A depositional sequence is a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities. This concept of a "sequence" is modified from Sloss. A depositional sequence is determined by a single objective criterion, the physical relations of the strata themselves. The combination of objective determination of sequence boundaries and the systematic patterns of deposition of the genetically related strata within sequences makes the sequence concept a fundamental and extremely practical basis for the interpretation of stratigraphy and depositional facies. Because distribution and facies of many sequences are controlled by global changes of sea level, sequenc s also provide an ideal basis for establishing comprehensive stratigraphic frameworks on regional or global scales.

A depositional sequence is chronostratigraphically significant because it was deposited during a given interval of geologic time limited by ages of the sequence boundaries where the boundaries are conformities; however the age range of the strata within the sequence may differ from place to place where the boundaries are unconformities. The hiatus along the unconformable part of a sequence boundary generally is variable in duration. The hiatus along the conformable part is not measureable, and the surface is practically synchronous. Stratal surfaces within a sequence are essentially synchronous in terms of geologic time.

Depositional sequences may range in thickness from hundreds of meters to a few centimeters. Sequences of different magnitudes may be recognized on seismic sections, well-log sections, and surface outcrops.

To define and correlate a depositional sequence accurately, the sequence boundaries must be defined and traced precisely. Usually the boundaries are defined at unconformities and traced to their correlative conformities. Discordance of strata is the main criterion used in the determination of sequence boundaries, and the type of discordant relation is the best indicator of whether an unconformity results from erosion or nondeposition. Onlap, downlap, and toplap indicate nondepositional hiatuses; truncation indicates an erosional hiatus unless the truncation is a result of structural disruption.

Examples of depositional sequences are presented on well-log and seismic sections. Both examples depend primarily on correlation of physical stratigraphic surfaces for identification of the unconformities bounding the sequences, and on biostratigraphic zonation for determination of the geologic ages of the sequences.