softsediment

Soft sediment refers to unconsolidated sediment in which particles are not yet lithified into solid rock. It typically includes clays, silts, and fine sands, often saturated with water or under elevated pore-water pressure. In many settings, organic-rich deposits such as peat are also considered soft sediment while they remain unconsolidated. These materials form in low-energy environments where fine particles settle slowly from suspension, including lakes, river mouths, deltas, marshes, continental shelves, and deep-water basins.

The physical properties of soft sediment are characterized by relatively high porosity and low shear strength compared with consolidated rocks. They can deform readily under overburden pressure, tidal loading, groundwater withdrawal, or seismic shaking. Over time, diagenetic processes such as compaction and cementation reduce porosity and strengthen the material, eventually transforming soft sediments into lithified rocks like mudstone, siltstone, or sandstone. In the near-surface, chemical and biological activity can further alter composition and structure.

Soft-sediment deformation structures are common in shallow deposits. Features such as load casts, flame structures, and convolute bedding form when overlying loads or gas escape cause buckling, sinking, or mixing of the layers. Liquefaction, in which saturated clays lose strength during earthquakes, can trigger rapid ground failure in soft sediments.

Engineering and hazard considerations are significant for soft sediment. Foundations on soft ground may experience excessive settlement or instability, and pipelines or buried cables near weak layers are at risk during seismic events. Understanding soft sediment properties aids in site characterization, dredging, and ground-improvement design.

In geology, soft sediments record depositional environments and early diagenesis, and they may preserve critical clues about past climate, hydrology, and biological activity before lithification.