The economically important black shale facies contains several discrete chemical groupings in visually similar rocks. We examined the elemental compositions of more than 300 black shales and anoxic sediments in order to understand the relation of their chemical variability to the depositional environment. The low-calcic shales were found to contain at least four discrete groupings whose depositional and early diagenetic redox conditions are suggested by their Fe, Mn and V concentrations (Quinby-Hunt and Wilde, 1994). The calcic shales (72 Paleozoic, 50 Mesozoic and 42 Cenozoic; > 50,000 ppm Al, > 4000 ppm Ca) were examined in terms of the four thermodynamic groups previously defined for the low-calcic shales. The Ca/Mn ratios of the calcic black shales fall into four discrete clusters (as opposed to a continuum). This differentiation reflects the stability of Mn minerals in overlying and interstitial waters under varying redox conditions. One cluster (Ca/Mn I) from a period of glaciation (Ordovician-Silurian boundary) have high Mn concentrations and fall on: [Mn] = 0.065 [Ca] + 1280 (ppm) (r = 0.95, n = 7). A second group (Ca/Mn II) from many different localities and ages fall on a line with a lower slope: [Mn] = 0.038 [Ca] + 187 (ppm) (r = 0.92, n = 21). The third group (Ca/Mn III) fall on the line: [Mn] = 0.011 [Ca] + 152 (ppm) (r = 0.95, n = 19). The greatest number of samples (Ca/Mn IV) fall on the line: [Mn] = 0.0027 [Ca] + 144 (ppm) (r = 0.85, n = 69). In order to establish the depositional environment associated with each of the groups, more modern sediments whose depositional settings are better defined were explored. As a result, the samples from both Ca/Mn I and II can be associated with deposition under oxic water, those from Ca/Mn I remained oxic during early diagenesis. After deposition, Ca/Mn II samples became anoxic, releasing Mn to interstitial waters. Ca/Mn II and IV were deposited under anoxic waters. In order to establish the thermodynamic environment of the shale component of the calcic shales, it is necessary to extrapolate to the no calcium intercept to determine the Mn content, then apply the rules determined for the low- calcic shales. This study demonstrates the wide variety of black shale depositional environments from oxic to methanogenic. Accordingly, black shales can not all be assigned to anoxic, sulfidic environments without addition chemical analysis and interpretation. The proper environmental assignment should be useful in assessing the economic potential of particular black shale deposits.
Black shales can be important economic deposits as they are a source of valuable metals such as manganese, copper, and gold and are a source rock for petroleum deposits and oil shales (Vine and Tourtelot, 1970, Armands, 1972; Gustafson and Williams, 1981; Andersson et al., 1982; Hallam, 1987; Force and Cannon, 1988; Vaughn et al., 1989; Ulmishek and Klemme, 1990; Shepherd et al., 1991; Frakes and Bolton, 1992; Roy, 1992; Moore et al., 1993). However, the economic potential of any given black shale can not be determined simply from its field lithologic description. Nor can its depositional environment or the potential economic viability of its mineral deposits be determined using the measured quantity of organic matter or C/S ratios determined in black shales (Berner and Raiswell, 1983; Raiswell and Berner, 1986, 1987; Pederson and Calvert, 1990). Detailed assays of spot samples are expensive and difficult to extrapolate to delineate the size of the ore body or the reservoir. Accordingly, relatively simple chemical tests which serve as a guide to the environmental conditions for their formation are of both geologic and economic interest.
The depositional environments of black shales and black shale facies were generally thought to be anoxic (Pettijohn, 1949; Vine and Tourtelot, 1970). However, black sediments can be deposited from oxic to anoxic conditions under fresh to estuarine to marine waters. Marine waters reflect a generally less variant water chemistry than do estuarine and fresh waters. However, even black shales formed under marine waters reflect varying degrees of productivity and sedimentation, and water column and sedimentary pH and Eh. Anoxic sediments form (1) under oxic bottom water where the biological or chemical oxygen demand consumes the oxygen in the interstitial waters producing anoxia in the sediment or (2) where anoxia already exists in the water column. The presence (oxic bottom water, >0.2 mL/L O2) or absence of bioturbation (anoxic bottom water, < 0.2 mL/L O2) (Rhoads and Morse, 1971), is commonly used to differentiate between oxic and anoxic bottom waters, although it is important to remember that the conditions that represent "anoxic" to an aerobic organism, do not indicate that all dissolved oxygen has been consumed. The black shales under discussion in this paper are all non-bioturbated, indicating that they were formed in sediments whose overlying waters contained < 0.2 mL/L O2. Thus to aerobic organisms, the marine black shales of this study would be considered anoxic, however, chemically their conditions of deposition range from oxic, to nitrate- and sulfate-reducing to possibly methanogenic (Quinby-Hunt and Wilde, 1994).
