Topset beds are horizontal layers of sediment deposited on top of a delta, often representing the most recent phase of deltaic deposition. These beds are significant in sedimentology for understanding the dynamics of delta formation, sediment transport, and the stratigraphic record of deltaic environments.
Reference: Reading, H. G., & Collinson, J. D. (1996). “Clastic Coasts.” Sedimentary Environments: Processes, Facies and Stratigraphy, 154-231.