The Alberta foreland basin: relationship between stratigraphy and Cordilleran terrane-accretion events

Abstract

A foreland-basin sequence resulting from accretion of a terrane to a continental passive margin ideally should be unconformity bounded, with a shallowing-upward pattern, like the classic Flysch to Molasse sequence of Alpine foreland basins. The basal unconformity is cut as the peripheral bulge associated with lithospheric flexure migrates cratonward ahead of the basin and the advancing overthrusts. The shallowing occurs because sediment supply at first is low – early stages of accretion near the continental slope generate little or no topography above sea level; later stages result in significant tectonic uplift, and much sediment is shed into the foreland, filling the basin. The upper unconformity is cut as lithospheric bending stresses are relaxed following overthrusting, and reduction of the flexural load on the lithosphere through erosion and (or) tectonic denudation of the overthrusts causes regional uplift or basin "rebound". Actual sequences show differences from this idealized version in that (i) basal unconformities may not develop under conditions of high eustatic sea level; and (ii) they may not shallow upward in all cases. These differences can occur because later terranes that accrete onto the seaward side of a previously accreted terrane may simply push it farther onboard, thus initiating sediment supply as rapidly as the load-induced subsidence. Also, in this way, a small terrane can influence the filling of a foreland basin that is more than one "lithospheric flexural half-wavelength" away from the site of accretion.The stratigraphy of the Alberta basin has been divided after comparison with the idealized sequence resulting from an individual accretion event. The six clastic wedges recognized (Fernie–Kootenay, Mannville, Dun vegan, Belly River, Edmonton, and Paskapoo) show a temporal correlation with the times of accretion of terranes (Intermontane superterrane, Bridge River, Cascadia, Insular superterrane, Pacific Rim – Chugach, and Olympic, respectively) in the Cordillera. Therefore, the stratigraphy of the foreland basin may be best interpreted in terms of Cordilleran tectonics rather than sea-level fluctuations. Eustatic sea-level variations are believed to affect the internal stratigraphy and sedimentology of some clastic wedges and are responsible for the deposition of some thin units, but they appear to operate on time scales that differ from those of the clastic wedges identified here.