Stratigraphy and palaeoceanography of the Cenomanian-Turonian Boundary Event in Oued Mellegue, north-western Tunisia

Abstract

An expanded Cenomanian-Turonian section along Oued Mellegue in north-western Tunisia provides a high-resolution record of the latest Cenomanian Oceanic Anoxic Event in an upper bathyal, pelagic to hemipelagic environment. A rapid >2|Pg positive excursion in |MD13C starts at the base of the organic-rich Bahloul Formation, which represents the Cenomanian-Turonian Boundary Event (CTBE) in Tunisia. The transition to a dysaerobic environment at the sea floor, as well as hostile conditions in the subsurface waters, was abrupt. Evolutionary turnover in planktonic foraminiferal faunas in the lowermost part of the CTBE, including the extinction ofRotalipora, is estimated to have lasted little more than 50ka. Maximum anoxic conditions are expressed higher in the section, representing surface-water conditions in which only small globular planktonic foraminifera were thriving. Nannofossils do not show as major an evolutionary change as planktonic foraminifera, but assemblage diversity is very low within the CTBE sediments, which is attributed to a combination of hostile conditions and dissolution during deposition. A decrease in abundance of high productivity indicators among the nannofossils could be real, or the result of dissolution. Overall, calcareous microfossils do not show evidence for a major increase in primary productivity during the CTBE. Sediment accumulation rates immediately after the CTBE in the Oued Mellegue area were much lower than further to the south on the palaeoshelf. This is consistent with a major sea-level highstand during which clastic sediments were trapped on the shelf. The sequence above the Bahloul Formation shows a gradual relaxation of hostile oceanic conditions, with diversification of planktonic foraminiferal and nannofossil populations.