ORIGIN OF SEQUENCES AND SEQUENCE BOUNDARIES: THE ATLANTIC MARGIN AS A STRATIGRAPHIC FRONTIER

Unconformity-related sequences are the building blocks of many sedimentary successions because their geometrically defined boundaries represent the principal discontinuities in sedimentation. Sequence boundaries are surfaces of subaerial degradation associated with such phenomena as valley incision, karst development, abrupt upward shoaling of marine facies, marine bypass and degradation and, in some cases, a reorganization of sediment dispersal patterns ("lowstand" sedimentation). They arise as a result of eustatic change, tectonically induced deformation, sedimentary and glacial loading of the crust, and climatically mediated changes in the equilibrium profiles of river systems. While sequence boundaries are conventionally dated at the genetically correlative conformities of defining unconformities, abundant geological evidence shows that they evolve over protracted intervals of geological time during progradation and progressive burial. That process is thought to begin in some settings before the development of sharp-based shorefaces/deltas and resolvable offlap, stratigraphic features that are commonly attributed to "relative sea-level" lowering. Offlap arises through wave-induced marine bypass and erosion as well as via subaerial degradation (so-called "forced regression"). For eustatically modulated stratigraphy, this makes it possible in some cases for "highstand" sedimentation to continue until close to the low stand (two words) of sea level, and in others, for "lowstand" sedimentation to begin close to the high stand of sea level. If the stratigraphic response to eustatic change varies spatially in this way, sequence boundaries are inevitably diachronous at some level (hypothetically by as much as half a cycle), and they must trace laterally in different places into different correlative conformities. Such anticipated complexities in the development of stratal architecture on the face of it indicate a flaw in the rationale for sequence stratigraphy, and a challenge for seismic and borehole loop ties. Instead, what may be needed is flexibility in rule-based interpretation and to move beyond qualitative assertions about eustatic change. High-resolution studies at locations such as the Atlantic margin are aimed at an achievable frontier in stratigraphic research.