The chemistry of oxic and anoxic waters differs vastly (Richards, 1965), resulting in wide variations in dissolved metal content and speciation. Similarly, the chemistry of anoxic waters varies depending on the most oxidative species present. That Mn is mobilized under reducing conditions has been reported in numerous articles (see, for example, Eaton, 1979; Bacon et al., 1980; Jacobs and Emerson, 1982; Emerson et al., 1983; Jacobs et al., 1985, 1987; Thomson et al., 1986; Landing and Bruland, 1987; Murray and Kuivila, 1990). The geochemistry of non-bioturbated black shales varies depending on the oxidative species - O2, nitrate, sulfate, and so on. As oxygen and oxygenated species are consumed in sea water, the pH and Eh of that water changes (Froelich et al., 1979; Wilde, 1987), resulting in black shales and anoxic sediments of very different compositions (Gee, 1981; Berry et al., 1986; Middleburg et al., 1987; Quinby-Hunt et al., 1989a; Vaughn et al., 1989; Wilde et al., 1989). Because ambient redox and pH conditions are important in determining thermodynamic stability fields (Garrels and Christ, 1965, Stumm and Morgan, 1970, Lee and SillÚn, 1959) during deposition and early diagenesis (Berner, 1981a,b), geochemical characteristics of black shales may provide evidence in their chemical composition of their chemical depositional environment.
The depositional environment of lithologically-described black shales must be investigated through means other than simple color or even the preservation of organic matter. As we noted in Quinby- Hunt and Wilde (1994), open ocean black shales with low rates of deposition offer an opportunity to determine the paleo-chemical environment during deposition and early diagenesis. As they are deposited slowly, such sediments were probably in equilibrium with the chemistry of the overlying waters and subsequently interstitial waters. Modern sediments deposited under anoxic bottom waters or experiencing anoxic conditions in interstitial waters reflect these conditions (Jacobs and Emerson, 1982; Emerson et al., 1983; Jacobs et al., 1985, 1987). After compaction, fine grained materials develop low porosity and permeability so that the remaining chemical signature is basically preserved. Thermal or chemical metamorphism or metasomatism can modify the chemical signature to some degree (Haack et al., 1984), however, the black shales examined in this study do not show signs of thermal metamorphism. The concentrations in the shales or sediments of elements sensitive to ambient Eh and pH in the shale should be a indicative of Eh-pH conditions during deposition and early diagenesis.
Depositional Environments of Low-Calcic Black Shales
As noted in Quinby-Hunt and Wilde (1994), the chemical composition of shales reflects (1) their initial (pre-erosional) source rocks for minerals stable in the depositional environment (Wood, 1980; Wood et al., 1979; Thompson et al., 1980; Quinby-Hunt et al., 1989b, 1991, Wilde and Quinby-Hunt, 1993), (2) the chemistry of the depositional environment as a function of early diagenetic processes and (3) post-lithofaction processes such as metamorphism, hydrothermal alteration, etc. We have examined the first factor, immobile elements, in Quinby-Hunt et al. (1991). In Quinby-Hunt and Wilde (1994), the chemistry of the depositional environment derived from mobile elements using a thermodynamic model as the indicator of the ambient oceanographic conditions during deposition in low-calcic, finely- laminated, non-bioturbated marine black shales.
