Modified for the World Wide Web April 1997 After U. S. Geological Circular 1037 Metalliferous Black Shales and Related Ore Deposits,
R. I. Grauch and J. S. Leventhal (eds.) p. 8-15 (1989)
Introduction
Anoxic sedimentary environments of the past are recorded in black shales. Wedepohl (1968) noted that "average" shales reflect shallow-water sediments that accumulated under oxidizing conditions. Thus, the composition of so-called "average" shales does not represent past conditions where anoxic water and sediments were a significant component of the potential geologic record. Secondarily, as sediments resulting in black shales can be deposited under both oxic and anoxic conditions and in shallow to deep water, a black-shale composite better indicates oceanic conditions during the time of deposition than composites based primarily on shallow-water oxic conditions. Accordingly, we analyzed 287 samples of stratigraphically well documented low calcic Paleozoic and Mesozoic black shale (table 1), using neutron activation analysis, for: Na, Mg, Al, K, Ca, Sc, Ti, V, Cr Mn, Fe, Co, Zn, Ga, As, Se, Rb, Sb, Cs, Ba, Hf. Ta, Ti and U. (table 2a)and for the rare-earth elements La, Ce, Nd, Sm. Eu, Tb, Dy, Yb, and Lu (table 2b).
This study expands the compilation of North American black shales by Vine and Tourtelot (1970) by the addition of 16 elements, including the first compilation of the rare-earth-element series for black shales. In this report, our data are compared with the black-shale compilation (V&T:BSC) of Vine and Tourtelot (1970) and with other general shale compilations such as Turekian and Wedepohl (1961) and Gromet and others (1984).
This study (called the Marine Sciences Group Black-Shale Composite or MSG:BSC) emphasizes the Paleozoic (Ordovician, Silurian, and Carboniferous) and middle Mesozoic (Jurassic), as most black shales were deposited then (Berry and Wilde, 1978). The MSG:BSC study includes samples (total 287, table 1) from the Cambrian (1), Lower Ordovician (90), Middle Ordovician (33), and Upper Ordovician (49), Lower Silurian (37), Carboniferous (19), and Jurassic (58). Samples were collected from continental Europe, Scandinavia, and North and South America. The samples were identified, stratigraphically and paleontologically, by W.B.N. Berry or were from well-documented collections (for example, Sedgwick Museum, Oxford, England; Museum of Paleontology, Berkeley, Calif.). The MSG:BSC study focused on field-identified black shales; nearly 90 percent of the samples contained less than 5 percent CaCO3. The study specifically excluded shaly limestone or limy shale containing greater than approximately 15 percent CaCO3 (Pettijohn, 1949, p. 291), in order to focus on the composition of the argillaceous black-shale facies. (MSG data available as Lotus 123 file).
Analytical Procedures
Neutron Activation Analysis
A suite of 41 elements was determined in the samples (table 2). All elemental abundances were determined at the Los Alamos National Laboratory using automated neutron activation analysis (Minor and others, 1982). Use of a single laboratory minimized the possibility of error commonly introduced by variations in analytical techniques or laboratories (Fairbairn and others, 1951; Quinby-Hunt and others, 1986). Most of the elemental concentrations were determined using conventional reduction of gamma-ray spectra of the radioactive isotopes.
Uranium was determined by delayed neutron counting. The automated system was calibrated using a collection of U.S. Geological Survey, U.S. National Bureau of Standards, and Canadian Geological Survey standard rocks. The system was checked periodically for stability against these standards.
