The Last Interglacial Rhine estuary - Sedimentary architecture, chronostratigraphy, preservation and analogue potential (original) (raw)
The aim of this paper is to reconstruct the evolution of the early to middle Holocene Rhine-Meuse river mouths in the western Netherlands and to understand the observed spatial and temporal changes in facies. This is achieved by constructing three delta wide cross-sections using a newly accumulated database with thousands of core descriptions and cone penetration test results, together with a large set of pollen/diatom analyses and OSL/14C-dates. Most of the studied deposits accumulated in the fluvial-to-marine transition zone, a highly complex area due to the interaction of terrestrial and marine processes. Understanding how the facies change within this zone, is necessary to make correct palaeogeographic interpretations. We find a well preserved early to middle Holocene coastal prism resting on lowstand valley floors. Aggradation started after 9 ka cal BP as a result of rapid sea-level rise. Around 8 ka most parts of the study area were permanently flooded and under tidal influence. After 8 ka a bay-head delta was formed near Delft, meaning that little sand could reach the North Sea. Several subsequent avulsions resulted in a shift from the constantly retreating Rhine river mouth to the north. When after 6.5 ka the most northerly river course was formed (Oude Rijn), the central part of the palaeovalley was quickly transgressed and transformed into a large tidal basin. Shortly before 6 ka retrogradation of the coastline halted and tidal inlets began to close, marking the end of the early-middle Holocene transgression. This paper describes the transition from a fluvial valley to an estuary in unprecedented detail and enables more precise palaeo-reconstructions, evaluation of relative importance of fluvial and coastal processes in rapid transgressed river mouths, and more accurate sediment-budget calculations. The described and well illustrated (changes in) facies are coupled to lithogenetic units. This will aid detailed palaeogeographic interpretations from sedimentary successions, not only in the Netherlands, but also in other estuarine and deltaic regions.
Most present day estuaries formed within incised fluvial valleys, created during the last glacial, that drowned during post-glacial sea-level rise. The sedimentary archive of the associated river-mouth areas contains important information on estuarine evolution under different rates of sea-level rise. This thesis presents a study on the development of the mouth of the Rhine-Meuse system in the Rotterdam area, western Netherlands, between 12000-6000 BP. During the study tens of thousands of core descriptions and cone penetration test results, as well as seismic data, pollen and diatom analyses, tens of OSL-dates and hundreds of radiocarbon dates were used. The objectives were to explain: 1) the early-mid Holocene sedimentary succession of the Rhine-Meuse river-mouth area; 2) the development of the river-mouth area in the early-mid Holocene in response to rapid sea-level rise (SLR) and 3) the interaction of the fluvial and coastal systems during the early-mid Holocene transgression. Between 10.5-8 ka BP, the effects of sea-level rise started to influence the study area: groundwater rise resulted in the formation of extensive wetlands, fluvial flood basins became more frequently flooded, sediment-aggradation rates increased and finally the river valley changed into an estuary with adjacent tidal basins. Sea level reached rates of 1 m/100 yr before 8 ka BP with a period of 2 m/100 yr between 8.5-8.3 ka BP as a result of sea-level jumping. During the latter period sea level rose 4.06 ± 0.5 m: 1.95 ± 0.74 m background sea-level rise and 2.11 ± 0.89 m sea-level jump. This jump is linked to the drainage of Lakes Agassiz and Ojibway in the Hudson Bay area and linked to the 8.2 event. After 8 ka BP the rate of SLR slowed down to 0.6 m/100 yr. North and south of the estuary, retrogradation of the coastline occurred at a faster rate than near the mouth at Hoek van Holland, leading to the formation of a promontory. In the upper estuary a bay-head delta was formed. Around 7.3 ka BP the main branch of the Rhine connected to a tidal inlet 20 km north of the promontory that was strongly eroded. The eroded sediment was partly used to fill in the abandoned estuary, but also to fill in tidal basins north of the promontory. The closure of the several tidal inlets resulted in poor drainage conditions and the wetland area increased rapidly after 6.5 ka BP and around 6 ka BP the coastline reached its most eastward position. The sedimentary succession was also described using sequence-stratigraphic concepts. The transition from lowstand to transgressive systems tract (TST) occurred when widespread basal-peat formation halted and gyttja accumulation and clastic deposition started. In large parts of the study area this occurred near-instantaneously during the sea-level jump. The highstand systems tract in the study area started after 6.5 ka BP with widespread peat formation in the closing tidal basins. The relatively late start of the TST is most likely typical for river-mouth areas along wide, low-gradient continental shelves where base-level changes are dominant for a relatively short period.
