Element distribution in some trough and platform types of black shale and associated rocks

Abstract

The distributions of some chemical elements in four sample sets of black shale and associated rocks were examined statistically. Pennsylvanian black shales deposited in a rapidly subsiding trough in central Colorado show few unusually high concentrations of minor elements, except lead, that can be attributed to adsorption by organic matter. High concentrations of boron, beryllium, gallium, lead, and possibly lithium may be related to Precambrian source rocks that are rich in the same elements. Minor elements in upper Paleozoic black shales of the Ouachita geosyncline show little or no association with organic matter. Local high concentrations of gallium and zirconium are associated with the detrital mineral fraction. High concentrations of barium show no association with major rock constituents, and barium may have been authigenically redistributed. Samples of shale from the Cherokee Group in the Western Interior coal province contain large amounts of silver, chromium, lead, vanadium, and zinc associated with organic matter and carbonate minerals. These elements may have been concentrated syngenetically by adsorption from sea water on decaying organic matter in a shallow-water platform environment. Samples of black shale and associated rocks from the Houy Formation of Devonian and Mississippian age in central Texas show unusually high concentrations of many minor elements, including cobalt, copper, molybdenum, nickel, lead, thallium, vanadium, and zinc. Of these, all but vanadium occur in close association with organic matter and pyrite. A very slow rate of deposition may have contributed to the concentration of these elements from sea water. However, many of these elements are toxic and could not coexist in abundance with living organisms. Some later addition of elements may have occurred.