Statistical Methods
Frequency distributions for each element were plotted against the midpoints of distribution bins used in determining the distribution (fig.1). Means and modes of elements in the MSG:BSC study are listed in table 2 with mean concentrations of Turekian and Wedepohl (1961) and Gromet and others (1984) for shales and means and modes of the V&T:BSC study. In determining the mode and to facilitate comparison with the study of black shales by Vine and Tourtelot (1970, p. 257), we used their geometric concentration ranges. Each bin (fig. 1) has boundaries of 1.2, 1.8, 2.6, 3.8, 5.6, 8.3, 12, 18 ppm, and so on. For example, the midpoint of the bin containing data ranging from 0.83 to 1.2 ppm is 1; the midpoint for the bin containing data from 1.2-1.8 ppm is 1.5 ppm. The next consecutive midpoints are 2, 3, 5, 7, 10 ppm, and so on. The mode, as the midpoint in the frequencydistribution bin, is presented because the mean is sensitive to a few extreme values and may not indicate the central tendency in a distribution.
If less than 60 percent of the samples did not contain detectable levels of an element, we have presented the mean of the detectable concentrations; for the mode, we have listed the percentage of all samples containing less than the detectable concentration. For example, 134 (47 percent) of the 287 samples contained measurable amounts of calcium. The average calcium concentration for samples in which it was detected (fig.1) was 17,000 ppm. However, the average sample containing undetectable quantities of calcium contained less than 800 ppm. In fact, 60 percent of all samples contained less than 2,600 ppm. Thus, the mode is given as "60 percent < 2,600 ppm" in table 2.
Discussion
The low-calcic shales analyzed for this study showed less variability of composition for the unimodal elements than was observed by Vine and Tourtelot (1970) because, in this study, the samples were basically clays and other detrital minerals having little carbonate to complicate the mineralogy. The multimodality seen in these low-calcic shale samples suggests that factors affecting composition of black shales, other than source composition, are present during deposition and diagenesis.
This study demonstrates that the black-shale facies is chemically complex and contains several chemofacies. Ideally, if the black-shale facies observed in the field was a single chemical-sedimentary quantity, the elements characteristic of that facies would show a unimodal distribution. When compared with the classic shale composites (Turekian and Wedepohl, 1961; Gromet and others, 1984; Taylor and McLennan, 1985) and with the study of black shales by Vine and Tourtelot (1970), this study shows excellent agreement for the elements generally associated with detrital minerals, such as aluminum, titanium, and scandium. The trimodality of manganese concentrations indicates the possibility of three redox zones in black shales. Multimodality in volatile elements such as arsenic and antimony suggests that, additionally, a volcanic source may have an important chemical impact on the black-shale faeies. The mean concentrations of elements of the organic and volatility indicators, vanadium, zinc, bromine, strontium, molybdenum, antimony, barium, and uranium, are significantly higher than the means for the shale composites and the Vine and Tourtelot (1970)
compilation. This difference is due to the inclusion of the Dictyonema-bearing black shales of Balto-Scandia in our composite. Several authors have noted that certain black shales are enriched in various metals (Goldschmidt, 1954; Wedepohl, 1964; Vine and Tourtelot, 1970; Tardy, 1975; Holland, 1979; Berry and others, 1986). The exclusion of these black shales from alternative compilations biases projections of the global oceanic conditions in the Paleozoic and Mesozoic. We prefer to include these samples, noting both the mean and the strong multimodal character of their frequency distributions. This approach identifies elements typifying an average black shale but also identifies elements with multiple modes that show that black shales can form under two or more chemical environments.
Conclusions
Black shales, which seem to be an easily identifiable and distinct sedimentary facies, represent a complex chemical system that contains several chemofacies and includes a wide range of redox conditions. The variability of composition observed in this study of lowcalcic black shales shows the need to examine distributional modes other than the mean when characterizing chemofacies associated with depositional environments. Comparisons with other composites, with respect to redox sensitive elements, indicate that the black-shale facies have a wider range of redox conditions than seen in published general composites. As such composites (for example, the North American shale composite) are used for normalization of chemical data, their usefulness as a standard, particularly for investigations of non-oxic shales, must be questioned. On the other hand, a well-documented black-shale composite may be a more valid standard for paleo-oceanographic and comparative chemical studies, especially in Paleozoic and earlier times when the redox conditions in the oceans were not as uniform or as oxic as in the modern oceans.
