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Symposium Abstracts: Storm-Dominated Shelves
HCS: Problems, Perspectives and Misunderstandings: Abstract
Hummocky cross-stratification (HCS) has become generally accepted as a structure reflecting sedimentation of coarse silt to medium sand entrained by storm-wave-induced currents, giving evidence of deposition between storm and fair weather wave bases. Dott and Bourgeois have proposed an idealized HFXM sequence thought to represent a waning flow sequence deposited by storm-waves that is succeeded by fair weather sediments. However, wave-rippled tops are often absent in HCS units from deltaic sequences and the X-zone is frequently composed of unidirectional current ripples. This suggests that oscillatory currents play, at best, a relatively minor role in the genesis of HCS. The absence of wave-rippled tops to HCS in deltaic sequences suggests that river flood processes were responsible for HCS deposition. It is possible that wave ripplied tops to HCS units in shoreface sequences may occur merely because the HCS was deposited there by storm-surge/wind-forced currents. Evidence will be presented showing that “hummocky” bed forms may be subdivided into three lamination types: basal laminae overstepping one another laterally at a very low angle across a flat basal surface (lateral accrctionary type); progressive thickening of laminae over hummock crests (pinch-and-swell type); and the mantling of a differentially scoured hummocky surface (scour-and-drape type). Walker and Leckie have proposed the term swaley cross-stratification (SCS) for shallow scours within amalgamated sandstone bodies containing only rare convex-upward surfaces. However, in practice, the distinction between SCS and amalgamated scour-and-drape type HCS is rather arbitrary. Furthermore, SCS conforms to descriptions of wave-produced scours produced during fair-weather periods in the high-energy, non-barred nearshore environment and may possibly be of a similar origin. In contrast, lateral accretionary and pinch-and-swell type HCS imply the existence of a hydro-dynamic pressure gradient during deposition and may possibly be of a different origin to scour-and-drape type HCS. In conclusion, it is proposed that SCS is not adopted as a sedimentological term. HCS appears to be a ‘bucket-term’ for a series of lamination styles, which may be polygenetic in origin and not necessarily related to storm-waves. Thus, the adoption of the Dott and Bourgeois conceptual sequence for HCS units appears to be premature.
Acknowledgments and Associated Footnotes
1 Department of Geology, University College, P. O. Box 78, Cardiff CF1 1XL, U.K.
Copyright © 2008 by the Canadian Society of Petroleum Geologists