A preassessment of coexisted submerged beachrock and a fossil-laden near-shore deposit on the coast of Hasir Island, SW Marmara Sea, is presented based on depositional characteristics, two-dimensional electrical resistivity tomography (ERT) survey and accelerator mass spectrometry (AMS) radiocarbon ages. ERT-derived geophysical images clearly showed the subsurface position and contact relationship of submerged beachrock under the studied beach. Textural features of beachrock are typical solely of marine-phreatic cementation, including consecutively developed cement fabrics, i.e. micrite coatings, radial aggregates consisting of scalenohedral high-Mg calcites, and reticulated needles of aragonite. The intertidal cementation of beachrock took place between 2940 and 2470 YBP when the level of the Marmara Sea was about 1.5 m lower than that of the present. Its purely submerged nature is likely concerned with rise in sea level in pursuit of the cementation period. Though dated between 2340 and 1590 YBP, the fossil-rich near-shore deposit behind the studied beach could be of a tsunamigenic origin based on its sequence characteristics typical of such a high-energy event.