References
Berry, W.B.N., and Wilde, Pat, 1978, Progressive ventilation of the oceans--an explanation for the distribution of the Lower Paleozoic black shales: American Journal of Science, v. 278, p. 257-275.
Berry, W.B.N., Wilde, Pat, Quinby-Hunt, M.S., and Orth, C.J., 1986, Trace element signatures in Dictyonema shales and their geochemical and stratigraphic significance: Norsk Geologisk Tidsskrift, v. 66, p. 45-51.
Fairbairn, H.W., and others, 1951, A cooperative investigation of precision and accuracy in chemical, spectrochemical and modal analysis of silicate rocks, in Contributions to geochemistry, 1950-51: U.S. Geological Survey Bulletin 980, p. 1-71.
Goldschmidt, V. M., 1954, Geochemistry: Oxford, England, Oxford University Press, 730 p.
Haskin, M.A., and Haskin, L.A., 1966, Rare earths in European shales--A redetermination: Science, v. 154, p. 507-509.
Holland, H.D., 1979, Metals in black shales--A reassessment: Economic Geology, v. 74, p. 1676-1680.
Minor, M.M., Hensley, W.K., Denton, M.M., and Garica, S.R., 1982, An automated activation analysis system: Journal of Radioanalytical Chemistry, v. 70, p. 459-471.
Pettijohn, F. J., 1949, Sedimentary rocks: New York, Harper and Brothers, 526 p.
Quinby-Hunt, M.S., McLaughlin, R.D., and Quintanilha, A.T., 1986, Instrumentation for environmental monitoring, Volume 2, in Greenberg, A.E., and Morton, G.A., eds., Water (2nd ed.): New York, Wiley, 982 p.
Tardy, Yves, 1975, Element partition reties in some sedimentary environments, I. Statistical treatments, II. Studies on North American black shales: Strasbourg Sciences Geologiques Bulletin, v. 28, p. 59-95.
Taylor, S.R., and McLennen, S.M., 1985, The continental crust--Its composition and evolution: Oxford, England, Blackwell Scientific Publication, 3i2 p.
Turekian, K.K., and Wedepohl, K.H., 1961, Distribution of the elements in some major units of the Earth's crust: Geological Society of America Bulletin, v, 72, p. 175-191.
Vine, J.D., and Tourtelot, E.B., 1970, Geocbemistry of black shale deposits--A summary report: Economic Geology, v. 65, p. 25~272.
Wedepohl, K.H., 1964, Untersuchen am Kupferschiefer in Nordwestdeutschland; Ein Beitrag zur Deutung der Genese bituminoser Sedimente: Geochimica et Cosmochimica Acta, v. 28, p. 305-364.
Wedepohl, K.H., 1968, Chemical fractionation in the sedimentary environment, in Ahrens, L.H., ea., Origin and distribution of the elements: Oxford, England, Pergamon Press, p. 999-1016.
