Autochthonous versus allochthonous Upper Triassic evaporites in the Sbiba graben, central Tunisia

Abstract

Two-dimensional seismic data and well data from the Sbiba graben in central Tunisia (North Africa) reveal its asymmetrical structural geometry affected by E–W, N–S, NW–SE, and NE–SW master strike-slip faults. These faults have been intruded by Upper Triassic evaporites, which can be classified into the following three types: (1) sealed evaporite anticlines and domes along folded structures; (2) extrusional wrinkles of “salt walls” at the borders of the graben related to along-strike displacement of the master fault zone; and (3) evaporite outpouring at the center of the graben interpreted as the result of fault and diapir movement. Detailed seismic-stratigraphic analysis highlights a complex interplay between fault growth and evaporite movement, which strongly controlled the evolution of the graben. During the Mesozoic, tectonic stresses induced multidirectional faulting along inherited faults. The vertical rising of the Upper Triassic evaporites was initiated locally at the intersection of master faults during regional extensional and transtensional events in the sub-evaporite basement. The interaction of normal faulting and evaporite diapirism caused lateral outpouring of surface-piercing evaporites. This phenomenon is well expressed in Upper Cretaceous (upper Cenomanian to lower Turonian) strata. The master faults, which cut the diapir at the border and the crest, have caused the extrusion of evaporites. The downward, gravity-driven flow of these evaporites across the slope surface and subsequent accumulation and preservation in Mesozoic sediments is related to the fault escarpment and the adjacent fault-induced depression. There is an apparent relation between tectonic inheritance and inversions of the graben, where interconnected deep faults and diapirs have been reactivated during extensional and contractional tectonic episodes, causing evolution of the structures with the associated sedimentary lapouts. Several autochthonous Triassic evaporites and associated fault escarpments have induced salt canopy structures.