Development of a mid-Holocene estuarine basin, Rhine-Meuse mouth area, offshore the Netherlands
A proper understanding of coastal development during periods of rapid sea-level rise is a prerequisite for the prediction of future coastal response to the expected acceleration in sea-level rise. However, the development of back-barrier basins, especially in river-mouth areas, during such periods is still not well understood. Here we show the response of back-barrier basins adjacent to the Rhine-Meuse river-mouth area, The Netherlands during the mid Holocene. A combination of high-resolution seismic data and cores was used to describe and explain late-Weichselian to early Holocene terrestrial and mid-Holocene back-barrier sequences. Along with dating and micropalaeontological analyses, these descriptions and explanations form the basis of a reconstruction of river-mouth and adjacent basin behaviour under conditions of rapid SLR. The nearby presence of the Rhine-Meuse estuary had a significant influence on the development of the adjacent back-barrier basin and especially its tidal channels. The back-barrier channels started to fill in between 8.3-7.4 ka BP due to decreasing rates of both sea-level rise and tidal-amplitude increase. During this time the Rhine relocated its mouth to the study area and the increased sediment delivery contributed to tidal-prism decrease and channel infill. Close to the Rhine-Meuse river mouth the back-barrier channels lay embedded in thick clayey, estuarine deposits and lateral migration of the tidal channels was limited. At greater distances, lateral migration rates increased as the thickness of cohesive back-barrier deposits decreases. The spatial pattern and stratigraphic setting of the northern channel fills suggests an episode of barrier overstepping between 7.5 and 6.6 ka BP. The mode and spatial limits of this overstepping are still not understood.
The Depositional Record
The long-term morphodynamic evolution of estuaries depends on a combination of antecedent topography and boundary conditions, including fluvial input, sea-level change and regional-landscape interactions. Identifying effects of such boundary conditions on estuary evolution is important to anticipate future changes in specific boundary conditions and for hindcasting with numerical and physical models. A comprehensive synthesis of the evolution of the former Old Rhine estuary is presented here, together with its boundary conditions over its full lifespan from 6500 to 1000 cal. yr BP. This system formed during a period of sea-level high stand, during which the estuary served as the main River Rhine outlet. The estuary went through three stages of evolution: a maturation phase in a wide infilling back-barrier basin, a stable mature phase and an abandoning phase, both in a laterally confined setting. The Old Rhine River formed by a river avulsion around 6500 BP that connected to a tidal channel within a large back-barrier basin. Decelerating sea-level rise caused the back-barrier basin to silt up around 5700 cal. yr BP, resulting in shoreline progradation by beach-barrier formation until ∼2000 cal. yr BP. Beach-barrier formation along the coast and natural levee formation along the river triggered peat formation in the coastal plain, laterally constraining the estuary and limiting overbank deposition, which caused most sediment to accumulate offshore. The abandoning phase started around 2200 cal. yr BP when a series of upstream avulsions led to a substantial reduction in fluvial input. This induced a period of enhanced estuarine overbank clay deposition that continued into near-complete silting up and estuary closure around 1200 AD. These findings exemplify how tidal systems, formed in wide coastal plains during sea-level high stand, depend on antecedent conditions, and how they respond to connection and disconnection of a large river over long, millennial timescales.
