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Early onset and rapid growth of sediment depocenters subsiding into Messinian salt in the Tyrrhenian Sea

Bruno C. Vendeville, Gaël Lymer, Virginie Gaullier, Frank Chanier, Agnès Maillard, Isabelle Thinon, Françoise Sage, Johanna Lofi, and Lies Loncke

The onset of rifting in the Tyrrhenian basin varied between Tortonian (to the West) and Messinian times or even later (to the East). Crustal-scale normal faulting during or right before Messinian times created some accommodation space that locally allowed for deposition of thick, mobile, halite-rich evaporites (The Mobile Unit) in areas such as the Cornaglia Terrace. This was followed by deposition of the comparatively more competent Upper Unit made of alternating layers of dolomitic marl and anhydrite, itself followed by deposition of silicoclastic Plio- Quaternary series.

Data from the METYSS* cruises (Messinian Event in the Tyrrhenian from Seismic Study) have evidenced that in some areas, where the Mobile Unit was thick, spontaneous mobilization of the mobile salt and its overburden started very early, right after salt deposition, as is attested by marked lateral thickness changes in the Upper Units. The overburden series spontaneously formed local depocenters that subsided into the Mobile Unit, even though the latter were very thin. Likely, this was made possible by the higher density of the Upper Unit. The fact that such depocenters formed only locally may be related to early salt or subsalt movements (causing local thin-skinned deformation, whether extension or shortening) during Messinian times, or to particular differential erosional or depositional processes.

One surprising finding is how fast this overburden subsidence occurred. Downbuilding was very rapid, as evidenced by the fact that most depocenters had grounded by the time the Uppermost Unit was deposited (a duration of about 200 000 years). Some depocenters had grounded even earlier in some parts of the region. The rate of depocenter subsidence exceeded 2 mm/y.

The implication is that, in other parts of the Western Mediterranean, spontaneous salt mobilization could too have started very early after salt deposition.

* METYSS cruises funded by the French Margin Programme (ACTION MARGES)

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013