For the low-calcic shales, four groups of shales were identified based on their Mn, Fe, and V concentrations (Table 1). These groups were related to five thermodynamic zones based on changing Eh and pH (Figure 1). Thus, for these low-calcic shales, variations in Mn and Fe concentrations can be used as indicators of Eh-pH conditions during deposition and early diagenesis. In group 1, corresponding to I in Figure 1 (oxic conditions: high Eh), Mn and Fe occur as Mn3+, 4+ and Fe3+ oxides and appear in relatively high concentrations (average concentrations 1310 and 56000 ppm, respectively). Group 2 corresponds to either IIa or III in Figure 1 indicating an environment that is nitrate-reducing to sulfate-reducing at intermediate pH. Mn is reduced to Mn2+ as is evidenced by the low Mn concentration (310 average). Probably most of the samples fall in zone III, as the nitrates that would be formed in zone IIa are relatively light in color. In the group 2 samples, Fe remains bound as oxides or sulfides with concentrations comparable to those in group 1 (52000 ppm average). In the samples of group 3, both Fe and Mn are reduced and relatively soluble, reflecting anoxic but non-sulfate reducing Eh and intermediate to low pH (Group 3 samples fall in zone IIb in Figure 1). The solubility of Mn2+ and Fe2+ and Mn and Fe mineral instability is reflected in the low average concentrations 170 and 23500 ppm respectively for Mn and Fe in the group 3 samples.
In group 4 low-calcic black shales, Mn and Fe concentrations are low - the Mn concentration is less than 260 ppm and averages 80 ppm and Fe is < 36000 ppm and averages 19000 ppm. Group 4 black shales have V concentration > 300 ppm, averaging ~1500 ppm, as compared to the V concentrations in groups 1, 2, and 3, which are generally less than 320 ppm, averaging from 130 to 170 ppm. As is explained in Quinby-Hunt and Wilde (1994), vanadium can be used as an indicator of the preservation of planktonic organic C. Vanadium often occurs in organic matter as V-tetrapyrrole complexes, which are best preserved under aphotic, low pH, highly-reducing conditions (Lewan and Maynard, 1982; Lewan, 1984). Organic matter derived from marine plankton are known to concentrate V. The highest contents of V in fossil fuels occurs in deposits that were originally plankton or algae (Breit, 1988 Lewan, 1984); bitumens derived from plankton are elevated in V content; those derived from higher plants are not enriched. Plankton accumulate V, primarily in chlorophyll, a precursor to tetrapyrroles (Yen, 1975; Lewan and Maynard, 1982; Breit, 1988).
In shales formed under oxic or mildly reducing conditions or with pH > ~ 7, organic matter and the tetrapyrroles would have been oxidized. The V content of shales formed under such conditions (Figure 1, zones I, II, and III) represents a baseline inorganic contribution (average ~140 ppm), such as is found in groups I, II and III. The high V concentration with the relatively low Mn and Fe content found for the group 4 samples, suggests low pH and low Eh for this group. The group 4 samples therefore, most probably fall in zone IV in Figure 1 (Quinby-Hunt and Wilde, 1994).
Samples and Procedure
The elemental concentrations of 121 black shales, of Cambrian, Ordovician, Silurian, Carboniferous, Jurassic, and Miocene (Table 2), were determined using neutron activation analysis at Los Alamos National Laboratory (Minor et al. 1982). Data are available on request. Additionally, 39 sediments from the Pleistocene San Francisco Bay were analyzed (Table 2). A suite of about 40 elements was determined. The advantages of using a single technique and single analytical laboratory and our methods of data handling, detection limits are described in Quinby-Hunt et al. (1989a,b). Only a large data set from a wide variety of well-characterized black shales, analyzed using modern, readily comparable analyses could assure that the data were comparable, and that the many different depositional environments resulting in marine black shales were represented. Our total collection contained some 400 field-identified "black" shales. For this report, we limited the discussion to samples containing more than 4000 ppm Ca and greater than 50000 ppm Al.
The Ca to Mn ratios of the samples were determined and Ca plotted vs Mn for the Paleozoic and Mesozoic black shales (Figure 2). Groups of samples were determined based on the plot in Figure 2. The correlation between Ca and Mn was determined for each group (Figure 3).
When considering the depositional environment for these black shales, it is useful to refer to an Eh-pH diagram to elucidate the various Mn species available under differing redox regimes (Figure 4). Because the stability fields derived in Eh-pH diagrams for common Mn oxides, sulfides and carbonates depend on a number of variables, including pH, Eh, cation and anion concentrations, pressure and temperature dependence of the carbonate system, problems of supersaturation, and the particular equilibrium expression chosen, such diagrams are necessarily not definitive - greatly simplifying the complexities of the Mn-sea water system. However, they can aid in understanding the complex factors under consideration. Construction of these diagrams is described in Garrels and Christ, 1965; Lee and SillÚn, 1959; and Brookins, 1987). The boundary conditions for Eh-pH diagrams for elucidation of the depositional environment of black shales was described in our treatment of low-calcic shales (Quinby- Hunt and Wilde, 1994). The pH range, 5-9, brackets natural marine to estuarine conditions: aerobic sea water, 7.8-8.5; anaerobic sea water, usually pH 6.8 or greater (Skirrow, 1975; Grasshoff, 1975; Jacobs et al., 1985); and interstitial waters, usually greater than pH 5, (Baas Becking et al., 1960). Eh is bounded by the stability of water.
