| Popis: |
The Marnes Bleues Formation from the Vocontian Basin (Southeastern France) shows many dark-colored levels, some concomitant to oceanic anoxic events OAE1a (the so-called Goguel Level) and OAE1b (the Jacob, Kilian, and, above all, Paquier levels). These levels are usually referred to as being organic-rich; they are scattered through a thick, rather homogeneous, succession of marls, that is poor in organic matter (OM). Through a multi-parameter approach, the dark levels from the Aptian-Albian were characterized. Our results show that all dark levels are not OM-rich and that the dark levels exhibit variable characteristics, such as OM nature (marine vs. continental), sedimentation and accumulation rates, redox conditions, surface-water productivity and relative sea level, but they all show low to modest enrichments in OM. Furthermore, all the levels share in common the fact that they formed under conditions of normal to low productivity and oxic to suboxic conditions. Thus, our results strongly suggest that, in the absence of high productivity and anoxic bottom conditions, the other factors reputed to favor OM accumulation only led to sporadic and low enrichments in organic contents. It is as if such factors could only enhance OM accumulation but could not induce it alone. What was true for the Vocontian Basin may be extended to other settings, regardless of their time of deposition or location. |
