Black shale organic content

Generally speaking, a direct relationship between uranium concentration and increasing organic carbon content exists in marine black shales. Uranium content is also directly related to the colloidal size ranges of such sediments. 

Site 41-368 is located on the Cape Verde Rise (17°30.4 N, 21°21.2 W water depth 3367 m) (Lancelot et al, 1975). The Turonian-Albian black shales were horizontally penetrated by two thin ( 20 cm) and one thick (15 m) diabase sills (Natland, 1977). The age of the major intrusive unit was determined by the K/Ar method to be approximately 19 Ma (early Miocene) (Duncan and Jackson, 1977). The Cretaceous section from Site 41-368 consists of black shale with variable organic carbon and carbonate contents (0.7-12% and 1-38%, respectively) due to lithologic differences e.g. Dennis etal, 1982 Simoneit etal, 1981). These values decrease to minima near the intrusions. The thermal stress from the intrusions affected the sediment to about 12 m above and 8 m below the major sill. 

Uraniferous black shales are of marine origin and, in addition to colour, are characterized by high sapropelic organic content, abundant pyrite or marcasite in thin lenses, nodules or disseminated particles, and the paucity of calcium and magnesium carbonates. The shales also contain small quantities of such other metals as Mn, Ti, V, Cu, Cr, Mo, P and REE. Most of these metals are dispersed in the shale, but phosphate may also occur as nodules or phosphate-rich layers. ... 

Figure 5.9 shows the strong correlation between the Th/U ratio and organic carbon content for Devonian black shales. 

FIGURE 5.9 ThAJ ratio versus the organic carbon content for the Devonian black shales (Virginia and Kentucky). Data from Fertl (1983). 

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