The lines representing the reduction of NO3- and SO42- are included to provide markers indicating the relative degree of anoxicity associated with the presence of certain Mn species. NO3- reduction to 2 roughly corresponds to the boundary between oxic and anoxic conditions and is independent of dissolved NO3- concentration. Reduction of SO42- to H2S or HS- (independent of dissolved sulfur concentration) approximates the redox conditions for the appearance of sulfide minerals, although their occurrence depends on both the Mn2+ and dissolved sulfur concentrations. Total dissolved sulfur was defined as the oceanic SO42- concentration at 35 ppt or 28mM/kg (Morris and Riley, 1966) with an activity coefficient of 0.12 (Garrels and Christ, 1965).
As the oceanic carbonate concentration is a function of atmospheric carbon dioxide pressure, which varied considerably over the time scale spanned by the black shales under consideration, Figure 4 shows a range of rhodochrosite solubility. The range is bounded by the maximum projected for the Phanerozoic in the Cambrian, about 18 times present, to a minimum in the modern world (Berner, 1990). The average observed Sigma CO2 in the modern ocean is 2.2 mM (Skirrow, 1975), activity coefficient of 0.47 (Garrels and Christ, 1965). With the exception of the Carboniferous samples, the Paleozoic samples in this study would probably have been formed under Sigma CO2 conditions significantly higher than present. The Mesozoic and Cenozoic samples probably experienced Sigma CO2 closer to those of the present.
The dissolved Mn concentration was varied from the maximum observed in oxic sea water ~10-8 (Pacific Ocean just above the oxygen minimum zone, Martin and Knauer, 1984) to that observed in interstitial waters of anoxic sediments and anoxic basins, ~10-5 (Jacobs and Emerson, 1982; Jacobs et al., 1987). The concentration boundaries were based on the solubilities of Mn(OH)2, Mn2O3, Mn3O4, d-MnO2, and MnCO3 (Bricker, 1964; Garrels and Christ, 1965; Brookins, 1987). [Reported solubility constants for rhodochrosite alone are highly variable (Middleburg et al., 1987), although for simplicity we have used the values in Garrels and Christ, 1965). The variability resulting in the diagram is small.] Other oxides were found to be even more soluble. Under the conditions presented here, alabandite (MnS) is not stable. Alabandite has been observed in the Landsport Deep, a permanently anoxic basin in the Baltic Sea (Suess, 1979). Suess attributes its occurrence to the high productivity conditions, resulting in supersaturation in the interstitial waters as the result of excessive terrestrially-dominated input of Mn to this basin.
Figure 4 shows that in oxic waters (above the NO3--reduction line), Mn oxides are the dominant species. Dissolution of these oxides occurs roughly concurrent with nitrate reduction. In the hatched region ("Mn dissolution"), the concentration of Mn dissolved in sea water would increase from 10-8 to 10-5. Below this line, depending on the concentration of both Mn2+ and dissolved carbonates, rhodochrosite (or mixed Ca/Mn carbonates) can form. At the concentrations of Mn2+ present in oxic sea water, 10-8, rhodochrosite is not stable, even under Cambrian total carbonate conditions. However, as Mn2+ is mobilized, rhodochrosite or other Ca/Mn carbonates may be expected to form. At the lower pH expected for anoxic waters, Mn carbonates are less soluble.
