In the Western Anatolian region, NE-SW, E-W directional basins were developed which were limited to the extension-related faults beginning in the late Oligocene to early Miocene. The fi llings of these basins consist of fl uvial-lacustrine deposits containing volcanic and volcaniclastic intercalations. These deposits include intensive local unconformities and soft sediment deformation structures. The fi lling of the Bigadiç Neogene Basin which is one of these basins, constitute base limestone unit, lower tuff unit, lower borate unit, upper tuff unit and upper borate unit. The base limestone unit composed of claystone, marl, limestone, dolomitic limestone facies was deposited in the deep lacustrine environment. The soft sediment deformation structures were defi ned in the base limestone unit, which outcroped in the south of Bigadiç. These are: slumps, rock falls, chaotic structures, clastic dykes, synsedimentary faults and breccia limestone. Deformation mechanisms are related essentially to the increase of slopes of layers, liquidization and fl uidization. In the study area; regional tectonics, sedimentological data, and deformation structures are evaluated together, it is concluded that these structures are formed by tectonic and seismic (earthquakes related to tectonic origin and syndepositional magmatic activities). Bulletin of the Mineral Research and Exploration http://bulletin.mta.gov.tr