Abstract--Black shale is a dark-colored mudrock containing organic matter that may have generated hydrocarbons in the subsurface or that may yield hydrocarbons by pyrolysis. Many black shale units are enriched in metals severalfold above expected amounts in ordinary shale. Some black shale units have served as host rocks for syngenetic metal deposits.Black shales have formed throughout the Earth's history and in all parts of the world. This suggests that geologic processes and not geologic settings are the controlling factors in the accumulation of black shale. Geologic processes are those of deposition by which the raw materials of black shale are accumulated and those of diagenesis in response to increasing depth of burial.Depositional processes involve a range of relationships among such factors as organic productivity, clastic sedimentation rate, and the intensity of oxidation by which organic matter is destroyed. If enough organic material is present to exhaust the oxygen in the environment, black shale results.Diagenetic processes involve chemical reactions controlled by the nature of the components and by the pressure and temperature regimens that continuing burial imposes. For a thickness of a few meters beneath the surface, sulfate is reduced and sulfide minerals may be deposited. Fermentation reactions in the next several hundred meters result in biogenic methane, followed successively at greater depths by decarboxylation reactions and thermal maturation that form additional hydrocarbons. Suites of newly formed minerals are characteristic for each of the zones of diagenesis.

Geochemical summary statistics for 18 elements in natural mateiials horn 117 landscape units have been compiled based on field and laboratory studies since 1958. Each landscape unit is brie f ly identified as to kind and location, and the expected concentration for one or more elements is gi\en together with factors indicating the degree of observed \ariation in the stuch and the degree of laboratory or "analytical" variation. Also listed are the observed lange and the total number of element analyses made in each study. The data on which these summaries are based have three attributes in common: They represent "large-scale" 01 legional geochemical studies; they represent background or "oidinary" natural geochemical \ariation; and they were collected according to objective sampling designs. The summaries clearly demonstrate the wide diveisity to be expected in elemental properties of landscape units and suggest that published element abundances for bioacl categoiies like1 "soil" or "carbonate rock" may be misleading. Fl F2 STATISTICAL STUDIES IN FIELD GEOCHEMISTRY One such study merits special mention. The U.S. Geological Survey has recently completed a geochemical survey of the State of Missouri, many aspects of which are unique to environmental geochemistry. It not only is a survey of a broad, geologically diverse area, but it was undertaken partly in support of active, trace-element related epidemiologic studies sponsored by the University of Missouri. Moreover, we think it is the first study of its kind in which an attempt has been made to characterize rocks, waters, soils, and plants chemically by a unified team approach using (and in part testing) efficient and objective sampling designs. Connor and others (1972) described this work in a preliminary way. Details of the study are available in a series of limited-distribution progress reports (U.S. Geological Survey, 1972a-f, 1973). The data tabulated in the present report represent the work of many people. Principal investigators are listed with a short description of each study, and authorship is cited for all published data. All unpublished data are preliminary. The reader is cautioned that some of the data summaries given here may be subject to minor revision. The sampling designs, data analyses, and geochemical summaries on which this report is based are statistical in nature; extended discussions of these subjects can be found in Miesch (1967a, b, 1972), Connor and others (1972), and Connor and Myers (1973). A proper list of acknowledgments for this report would comprise more than 100 people including computer programmers, specialists in data handling, and assistants in the field, laboratory, and office. Unquestionably, the most important contributors are the chemists, spectrographers, and other laboratory personnel who catalogued, prepared, and measured the concentrations of up to 69 elements in more than 8,000 samples of rocks, soils, and plant material over a period of more than 10 years.

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