Depositional Environments of Calcic Black Shales
Establishing the depositional environment for the calcic black shales is more complex than for the low- calcic shales, as Ca mineral matrices must be considered. An examination of Figure 2 demonstrates that the calcic shales fall into 4 Ca/Mn groups based on the relationship between Ca and Mn concentration. Each of the Ca/Mn groups fall on lines described by regression formulas as can be seen in Figure 3. The precise nature of the Ca,Mn-related mineral matrices, their formation mechanisms, thermodynamics and kinetics in anoxic sediments and shales is a matter of considerable discussion (Lorens, 1981; Franklin and Morse, 1983; Emerson et al., 1980; Thomson et al., 1986; Middleburg et al., 1987; Mucci, 1988; Dromgoole and Walter, 1990; Morse, 1990; Delian et al., 1992). Various controlling phases have been suggested by these researchers and others: rhodochrosite, various mixed Ca-Mn phases, and calcite. Boyle (1983) reports that in reducing environments the Mn/Ca increases on foraminifera tests, presumably due to formation of MnCO3 overgrowths. The range of species that can be described as CaxMnyCO3 is vast and influenced by many factors such as degree of anoxicity, pH, temperature, pressure, rate of deposition, and sediment grain size. For the finely-divided, marine, black shale systems investigated for this study, x and y are fixed as defined by the four regression formulae appearing in Figure 3. An examination of the samples that fall into each group helps define the probable depositional and early diagenetic environment for each group.
None of the Ca/Mn groups correspond neatly with the groups defined for the low-calcic shales. The samples in Ca/Mn I roughly correspond with those in the low-calcic group 1 (Table 1), although several samples have lower [Fe] than those in low-calcic group 1. None have high V concentrations. At least some samples in all of the other Ca/Mn groups could be placed in low-calcic groups 2, 3 and 4. If the Ca-correlated Mn contribution, as determined by the Mn-intercept, is eliminated then the shale component for Mn, plus the [Fe] and [V] determine the sample's placement in low-calcic group 2, 3 or 4. Each group contains at least two samples containing high concentrations of V - indicating that these three groups formed under sufficiently anoxic conditions to permit preservation of V-containing porphyrins.
In evaluating the meaning of each of the regression lines, a number of hypotheses were tested. The locales and ages of the samples associated with each line is shown in Figure 5. As is clear from this figure, only Ca/Mn group I is associated with Paleozoic and Mesozoic samples from a single time and location (the upper Ordovician Moffat Shales from Scotland). All the other regressions fit samples from multiple times and places. Samples in groups Ca/Mn II, III and IV, apparently formed under conditions reflecting highly varying atmospheric concentrations of CO2. Analytical artifacts also were eliminated; even samples analyzed at a different location by a different analyst fall into one of the correlations [see discussion of samples reported in Hirst, 1974; Coveney et al., 1987, below]. In performing this analysis, it became evident that the geochemical conditions of the depositional or early diagenetic environment probably played an important part in the sample groupings. This hypothesis is supported by the high correlations associated with each group which suggests a chemical rather than physical or geological sorting mechanism.
To elucidate the chemical conditions represented by each group, evidence for each environment was considered and to further clarify the environments involved, some more modern samples were analyzed. Thirty-nine sediment samples from the Pleistocene San Francisco Bay and three Miocene samples from the Monterey Shale were analyzed using NAA at LANL and their Ca and Mn concentrations plotted on the Ca-Mn graph (Figure 6). In addition, analyses of 20 Pleistocene Black Sea sediments and 5 Pennsylvanian samples were obtained from the literature (Hirst, 1974; Coveney et al., 1987) and also added to the graph (Figure 6). Based on information regarding the environments of all of these samples (Paleozoic, Mesozoic, Cenozoic, analyzed in association with this project and from the literature), the chemical depositional environment during deposition and early diagenesis can be elucidated (Figure 7).
