Evidence of Green Sulfur Bacteria and Anoxia From Campanian/Maastrichtian Cretaceous Egypt

Palaeoecological and post-depositional changes recorded in Campanian–Maastrichtian black shales, Abu Tartur plateau, Egypt

Authors:

El-Shafeiy et al

Abstract:

The Upper Cretaceous black shales of Egypt are part of the worldwide belt of Late Cretaceous organic-rich shales. Black shales are particularly prominent in North Africa and the Middle East. In Egypt, these shales occur in an east-west trending belt extending from the Quseir-Safaga district along the Red Sea to the Kharga-Dakhla land-stretch passing through the Nile Valley. The black shales are hosted mainly in the Campanian to Maastrichtian Duwi and Dakhla formations. In order to reconstruct palaeoenvironmental conditions, the present work focuses on the distribution of organic matter including lipid biomarkers within the Abu Tartur borehole section, which was drilled in 2007 in the Maghrabi-Liffya area. The kerogen in the Abu Tartur section is of type III with the exception of sedimentary deposits at the Duwi/Dakhla transition. Low Tmax, odd-over-even predominance of n-alkanes with a commonly high Carbon Preference Index, good preservation of carboxylic acids and abundant 17β,21β-hopanes and -hopanoic acids indicate immaturity of the organic constituents in the bitumen. Although thermal maturation was only low, the preponderance of rearranged steranes (diasterenes) over regular steranes indicates enhanced clay catalysis. Significant allochthonous input typifies the Abu Tartur section deposits, which are characterized by high contents of long-chain n-alkanes and low carbonate contents. The high content of desmethyl steranes and diasterenes suggests that marine algae were the main marine primary producers. The presence of different isomers of hopanes (C27, C29–C31) and hopanoic acids (C31–C33) reveals input from various bacteria. The observed variation in the abundance of biomarkers corresponds to changes in planktic algal assemblages associated with sea level change and episodic photic zone anoxia, which are indicated by the occurrence of aryl isoprenoids, biomarkers of green sulphur bacteria.