Although generally called "black shales," "sheety shales," or "paper shales," thinly bedded black rocks in upper Carboniferous successions in the Illinois basin and Mid-Continent are actually fissile coals. Thin section examination demonstrates that these rocks are as much as 90% carbon by volume. In addition to carbon, all these rocks contain large amounts of non-skeletal and much skeletal phosphate as fecal masses, gastric residues, teeth, bones, cartilage, brachiopod shells, and conodonts. We have elected the term "organophosphorite" for these rocks rather than "shale," because many of them contain less than 10% terrigenous clay.

Sources of the organic carbon, including kerogens, were varied and distributed among several plant and animal groups. Driftwood is common and testifies to input from terrestrial plants. Algal components probably dominated the plant fraction, but have left only vague traces. Animal phyla from Protozoa through Chordata are represented, but with heavy bias toward

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those with siliceous (radiolarians, sponges), or phosphatic (brachiopods, conodonts, bony fish and sharks) skeletons. This indicates an abundance of mobile organisms in the depositional environment, mostly nekton with some plankton and pseudoplankton.

In addition to the production of diverse hydrocarbons, living organisms were responsible for the biogenic phosphate, some biogenic silica, and such heavy metals as zinc (<4,000 ppm) and uranium (<10 ppm).

Many lines of evidence, including stratigraphic distribution, paleoecology, and taphonomy, point convincingly to an origin in coastal marine environments analogous to modern salt marshes but with mostly floating vegetation.

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