Fluvio-deltaic floodbasin deposits recording differential subsidence within a coastal prism (central Rhine–Meuse delta, The Netherlands)

Abstract

In the central Netherlands, the Rhine follows a course imposed by Late Quaternary glaciation, forcing it to cross several tectonic blocks of the Roer Valley Graben system before entering the south-eastern North Sea Basin. Holocene sealevel rise resulted in the formation of a coastal prism (Holocene Rhine–Meuse delta). Across the Peel Boundary Fault-zone (PBF) in the central delta, differences in subsidence between the downstream Roer Valley Graben and its upstream shoulder influenced fluvial deposition. This study examines the sedimentary response upstream and downstream of the PBF and uses that to quantify differential subsidence rates. The local identification and quantification of the subsidence component within deltaic relative base-level rise may serve to determine to what extent coastal prism aggradation and resulting architecture are controlled by downstream sea-level rise, local tectonics and discharge coming from upstream. A floodbasin-section shows both syn-depositional and post-depositional tectonic effects in the Late-glacial to Holocene (marine oxygen isotope stages 2 and 1) sedimentary record, and reveals an active fault of the Peel Boundary Fault-zone. Differential subsidence across the PBF is quantified for the Last Glacial Maximum subsurface (averaged rate c. 0.06 m kyr−1 over the past 15 kyr). The offset in Holocene basal peat yields a similar value (0.03–0.07 m kyr−1, averaged over the past 7 kyr). Groundwater-level rise in the study area (as reconstructed from series of radiocarbon-dated basal peats) is used to quantify subsidence by comparison with relative sea-level rise at the river mouth. High rates of subsidence between 7000 and 5500 cal. yr BP together with syn-depositional sedimentary evidence suggest that the last major activity (palaeo-earthquakes) along this fault occurred in the middle Holocene. The timing of this and earlier fault activity may be related to the deglaciation and sea-level rise history. The results imply that early–middle Holocene deltaic back-filling should not be attributed solely to downstream relative sea-level rise dominating upstream controls, but that local differential subsidence is another independent control. Similar effects of differential subsidence in coastal prism sedimentary architecture can be expected in other coastal prisms.