Group Ca/Mn I
The correlation expression for the samples in Ca/Mn I is: [Mn] = 0.065 [Ca] + 1280 (ppm). The correlation coefficient is r = 0.95, however, only 7 samples fall on this line. Its statistical significance might be questioned. An examination of Figure 2 reveals that the 7 samples clearly form a line and fall nowhere near the other three clusters of samples. The intersection with the Ca axis of Figure 3 at [Mn] = 1280 ppm is representative of the concentration in the shale component of the sample and suggests that with no calcium-containing facies present, > 1200 ppm Mn would be present in these samples. Referring to the low-calcic zonations (Quinby-Hunt and Wilde, 1994 and above, Table 1), this high Mn concentration suggests an oxic (for black shales) depositional environment. There was high Mn deposition, which remained largely insoluble after burial. However, the high slope of the regression line indicates that Mn2+ was available to substitute for Ca2+ in the calcium mineral lattice or to form overgrowths. Pederson and Price (1982) noted that the availability of Mn oxides for dissolution during early diagenesis is important for the formation of Ca,Mn carbonates from reducing environments. The lack of bioturbation in these samples indicates that biologically, if not chemically the sediments were anoxic. Thus for these samples, it is reasonable to postulate that sedimentation and early diagenesis took place near the Mn oxide/Mn2+ redox boundary. An examination of Figure 4 indicates that in the hatched area below the Mn oxides, the Mn2+ concentration increases from 10-8 to 10-5. In this field, rhodochrosite is soluble, leaving Mn2+ available for incorporation into the Ca mineral lattice or to form overgrowths. However, at the pH of mildly anoxic sea water ~ 7.8, MnCO3 is insoluble at slightly lower Eh, under most atmospheric CO2 conditions. Thus, if the Mn in these samples was formed in somewhat oxic waters, sufficient Mn would be available for dissolution and incorporation into or on the Ca mineral lattice leaving a significant component insoluble (hence the high Mn intercept; Figure 3 and Figure 7). If the sediments and interstitial waters remained only slightly anoxic during early diagenesis, the pH could have been high enough to support precipitation of MnCO3 while maintaining a high concentration of Mn in the shale component.
An examination of the samples involved supports this hypothesis. All the Paleozoic samples are from the Moffat Shales in Scotland. They are from the complanatus zone, the barrens or the extraordinarius zone, which were from a period of Late Ordovician glaciation, or from the maximus zone, when the formation was shoaling (Brenchley and Newall, 1984). The glaciations would have resulted in a ventilation of deep waters and greater oxygen solubility in the water column (Berry and Wilde, 1978; Wilde, 1987). The shoaler sample also could tend to be in more oxygenated conditions. The only modern sample was an alluvial sample from San Francisco Bay, California, which is thought to have been deposited under conditions that were oxic to mildly anoxic (Sloan, 1981 and personal communication).
Group Ca/Mn II
The regression for the group Ca/Mn II samples is [Mn] = 0.038 [Ca] + 187 (ppm) with a correlation constant of r = 0.92. Twenty-one samples can be placed in this group. The relatively steep slope of the line suggests that sufficient Mn2+ was available for incorporation into or on the calcium mineral lattice, but that the [Mn] and pH in the interstitial waters were such that Ca/Mn carbonates were stabilized. If the water column were relatively oxic, possibly in the Mn2O3 region, sufficient Mn would be present in the sediments to dissolve under increasingly reducing conditions and subsequently form Ca,Mn carbonates. The low intercept of the line, [Mn] = 187 ppm, suggests that conditions during early diagenesis were sufficiently reducing conditions that Mn2+ was mobilized even from the shale component (Figure 7).
Two of the samples (Dictyonema beds from Scania) contained high concentrations of V, indicating that in the sediments, the Eh (sulfate-reducing) and pH (low, probably below 7, Lewan, 1984) were sufficient to permit preservation of V-containing porphyrins. These samples were deposited in the lower Ordovician when the atmospheric CO2 concentration was roughly 14x present (Berner, 1990). Under these conditions, even at low pH (~6.5-7), MnCO3 would be stable if Mn2+ concentrations were of the order of 10-5. The rest of the samples (including samples from the Tremadoc in Wales and Levis, Canada) in this group contained an average of 140 ppm V, suggesting that productivity was insufficient, the pH was too high, or the Eh too high to allow V accumulation. The majority of the Paleozoic samples came from the Snake Hill and Utica shales, deposited in the upper to middle Ordovician (atmospheric CO2 ~10x present, Berner, 1990). Other samples were from the Moffat shale (gracilis, upper Ordovician), Newfoundland (middle Ordovician), and the Salop (Carboniferous, atmospheric CO2 concentration ╗ present).
It appears that the samples in Ca/Mn group II were precipitated under an oxic to mildly anoxic water column, but during early diagenesis experienced more strongly reducing conditions, that those samples in Ca/Mn group I. The samples from San Francisco Bay in this group, support this hypothesis. They were deposited under oxic bottom water, but the sediments were probably sulfidic (Sloan, 1981 and personal communication).
