Publication : Palaeogeography, Palaeoclimatology, Palaeoecology

Publication de l’article sur l’origine des vallées tunnel

Does porewater or meltwater control tunnel valley genesis? Case studies from the Hirnantian of Morocco

ravier et al 2015 pal pal pal

Highlights

  • •We compared processes involved in the formation of two Upper Ordovician tunnel valleys.
  • •Models of tunnel valley formation are driven by porewater pressures or meltwater flows.
  • •The distribution of ice streams controls tunnel valley formations and morphologies

Abstract :

Several Ordovician tunnel valleys are exposed in the Moroccan Anti-Atlas Mountains, including the Alnif and the Foum Larjamme tunnel valleys, located 150 km away from each other. Sedimentological and deformational analyses of these two glacial troughs reveal that differing processes lead to their formations.

The Alnif tunnel valley contains numerous deformation structures within sediments both below and above the main glacial erosion contact surface. Ball-structures and clastic dykes occur within preglacial sediments down to 35 m below glacial incisions while overlying glacial sediments contain fluted surfaces, clastic dykes, dewatering structures, folds and radial step normal faults. The characteristics of the Alnif tunnel valley can be explained by a porewater pressure-driven model of formation where the localized increase of basal shear stress and porewater pressure underneath subglacial deforming zones lead to the development of a dense hydrofracture network in the preglacial bed. These processes of hydraulic brecciation promoted subglacial remobilization of the preglacial material and contributed to the formation of the tunnel valley.

The Foum Larjamme tunnel valley displays undisturbed preglacial sediments and few dewatering structures at the base of the glacial sedimentary infill which suggests relatively low porewater pressures within the tunnel valley during formation. This second type of tunnel valley where porewater pressure remained relatively low appears to have been formed by meltwater erosion. The undulating base of the Foum Larjamme tunnel valley implies progressive erosion by a stable subglacial braided network of Nye-channels, or alternatively by channels migrating laterally during episodic minor subglacial outbursts.

These two tunnel valleys highlight the regional variability of processes involved in the formation of tunnel valleys. The distribution of palaeo-ice streams in North Africa illustrate that morphologies and processes involved in the formation of tunnel valleys vary between ice stream and inter-ice stream zones due to variations in meltwater availability, the topography and bed lithological properties.

Publicités

Publication : Climate of the Past

Premiers résultats des modélisations climatiques sur la glaciation Ordovicienne. En collaboration avec le CEA et l’IPGP

climate past

Abstract. The Ordovician is a particular Period during Earth History highlighted by abundant evidence for continental-size polar ice-sheets. Modelling studies published so far require a sharp CO2 drawdown to initiate this glaciation. They mostly used non-dynamic slab mixed-layer ocean models. Here, we use a general circulation model with coupled components for ocean, atmosphere and sea ice to examine the response of Ordovician climate to changes in CO2 and paleogeography. We conduct experiments for a wide range of CO2 (from 16 to 2 times the preindustrial atmospheric CO2 level (PAL)) and for two continental configurations (at 470 Ma and at 450 Ma) mimicking the Middle and the Late Ordovician conditions. We find that the temperature–CO2 relationship is highly non-linear when ocean dynamics is taken into account. Two climatic modes are simulated as radiative forcing decreases. For high CO2 concentrations (≥ 12 PAL at 470 Ma and ≥ 8 PAL at 450 Ma), a relative hot climate with no sea ice characterises the warm mode. When CO2 is decreased to 8 PAL and 6 PAL at 470 and 450 Ma, a tipping-point is crossed and climate abruptly enters a runaway icehouse leading to a cold mode marked by the extension of the sea ice cover down to the mid-latitudes. At 450 Ma, the transition from the warm to the cold mode is reached for a decrease in atmospheric CO2from 8 to 6 PAL and induces a ~ 9 °C global cooling. We show that the tipping-point is due to the existence of a quasi-oceanic Northern Hemisphere, which in turn induces a minimum in oceanic heat transport located around 40° N. The peculiar shape of the oceanic heat transport in the Northern Hemisphere explains the potential existence of the warm and of the cold climatic modes. This major climatic instability potentially brings a new explanation to the sudden Late Ordovician Hirnantian glacial pulse that does not require any large CO2drawdown.

Pohl, A., Donnadieu, Y., Le Hir, G., Buoncristiani, J.-F., and Vennin, E. 2014. Effect of the Ordovician paleogeography on the (in)stability of the climate, Clim. Past Discuss., 10, 2767-2804, doi:10.5194/cpd-10-2767-2014

Publication : sedimentary Geology

Sortie du papier sur le creusement des vallée tunnel durant l’Ordovicien

en tete papier sed geol maroc edouard

Edouard Ravier, Jean-François Buoncristiani, Michel Guiraud , John Menzies , Sylvain Clerc , Bastien Goupy , Eric Portier 2014. Porewater pressure control on subglacial soft sediment remobilization and tunnel valley formation : A case study from the Alnif tunnel valley (Morocco). Sedimentary Geology 304, 71-95.

