Sulfur isotope compositions were determined for pyrite and sphalerite grains isolated from 8 metalliferous Missourian, Desmoinesian, and Atokan black shales from Missouri, Illinois, Kansas, and Indiana.

The younger Missourian shales from the Forest City and Illinois basins contain consistently light sulfur as expected for euxinic conditions, but sulfides from the older shales show heavier and more erratic sulfur values. These isotope patterns suggest that younger shales accumulated slowly offshore, although older shales may have been deposited more quickly in shallower water. Isotope values, which also correlate with heavy metal patterns (e.g., higher molybdenum values for the easternmost older shales), may therefore reflect gradually deepening conditions for the epicontinental Pennsylvanian seas of the United States Mid-Continent.

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