Salt Deformation and Sub-salt Fluid Circulation in the Algero-Balearic Abyssal Plain

Roger Urgeles, Angelo Camerlenghi, Alejandra L. Cameselle, Arianna Mocnik, Anna Del Ben, Riccardo Geletti, Fabrizio Zgur, Gert De Lange, Cesar R. Ranero, Yizhaq Makovsky, Nigel Wardell, Renata G. Lucchi, and Giuliana Panieri

Subsurface salt deformation is known to affect the circulation of hydrocarbon fluids on continental margins. The tectonic quiescence of the Central and Western Mediterranean extensional basins has not been considered until now to generate salt-fluid interaction other than the one deriving from de-watering of the post Messinian clastic wedges. During the summer of 2012 a geophysical cruise in the Algero-Balearic Abyssal Plain (ABAP) was undertaken to test the hypothesis that faulting or lateral migration in the Messinian salt have allowed deep seated sub-salt fluids to migrate upwards into and through the Plio-Quaternary sedimentary section, and to the seafloor. During the cruise multibeam bathymetry, chirp data, seismic multichannel data (two arrays of 4 GI guns each with a total volume of 1680 cu. in. and 3 km digital streamer with an active group length of 12.5 m) were acquired together with a couple of sediment cores. The bathymetric data shows that the central part of the ABAP is dominated by a series of small hills that rarely exceed 80 m above the surrounding abyssal plain sediments. There is an increasing organization in distribution of these hills from South to North, with those in the South being more rounded and randomly distributed, and those in the North being more elongated and arranged parallel to the Balearic Margin. Chirp data highlight a relatively complex structure of the ABAP that is far from the normally well-layered abyssal plain sediments and suggest increasing deformation from South to North. In preliminary processed seismic data the base of the Messinian sequence is clearly visible in areas where the salt deformation is reduced. Such reflector is characterized by its reverse polarity with respect to the seafloor reflector, and by a marked discordance with the top of Messinian reflector, which is mostly deformed by incipient diapiric deformation and lateral salt flow. Messinian and pre-Messinian reflectors are often characterized by reverse polarity and/or amplitude anomalies, which suggests the occurrence of overpressured fluids and gas below the evaporites. The string interstitial water salinity gradient observed in a core from the top of a mound ranged from 39 ppt at 10 cm below the seafloor to 46 ppt at 208 cm. Further geochemistry analyses are underway to determine whether the salinity gradient reflects diffusion or advection of interstitial fluids.

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