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Formation of dolomite in the Coorong region, South Australia
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Formation of dolomite in the Coorong region, South Australia MICHAEL R. ROSEN, DONALD E. MISER*, MICHAEL A. STARCHERand JOHN IL WARREN+* Department of Geological Sciences, University of Texas at Austin, Austin, TX 787 13-7909, U.S.A.
(Received August 15, 1988; accepted in revised form December 19, 1988) Abstract-Cores
of unlithitied Holocene sediment from shallow ephemeral lakes in the Coorong region, S.A., contain dolomite at various stratigraphic intervals. Two distinct dolomite types arc recognized using stable carbon and oxygen isotope data, unit cell calculations, transmission eleqron microscopy (TEM), and bulk composition data. Detailed X-ray diffraction (XRD) analyses show that the dolomite is ordered, but that ordering is variable. Average unit cell parameters indicate that the crystal lattice of one type of dolomite (type A) is expanded in the c,, direction (c, = 16.08 A) and contracted in the a, direction (p, = 4.799 A) relative to ideal dolomite (c,, = 16.02 A and o. = 4.812 A, respectively). Type A dolomite occurs in the centers of the larger basins. The mole fraction of MgCO, is as much as 53 f 1%. The unusual crystal chemistry and (TEM) data indicating a heterogeneous microstructure suggestthat this dolomite is generated by rapid precipitation from brines which have highly elevated Mg/Ca ratios. Dolomite ordering reflections are present in electron difiiaction patterns but are weak. Stable oxygen and carbon isotope values are tightly grouped (ave. 6’*0 = +7.6%0, s.d. = 0.6%; 6°C = +3.5%, rd. = 0.7%0). The other type of dolomite (type B) is Ca-rich, has lower stable isotope values (ave. 6”O = +6.4%, b13Ca - 1.2%) than type A dolomite, has a more homogeneous microstructbre, is expanded in both the a. and c. axes, and is generally better ordered than type A dolomite. These data suggesttype B dolomite is precipitated more slowly from lessevaporitic brines than type A dolomite. INTRODUCTION
connected by interlake corridon and the only outlet to the Early Holocene Coorong Lagoon was through North Stromatolite Lake to the east of Pellet Lake. The present-day interlake corridors are now filled with sediment and covered by halophytes and grasses,while the dune ridge system and areas marginal to the ephemeral lake depressions are covered with a dense scrub of eucalypt trees and shrubs. The fifth lake studied (Milne Lake) has had no Holocene marine connection, or any connection to the other four lakes in its development, and yet has the most dolomite in the basin fill. The stratigraphic distribution of the sediments is listed in Table I. All the lakes in the Coorong region can be divided into four basic units regardlessof the mineralogy of the lake. From bottom to top these units are 1) the basal unif consistingofdetrital siliciclastic sand, shell material, and dolomite, 2) the organic rich unit which may have up to 12% total organic carbon, 3) the laminufed unit which compriss the bulk of the sediment in the lakes, and 4) the massive unif. which contains much of the dolomite. More detailed information on the sedimentology of the units in these lakes can be found in ROSEN et al. (1988) and WARREN (1989). An extensive description of the climate and general hydrology of the Coorong area can be found in the above references,and a summary of this information is presented by ROSEN et al. (1988).
THE DISCOVERY OF Holocene dolomite in the Coorong region in the late 1950s has made the Coorong an important natural laboratory to determine the conditions under which sedimentary dolomite forms. Studies by ALDERMAN (1959), SKINNER (1963), VON DER BORCH (1965, 1976), VON DER BORCH and LOCK (1979), and LOCK (1982) have discussed various aspects of the regional chemistry and distribution of dolomite in the Coorong region. Recent papers by BOTZ and VON DER E~ORCH(1984) and ROSEN ef al. (1988) have discussed the stratigraphy and geochemistry of individual dolomite-forming lakes. This paper presents a detailed study of dolomite from cores from five lakes in the Coorong region (Fig. 1). Dolomite has been analyzed from all stratigraphic horizons in the lakes. These horizons represent the extremes of chemical conditions under which dolomite forms in the Coorong region. By determining the chemistry of the dolomite from these known stratigraphic positions, insights can be drawn about the conditions under which the dolomite forms. This leads to a predictive model of the chemistry of Coorong dolomite based on stratigraphic occurrence. GEOLOGIC
METHODS
Forty-two cores along several transectswere taken in the five lakes by pushing4 cm diameter plastic pipe into the soft Holocene sediment until it bottomed on Pleistocene calcrete (Fig. I). Dolomite was iden-
SElTlNG
tified in over 30 of these cores. Based on the depth of penetration of the core tubes to the calcrete, divided by the actual recovered nonrepeated core, we determined a compaction of about 50% due to coring. Cores were wrapped in plastic, shipped back to the United States, split, then sampled at 4 cm intervals. Estimates of mineral abundances were based on relative peak intensities from the XRD charts. All samples were run at 35 KV and 20 ma using CuK a radiation and a scan speed of I .2 degrees2-theta per minute. Some samples were also run at 0.6 degrees 2-theta per minute to determine if the slower scan rate gave more accurate unitcell dimensions. The results for both scan speedsare within the error of the method (see Table 4) and so the faster scan speed was used throughout the rest of the study. Detailed description of the sample preparation. isotope proadurcg TEM, and XRD methods used can be found in ROSEN et ui. (1988). All samples which contained carbonate phases other than dolomite were treated with EDTA, using modifications ofthe methods of BAB.
The lakes analyzed for this study are situated in the northern string of ephemeral lakes (or salinas) behind the Coorong Lagoon (Fig. I). The lakes are separated from the present lagoon by the most seawardridge of a shore-parallelPleistocenedune barrier systemwhich controls the location of the Holocene lakes and interlake corridors along the Southeastern Coastal Plain of South Australia. Four of the five lakes (North Stromatolite Lake, Pellet Lake, Dolomite Lake, and Halite Lake), informally termed the Salt Creek Chain. form a chain of lakes which were ai extension of the Coorong L&on during Early Holocene time (;s6,000 years B.P.). The salt Creek Chain was
Presenf addresses: *Departments of Geology and Chemistry, Arizona State University, Tempe, AZ 85287-1404, U.S.A. l * National Centre for Petroleum Geology and Geophysics.G.P.O. Box 498. Adelaide, S.A. 5001, Australia.
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in this lake may reflect a late Pleistocene marine connection that was quickly closed. Alternatively, the sand may represent the initial deflationary sediments carried into the lake at the beginning of the Holocene. Pellet lake. which was studied in detail by ROSEN ef a/. (198X).has Mg-rich dolomite in the upper 0.2 to 0.6 meters of the lake and Carich dolomite in places around the margin of the lake and in the basal sand unit. Dolomite Lake has only Ca-rich dolomite in the top 0.2 meters and in the basal sand. Halite Lake has only the basal sand unit, and the dolomite in this unit is also Ca-rich. Milne Lake is dominated by Mg-rich dolomite and magnesite throughout the bulk of the sediment in the core. In all the lakes studied, Ca-rich dolomite may coexist or be stratigraphically associatedwith aragonite, calcite and/or Mg-calcite phases. Magnesium-rich dolomite coexists or is stratigraphically associated with magnesite or hydromagnesite phases. FIG. 1. Location map of lakes in the Salt Creek Chain and Mime Lake. The positions of the cores taken are marked for reference.
COCK et al. (1967), to eliminate overlapping peak positions. To determine the effect of EDTA on the dolomite in the sample, two samples of pure Mg-rich dolomite from Mime Lake and two samples of C&rich dolomite from Dolomite Lake were split into five subsamples. The first subsample was untreated and used as a reference sample. the next three subsampleswere treated with EDTA for one minute. three minutes, and five minutes, respectively. The last subsample was treated with fresh EDTA three times at two-minute intervals. After each two-minute treatment the subsample was washed and analyzed for unit-cell parameters and isotopic composition. The results showed that EDTA had little or no effect on the isotopic composition or the unit cell dimensions of any of the samples (Table 2). There may be a slight increase in the MgC03 mole percent (about I mole percent) of the Ca-rich samplesafter three two-minute washings in EDTA. However, this variation is close to the error limit of the method and so is hardly significant. We conclude that only a single kind of dolomite seems to characterize each sample. In this study, all MgCO, mole percent values have been calculated using LUMDSEN and CHIMANSKY’S (I 980) equation. Other equations by HUTCHISON (1974) from the data of GOLDSMITH a al. (1955) and MISER (1987) have been developed from various experimental and natural carbonate phases and may yield significantly different results.Therefore, the determination of Mg mole percent from XRD patterns should be treated with some caution. When possible, XRD valuesshould be correlated with other independent chemical analyses. Unfortunately, in the Coorong dolomite, grain-size is too small (< 1 pm) for microprobe analysis, and small amounts of contaminating Mg and Cabearing minerals make chemical analysessuspect.Therefore, XRD analysis is the most reproducible method of determining MgCO, mole percent in dolomite from the Coorong. Recently, it has been suggestedthat line-grained calcite (~5 pm) washed in distilled water will dissolve and reprecipitate when dried at temperatures <6O”C (BARRERA and SAVIN, 1987). Under these conditions the calcite reprecipitatesfrom water enriched in ‘ti during evaporation and shifts measured oxygen isotopic values to heavier values. The few calcite and aragonite samples in this study were washed and dried following the procedures outlined by BARRERA and SAVIN (1987). Dolomite samples were washed in distilled water and dried at low temperatures; however, there is no evidence at this time to suggestthat this processof dissolution-reprecipitation is op erating on tine-gmined dolomite samples.
