Sequence Stratigraphic Significance of Continental Shelf Sand Ridges (Gulf of Mexico, East China Sea, North Sea)

Dag Nummedal: University of Wyoming; and John Suter: ExxonMobil

Abstract

The origin of isolated shallow marine sand bodies remains controversial. The SEPM research conference in Wyoming in 1995 has helped to identify many of the issues, but hardly has resolved the conflicts surrounding such well-studied stratigraphic units as the Cretaceous Shannon Sandstone (Suter and Clifton, 1999). The lowstand to transgressive evolution of clastic shelves, therefore, is clearly inadequately understood, in spite of a number of excellent case studies of shelves ranging from the East China Sea, through the Gulf of Mexico to the North Sea and many places between.

It is undoubtedly true that estuarine parts of incised valley fills, erosional remnants, lowstand deltas, and other systems may form isolated marine sand bodies. It is also true, however, that modern clastic shelves contain an abundance of transgressive shelf sand ridges (Snedden et al., 1994). So why are there so few (any?) convincingly documented ancient analogs? One of us has interpreted the upper part of the Cretaceous Tocito Sandstone in the San Juan basin as a transgressive, tide-dominated shelf sand rigde (Nummedal and Riley, 1999), but this interpretation is certainly not universally accepted.

We rarely encounter subsurface reservoir sand bodies interpreted as shelf sand ridges. This state of affairs reflects a number of factors, including inadequate sampling of Quaternary sand ridges. They are abundant on today's mostly transgressive shelves around the world, but what is their preservation potential during subsequent sea level falls? Secondly, do we have unambiguous criteria to differentiate inshore versus offshore deposits? Thirdly, what are the scales of transgressive sand bodies, and what are their diagnostic features at different scales? What is their expression in seismic, core and outcrop?

References

Nummedal, D., and G.W. Riley, 1999, The origin of the Tocito Sandstone and its sequence stratigraphic lessons: in, Isolated Shallow Marine Sand Bodies: Sequence Stratigraphic Analysis and Sedimentologic Interpretation. SEPM Special Publication 64, pp. 227-254.

Snedden, J.W., R.D. Kreisa, R.W. Tillman, S.J. Culver and W.J. Schweller, 1999, An expanded model for modern shelf sand ridge genesis and evolution on the New Jersey Atlantic shelf: in, Isolated Shallow Marine Sand Bodies: Sequence Stratigraphic Analysis and Sedimentologic Interpretation. SEPM Special Publication 64, pp. 147-163.

Suter, J.R. and H.E. Clifton, 1999, The Shannon Sandstone and isolated linear sand bodies: interpretations and realizations; in, Isolated Shallow Marine Sand Bodies: Sequence Stratigraphic Analysis and Sedimentologic Interpretation. SEPM Special Publication 64, pp. 321-356.