ANOXIA IN SHALLOW SHELF ENVIRONMENTS: THE CASE OF OAE2 IN MOROCCO AND EGYPT

The Late Cenomanian to early Turonian oceanic anoxic event (OAE2) was examined in shallow shelf environments of Morocco and Egypt, to test the hypothesis that this anoxic event was largely restricted to deeper shelf and upper slope environments. A multi-disciplinary study, involving sedimentologic and biostratigraphic analyses, carbon isotopes, bulk rock and clay mineralogy, rock-eval and total phosphorus quantification, shows that shallow water sequences can be correlated with section of deeper environments, show similar though muted responses to oceanic anoxic events and can improve our understanding of oceanic anoxic events. Age control in these shallow shelf environments is based on integrated ammonite, foraminifera and nannofossil biostratigraphies and carbon isotope stratigraphy. The age control achieved allows for correlation with the OAE2 in deeper water sequences of the Tarfaya basin in Morocco and Pueblo, Colorado. Environmental conditions are assessed based on presence and relative abundances of low-oxygen tolerant benthic and planktic foraminifera and nannofossils. The results show that the characteristic OAE2 carbon isotope excursion reached into shallow oyster-dominated coastal areas and is not restricted to deeper upwelling regions. Low oxygen benthic and planktic foraminifera indicate dysoxic environments with high nutrient influx from terrestrial runoff. Clay mineralogy suggests increasing humidity expressed by the high kaolinite content. Oyster beds, which require nutrient-rich waters to grow, coincide with intervals of high humidity and increased continental runoff providing the necessary nutrient influx. Total phosphorus quantification indicates high concentrations at the onset of the carbon isotope excursion and diminishing as the excursion progresses. The apparent decoupling of the carbon and phosphorus cycle during the OAE2 supports Mort et al.* (2005) model of sustaining productivity via increased flux of organic matter and phosphorus into the sediments, and phosphorus recycling. The increase in nutrients and higher productivity in marginal shelf areas may be linked to an increasingly humid climate coupled with high terrestrial runoff and the sea level transgression during the Late Cenomanian. *Mort et al., 2005, 7th ISC, Neuchâtel, p.149.