What does the presence of sustained net progradation of the shoreline imply about the ancient water body?

Answer

The ocean persisted for tens of millions of years

Sustaining a net outward building (progradation) of the shoreline through sediment influx from distant rivers suggests the large water reservoir was stable and persistent, implying an existence spanning tens of millions of years.

What does the presence of sustained net progradation of the shoreline imply about the ancient water body?
ancientMarsGeologyevidenceshoreline