Deep basin Messinian evaporites on the southern Balearic margin, Western Mediterranean

Abstract

This study is based on multibeam bathymetry and multi-channel seismic reflection data collected during Eurofleets R/V OGS Explora, Cruise No. E12 (SALTFLU), Valencia (Spain) 22/06 - 04/07/2012. Modelling of overpressure evoltion is based on the resulting stratigraphy. The study area encompasses proposed drill sites BAL-02 and BAL-02 of IODP pre-proposal 857B-Pre, Deep-Sea Records of the Messinian Salinity Crisis (DREAM), proposed by J. Lofi et al in 2015. We identify four types of salt-induced abyssal plain sediment deformation structures: (1) a belt of elongated abyssal plain seahills thought to be the attenuated surface expression of salt rollers either produced by reactivated right strike slip faults synthetic to the master fault of the EBE or by differential compaction above such faults; (2) anastomosing knolls in the Menorca Fan area thought to be the product of subsurface salt deformation and gliding triggered by differential sedimentary load; (3) abyssal knolls and seahills spread in the entire abyssal plain across the basin thought to be the product of subsurface salt anticlines and pillows in a relatively more evolved stage of basin evolution affected by basement faulting, perhaps northward propagation of the shortening affecting the Algerian margin; and (4) a small flat topped circular mound in the middle of the abyssal plain interpreted as a mud volcano extruding sediments and fluids of likely pre-Messinian origin. The structures suggest that the crustal transition, not imaged in the seismic data due to intense deformation does not correspond to the Emile Baudot Escarpment (the present day continental tectonically driven continental slope). Rather it is located farther offshore in the deep basin, approximately where intense halokinesis occurs in the abyssal plain, marking the edge of the deep Messinian basin filled with kilometer-thick evaporites. Messinian marginal basins, therefore, are presently located also below the abyssal plain, at the floor of the Emile Baudot Esacarpment. Evidence for sub-salt overpressure is provided by pearchment structures in the deep Messinian basin. A compaction disequilibrium modeling exercise demonstrates that the high sedimentation rate of the thick evaporitic succession in the deep Messinian basin, may indeed generate considerable overpressure, persisting today (overpressure coefficient up to 0.8). However, the reduced evaporite thickness in the marginal basins reduces considerably the likelihood of sub-salt overpressure (overpressure coefficient up to 0.2). Therefore, we conclude that marginal basins belong to different geological domains compared to the deep basin, being the difference given by different crustal structure, evaporite thickness, in some cases evaporite composition (absence of halite layer), and pore pressure regime below and within the evaporitic formations. An upward pore water expulsion system is inferred through the Plio-Quaternary sedimentary cover induced by gypsum dehydration to anhydrite