‘Black shales’ is an old term for argillaceous rocks rich in organic matter. The characteristics of the depositional environment of such deposits have long intrigued geologists, particularly the extent to which actualistic models can be applied. The revived interest in the last few years stems principally from the wide-spread recognition that black shales may be important source rocks for petroleum, and from the discovery of organic-rich Cretaceous deposits in the deep ocean.
In view of this burgeoning interest, the need for more precise definition and classification has become imperative (see the paper by Spears), the more so because ‘black shales’ is now entering the realm of Franglais. The kind of rocks I have in mind need be neither black nor shaly, although they usually are.
In my opinion the most characteristic lithological feature is laminae of organic matter arranged parallel to the bedding and responsible for the pronounced fissility of the rocks, which may weather to so-called paper shales. The content of organic matter rarely exceeds a few percent; when it is considerably higher the rock may yield petroleum upon distillation and hence can be described as oil shale. Structureless and chemically inert kerogen dominates, but there is a variable admixture of terrestrially-derived spores, pollen and woody matter together with non-calcareous microplankton such as dinoflagellates. Actualistic analogues suggest that the fine rhythmic kerogen-clay laminae, of the order of 20–30 µ m in thickness, are seasonally-produced varves, which can therefore be used to estimate rates of sedimentation.
The dominant inorganic constituents