The Last Interglacial Rhine estuary: Sedimentary architecture, chronostratigraphy, preservation and analogue potential

Estuaries and deltas reflect complex sedimentary environments, where fluvial and marine processes alternate both in space and time. Their records, that make up an important part of the infill of sedimentary basins, are highly heterogeneous, with alternations of sand, silt, clay and organic material, as well as mixtures thereof. Understanding of the heterogenic nature of these deposits is crucial in applied geosciences, as it largely determines the physical characteristics of geohydrological and petroleum geological parameters such as porosity, permeability and sand-body connectivity. Obtaining quantitative insights in these complex environments in petroleum geological studies is often difficult because of incomplete stratigraphic preservation caused by erosion, faulting and compaction processes, as well as to the limited number of available wells. These difficulties can be partly overcome by using (modern) analogues, which are generally more extensively studied, such as for example the Holocene and Pleistocene records of the Rhine system in the Netherlands. Here, the current state of research demonstrates that such records can be studied in great detail, while offering the advantage that deposits are relatively easily accessible, well described in past research, and supported with unparalleled dating control. This study provides new data and insights on the evolution and depositional history of the River Rhine in the southern part of the North Sea Basin during the late Middle and Late Pleistocene and its response to climate and sea-level change, and glacio-isostasy. To study all this, a reconstruction based on six newly constructed geological transects, luminescence dating, biostratigraphical records and a 3D geological model was made. The late Middle to Late Pleistocene incised-Rhine valley-fill in the central Netherlands holds a circa 50-metre-thick and 10 to 20 kilometre wide sand-dominated record. Basically, it consists of three stacked successions. The lower part of the valley infill consists of coarse-grained deposits of the Late Saalian braided Rhine river. These sediments are overlain by medium-grained sands with clay and organic admixtures, deposited by a meandering river that was in existence from the latest stages of the Saalian to very early-Eemian. These lower deposits are overlain by the central fill dating from the Last Interglacial, which consists of peat and gyttja-layered silty and loamy floodbasin sediments. This basal part of the transgressive sequence gradually changes into estuarine deposits towards its top. Coarse-grained fluvial deposits generally overlay these transgressive deposits and mark the truncation of the highstand and transgressive systems tracts. These fluvial deposits correlate to the Early and Middle Weichselian relative sea-level fall. The Last Interglacial and Holocene transgressive deposits in the Netherlands were quantitatively compared and their preservation potential evaluated. Subsequently, their applicability as a North Sea Basin analogue for the Early Jurassic Åre Formation in the Norwegian offshore was assessed. For this purpose, new and existing data from both transgressive Rhine records was reviewed in a sequence stratigraphic framework, before volumetrics were calculated and longitudinal trends quantified. Major differences between the Last Interglacial and Holocene transgressive records were found. Differences in initial valley configuration and relative sea-level rise-rates during both transgressions – due to eustasy and glacio-isostasy – are considered key controls of the volumes and spatial arrangements of the transgressive deposits. Comparison of the Late Quaternary Rhine records with the Early Jurassic Åre Formation off Norway shows the potential of the Last Interglacial and Holocene transgressive systems tracts as analogues for ancient stratigraphic records