干旱区生态环境知识资源中心

The early Toarcian is marked worldwide by major environmental changes that resulted in organic-rich black shale deposition, the Toarcian Oceanic Anoxic Event (T-OAE). This organic-rich sedimentation is particularly recorded in the Grands Causses (GCB) and Quercy (QB) basins. The main objectives of this study are the characterization, through organic petrology and geochemistry techniques, of the organic matter (OM) of the late Pliensbachian - early Toarcian sedimentary successions of these basins and the definition of the organic fades, assessing their implications in the evolution of depositional paleoenvironments.

In the GCB sedimentation occurred, during the late Pliensbachian, in a well oxygenated water body (Total Organic Carbon - TOC 0.6 wt.% and low Amorphous Organic Matter - AOM) proximal to the terrestrial source area (high terrestrial contribution) with marine influence. In the lattermost Pliensbachian a shallowing of the water column is observed with decrease of marine influence and separation from the terrestrial source area (TOC 0.9 wt%, AOM and zooclasts co-dominate) under an arid climate (Classopollis sap.). From the Tenuicostatum to early Serpentinum chronozones a restricted and stagnated environment (TOC 5.7 wt.%, AOM dominates) is implemented. From middle to late Serpentinum Chronozone the reestablishment of the oxygen levels (decrease in TOC and AOM) takes place, as well as paleoceanographic circulation patterns.

In the QB, the late Pliensbachian to earliest Toarcian is characterized by low TOC (0.2 wt.%) with sedimentation occurring in a shallow oxygenated proximal water body (amorphous Hydrozoans dominate) separated from the terrestrial source area with arid climate (Classopollis ssp.), and with episodes of emersion. From the Semicelatum Subchronozone to Serpentinum Chronozone the development of dysoxic to anoxic conditions (TOC 4.2 wt.%) takes place, associated with water column stratification and more effective non-carbonate sedimentation, and with an increase in water level. For the lattermost Serpentinum Chronozone shallowing of a more oxidizing water body, with some oxygen depletion still present is proposed.

The differences in these paleoenvironmental depositional contexts further demonstrate that, although the TOAE has a global character, local control mechanisms in these basins play a pivotal role. Furthermore, the first occurrence of Hydrozoans, namely its free-swimming medusoid forms, is described in organopalynological preparations of sediments from the Pliensbachian-Toarcian and their first paleoenvironmental application in a palynofacies based study is made.