An unconformable relationship in which a transgressive sequence (see transgression) of progressively younger members of an upper series of strata onlap and rest upon the underlying (oldest) stratum. Strictly, ‘onlap’ refers to the process and ‘overlap’ to the resulting structural relationship, but the two terms are often used synonymously. The opposite of overlap is ‘offlap’. See also overstep.
Subjects: Earth Sciences and Geography.