Abstract: Relative Sea-Level Change and the Origin of Sequence Boundaries in Tectonically Active Settings

Nicholas Christie-Blick, Neal W. Driscoll

The conventional interpretation of sequence boundaries is that they are due to a "relative fall" in sea level, usually at some specified break in depositional slope. This idea is appealing because it appears to account for both subsidence and eustasy. In the conventional view, the magnitude of relative sea-level change may vary spatially and temporally, reflecting lateral variations in the assumed long-term subsidence rate. However, it is assumed that an increase in the rate of fall of sea level affects all points by either increasing the rate of the relative sea-level fall or decreasing the rate of the relative sea-level rise. In tectonically active settings associated with episodic deformation and tilting, this is not necessarily the case. Instead, different portions of a basin may imultaneously experience either an increase or a decrease in the rate of relative sea-level fall. Consequently, in both extensional and foreland basins, sequence boundaries may pass laterally from local subaerial unconformities into marine onlap surfaces.

Examples of this phenomenon are drawn from Jeanne d'Arc basin, a half-graben of Mesozoic age beneath the continental margin of eastern Canada, and the Cretaceous interior seaway, a foreland basin in western North America. In Jeanne d'Arc basin, the early Aptian rift-onset unconformity is characterized not only by prominent onlap but also by deepening rather than shoaling of the sedimentary facies in depocenters. At the same time, the unloaded footwall of the basin-bounding fault system experienced erosion and bypass. Similarly, in the Cretaceous interior seaway, times of crustal shortening and orogenic loading are marked by simultaneous deepening in depocenters and uplift and erosion along the orogenic margin and peripheral bulge. Although the differences between extensional and forel nd basins are normally emphasized, with respect to the origin and characteristics of sequence boundaries we think that they have much in common. Recognition of this phenomenon influences expectations about the sedimentary facies that might be associated with sequence boundaries interpreted from seismic reflection profiles.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994