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Gravity-Driven Deformation in the Miocene Sequence: An Interplay between Tectonics and Eustasy Case Study from Romanian Offshore, Western Black Sea

Alexandru Lăpădat, Z. Schleder, Zamir Bega, D. Stefaniuc, D. Leslie, R. Rusby, C. Krezsek, and G. Ingram

The Late Miocene passive margin evolution of the Romanian offshore area, Western Black Sea, is characterized by thin-skinned gravitational gliding/spreading of the 2 km thick clastic sediment pile overlying upper Oligocene overpressured shales. Most of the faults related to the gliding are healed by a major overlying unconformity that is associated with the Messinian sea-level drop.

The gravitational failure, localized on the northern flank of the Histria Trough, generated up-dip arcuate listric growth faults, which nucleated preferentially on top of some pre-existing structures. This suggests that reactivation of earlier basement features may have played an important role in initiating the gravity collapse. In the up-dip domain the basinward-induced translation generated numerous highly tilted fault blocks, rafts and pre-rafts. In the extensional domain, besides the rafts, we identify keystone grabens, regional and counter-regional faults. At the keystone grabens and counter-regional faults the ductile Oligocene shale forms shale pillows akin to reactive diapirs seen in salt tectonics. Surprisingly, in the down-dip domain we have one single toe thrust structure to balance all the up-dip extension. We present geometrically balanced cross sections to illustrate the deformation sequence.

Contemporaneous with the gravitational deformation, massive Mass Transport Complexes (MTCs) were deposited in the deep water parts of the basin. We explore the possibility that the significant sea-level drop during Messinian generated slope instability, gravitational failure and associated deposition of MTCs. Some evidence of direct relations between basement architecture, reactivation and thin-skinned deformation suggests that there may have been more contributing factors triggering the gravity transport.

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