High-Frequency Sequences Reveal Complex Patterns of Plate Flexure: Late Cenomanian of Western Canada Foreland Basin

A. Guy Plint¹ and Michael A. Kreitner^2
1 Department of Earth Sciences, University of Western Ontario, London, ON
² Petro Canada Ltd, Calgary, AB

Late Cenomanian shallow marine mudstones of the Kaskapau Fm. form a wedge, tapering eastwards across the foreland basin, from 250 to 10 m over 300 km. The rocks overlie deltaic strata of the Dunvegan Fm, and were deposited during the largest eustatic rise of the Cretaceous. 21 high-frequency sequences, 5-20 m thick were mapped on the basis of subaerial/marine erosion surfaces. Lowstand shorelines in sequences 1-13 were initially oriented NE-SW, but gradually rotate to N-S, with progradation to the SE and E. In contrast, sequences 14-21 prograded from E to W (i.e. towards the orogen!) in response to regional uplift in the east and cannibalization of older Kaskapau sequences. Uplift was focused along a narrow N-S flexure zone that coincides with a major terrane boundary in the Precambrian basement. Isopach maps show that some sequences form short (~80 km) wedges whereas others taper very gradually over 300 km. The variable taper length of sequences may reflect the interaction of high-frequency eustasy with changes in the radius of plate flexure. Many sequences show thickening into arcuate depocentres flanking the orogen. Successive depocentres show rapid lateral migration of 100-200 km along the margin of the orogen. The rapid lateral shifts in depocentres are interpreted to reflect lateral migration of zones of active thrusting on timescales of ~50 ky. The regional uplift in the east during sequence 14-21 time may possibly be related to stresses generated by terrane docking events on the plate margin.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005