Variations of organo-mineral microfacies of Posidonia Shale from the Lower Saxony Basin and the West Netherlands Basin: Application to paleoenvironmental reconstruction

Abstract

High resolution organic petrological analysis has been carried out on core samples of the Posidonia Shale from four wells, two located in northwestern Germany (Lower Saxony Basin) and two in the western Netherlands (West Netherlands Basin). The organo-mineral microfacies defined by maceral composition and associated mineral matter show a wide range of variations related to the proportions of individual liptinite macerals of marine origin and of terrestrial macerals (macerals of vitrinite, inertinite groups and sporinite). Variations in organo-mineral microfacies, in which the organic components are sensitive to paleoenvironmental changes, are considered as a potential indicator of changes in depositional conditions.Sedimentation of organic matter is controlled by several factors, the most important in the marine environment being primary biomass productivity and influx of transported terrestrial organic matter, biological degradation related to bacterial activity, and the oxygen content of the bottom water. The level of degradation in organic-rich sediments is mainly related to oxygen availability which, in turn, influences the diversity of algae and the quality and quantity of the preserved organic matter. These two factors can be directly linked to maceral preservation and the organo-mineral microfacies. In order to evaluate the relationships between different types of organic matter and paleoenvironmental conditions, two indices, the oxidation index and the degradation index have been proposed. These indices illustrate the transition from highly degraded to well preserved macerals, provide information on the types of organic matter, and permit recognition of precursors for degraded amorphous organic matter (AOM).Use of these indices not only enables determination of the organo-mineral microfacies, but also sheds light on the AOM precursors in the Posidonia Shale. Information derived from the indices can also be used to reconstruct the paleoenvironments.