ABSTRACT: Upper Devonian Platform and Buildup Megasequences, Alberta Rocky Mountains

MOUNTJOY, ERIC, and DAVID MCLEAN, McGill University, Montreal, Quebec, Canada

Large-scale depositional sequences make up the Upper Devonian succession of western Canada. With improved conodont biostratigraphy, these sequences can now be correlated with the Johnson et

al. estimated Devonian eustatic curve and compared with sections in other parts of North America and the world.

The Upper Devonian is divisible into three major transgressive successions: (1) Leduc-Cairn and Peechee, (2) Grotto-Arcs-Ronde, and (3) Sassenach-Palliser-Exshaw. The first two major Frasnian cycles represent progressive deepening followed by shallowing-upward phases. The third major succession is Famennian and in its upper part appears to be an overall deepening-upward cycle that continues into the Carboniferous. This contrasts with the generally shallowing-upward sequences and sea level lowerings depicted in the Johnson eustatic curve.

Locally the Frasnian Cairn-Peechee and Grotto-Arcs successions have been studied in detail. They contain meter-scale, shallowing-upward, fifth-order hemicycles. Superimposed on these meter-scale cycles are larger intermediate scale shallowing-upward trends. These fourth-order sequences also occur in Peechee buildup margins as shown by distinct retrograding, aggrading, and minor prograding phases of these margins.

The Grotto-Arcs-Ronde succession represents two shallowing-upward, third-order cycles. Distinctive megabreccias associated with the Grotto-Arcs are clearly not lowstand deposits but were deposited during a sea level rise. This is supported by the lack of exposure and diagenetic textures in these breccias.

Minor modifications of the Frasnian and major changes of the Famennian portions of the Johnson et al. curve will be discussed.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)