* Sources:
Table 1 Summary of samples for study of elemental chemistry of black shales Age Location Samples Source* Jurassic Oxfordian Switzerland 6 WBNB Liassic England, UK 52 WBNB Carboniferous Wesphalian Wales, UK 9 RAR Silurian Landovery Scotland, UK 4 WBNB Landovery Scotland, UK 3 SMCU Landovery Wales, UK 2 WBNB Landovery Norway 1 WBNB Landovery New Brunswick, Canada 1 WBNB Landovery Maine, USA 1 WBNB Ordovician Ashgill Idaho 1 WBNB Ashgill Scotland, UK 48 WBNB Ashgill Scotland, UK 1 SMCU upper-middle New York, USA 4 WBNB upper-middle New Jersey, USA 7 UCMP middle Wales, UK 1 WBNB middle Penna, USA 6 LP middle Penna, USA 5 UCMP middle Norway 3 WBNB middle New York 4 PW middle New Jersey 1 WBNB middle Maine 1 WBNB early-middle Newfoundland 1 SMCU Tremadoc Norway 23 WBNB Tremadoc Sweden 21 LUC Tremadoc Levis, Canada 15 WBNB Tremadoc Wales, UK 14 WBNB Tremadoc Bolivia 6 BCPC Tremadoc New York, USA 4 WBNB Tremadoc Estonia 2 SMCU Tremadoc Belgium 2 SMCU Tremadoc New Brunswick, Canada 2 SMCU Tremadoc Denmark 1 SMCU Cambrian upper-middle Norway 1 WBNB
WBNB: William B. N. Berry, University of California, Berkeley
SMCU: Sedgwick Museum, Cambridge University (mainly from theBulman collection)
UCMP: Museum of Paleontology, University of California, Berkeley
LP: Lucien Platt, Bryn Mawr University
RAR: Robert A. Raisewell, Leeds University
PW: Pat Wilde, University of California, Berkeley
LUC: Lund University Collection, Sweden
BCPC: Bolivian California Petroleum Company, La Paz, Bolivia
Table 2a Marine Science Group Composite compared to other shale and black shale composites NON-RARE EARTH ELEMENTS Marine Sciences Group Black Shale Composite Black Shale Shale ELEMENT This Report V&T (1970) T&W (1961) T&M (1985) NASC (1984) Mean Mode* Minimum Maximum N Mean Mode* Mean Mean Mean Na 5,260 7,000 380 24,800 287 7,000 10,000 9,600 8,900 7,500 Mg 10,400 10,000 2,490 40,200 284 7,000 7,000 15,000 13,000 17,000 Al 82,1000 70,000 14,700 129,900 287 70,000 70,000 80,000 100,000 89,000 Cl 240+ <120 (75) 30 1,780 99 NR NR NR NR NR K 29,900 20,000 3,500 83,600 286 20,000 30,000 26,600 31,000 32,000 Ca 17,100+ <2,600 (60) 740 73,300 134 15,000 10,000 22,100 9,300 25,000 Sc 15.6 15 1.5 30.7 287 10 10 13 16 14.9 Ti 4,340 5,000 850 7,360 286 2,000 2,000 4,600 6,000 4,200 V 500 150 39 6,260 187 150 150 130 150 NR Cr 111 100 10 418 278 100 70 90 110 124.5 Mn 383 200 15 3,780 287 150 100 850 850 465 Fe 36,800 50,000 2,500 99,800 287 20,000 30,000 47,200 50,000 40,000 Co 16.9 20 0.4 108 287 10 15 19 23 25.7 Zn 310+ <38 (65) 7 3,800 106 <300 ++ 95 85 NR Ga 21.9+ <38 (80) 7.8 43.2 119 20 20 19 20 NR As 28.8 20 1.3 152.6 279 NR NR NR NR 28.4 Se 5.6+ <5.6 (80) 1.0 29.8 76 NR NR NR NR NR Br 4.0+ <2.6 (76) 0.9 36.1 106 NR NR NR NR 0.69 Rb 131 150 18 322 287 NR NR NR 160 125 Sr 310+ <260 (75) 150 530 21 200 200 300 200 142 Zr 230+ <380 (90) 39 1,010 99 70 100 160 210 200 Mo 65+ <18 (65) 0.1 600 118 10 ++ 2.6 1.0 NR In 0.21+ <0.18 (76) 0.08 0.6 66 NR NR NR NR NR Sb 5.7 1.5 0.2 70.6 260 NR NR NR NR 2.09 Cs 8.6 10 0.5 19.6 287 NR NR NR 15 5.16 Ba 1,120 300 140 50,200 279 300 300 139 650 636 Hf 4.3 5 0.8 18.4 287 NR NR NR 5.0 6.3 Ta 0.9 0.7 0.3 1.8 275 NR NR NR NR 1.12 W 3.3+ <3.8 (82) 1.0 12.5 61 NR NR NR 2.7 2.1 Au 0.023 ** 0.007 0.053 14 NR NR NR NR NR Th 11.6 10 1.1 32.3 287 NR NR NR 14.6 12.3 U 15.2 3 1.2 442.6 287 NR NR NR 3.1 2.66 EXPLANATION OF ANNOTATION
Concentrations in parts per million.
