Search

The thickness and distribution of early syn-rift deposits record the evolution of structures accommodating the earliest phases of continental extension. However, our understanding of the detailed tectono-sedimentary evolution of these deposits is poor, because in the subsurface, they are often deeply buried and below seismic resolution and sparsely sampled by borehole data. Furthermore, early syn-rift deposits are typically poorly exposed in the field, being buried beneath thick, late syn-rift and post-rift deposits. To improve our understanding of the tectono-sedimentary development of early syn-rift strata during the initial stages of rifting, we examined quasi-3D exposures in the Abura Graben, Suez Rift, Egypt. During the earliest stage of extension, forced folding above blind normal fault segments, rather than half-graben formation adjacent to surface-breaking faults, controlled rift physiography, accommodation development and the stratigraphic architecture of non-marine, early syn-rift deposits. Fluvial systems incised into underlying pre-rift deposits and were structurally focused in the axis of the embryonic depocentre, which, at this time, was characterized by a fold-bound syncline rather than a fault-bound half graben. During this earliest phase of extension, sediment was sourced from the rift shoulder some 3 km to the NE of the depocentre, rather than from the crests of the flanking, intra-basin extensional forced folds. Fault-driven subsidence, perhaps augmented by a eustatic sea-level rise, resulted in basin deepening and the deposition of a series of fluvial-dominated mouth bars, which, like the preceding fluvial systems, were structurally pinned within the axis of the growing depocentre, which was still bound by extensional forced folds rather than faults. The extensional forced folds were eventually locally breached by surface-breaking faults, resulting in the establishment of a half graben, basin deepening and the deposition of shallow marine sandstone and fan-delta conglomerates. Because growth folding and faulting were coeval along-strike, syn-rift stratal units deposited at this time show a highly variable along-strike stratigraphic architecture, locally thinning towards the growth fold but, only a few kilometres along-strike, thickening towards the surface-breaking fault. Despite displaying the classic early syn-rift stratigraphic motif recording net upward-deepening, extensional forced folding rather than surface faulting played a key role in controlling basin physiography, accommodation development, and syn-rift stratal architecture and facies development during the early stages of extension. This structural and stratigraphic observations required to make this interpretation are relatively subtle and may go unrecognized in low-resolution subsurface data sets.

Because salt can decouple sub- and supra-salt deformation, the structural style and evolution of salt-influenced rifts differs from those developed in megoscopically homogenous and brittle crust. Our understanding of the structural style and evolution of salt-influenced rifts comes from scaled physical models, or subsurface-based studies that have utilised moderate-quality 2D seismic reflection data. Relatively few studies have used high-quality 3D seismic reflection data, constrained by borehole data, to explicitly focus on the role that along-strike displacement variations on sub-salt fault systems, or changes in salt composition and thickness, play in controlling the four-dimensional evolution of supra-salt structural styles. In this study, we use 3D seismic reflection and borehole data from the Sele High Fault System (SHFS), offshore Norway to determine how rift-related relief controlled the thickness and lithology of an Upper Permian salt-bearing layer (Zechstein Supergroup), and how the associated variations in the mechanical properties of this unit influenced the degree of coupling between sub- and supra-salt deformation during subsequent extension. Seismic and borehole data indicate that the Zechstein Supergroup is thin, carbonate- dominated and immobile at the footwall apex, but thick, halite-dominated and relatively mobile in high accommodation areas, such as near the lateral fault tips and in the immediate hangingwall of the fault system. We infer that these variations reflect bathymetric changes related to either syn-depositional (i.e. Late Permian) growth of the SHFS or underfilled, fault scarprelated relief inherited from a preceding (i.e. Early Permian) rift phase. After a period of tectonic quiescence in the Early Triassic, regional extension during the Late Triassic triggered halokinesis and growth of a fault-parallel salt wall, which was followed by mild extension in the Jurassic and forced folding of Triassic overburden above the fault systems upper tip. During the Early Cretaceous, basement-involved extension resulted in noncoaxial tilting of the footwall, and the development of an supra-salt normal fault array, which was restricted to footwall areas underlain by relatively thick mobile salt; in contrast, at the footwall apex, no deformation occurred because salt was thin and immobile. The results of our study demonstrate close coupling between tectonics, salt deposition and the style of overburden deformation for >180 Myr of the rift history. Furthermore, we show that rift basin tectono-stratigraphic models based on relatively megascopically homogeneous and brittle crust do not appropriately describe the range of structural styles that occur in salt-influenced rifts.