Onlap

Onlap is a term used in sedimentary geology and stratigraphy to describe a depositional contact in which progressively younger strata overlie an older surface by lapping onto it. The edge of each successive bed reaches farther across the surface, creating a pattern in which the succession thickens toward the basin and thins toward the source. Onlap commonly forms during transgressive sequences, when relative sea level rises and shoreline position shifts landward, allowing offshore deposits to overlie and progressively cover the underlying topography.

In practice, onlap is recognized in outcrops and seismic profiles by wedges of sediment that wedge upward and toward the basin, with beds that appear to “lap” onto an irregular surface. It indicates a history of relative rise in sea level or subsidence that permits successive depositional episodes to overlie the exposed surface.

Relation to other stratigraphic terms: onlap is one pattern used to describe how beds terminate or contact one another. It is contrasted with toplap, offlap, and downlap, which describe different ways in which successive beds terminate along surfaces or clinoforms and reflect different combinations of sea-level change and sediment supply. Together, these terms help engineers and geologists interpret sequence architecture, facies changes, and past environmental conditions.

Applications: onlap is widely used in sequence stratigraphy and basin analysis, including petroleum geology and academic research, to reconstruct relative sea-level curves, sediment transport, and the geological history of a basin.