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Hydrocarbon-seep Scenarios in the Western Mediterranean Margin During the Messinian Salinity Crisis: Onland Case Studies from the Italian Peninsula

Annalisa Iadanza, Gianluca Sampalmieri, Paola Cipollari, Domenico Cosentino, and Marco Mola

Brecciated fabrics extensively affect the carbonatic units developed during the Messinian Salinity Crisis (MSC) of the Mediterranean Sea. In our study - encountering a combined sedimentological, textural and geochemical approach - we investigated the Messinian brecciated bodies in correspondence to some key-sections, representative of different geodynamic contexts. The study areas, cropping out in the Italian Peninsula, are located: in the Maiella foreland basin (Central Italy); in the Rossano and Crotone forearc basins (Calabrian Arc); in the Caltanissetta foredeep basin (Sicily).

Brecciation mainly affects microbial mudstones, resting at different stratigraphic levels and always associated to the Messinian Erosional Surface (MES), and gypsum units to a minor extent.

From a sedimentological point of view, breccias variously consist of: patchy concretions, massive buildups and stratified thick bodies. They consist of fabric-retentive mud breccias, devoid of gravity segregation and any preferential distribution. From a textural point of view, they show scale- and lithology-independent patterns. Interestingly, the brecciated bodies are accompanied by fluid migration pathways occurring at any scale of observation. These characteristics clearly point to in situ brecciation phenomena, where hydrofracturing is held responsible for the widespread brecciation.

The fluid-driven hypothesis was eventually endorsed by the 13C-depleted values (down to -43.7 ‰ PDB ) yielded by the carbonates, pointing to the occurrence of hydrocarbon-enriched fluids involved in authigenesis, at least to a certain extent.

In view of that, there is no evidence to consider the upper Messinian breccias as a stratigraphic unit. Since the brecciated lithofacies are sealed by Lago-Mare deposits (late post-evaporitic), the age of this processes is constrained to the early post-evaporitic phase.

By considering the different geodynamic settings of the investigated basins, we rule out a common tectonic trigger. Conversely, we suggest the major drawdown of the Mediterranean Sea recorded by the MES as the primary trigger for fluid migration: the high depressurization experienced by the sedimentary column after the removal of at least 1 km of water column, likely favoured a catastrophic migration of overpressured fluids from below. Similar data and interpretations, already suggested in the Tertiary Piedmont Basin, SE Spain and Northern Marocco, point to a regional extent of the event, that conceivably occurred throughout the Western Mediterranean margins in Messinian times.

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