Sequence Boundary. The sequence boundary is an unconformity updip and a correlative conformity downdip. Where it is an unconformity, it is a surface of subaerial exposure and erosion; however, the expression of those features in an individual outcrop may or may not be obvious.he transgressive surface represents the first major flooding surface to follow the sequence boundary and is usually distinct from the relatively minor flooding surfaces that separate parasequences in the lowstand systems tract.