ARCHITECTURE AND FACIES DISTRIBUTION OF ORGANIC-CLASTIC LAKE FILLS IN THE FLUVIO-DELTAIC RHINE-MEUSE SYSTEM, THE NETHERLANDS

Lakes are prominent features in distal zones of delta plains. Organic-clastic lake fills may form in these lakes, which include gyttja and lacustrine deltas. Understanding of the geometrical properties, the facies distribution, and the origin of organic-elastic lake fills is limited. Knowledge of their development and composition, however, will improve our fundamental understanding of both sediment distribution in and evolution of distal delta plains. Furthermore, organic-elastic lake fills potentially form connectors in hydrocarbon reservoirs. This study aims to describe and explain the characteristics of organic-elastic lake fills and thereby to contribute to the understanding of delta evolution, especially of distal delta plains. A field study conducted in the Schoonhoven and Angstel-Vecht areas, located in the distal part of the Rhine-Meuse delta, the Netherlands, indicates that organic-elastic lake fills developed in peat-bounded lakes after an avulsion initiated fluvial sedimentation. The characteristics of organic-elastic lake fills, such as geometry, sedimentary facies, and sedimentation rates, were both qualitatively and quantitatively determined by cross sections, geomorphogenetic maps, and logs. The results show that in the Angstel-Vecht area as much as 30% of the fluvial-sand volume is stored in organic-clastic lake fills. This contrasts to the general notion that overbank deposits do not contain significant portions of sand. Further, the lacustrine setting often results in vertically bounded sediment bodies that include gyttja and underlies elastic depositional fades, which show a prominent coarsening-upward succession. It is concluded that the presence of organic-elastic lake fills enlarges the reservoir capacity of ancient fluvio-deltaic successions as they contribute to the reservoir volume and also can form connectors between isolated channel-belt sand bodies.