New Evidence for Cretaceous Depositional Changes in the Alberta Foreland Basin Triggered by Tectonism in the Rockies

A new set of muscovite, biotite, feldspar, and whole rock 40Ar/39Ar ages has been obtained from the Main Ranges of the Rocky Mountain fold-and-thrust belt (RM-FTB) and from its western margin along the northern segment of the Southern Rocky Mountain Trench (SRMT). The tectonic pulses identified in this study document the kinematic link between the Alberta Foreland Belt (southern Canadian Rockies) and the Omineca hinterland, and are contemporaneous with important depositional changes in the Alberta Foreland Basin. 

Of major regional significance is the identification of previously unrecognized Early Cretaceous tectonic pulses that produced out-of-sequence thrusting in the western Rocky Mountains fold-thrust belt: a) Valanginian ages of ca. 135–131 Ma may record a phase of tectonic loading of the North American margin accompanied by the development of a forebulge that corresponds to the extensive pre-Cadomin–pre-Mannville (“sub-Cretaceous”) unconformity in the foreland basin. The early Aptian ages of 128–123 Ma obtained in this study may record the subsequent orogenic uplift accompanied and outlasted by vigorous erosion in the proto-Rockies and deposition of extensive Cadomin gravel sheets in the proximal portion of the foreland. The late Aptian–earliest Albian ages of 115–111 Ma are contemporaneous with the Moosebar sea transgression, which suggests a new phase of tectonic loading that triggered downwarping of the foreland basin floor (i.e., creation of accommodation space). The mid-Cretaceous tectonic pulse (101–96 Ma) is quasi-contemporaneous with the development of the Dunvegan fluvial system. Finally, the latest Cretaceous compressional pulses at the western margin of the Rockies (79–71 Ma), best recorded by the Bear Foot thrust and the Valemount strain zone, are contemporaneous with major Campanian thrusting in the Front Ranges (tectonic loading), that triggered crustal down-warping accompanied by the last major transgression (“Bearpaw sea”) in the Alberta Basin.