GEOCHEMICAL STUDY OF ORGANIC-MATTER RICH CYCLES FROM THE KIMMERIDGE CLAY FORMATION OF YORKSHIRE (UK) - PRODUCTIVITY VERSUS ANOXIA

In this contribution, we study two meter-scale cycles from the Kimmeri dge Clay Formation (cored near Marton, Yorkshire) which shows cyclic o rganic matter (OM) distribution. Our aim is to try to understand the f actors responsible for OM accumulation. The first cycle, called lower cycle, shows a total organic carbon (TOC) content fluctuating between 1 and 10% whereas the second, called upper cycle, shows a TOC content varying between 5 and 35%. The geochemical composition (major elements and trace elements), the organic geochemistry (TOC, HI, palynofacies) and mineralogy of the sediments have been determined. In both cycles, the cyclicity is expressed through variations in the nature and in th e relative abundance of the various types of organic-matter constituen ts. Furthermore, dilution effects by inorganic components of the sedim ent cannot account for the TOC cyclicity. For the lower cycle, the Mo, V and U content is low and little variable as is the intensity of the oxidation which OM suffered from. This indicates that variations in p hytoplanktonic productivity may be held responsible for the cyclicity in steady and mildly reducing redox conditions. In the upper cycle, cy clicity also appears to depend on productivity but variations in the c oncentration of Mo, V and U and in the oxidation state of the OM sugge st the environment was temporarily more reducing. It is proposed that larger amounts of H2S were released into marine bottom waters as a res ult of initial OM decomposition, forced the oxic-anoxic boundary to ri se in the water column and thus favoured OM storage. The main driving force for variations in the OM concentration was the productivity of o rganic-matter-walled phytoplankton. Redox conditions of the deposition al environment could have had a positive action, but only by acting as a positive feedback effect.