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CSPG Special Publications

Abstract


Shelf Sands and Sandstones — Memoir 11, 1986
Pages 344-344
Symposium Abstracts: Storm-Dominated Shelves

Wave-Dominated Shelves — A Geological Point of View: Abstract

Roger G. Walker1

Abstract

The geologist identifies two main problems of wave-(storm) dominated shelf deposition — what are the characteristic associations of sedimentary structures (i.e., facies), and how was the sand emplaced in the first place? The structure which has received the most attention recently is hummocky cross-stratification (HCS), and there is now fair agreement that it is formed by storm waves, possibly in combination with unidirectional flows. To preserve abundant HCS requires deposition below fair weather wave base. In one of the commonest HCS facies, namely alternations of bioturbated mudstone with sharp-based HCS sandstone 5 to 100 cm thick, angle-of-repose, medium-scale, cross-bedding is exceedingly rare to absent. Counts of the number of sharp-based sandstones within sections whose absolute ages can be estimated, suggest sandstone recurrence intervals of between 400 to 15 000 years (average of 5 studies is about 5000 years). The geologist therefore recognizes the possibility that these HCS sandstones represent events that marine geologists and oceanographers may never have observed or measured. At least three studies (Fernie-Kootenay transition, Cardium at Ricimus, Wapiabi-Chungo transition) have demonstrated the intimate association of sharp-based HCS sandstones with classical turbidites, with evidence that emplacing flows for both of these facies used the same paleoslope. The implication is that turbidity currents may have been important in emplacing HCS sandstones into the basin, with some paleoflow evidence indicating dispersal down the paleoslope and perpendicular to isobaths. Marine geologists, on the other hand, have emphasized that wind-forced currents and storm-surge ebb currents are deflected by Coriolis forces and mostly flow parallel to isobaths as geostrophic flows. Thus sand is dispersed mostly parallel to the shore, incrementally, over a long period of time. There is little positive evidence in the geological record for incremental geostrophic dispersal of sand, perhaps because the final storm imprint erases evidence for the long term sand transport processes. Clearly, the different points of view both contribute new insights to problems of shelf sand facies and their origin.


 

Acknowledgments and Associated Footnotes

1 Department of Geology, McMaster University, Hamilton, Ontario, Canada L8S 4M1

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