Group Ca/Mn III
The twenty Ca/Mn III group samples fall on the line, [Mn] = 0.011 [Ca] + 152 (ppm); the correlation constant is 0.95. The low intercept at [Mn] = 152 ppm suggests either low preservation of Mn in the shale component of the shales or low initial deposition or both. The Paleozoic and Mesozoic samples in the group are from the organic-rich Lias (Toarcian, Jurassic, Gad et al., 1969) in Yorkshire (Whitby) and the Flagabro and Fagelsang cores from the Dictyonema Shales of Scania. The Scanian samples were all rich in vanadium, indicating strongly reducing conditions and relatively low pH. As the atmospheric CO2 in the lower Ordovician (~14x present) was vastly different from that in the Jurassic (~3-4 x present) (Berner, 1990), clearly factors other than Sigma CO2 are important. Samples from the Utica Shales, the Tremadoc at Levis, and Dictyonema Shales in Scania appear in both Ca/Mn groups II and III, further substantiating the idea that dissolved carbonate concentration is not the only important factor to be considered. Most probably the water column from which this group was deposited was more strongly anoxic, and therefore at lower pH than was the water column from which the Ca/Mn group II samples precipitated. If this were the case, then significantly less Mn would be available for dissolution into the interstitial waters for incorporation into Ca/Mn carbonates. All but two of the Black Sea samples (Hirst, 1974), which formed under anoxic (sulfate-reducing) bottom waters and remained in anoxic sediments through early diagenesis, fall near this line (Figure 6).
Group Ca/Mn IV
The majority of the calcic black shales (69) fall into this group. The Mn, Ca, and V concentrations of these samples are very similar to those in Ca/Mn group III. The regression line is [Mn] = 0.0027 [Ca] + 144 (ppm), with a correlation constant of r = 0.85. As with Ca/Mn group III, the low slope of the line and intercept at [Mn] of 144 ppm suggests low Mn deposition from the water column, and possibly even less Mn available to be released as Mn2+ into interstitial water to be incorporated into Ca/Mn carbonates. Although the MnCO3 preservation is low, this group contained the highest levels of Ca, indicating that CaCO3 is preserved under the depositional and diagenetic conditions experienced by these shales. As with the Ca/Mn III shales, samples come from the lower Ordovician (many sites) through the Jurassic, indicating a wide range of atmospheric CO2. Since the samples in both groups (Ca/Mn groups III and IV) come from similarly variable locales and times (samples from both the Lias in Yorkshire and the Flagabro and Fagelsang cores, as well as from Levis and the Utica Shale fall in both groups), it is difficult to assume differing source compositions or varying atmospheric CO2 concentration to explain the different regressions. The very low concentrations of Mn in these samples - all contain less than 750 ppm Mn and average 250 ppm, coupled with the low intercept and slope of regression formula, suggest that both lower Eh in the water column and in the interstitial waters may be important.
The Paleozoic and Mesozoic samples in the group are largely organic-rich from the Lias (Toarcian, Jurassic, Gad et al., 1969) in Yorkshire (Whitby) and the Flagabro and Fagelsang cores from the Dictyonema shales of Scania. The Scanian samples are rich in vanadium. The oil-bearing shales of the Monterey formation fall on this line, as did four of the five shales from the Pennsylvanian. Both are thought to have formed under highly anoxic conditions. The Monterey shales are also vanadium-rich with a signature comparable to that of the Dictyonema Shales (Wilde et al., 1989). Preservation of the organic matter and V in the Monterey and Scanian samples suggests strongly reducing, low pH conditions in the sediments (Figure 7).
Summary and Conclusions
An examination of the slope of Ca/Mn regression lines derived from nearly 120 Paleozoic and Mesozoic augmented by 42 Cenozoic black shales and anoxic sediments is seen to be a function of carbonate chemistry with variations related to Eh/pH conditions. The intercept of Ca/Mn regression lines is a function of the Mn concentration in the shale component and can be used to determine the depositional/early diagenetic redox environment of the sediment.
