ABSTRACT

Black fissile "shales," from some Missourian and Virgilian cyclothems of central Iowa and eastern Nebraska, have been analyzed to determine their chemical and physical properties, the stratigraphic, petrographic, and paleontologic successions over wide geographic areas, and the environment of deposition suggested by these parameters.

The stratigraphic succession commonly consists of three basic elements: a lower black fissile siltstone; a medial black "varved" siltstone; and, an upper gray to gray-green shaly siltstone.

Organic contents range from 2 to 26 percent. Acid-soluble contents range from 11 to 38 percent. An inverse relationship between acid-soluble constituents present, and organic content exists. A direct relationship between organic content, fissility, and radioactivity occurs.

Petrographically, the silts are predominantly quartz, with lesser amounts of illite, kaolinite, vermiculite, pyrite, and carbonates. The siltstones have an average mineral composition within the range of sub-graywacke to graywacke. Sorting coefficients range from 1.9 to 4.3. No general trend in the degree of sorting is apparent.

A definable stratigraphic, petrographic, and paleontologic succession occurs, and can be traced over a wide geographic range. The stratigraphic and paleontologic characteristics, as well as many other features, have their closest association with modern tidal flat deposits. The deposits apparently accumulated in both regressive and transgressive seas. The cyclothem boundary occurs within these black silt intervals.