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"Cycle Definition and Controls, Nisku-Equivalent Strata, Southeast British Columbia [Abstract]"
Upper Frasnian, Nisku-equivalent strata in the southern Canadian Rocky Mountains consists of open shelf (Grotto Member) and peritidal (Arcs Member) carbonates that prograded basinward during an overall regressive phase of deposition. Both the 100 m thick Grotto and 12 m thick Arcs members consist of stacked, metre-scale, shallowing-upward cycles. Cycles in the Grotto Member are entirely subtidal and laterally continuous over the study area. Each cycle exhibits a gradual upward increase in the abundance, diversity and size of fossils, and a transition from a crinoid-brachiopod assemblage to a coral-stromatoporoid assemblage. Tops of the twenty cycles present are characterized by a sharp contact or by a subaqueous hardground. Seven peritidal cycles comprising the Arcs Member typically consist of a restricted mudstone that passes upward into a biolaminite which is, in turn, capped by a karstic erosion surface.
In the past, subtidal shallowing-upward cycles have generally been interpreted as reflecting eustacy (allocyclic) whereas peritidal cycles have been interpreted as the result of tidal flat progradation (autocyclic). Similarity in style and scale of the Grotto and Arcs cycles and regional facies-equivalency of these two members implies a single dominant controlling mechanism for the cyclicity. A glacio-eustatic cyclic control is most consistent with the nature of Grotto/Arcs cycles.
Correlation using these cycles provides a precise tool for paleoenvironmental and stratigraphic analysis within the Grotto-Arcs and equivalent strata of Western Canada.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
1 Shell Canada Limited, Calgary, Alberta T2P 3S6
2 Queen's University, Kingston, Ontario K7L 3N6
Copyright © 2003 by The Society of Canadian Petroleum Geologists. All Rights Reserved.