher, W.L., Rodda, P.U., 1969. Edwards FormationŽ lower and Gas Fields. pp. 369±388. Cretaceous. Texas: dolomitization in a carbonate platform Dix, G.R., 1993. Patterns of burial and tectonically controlled system. Am. Assoc. Petrol. Geol. Bull. 52, 55±72. dolomitization in an Upper Devonian fringing-reef complex Folk, R.L., Land, L.S., 1975. MgrCa ratio and salinity: two Ð Leduc Formation, Peace River Arch area, Alberta, Canada. controls on crystallization of dolomite. Am. Assoc. Petrol. J. Sediment. Petrol. 63, 628±640. Geol. Bull. 59, 60±68. Dravis, J., Yurewicz, D.A., 1985. Enhanced carbonate petrogra- Folk, R.L., 1987. Detection of organic matter in thin sections of phy using fluorescence microscopy. J. Sediment. Petrol. 55, carbonate rocks using a white card. Sediment. Geol. 54, 795±804. 193±200. Dunham, R.J., 1962. Classification of carbonate rocks according Folk, R.L., 1993. SEM imaging of bacteria and nannobacteria in to depositional texture. AAPG Mem. 1, 108±121. carbonate sediments and rocks. J. Sediment. Petrol. 63Ž. 5 , Durocher, S., Alaasm, I.S., 1997. Dolomitization and neomor- 990±999. phism of MississippianŽ. Visean upper Debolt Formation, Folk, R.L., 1994. Nannobacteria: major participants in the precipi- Blueberry Field, northeastern British Columbia; geologic, tation of carbonates and other minerals. In: Sasowsky, I.D., petrologic, and chemical evidence. Am. Assoc. Petrol. Geol. Palmer, M.V.Ž. Eds. , Breakthroughs in Karst Geomicrobiology Bull. 81Ž. 6 , 954±977. and Redox Geochemistry; Abstracts and Field-Trip Guide vol. Ebanks, W.J., 1991. Bindley field Ð USA Anadarko Basin, 1. Karst Waters Institute, Charles Town, WV, pp. 17±18. J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 73

Fookes, E., 1995. Development and eustatic control of an Upper Ghazban, F., McNutt, R., Schwarcz, H., 1994. Genesis of sedi- Jurassic reef complexŽ. Saint Germain-de-Joux, eartern France . ment-hosted Zn±Pb±Ba deposits in the Iranuh District, Esfa- Facies 33, 129±149. han Area, west-central Iran. Econ. Geol. 89Ž. 6 , 1262±1278. Fouke, B.W., Beets, C.J., Meyers, W.J., Hanson, G.N., Melillo, Ghosh, S.K., Friedman, G.M., 1989. Petrophysics of a dolostone A.J., 1996. Sr-87rSr-86 chronostratigraphy and dolomitization reservoir; San Andres FormationŽ. Permian , West Texas. Car- history of the Seroe Domi Formation, CuracaoŽ Netherlands bonates Evaporites 4Ž. 1 , 45±119. Antilles. . Facies 35, 293±320. Gill, D., 1985. Depositional facies of Middle SilurianŽ. Niagaran Freeze, R.A., Cherry, J.A., 1979. Groundwater. Prentice-Hall, pinnacle reefs, Belle River Mills Gas field, Michigan Basin, Englewood Cliffs, NJ, 604 pp. southeastern Michigan. In: Roehl, P.O., Choquette, P.W. Friedman, G.M., 1965. Terminology of crystallization textures Ž.Eds. , Carbonate Petroleum Reservoirs. Springer, New York, and fabrics in sedimentary rocks. J. Sediment. Petrol. 35, pp. 121±139. 643±655. Gill, I.P., Moore, C.H., Aharon, P., 1995. Evaporitic mixed-water Friedman, G.M., 1980. Dolomite is an evaporite mineral: evidence dolomitization on St. Croix, U.S.V.I. J. Sediment. Res., Sect. from the rock record and from sea-marginal ponds of the Red A 65Ž. 4 , 591±604. Sea. In: Zenger, E.H., Dunham, J.B., Ethington, R.L.Ž. Eds. , Given, R.K., Wilkinson, B.H., 1987. Dolomite abundance and Concepts and Models of Dolomitization. Spec. Publ.-SEPM stratigraphic age, constraints on rates and mechanisms of vol. 28, pp. 69±80. dolostone formation. J. Sediment. Petrol. 57, 1068±1078. Friedman, G.M., Sanders, J.E., 1967. Origin and occurrence of Graf, D.L., Goldsmith, G.R., 1956. Some hydrothermal syntheses dolostones. In: Chilingar, G.V., Bissell, H.J., Fairbridge, R.W. of dolomite and protodolomite. J. Geol. 64, 173±186. Ž.Eds. , Carbonate Rocks, Origin, Occurrence, and Classifica- Gregg, J.M., Sibley, D.F., 1984. Epigenetic dolomitization the tion. Elsevier, Amsterdam, pp. 267±348. origin of xenotopic dolomite texture. J. Sediment. Petrol. 54, Gao, G.Q., Land, L.S., 1991. Early Ordovician Cool Creek 907±931. dolomite, Middle Arbuckle Group, Slick Hills, SW Oklahoma, Gregg, J.M., Shelton, K.L., 1990. Dolomitization and dolomite USA. J. Sediment. Petrol. 61, 161±173. neomorphism in the back reef facies of the Bonneterre and Gao, G., Land, L.S., Folk, R.L., 1992. Meteoric modification of Davis formationsŽ. Cambrian , Southeastern Missouri. J. Sedi- early dolomite and late dolomitization by basinal fluids, upper ment. Petrol. 60Ž. 4 , 549±562. Arbuckle Group, Slick Hills, Southwestern Oklahoma. Am. Gregg, J.M., Howard, S.A., Mazzullo, S.J., 1992. Early diagenetic Assoc. Petrol. Geol. Bull. 76, 1649±1664. recrystallization of Holocene Ž.-3000 years old peritidal Gao, G., Land, L.S., Elmore, R.D., 1995. Multiple episodes of dolomites, Ambergis Cay, Belize. Sedimentology 39, 143±160. dolomitization in the Arbuckle Group, Arbuckle Mountains, Gregg, J.M., Laudon, P.R., Woody, R.E., Shelton, K.L., 1993. south-central Oklahoma; field, petrographic, and geochemical Porosity evolution of the Cambrian Bonneterre Dolomite, evidence. J. Sediment. Res., Sect. A 65Ž. 2 , 321±331. southeastern Missouri. Sedimentology 40Ž. 6 , 1153±1169. Garber, R.A., Harris, P.M., Borer, J.M., 1990. Occurrence and Gunatilaka, A., 1989. Spheroidal dolomites; origin by hydrocar- significance of magnesite in Upper PermianŽ. Guadalupian bon seepage? Sedimentology 36Ž. 4 , 701±710. Tansill and Yates formations, Delaware Basin, New Mexico. Gunatilaka, A., Mwango, S., 1987. Continental sabkha pans and Am. Assoc. Petrol. Geol. Bull. 74Ž. 2 , 119±134. associated nebkhas in southern Kuwait, Arabian Gulf. In: Garea, B.B., Braithwaite, C.J.R., 1996. Geochemistry, isotopic Frostrick, L., Reid, I.Ž. Eds. , Desert Sediments: Ancient and composition and origin of the Beda dolomites, block NC74F, Modern. Geol. Soc. Spec. Publ. vol. 35, pp. 187±203. SW Sirt Basin, Libya. J. Petrol. Geol. 19Ž. 3 , 289±304. Gunatilaka, A., Saleh, A., Al-Temeemi, A., Nassar, N., 1987. Garrison, R.E., Graham, S.A., 1984. Early diagenetic dolomites Calcium-poor dolomite from the sabkhas of Kuwait. Sedimen- and the origin of dolomite-bearing breccias, lower Monterey tology 34, 999±1006. formation, Arroyo Seco, Monterey County, California. In: Halley, R.B., Schmoker, J.W., 1983. High porosity Cenozoic Garrison, R.E., Kastner, M., Zenger, D.H.Ž. Eds. , Dolomites rocks of South Florida: progressive loss of porosity with of the Monterey Formation and Other Organic-Rich Units. depth. Am. Assoc. Petrol. Geol. Bull. 67, 191±200. Spec. Publ.