An examination of paleontological information concerning the samples in the four groups and comparison with more modern shales helped elucidate the chemical nature of the depositional environment for each group (Figure 7). The Ca/Mn I samples had a high slope and high Mn-intercept. These black shales are unusual and most probably derive from conditions in the water column near a redox boundary where MnO2 or Mn2O3 are insoluble. In the sediments, conditions are sufficiently reducing for some Mn2+ to be available to substitute for Ca2+ in calcite or to form overgrowths. The pH is sufficiently high to permit preservation of the Ca/Mn carbonates. The Ca/Mn II shales formed under oxic/nitrate-reducing bottom waters supporting relatively high levels of Mn oxide deposition. However, the [Mn2+] in the shale component was mobilized after deposition, releasing high levels of Mn2+ to combine with calcium mineral lattices. The pH or Sigma CO2 is sufficient to permit preservation of the Ca,Mn carbonate species. Ca III/IV shales are formed under sulfate-reducing waters, similar to conditions in the Black Sea and experience similar or more strongly reducing conditions in the sediments. The Ca/Mn II, III and IV all experienced strongly reducing conditions in the sediments, however the Fe and V concentrations for each can be used to differentiate between which of the redox zones in Figure 1 resulted in each of the samples.
Using the easily obtained Mn, Fe, V, and Ca concentrations can provide a useful, method of postulating the depositional and early diagenetic environment of finely-grained marine black shales. Further investigations of the interaction of these facies with both metal concentrations and organically- linked elements, offer a potential investigative tool in the evaluation of the economic potential of various black shale deposits. Such procedures continue the pioneering work of Vine and Tourtelot (1970).
Acknowledgements. Initial discussions of this topic were presented at the meeting of International Geological Correlation Programme 254 Symposium on Metalliferous Black Shales and Related Ore Deposits at the Annual Meeting of the Geological Society of America 1990 in Dallas, Texas and the International Geological Congress in Kyoto, Japan in 1992. We thank George Breit for his helpful discussions on vanadium. Robert Berner, John Morse, and Alfonso Mucci offered valuable comments and much discussion. Carl Orth provided the NAA analyses, for which we profoundly thank him. W.B.N. Berry gave continuous encouragement in the early stages of this study and provided many of the samples. P. Wilde acknowledges the support of B.-D. Erdtmann and the von Humboldt Foundation while he was a Senior Fellow at the Technical University-Berlin.
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|Group 1 Oxic||Group 2 Mn-Soluble||Group 3 Mn,Fe-Sol||Group 4 V-High|
|Mn > 800||Mn < 750||Mn < 750||Mn < 750|
|Fe > 37500||Fe > 37500||Fe < 37500||Fe < 37500|
|V < 320||V < 320||V < 320||V > 320|
|V||130||140||170||1500|Table 2. Summary of samples for study of depositional environments of calcic black shales.
AGE LOCATION SAMPLES SOURCE
CENOZOIC Pleistocene San Francisco Bay 39 DS Pleistocene Black Sea 20 Hirst Miocene Monterey Shale 3 JD MESOZOIC JURASSIC Oxfordian Switzerland 3 WBNB Liassic England 47 WBNB PALEOZOIC CARBONIFEROUS Westphalian Salop 1 RAR Pennsylvanian Mecca Quarry Member 5 Coveney and strat. equiv. SILURIAN Llandovery Scotland (Moffat) 1 SMCU Llandovery Norway 1 SMCU ORDOVICIAN Ashgill Scotland (Moffat) 6 WBNB Ashgill Scotland (Moffat) 1 SMCU Upper-Middle New York (Snake Hill and Utica) 17 BH Upper-Middle New York (Snake Hill) 5 WBNB Middle New York (Fisher Rd) 1 PW Middle Maine 1 WBNB Middle Newfoundland 1 WBNB Tremadoc Norway 4 WBNB Tremadoc Sweden 13 LUC Tremadoc Quebec (Levis) 11 WBNB Tremadoc Quebec (Matane) 1 WBNB Tremadoc Canada (Gaspe) 1 WBNB Tremadoc Wales 1 WBNB Tremadoc New York 1 PW
SOURCE OF SAMPLES: WBNB - William Berry, Univerity of California, Berkeley; SMCU - Sedgwick Museum, Cambridge University, mainly from the Bulman Collection; RAR - Robert Raiswell, Leeds University, Leeds, UK; LUC - Lund University Collection; BH - Bernward Hay, Woods Hole Oceanographic Institute; PW - Pat Wilde, Office of Naval Research, Asian Office, Tokyo, Japan; DS- Doris Sloan, University of California, Berkeley; Coveney- (Coveney et al., 1987); Hirst- (Hirst, 1974); JD - John Dunham, Union Oil. * A word on notation. A Roman numeral in parenthesis after the elemental symbol indicates the oxidation state of the element in a compound. A superscript indicates the valence of an ion in solution. Depositional Environments of Calcic Black Shales (9/16/94)