Field trip au Maroc 2014 – projet ANR SeqStrat-Ice

Février 2014, une mission de terrain dans le sud du Maroc a été organisée dans le cadre du projet de recherche SeqStrat-Ice: « Les glaciations du passé: leçons pour un modèle de stratigraphie séquentielle dédié aux systèmes glaciaires ». Réunissant 19 chercheurs ce fieldtrip avait pour objectif d’établir in situ une synthèse des premiers résultats du projet.

Financé par l’ANR (2012-2016) le projet ANR SeqStrat-Ice est un consortium de recherche entre 4 Unités de recherche académique (l’Institut de Physique du Globe de Strasbourg; Biogéosciences, Dijon; Géosystèmes, Lille, l’université d’El Jadida) et  l’IFP-Energies Nouvelles. Ce projet est réalisé en étroite collaboration avec l’université Chouaib Doukkali d’El Jadida (partenariat avec Mohamed El Houicha) .

Stage master 2 – 2013/2014 : Caractérisation des structures de déformations à micro et macro échelle de deux enregistrements glaciaires Ordoviciens

Dans les enregistrements sédimentaires d’origine glaciaires, de nombreuse structures de déformations provenant de l’interaction entre la glace et un substrat meuble sont observés à macro et micro-échelle. Ces déformations résultent de l’action cissaillante de la glace sur des sédiments gorgés d’eau et entrainent une modification notable de la structure du sédiment et de ses propriétés pétrophysiques. Ces dépôts clastiques Ordoviciens sont connus pour leur fort potentiel réservoir et font donc l’objet de nombreuses études, principalement d’ordre sédimentologique, stratigraphique et diagénétique (Deynoux et ghienne, 1998; Le Heron, 2007; Tournier et al., 2010 ; Clerc et al., 2013). Néanmoins l’impact des déformations glaciotectoniques sur la modification des propriétés réservoirs est encore mal connu, notamment l’impact des fortes variations intrinsèques de pressions de fluides (Pf) induites par des épisodes de couplage entre la glace et les sédiments.

Les objectifs de ce travail sont les suivants :

  •  Etablir un inventaire des structures de déformations observables à micro et macro-échelle.
  • Interpréter les données en termes de variations de Pf et de dynamique glaciaire (vitesse d’écoulement/couplage/découplage)
  • Etablir la relation en modifications précoces liées aux variations de Pf et histoire diagénétique.

Les outils: Microscope polarisant, MEB, analyse d’image (JM Microvision), Porosimètre mercure.

Compétences requises : Sédimentologie/tectonique/diagenèse.

Encadrement : E. Ravier, JF. Buoncristiani, M. Guiraud, E. Vennin, E. Portier (GDFSUEZ), et G. Desaubliaux (GDFSUEZ)

Mission Maroc Décembre 2012

Une nouvelle mission au Maroc dans l’Anti-Atlas marocain sur les dépôts glaciaires Ordoviciens permettant la reconnaissance et l’échantillonnage des structures de déformations sous-glaciaires.

Ce diaporama nécessite JavaScript.

Publication :

Notre dernier papier sur la glaciation Ordovicien est en ligne

Subglacial to proglacial depositional environments in an Ordovician glacial tunnel valley, Alnif, Morocco

  • a Laboratoire Biogeosciences UMR/CNRS 6282 Université de Bourgogne, 6 Bd Gabriel, 21000 DIJON, France
  • b GDF Suez EPI, 1 place Samuel de Champlain – Faubourg de l’Arche 92930 Paris La Défense Cedex – France

Abstract

This paper presents the sedimentary analysis of an exceptional Ordovician glacial tunnel valley in the eastern part of the Anti-Atlas. The valley infill comprises two major glacial erosion surfaces (striated pavements) each overlain by a fining-upward glacial unit. These units are composed of five distinct facies associations, recording the evolution from subglacial to proglacial environments, and an additional sixth facies association, overtopping the tunnel valley infill, and associated with post-glacial environments. The tunnel valley infill also records a transitional environment between the subglacial and proglacial settings, which is compared with the Antarctic ice-sheet margin. These three environments are defined by the position of the grounding line and the coupling line. The new proposed depositional model also differs from usual Ordovician depositional models in which the main tunnel valley infill is interpreted as essentially proglacial outwash deposits, in a range of glaciomarine to glaciofluvial environments. Overall, a substantial part of the valley infill (~ 50% of volume) was deposited in a subglacial setting. The sedimentary bodies could form potentially thick and laterally extended, although these were limited by the shape and extent of the subglacial accommodation space. Finally, the sedimentary record, when compared with regional analogues, also provides information for the palaeogeographic reconstruction of the Ordovician ice-sheet in this region.


Highlights

► An Ordovician tunnel valley infill is described in detail ► Subglacial and proglacial depositional environments are determined ► An additional environment is proposed associated with a lightly grounded ice-sheet ► Accommodation space is defined for proglacial and subglacial settings ► The study provides additional data for Hirnantian ice-sheet reconstruction