Late Pleistocene-Holocene Sedimentation and Sea Level Changes in the Gulf of Kuşadası: Evidence from High-Resolution Seismic and Sediment Core Data

The Late Quaternary sedimentary succession and sea-level changes in the Gulf of Kuşadası, located in the Aegean Sea, have been comprehensively examined using high-resolution seismic reflection profiles and sediment cores collected by R/V TUBITAK Marmara in 2022. The seismic stratigraphy reveals four main depositional units, each bounded by distinct reflection surfaces that reflect significant sea-level fluctuations since the Last Glacial Maximum (LGM). Correlation of the seismic profiles with the ¹⁴C-dated sediment cores provides the robust chronology of seismic stratigraphic units, seismic boundaries, paleo wave-abraded platforms, and marine terraces. The depths of the paleoshorelines observed in the seismic profiles were compared with the global sea-level curve to more accurately determine the timing of sea-level changes in the gulf. The deepest wave-abraded platform observed in the seismic profiles is at a depth of -172 m. According to chronology of the depositional units in the seismics adjusted with ¹⁴C-datings from the cores, the deepest wave-abraded platform at -172 m in the seismic profile conforms with the sea-level lowstand (-135 m) at ca. 21.5 cal ka BP during the LGM based on the global sea-level curve. Such comparison reveals the subsidence of the submerged seafloor due to vertical displacement along active normal faults in the Gulf of Kuşadası since the LGM. Subsequent sea-level rise triggered by post-glacial warming led to the deposition of transgressive units characterized by coastal onlaps and localized channel fills. Brief sea-level stillstands disrupted this transgressive phase at approximately 17 cal ka BP and 14.6 cal ka BP, forming younger wave-abraded platforms at -135 m and -112.5 m, respectively. The depths of these platforms, compared with the global sea-level curve, suggest ongoing subsidence at a slower rate, indicating a complex interplay between sea-level changes and tectonic activity in the Gulf. The subsidence is likely attributed to tectonic movements along the seafloor rather than hydrostatic loading.

The acoustic reflection characteristics, together with the geometry and spatial extents of the seismic stratigraphic units, also provide important insights into the depositional processes during the changing sea-level. The most prominent depositional facies can be presented in the seismic profiles as two amalgamated deltaic sequences of the paleo-Küçük Menderes River. Their depositional periods can be confidently deduced from the correlation of the seismic stratigraphic units with the chronostratigraphic units in the cores. The topset/foreset transitions of these deltaic sequences, located at depths of -37.5 m and -112.5 m in the seismic profiles, correspond to estimated ages of 9.3 cal ka BP and 14.6 cal ka BP, respectively.