Submarine slope processes in rift-margin basins, Miocene Suez Rift, Egypt

Abstract

New, high-resolution lithofacies data from hanging-wall Miocene synrift (Rudeis Formation) exposures of the eastern Suez Rift margin, Egypt, reveal a submarine slope depositional system dominated by coarse-grained (pebble), heterogeneous, lenticular beds, formed by coalescing turbidity currents, slumps, and debris flows deposited on deforming submarine substrate. Flows include prerift clasts and contemporaneous shallow-marine fossil fragments originating from an uplifted eastern hinterland. Multiple terrestrial drainages debouched onto faulted offshore slopes or fed small fan deltas on narrow shelves ( 360 m Rudeis Formation is divided into stratigraphic units R1 and R2, which exhibit upward-coarsening and unordered vertical motifs. Ongoing faulting influenced synrift deposition by controlling the locus of subsidence and gravity base level. Mesoscale faults became inactive during Rudeis Formation times, with strain localized on the large rift border fault system, leading to a wider basin with time. We compare 16 subaqueous rift-margin basin fills from various tectonic and geographic settings and show they generally represent proximal gravity-flow deposits dominated by nongraded beds. We find little commonality in vertical grain-size trends, highlighting the diversity of stratal architectures. Most basin fills show an inverse relationship between maximum clast size and shelf width. We propose a new model to capture the spectrum of sedimentary responses to rifting within rift-margin basins, varying as a function of shelf width, slope gradient, maximum grain size, and textural maturity. Rudeis Formation strata at north Wadi Baba represent a particularly coarse-grained end member, deposited on steep slopes, with a narrow shelf.