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Department of Geology, Royal School of Mines, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK
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 (Mitchum et al. 1977). The idea that sequence boundaries are so areally extensive is closely connected to the ideology favoured 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. This paper examines the stratigraphic conditions necessary for the development of regionally extensive and discrete surfaces of onlap, downlap and toplap. The main conclusions 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 sequence boundaries probably owes more to the limited resolution of seismic data and the map-driven need to correlate over large areas than to any physical continuity of a single surface.
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  N. H. Woodcock, A. J. Butler, J. R. Davies, and R. A. Waters Sequence stratigraphical analysis of late ordovician and early Silurian depositional systems in the Welsh Basin: a critical assessment Geological Society, London, Special Publications, 1996; 103: 197 - 208. [Abstract] [PDF]
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