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The AAPG/Datapages Combined Publications Database

CSPG Special Publications

Abstract


Shelf Sands and Sandstones — Memoir 11, 1986
Pages 338-338
Symposium Abstracts: Storm-Dominated Shelves

Paleohydraulic Significance of Hummocky Stratification: Abstract

Joanne Bourgeois1, J. Dungan Smith2

Abstract

The distinctive characteristics of hummocky stratification are: 1. erosional lower bounding surfaces (of laminasets) with typical slope angles of less than 10°; 2. laminae parallel or subparallel to the lower bounding surface and scattered dip directions of laminae; 3. laminae thickening into swales; and 4. irregularly spaced swales and intervening hummocks at intervals of one to several metres with a relief mostly of 5 to 30 cm. Commonly, hummocky-laminated beds alternate with bioturbated strata. Hummocky stratification is usually found in fine-grained sandstone located stiatigraphically between offshore and shoreface deposits, and it has been linked genetically to storm-wave processes. There remain major questions concerning the paleohydraulic significance of hummocky stratification, particularly whether it is produced by random scour and drape, or by regular bed-form development. Hummocky stratification is not a ripple-scale phenomenon (see 4, above); ripples, controlled by the mean saltation length or by the orbital diameter of the wave, if this is smaller, are 5 to 20 cm in wavelength in fine sand. Larger bed forms, controlled by orbital diameter alone, could exist, but the time necessary to build a dune of the dimensions cited above is apparently greater than one wave period. Most critically, there is no microstratigraphic evidence for regular migration of bed forms, nor for regular scaling in the geometry of the laminae. We propose that hummocky stratification is a scour-and-drape phenomenon with the following controls: wave conditions must be sufficient to produce properly scaled scours (swales); waves must be breaking nearby in order to produce the large fluxes of suspended sediment that supply the drapes, and there must be a local convergence in the sediment-transport field so that deposition will occur. These conditions are met seaward of the offshore bar during severe storms. Detailed stratigraphic analyses employed in conjunction with models for flow processes in this environment can lead to estimates of paleohydraulic conditions.


 

Acknowledgments and Associated Footnotes

1 Department of Geological Sciences, University of Washington, Seattle, Washington 98195, U.S.A.

2 Department of Geological Sciences, University of Washington, Seattle, Washington 98195, U.S.A.

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