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onlap The progressive spreading of successive beds of sediment over an increasingly wider area, usually as a result of rising sea level or the gradual subsidence of a land area. Onlap results in an overlapping (see OVERLAP) stratigraphical relationship between the sedimentary succession and the underlying basement rocks. The upper, younger beds ‘pinch out’ updip against the older, inclined surface on which they rest. Compare OFFLAP. See also BASELAP; COASTAL ONLAP; DOWNLAP; and TOPLAP.

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