ABSTRACT: Allocyclic(?) Controls on Upper Devonian Platform and Buildup Development, Canadian Rocky Mountains
MCLEAN, DAVID
J., McGill University, Montreal, Quebec, Canada
Cycle stacking patterns in Frasnian Fairholme Group reef complexes of the southern Canadian Cordillera suggest that superimposed short-term and long-term fluctuations in relative sea level controlled platform and buildup development. Regional correlation of these stacking patterns, and assuming uniform cycle periodicity, reveals an ordered heirarchy of three magnitudes of sea level change: fifth (0.01-0.1 Ma) and fourth (0.1-1 Ma) order, and two types of third order (1-10 Ma).
The platform-interior portions of the Cairn Formation consist of meter-scale, shallowing-upward cycles. These are interpreted to have been deposited during short-term, high-frequency (fifth order) sea level oscillations. Superimposed on this cyclicity are larger shallowing-upward trends in which subtidal meter-scale cycles or subtidal noncyclic intervals gradually (sometimes abruptly) pass upward into peritidal cycles of comparable thickness. These intermediate-scale sequences are the product of both fourth- and third-order driving mechanisms of possible tectonic origin. Collectively, the Flume, Upper Cairn, and Peechee Members represent a larger-scale third-order depositional sequence.
The development of entirely subtidal cycles, and the regional correlation of groups of vertically stacked subtidal and peritidal cycles, suggest a syndepositional allocyclic control on platform and buildup development. Uncertainties in the temporal durations of both the high-frequency cycles and the larger-scale sequences, and assumptions inherent in these calculations, mask the presence of definitive orbitally forced Milankovitch rhythms within the Cairn Formation. In the absence of an unequivocal astronomical mechanism, tectonic mechanisms are postulated to explain the regionally correlative large-scale sequences.
AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)