The Hesperian Massif: From Iapetus aulacogen to ensialic orogen a model for its development

The mantle plume/rift system model developed by three of the present authors (v. C., d. T., K.) for the northern Hesperian Massif is expanded to that of an aulacogen/ensialic orogen for the entire massif. The first author (v. d. M. M.) contributes a review of the supracrustal history of the massif and its predrift adjacent areas, summarized in a time-stratigraphic table of the Cambro-Ordovician and a palinspastic-palaeogeographic sketch map of the general region in Middle Cambrian times. The infrastructural history is also reviewed, and it is concluded that the early Palaeozoic features of the Massif are consistent with those of an aulacogen oriented at a high angle to the Iapetus ocean. In the Late Palaeozoic its features changed into those of an ensialic orogen without appreciable loss of continuity as shown by the facies- and age record of sedimentary, igneous and metamorphic events.

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Zusammenfassung Das Graben-Mantelkissen-Modell, welches für das nördliche Hesperische Massiv von drei der Autoren (v. C., d. T., K.) entwickelt wurde, wird zu dem eines aulacogenen ensialischen Orogens für das ganze Massiv weiterentwickelt. Der erste Autor (v. d. M. M.) liefert eine Übersicht der suprakrustalen Entwicklungsgeschichte des Massivs und seiner Umrandung vor der Driftung, zusammengefaßt in einer stratigraphischen Tabelle des Kambro-Ordoviziums und in einer palinspastisch-paleogeographischen Skizze der Region während des Mittelkambriums.Die infrastrukturale Entwicklungsgeschichte wird ebenfalls beleuchtet, und es wird daraus geschlossen, daß die frühpaläozoischen Konfigurationen des Massivs mit denen eines Aulacogens übereinstimmen, unter Einschluß eines großen Winkels mit dem Iapetus Ozean. Im Jungpaläozoikum veränderten sich die Charakteristika zu denen eines ensialischen Orogens, ohne daß, im weiteren Verlauf der Erdgeschichte, dieser Prozeß unterbrochen wurde, wie sedimentäre Fazies, Altersdatierungen und kristalline und metamorphe Entwicklung dokumentieren.

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Résumé Le modèle d'un panache mantélique avec son système de rift, développé par les trois derniers auteurs pour le Massif Hespérique septentrional, est étendu à un modèle d'aulacogène/orogène ensialique pour le massif entier. Le premier auteur y contribue par un aperçu de l'histoire supracrustale du massif et de ses régions limitrophes avant l'ouverture du présent Océan Atlantique. Cette histoire est résumée dans un tableau chronostratigraphique et exprimée dans un croquis palinspastique/paléogéographique de la région circum-Hespérique à l'époque du Cambrien Moyen. L'histoire infracrustale est passée également en revue. On en conclut que les traits caractéristiques du massif pendant le Paléozoique inférieur correspondent avec ceux d'un aulacogène orienté quasi-perpendiculairement à l'Océan proto-Atlantique, dit Iapetus. Au Paléozoique supérieur ces traits se sont transformés de façon à ressembler à ceux d'un orogène ensialique sans perdre sa continuité, ce que montre l'étalement ininterrompu, dans le temps comme dans l'espace, des événements sédimentaires, ignés et métamorphiques.

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Analytical and numerical analysis of tides and salinities in estuaries; part II: salinity distributions in prismatic and convergent tidal channels

Estuaries, commonly, are densely populated areas serving the needs of the inhabitants in multiple ways. Often the interests are conflicting and decisions need to be made by the local managers. Intake of fresh water for consumption, agricultural purposes or use by industries may take place within a region not far landward of the limit of salt intrusion. Human interventions (e.g. deepening of the navigation channels) or climate changes (sea level rise, reduction of the river discharge) can bring these intake locations within the reach of saline or brackish water and consequently endanger their function. To support policy and managerial decisions, a profound knowledge of processes associated with the salinity structure in estuaries is required. Although nowadays advanced numerical three-dimensional models are available that are able to cope with the complexity of the physics there is still a need for relatively simple tools for quick-scan actions in a pre-phase of a project or for instructive purposes. The analytical model described in this paper may serve these needs. It computes the maximum salinity distribution using the dispersion coefficient in the mouth as the only model parameter. The model has been calibrated using observational data in a large number of estuaries and experimental data in a tidal flume. The dispersion coefficient was successfully related to geometric and hydrodynamic parameters resulting in an expression that can be used for convergent estuaries as well as prismatic channels, see Eqs. 25a and 25b. Application of the model in a predictive mode showed its promising capabilities. Comparison with three-dimensional numerical models indicates that the channel geometry in the estuary mouth largely influences dispersive processes. The analytical model for salt intrusion may be used in combination with the analytical model for tidal propagation in convergent estuaries and tidal channels by Van Rijn (part I). In this way, input is obtained on the tidal velocity amplitude and the Chézy roughness following calibration of this model on tidal amplitudes along the estuary.     

Groundwater impact on environmental flow needs of streams in sandy catchments in the Netherlands

Abstract

During recent years, water managers and water users in the Netherlands experienced water shortages in numerous streams. Besides low rainfall amounts and high temperatures, anthropogenic alterations to the groundwater system are also responsible for the reduced baseflow in streams. These alterations may reduce resilience and increase a risk to streams as more droughts are expected in the Netherlands due to climate change. We propose a methodology to assess the impact of groundwater-related alterations and climate change on baseflow and environmental flow needs (EFN). Application of this methodology for two sandy catchments showed that, under average meteorological conditions, baseflow in the main streams still meets the EFN requirements. During dry years, baseflow is probably insufficient in the upper parts of the catchments. Anthropogenic alterations show a significant impact: drainage caused 25–40% baseflow reduction, groundwater abstractions caused 5–28% and climate change will potentially cause an additional reduction of 33–70% by 2050.

Editor D. Koutsoyiannis; Guest editor M. Acreman