Soft‐sediment deformation structures in the Late Messinian Abu Madi Formation, onshore Nile Delta, Egypt: Triggers and tectonostratigraphic implications

Abstract

The Upper Messinian Abu Madi Formation of the Nile Delta constitutes sediments deposited during the final stage of the Messinian salinity crisis (MSC). Several levels of soft‐sediment deformation structures (SSDS) were observed in the transgressive heterolithic fluvial facies deposited adjacent to the deep‐seated faults. The most common deformation features include diapiric structures, intraformational lithoclastic breccia, small‐scale normal faults, slump folds, and liquefied beds. Such association of SSDS typically resembles those described elsewhere as generated by a complex interplay of gravity‐driven and seismically induced liquefaction processes. The pore pressure measurements revealed high pore fluid density ~ 9.1 ppg equivalent density values which reflect a mild pressure disequilibrium in Abu Madi sediments. Pressure disequilibration is observed all over the studied section of the Abu Madi Formation and is not only restricted to the fluvial channel sandstones deposited during rapid loading. Therefore, allogenic seismic activity has been proposed as the main trigger for pressure disequilibrium and the development of SSDS in the Abu Madi Formation. The heterolithic nature of the Abu Madi sediments as well as the scarcity of bioturbation provides the favourable conditions for the preservation of the seismically induced deformation. The lateral variation in thickness suggests deposition of the Abu Madi Formation during periods of active subsidence which promotes the generation of the SSDS. Late MSC tectonism likely controlled the evolution and sedimentary facies variability of the Abu Madi canyon‐infill system, and therefore the distribution of Abu Madi facies varies significantly over very short distances. Abu Madi SSDS follows the spatial distribution observed in other “lago‐mare” deposits (e.g., Foes Formation and SE Spain), characterized by spatial variability and vertical rhythmic alternation between deformed and non‐deformed layers. Accordingly, regional tectonic instability in the circum‐Mediterranean margins during the late stage of the MSC is proposed.