DO STRATIGRAPHIC SEQUENCES EXIST IN DEEP-WATER SETTINGS? DOES IT MATTER? (Invited Presentation)

Stratigraphic discontinuities arise for a host of reasons, and at a great range of length scales and timescales. Some of the most prominent and laterally persistent surfaces are unconformities associated with subaerial degradation, and they are the features that Vail et al. (1984, AAPG Memoir 36, 129-144) designated as sequence boundaries. At updip locations, where evidence is typically best developed, characteristics include angular discordance between bedding in superposed sediments or rocks, or with respect to an intervening surface; valley incision; evidence for the subaerial exposure of marine sediments; abrupt upward shoaling of depositional facies; and biostratigraphic, chemostratigraphic or other evidence for a break in sedimentation. In some cases, sequence boundaries pass laterally into generally concordant and more or less conformable deep-marine deposits. In other cases, a change from shelf progradation to the delivery of sediment preferentially through canyon systems results in a mappable change in 3D stratal geometry (from offlap to onlap), with concomitant upward coarsening of deep-water facies. However, the reciprocal sedimentation model, wherein deposition occurs primarily in either non-marine to shelf settings or in deep water, but not simultaneously in both, has been over-emphasized. Stratigraphic discontinuities develop independently in slope to basinal settings through mass wasting, avulsion, erosion by contour currents, dissolution, etc. – phenomena that commonly have little to do with those controlling nearshore sedimentation. That doesn’t matter if the objective is to interpret facies in the context of stratal geometry – the essence of sequence stratigraphy, even if standard sequence stratigraphic terminology is inappropriate. Off-shelf discontinuities and the intervals they bound may nonetheless relate only loosely if at all to sea-level change. Fudging deep-water stratigraphy to match perceived global sea-level cycles is therefore unlikely to be useful. Examples for this presentation will be drawn from diverse marine seismic and outcrop data.