2016
This paper describes the sedimentary architecture, chronostratigraphy and palaeogeography of the late Middle and Late Pleistocene (Marine Isotope Stage / MIS 6-2) incised Rhine-valley fill in the central Netherlands based on six geological transects, luminescence dating, biostratigraphical data and a 3D geological model. The incised-valley fill consists of a ca. 50 m thick and 10-20 km wide sand-dominated succession and includes a well-developed sequence dating from the Eemian interglacial (Last Interglacial). The lower part of the valley fill contains coarse-grained fluvio-glacial and fluvial Rhine sediments that were deposited under Late Saalian (MIS 6) cold-climatic periglacial conditions and during the transition into the warm Eemian interglacial (MIS 5e-d). This unit is overlain by fine-grained fresh-water flood-basin deposits, which are transgressed by a fine-grained estuarine unit that formed during marine high-stand. This ca. 10 m thick sequence reflects gradual drowning of the Eemian interglacial fluvial Rhine system and transformation into an estuary due to relative sea-level rise. The chronological data suggests a delay in timing of regional Eemian interglacial transgression and sea-level high-stand of several thousand years, when compared to eustatic sea-level. As a result of this glacio-isostatic controlled delay, formation of the interglacial lower delta took only place for a relative short period of time and delta progradation was limited. During the cooler Weichselian Early Glacial period (MIS 5d-a) deposition of deltaic sediments continued and extensive westward progradation of the Rhine delta occurred. Major parts of the Eemian and Weichselian Early Glacial deposits were eroded and buried as a result of sea-level lowering and climate cooling during the early Middle Weichselian (MIS 4-3). Near complete sedimentary preservation occurred along the margins of the incised valley allowing the detailed reconstruction presented here.
The Depositional Record, 2018
The long-term morphodynamic evolution of estuaries depends on a combination of antecedent topography and boundary conditions, including fluvial input, sea-level change and regional-landscape interactions. Identifying effects of such boundary conditions on estuary evolution is important to anticipate future changes in specific boundary conditions and for hindcasting with numerical and physical models. A comprehensive synthesis of the evolution of the former Old Rhine estuary is presented here, together with its boundary conditions over its full lifespan from 6500 to 1000 cal. yr BP. This system formed during a period of sea-level high stand, during which the estuary served as the main River Rhine outlet. The estuary went through three stages of evolution: a maturation phase in a wide infilling back-barrier basin, a stable mature phase and an abandoning phase, both in a laterally confined setting. The Old Rhine River formed by a river avulsion around 6500 cal. yr
This paper presents the current state of knowledge on the evolution and depositional history of the River Rhine in the southern part of the North Sea basin during the upper Middle and Late Pleistocene, and its response to climate change, sea-level oscillation and glacio-isostasy. The study focuses on the development of the Eemian interglacial lower-delta in the central Netherlands and its relation to records of climate and sea-level rise, and uses the Saalian and Weichselian pre- and postdating periods to place its development in context. The Rhine fluvial system fills the gradually subsiding North Sea basin, but its development has strongly been affected by the Saalian glaciation and its remaining topography. Ice-pushed ridges originating off the limit of maximum glaciation basically divided the central Netherlands into two sedimentary depocentres: a central depocentre within the former ice-limit, and a southern depocentre south of it. The sedimentary record of the central depocentre, including an incised-valley fill, shows a 20e40 m thick stacked sequence consisting of three units. The incised-valley fill consists of a Late Saalian to early Eemian age lower fluvial unit and aWeichselian age upper fluvial unit, both composed of coarse-grained channel deposits. Sandwiched in-between is a 5e15 m thick record composed of fine-grained fluvial and estuarine (tidal) floodbasin and shallow-marine deposits. It is of Eemian interglacial and Early Weichselian age, and comprises transgressive and highstand deposits that show the drowning of a fluvial system. Inland parts transformed from fluvial to deltaic and estuarine environments, and the most downstream parts transformed to a shallow-marine embayment. Preservation of these units occurred, despite considerable sea-level fall and climate-controlled erosion taking place in the last-glacial. Preservation potential was increased by the fact that the Rhine system avulsed away to the southern depocentre, halfway the Weichselian Pleniglacial. Consequently, the infill of the southern depocentre is of an entire different nature, and last-interglacial transgressive or highstand units are hardly preserved. Because of glaciation and resulting depocentre configuration, the Netherlands in NW Europe thus offers a very good opportunity to study the transgressive interglacial lower-deltaic records and fallingstage preservation thereof e both key elements for understanding sedimentary development over full 100-ky glacial-interglacial cycles of climate and base-level change.