| Abstract
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Various geochemical parameters including type and abundance of organic matter (TOC), sulfide-sulfur quantities, fluctuations in bottom-water anoxicity (DOP), metal content differences, and sulfur isotope variations have been assessed in order to characterize Midcontinent Pennsylvanian black shales. Based on these geochemical parameters, the deposits can be grouped into three types: Mecca-type, Heebner-type, and Shanghai-type. Mecca-type black shales are characterized by at least 20% TOC (mostly terrestrial-type), high metal content (especially V, Cr, Ni, Mo, U, and Sn), erratic and heavy usd\delta\sp{34}usdS values, high DOP, and an association with underlying coals and siliciclastics. Heebner-type black shales are characterized by less than 20% TOC (mostly marine-type, although somewhat mixed), moderate metal content, light, consistent usd\delta\sp{34}usdS values, moderate DOP, and an association with phosphatic nodules and laminae. A sedimentary association with siliciclastics (Missourian) and carbonates (Virgilian) is evident. Shanghai-type deposits are characterized by less than 15% TOC (mostly mixed terrestrial and marine types), low metal contents, erratic and heavy usd\delta\sp{34}usdS values, low DOP, and a mostly phosphatic fauna. Multivariate statistical analyses performed on geochemical data indicate that a 94% probability exists that Pennsylvanian black shales of Midcontinent North America can be classified into one of these groupings. Metal constituents in the Mecca-type shales and metal-liferous Heebner-type shales developed in response to repeated contact of black shales and organic matter with various types of fluids: brackish seawater, connate water, basinal brines, and modern groundwater. It is probable that a redox interface existed so that metals present in fluids were attracted to the nearshore organic materials in Mecca-type (Desmoinesian) shales. Favorable conditions for the emplacement of metals in the shales includes a semieuxinic to euxinic basinal setting, DOP between 7.0 and 1.00, the presence of available metals, an abundance of organic material to absorb the metals, and a predominance of quiescence to fixate metals. A model for time-dependent changes of Midcontinent Pennsylvanian black shales is offered that shows a progressive ventilation for deposition of the black shales from the Desmoinesian through the Virgilian in the Midcontinent so that metal supplies, DOP, and TOC contents decrease with time as a result of decreased sediment supply or a fluctuation in sediment source.
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