Onlap or ''overlap'' is the
geological phenomenon of successively wedge-shaped younger rock
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
extending progressively further across an
erosion surface cut in older rocks. It is generally associated with a
marine transgression. It is a more general term than ''
overstep'', in which the younger beds overlap onto successively older beds.
The opposite is offlap, in which each younger rock bed
pinches out short of the full extent of the underlying older bed, typically due to a
marine regression
A marine regression is a geological process occurring when areas of submerged seafloor are exposed above the sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously-exposed land.
Evidence of marine ...
.
References
Stratigraphy
{{Geology-stub
