Soft-Sediment Deformation Structures: Insights into Earthquake History Related to Tectonic Events in the Marhouma Zone, Southwestern Algeria

The deformation structures of the soft-sediments of the Marhouma Formation (Upper Devonian) in the Ougarta range of Southwestern Algeria, include: (i) contorted structures, including convoluted folds, pillow and boulder structures , concave paths with consolidation laminations, cross-bedding with recumbent folds, and irregular convoluted bedding gradually transitioning to massive beds; (ii) intrusive structures, including pillars, dikes, cusps, and subsidence lobes; (iii) brittle structures, represented by fractures and faults with nearly horizontal planes but of normal or thrust nature. The observed syn-sedimentary deformation structures (SSDS) result primarily from differential liquefaction and fluidization of saturated sediments, controlled by density and compaction contrasts between lithological levels. These processes led to the development of the convoluted folds, which locally evolved into pillow and ball structures, pseudonodules, and chaotic structures during phases of maximum deformation. The subsidence of the layers was accompanied by lateral sediment migration and fluid escape, generating intrusive structures and massive levels. The presence of faults and fractures affecting poorly consolidated sediments attests to brittle syn-sedimentary deformation that occurred under at least three regimes: (i) the early extensional tectonic regime marked by SE-verging normal faults; (ii) compressional regime represented by the reactivation of normal faults as thrust faults; (iii) the final tangential tectonic regime marked by NE-verging thrust faults. The spatial and stratigraphic association of soft-sediment deformation structures, their lateral continuity, vertical recurrence, and similarity to other structures indicate seismic triggering, that is, it assumes formation of seismites linked to repeated episodes of paleo-seismicity characterized by depth of <20 km, proximity to the seismic source 10 to 20 km, and probable magnitude 5 ≤ M ≤ 7.2.