The sediments are present at the bottom of the ocean. They break due to erosion and weathering. These are transported to the beach part by the action of water, wind and gravity. The sea water level rises that inundates the beach covering it with the new clay over the old beach material. This sea rise could be due to multiple reasons ,such as tectonic movements. This process of deposition of the clay on the beach is known as transgression.Hence, the answer is 'transgression'.
Transgression are the new clay sediment deposited on top of old beach material. Hope this helps!!