RESULTS
Stable oxygen and carbon isotopes
We analyzed oxygen and carbon isotopic ratios from the various dolomitic units. The results from over 100 analyses (Fig. 3, Table 3) indicate that two types of dolomite, type A and type B, can be recognized from their isotopic signature. Type A dolomite is, in general, slightly heavier in oxygen (mean = +7.6) than type B dolomite (mean = +6.4) and is 3 to 6% heavier in carbon than type B dolomite (mean, type A = +3.5; mean, type B = -1.2). Figure 3 shows a fairly tight fit of type A dolomite along a linear trend (correlation coefficient = 0.75). Type B dolomite values are variable and do not appear to follow a simple trend. Where we could find single phases of other carbonate minerals (i.e. aragonite and calcite), they were analyzed to see if they bore any relation to the dolomite isotopic signatures. The samples analyzed do not coexist with dolomite (i.e. they are from a non-dolomitic stratigraphic interval). Very few single phases uncontaminated by organic matter or sulfate minerals could be found. Five aragonite samples from Halite Lake and one calcite and one aragonite sample from Dolomite Lake were analyzed (Table 3). The aragonite from both Dolomite and Halite Lake is I to 3%~lighter in oxygen than the Type B dolomite from the core from which the sample was taken, and the carbon values are 2 to 4%0 heavier. Similarly, the calcite value from Dolomite Lake is almost 7%0 lighter in oxygen than the dolomite from the same core, though the carbon values are similar.
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Dolomite has been found at various stratigraphic intervals in the five lakes studied (Fig. 2). In North Stromatolite Lake, Ca-rich dolomite occurs only in a basal siliciclastic sand unit. This unit can be found in all the lakes studied,and probably representsa sand deposited ‘ve phaseof evolution in the area at the beginning during the tmmgmast of the Holocene when all four lakes in the Salt Creek Chain were floodedby marine water from the Coorong Lagoon. Although Milne Lake never had a Holocene marine connection, the thin basal sand
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Unit cell calculations Results from over 150 unit cell calculations (Fig. 4, Table 4) show that dolomite unit cell dimensions can also be grouped into two categories. These two categories correspond to the type A and type B dolomite groups determined from the isotopic analysis. Type A dolomite (which has heavy oxygen and carbon values) has unit cells expanded in the c,
direction (average of 16.08 A) and contracted in the a, direction (average of 4.799 A) with respect to ideal dolomite (a~ = 4.8 12 c, = 16.02). Most samples from type B dolomite (which has lighter oxygen and carbon values), particularly those from the margins of the lakes, are expanded in both the c, and a, directions. Average unit cell dimensions for type B dolomite are a, = 4.827 and c, = 16.15. Type A dolomite is dominated by Mg-rich dolomite, containing up to 3 mole percent excess MgCOj . Type B dolomite samples from the basal siliciclastic sand may have nearly stoichiometric compositions, whereas type B dolomite from
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FIG. 2. Idealized east-west cross-section through the salt Creek Lake Chain and Milne Lake showing the position of the geochemical dolomite types in the lakes and in the interlake corridors. Note the occurrence of type B dolomite in the center of Dolomite Lake (see isotope discussion section for explanation). WARREN (1989, p. I74I8 1) givesa full description of the Coorong stratigraphy.
Fro. 3. Cross-plot of stable oxygen and carbon isotopes. Notice the relatively tight grouping of type A dolomite (squares) which appear to follow a covariant trend (arrow). Type B dolomite values are more variable and do not follow any simple trends.
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the margins of the lakes or lake corridors are invariably more &rich. This difference is also reflected in the calculated unit cell vahles. Possible depth-related trends are present in type A and type B dolomite in Pellet Lake, but only within individual cores (ROSEN et d., 1988). However, lake-wide trends or interlake trends have not been observed for either type A or type B dolomite.
ically throughout all samples. To our knowledge, no ancient dolomite with similar microstructure has been observed. The lattice strain associated with such an unusually high defect density should render these crystals highly unstable. It is unlikely that the crystals would persist for geologically long periods of time without being replaced by a dolomite with a more homogeneous microstructure. Despite these defects, electron diffraction patterns of type A dolomite show fairly sharp, discrete diffraction maxima, usually without any significant streaking. Thus, the material is well crystalline and devoid of amorphous domains. Similarly, there is no evidence of Ca- or Mg- enriched domains, although some concentration along defects is likely. Furthermore, none of the dolomite is intergrown with any other carbonate phase, such as aragonite, magnesite, or hydromagnesite. Other carbonates, when present, occur as separate and distinct crystal phases. Ordering reflections can be observed in type A dolomite, but they are extremely weak (Fig. 5b). A great deal of the intensity observed in electron diffraction patterns is due to multiple diffraction. Thus, the materials, although technically dolomite, demonstrate considerable cation disorder. The variability in the degree of cation ordering probably causes the spread in calculated c, values (see Unit ceil calculations). Type B dolomite, although Ca-rich, generally does not display a modulated microstructure typical of other calcian dolomites, though it has been found rarely in some samples (Fig. 5~). When found, the modulated contrast is generally weaker than that found in ancient, calcian-rich dolomites. This weak modulated microstructure may represent the regular arrangement of the defects seen to be random in other crystals from these lakes, indicating a slower growth of these crystals when compared to most of the Coorong dolomite. Type B dolomite crystals from the margins of the lakes and Dolomite Lake possess a relatively homogeneous microstructure (Fig. 5d). The excess calcium must therefore be distributed evenly throughout the lattice. Electron diIEaction patterns and stronger ordering reflections observed in XRD patterns demonstrate that this dolomite possesses stronger ordering reflections than type A dolomite. Type B dolomite
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Transmission electron microscopy In TEM, both type A and B dolomite crystals exhibit large variations within a given sample. Microstructures range from heterogeneous, with closely spaced, random defects, to relatively featureless and homogeneous crystals. The heterogeneous crystals (Fii 5a) are similar to those described by WENK et al. (1983). Due to the closely spaced nature of the defects in the heterogeneous crystals, exact identification of individual features cannot be determined, however, they probably consist largely of dislocation tangles and stacking faults. These structures probably represent “mistakes” incorpomted during growth. The high defect density of the heterogeneous dolomite can produce overall warping of the crystals and may divide them into smaller subcrystals. The latter are documented by the presence of MoW fringes in some samples. Type A dolomite is dominated by this type of heterogeneous microstructure, although it occurs sporad-
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a0 FIG. 4. Unit cell dimensions for both type A and type B dolomite. The line connects the unit cell dimensions of calcite to magnesite. Notice that ideal dolomite falls below this line due to the contraction of the unit cell when dolomite is well ordered. In contrast to the isotope plot, type A dolomite unit cell dimensions are scattered, indicating rapid precipitation from solution. Type B dolomite unit cell dimensions
are tightly grouped, indicating slower precipitation.
