Terminations: Base of a Sequence Older Beds Older Beds SLIDE 17 Vail & Mitchum gave us a set of terms to use when marking seismic unconformities There are two types of terminations we might observe at the base of a depositional package, a sequence If the reflections that terminate are at a lower dip than the surface against which they terminate, then we call these onlap terminations If the reflections that terminate are at a higher dip than the surface against which they terminate, then we call these downlap terminations NOTE: These definitions are based on observed geometric relationships, not on inferred depositional processes Onlap Downlap A base-discordant relationship in which younger strata terminate progressively against a surface of greater inclination A base-discordant relationship in which younger inclined strata terminate progressively against a surface of lower inclination NOTE: These definitions are based on observed geometric relationships, not on inferred depositional processes Courtesy of ExxonMobil L 11 – Stratigraphic Analysis