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UN MODELE GEOMETRIQUE ET STRATIGRAPHIQUE DES SEQUENCES DE DEPÔTS QUATERNAIRES SUR LA MARGE DU GOLFE DU LION : ENREGISTREMENT DES CYCLES CLIMATIQUES DE 100 000 ANS

Abstract : The up to 70 km wide continental shelf of the Golfe du Lion has been the object of numerous investigationss since the early days of Oceanography. Yet several key questions have remained unanswered and are still controversial. Among these questions are: What are the relationships between sedimentary sequences, Quaternary glacioeustatic cycles and tectonics? What is the sequence duration? What are the mechanisms of deposition?
A study of the Very High Resolution (VHR) seismic reflection (Sparker) data collected by Ifremer between 1992 and 1998 on the outer shelf and upper slope offshore of Sète in the Languedoc region gives a new insight into these issues. The surveyed area covers 40 x 40 km with a line spacing of 400-1000 m. Useful depth of penetration is 150-250ms twtt (100-200m). Information on recent sediments is provided by a dozen short (up to 10m) cores collected in the same area.
The approach used throughout the study involved a constant confrontation of modelling and field data. In co-operation with different laboratories and institutions several softwares were used and/or adapted (Sithere-Ifremer; Isatis-Géovariances and Dionisos-Ifp...) for 3D numerical cartography of successive seismic units and interfaces and for stratigraphic simulation of the sequences.
Analysis of the 3D geometry reveals a basic depositional pattern consisting of a pair of horizontally juxtaposed prisms. Prism PI with low angle clinoforms (<1°) lies on the upper part of the shelf and corresponds to prodeltaïc-offshore deposits. Prism PII with steeply dipping clinoforms (~4°) lies on the outer shelf between 40 and 70 km from the present day coastline and corrresponds to littoral deposits.
Results obtained from the seismic and core data and from numerical modelling in an inter-disciplinary and collaborative context (Seismic Stratigraphy, Stratigraphic Simulation, lithology, palynology, micropaleontology...) indicate that the basic depositional pattern records a 100 000 years glacioeustatic (interglacial/glacial) cycle. As previously suggested by Aloïsi [1986], the prism PI corresponds to deposition at «high to middle» sea level and prism PII to deposition during low sea level at the glacial maximum. A total of five sequences of paired prisms are recognised, showing that the corresponding glacioeustatic cycles (the last five cycles) are recorded in the shelf sediments of the western part of the Golfe du Lion. The age of the base of the series analysed is taken to be 540 000 years (isotopic stage 12). A shorter sequence duration would imply unacceptably rapid subsidence rates. The shorter duration cycles (40 000 y, 20 000 y or less) are not clearly identifiable in the geometry of the deposits; this calls into question to what extent such short climatic cycles are recorded in the sediment geometries of passive margins of the Golfe du Lion type with similar subsidence and sedimentation rates.
On the other hand, hydrodynamic regim variations have an impact on the 3D sediments geometries. The activity of submarine canyons (infilling, transport, erosion), the importance of currents have been variable in space and time. The Languedoc canyons (Aude and Hérault), quiescent at present, were very active during glacial times; their activity is different from one canyon head to another.
The stratigraphic simulation of exceptionally well preserved and well defined sets of prisms in the study area gives geological meaning to the «simple» transformation of isotopic curves to eustatic curves and their global significance.
This study also provides the first estimate of the average total subsidence rate of the shelf in the western part of the Golfe du Lion during the upper Quaternary. The subsidence reflects a seaward tilting of the shelf and reaches a maximum rate of 250 m/Ma at the shelf break (70 km from the present day coastline). The coherence of this quantitative model is confirmed by independent calculation of the same average rate of subsidence for the entire Plioquaternary (240-260 m/Ma) based on evidence of deeper penetration seismic profiles acquired by Elf Aquitaine in 1996. This suggest that subsidence rate has been constant during the Plioquaternary. The subsidence does not create any individual sequence but the rate controls sequences accumulation.
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https://tel.archives-ouvertes.fr/tel-00136517
Contributor : Marina Rabineau <>
Submitted on : Wednesday, March 14, 2007 - 11:57:24 AM
Last modification on : Tuesday, February 2, 2021 - 9:52:02 AM
Long-term archiving on: : Tuesday, April 6, 2010 - 10:20:44 PM

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  • HAL Id : tel-00136517, version 1

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Marina Rabineau. UN MODELE GEOMETRIQUE ET STRATIGRAPHIQUE DES SEQUENCES DE DEPÔTS QUATERNAIRES SUR LA MARGE DU GOLFE DU LION : ENREGISTREMENT DES CYCLES CLIMATIQUES DE 100 000 ANS. Géologie appliquée. Université Rennes 1, 2001. Français. ⟨tel-00136517⟩

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