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samples, particularly from the basal siliciclastic unit, typically show more sharply defined superstructure ordering peaks than type A dolomite. Type B dolomite from the siliciclastic basal unit is variable but exhibits two crystal populations. Most of the crystah show a heterogeneous microstructure similar to that of type A dolomite, but ordering reflections are stronger. In addition, a large number of crystals are homogeneous and are larger than the heterogeneous ones. DISCUSSION Isotopes ROSENet al. (1988) attributed heavy carbon values in Pellet Lake to an inorganic Rayleigh distillation process rather than organically influenced methanogenetic processes. Evidence for organic or inorganic control on the carbon reservoir in the lake centers is still equivocable; however, the most compelling evidence for an inorganic origin for heavy carbon values in these lakes is that dolomite is most abundant at the sediment-water interface. If organically influenced methanogenetic processes were creating heavy carbon values, dolomite would be expected to be most abundant below the sediment-water interface and decrease in abundance toward the top of the core. Heavy 613C values identical to the heaviest Coorong values have also been reported by CLAYTON et al. (1968) from Deep Springs Lake in California; a continental playa. They attributed the heavy carbon values to strictly inorganic processes. Thus, it seems likely that a biological contribution to the carbon reservoir in the lake centers is
minor. However, organically influenced meteoric or soil CO2 must contribute to the lighter carbon ratios for type B dolomite. This is consistent with the stratigraphic position of type B dolomite which is around the margins of the lakes, in the interlake cqrridors, and in the siliciclastic basal unit. Although evaporitic in nature, the oxygen isotope. values for type B dolomite from the siliciclastic base are on average lighter than those of type A dolomite (Fig. 3). Restricted evaporitic conditions most likely existed during deposition of the siliciclastic basal unit, but the area had not evolved into a completely closed basin. In addition, the water body would be necessarily deeper and larger and thus less susceptible to extreme evaporative influences on its isotopic composition. Average oxygen and carbon isotopic values for cores from the basin centers are slightly heavier than for cores closer to the margins and much heavier than type B dolomite in the interlake corridors. This is consistent with an inorganic evaporitic origin for the heavy values. Within the Salt Creek Chain, type B dolomite seems to become lighter in both oxygen and carbon toward Halite Lake. This may be due to the proximity of a more meteoric influence in Halite Lake during the Early Holocene connection to the Coorong Lagoon. The relatively consistent isotopic values for type A dolomite suggest a relatively homogeneous isotopic reservoir produced by an evaporating brine. The covariant trend demonstrated earlier (Fig. 3) also suggests an evolving brine. The relative scatter of type B dolomite isotopic values, particularly in the carbon signatures, suggests some sort of mixing zone between evaporitic brines and meteoric water (CONIGLIOet al., 1988).
666
M. R. Rosen e( al.
FIG. 5. a) TEM dark field photomicrograph of a crystal from type A dolomite (core PM). The heterogeneous appearance of the crystal is the result of extremely closely spaced lattice defects. The small, pseudo-lamellar features at the top of the crystal are interference fringes caused by the overlapping ofdomains ofthe crystal with slightly different orientations. This microstructure is very common in type A dolomite and is typical of those that are largely cation disordered (g = 01.2 s = 0). Photograph from ROSENet a/. (1988). b) Electron diffraction nattem of a tvpe B dolomite crystal from Dolomite Lake (core DA). Half of the spots in this pattern, such as the 00.3 reflection, would be missing if the crystal had a calcite-like structure even if it had a dolomite composition. The extra spots are the result of the change in symmetry from R3c to R3 that occurs as a result of dolomite-type cation order. These “ordering” reflections verify that the crystals have the dolomite structure. In addition, they are much more intense type B dolomite than in Type A dolomite, suggesting a greater degree of cation ordering in the former (see ROSENet al,, 1988, for a type A dolomite electron diffraction pattern). In this orientation, none of these “ordering” reflections can be derived solely from multiple diffraction events or other spurious effects. c) Dark field electron photomicrograph taken in a two-beam condition with g = (108) (core DA). The diffuse light and dark bands represent the modulated microstructure found only in type B dolomite. This microstructure is common to ancient and some modem marine dolomite but is relatively rare in samples from the Coorong and unknown from Holocene dolomite from other continental settings. Notice that this growth feature changes orientation in different sectors of the crystal with its orientation governed by the orientation of the nearest crystal face. Scale bar is 0.2 Frn. d) TEM dark field photomicrograph of a type B dolomite crystal (core PA). Although the core appears similar to 5a, the rims are homogeneous in appearance, and thus contain far fewer lattice defects. Notice the well developed rhombohedral crystal faces. Ordering is still imperfect, but better than most of type A dolomite. Scale bar = 0. I pm, g = 10.4, s = 0. Photograph from ROSENe( al. (1988).
The mineral associations and the stratigraphic positions of type B dolomite (i.e. around the edges of the lakes and between lakes) also suggest a mixed-water influence. The occurrence of type B dolomite in the center of Dolomite Lake can be attributed to the relatively small area of lake surface. Because the lake is so small, the entire lake surface is influ-
enced by meteoric input and so forms type B dolomite rather than the more evaporitic type A dolomite. Oxygen values in type B dolomite are heavy compared with typical values for dolomite (LAND, 1980) but are consistently lighter than type A dolomite. It may be that incoming groundwater, affected by biologic activity, is relatively light
Formation of dolomite in the Coorong
in carbon, yet the water is still a relatively concentrated brine and so retains relatively heavy oxygen values. Following the equations in LAND (1985), it can be shown that the 8”O isotopic value (in SMOW) of the water which precipitated the dolomite. can be constrained to around +4 to +6% (Fig. 6) given the temperature range measured by VON DER BIRCH (1965) in lakes of the region. This value is heavy for meteoric water, but considering the evaporitic nature of the Coorong region, it is not unreasonable. Oxygen isotopic ratios of > +7%0 (SMOW) have bee.n reported from continental groundwater inland of the Abu Dhabi sabkha (MCKENZIE et al., 1980). Because of the presence of magnesite or hydromagnesite and lack of calcite or aragonite, it seems likely that the solution which precipitated phases in type A dolomite unit had a higher Mg:Ca ratio than the solutions which precipitated type B dolomite. VONDER BIRCH (1965) and B~TZ and VON DER BIRCH (1984) have both observed that Mg-rich dolomite (our type A dolomite) in the Coorong area correlates with the presence of magnesite (i.e. Milne Lake). This suggests a genetic relationship between co-existing Mg-rich dolomite and magnesite, possibly due to high Mg:Ca ratios of the precipitating fluid. In Pellet Lake, dolomite occurs with hydromagnesite, another Mg-rich mineral phase, so that high Mg:Ca ratios of the precipitating fluid can be called upon to explain the excess of Mg in the dolomite crystal lattice. The suggestion that a high Mg:Ca ratio is an important controlling factor in dolomite formation is not new. VON DER BORCH (1965) and FOLK and LAND (1975) suggested high Mg:Ca ratios to explain dolomite formation. Experimental data from “protodolomite” formed under very high Mg:Ca ratios but at low temperatures (33“C) also suggests the importance of high Mg:Ca ratios on the formation of sedimentary dolomite (00~0~1 et al., 1983; OOMORI and KITANO, 1987). The few isotopic values from aragonite in Halite Lake suggest that isotopically different fluids precipitated dolomite and aragonite at the base of the core. If the aragonite and dolomite precipitated in isotopic equilibrium, then the do-
10
8
6
4
2
‘80 (PDB) FIG. 6. Plot of oxygen isotopic composition of the dolomite (in PDB) versus temperature. The curved lines represent isotopic compositions of the water (in SMOW) in equilibrium with dolomite at the given temperatures (data from LAND, 1985). Given the range of isotopic compositions for both type A and type B dolomite (see Table 3) and a temperature range of 10 to 28°C (see text), the isotopic composition of the water precipitating the dolomite is constrained within the black box (between +4 to +6% SMOW).
667
lomite should be at least 3460heavier in ‘*O than the other carbonate phase (LAND, 1985). The aragonite and dolomite samples that are 0.04 m apart are only about 1960different. The calcite sample from Dolomite Lake is almost 7%~lighter than the dolomite from the same core. This fractionation is too large to be in equilibrium with dolomite. Therefore, it seems unlikely that dolomite is forming from the diagenetic replacement of precursor calcite and aragonite phases. These data, although sketchy, also support the other data suggesting that dolomite precipitated directly out of solution and had no other carbonate precursor phase.. Unit cell parameters and microstructures Type A dolomite is overwhelmingly dominated by crystals with unit cells contracted in the a, direction and expanded in the c, direction relative to ideal dolomite (Fig. 4). The incorporation of small Mg ions replacing the larger Ca ions in the unit cell has been proposed as the cause of the contraction in the a, direction (ROSEN et al., 1988), whereas expansion of the c,, parameter may be related to cation disordering (cJ REEDER and WENK, 1983) or other lattice defects (MISER et al., 1987). Variations in c, are more variable, particularly for type A dolomite, suggesting that ordering may be a more important control than composition for this unit cell dimension. The incorporation of excess Mg in type A dolomite crystals and its association with magnesite or hydromagnesite suggest precipitation from a highly evaporitic solution with a high Mg/Ca ratio. Crystallization of type A dolomite from highly evaporitic fluids may have been very rapid, causing the incorporation of excess magnesium into the lattice and some cation disorder. The degree of cation disorder and variability from crystal to crystal is further evidence that the dolomite precipitated rapidly from solution. In Ca-rich (type B) dolomite, the presence of calcite and aragonite phases coexisting with dolomite suggests that the Mg/Ca ratio was much lower. Dolomite crystallization may have been much slower with fewer Mg ions incorporated into the lattice. The basal siliciclastic type B dolomite, which may be nearly stoichiometric and only slightly expanded in the c, direction, may represent an intermediate state of fluid saturation (and rate of crystal growth) between type A dolomite and some of the more Carich type B dolomite, or it may represent a diagenetic stabilization of a previous Mg-rich dolomite. The occurrence of a modulated microstructure rather than random defects in type B dolomite may also reflect slower growth of these crystals. Type B dolomite from the margins of the lakes and the interlake corridors generally is less ordered, and has a more expanded unit cell (in both directions) than the basal type B dolomite. In TEM, replacement textures were not observed. This suggests that where the dolomite is most influenced by meteoric water input, the pore water has little tendency to remain within the dolomite precipitating stability field so that only poorly ordered, Ca-rich dolomite can form. These data also indicate that differences between the dolomite types are the result of the chemistry of the precipitating solutions and are not related to progressive stabilization by dissolutionreprecipitation reactions with increasing age.
