DISTRIBUTION OF SEDIMENTARY ORGANIC-MATTER IN A MIXED CARBONATE-SILICICLASTIC PLATFORM ENVIRONMENT - OXFORDIAN OF THE SWISS JURA MOUNTAINS

Outcrop sections from the Swiss Jura, consisting of carbonate-silicicl astic deposits spanning the Middle-Late Oxfordian boundary, provide a palaeogeographical cross-section ranging from coastal to shallow platf orm and intra-platform basin environments. Using a sedimentological an d a 3rd order sequence stratigraphic framework based on those sections , the distribution of sedimentary organic matter (palynofacies) has be en spatially studied in relation to sequence stratigraphy. The main fa ctors influencing the spatial variations of sedimentary organic matter are the proximity of land, organic productivity, level of biodegradat ion and hydrodynamic conditions of the palaeoenvironment. These factor s determine the distribution of the land-derived, relatively allochtho nous organic constituents (phytoclasts, pollen and spores), and marine , relatively autochthonous constituents (phytoplankton and foraminifer a linings). Five main palynofacies parameters appear as good indicator s of proximal-distal trends: the relative proportion of total phytocla sts; the proportion of fresh, translucent fragments among these phytoc lasts; the ratio of marine to continental palynomorphs; and the relati ve amount and species diversity of dinoflagellate cysts. In lowstand d eposits, high-energy hydrodynamic conditions may affect these trends o n the shallow platform, where storms tend to increase the proportion o f the marine organic fraction landwards. These five palynofacies param eters are largely related to water depth. Therefore, when compared wit h the established 3rd order sequence stratigraphic framework, their st ratigraphic variations indicate relative sea-level changes or, alterna tively, proximality changes. Parameters indicative of more distal cond itions increase in the transgressive and early highstand, and decrease in the late highstand. The opposite occurs for parameters indicative of more proximal conditions. Foraminifera linings seem to be particula rly concentrated near the shelf edge and on the slope, Finally, bisacc ate pollen may display cyclicity associated with alternating humid and dry periods. This palaeoclimatic signature may be related to orbital cyclicity.