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The AAPG/Datapages Combined Publications Database

CSPG Special Publications

Abstract


Shelf Sands and Sandstones — Memoir 11, 1986
Pages 332-332
Symposium Abstracts: Sediment Source, Supply and Dispersal

On the Dynamics of Continental Shelf Storm Flows: Abstract

Charles E. Adams Jr.1

Abstract

Storm layer deposits are important sedimentary components formed on continental shelves by waves and semipermanent wind-driven currents. They are prominent in the rock record as well. The structural and textural characteristics of deposits are a function principally of the dynamic conditions existing in the benthic boundary layer both during and after the period of full flow development. Studies of steady-state benthic boundary layer dynamics can provide a better understanding of ancient shelf deposits and paleoceanography. Ekman layers are increasingly recognized as diagnostic features of turbulent benthic boundary layer flows on the shelf, developing during the waning stages of storm activity. Variations in magnitude and direction of the velocity and shear stress vectors with height above the bottom are distinctive; they may or may not be colinear. In an unstratified Ekman layer, the total veering angle or the angular difference between the direction of the bottom shear stress vector, τo, and the current, U, above the layer is given in sin2 αo = 200/ρ|U2|). Nominal values of boundary layer parameters yield typical veering angles of 10° to 20° in an anticlockwise sense looking downward. With shear stress decreasing upward, a flow carries a suspended load of increasingly larger size grades as the bottom is approached. The result is a selective sorting of the sediment by size away from the source. Structures and textures imparted by the sorting process may be detectable in storm deposits. In a sediment-laden flow, suspended sediment induced stratification effects lead to an increase in the angle of veering with a decrease in the effective bottom stress. Scale analyses suggest a limiting value of αo = 90°; sediment-induced reductions of τo greater than 50% have been noted. Information from a numerical model is compared with observations from a shallow, depth-limited Ekman layer. The results indicate the importance of geological observations to an understanding of boundary layer dynamics and to the interpretation of them with respect to sediment distribution. A field study of storm layer deposition dynamics using a new boundary layer profiling instrumentation system is outlined.


 

Acknowledgments and Associated Footnotes

1 Coastal Studies Institute, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A.

Copyright © 2008 by the Canadian Society of Petroleum Geologists