NR, not reported;
<, less than;
( ), percent of samples containing less than the modal value of the element
* The mode is the midpoint of distribution bins used for determining the frequency distribution by the method of Vine and Tourtelot (1970) (see text). If less than 60 percent of the samples contained detectable amounts of the element, the frequency distribution of the detection limit was examined in conjunction with the detectable concentrations and the percentage of samples below a certain concentration determined.
+ These values are means of the samples in which less than 60 percent of the samples contained detectable concentrations.
++ For these elements Vine and Tourtelot (1970) indicated a majority of samples less than the lower limit of detection using spectrography.
** Too few values available for calculation of meaningful mode.SOURCES OF SAMPLES
This Report: Quinby-Hunt and others, (1989) Data available as Lotus 123 file.
V&T 1970: Black Shale Composite Vine and Tourtelot (1970).
T&W 1961: Shale Composite Turekian and Wedepohl (1961).
T&M 1985: Post-Archean Australian shales Taylor and McLennan (1985).
NASC 1984: North American Shale Composite, Gromet and others (1984).
H&H 1966: European shales, Haskin and Haskin (1966)
Chondrite: Rare Earth Reference, Haskin and others (1966).
Table 2b Marine Science Group Composite compared to other shale and black shale composites RARE EARTH ELEMENTS Marine Sciences Group Black Shale Composite Black Shale Shale Chondrite ELEMENT This Report V&T (1970) T&M (1985) NASC (1984) H&H (1966) H et al. (1966) Mean Mode* Minimum Maximum N Mean Mode* Mean Mean Mean Mean La 44 50 10 111 286 30 30 38 31.1 41.1 0.30 Ce 80 100 13 197 282 NR NR 80 66.7 81.3 0.84 Nd 55 50 14 130 214 NR NR 32 27.4 40.1 0.58 Sm 6.2 7 0.8 27.5 280 NR NR 5.6 5.59 7.3 0.21 Eu 1.27 1.5 0.21 5.26 285 NR NR 1.1 1.18 1.52 0.074 Tb 0.95 1 0.06 2.42 265 NR NR 0.77 0.85 1.05 0.049 Dy 4.85 5 0.25 10.73 285 NR NR 4.4 NR NR 0.31 Yb 3.10 3 1.29 6.21 286 NR NR 2.8 3.06 3.29 0.17 Lu 0.47 0.5 0.07 0.90 270 NR NR 0.43 0.456 0.58 0.031 EXPLANATION OF ANNOTATION
Concentrations in parts per million.
NR, not reported;
* The mode is the midpoint of distribution bins used for determining the frequency distribution by the method of Vine and Tourtelot (1970) (see text). If less than 60 percent of the samples contained detectable amounts of the element, the frequency distribution of the detection limit was examined in conjunction with the detectable concentrations and the percentage of samples below a certain concentration determined.
SOURCES OF SAMPLES
This Report: Quinby-Hunt and others, (1989) Data available as Lotus 123 file.
V&T 1970: Black Shale Composite Vine and Tourtelot (1970).
T&W 1961: Shale Composite Turekian and Wedepohl (1961).
T&M 1985: Post-Archean Australian shales Taylor and McLennan (1985).
NASC 1984: North American Shale Composite, Gromet and others (1984).
H&H 1966: European shales, Haskin and Haskin (1966)
Chondrite: Rare Earth Reference, Haskin and others (1966).