Quantification des flux sédimentaires et de la subsidence du bassin ...

tel-00790852, version 1 - 21 Feb 2013
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this seismic unit can be followed within the upper part of LU1 and its top corresponds to the
top of UU. This configuration means that the upper part of LU1 and the overlying MU and
UU units were deposited in a transgressive context, during the landward migration of the
shoreline that led to the formation of the TRS. An heterogenous evaporitic body (thick of 266
m) has been drilled in GLP2 well between 3703 and 3437 m depth (Fig. 7; Brun et al., 1984).
It is made of an alternation of halite, clays and anhydrite. The Messinian age of these
evaporites is supported by the occurrence of Amaurolithus tricorniculatus in the overlying
unit and attested by that of Globigerina nepenthes at 3508.70 m depth (Brun et al., 1984).
This interval probably corresponds to the seismic facies mapped above the TRS (Figs. 7 and
9). In addition the clays sandwiched between the evaporitic bodies are characterized by 80%
of planktonic foraminifers (at 3508.70 m from a side well core; Fig. 7), that is consistent with
their deposition in a context of sea-level rise (Brun et al., 1984).
5. Scenario of the peak of the Messinian Salinity Crisis
These new observations lead to subdivide the peak of the MSC (phase 2) into three distinct
steps including instantaneous events at the geological scale (Figs. 8, 10 and 11): (i) the sea
level drop at 5.60 Ma which followed the high sea-level episode of Lago Mare 1, (ii) a slow
rise in sea level at the origin of the TRS, and (iii) the very fast catastrophic marine reflooding
at 5.46 Ma which caused the high sea-level episode of Lago Mare 3 (Clauzon et al., 2005;
Bache et al., 2012). Lago Mare 2 is still an imperfectly explained episode of unknown
duration, which only concerned the central basins (Clauzon et al., 2005; Fig. 6).
5.1. Step I, sea level drop: subaerial erosion and detrital deposition
The lower part of Dm prisms and its distal counterpart have been interpreted as deposited
before the initiation of the Messinian drawdown dated at 5.60 Ma, i.e. during the Middle-
Upper Miocene (Lofi and Berné, 2008), maybe at the Serravallian/Tortonian transition
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