-SEPMŽ. Pacific Section, Los Angeles vol. 41, pp. Halley, S.W., Walshe, J.L., 1995. A reexamination of the Mount 87±101. Bischoff cassiterite sulphide skarn, Western Tasmania. Econ. Garven, G., Appold, M.S., Toptygina, V.I., Hazlett, T.J., 1999. Geol. 90Ž. 6 , 1676±1693. Hydrogeologic modeling of the genesis of carbonate-hosted Hanshow, B.B., Back, W., Deike, R.G., 1971. A geochemical lead±zinc ores. Hydrogeol. J. 7, 108±126. hypothesis of dolomitization by ground water. Econ. Geol. 66, Gawthorpe, R.L., 1987. Burial dolomitization and porosity devel- 710±724. opment in a mixed carbonate-clastic sequence: an example Hardie, L.A., 1987. Dolomitization; a critical view of some from Bowland Basin, northern England. Sedimentology 34, current views. J. Sediment. Petrol. 57Ž. 1 , 166±183. 533±558. Hardie, L.A., 1991. On the significance of evaporites. Annu. Rev. Gebelein, C.D., Hoffman, P., 1973. Algal origin of dolomite Earth Planetary Sci. 19, 131±168. lamination in stromatolitic limestone. J. Sediment. Petrol. 43, Hardie, L.A., 1996. Secular variation in seawater chemistry: an 603±611. explanation for the coupled secular variation in the mineralo- 74 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

gies of marine limestones and potash evaporites over the past tization in Sabkha Faishakh, Qatar Ð evidence from changes 600 m.y. Geology 24Ž. 3 , 279±283. in the chemistry of the interstitial brines. J. Sediment. Petrol. Harris, P.M., Walker, S.D., 1990. McElroy field Ð USA Central 63Ž. 6 , 1042±1048. Basin platform Permian basin, Texas. In: Beaumont, E.A., James, N.P., Bone, Y., Kyser, T.K., 1993. Shallow burial dolomi- Foster, N.H.Ž. Eds. , Stratigraphic Traps I: AAPG Treatise of tization and dedolomitization of mid-Cenozoic, cool-water, Petroleum Geology, Atlas of Oil and Gas Fields, Tulsa, OK. calcitic, deep-shelf limestones, southern Australia. J. Sedi- pp. 195±223. ment. Petrol. 63, 528±538. Haynes, F.M., Kesler, S.E., 1994. Relation of mineralization to Jodry, R.L., 1969. Growth and dolomitization of Silurian reefs, St. wall-rock alteration and brecciation, Mascot Jefferson City Clair County, Michigan. Am. Assoc. Petrol. Geol. Bull. 53, Mississippi Valley Type district, Tennessee. Econ. Geol. 89 957±981. Ž.1 , 51±66. Johnson, R.A., Budd, D.A., 1994. The utility of continued reser- Heroux, Y., Chagnon, A., Savard, M., 1996. Organic matter and voir description: an example from Bindley field, western clay anomalies associated with base-metal sulphide deposits. Kansas. Am. Assoc. Petrol. Geol. Bull. 78, 722±743. Ore Geol. Rev. 11Ž. 1±3 , 157±173. Jones, H.D., Kesler, S.E., Furman, F.C., Kyle, J.R., 1996. Sulfur Hewett, D.F., 1928. Dolomitization and ore deposition. Econ. isotope geochemistry of southern Appalachian Mississippi Geol. 23, 821±863. Valley-Type deposits. Econ. Geol. 91Ž. 2 , 355±367.

Hill, C.A., 1995. H2 S-related porosity and sulfuric acid oil-field Kahle, C.F., 1965. Possible roles of clay minerals in the formation karst. In: Budd, D.A., Saller, A.H., Harris, P.M.Ž. Eds. , Un- of dolomite. J. Sediment. Petrol. 35, 448±453. conformities and Porosity in Carbonate Strata. AAPG Mem. Kaldi, J., Gidman, J., 1982. Early diagenetic dolomite cements: vol. 63, pp. 301±306. examples from the Permian Lower Magnesian Limestone of Hitzman, M.W., Allan, J.R., Beaty, D.W., 1998. Regional dolomi- England and the Pleistocene carbonates of the Bahamas. J. tization of the Waulsortian limestone in southeastern Ireland: Sediment. Petrol. 52, 1073±1085. evidence of large scale fluid flow driven by the Hercynian Kamali, M.R., Lemon, N.M., Apak, S.N., 1995. Porosity genera- orogeny. Geology 26Ž. 6 , 547±550. tion and reservoir potential of Ouldburra Formation carbon- Holland, H.D., Horita, J., Seyfried, W., 1996. On the secular ates, Officer Basin, South Australia. APEA J. 35, 106±120. variations in the composition of Phanerozoic marine potash Karajas, J., Kernick, N., 1984. A prospective Nita Formation evaporites. Geology 24Ž. 11 , 993±996. reservoir trend on the Broome Platform. In: Purcell, P.G. Horn, M.K., 1990. Renqui field. In: Beaumont, E.A., Foster, N.H. Ž.Ed. , The Canning Basin, Western Australia. Proceedings Ž.Eds. , Structural Traps II: AAPG Treatise of Petroleum Geol- Geological Society of Australia and Petroleum Exploration ogy, Atlas of Oil and Gas Fields. pp. 227±252. Society of Australia, Canning Basin Symposium, Perth, WA, Hsu, K.J., 1967. Chemistry of dolomite formation. Elsevier Dev. pp. 169±177. Sediment. 9b, 169±191. Kastner, M., 1984. Control of dolomite formation. Nature 311 Hsu, K.J., Siegenthaler, C., 1969. Preliminary experiments on Ž.5985 , 410±411. hydrodynamic movement induced by evaporation and their Kaufman, J., 1994. Numerical models of fluid flow in carbonate bearing on the dolomite problem. Sedimentology 12, 448±453. platforms Ð implications for dolomitization. J. Sediment. Hsu, K.J., Schneider, J., 1973. Progress report on dolomitization- Res., Sect. A 64Ž. 1 , 128±139. hydrology of Abu Dhabi Sabkhas, Arabian Gulf. The Persian Kaufman, J., Harrison, G.N., Meyers, W.J., 1991. Dolomitization Gulf. Springer, New York, pp. 409±422. of the Swan Hills Formation, Rosevear field, Alberta, Canada. Hu, B.L., 1992. Petroleum geology and prospects of the Tarim Sedimentology 38, 41±66. basin, China. In: Halbouty, M.T.Ž. Ed. , Giant Oil and Gas Kelts, K.R., Mackenzie, J.A., 1982. Diagenetic dolomite forma- Fields of the Decade 1978±1988. AAPG Mem. vol. 54, pp. tion in Quaternary anoxic diatomaceous muds of Deep Sea 493±510. Drilling Project Leg 64, Gulf of California. Initial Rep. Deep Hull, C.E., Warman, H.R., 1979. Asmari oil fields of Iran. Am. Sea Drill. Proj. 64Ž. 2 , 553±569. Assoc. Petrol. Geol. Bull. 54, 428±437. Kendall, A.C., 1988. Aspects of evaporite basin stratigraphy. In: Humphrey, J.D., Quinn, T.M., 1989. Coastal mixing zone Schreiber, B.C.Ž. Ed. , Evaporites and Hydrocarbons. Columbia dolomite, forward modeling, and massive dolomitization of University Press, New York, pp. 11±65. platform-margin carbonates. J. Sediment. Petrol. 59, 438±454. Kendall, A.C., 1989. Brine mixing in the Middle Devonian of Hurley, N.F., Budros, R., 1990. Albion-Scipio and Stoney Point Western Canada and its possible significance to regional fields, U.S.A., Michigan Basin. In: Beaumont, E.A., Foster, dolomitization. Sediment. Geol. 64, 271±285. N.H.Ž. Eds. , Stratigraphic Traps I: AAPG Treatise of Petroleum Kendall, A.C., 1992. Evaporites. In: Walker, R.G., James, N.P. Geology, Atlas of Oil and Gas Fields, Tulsa, OK. pp. 1±37. Ž.Eds. , Facies Models: Responses to Sea Level Change. Geo- Illing, L.V., Wells, A.J., Taylor, J.C.M., 1965. Penecontempora- logical Association of Canada, pp. 375±409. neous dolomite in the Persian Gulf. In: Pray, L.C., Murray, Kerans, C., 1988. Karst-controlled reservoir heterogeneity in El- R.C.Ž. Eds. , Dolomitization and Limestone Diagenesis. Spec. lenberger Group carbonates of west Texas. Am. Assoc. Petrol. Publ.-Soc. Econ. Paleont. Mineral. vol. 13, pp. 89±111. Geol. Bull. 72, 1160±1183. Illing, L.V., Taylor, J.C.M., 1993. Penecontemporaneous dolomi- Khalaf, F.I., 1990. Occurrence of phreatic dolocrete within Ter- J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 75

tiary clastic deposits of Kuwait, Arabian Gulf. Sediment. Geol. ern, Holocene, and Pleistocene occurrences. Earth Sci. Rev. 27 68Ž. 3 , 223±239. Ž.3 , 221±263. Kinsman, D.J.J., 1965. Dolomitization and evaporite development, Leary, D.A., Vogt, J.N., 1986. Diagenesis of the San Andres including anhydrite, Lagoonal Sediments, Persian Gulf. Geo- FormationŽ. Guadalupian , Central Basin Platform, Permian logical Society of America. Basin. In: Bebout, D.G., Harris, P.M.Ž. Eds. , Hydrocarbon Kocurko, M.J., 1979. Dolomitization by spray-zone brine seepage, Reservoir Studies San AndresrGrayburg Formations, Permian San Andres, Colombia. J. Sediment. Petrol. 49, 209±214. Basin. Spec. Publ.-SEPM vol. 26, pp. 67±68. Kontak, D.J., 1998. A study of fluid inclusions in sulphide and Lee, Y.I., Friedman, G.M., 1987. Deep-burial dolomitization in nonsulphide mineral phases from a carbonate-hosted Zn±Pb the Ordovician Ellenberger Group carbonates, west Texas and deposit, Gays River, Nova Scotia, Canada. Econ. Geol. 93Ž. 6 , southeastern New Mexico. J. Sediment. Petrol. 57, 544±577. 793±816. Lee, Y.I., Friedman, G.M., 1988. Deep-burial dolomitization in Kopaska-Merkel, D.C., Friedman, G.M., 1989. Petrofacies analy- the Ordovician Ellenberger Group carbonates, west Texas and sis of carbonate rocks; example from lower Paleozoic Hunton southeastern New Mexico Ð reply. J. Sediment. Petrol. 58, Group of Oklahoma and Texas. Am. Assoc. Petrol. Geol. Bull. 910±913. 73Ž. 11 , 1289±1306. Leighton, M.W., Pendexter, C., 1962. Carbonate rock types. Kretz, R., 1992. Carousel model for the crystallization of saddle AAPG Mem. 33±61. dolomite. J. Sediment. Petrol. 62, 190±195. Lindsay, R.F., Kendall, C.G.St.C., 1985. Depositional facies, dia- Krumbein, W.E., 1979. Calcification by bacteria and algae. In: genesis and reservoir character of Mississippian cyclic carbon- Trudinger, P.A., Swaine, D.J.Ž. Eds. , Biogeochemical Cycling ates in the Mission Canyon Formation, Little Knife field, of Mineral-Forming Elements. Elsevier, Amsterdam, pp. 47± Williston Basin, North Dakota. In: Roehl, P.O., Choquette, 62. P.W.Ž. Eds. , Carbonate Petroleum Reservoirs. Springer, New Kupecz, J.A., Land, L.S., 1994. Progressive recrystallization and York, pp. 175±190. stabilization of early-stage dolomite: lower Ordovician Ellen- Lippmann, F., 1973. Sedimentary Carbonate Minerals. Springer, burger Group, west Texas. In: Purser, B., Tucker, M., Zenger, New York. D.Ž. Eds. , Dolomites Ð A Volume in Honor of Dolomieu. Lippmann, F., 1982. Stable and metastable solubility diagrams for

Spec. Publ. Int. Assoc. Sedimentol. vol. 21. Blackwell Scien- the system CaCO332 ±MgCO ±H O at ordinary temperatures. tific Publications, Cambridge, pp. 255±279. Bull. Mineral. 105, 273±279. Land, L.S., 1973. Holocene meteoric dolomitization of Pleis- Lopez-Gomez, J., Mas, R., Arche, A., 1993. The evolution of the tocene limestones, North Jamaica. Sedimentology 20, 411± Middle TriassicŽ. Muschelkalk carbonate ramp in the SE 424. Iberian Ranges, eastern Spain: sequence stratigraphy, dolomi- Land, L.S., 1980. The isotopic and trace element geochemistry of tization processes and dynamic controls. Sediment. Geol. 87 dolomite: the state of the art. In: Zenger, D.H., Dunham, J.B., Ž.3±4 , 165±193. Ethington, R.L.Ž. Eds. , Concepts and Models of Dolomitiza- Longman, M.W., Schnidtman, K.H., 1985. Deposition and diagen- tion. Spec. Publ.-SEPM vol. 28, pp. 87±110. esis of Mississippian Charles ACB Ž.Ratcliffe reservoir in Land, L.S., 1984. Frio Sandstone diagenesis, Texas Gulf Coast. Lustre field, Valley County, Montana. In: Longman, M.W., In: McDonald, D.A., Surdam, R.C.Ž. Eds. , Clastic Diagenesis. Shanley, K.W., Lindsay, R.F., Eby, D.E.Ž. Eds. , Rocky Moun- AAPG Mem. vol. 37, pp. 47±62. tain Carbonate Reservoirs. SEPM Core Workshop, pp. 265± Land, L.S., 1985. The origin of massive dolomite. J. Geol. Educ. 310. 33, 112±125. Longman, M.W., Fertal, T.G., Stell, J.R., 1992. Reservoir perfor- Land, L.S., 1991. Dolomitization of the Hope Gate Formation mance in Ordovician Red River Formation, Horse Creek and Ž.North Jamaica by seawater: reassessment of mixing zone South Horse Creek fields, Bowman County, North Dakota. dolomite. In: Taylor, H.P., O'Neil, J.R., Kaplan, I.R.Ž. Eds. , Am. Assoc. Petrol. Geol. Bull. 76, 449±467. Stable Isotope Geochemistry: A Tribute to Samuel Epstein. Loucks, R.G., Anderson, J.H., 1985. Depositional facies, diage- Geochem. Soc. Spec. Publ. vol. 3, pp. 121±133. netic terranes and porosity development in Lower Ordovician Land, L.S., 1998. Failure to precipitate dolomite at 25 degrees C Ellenberger Dolomite, Puckett field, west Texas. In: Roehl, from dilute solution despite 1000-fold oversaturation after 32 P.O., Choquette, P.W.Ž. Eds. , Carbonate Petroleum Reser- years. Aquat. Geochem. 4Ž. 3±4 , 361±368. voirs. Springer, New York, pp. 19±37. Land, L.S., Hoops, G.K., 1973. Sodium in carbonate sediments Lucia, F.J., 1968. Recent sediments and diagenesis of south and rocks: a possible index to the salinity of diagenetic Bonaire, Netherlands Antilles. J. Sediment. Petrol. 845±858. solutions. J. Sediment. Petrol. 43, 614±617. Lucia, F.J., 1972. Recognition of evaporite±carbonate shoreline Land, L.S., Salem, M., Morrow, D., 1975. Paleohydrology of sedimentation. In: Rigby, J.K., Hamblin, W.K.Ž. Eds. , Recog- ancient dolomites: geochemical evidence. Am. Assoc. Petrol. nition of Ancient Sedimentary Environments. Spec. Publ.-Soc. Geol. Bull. 59, 1602±1625. Econ. Paleontol. Mineral. vol. 16, pp. 190±191. Landes, K.K., 1946. Porosity through dolomitization. Am. Assoc. Lucia, F.J., 1983. Petrophysical parameters estimated from visual Petrol. Geol. Bull. 30, 305±318. description of carbonate rocks: a field classification of pore Last, W.M., 1990. Lacustrine dolomite Ð an overview of mod- space. J. Petrol. Technol. 35, 626±637,Ž. March . 76 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Lucia, F.J., 1995. Rock-fabricrpetrophysical classification of car- Mawson, S.D., 1929. Some South Australian limestones in the bonate pore space for reservoir characterization. Am. Assoc. process of formation. Q. J. Geol. Soc. London 85, 613±623. Petrol. Geol. Bull. 79, 1275±1300. Mazurov, M.P., Titov, A.T., 1999. Magnesian skarns from the Lucia, F.J., 1999. Carbonate Reservoir Characterization. Springer, sites of layer-by-layer injections of basic magma into the Berlin, 226 pp. evaporites of the platform's sedimentary cover. Geol. Geofiz. Lucia, F.J., Major, R.P., 1994. Porosity evolution through hyper- 40Ž. 1 , 82±89, Russian. saline reflux dolomitization. In: Purser, B., Tucker, M., Zenger, Mazzullo, S.J., Reid, A.M., Gregg, J.M., 1987. Dolomitization of D.Ž. Eds. , Dolomites Ð A Volume in Honor of Dolomieu. Holocene supratidal deposits, Ambergis Cay, Belize. Geol. Spec. Publ.-Int. Assoc. Sedimentol. vol. 21. Blackwell Scien- Soc. Am. Bull. 98, 224±231. tific Publications, Cambridge, pp. 325±341. Mazzullo, S.J., Bischoff, W.D., Teal, C.S., 1995. Holocene shal- Lumsden, D.N., 1988. Characteristics of deep marine dolomite. J. low-subtidal dolomitization by near-normal seawater, northern Sediment. Petrol. 58, 1023±1051. Belize. Geology 23Ž. 4 , 341±344. Lumsden, D.N., Chimahusky, J.S., 1980. Relationship between McCullough, M.L., Land, L.S., 1992. Diagenetic history of a dolomite nonstoichiometry and carbonate facies parameters. submerged Pleistocene Jamaican fringing reef. Carbonates Spec. Publ.-SEPM 28, 123±138. Evaporites 7, 2±10. Machel, H.G., 1987. Saddle dolomite as a by-product of chemical McManus, A., Wallace, M.W., 1992. Age of Mississippi Valley compaction and thermochemical sulfate reduction. Geology 15 type sulfides determined using cathodoluminesence cement Ž.10 , 936±940. stratigraphy, Lennard Shelf, Canning Basin, Western Aus- Machel, H.G., Anderson, J.H., 1989. Pervasive subsurface dolomi- tralia. Econ. Geol. 87, 189±193. tization of the Nisku Formation in central Alberta. J. Sedi- McQuillan, H., 1985. Fracture-controlled production from the ment. Petrol. 59, 891±911. Oligo-Miocene Asmari Formation in Gachsaran and Bibi Machel, H.G., Burton, E.A., 1991. Burial-diagenetic sabkha-like Hakimeh fields, southwest Iran. In: Roehl, P.O., Choquette, gypsum and anhydrite nodules. Chem. Geol. 90, 211±231. P.W.Ž. Eds. , Carbonate Petroleum Reservoirs. Springer, New Machel, H.G., Burton, E.A., 1994. Golden Grove Dolomite, Bar- York, pp. 511±523. bados; origin from modified seawater. J. Sediment. Res., Sect. Medlin, W.L., 1959. Preparation of synthetic dolomite. Am. Min- A 64Ž. 4 , 741±751. eral. 44, 979±986. Machel, H.G., Mountjoy, E.W., 1986. Chemistry and environ- Meinert, L.D., 1993. Igneous petrogenesis and skarn deposits. In: ments of dolomitization Ð a reappraisal. Earth Sci. Rev. 23, Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., Duke, J.M. 175±222. Ž.Eds. , Mineral Deposit Modeling. Geol. Assoc. Can. Spec. Mackenzie, J., 1981. Holocene dolomitzation of calcium carbon- Paper vol. 40, pp. 569±583. ate sediments from the coastal sabkhas of Abu Dhabi, U.A.E. Metherell, R.G., Workman, L.E., 1969. Geology of Crossfield J. Geol. 89, 185±198. East and Lone Pine Creek gas field, Alberta. Can. Soc. Petrol. Mackenzie, J., Hsu, K.J., Schneider, J.F., 1980. Movement of Geol. Bull. 17, 92±108. subsurface waters under the sabkha, Abu Dhabi, UAE and its Meyers, W.J., Lu, F.H., Zachariah, J.K., 1997. Dolomitization by relation to evaporative dolomite genesis. Spec. Publ.-SEPM mixed evaporative brines and freshwater, upper Miocene car- 28, 11±30. bonates, Nijar, Spain. J. Sediment. Res., Sect. A 67Ž. 5 , Magaritz, M., Goldenberg, L., Kafri, U., Arad, A., 1980. Dolomite 898±912. formation in the seawater±freshwater interface. Nature 287, Middleburg, J.J., de Lange, G.J., Kreulen, R., 1990. Dolomite 622±624. formation in anoxic sediments of Kau Bay, Indonesia. Geol- Magaritz, M., Peryt, T.M., 1994. Mixed evaporative and meteoric ogy 18, 399±402. water dolomitization; isotope study of the Zechstein Lime- Middleton, K., Coniglio, M., Sherlock, R., Frape, S.K., 1993. stoneŽ. Upper Permian , southwestern Poland. Sediment. Geol. Dolomitization of Middle Ordovician carbonate reservoirs, 92, 257±272. southwestern Ontario. Bull. Can. Petrol. Geol. 41Ž. 2 , 150±163. Maiklem, W.R., 1971. Evaporative drawdown Ð a mechanism Miller, J.A., 1985. Depositional and reservoir facies of the Missis- for water level lowering and diagenesis in the Elk Point Basin. sippian Leadville Formation, Northwest Lisbon field, Utah. In: Bull. Can. Assoc. Petrol. Geol. 19, 487±503. Roehl, P.O., Choquette, P.W.Ž. Eds. , Carbonate Petroleum Malone, M.J., Baker, P.A., Burns, S.J., 1994. Recrystallization of Reservoirs. Springer, New York, pp. 161±173. dolomite Ð evidence from the Monterey FormationŽ. Miocene . Milliken, K.L., Land, L.S., Loucks, R.G., 1981. History of burial Sedimentology 41Ž. 6 , 1223±1239. diagenesis determined from isotopic geochemistry, Brazoria Mattavelli, L., Margarucci, V., 1990. Malossa field, Italy Po County, Texas. Am. Assoc. Petrol. Geol. Bull. 65, 1397±1413. basin. In: Beaumont, E.A., Foster, N.H.Ž. Eds. , Structural Miser, D.E., Swinnea, J.S., Steinfink, H., 1987. TEM observations Traps VII: AAPG Treatise of Petroleum Geology, Atlas of Oil and X-ray structure refinement of a twinned dolomite mi- and Gas Fields. pp. 119±137. crostructure. Am. Mineral. 72, 188±193. Mattes, B.W., Conway, M.S., 1990. Carbonaterevaporite deposi- Mitchell, J.T., Land, L.S., Miser, D.E., 1987. Modern marine tion in the Late Precambrian±Early Cambrian Ara Formation dolomite cement in a north Jamaican fringing reef. Geology of southern Oman. In: Robertson, A.H.F., Searle, M.P., Ries, 15, 557±560. A.C.Ž. Eds. , The Geology and Tectonics of the Oman Region. Montanez,Ä I.P., 1994. Late diagenetic dolomitization of Lower Geol. Soc. Spec. Publ. vol. 49, pp. 617±636. Ordovician, Upper Knox carbonates: a record of the hydrody- J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 77

namic evolution of the south Appalachian Basin. Am. Assoc. Murray, R.C., 1960. Origin of porosity in carbonate rocks. J. Petrol. Geol. Bull. 78Ž. 8 , 1210±1239. Sediment. Petrol. 30, 59±84. Monty, C.L.V., Rouchy, J.M., Maurin, A., Bernet-Rollande, M.C., Murray, R.C., 1969. Hydrology of South Bonaire, Netherlands Perthuisot, J.P., 1987. Reef-stromatolites-evaporites facies re- Antilles; a rock selective dolomitization model. J. Sediment. lationships from middle Miocene examples of the Gulf of Petrol. 39, 1007±1013. Suez and the Red Sea. In: Peryt, T.M.Ž. Ed. , Evaporite Basins. Muller,È S., Teitz, G., 1971. Dolomite replacing Acement AB in Lect. Notes Earth Sci. vol. 13. Springer, Berlin, pp. 133±188. biocalcarenites from Fuerteventura, Canary Islands, Spain. In: Moore, C.H., Chowdhury, A., Chan, L., 1988. Upper Jurassic Bricker, D.P.Ž. Ed. , Carbonate Cements. John Hopkins Press, Smackover dolomitization, Gulf of Mexico; a tale of two Baltimore, 376. waters. In: Skula, V., Baker, P.A.Ž. Eds. , Sedimentology and Nesbitt, B.E., Muehlenbachs, K., 1994. Paleohydrology of the Geochemistry of Dolostones. Spec. Publ.-SEPM vol. 43. Canadian Rockies and origins of brines, Pb±Zn deposits and Morgan, W.A., 1985. Silurian reservoirs in upward-shoaling cy- dolomitization in the Western Canada Sedimentary Basin. cles of the Hunton Group, Mt. Everette and southwest Reeding Geology 22, 243±246. fields, Kingfisher County, Oklahoma. In: Roehl, P.O., Cho- Nesbitt, B.E., Prochaska, W., 1998. Solute chemistry of inclusion quette, P.W.Ž. Eds. , Carbonate Petroleum Reservoirs. Springer, fluids from sparry dolomites and magnesites in Middle Cam- New York, pp. 107±120. brian carbonat rocks of the southern Canadian Rocky Moun- Morrow, D.W., 1982. Diagenesis 2; Dolomite: Part 2. Dolomitiza- tains. Can. J. Earth Sci. 35Ž. 5 , 546±555. tion models and ancient dolostones. Geosci. Can. 9Ž. 2 , 95± Nicolaides, S., 1995. Origin and modification of Cambrian 107. dolomitesŽ. Red Heart Dolomite and Arthur Creek Formation , Morrow, D., 1998. Regional subsurface dolomitization: models Georgina Basin, central Australia. Sedimentology 42Ž. 2 , 249± and constraints. Geosci. Can. 25Ž. 2 , 57±70. 266. Morrow, D.W., Ricketts, B.D., 1988. Experimental investigation Nielsen, P., Swennen, R., Dickson, J.A.D., Fallick, A.E., Kep- of sulfate inhibition of dolomite and its mineral analogues. In: pens, E., 1997. Spheroidal dolomites in a Visean karst system Shukla, V., Baker, P.A.Ž. Eds. , Sedimentology and Geochem- Ð bacterial induced origin. Sedimentology 44Ž. 1 , 177±195. istry of Dolostones. Spec. Publ.-SEPM vol. 43, pp. 27±38. Ohde, S., Kitano, Y., 1981. Protodolomite in Daito-jima, Oki- Morse, J.W., Casey, W.H., 1988. Ostwald processes and mineral nawa. Geochem. J. 15, 199±207. paragenesis in sediments. Am. J. Sci. 288, 537±560. Oliver, J., 1986. Fluids expelled tectonically from orogenic belts: Morse, J.W., Mackenzie, F.T., 1990. Geochemistry of Sedimen- their role in hydrocarbon migration and other geologic phe- tary Carbonates. Elsevier, Amsterdam, 707 pp. nomena. Geology 14, 99±102. Mountjoy, E.W., Halim-Dihardja, M.K., 1991. Multiple phase Patterson, R.J., Kinsman, D.J.J., 1981. Hydrologic framework of a fracture and fault controlled burial dolomitization, Upper De- sabkha along the Arabian Gulf. Am. Assoc. Petrol. Geol. Bull. vonian Wabamum Group, Alberta. J. Sediment. Petrol. 61, 65, 1457±1475. 590±612. Patterson, R.J., Kinsman, D.J.J., 1982. Formation of diagenetic Mountjoy, E.W., Amthor, J.E., 1994. Has burial dolomitization dolomite in coastal sabkhas along the ArabianŽ. Persian Gulf. come of age? Some answers from the West Canada Sedimen- Am. Assoc. Petrol. Geol. Bull. 66, 28±43. tary Basin. In: Purser, B., Tucker, M., Zenger, D.Ž. Eds. , Peryt, T.M., Scholle, P.A., 1996. Regional setting and role of Dolomites Ð A Volume in Honor of Dolomieu. Spec. Publ.- meteoric water in dolomite formation and diagenesis in an Int. Assoc. Sedimentol. vol. 21, pp. 203±229. evaporite basin; studies in the ZechsteinŽ. Permian deposits of Mountjoy, E.W., Machel, H.G., Green, D., Duggan, J., Williams- Poland. Sedimentology 43Ž. 6 , 1005±1023. Jones, A.E., 2000. Devonian matrix dolomites and deep burial Peterson, J.A., 1983. Petroleum geology and resources of south- carbonate cements: a comparison between the Rimbey- eastern Mexico, northern Guatemala, and Belize. Geol. Surv. Meadowbrook reef trend and the deep basin of west-central Circ.Ž. U. S. 760, 44 pp. Alberta. Bull. Can. Petrol. Geol. 47Ž. 4 , 487±509. Peterson, J.A., 1992. Aneth field Ð USA Paradox basin, Utah. In: Mriheel, I.Y., Anketell, J.M., 1995. Origin of the mid-Ypresian Foster, N.H., Beaumont, E.A.Ž. Eds. , Stratigraphic Traps III: Jirani Dolomite; a major reservoir rock in the NW Libya AAPG Treatise of Petroleum Geology, Atlas of Oil and Gas offshore. J. Petrol. Geol. 18Ž. 4 , 439±451. Fields. pp. 41±81. Muchez, P., Viaene, W., 1994. Dolomitization caused by water Petta, T.J., Rapp, S.D., 1990. Appleton field Ð USA Gulf of circulation near the mixing zone: an example from the Lower Mexico basin, Alabama. In: Beaumont, E.A., Foster, N.H. ViseanÂ of the Campine BasinŽ. northern Belgium . In: Purser, Ž.Eds. , Structural Traps IV: AAPG Treatise of Petroleum B., Tucker, M., Zenger, D.Ž. Eds. , Dolomites Ð A Volume in Geology, Atlas of Oil and Gas Fields. pp. 299±318. Honor of Dolomieu. Spec. Publ.-Int. Assoc. Sedimentol. vol. Pilkey, O.H., 1966. Carbonate and clay mineralogy of the Persian 21. Blackwell Scientific Publications, Cambridge, pp. 155± Gulf. Deep Sea Res. 13, 1±16. 166. Phipps, G.G., 1989. Exploring for dolomitized Slave Point car- Mullins, H.T., Dix, G.R., Gardulski, A.F., Land, L.S., 1988. bonates in northeastern British Columbia. Geophysics 54, Neogene deep-water dolomite from the Florida±Bahamas Plat- 806±814. form. In: Shukla, V., Baker, P.A.Ž. Eds. , Sedimentology and Powers, R.W., 1962. Arabian Upper Jurassic carbonate reservoirs. Geochemistry of Dolostones, Based on a Symposium. Spec. In: Ham, W.E.Ž. Ed. , Classification of Carbonate Rocks. AAPG Publ.-SEPM vol. 43, pp. 235±243. Mem. pp. 122±192. 78 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Purvis, K., 1989. Zoned authigenic magnesites in the Rotliegend Roehl, P.O., Weinbrandt, R.M., 1985. West Cat Canyon Field. In: Lower Permian, southern North Sea. Sediment. Geol. 65Ž. 3±4 , Roehl, P.O., Choquette, P.W.Ž. Eds. , Carbonate Petroleum 307±318. Reservoirs. Springer, New York, pp. 525±545. Qing, H., Mountjoy, E.W., 1994a. Formation of coarsely crys- Rosen, M.R., Miser, D.E., Warren, J.K., 1988. Sedimentology, talline, hydrothermal dolomite reservoirs in the Presquile Bar- mineralogy and isotopic analysis of Pellet Lake, Coorong rier, Western Canada Sedimentary Basin. Am. Assoc. Petrol. region, South Australia. Sedimentology 35Ž. 1 , 105±122. Geol. Bull. 78Ž. 1 , 55±77. Rosen, M.R., Miser, D.E., Starcher, M.A., Warren, J.K., 1989. Qing, H.U., Mountjoy, E.W., 1994b. Rare earth element geochem- Formation of dolomite in the Coorong region, South Australia. istry of dolomites in the Middle Devonian Presqu'ile barrier, Geochim. Cosmochim. Acta 53Ž. 3 , 661±669. Western Canada Sedimentary Basin: implications for fluid-rock Rougerie, F., Wauthy, B., 1988. The endo-upwelling concept: a ratios during dolomitization. Sedimentology 41Ž. 4 , 787±804. new paradigm for solving an old paradox. Proceedings of the Qing, H., Mountjoy, E.W., 1994c. Origin of dissolution vugs, Sixth International Reef Symposium Ð Australia. pp. 1±6. caverns, and breccias in the Middle Devonian Presqu'ile bar- Runnells, D.D., 1969. Diagenesis, chemical sediments, and the rier, host of Pine Point Mississippi Valley-type deposits. Econ. mixing of natural waters. J. Sediment. Petrol. 39, 1188±1201. Geol. 89Ž. 4 , 858±876. Ruppel, S.C., Cander, H.S., 1988. Dolomitization of shallow QGPCŽ Qatar General Petroleum Corporation and Amoco Qatar water carbonates by seawater and seawater-derived brines, San Petroleum Company. , 1991. Dukhan field. In: Foster, N.H., Andres FormationŽ. Guadalupian , West Texas. In: Shukla, Beaumont, E.A.Ž. Eds. , Structural Traps V: AAPG Treatise of V.J., Baker, P.A.Ž. Eds. , Sedimentology and Geochemistry of Petroleum Geology, Atlas of Oil and Gas Fields. pp. 103±120. Dolostones. Spec. Publ.-SEPM vol. 43, pp. 245±262. Radke, B.M., Mathis, R.L., 1980. On the formation and occur- Ruzla, K., Friedman, G.M., 1985. Factors controlling porosity in rence of saddle dolomite. J. Sediment. Petrol. 50Ž. 4 , 1149± dolomite reservoirs of the Ordovician Red River Formation, 1168. Cabin Creek field, Montana. In: Roehl, P.O., Choquette, P.W. Raiswell, R., 1982. Pyrite texture, isotopic composition and the Ž.Eds. , Carbonate Petroleum Reservoirs. Springer, New York, availability of iron. Am. J. Sci. 282Ž. 8 , 1244±1263. pp. 39±58. Ramsey, D.W., Onasch, C.M., 1999. Fluid migration in a cratonic Saller, A.H., 1984. Petrologic and geochemical constraints on the setting: the fluid histories of two fault zones in the eastern origin of subsurface dolomite, Enewetak Atoll; an example of midcontinent. Tectonophysics 305Ž. 1±3 , 307±323. dolomitization by normal seawater. Geology 12Ž. 4 , 217±220. Randell, M.W., 1994. Geology of the Polaris Pb±Zn Mississippi Saller, A.H., Van Hoorn, D., Miller, J.A., Guy, B.T., 1991. Valley-Type deposit, Canadian Arctic Archipelago. Doctoral Reservoir geology of Devonian carbonates and chert Ð impli- thesis, University of TorontoŽ. Canada . cations for Tertiary recovery, Dollarhide field, Andrews Reeder, R.J., 1981. Electron optical investigation of sedimentary County, Texas. Am. Assoc. Petrol. Geol. Bull. 75, 86±107. dolomites. Contrib. Mineral. Petrol. 76, 148±157. Saller, A.H., Henderson, N., 1998. Distribution of porosity and Reeder, R.J., 1983. Crystal chemistry of rhombohedral carbonates. permeability in platform dolomites Ð insight from the Per- Mineral. Soc. Am.: Rev. Mineral. 11, 1±48. mian of west Texas. Am. Assoc. Petrol. Geol. Bull. 82Ž. 8 , Reeder, R.J., Prosky, J.L., 1986. Compositional sector zoning in 1528±1550. dolomite. J. Sediment. Petrol. 56, 237±247. Sangster, D.F., Nowlan, G.S., McCracken, A.D., 1994. Thermal Reeder, R.J., Sheppard, C.E., 1982. Variation of lattice parameters comparison of Mississippi Valley-Type lead±zinc deposits and in some sedimentary dolomite. Am. Mineral. 69, 520±527. their host rocks using fluid inclusion and conodont alteration Reinhold, C., 1998. Multiple episodes of dolomitization and index data. Econ. Geol. 89Ž. 3 , 493±514. dolomite recrystallization during shallow burial in Upper Santiago, J., Baro, A., 1992. Mexico's giant fields, 1978±1988 Jurassic shelf carbonates: eastern Swabian Alb, southern Ger- decade. In: Halbouty, M.T.Ž. Ed. , Giant Oil and Gas Fields of many. Sediment. Geol. 121Ž. 1±2 , 71±95. the Decade 1978±1988. AAPG Mem. pp. 73±99. Riviere, A., 1939. Sur la dolomitization des sediments calcaires. Sass, E., Bein, A., 1988. Dolomites and salinity; a comparative Comptes Rendus 209, 597±599. geochemical study. In: Shukla, V., Baker, P.A.Ž. Eds. , Sedi- Rizzi, G., Braithwaite, C.J.R., 1997. Sedimentary cycles and mentology and geochemistry of dolostones, based on a sympo- selective dolomitization in limestones hosting the giant Navan sium. Spec. Publ.-SEPM vol. 43, pp. 223±233. Pb±Zn ore deposit, Ireland. Explor. Min. Geol. 6Ž. 1 , 63±77. Savard, M.M., 1996. Pre-ore burial dolomitization adjacent to the Rodgers, K.A., Easton, A.J., Downes, C.J., 1982. The chemistry carbonate-hosted Gays River Zn±Pb deposit, Nova Scotia. of carbonate rocks of Niue Island, South Pacific. J. Geol. 90, Can. J. Earth Sci. 33Ž. 2 , 302±315. 645±662. Schmidt, V., McIlreath, I.A., Budwill, A.E., 1985. Origin and Roedder, E.S., 1984. Fluid inclusions. Mineral. Soc. Am.: Rev. diagenesis of Middle Devonian pinnacle reefs encased in Mineral. 12, 646 pp. evaporites, AAB and AEB pools, Rainbow field, Alberta. In: Roehl, P.O., 1985. Depositional and diagenetic controls on reser- Roehl, P.O., Choquette, P.W.Ž. Eds. , Carbonate Petroleum voir rock development and petrophysics in Silurian tidalites, Reservoirs. Springer, New York, pp. 141±160. Interlake Formation, Cabin Creek field area, Montana. In: Schmoker, J.W., Krystinik, K.B., Halley, R.B., 1985. Selected Roehl, P.O., Choquette, P.W.Ž. Eds. , Carbonate Petroleum characteristics of limestone and dolomite reservoirs in the Reservoirs. Springer, New York, pp. 85±105. United States. Am. Assoc. Petrol. Geol. Bull. 69Ž. 5 , 733±741. J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 79

Schramm, M.W., Livraga, G., 1986. Vega field and the potential an arid, continental diagenetic facies. Sedimentology 39Ž. 6 , of the Ragusa Basin, offshore Sicily. In: Halbouty, M.T.Ž. Ed. , 1119±1136. Future Petroleum Provinces of the World. AAPG Mem. pp. Steidtmann, E., 1926. The origin of dolomites. In: Twenhofel, 559±566. W.H.Ž. Ed. , Treatise on Sedimentation. Williams and Wilkins, Schreiber, B.C., 1988. Evaporites and Hydrocarbons. Columbia Baltimore, MD, pp. 256±265. University Press, New York, 475 pp. Strakhov, N.M., 1958. Facts and hypotheses in the problem of the Sears, S.O., Lucia, F.J., 1980. Dolomitization of Northern Michi- formation of dolomite rocks. Izv. Akad. Nauk S.S.S.R. Ser. gan Niagara Reefs by Brine Refluxion and Freshwaterr Geol. 6, 3±22. Seawater Mixing. Spec. Publ.-SEPM vol. 28, pp. 215±235. Suchy, V., Rozkosny, I., Zak, K., Francu, J., 1996. Epigenetic Shields, M.J., Brady, P.V., 1995. Mass balance and fluid flow dolomitization of the Pridoli FormationŽ. Upper Silurian , the constraints on regional-scale dolomitization, Late Devonian, Barrandian Basin, Czech Republic Ð implications for burial Western Canada Sedimentary Basin. Bull. Can. Petrol. Geol. history of Lower Palaeozoic strata. Geol. Rundsch. 85Ž. 2 , 43Ž. 4 , 371±392. 264±277. Shikazono, N., Hoshino, M., Utada, M., Nakata, M., Ueda, A., Suess, E., von Huene, R., Emeis, K.-C., Bourgois, J., Cruzado 1998. Hydrothermal carbonates in altered wall rocks at the Castaneda, J.d.C., De Wever, P., Eglinton, G., Fernandez, Uwamuki Kuroko deposits, Japan. Mineral. Deposita 33Ž. 4 , A.W.S., Garrison, R., Greenberg, M., Hill, P.R., Ibaraki, M., 346±358. Kastner, M., Kemp, A.E.S., Kvenvolden, K.A., Langridge, R., Shinn, E.A., 1983. Tidal flat environment. AAPG Mem. 33, Lindsley-Griffin, N., McCabe, R., Marsters, J., Martini, E., 173±310. Ocola, L., Paz, E.H., Resig, J., Schrader, H.-J., Thornburg, Sibley, D.F., 1980. Climatic control of dolomitization, Seroe T.M., Yamano, M., 1988. Ocean Drilling Program Leg 112, Domi FormationŽ. Pliocene , Bonaire, N.A. In: Zenfger, D.H., Peru continental margin: Part 2. Sedimentary history and Dunham, J.B., Ethington, R.L.Ž. Eds. , Concepts and Models of diagenesis in a coastal upwelling environment. Geology 16 Dolomitization. Spec. Publ.-SEPM vol. 28, pp. 247±258. Ž.10 , 939±943. Sibley, D.F., Gregg, J.M., 1987. Classification of dolomite rock Sun, S.Q., 1994. A reappraisal of dolomite abundance and occur- textures. J. Sediment. Petrol. 57, 967±975. rence in the Phanerozoic. J. Sediment. Res., Sect. A 64Ž. 2 , Sibley, D.F., Nordeng, S.H., Borkowski, M.L., 1994. Dolomitiza- 396±404. tion kinetics in hydrothermal bombs and natural settings. J. Sun, S.Q., 1995. Dolomite reservoirs; porosity evolution and Sediment. Res., Sect. A 64Ž. 3 , 630±637. reservoir characteristics. Am. Assoc. Petrol. Geol. Bull. 79Ž. 2 , Sieverding, J.L., Royse, F., 1990. Whitney Canyon Ð Carter 186±204. Creek field Ð USA western Wyoming thrust belt, Wyoming. Sun, S.Q., Esteban, M., 1994. Paleoclimatic controls on sedimen- In: Beaumont, E.A., Foster, N.H.Ž. Eds. , Stratigraphic Traps tation, diagenesis, and reservoir quality Ð lessons from III: AAPG Treatise of Petroleum Geology, Atlas of Oil and Miocene carbonates. Am. Assoc. Petrol. Geol. Bull. 78Ž. 4 , Gas Fields. pp. 1±29. 519±543. Simms, M., 1984. Dolomitization by groundwater Ð flow sys- Taylor, T.R., Sibley, D.F., 1986. Petrographic and geochemical tems in carbonate platforms. Trans. Gulf Coast Assoc. Geol. characteristics of dolomite in the Trenton Formation, Ordovi- Soc. XXXIV, 411±420. cian, Michigan Basin. Sedimentology 33, 61±86. Skall, H., 1975. The paleoenvironment of the Pine Point lead±zinc Teisserene, P., Villemin, J., 1989. Sedimentary basin of Gabon Ð district. Econ. Geol. 70Ž. 1 , 22±47. geology and oil systems. In: Edwards, J.D., Santogrossi, P.A. Skeats, E.W., 1905. The chemical and mineralogical evidence as Ž.Eds. , DivergantrPassive Margin Basins. AAPG Mem. vol. to the origin of dolomites of southern Tyrol. Q. J. Geol. Soc. 48, pp. 117±199. London 61, 97±141. Thompson, T.B., 1998. Geology and geochemistry of the Kokomo Skinner, H.C.W., 1963. Precipitation of calcian dolomites and Mining District, Colorado. Econ. Geol. 93Ž. 5 , 617±638. magnesian calcites in the southeast of South Australia. Am. J. Tompkins, L.A., Rayner, M.J., Groves, D.I., Roche, M.T., 1994. Sci. 261, 449±472. Evaporites; in situ sulfur source for rhythmically banded ore in Smith, D.B., 1981. The magnesian LimestoneŽ. Upper Permian the Cadjebut Mississippi Valley-type Zn±Pb deposit, Western reef complex of northeastern England. Spec. Publ.-SEPM 30, Australia. Econ. Geol. 89Ž. 3 , 467±492. 187±202. Tucker, M., Wright, V.P., 1990. Carbonate Sedimentology. Black- Spencer, R.J., Hardie, L.A., 1990. Contol of seawater composition well Scientific Publications, Oxford, 482 pp. by mixing of river waters and mid-ocean ridge hydrothermal Vahrenkamp, V.C., Swart, P.K., 1994. Late Cenozoic dolomites brines. In: Spencer, R.J., Chou, I.M.Ž. Eds. , Fluid Mineral of the Bahamas: metastable analogues for the genesis of Interactions: A Tribute to H.P. Eugster. Geochem. Soc. Spec. ancient platform dolomites. In: Purser, B., Tucker, M., Zenger, Publ. vol. 2, pp. 409±419. D.Ž. Eds. , Dolomites Ð A Volume in Honor of Dolomieu. Sperber, C.M., Wilkinson, B.H., Peacor, D.R., 1984. Rock com- Spec. Publ.-Int. Assoc. Sedimentol. vol. 21, pp. 133±153. position, dolomite stoichiometry and rockrwater reactions in Vasconcelos, C., Mackenzie, J.A., 1997. Microbial mediation of dolomitic carbonate rocks. J. Geol. 92, 609±622. modern dolomite precipitation and diagenesis under anoxic Spotl, C., Wright, V.P., 1992. Groundwater Dolocretes from the conditionsŽ. Lagoa Vermelha, Rio de Janeiro, Brazil . J. Sedi- Upper Triassic of the Paris Basin, France Ð a case study of ment. Res., Sect. A 67Ž. 3 , 378±390, Ž Part A . . 80 J. WarrenrEarth-Science ReÕiews 52() 2000 1±81

Vasconcelos, C., Mackenzie, J.A., Bernasconi, S., Grujic, D., formed in marine sabkhaŽ. subaerial and salina Ž subaqueous . Tien, A.J., 1995. Microbial mediation as a possible mecha- settings; modern and ancient. Am. Assoc. Petrol. Geol. Bull. nism for natural dolomite formation at low temperatures. 69Ž. 6 , 1013±1023. Nature 377Ž. 6546 , 220±222. Warren, J.K., Kempton, R.H., 1997. Evaporite Sedimentology and Veizer, J., 1983. Trace elements and isotopes in sedimentary the Origin of Evaporite-Associated Mississippi Valley-type carbonates. In: Reeder, R.J.Ž. Ed. , Carbonates: Mineralogy and Sulfides in the Cadjebut Mine Area, Lennard Shelf, Canning Chemistry. Mineral. Soc. Am.: Rev. Mineral. vol. 11, pp. Basin, Western Australia. In: Montanez, I.P., Gregg, J.M., 265±300. Shelton, K.L.Ž. Eds. , Basinwide Diagenetic Patterns: Inte- Van Tuyl, F.M., 1918. Depth of dolomitization. Science 48, grated Petrologic, Geochemical, and Hydrologic Considera- 350±352. tions. Spec. Publ.-SEPM vol. 57, pp. 183±205. von der Borch, C.C., 1965. Source of ions for Coorong dolomite Watson, H.J., 1982. Casablanca field offshore Spain: a paleogeo- formation. Am. J. Sci. 684±688. graphic trap. In: Halbouty, M.T.Ž. Ed. , The Deliberate Search Von der Borch, C.C., 1976. Stratigraphy and formation of for the Subtle Trap. AAPG Mem. vol. 32, pp. 237±250. Holocene dolomitic carbonate deposits of the Coorong area, Watts, N.R., Coppold, M.P., Douglas, J.L., 1994. Application of South Australia. J. Sediment. Petrol. 46Ž. 4 , 952±966. reservoir geology to enhanced oil recovery from Upper Devo- Von der Borch, C.C., Lock, D., 1979. Geological significance of nian Nisku Reefs, Alberta, Canada. Am. Assoc. Petrol. Geol. the Coorong dolomites. Sedimentology 26, 813±824. Bull. 78Ž. 1 , 78±101. Von der Borch, C.C., Lock, D., Schwebel, D., 1975. Ground-water Wells, A.J., 1962. Recent dolomite in the Persian Gulf. Nature formation of dolomite in the Coorong region of South Aus- 194, 274±275. tralia. Geology 3, 283±285. Wenk, H.R., Barber, D.J., Reeder, R.J., 1983. Microstructures in Von Morlot, A., 1847. Uber die dolomit und seine kunstliche carbonates. Mineral. Soc. Am.: Rev. Mineral. 11, 261±301. darstellung aus kalkstein. Haidinger Naturwiss, Abhandl. 1, Wenk, H.R., Hu, M., Frisia, S., 1993. Partially disordered 305. dolomite: microstructural characterization of Abu Dhabi sabkha Wallace, M.W., 1990. Origin of dolomitization on the Barbwire carbonates. Am. Mineral. 78Ž. 7±8 , 769±774. Terrace, Canning Basin, Western Australia. Sedimentology 37, Weyl, P.K., 1960. Porosity through dolomitization:conservation of 105±122. mass requirements. J. Sediment. Petrol. 30, 85±90. Ward, W.C., Halley, R.B., 1985. Dolomitization in a mixing zone Whitaker, F.F., Smart, P.L., Vahrenkamp, V.C., Nicholson, H., of near-seawater composition, Late Pleistocene, Northeastern Wogelius, R.A., 1994. Dolomitization by near-normal seawa- Yucatan Peninsula. J. Sediment. Petrol. 55, 407±420. ter? Field evidence from the Bahamas. In: Purser, B., Tucker, Wardlaw, N.C., 1976. Pore geometry of carbonate rocks as re- M., Zenger, D.Ž. Eds. , Dolomites Ð A Volume in Honor of vealed by pore casts and capillary pressure. Am. Assoc. Petrol. Dolomieu. Spec. Publ.-Int. Assoc. Sedimentol. vol. 21. Black- Geol. Bull. 60Ž. 2 , 245±257. well Scientific Publications, Cambridge, pp. 111±132. Wardlaw, N.C., Cassan, J.P., 1978. Estimation of recovery effi- Whittaker, S.G., Mountjoy, E.W., 1996. Diagenesis of an Upper ciency by visual observation of pore systems in reservoir Devonian carbonate±evaporite sequence: Birdbear Formation, rocks. Bull. Can. Petrol. Geol. 26Ž. 4 , 572±585. southern Interior Plains, Canada. J. Sediment. Res., Sect. A 66 Wardlaw, N.C., Taylor, R.P., 1976. Mercury capillary pressure Ž.5 , 965±975. curves and the interpretation of pore structure and capillary Wilson, A.O., 1985. Depositional and diagenetic facies in the behaviour in reservoir rocks. Bull. Can. Petrol. Geol. 24Ž. 2 , Arab-C and -D reservoirs, Qatif field, Saudi Arabia. In: Roehl, 225±262. P.O., Choquette, P.W.Ž. Eds. , Carbonate Petroleum Reser- Wardlaw, N.C., McKellar, M., Yu, L., 1988. Pore and throat size voirs. Springer, New York, pp. 319±340. distributions determined by mercury porosimetry and by direct Wilson, J.L., 1975. Carbonate Facies in Geologic History. observation. Carbonates Evaporites 3Ž. 1 , 1±15. Springer, Berlin, 471 pp. Warren, J.K., 1988. Sedimentology of Coorong dolomite in the Wilson, E.N., Hardie, L.A., Philips, O.M., 1990. Dolomitization Salt Creek region, South Australia. Carbonates Evaporites 3 from geometry, flow patterns, and the origin of massive Ž.2 , 175±199. dolomite: the Triassic Latemar buildup, northern Italy. Am. J. Warren, J.K., 1989. Evaporite Sedimentology: Importance in Hy- Sci. 290, 741±796. drocarbon Accumulation. Prentice-Hall, Englewood Cliffs, NJ, Wojcik, K.M., Goldstein, R.H., Walton, A.W., 1994. History of 285 pp. diagenetic fluids in a distant foreland area, Middle and Upper Warren, J.K., 1990. Sedimentology and mineralogy of dolomitic Pennsylvanian, Cherokee Basin, Kansas, USA Ð Fluid inclu- Coorong lakes, South Australia. J. Sediment. Petrol. 60Ž. 6 , sion evidence. Geochim. Cosmochim. Acta 58Ž. 3 , 1175±1191. 843±858. Wolfbauer, C.A., Surdam, R.C., 1974. Origin of non-marine Warren, J.K., 1991. Sulfate dominated sea-marginal and platform dolomites Eocene Lake Gosiute, Green River Basin, Wyoming. evaporative settings. In: Melvin, J.L.Ž. Ed. , Evaporites, Geol. Soc. Am. Bull. 85, 1733±1740. Petroleum and Mineral Resources. Dev. Sedimentol. vol. 50. Woody, R.E., Gregg, J.M., Koederitz, L.F., 1996. Effect of tex- Elsevier, Amsterdam, pp. 477±533. ture on the petrophysical properties of dolomite Ð evidence Warren, J.K., 1999. Evaporites: Their Evolution and Economics. from the Cambrian±Ordovician of southeastern Missouri. Am. Blackwell Scientific Publications, Oxford, 438 pp. Assoc. Petrol. Geol. Bull. 80Ž. 1 , 119±132. Warren, J.K., Kendall, C.G.St.C., 1985. Comparison of sequences Wright, D.T., 1999. The role of sulphate-reducing bacteria and J. WarrenrEarth-Science ReÕiews 52() 2000 1±81 81

cyanobacteria in dolomite formation in distal ephemeral lakes the Sinian Qigebulake Formation dolomite in northeast Tarim of the Coorong region, South Australia. Sediment. Geol. 126 basin. In: Jia, R.X.Ž. Ed. , Petroleum Research in the Northern Ž.1±4 , 147±157. Tarim Basin, China: Part I. Stratigraphy and Sedimentology. Xun, Z., Fairchild, I.J., 1987. Mixing zone dolomitization of China University Geosciences Press, Beijing, pp. 247±255. Devonian carbonates, Guangxi, south China. In: Marshall, J.D. Yoo, C.M., Gregg, J.M., Shelton, K.L., 2000. Dolomitization and Ž.Ed. , Diagenesis of Sedimentary Sequences. Spec. Publ.-Geol. dolomite neomorphism: Trenton and Black River Limestones Soc. London vol. 36, pp. 157±170. Ž.Middle Ordovician northern Indiana, USA. J. Sediment. Yang, B.X., 1986. Diagenetic history and reservoir characteristics Res., Sect. A 70Ž. 1 , 265±274. of a Middle Carboniferous dolomite reservoir, eastern Sichuan, Young, R.B., 1933. Conditions of deposition of the Dolomite China. Sediment. Geol. 46, 275±291. Series. Trans. Geol. Soc. S. Afr. 36, 135±1221. Yao, Q.J., Demicco, R.V., 1995. Paleoflow patterns of dolomitiz- Zenger, D.H., 1996. Dolomitization patterns in the widespread ing fluids and paleohydrology of the southern Canadian Rocky Bighorn FormationŽ. Upper Odovician , Western Craton, USA. mountains Ð evidence from dolomite geometry and numeri- Carbonates Evaporites 11Ž. 2 , 219±225. cal modelling. Geology 23Ž. 9 , 7691±7694. Zenger, D.H., Dunham, J.B., Ethington, R.L., 1980. Concepts and Ye, Q.C., Mazzullo, S.J., 1993. Dolomitization of Lower Permian models of dolomitization. Spec. Publ.-SEPM 28, 320 pp. platform facies, Wichita Formation, North Platform, Midland Zhai, G.M., Zha, Q.H., 1982. Buried hill oil and gas pools in the Basin. Carbonates Evaporites 8Ž. 1 , 55±70. North China basin. In: Halbouty, M.T.Ž. Ed. , The Deliberate Yie, D.S., 1991. Diagenesis and reservoir characteristics of Cam- Search for the Subtle Trap. AAPG Mem. vol. 32, pp. 317±335. brian through Early Ordovician dolomite in the northern Tarim Zimmermann, H., 2000. On the origin of fluid inclusions in basin. In: Jia, R.X.Ž. Ed. , Petroleum Research in the Northern ancient halite Ð basic interpretation strategies. In: Geertmann, Tarim Basin, China: Part I. Stratigraphy and Sedimentology. R.M.Ž. Ed. , Salt 2000 Ð 8th World Salt Symposium vol. 1. China University Geosciences Press, Beijing, pp. 263±273. Elsevier, Amsterdam, pp. 199±203. Yie, D.S., Liu, S.H., 1991. Diagenesis and reservoir property of