coastal onlap The deposition and onlap of coastal non-marine and littoral deposits further and further inland, due to a relative rise in sea level. If, subsequently, relative sea level falls, the base level is lowered and erosion probably occurs at the top of the sequence. At the next relative sea level rise, coastal onlap will recommence but from a lower level. This downward shift in coastal onlap is thus an indication that there has been a relative fall in sea level. See also SEISMIC STRATIGRAPHY.

coastal onlap The deposition and advance of coastal non-marine and littoral deposits further and further inland, owing to a relative rise in sea level. If, subsequently, relative sea level falls, the base level is lowered and erosion probably occurs at the top of the sequence. At the next relative sea-level rise, coastal onlap will recommence but from a lower level. This downward shift in coastal onlap is thus an indication that there has been a relative fall in sea level.