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depositional sequence A discrete succession of strata, deposited more or less continuously, and bounded at top and bottom by either an unconformity surface or the equivalent, correlative disconformity or conformity surfaces (see CONFORMABLE). The ancient stratal surfaces can be traced laterally by seismic reflection profiling. A depositional sequence is the basic, operational, stratigraphic unit in seismic stratigraphy. Where top and bottom boundaries can be traced to conformity surfaces a depositional sequence becomes of chronostratigraphic significance, as it represents a specific interval of geologic time during which the particular unit was deposited. See BASELAP; DOWNLAP; ONLAP; and TOPLAP.