Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis.

TLDR: In this article, seismic reflection profiles in the western part of the Gulf of Lions were used to confirm the basinward extension of the Messinian erosion and enable the mapping of distinctive seismic markers indicating Messinian Erosional Surface (or Messinian unconformity), the basin margin detrital deposits, and the deep evaporite sequence.

About: This article is published in Marine Geology.The article was published on 2005-05-30 and is currently open access. It has received 223 citations till now. The article focuses on the topics: Late Miocene & Evaporite.

Figures

Fig. 4 – A: Seismic line illustrating the method (Mauffret et al., 2001) used to calculate the volume of sediments eroded from the shelf during the Messinian Salinity Crisis. Black bold line: the Messinian Erosional Surface (MES); Black short dotted line: a Miocene horizon used as a reference to draw the “ghost” of the youngest Miocene reflector preserved from erosion (black large dotted line). The eroded volume is estimated between the ghost horizon and the MES using a constant velocity of 2000 m/sec. B: Study area. The volume eroded has been estimated beneath the present-day platform.

Fig. 5 – Portion of the Calmar75 seismic line illustrating the classical seismic facies of the Messinian deep-basin evaporite succession composed of the Lower Evaporites at the bottom, the Salt, and the Upper Evaporites at the top. Reflector references L, K and M, are respectively after Biju-Duval et al. (1978), Montadert et al. (1970) and Ryan et al. (1973).

Fig. 6 – Strike seismic profile LRM15 (A) and dip seismic profile LRM12 (B) across the Gulf of Lions continental shelf, illustrating the erosional character of the Messinian Erosional Surface (MES) at the base of the Plio-Quaternary prograding sequence.

Fig. 7 – A: Dip seismic profile (Marion10, Ifremer) across the present-day slope (see position in Fig. 1). The Messinian Erosional Surface (MES) at the base of the prograding Plio-Quaternary sequence has a sharp erosional character. Downslope, the Plio-Quaternary deposits are faulted by salt tectonism. B: Zoom on the MES illustrating the angular discordance between the toplaps of the eroded Miocene reflectors and the downlaps of the prograding Lower Pliocene reflectors.

Fig. 12 – Line drawings of the Messinian Erosional Surface (LRM profiles), illustrating the upstreamdownstream morphological changes in the Languedoc-Roussillon valleys. See Fig. 1 for profile locations.

Fig. 17 – LRM14 dip seismic line section beneath the outer shelf. Discontinuous high-amplitude reflectors above the Messinian Erosional Surface are interpreted as the landward continuation of Unit D, overlain by draped Pliocene bottomsets. See Fig. 1 for profile location.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Erosional processes and paleo-environmental changes in the western gulf of lions (sw france) during the messinian salinity crisis" ?

The geometrical relationship between these three elements and their relationship to the paleogeography of the margin during the MSC provide new information about the evolution of the study area during the Messinian. A chaotic seismic unit ( Unit D ) filling Messinian lows and extending beneath the Salt within the study area is interpreted as a Messinian clastic unit. The authors propose a polyphase scenario of detrital fan deposition involving pre-, syn-, and post-Salt deposition in subaqueous/subaerial environments. Their results also suggest that large submarine gravity flows occurred prior to any significant accumulation of Salt in the basin and prior to the Upper Evaporites. 

Q2. What is the axis of the Messinian paleo-Nile thalweg?

In the Nile system, for example, a seismic chaotic unit is present in the axis of the Messinian paleo-Nile thalweg, lying above the MES that here truncates underlying Tortonian prodelta shales (Barber, 1981). 

Q3. What would result in the high relief of the MES beneath the shelf?

The high relief of the MES beneath the shelf would then result from prolonged exposure to subaerial erosion (compared to the slope domain that remained submerged during the first phase). 

Q4. What is the likely explanation for the sapping of the groundwater?

The sediments may have dewatered through aquifers, with the sapping of the groundwater triggering avalanches and enhanced mass-wasting. 

Q5. What is the cause of the extension of the Gulf of Lions shelf?

Over the shelf, the post-Messinian sequence consists of a thick Plio-Quaternary succession whose deposition began in the Early Pliocene and led to a progradation of the Gulf of Lions margin by as much as 120 km (Lofi et al., 2003a). 

Q6. Why are the MES not noted in the Valencia Trough?

The fact that they are noted in the Valencia Trough could be because its very low basin-floor gradient favoured and enhanced the registration of even very slight variations during the drop in sea level. 

Q7. What could be the reason for the multiple phases of MES in the Mediterranean?

These multiple phases could nevertheless also reflect variations in the western Mediterranean base-level due to climatic changes, such as increased/decreased periods of runoff. 

Q8. How many km3 of sediments were eroded during the MSC?

The authors have estimated the volume of sediments eroded from the Languedoc-Roussillon shelf during the MSC as 3000 km3 (Fig. 15), which is equivalent to a 500-m-thick column of uncompacted sediments over the study area. 

Q9. How can the authors account for the morphological similarity of the Messinian valleys?

The (flat-)steep-flat-steep profiles of the Messinian valleys can be accounted for by applying Schuum's (1977) concept of the dynamic metastable equilibrium of rivers. 

Q10. What is the MES under the shelf and slope?

It appears evident that the Gulf of Lions margin was exposed during the “desiccation” phase, i.e. after 5.6 Ma, and that consequently the MES beneath the shelf and slope was shaped by subaerial processes. 

Q11. How much was the volume between the ghost horizon and the MES?

The authors then estimated the volume between their ghost horizon and the MES using a constant velocity of 2000 m/sec for the corresponding deposits. 

Q12. What are the main factors that account for the peculiar profiles of the MSC?

In an attempt to understand the major driving forces that prevailed during their formation, two principal factors can be considered to account for these peculiar profiles: tectonism and/or eustasy.