GEOCHEMICAL IMPRINT OF DEPOSITIONAL CONDITIONS ON ORGANIC-MATTER IN LAMINATED BIOTURBATED INTERBEDS FROM FINE-GRAINED MARINE SEQUENCES

For rocks that are still in a diagenetic stage of thermal alteration, the relative abundance of biomarker compounds and specific biomarker indices can be useful indicators of depositional and early diagenetic conditions. Thermally immature laminated rocks are characterized by relatively high contents of 17 beta (H), 21 beta (H)-hopanes, hopenes, sterenes and diasterenes, and by strong predominance of the 22R over 22S homohopane isomers. Thermally immature bioturbated samples are characterized by absence of the beta , beta -hopanes, by low contents of both saturated and unsaturated polycyclic hydrocarbons, and by slight or no predominance of the 22R over 22S homohopane isomers. For both the thermally mature and immature laminated samples, the degree of isomerization at the **2**2C position for hopanes and at the **2**0C position for steranes is generally consistent with the degree of thermal maturity interpreted from other properties of the organic matter. The bioturbated samples, however, exhibit inconsistent and anomalously high degrees of isomerization for the homohopanes.