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The AAPG/Datapages Combined Publications Database

CSPG Special Publications

Abstract


Shelf Sands and Sandstones — Memoir 11, 1986
Pages 339-339
Symposium Abstracts: Storm-Dominated Shelves

Origin of Hummocky Cross-Stratification: Part 2. Paleohydraulic Analysis Indicates Formation by Orbital Ripples Within the Wave-Formed Flat-Bed Field: Abstract

William L. Duke1, Dale A. Leckie2

Abstract

Numerous studies of hummocky cross-stratification (HCS) suggest an origin due to storm-generated surface gravity waves. On the shelf, storm waves generate various combinations of unidirectional, oscillatory, or multidirectional flow components; the relative importance of these in the formation of HCS is unknown. In part 1 of this talk (Leckie and Duke), geological evidence is presented indicating that many ancient examples of HCS, which occurred in very fine to fine sandstone, formed under identical hydrodynamic conditions to straight-crested symmetrical gravel dunes. We can precisely determine generative conditions for the gravel dunes, and thus also for the HCS. The gravel dunes are traditional orbital ripples formed under oscillatory-dominant flow; their spacing (λG) is related to bottom orbital diameter (do) by λG = 0.65 do. Knowing grain size, grain density, and fluid density, we can determine the ratio of maximum orbital speed (Um) to period (T) from empirical studies. On a graph of Um vs. T, isograds of do and Um/T have opposite slopes; using this nomogram, the intersection of the appropriate isograds will alone determine generative values of Um and T for the gravel dunes without making prior assumptions about wave parameters or water depth. This method has been successfully tested with recent occurrences for which depth and wave parameters are known. For the ancient gravel dunes, Um = 0.8 to 1.3 m/s and T = 5.3 to 6.4 s. These calculations are for incipient motion; actual Um values may have been slightly larger, with slightly smaller T values. These conditions fall within the wave-formed flat-bed field for very fine to fine sand. Form wavelength of HCS (λH) associated with these gravel dunes was measured; the ratio λGH = 0.65; thus, λH = do. These conclusions are consistent with an independent paleohydraulic analysis of HCS from an ancient lake (Duke, 1984). Thus, independent lines of evidence indicate that HCS is formed under oscillatory-dominant flow within the flat-bed field by low-amplitude, three-dimensional orbital ripples possessing a characteristic spacing approximately equal to do.


 

Acknowledgments and Associated Footnotes

1 Department of Geology, McMaster University, Hamilton, Ontario, Canada L8S 4M1; present address: Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A.

2 Petro-Canada, Calgary, Alberta, Canada T2P 3E3; Institute of Sedimentary and Petroleum Geology, 3303 - 33rd Street N.W., Calgary, Alberta, Canada T2L 2A7

Copyright © 2008 by the Canadian Society of Petroleum Geologists