A diachronous sediment layer or stratigraphic surface is a sedimentary layer and/or depositional or erosional surface that is time transgressive

An example of this includes the erosional ravinement surface that parallels the migration of the shoreface across previously deposited coastal deposits during the transgressive movement of the landward margin of the Lowstand Systems Tract with a subaerial landward margin and/or the Transgressive Systems Tract. The attached movie in the vertical menu bar shows how this erosional surface transgresses time, or is diachronous, wherever the landward edge of the sea rises over an underlying sedimentary surface.

If the transgressive surface is equated to the ravinement surfaces in outcrop, then it should be recognized that these are diachronous too!

Further, the attached movie shows how the erosion associated with the Early Lowstand Systems Tract forced regression is time transgressive or diachronous. Thus if in outcrop this erosion is ascribed to a Sequence Boundary unconformity then this too is time transgressive or diachronous.

The animated Gifs are provided below if you are unable to operate the Quick Time movie in the left vertical menu bar.