Mid-Cretaceous Oceanic Anoxic Events recorded in SE France and central Italy

Abstract

The mid-Cretaceous is characterized by prominent global volcanism and a climax of the Mesozoic greenhouse climate. Widespread anox- ic conditions in the ocean, termed “Oceanic Anoxic Events” （OAEs） , developed repeatedly during this time, and resulted in numerous, widespread accumulations of laminated, organic-rich sediments（so-called “black shales”). Although many local names for the black shales have been established, the major mid-Cretaceous OAEs are given the generic names OAE 1a-d and 2. OAEs have been critical in Earth history because 1） enhanced carbon burial acts as a global thermostat during times of greenhouse climate（producing black shale）; 2）oxygen depletion in the oceans caused mass extinctions of marine biota; and 3） most Cretaceous hydrocarbon source-rocks were deposited under anoxic oceanic conditions. The mid-Cretaceous strata of SE France（hemipelagic Vocontian Basin）and central Italy（pelagic Umbria-Marche Basin） consist predominantly of pelagic-hemipelagic limestone and marlstone, commonly with intercalated black shales indicating OAEs. Although most of the black shales lack fossils, some contain abundant macro- and microfossils. This may reflect variation in the position of the nutricline, as well as the depth and areal extent of anoxia in the ocean.