"the deposition of sediments," 1845; see sediment + -ation, ending used in forming nouns of action. The uncommon verb sediment is attested only from 1899 and could be a back-formation.
Entries linking to sedimentation
sediment n.
1540s, "matter which settles by gravity to the bottom of water or other liquid," from French sédiment (16c.) and directly from Latin sedimentum "a settling, sinking down, subsidence," from stem of sedere "to settle, sit" (from PIE root *sed- (1) "to sit"). Especially in geology after 1680s.