The frontal facies and sedimentation processes of a shoal-water fan delta in the Köprü Basin of southern Türkiye

This paper focuses on the delta front deposits of well-exposed, progradational Serravallian fan delta complexes in the Köprü Basin, southern Türkiye. These shoal-water fan deltas were lobate, moderate to large (6–20 km in radius), built by fluvial processes and characterised by a high sediment supply, high nearshore wave energy and minimal tidal influence. The alluvial facies include channel-floor gravel lags, transverse/oblique and longitudinal gravel bars, local sandy cappings of inactive bars and abandoned channels, and minor gravelly deposits of hyperconcentrated sheetfloods. The alluvium in the middle to lower fan reaches bears little evidence of marine influence, except for isolated transgressive sheets of wave-worked sediment and occasional deep palaeochannels attributed to trunk river incision during forced regression episodes. The record of a short-term interplay of fluvial and marine processes is limited to the palaeoshoreline facies assemblages, including well-developed gravelly beaches and wave-modified mouth bars. The shoreface facies belt of wave-worked sandstones fringes both deltaic and interdeltaic shorelines but is generally wider and richer in gravel in the former case. The shoreface facies assemblage bears evidence of strong episodic seaward currents, often highly oblique to the shoreline, attributed to storm-generated rip currents and the compensation currents of storm and tsunami events. The well-sorted, shell-bearing shoreface sand was spread by storms into a wide offshore-transition zone and deposited as tempestites intercalated with hemipelagic mud. The seafloor had a significant inclination, and the tempestites were deposited by unidirectional compensation currents combined with waves and boosted by the gravity effect of high sediment concentration. Sporadic emplacement of thick gravelly mass flows in the offshore-transition zone is attributed to seismic tsunami events. The seaward record of fluvial floods includes progradational mouth-bar wedges formed by frictional stream effluent; nearshore mounds of beach-derived gravel, emplaced as cohesionless debris flows by the outbreaks of wave-dammed streams; and solitary turbiditic palaeochannels formed by a hyperpycnal effluent during forced regressions. The submarine channels were formed in only one fan deltas, with high discharges and somewhat steeper seafloor gradients, suggesting a morphodynamic threshold for channelised underflow. In the other fan deltas, mouth bars prograded by default in response to a sea-level fall, incised by streams and curtailed by marine processes. The study concurs with the prevailing opinion that the fronts of shoal-water fan deltas are tempestite- rather than turbidite-prone and points further to a heavy reworking of shoreface by storms, to the impact of sea-level fall on the pattern of delta front sedimentation, and to the occurrence of tsunami events in tectonically active basins.