downlap

Downlap is a stratigraphic term used in sedimentology and sequence stratigraphy to describe a condition where a depositional unit terminates downward, or down-dip, against an inclined surface. In practice, successive beds within the unit thin and end on or against the underlying surface as you move in the down-dip direction, creating a downlap surface at the base of the overlying package. The feature marks the lower limit of the overlying strata and is commonly interpreted as evidence of progradation and changing accommodation space or sediment supply during deposition.

Downlap surfaces are typically identified in outcrops, cores, and seismic data. In outcrops, they appear as a coherent plane along which the tops of beds truncate downward toward the basin or slope. In seismic profiles, downlap is seen when continuous reflectors terminate downward against an inclined surface, producing a wedge-like or clinoform geometry. Recognizing downlap helps geologists reconstruct depositional environments and the direction of sediment advance within a sequence.

Relation to other stratigraphic terms: downlap is distinct from onlap, where beds progressively overlie earlier surfaces in the updip direction, and toplap or offlap, which describe different termination patterns of beds along their edges. Downlap is most commonly associated with progradational or basinward-depositing sequences, where the reach of deposition thins and ends toward the distal end.