ABSTRACT: Sequence Boundaries--How Laterally Correlatable Are They?

CARTWRIGHT, JOSEPH, Royal School of Mines, Imperial College, London, U.K.

Sequence boundaries, as defined by the Exxon Group, are regionally extensive surfaces (basin-wide scales) that are characterized in part by stratal discontinuity surfaces in the form of onlap, downlap, and toplap. The idea that sequence boundaries are so areally extensive is closely connected to the ideology favored by the Exxon Group that sequence boundaries are eustatic in origin. This concept of sequence boundaries was initially based on seismic stratigraphic observations of stratal reflection geometry but has now been extended to rock stratigraphic relationships in general. One of the important issues in sequence stratigraphy is to decide whether concepts based on the low resolution seismic method are applicable at much higher orders of resolution, such as in outcrop.

The aim of this presentation is to examine the stratigraphic conditions necessary in order for regionally extensive and discrete surfaces of onlap, downlap, and toplap to develop. The main conclusions presented here are that in order for onlap, downlap, and toplap surfaces to develop as discrete surfaces there must be no contemporaneous sediment accumulation beyond the lap-out position. If this condition is not met, then discrete lap-out surfaces do not develop. Instead, closely related families of surfaces that individually are of limited areal extent are more likely to form.

It is argued that in most depositional environments some form of sedimentation is likely to occur beyond the lap-out position. This suggests that sequence boundaries should only develop over extremely limited areas and should not be expected to form regionally correlatable stratigraphic surfaces. The apparent regional extent of some sequence boundaries and their genetic linkage with eustatic changes is in conflict with predictions based on simple stratigraphic principles. The regional correlatability of any geological surface is more a product of the correlation methodology than physical reality. The concept of sequence boundaries as regional surfaces may thus owe more to the correlation approach adopted by the Exxon Group than to stratigraphic reality.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)