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Five million years of sedimentation in the Gulf of Lions

For sediments to accumulate in the sea, enough volume must be available to them. In the Gulf of Lions, stratigraphic models confirm that this volume was mainly provided by the slow seaward tilt of the margin, which gave the sedimentary layers their spatial configuration.

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The Gulf of Lions formed about 25 million years (Ma) ago by the anticlockwise rotation of the Corsica-Sardinia block towards its present location. In this Western Mediterranean basin, the deposit of sediments on the continental margins occurred under the influence of the regional geological history: opening or closing of the connection with the Atlantic ocean (Strait of Gibraltar), high sedimentation rate, subsidence (downward shift of the lithosphere). The Gulf of Lions, where a large amount of data is available, is thus a natural laboratory for stratigraphic research.

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The Gulf of Lions (white frame) and its margin (continental shelf, in beige).Red line shows the location of the stratigraphic profiles simulated by the models.

A seismic prospection survey conducted in 1996 on the continental shelf (margin) of the Gulf of Lions provided a description of the overall geometry of the strata which deposited since the beginning of Pliocene, 5.4 Ma ago. These layers extended farther and farther offshore (progradation) then, from 2.6 Ma, also overlaid the preceding ones (progradation-aggradation). In order to explain this change, different hypotheses are put forward about the rate of subsidence of the margin: either constant (seaward tilt with a rotation point 13 km upstream from the present-day coastline) or increasing after 2.6 Ma.

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Interpretation of the seismic profiles from the coastline (left) seawards (right). The pattern of strata limits show that they pile up during a second phase only (aggradation)

The paper contributes to this debate with numerical stratigraphic models. These tools make possible to reproduce sedimentological geometries (the arrangement of layers) according to the main parameters governing their deposit: volume available for the sedimentation (controlled by sea-level and subsidence), sedimentary flux and transport. The comparison with seismical data leads to validate or reject hypotheses. Two sets of simulations were conducted. To study the role of its variations, subsidence was simulated with a constant rate since 5.33 Ma (255 meters per million years at a point located 70 km offshore) and with three scenarios of increasing rate from 2.6 Ma onwards. To test the influence of sea-level variations, two simulations were conducted with different curves (subsidence rate and water and sediment fluxes being identical).

The results are shown as vertical profiles showing the depth of layers when they deposited (paleobathymetry), the shape and age of the main limits between strata, and the offlap breaks (the edges of strata, which indicate their stacking pattern). It is then possible to compare the geometry of surfaces and the disposition of offlap breaks with the geological interpretations of seismic profiles.

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Simulation results with a constant subsidence rate: paleobathymetry (left) and comparison with the geological interpretations of seismic profiles (right)

Only the simulation using a constant rate of subsidence yields a good reproduction of prograding, then prograding–aggrading geometries; the height of layer surfaces and the offlapbreak positions are not reproduced when subsidence is very small or negligible. Therefore the change observed in the stacking pattern (progradation to progradation-aggradation) is not linked to a sudden change in subsidence, but to the combination of ongoing tilt of the margin together with variations in sediment fluxes and sea-level. The frequency and amplitude of sea-level cycles are also an important factor in stratigraphic simulation. A low-resolution description with five major cycles of high amplitude does not lead to the observed geometries: the offlap-break trajectory line is segmented, with alternating seawards and landwards migrations (progradation/retrogradation). With a different, high-resolution, curve, the offlap-break trajectory line and the transition from prograding to prograding–aggrading stacking pattern is more consistent with seismic data. The simulations allowed the computation of sediment flux and water discharge. In all regimes of sea-level fluctuation, fluxes considerably increase around 3.8 Ma, shifting from less than 25 to almost 100 km3 of sediments per million years, and from 100 to 300 m3 of water per second of fluvial discharge to the sea.

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Simulations results with the two hypotheses on sea-level evolution (curve left of each profile). Offlap breaks are shown by black dots.

The main result is that a constant subsidence rate reproduces the geometry of sedimentary filling. The amount of space created on the inner shelf is smaller than that created on the outer shelf because of the global seaward tilt of the margin: there is no need to infer a variation in subsidence rate through time, since it varies spatially. Therefore, a regular ongoing process (the tilt of the margin) in conjunction with sea-level and sediment-supply fluctuations can produce an important change in the resulting geometries of the margin (pro to pro-aggrading pattern). The most consistent simulation implies an increase in sediment and water fluxes at 3.8 Ma. The implied evolution of water discharge is consistent with climate studies indicating high precipitation in north-western Europe from 3.6 to 2.6 Ma. The increase in sediment supply on the shelf around 3.8 Ma might be related to the beginning of global climate cooling in the Northern Hemisphere and/or to the end of upstream trapping of sediments on land in deltas, after the "Messinian crisis" where the level of the Mediterranean, then isolated from the Atlantic, was very low.


The paper

Leroux E., Rabineau M., Aslanian D., Granjeon D., Droz L., Gorini C., 2014. Stratigraphic simulations of the shelf of the Gulf of Lions: testing subsidence rates and sea-level curves during the Pliocene and Quaternary. Terra Nova 26(3) : 230-238.

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The authors

This work was done by researchers from the laboratory Domaines océaniques of IUEM, the Géosciences marines Department of Ifremer, IFP-Energies Nouvelles and the Institut des sciences de la Terre de Paris


The journal

Terra Nova is the official journal of the European Union of Geosciences and is published by Blackwell. It publishes short, innovative and provocative papers of interest to a wide readership and covering the broadest spectrum of the Solid Earth and Planetary Sciences. Terra Nova encompasses geology, geophysics and geochemistry, and extends to the fluid envelopes (atmosphere, ocean, environment) whenever coupling with the Solid Earth is involved. 



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