Holocene tidal back-barrier development at decelerating sea-level rise: a 5 millennia record, exposed in the western Netherlands

Abstract

A succession of tidal back-barrier deposits, ranging in age from about 6000 to 0 cal BC, was recently exposed in an up to 17-m-deep excavation in the western Netherlands. The lower part of the succession shows the flooding of the area and the establishment of a tidal back-barrier basin during high rates of relative sea-level rise; the upper part of the succession consists mainly of estuarine channel and tidal flat deposits. Medieval lake deposits cover the estuarine sequence. The development of the Holocene Holland Tidal Basin took place in the Atlantic and Early Subboreal sub-stages between ∼7000 and 3500 cal BC after which the tidal inlets closed and fresh water marshes developed. Its evolution was largely controlled by both the rates of sea-level rise and of sediment supply. All the sediment was derived from the North Sea and the shoreface. During initial flooding of the outcrop area, the rate of sea-level rise (0.1–0.06 cm/year) far outran that of sediment supply. Facies zones developed controlled by the inundated topography and sediment supply. Fresh water reed marshes evolved along the landward side of the tidal basin. The Basal Peat that formed in these marshes is overlain by a thin layer of organic-rich clays, which represent the deposits in a lagoon separating the marshes from the tide-influenced sands and clays of the proximal zone of the tidal basin. The latter consisted of sand-rich channel sequences separated by mud-rich interchannel areas. Position of the channels was controlled by the pre-existing topography; the channels were confined by levees consisting of overbank and crevasse deposits. The interchannel areas consisted of sub-tidal mudflats. The studied outcrop showed a subtidal fan or delta, connecting a non-exposed channel to a mud-rich sequence of subtidal interchannel sediments. The latter graded upwards into inter- and supratidal flats deposited when the rate of sea-level rise decelerated after ±5000 cal BC and sediment supply eventually outran the creation of accommodation space. This sequence is overlain and incised by an estuarine system of mainly Subboreal and Subatlantic age with well-developed inclined heterolithic stratification showing in particular the equilibrium variations of the channel system under influence of changing storm/fair weather conditions. The excavation shows an almost complete succession of the channel, from its origin to its final abandonment stage with a number of different channel sub-facies. Upwards, the pointbar deposits merge into intertidal flats. A thin veneer of medieval lake deposits overlies the latter.