ABSTRACT

Accretion of an allochthonous terrane onto a pre-existing continental margin results in flexural loading of the lithosphere and development of a foredeep on the edge of the craton. An unconformity-bounded shallowing-upward sequence like the classic Alpine flysch to molasse is generated within the foredeep. The shallowing is a result of low sediment supply rates from the initially submarine orogenic belt, followed by higher rates of sediment supply as thrusting uplifts material above sea level. The lower unconformity is a result of uplift caused by passage of the peripheral bulge across the pre-existing platform sequence. The upper unconformity results from "rebound" of the basin as the accretion phase wanes, and the tectonic load is eroded.

The Cordillera is composed of a number of terranes accreted in succession, which resulted in repeated influxes of coarse clastics into the foreland. Later accretionary events are not associated with deep-water facies because the subsequent accretions translated already accreted terranes and telescoped miogeoclinal rocks farther onto the continental margin, initiating sedimentation nearly coincident in time with subsidence.

The evolution of the Alberta Foreland Basin can therefore be interpreted in terms of the history of terrane accretion events in the Canadian Cordillera. Six major clastic wedges (Fernie-Kootenay, Mannville, Dunvegan, Belly River, Edmonton, and Paskapoo) correlate roughly in time to the accretion of the six principal terranes mapped in the Cordillera (Intermontane, Bridge River, Cascadia, Insular, Pacific Rim - Chugash, Olympic). Unconformities representing long time intervals are interpreted as resulting principally from basin "rebound", but shorter intervals may have been the result of the peripheral bulge retreat or eustatic fluctuations.

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