External Controls on Fluvial Processes: The Scollard Sequence, Western Canada Foredeep

The Scollard-age (Late Maastrichtian – Early Paleocene) strata in the Western Canada foredeep form a fully nonmarine depositional sequence bounded by regional subaerial unconformities, and include the relatively conformable successions of the Scollard, Coalspur and Willow Creek formations. The paleo-depositional environment was dominated by fluvial systems sourced from the adjacent Cordilleran belt, with additional sedimentation in high watertable floodplains and lacustrine environments.

In the absence of syn-depositional marine influences, fluvial processes responded to a combination of tectonic and climatic (‘upstream') controls. The extent of the tectonic control was investigated based on reconstructions of paleo-slope gradients, changes in the direction of syn-depositional tilt, and variations in the burial depths recorded by the Scollard sequence across the foredeep. The shift in fluvial styles from gravel- and sand-bed braided, at the base of the sequence, to sand-bed and fine-grained meandering toward the top suggests a decrease in slope gradients with time, as expected from a stage of orogenic loading. Changes in the direction of syn-depositional tilt, induced by differential subsidence along strike, are also documented from paleo-current data. Differential subsidence is independently confirmed by the study of late diagenetic clays, which indicate increasing burial depths to the south, toward the center of loading.

Superimposed on the effects of tectonism, several cycles of climatic fluctuations between wetter and drier conditions controlled the composition of sandstones in terms of framework and early diagenetic constituents, as well as the occurrence of coal seams. This study suggests that the early diagenetic clay mineral assemblages in the stratigraphic section depend on the position of sandstones relative to the sequence boundaries. The observed increase in the abundance of authigenic kaolinite immediately below sequence boundaries indicates infiltration of meteoric water during times of subaerial erosion, accompanied by the dissolution of unstable minerals (e.g., micas, feldspar) and the formation of secondary porosity.