M. R. Rosen et al.
668 GEOLOGIC SIGNIFICANCE
The variability of unit cell parameters, and the degree of disorder relative to ideal dolomite, indicates that most, if not all, dolomite in the Coorong region is metastable. With time and burial, Coorong dolomite is likely to be subject to dissolution and reprecipitation as a more stable diagenetic dolomite phase. This stabilization will destroy most of the primary microstructures and alter the isotopic composition and unit cell dimensions of the dolomite. So what is the significance of this study to examples of ancient dolomites? Type B dolomite is similar to modern dolomite from other regions except that it lacks abundant modulated microstructures. Although metastable, it may be more stable than type A dolomite and so retain some of its characteristics even when diagenetically altered. In addition, if Type A and Type B dolomite are affected by the same fluid, the resulting geochemical parameters may retain their relative differences. In a relatively open diagenetic system that is rock-buffered, diagenetic type A dolomite should be relatively heavier in carbon and oxygen than diagenetic type B dolomite given a more or less homogeneous diagenetic fluid. If diagenesis occurs in a closed or semi-closed system retention ofthe relative isotopic differences would be more likely. In rocks that may have had limited access to diagenetic fluids, microstructures of the primary dolomite may be retained in the cores of crystals which subsequently acquire overgrowths. For example, the recognition of dominantly heterogeneous cores in a particular dolomitic facies would indicate relatively rapid crystal growth. In addition it suggests that the dolomite formed from an evaporitic brine directly from solution (i.e. type A dolomite). Examples of an environment where relicts of the primary fabric could be retained would be dolomite enclosed by less permeable evaporite seals. Sabkhas or evaporite brine pans may create such a seal. Future detailed study of dolomites encased in Jurassic evaporites in the Buckner Formation (Gulf Coast, U.S.A.) or in the Arab sabkha cycles of the Arabian Gulf may reveal evidence of their original depositional microstructure. An example of the application of unit cell parameters to an ancient environment is illustrated by VOGT (1986, p. 6775). He demonstrated slight, but statistically valid, differences in unit cell parameters of peritidal and subtidal dolomite from the San Andres Formation (Permian) of Texas, U.S.A. Although this dolomite has been interpreted as a pervasive early replacement dolomite rather than a primary precipitate, the method illustrates a difference in the composition of the dolomite from two environments related to the fluids from which they were precipitated. CONCLUSIONS The geochemistry of Coorong dolomite systematically varies within a wide range of compositions, microstructures and isotopic values within a single basin, and thus constrains the conditions under which dolomite can be expected to form. Two types of dolomite have been recognized which characterize the dolomite forming in the Coorong region. Type A dolomite is characterized by: 1) the incorporation of excess Mg into the crystal lattice; 2) the presence of a heterogeneous microstructure; 3) poor degree of cation ordering; 4) asso-
ciation with the magnesium carbonates hydromagnesite and magnesite; 5) relatively heavy oxygen and carbon isotope ratios; and, 6) the absence of associated CaCOs phases. These characteristics suggest rapid crystallization from evaporitic basin center brines with a relatively high Mg/Ca ratio. In addition, the absence of replacement textures, such as syntaxial, submicron calcite domains in dolomite, or the partial dolomitization of shell and fecal material, support the suggestion of VON DER BORCH (1965) that type A dolomite in the Coorong precipitates directly from these brine solutions. Type B dolomite is characterized by: 1) incorporation of excess Ca into the crystal lattice; 2) variable but more homogeneous microstructures with occasional modulated microstructures present; 3) relatively better cation ordering than type A dolomite; 4) association with the calcium carbonate phases aragonite, calcite, and Mg-calcite; 5) slightly lighter oxygen isotopic values and much lighter carbon isotope values than type A dolomite; and, 6) the absence of associated MgCOJ phases. These characteristics suggest that type B dolomite formed from fluids under lower Mg:Ca conditions than type A dolomite. The more homogeneous microstructures, minor modulated microstructures, stronger ordering reflections, and small crystal overgrowths suggest that the dolomite formed more slowly than type A dolomite. The carbon isotopic values along with the stratigraphic position of the dolomite suggest that type B dolomite formed from a mixture of evaporitic basin center brines and less evaporitic organically influenced meteoric water. Acknowledgements-We would like to thank Chris von der Borch of Blinders University who provided the facilities of the Earth Sciences Department during the field stages of this study, and Lynton Land of the University of Texas-Austin who provided stable isotopic analyses and reviewed earlier drafts of this manuscript. Discussions with Patti &anger were very helpful in focusing our thoughts on various aspects of Coorong sedimentology. We would also like to thank Jeff Horowitz for expertly drafting Figs. 1 and 2. This work was funded by N.S.F. Grant No. EAR-8206146, and their support is gratefully acknowledged. D.E.M. acknowledges the support of the Mobil Field Research Laboratory. The Owen Coates Foundation of the University of Texas-Austin funded part of the publication costs. Editorial handling: S. E. Calvert REFERENCES
ALDERMAN A. R. (1959) Aspects of carbonate sedimentation. J. Geol. Aust. 6, I-10. BABCOCKR. S., ATWOODD. K. and PERRYD. (1967) Separation of dolomite from fine grained recent sediments. Amer. Mineral. 52, 1563-1567. BARRERAE. and SAWN S. M. (1987) Effect of sample preparation on the G’xO-value of line-grained calcite. Chem. Geol. 66, 301305. BOTZR. W. and VONDERBORC~ C. C. (1984) Stable isotope study of carbonate sediments from the Coorong area, South Australia. Sedimentology 31, 837-849. CLAYTON R. N., JONES B. F. and BERNERR. A. (1968) Isotope studies ofdolomite formation under sedimentary conditions. Geochim. Cosmochim. Acta 32,4 15-432. CONIGLIO M., JAMESN. P. and AISSAOUID. M. (1988) Dolomitization of Miocene carbonates, Gulf of Suez, Egypt. J. Sediment. Petrol. 58, 100-l 19. FOLK R. L. and LAND L. S. (1975) MgjCa ratio and salinity: Two controls over crystallization of dolomite. Amer. Assoc. Petrol. Geol. Bull. 59, 60-69.
Formation of dolomite in the Coorong GOLDSMITH
J. R., GRAF D. A. and JOENSUU0.1. (1955) The occurrence of magnesian calcites in nature. Geochim. Cosmochim. Acta I,2 12-230. HUTCHISONC. S. (1974) Laboratory Handbook of Petrographic Techniques. Wiley-Interscience, New York, 527~. LAND L. S. (1980) The isotope and trace element geochemistry of dolomite: The state of the art. In Concepts and Models of Dolo mitization(eds. D. H. ZENGER,J. B. DUNHAMand R. L. ETHINGTON). Spec. Pub. Sot. Econ. Paleom. Mineral., Tulsa 28,87-
110. L. S. (1985) The origin of massive dolomite. /. Geol. Ed. 33, 112-125. Lock D. E. (1982) Groundwater controls on dolomite formation in the Coorong Region of South Australia and in ancient analogues. Ph.D. dissertation. The Flinders Univ. of South Australia. LUMSDEND. N. and’CHIMAHUSKY J. S. (1980) Relationship between dolomite nonstoichiometry and carbonate facies parameters. In Conceptsand Models of Dolomitization(eds. D. H. ZENGER,J. B. DUNHAMand R. L. ETHINGTON).Spec. Pub. Sot. Econ. Paleont. Mineral., Tulsa 28, 123- 137. MCKENZIE J. A., HsO K. J. and SCHNEIDER J. F. (1980) Movement of subsurface waters under the sabkha, Abu Dhabi, UAE, and its relation to evaporative dolomite genesis. In Conceprsand Models of Dolomilization(eds. D. H. ZENGER,J. B. DUNHAMand R. L. ETHINGTON).Spec. Pub. Sot. Econ. Paleont.Mineral., Tulsa 28, 1l-30. MISER D. E. (1987) Microstructures in natural and synthetic dolo mites. Ph.D. dissertation, Univ. Texas-Austin. MISERD. E., SWINNEAJ. S. and STEINFINKH. (1987) TEM observations and X-my crystal structure refinement of a twinned dolomite with a modulated microstructure. Amer. Mineral. 72, 188193. LAND
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OOMORIT. and KITANO Y. (1987) Synthesis of protodolomite from seawater containing dioxane. Geochem. J. 21.59-65. OOMORIT., KANESHIMA K., TAIRAT. and KITANO Y. (1983) Synthetic studies of protodolomite from brine waters. Geochem. J. 17, 147-152. REEDERR. J. and WENKH.-R. (1983) Structure refinements of some thermally disordered dolomites. Amer. Mineral. 68.769-776. ROSENM. R. and WARREN1. K. (1988) Dolomite occumnce in the Coorong Region, South Australia (abstr.). Amer. Assoc. Petrol. Geol. Bull. 79, 242. ROSEN M. R., MISERD. E. and WARRENJ. K. (1988) Sedimentology, mineralogy and isotopic analysis of Pellet Lake, Coorong Region, South Australia. Sedimenfology35, 105- 122. SKINNERH. C. W. (1963) Pmcioitation of calcian dolomites and magnesian calcites‘in the southeast of Australia. Amer. J. Sci. 261, 449-472. V~GT J. N. (1986) Dolomitixation and anhydrite diagenesis of the San Andres (Permian) Formation, Gaines County, Texas. M. A. Thesis, Univ. Texas-Austin. VON DERBIRCH C. C. (1965) The distribution and preliminary geochemistry of modem carbonate sediments of the Coorong area, South Australia. Geochim. Cosmochim. Acta 29.78 l-799. VOXDERBIRCH C. C. ( 1976) Stmtigmphy and formation of Holocene dolomitic carbonate deposits of the Coorong area, South Australia. J. Sedimenl. Petrol.46, 952-966. VON DERBIRCH C. C. and LOCKD. E. ( 1979) Geological significance of Coorong dolomites. Sedimeruology26.8 13-824. WARRENJ. K. (1989) Evaporife Sedimemology. Prentice Hall, Englewood Cliffs, NJ, 285~. WENKH.-R., BARBERD. J. and REEDERR. J. (1983) Microstructums in carbonates. In Carbonates: Mineralogy and Chemistry (cd. R. J. REEDER);Reviewsin Mineralogy, Vol. I I, pp. 301-367.
+ .OO
Formation of dolomite in the Coorong region, South Australia MICHAEL R. ROSEN, DONALD E. MISER*, MICHAEL A. STARCHERand JOHN IL WARREN+* Department of Geological Sciences, University of Texas at Austin, Austin, TX 787 13-7909, U.S.A.
(Received August 15, 1988; accepted in revised form December 19, 1988) Abstract-Cores
of unlithitied Holocene sediment from shallow ephemeral lakes in the Coorong region, S.A., contain dolomite at various stratigraphic intervals. Two distinct dolomite types arc recognized using stable carbon and oxygen isotope data, unit cell calculations, transmission eleqron microscopy (TEM), and bulk composition data. Detailed X-ray diffraction (XRD) analyses show that the dolomite is ordered, but that ordering is variable. Average unit cell parameters indicate that the crystal lattice of one type of dolomite (type A) is expanded in the c,, direction (c, = 16.08 A) and contracted in the a, direction (p, = 4.799 A) relative to ideal dolomite (c,, = 16.02 A and o. = 4.812 A, respectively). Type A dolomite occurs in the centers of the larger basins. The mole fraction of MgCO, is as much as 53 f 1%. The unusual crystal chemistry and (TEM) data indicating a heterogeneous microstructure suggestthat this dolomite is generated by rapid precipitation from brines which have highly elevated Mg/Ca ratios. Dolomite ordering reflections are present in electron difiiaction patterns but are weak. Stable oxygen and carbon isotope values are tightly grouped (ave. 6’*0 = +7.6%0, s.d. = 0.6%; 6°C = +3.5%, rd. = 0.7%0). The other type of dolomite (type B) is Ca-rich, has lower stable isotope values (ave. 6”O = +6.4%, b13Ca - 1.2%) than type A dolomite, has a more homogeneous microstructbre, is expanded in both the a. and c. axes, and is generally better ordered than type A dolomite. These data suggesttype B dolomite is precipitated more slowly from lessevaporitic brines than type A dolomite. INTRODUCTION
connected by interlake corridon and the only outlet to the Early Holocene Coorong Lagoon was through North Stromatolite Lake to the east of Pellet Lake. The present-day interlake corridors are now filled with sediment and covered by halophytes and grasses,while the dune ridge system and areas marginal to the ephemeral lake depressions are covered with a dense scrub of eucalypt trees and shrubs. The fifth lake studied (Milne Lake) has had no Holocene marine connection, or any connection to the other four lakes in its development, and yet has the most dolomite in the basin fill. The stratigraphic distribution of the sediments is listed in Table I. All the lakes in the Coorong region can be divided into four basic units regardlessof the mineralogy of the lake. From bottom to top these units are 1) the basal unif consistingofdetrital siliciclastic sand, shell material, and dolomite, 2) the organic rich unit which may have up to 12% total organic carbon, 3) the laminufed unit which compriss the bulk of the sediment in the lakes, and 4) the massive unif. which contains much of the dolomite. More detailed information on the sedimentology of the units in these lakes can be found in ROSEN et al. (1988) and WARREN (1989). An extensive description of the climate and general hydrology of the Coorong area can be found in the above references,and a summary of this information is presented by ROSEN et al. (1988).
THE DISCOVERY OF Holocene dolomite in the Coorong region in the late 1950s has made the Coorong an important natural laboratory to determine the conditions under which sedimentary dolomite forms. Studies by ALDERMAN (1959), SKINNER (1963), VON DER BORCH (1965, 1976), VON DER BORCH and LOCK (1979), and LOCK (1982) have discussed various aspects of the regional chemistry and distribution of dolomite in the Coorong region. Recent papers by BOTZ and VON DER E~ORCH(1984) and ROSEN ef al. (1988) have discussed the stratigraphy and geochemistry of individual dolomite-forming lakes. This paper presents a detailed study of dolomite from cores from five lakes in the Coorong region (Fig. 1). Dolomite has been analyzed from all stratigraphic horizons in the lakes. These horizons represent the extremes of chemical conditions under which dolomite forms in the Coorong region. By determining the chemistry of the dolomite from these known stratigraphic positions, insights can be drawn about the conditions under which the dolomite forms. This leads to a predictive model of the chemistry of Coorong dolomite based on stratigraphic occurrence. GEOLOGIC
METHODS
Forty-two cores along several transectswere taken in the five lakes by pushing4 cm diameter plastic pipe into the soft Holocene sediment until it bottomed on Pleistocene calcrete (Fig. I). Dolomite was iden-
SElTlNG
tified in over 30 of these cores. Based on the depth of penetration of the core tubes to the calcrete, divided by the actual recovered nonrepeated core, we determined a compaction of about 50% due to coring. Cores were wrapped in plastic, shipped back to the United States, split, then sampled at 4 cm intervals. Estimates of mineral abundances were based on relative peak intensities from the XRD charts. All samples were run at 35 KV and 20 ma using CuK a radiation and a scan speed of I .2 degrees2-theta per minute. Some samples were also run at 0.6 degrees 2-theta per minute to determine if the slower scan rate gave more accurate unitcell dimensions. The results for both scan speedsare within the error of the method (see Table 4) and so the faster scan speed was used throughout the rest of the study. Detailed description of the sample preparation. isotope proadurcg TEM, and XRD methods used can be found in ROSEN et ui. (1988). All samples which contained carbonate phases other than dolomite were treated with EDTA, using modifications ofthe methods of BAB.
The lakes analyzed for this study are situated in the northern string of ephemeral lakes (or salinas) behind the Coorong Lagoon (Fig. I). The lakes are separated from the present lagoon by the most seawardridge of a shore-parallelPleistocenedune barrier systemwhich controls the location of the Holocene lakes and interlake corridors along the Southeastern Coastal Plain of South Australia. Four of the five lakes (North Stromatolite Lake, Pellet Lake, Dolomite Lake, and Halite Lake), informally termed the Salt Creek Chain. form a chain of lakes which were ai extension of the Coorong L&on during Early Holocene time (;s6,000 years B.P.). The salt Creek Chain was
Presenf addresses: *Departments of Geology and Chemistry, Arizona State University, Tempe, AZ 85287-1404, U.S.A. l * National Centre for Petroleum Geology and Geophysics.G.P.O. Box 498. Adelaide, S.A. 5001, Australia.
661
M. R. Rosen et al.
662
in this lake may reflect a late Pleistocene marine connection that was quickly closed. Alternatively, the sand may represent the initial deflationary sediments carried into the lake at the beginning of the Holocene. Pellet lake. which was studied in detail by ROSEN ef a/. (198X).has Mg-rich dolomite in the upper 0.2 to 0.6 meters of the lake and Carich dolomite in places around the margin of the lake and in the basal sand unit. Dolomite Lake has only Ca-rich dolomite in the top 0.2 meters and in the basal sand. Halite Lake has only the basal sand unit, and the dolomite in this unit is also Ca-rich. Milne Lake is dominated by Mg-rich dolomite and magnesite throughout the bulk of the sediment in the core. In all the lakes studied, Ca-rich dolomite may coexist or be stratigraphically associatedwith aragonite, calcite and/or Mg-calcite phases. Magnesium-rich dolomite coexists or is stratigraphically associated with magnesite or hydromagnesite phases. FIG. 1. Location map of lakes in the Salt Creek Chain and Mime Lake. The positions of the cores taken are marked for reference.
COCK et al. (1967), to eliminate overlapping peak positions. To determine the effect of EDTA on the dolomite in the sample, two samples of pure Mg-rich dolomite from Mime Lake and two samples of C&rich dolomite from Dolomite Lake were split into five subsamples. The first subsample was untreated and used as a reference sample. the next three subsampleswere treated with EDTA for one minute. three minutes, and five minutes, respectively. The last subsample was treated with fresh EDTA three times at two-minute intervals. After each two-minute treatment the subsample was washed and analyzed for unit-cell parameters and isotopic composition. The results showed that EDTA had little or no effect on the isotopic composition or the unit cell dimensions of any of the samples (Table 2). There may be a slight increase in the MgC03 mole percent (about I mole percent) of the Ca-rich samplesafter three two-minute washings in EDTA. However, this variation is close to the error limit of the method and so is hardly significant. We conclude that only a single kind of dolomite seems to characterize each sample. In this study, all MgCO, mole percent values have been calculated using LUMDSEN and CHIMANSKY’S (I 980) equation. Other equations by HUTCHISON (1974) from the data of GOLDSMITH a al. (1955) and MISER (1987) have been developed from various experimental and natural carbonate phases and may yield significantly different results.Therefore, the determination of Mg mole percent from XRD patterns should be treated with some caution. When possible, XRD valuesshould be correlated with other independent chemical analyses. Unfortunately, in the Coorong dolomite, grain-size is too small (< 1 pm) for microprobe analysis, and small amounts of contaminating Mg and Cabearing minerals make chemical analysessuspect.Therefore, XRD analysis is the most reproducible method of determining MgCO, mole percent in dolomite from the Coorong. Recently, it has been suggestedthat line-grained calcite (~5 pm) washed in distilled water will dissolve and reprecipitate when dried at temperatures <6O”C (BARRERA and SAVIN, 1987). Under these conditions the calcite reprecipitatesfrom water enriched in ‘ti during evaporation and shifts measured oxygen isotopic values to heavier values. The few calcite and aragonite samples in this study were washed and dried following the procedures outlined by BARRERA and SAVIN (1987). Dolomite samples were washed in distilled water and dried at low temperatures; however, there is no evidence at this time to suggestthat this processof dissolution-reprecipitation is op erating on tine-gmined dolomite samples.
RESULTS
Stable oxygen and carbon isotopes
We analyzed oxygen and carbon isotopic ratios from the various dolomitic units. The results from over 100 analyses (Fig. 3, Table 3) indicate that two types of dolomite, type A and type B, can be recognized from their isotopic signature. Type A dolomite is, in general, slightly heavier in oxygen (mean = +7.6) than type B dolomite (mean = +6.4) and is 3 to 6% heavier in carbon than type B dolomite (mean, type A = +3.5; mean, type B = -1.2). Figure 3 shows a fairly tight fit of type A dolomite along a linear trend (correlation coefficient = 0.75). Type B dolomite values are variable and do not appear to follow a simple trend. Where we could find single phases of other carbonate minerals (i.e. aragonite and calcite), they were analyzed to see if they bore any relation to the dolomite isotopic signatures. The samples analyzed do not coexist with dolomite (i.e. they are from a non-dolomitic stratigraphic interval). Very few single phases uncontaminated by organic matter or sulfate minerals could be found. Five aragonite samples from Halite Lake and one calcite and one aragonite sample from Dolomite Lake were analyzed (Table 3). The aragonite from both Dolomite and Halite Lake is I to 3%~lighter in oxygen than the Type B dolomite from the core from which the sample was taken, and the carbon values are 2 to 4%0 heavier. Similarly, the calcite value from Dolomite Lake is almost 7%0 lighter in oxygen than the dolomite from the same core, though the carbon values are similar.
hydlhcS.hCm&W;wmdM,lreL&tiwn~ lalc I. Mmaal* smm thickness of unitsin nxtas.% iben d Wlmn (1988) forcxplarmtim of unm .._~. nl*l ~_mc-__. N. SoomuoLe HaJib l&z IJC IA& ldkf
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DISTRlBUTION
OF DOLOMITE
Dolomite has been found at various stratigraphic intervals in the five lakes studied (Fig. 2). In North Stromatolite Lake, Ca-rich dolomite occurs only in a basal siliciclastic sand unit. This unit can be found in all the lakes studied,and probably representsa sand deposited ‘ve phaseof evolution in the area at the beginning during the tmmgmast of the Holocene when all four lakes in the Salt Creek Chain were floodedby marine water from the Coorong Lagoon. Although Milne Lake never had a Holocene marine connection, the thin basal sand
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663
Formation of dolomite in the Coorong
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Unit cell calculations Results from over 150 unit cell calculations (Fig. 4, Table 4) show that dolomite unit cell dimensions can also be grouped into two categories. These two categories correspond to the type A and type B dolomite groups determined from the isotopic analysis. Type A dolomite (which has heavy oxygen and carbon values) has unit cells expanded in the c,
direction (average of 16.08 A) and contracted in the a, direction (average of 4.799 A) with respect to ideal dolomite (a~ = 4.8 12 c, = 16.02). Most samples from type B dolomite (which has lighter oxygen and carbon values), particularly those from the margins of the lakes, are expanded in both the c, and a, directions. Average unit cell dimensions for type B dolomite are a, = 4.827 and c, = 16.15. Type A dolomite is dominated by Mg-rich dolomite, containing up to 3 mole percent excess MgCOj . Type B dolomite samples from the basal siliciclastic sand may have nearly stoichiometric compositions, whereas type B dolomite from
6-
;w
FIG. 2. Idealized east-west cross-section through the salt Creek Lake Chain and Milne Lake showing the position of the geochemical dolomite types in the lakes and in the interlake corridors. Note the occurrence of type B dolomite in the center of Dolomite Lake (see isotope discussion section for explanation). WARREN (1989, p. I74I8 1) givesa full description of the Coorong stratigraphy.
Fro. 3. Cross-plot of stable oxygen and carbon isotopes. Notice the relatively tight grouping of type A dolomite (squares) which appear to follow a covariant trend (arrow). Type B dolomite values are more variable and do not follow any simple trends.
M. R. Rosen et al.
664
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the margins of the lakes or lake corridors are invariably more &rich. This difference is also reflected in the calculated unit cell vahles. Possible depth-related trends are present in type A and type B dolomite in Pellet Lake, but only within individual cores (ROSEN et d., 1988). However, lake-wide trends or interlake trends have not been observed for either type A or type B dolomite.
ically throughout all samples. To our knowledge, no ancient dolomite with similar microstructure has been observed. The lattice strain associated with such an unusually high defect density should render these crystals highly unstable. It is unlikely that the crystals would persist for geologically long periods of time without being replaced by a dolomite with a more homogeneous microstructure. Despite these defects, electron diffraction patterns of type A dolomite show fairly sharp, discrete diffraction maxima, usually without any significant streaking. Thus, the material is well crystalline and devoid of amorphous domains. Similarly, there is no evidence of Ca- or Mg- enriched domains, although some concentration along defects is likely. Furthermore, none of the dolomite is intergrown with any other carbonate phase, such as aragonite, magnesite, or hydromagnesite. Other carbonates, when present, occur as separate and distinct crystal phases. Ordering reflections can be observed in type A dolomite, but they are extremely weak (Fig. 5b). A great deal of the intensity observed in electron diffraction patterns is due to multiple diffraction. Thus, the materials, although technically dolomite, demonstrate considerable cation disorder. The variability in the degree of cation ordering probably causes the spread in calculated c, values (see Unit ceil calculations). Type B dolomite, although Ca-rich, generally does not display a modulated microstructure typical of other calcian dolomites, though it has been found rarely in some samples (Fig. 5~). When found, the modulated contrast is generally weaker than that found in ancient, calcian-rich dolomites. This weak modulated microstructure may represent the regular arrangement of the defects seen to be random in other crystals from these lakes, indicating a slower growth of these crystals when compared to most of the Coorong dolomite. Type B dolomite crystals from the margins of the lakes and Dolomite Lake possess a relatively homogeneous microstructure (Fig. 5d). The excess calcium must therefore be distributed evenly throughout the lattice. Electron diIEaction patterns and stronger ordering reflections observed in XRD patterns demonstrate that this dolomite possesses stronger ordering reflections than type A dolomite. Type B dolomite
16.3-)
k
Transmission electron microscopy In TEM, both type A and B dolomite crystals exhibit large variations within a given sample. Microstructures range from heterogeneous, with closely spaced, random defects, to relatively featureless and homogeneous crystals. The heterogeneous crystals (Fii 5a) are similar to those described by WENK et al. (1983). Due to the closely spaced nature of the defects in the heterogeneous crystals, exact identification of individual features cannot be determined, however, they probably consist largely of dislocation tangles and stacking faults. These structures probably represent “mistakes” incorpomted during growth. The high defect density of the heterogeneous dolomite can produce overall warping of the crystals and may divide them into smaller subcrystals. The latter are documented by the presence of MoW fringes in some samples. Type A dolomite is dominated by this type of heterogeneous microstructure, although it occurs sporad-
.
