Uniform Mud (Unifite) Deposition in the Hellenic Trench, Eastern Mediterranean

Abstract

Unifites are nearly structureless, often thick, layers of clayey silt and silty clay that appear compositionally homogeneous and generally show a subtle fining-upward trend. Formed by uniform and faintly laminated muds, unifites are deposited from rapidly emplaced single gravity-flow events. Along the Hellenic Arc, unifites are restricted to small trench basins and interpreted as an end-member gravity-emplaced facies. Unifites are not truly homogeneous and the petrological distinctions observed are closely related with the trench basin depositional site relative to steep margins bounding the trench plain. The faintly laminated portions of unifites contain a higher silt content; the uniform mud portions are slightly better sorted and display an upward increase of planktonic tests. The sand fraction is dominated by clastic aggregates eroded from older margin sediments; unifites also comprise a large silt-size nannofossil content (including reworked forms).The increased uniformity basinward of unifites records deposition from turbidity current-related flows of diminished concentration that spread over large areas of a flat trench floor. Faint laminae may be related to phases of flocculation and depositional sorting of the sediment load during transport, and to the hydraulic jump affecting a flow upon its arrival on a near-flat basin floor. The slower-moving tail releases the uppermost nonlaminated, graded unifite mud term. The thickness of Hellenic unifites is a function of entrapment of moderate amounts of material in small trench plains. The homogenization process essential for unifite deposition involves relief bypass, i.e., the preferential entrapment of coarser or denser fractions in slope depressions, while finer or less dense particles are transported further downslope across irregular seafloor features. Unifite deposition records the interplay of: (1) complexity of dispersal paths and accessibility of sediment to the trench basin, (2) redepositional processes, grain-support mechanisms and gravity-induced flow characteristics, (3) type of material transported, (4) extent of textural segregation and compositional sorting during flow, (5) slope relief bypassing process, and (6) selective entrapment of essentially fine-grained particles in the more distal trench catchment basins. Mediterranean unifites can serve to interpret uniform mud facies on both active and passive margins and in the rock record.