Cenomanian-Turonian black shales in North Africa
Sebastian Lüning
Currently, I am mainly working on black shales of the Cenomanian-Turonian (CT) anoxic event across North Africa. One of the aims is to get a better idea of the vertical and lateral distribution patterns of the organic matter and from this deduce the depositional mechanisms and ecological processes which have led to the accumulation of this organic-rich strata. Despite (or becauseof?) intensive research by numerous geoscientists in the past 30 years on this subject worldwide, the underlying causes for the observed anoxia are still controversially discussed. As with other black shales in earth history, the two main models for the CT are (1) high primary productivity and (2) stagnation, both potentially assisted by various 'helpers' such as increased volcanism, mid ocean spreading (e.g. Sinninghe Damsté), and meteorite impacts. While some groups favour the productivity scenario for black shales in general (e.g. Pedersen & Calvert 1990), others prefer the stagnation model (e.g. Tyson).
For the organic-rich CT deposits in Northwestern Africa, the productivity model had been established for a while (Kuhnt/Thurow, Morocco). New results from the Atlantic deep sea (e.g. Sinninghe Damsté & Köster 1998) and the Cretaceous shelfal sea in Tunisia (Nederbragt & Fiorentino 1999), however, yield some evidence against high productivity during the CT interval. In recent years, calcareous nannofossils turned out to be good proxies to indicate low, medium or high productivity and other water mass properties. While benthic organisms (e.g. benthic foraminifera, ostracods) reflect the conditions at or near the sediment surface and simply disappear under anoxic bottom water conditions, the nannofossils indicate the conditions in the upper parts of the water column (partly above the oxygen minimum zone) and, together with planktic foraminifera, provide important additional information. Calcareous nannofossils, studied by Dr Akaml Marzouk (Univ. Tanata) are therefore an integral part of the investigations.
In our current study we try to further investigate the depositional mechanisms and ecological causes of deposition of organic-rich strata in North Africa. One aim is to develop a general, predictive depositional model applicable for the whole of the North African region. The model will include important parameters such as distribution and organic richness of CT strata, pre- and syndepositional tectonic activities, diachronities in the development of anoxia etc. The study is carried out in cooperation with several partners, and includes gamma-ray spectroscopy, framboid grain size analysis, sedimentology, the analysis of foraminifera, calcareous nannofossils and ostracoda, and geochemistry. An integration of hitherto unpublished data from petroleum exploration activities in the region now allows refined interpretations using a much larger data base than in previous studies.