I To calcite
0 0
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A dolomite B dolomite) 4.630
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a0 FIG. 4. Unit cell dimensions for both type A and type B dolomite. The line connects the unit cell dimensions of calcite to magnesite. Notice that ideal dolomite falls below this line due to the contraction of the unit cell when dolomite is well ordered. In contrast to the isotope plot, type A dolomite unit cell dimensions are scattered, indicating rapid precipitation from solution. Type B dolomite unit cell dimensions
are tightly grouped, indicating slower precipitation.
665
Formation of dolomite in the Coorong T&lr,4. Hcugonaluni~celldimensbininank(momr~MgCQ3mlnole%
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samples, particularly from the basal siliciclastic unit, typically show more sharply defined superstructure ordering peaks than type A dolomite. Type B dolomite from the siliciclastic basal unit is variable but exhibits two crystal populations. Most of the crystah show a heterogeneous microstructure similar to that of type A dolomite, but ordering reflections are stronger. In addition, a large number of crystals are homogeneous and are larger than the heterogeneous ones. DISCUSSION Isotopes ROSENet al. (1988) attributed heavy carbon values in Pellet Lake to an inorganic Rayleigh distillation process rather than organically influenced methanogenetic processes. Evidence for organic or inorganic control on the carbon reservoir in the lake centers is still equivocable; however, the most compelling evidence for an inorganic origin for heavy carbon values in these lakes is that dolomite is most abundant at the sediment-water interface. If organically influenced methanogenetic processes were creating heavy carbon values, dolomite would be expected to be most abundant below the sediment-water interface and decrease in abundance toward the top of the core. Heavy 613C values identical to the heaviest Coorong values have also been reported by CLAYTON et al. (1968) from Deep Springs Lake in California; a continental playa. They attributed the heavy carbon values to strictly inorganic processes. Thus, it seems likely that a biological contribution to the carbon reservoir in the lake centers is
minor. However, organically influenced meteoric or soil CO2 must contribute to the lighter carbon ratios for type B dolomite. This is consistent with the stratigraphic position of type B dolomite which is around the margins of the lakes, in the interlake cqrridors, and in the siliciclastic basal unit. Although evaporitic in nature, the oxygen isotope. values for type B dolomite from the siliciclastic base are on average lighter than those of type A dolomite (Fig. 3). Restricted evaporitic conditions most likely existed during deposition of the siliciclastic basal unit, but the area had not evolved into a completely closed basin. In addition, the water body would be necessarily deeper and larger and thus less susceptible to extreme evaporative influences on its isotopic composition. Average oxygen and carbon isotopic values for cores from the basin centers are slightly heavier than for cores closer to the margins and much heavier than type B dolomite in the interlake corridors. This is consistent with an inorganic evaporitic origin for the heavy values. Within the Salt Creek Chain, type B dolomite seems to become lighter in both oxygen and carbon toward Halite Lake. This may be due to the proximity of a more meteoric influence in Halite Lake during the Early Holocene connection to the Coorong Lagoon. The relatively consistent isotopic values for type A dolomite suggest a relatively homogeneous isotopic reservoir produced by an evaporating brine. The covariant trend demonstrated earlier (Fig. 3) also suggests an evolving brine. The relative scatter of type B dolomite isotopic values, particularly in the carbon signatures, suggests some sort of mixing zone between evaporitic brines and meteoric water (CONIGLIOet al., 1988).
666
M. R. Rosen e( al.
FIG. 5. a) TEM dark field photomicrograph of a crystal from type A dolomite (core PM). The heterogeneous appearance of the crystal is the result of extremely closely spaced lattice defects. The small, pseudo-lamellar features at the top of the crystal are interference fringes caused by the overlapping ofdomains ofthe crystal with slightly different orientations. This microstructure is very common in type A dolomite and is typical of those that are largely cation disordered (g = 01.2 s = 0). Photograph from ROSENet a/. (1988). b) Electron diffraction nattem of a tvpe B dolomite crystal from Dolomite Lake (core DA). Half of the spots in this pattern, such as the 00.3 reflection, would be missing if the crystal had a calcite-like structure even if it had a dolomite composition. The extra spots are the result of the change in symmetry from R3c to R3 that occurs as a result of dolomite-type cation order. These “ordering” reflections verify that the crystals have the dolomite structure. In addition, they are much more intense type B dolomite than in Type A dolomite, suggesting a greater degree of cation ordering in the former (see ROSENet al,, 1988, for a type A dolomite electron diffraction pattern). In this orientation, none of these “ordering” reflections can be derived solely from multiple diffraction events or other spurious effects. c) Dark field electron photomicrograph taken in a two-beam condition with g = (108) (core DA). The diffuse light and dark bands represent the modulated microstructure found only in type B dolomite. This microstructure is common to ancient and some modem marine dolomite but is relatively rare in samples from the Coorong and unknown from Holocene dolomite from other continental settings. Notice that this growth feature changes orientation in different sectors of the crystal with its orientation governed by the orientation of the nearest crystal face. Scale bar is 0.2 Frn. d) TEM dark field photomicrograph of a type B dolomite crystal (core PA). Although the core appears similar to 5a, the rims are homogeneous in appearance, and thus contain far fewer lattice defects. Notice the well developed rhombohedral crystal faces. Ordering is still imperfect, but better than most of type A dolomite. Scale bar = 0. I pm, g = 10.4, s = 0. Photograph from ROSENe( al. (1988).
The mineral associations and the stratigraphic positions of type B dolomite (i.e. around the edges of the lakes and between lakes) also suggest a mixed-water influence. The occurrence of type B dolomite in the center of Dolomite Lake can be attributed to the relatively small area of lake surface. Because the lake is so small, the entire lake surface is influ-
enced by meteoric input and so forms type B dolomite rather than the more evaporitic type A dolomite. Oxygen values in type B dolomite are heavy compared with typical values for dolomite (LAND, 1980) but are consistently lighter than type A dolomite. It may be that incoming groundwater, affected by biologic activity, is relatively light
Formation of dolomite in the Coorong
in carbon, yet the water is still a relatively concentrated brine and so retains relatively heavy oxygen values. Following the equations in LAND (1985), it can be shown that the 8”O isotopic value (in SMOW) of the water which precipitated the dolomite. can be constrained to around +4 to +6% (Fig. 6) given the temperature range measured by VON DER BIRCH (1965) in lakes of the region. This value is heavy for meteoric water, but considering the evaporitic nature of the Coorong region, it is not unreasonable. Oxygen isotopic ratios of > +7%0 (SMOW) have bee.n reported from continental groundwater inland of the Abu Dhabi sabkha (MCKENZIE et al., 1980). Because of the presence of magnesite or hydromagnesite and lack of calcite or aragonite, it seems likely that the solution which precipitated phases in type A dolomite unit had a higher Mg:Ca ratio than the solutions which precipitated type B dolomite. VONDER BIRCH (1965) and B~TZ and VON DER BIRCH (1984) have both observed that Mg-rich dolomite (our type A dolomite) in the Coorong area correlates with the presence of magnesite (i.e. Milne Lake). This suggests a genetic relationship between co-existing Mg-rich dolomite and magnesite, possibly due to high Mg:Ca ratios of the precipitating fluid. In Pellet Lake, dolomite occurs with hydromagnesite, another Mg-rich mineral phase, so that high Mg:Ca ratios of the precipitating fluid can be called upon to explain the excess of Mg in the dolomite crystal lattice. The suggestion that a high Mg:Ca ratio is an important controlling factor in dolomite formation is not new. VON DER BORCH (1965) and FOLK and LAND (1975) suggested high Mg:Ca ratios to explain dolomite formation. Experimental data from “protodolomite” formed under very high Mg:Ca ratios but at low temperatures (33“C) also suggests the importance of high Mg:Ca ratios on the formation of sedimentary dolomite (00~0~1 et al., 1983; OOMORI and KITANO, 1987). The few isotopic values from aragonite in Halite Lake suggest that isotopically different fluids precipitated dolomite and aragonite at the base of the core. If the aragonite and dolomite precipitated in isotopic equilibrium, then the do-
10
8
6
4
2
‘80 (PDB) FIG. 6. Plot of oxygen isotopic composition of the dolomite (in PDB) versus temperature. The curved lines represent isotopic compositions of the water (in SMOW) in equilibrium with dolomite at the given temperatures (data from LAND, 1985). Given the range of isotopic compositions for both type A and type B dolomite (see Table 3) and a temperature range of 10 to 28°C (see text), the isotopic composition of the water precipitating the dolomite is constrained within the black box (between +4 to +6% SMOW).
