Octavian Catuneanu¹, Arthur, R. Sweet², Ahmed Khidir¹

(1) University of Alberta, Edmonton, AB
(2) Geological Survey of Canada, Calgary, AB

ABSTRACT: Changing Fluvial Styles in a Dynamic Foreland System: The Scollard Formation of the Western Canada Foredeep

The Scollard Formation represents a fully fluvial succession which accumulated during the Late Maastrichtian-Early Paleocene interval in the foredeep of the Western Canada foreland system. The timing of Scollard fluvial aggradation corresponds to a stage of loading in the thrust-fold belt that resulted in foredeep subsidence and forebulge uplift. The correlative forebulge stratigraphy is mostly reduced to an unconformity that marks a maximum regression of the Interior Seaway, i.e. the turnaround between the Bearpaw regression and the Cannonball transgression. The Scollard therefore accumulated in isolation from marine influences, on a dynamic topography controlled by tectonic mechanisms. Differential subsidence across the foredeep, with higher rates towards the center of loading, produced changes with time in the amount and direction of topographic tilt. Gradual shallowing of the topographic gradient during orogenic loading resulted in a shift in fluvial styles from braided (lower Scollard) to meandering (upper Scollard). At the same time, the direction of topographic tilt changed from ENE (away from the orogenic load) during the lower Scollard, to SSE (towards the load) during the upper Scollard, as inferred from paleoflow measurements. Subsidence in the foredeep triggered a relative rising of the water table, which resulted in coal swamp development in the upper part of the profile. The upwards increase in the abundance of coal within the Scollard Formation was previously interpreted to reflect a shift towards more humid climatic conditions. The new tectonic model offers an alternative to the inferred Late Maastrichtian to Early Paleocene long-term climatic change.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado