Onlap, offlap, and the origin of unconformity-bounded depositional sequences

Unconformity-bounded depositional sequences represent the fundamental building blocks of sedimentary successions.They are typically characterized by onlap ** at the base and by offlap at the top, and they tend to be markedly asymmetrical, with onlap accounting for a larger part of any cycle of sedimentation than offlap.This geometric asymmetry is responsible for the familiar sawtooth shape of the coastal onlap curve.However, the existence of offlap demonstrates that in general sequence boundaries develop over a finite interval of time, and not instantaneously.Offlap cannot be attributed solely to erosional truncation of originally sigrnoid clinoforms.In many cases this would require an unreasonable amount of erosion, as well as a marked increase in the sediment supply in order to account for the observed progradation.The horizontal segments of the coastal onlap curve are therefore artifacts, representing no more than the conventional ages of unconformities, and they do not indicate the manner in which sequence boundaries actually form.Depositional sequences are commonly associated with a cyclic arrangement of facies, but trans-* This is an extended abstract of an article originally submitted to this Special Issue on "The Record of Sea-Level Fluctuation".When the special issue was transferred fromMarine Geology to Sedimentary Geology, the author requested that the full version of the paper be published in a regular issue of Marine Geology (March 1991).** The terms onlap and offlap refer to the progressive up-dip termination of strata against an underlying and overlying surface, respectively.