667
lomite should be at least 3460heavier in ‘*O than the other carbonate phase (LAND, 1985). The aragonite and dolomite samples that are 0.04 m apart are only about 1960different. The calcite sample from Dolomite Lake is almost 7%~lighter than the dolomite from the same core. This fractionation is too large to be in equilibrium with dolomite. Therefore, it seems unlikely that dolomite is forming from the diagenetic replacement of precursor calcite and aragonite phases. These data, although sketchy, also support the other data suggesting that dolomite precipitated directly out of solution and had no other carbonate precursor phase.. Unit cell parameters and microstructures Type A dolomite is overwhelmingly dominated by crystals with unit cells contracted in the a, direction and expanded in the c, direction relative to ideal dolomite (Fig. 4). The incorporation of small Mg ions replacing the larger Ca ions in the unit cell has been proposed as the cause of the contraction in the a, direction (ROSEN et al., 1988), whereas expansion of the c,, parameter may be related to cation disordering (cJ REEDER and WENK, 1983) or other lattice defects (MISER et al., 1987). Variations in c, are more variable, particularly for type A dolomite, suggesting that ordering may be a more important control than composition for this unit cell dimension. The incorporation of excess Mg in type A dolomite crystals and its association with magnesite or hydromagnesite suggest precipitation from a highly evaporitic solution with a high Mg/Ca ratio. Crystallization of type A dolomite from highly evaporitic fluids may have been very rapid, causing the incorporation of excess magnesium into the lattice and some cation disorder. The degree of cation disorder and variability from crystal to crystal is further evidence that the dolomite precipitated rapidly from solution. In Ca-rich (type B) dolomite, the presence of calcite and aragonite phases coexisting with dolomite suggests that the Mg/Ca ratio was much lower. Dolomite crystallization may have been much slower with fewer Mg ions incorporated into the lattice. The basal siliciclastic type B dolomite, which may be nearly stoichiometric and only slightly expanded in the c, direction, may represent an intermediate state of fluid saturation (and rate of crystal growth) between type A dolomite and some of the more Carich type B dolomite, or it may represent a diagenetic stabilization of a previous Mg-rich dolomite. The occurrence of a modulated microstructure rather than random defects in type B dolomite may also reflect slower growth of these crystals. Type B dolomite from the margins of the lakes and the interlake corridors generally is less ordered, and has a more expanded unit cell (in both directions) than the basal type B dolomite. In TEM, replacement textures were not observed. This suggests that where the dolomite is most influenced by meteoric water input, the pore water has little tendency to remain within the dolomite precipitating stability field so that only poorly ordered, Ca-rich dolomite can form. These data also indicate that differences between the dolomite types are the result of the chemistry of the precipitating solutions and are not related to progressive stabilization by dissolutionreprecipitation reactions with increasing age.
M. R. Rosen et al.
668 GEOLOGIC SIGNIFICANCE
The variability of unit cell parameters, and the degree of disorder relative to ideal dolomite, indicates that most, if not all, dolomite in the Coorong region is metastable. With time and burial, Coorong dolomite is likely to be subject to dissolution and reprecipitation as a more stable diagenetic dolomite phase. This stabilization will destroy most of the primary microstructures and alter the isotopic composition and unit cell dimensions of the dolomite. So what is the significance of this study to examples of ancient dolomites? Type B dolomite is similar to modern dolomite from other regions except that it lacks abundant modulated microstructures. Although metastable, it may be more stable than type A dolomite and so retain some of its characteristics even when diagenetically altered. In addition, if Type A and Type B dolomite are affected by the same fluid, the resulting geochemical parameters may retain their relative differences. In a relatively open diagenetic system that is rock-buffered, diagenetic type A dolomite should be relatively heavier in carbon and oxygen than diagenetic type B dolomite given a more or less homogeneous diagenetic fluid. If diagenesis occurs in a closed or semi-closed system retention ofthe relative isotopic differences would be more likely. In rocks that may have had limited access to diagenetic fluids, microstructures of the primary dolomite may be retained in the cores of crystals which subsequently acquire overgrowths. For example, the recognition of dominantly heterogeneous cores in a particular dolomitic facies would indicate relatively rapid crystal growth. In addition it suggests that the dolomite formed from an evaporitic brine directly from solution (i.e. type A dolomite). Examples of an environment where relicts of the primary fabric could be retained would be dolomite enclosed by less permeable evaporite seals. Sabkhas or evaporite brine pans may create such a seal. Future detailed study of dolomites encased in Jurassic evaporites in the Buckner Formation (Gulf Coast, U.S.A.) or in the Arab sabkha cycles of the Arabian Gulf may reveal evidence of their original depositional microstructure. An example of the application of unit cell parameters to an ancient environment is illustrated by VOGT (1986, p. 6775). He demonstrated slight, but statistically valid, differences in unit cell parameters of peritidal and subtidal dolomite from the San Andres Formation (Permian) of Texas, U.S.A. Although this dolomite has been interpreted as a pervasive early replacement dolomite rather than a primary precipitate, the method illustrates a difference in the composition of the dolomite from two environments related to the fluids from which they were precipitated. CONCLUSIONS The geochemistry of Coorong dolomite systematically varies within a wide range of compositions, microstructures and isotopic values within a single basin, and thus constrains the conditions under which dolomite can be expected to form. Two types of dolomite have been recognized which characterize the dolomite forming in the Coorong region. Type A dolomite is characterized by: 1) the incorporation of excess Mg into the crystal lattice; 2) the presence of a heterogeneous microstructure; 3) poor degree of cation ordering; 4) asso-
ciation with the magnesium carbonates hydromagnesite and magnesite; 5) relatively heavy oxygen and carbon isotope ratios; and, 6) the absence of associated CaCOs phases. These characteristics suggest rapid crystallization from evaporitic basin center brines with a relatively high Mg/Ca ratio. In addition, the absence of replacement textures, such as syntaxial, submicron calcite domains in dolomite, or the partial dolomitization of shell and fecal material, support the suggestion of VON DER BORCH (1965) that type A dolomite in the Coorong precipitates directly from these brine solutions. Type B dolomite is characterized by: 1) incorporation of excess Ca into the crystal lattice; 2) variable but more homogeneous microstructures with occasional modulated microstructures present; 3) relatively better cation ordering than type A dolomite; 4) association with the calcium carbonate phases aragonite, calcite, and Mg-calcite; 5) slightly lighter oxygen isotopic values and much lighter carbon isotope values than type A dolomite; and, 6) the absence of associated MgCOJ phases. These characteristics suggest that type B dolomite formed from fluids under lower Mg:Ca conditions than type A dolomite. The more homogeneous microstructures, minor modulated microstructures, stronger ordering reflections, and small crystal overgrowths suggest that the dolomite formed more slowly than type A dolomite. The carbon isotopic values along with the stratigraphic position of the dolomite suggest that type B dolomite formed from a mixture of evaporitic basin center brines and less evaporitic organically influenced meteoric water. Acknowledgements-We would like to thank Chris von der Borch of Blinders University who provided the facilities of the Earth Sciences Department during the field stages of this study, and Lynton Land of the University of Texas-Austin who provided stable isotopic analyses and reviewed earlier drafts of this manuscript. Discussions with Patti &anger were very helpful in focusing our thoughts on various aspects of Coorong sedimentology. We would also like to thank Jeff Horowitz for expertly drafting Figs. 1 and 2. This work was funded by N.S.F. Grant No. EAR-8206146, and their support is gratefully acknowledged. D.E.M. acknowledges the support of the Mobil Field Research Laboratory. The Owen Coates Foundation of the University of Texas-Austin funded part of the publication costs. Editorial handling: S. E. Calvert REFERENCES
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Formation of dolomite in the Coorong GOLDSMITH
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