SEDIMENTARY ORGANIC-MATTER IN CONDENSED SECTIONS FROM DISTAL OXIC ENVIRONMENTS - EXAMPLES FROM THE MESOZOIC OF SE FRANCE

A detailed analysis of sedimentary organic matter (or palynofacies) wa s carried out on thermally immature to early mature Upper Jurassic and Hauterivian condensed intervals in deep-sea carbonate-marl alternatio ns outcropping in the Vocontian Basin (SE France). All the condensed s ections studied are characterized by intense bioturbation and very low organic carbon content (< 0.25 wt.%), indicative of oxic depositional conditions. Oxic condensed sections display variable palynofacies sig natures, which are best illustrated by: (1) the ratio of continental t o marine constituents; (2) the ratio of opaque to translucent phytocla sts (i.e. woody debris) and (3) the preservation of palynomorphs (base d on fluorescence intensity and morphological preservation state in tr ansmitted light microscopy). Both of the ratios increase with the degr ee of palynomorph degradation, which shows that phytoclasts, especiall y the opaque ones, become relatively concentrated in the most degraded facies. These observations lead to the classification of oxic condens ed sections into three organic facies types showing different degrees of preservation and palynofacies signatures. Type 1 organic facies dis play intense degradation and are characterized by high values of the r atio of continental to marine fraction. They record unfavourable depos itional environments for preservation of organic matter. Type 2 organi c facies are most common and are characterized by a decreasing value o f the ratio of continental to marine fraction. Type 3 organic facies d isplay the same trend of the ratio of continental to marine fraction a s type 2, but the palynomorph assemblage is better preserved. Type 1 a nd type 3 organic facies are relatively rare. Recognizing these organi c facies types is important when analysing the relationship between se dimentary organic matter and sequence stratigraphy, because it allows the use of the appropriate palynofacies parameters. In particular, the use of the ratio of continental to marine constituents, usually a ver y good indicator of regressive-transgressive trends, becomes questiona ble in highly degraded intervals. Moreover, distinguishing between wel l-preserved or highly degraded palynofacies in condensed intervals pro vides valuable information on the oxicity of the depositional environm ent.