Stratigraphical and lateral distribution of sedimentary organic matter in Upper Jurassic carbonates of SE France

L. Bombardiere et Ge. Gorin, Stratigraphical and lateral distribution of sedimentary organic matter in Upper Jurassic carbonates of SE France, SEDIMENT GE, 132(3-4), 2000, pp. 177-203
Citations number
Categorie Soggetti
Earth Sciences
Journal title
ISSN journal
0037-0738 → ACNP
Year of publication
177 - 203
SICI code
A detailed analysis of sedimentary organic matter (SOM) has been carried ou t in Upper Jurassic deep-water marls and carbonates of the Vocontian Basin (SE France). Its main purpose is to analyse the vertical and lateral trends of SOM distribution, in order to better understand processes and factors c ontrolling the accumulation and preservation of organic constituents in bas inal environments. The Vocontian Basin is characterized by widespread oxic depositional environments in the Upper Jurassic, as confirmed by the very l ow total organic carbon content (less than 0.25 wt.%). For such organic-poo r deposits, palynofacies analysis represents the best tool to investigate S OM. Stratigraphic analysis of palynofacies trends has been performed within a pre-established sequence stratigraphic framework, defined by means of fi eld sedimentology and, subsequently, complemented through lateral correlati ons, biostratigraphical constraints and geochemistry. This approach highlig hts the vertical signatures which are directly related to relative sea leve l changes. Furthermore, lateral correlations of palynofacies trends, at the scale of 3rd and 2nd order cyclicity, permit the signatures observable at basin scale to be distinguished from those detectable only at local scale. The former are controlled by factors affecting the whole basin, such as eus tasy, regional tectonics and climate. The palynofacies signatures at the scale of 3rd order cyclicity reflect rel ative sea level changes and the degradation state of SOM. Taking into accou nt the preservation state of organic constituents leads to a better underst anding of the relationship between SOM and sequence stratigraphy. Usually, condensed sections about maximum flooding surfaces are characterized by a d ecrease in the relative proportion of the terrestrial organic constituents. As far as sequence boundaries are concerned, they are marked by an increas e in the relative proportion of continental organic matter, which results f rom a renewal of the erosion related to relative sea level fall. An increas e in fresh woody debris has not been systematically observed at sequence bo undaries. Nevertheless, the abundance of fresh woody debris tends to decrea se stratigraphically towards maximum flooding intervals and laterally in a distal direction. The vertical palynofacies signatures at the scale of 2nd order cyclicity appear to be controlled by tectonics. In the Upper Oxfordia n to Lower Kimmeridgian interval, these signatures confirm the hypothesis o f a regional uplift of the hinterland and/or a differential basin subsidenc e related to the intense rifting activity in the Proto-Atlantic or to the w estward opening of the Tethys. (C) 2000 Elsevier Science B.V. All rights re served.