Tectonic Controls of Depositional Environments and Sedimentary Sequences in Foreland Basins

Douglas J. Cant, Glen S. Stockmal

A typical foreland basin sequence results from accretion of a terrane to a continent because of load-induced subsidence, migration of a peripheral bulge, and uplift of sediment sources. The sequence should be unconformity-bounded, with a deep to shallow water pattern of deposition. The lower unconformity is cut as the peripheral bulge migrates ahead of the basin. The deep water occurs because accretion into the continental slope causes subsidence before much sediment is supplied. This phase may be turbidites like the Alpine Flysch, but in the Western Interior of North America it is generally open shelf. The rigid Precambrian crest prevented subsidence to great depths. The shallow-water phase is marginal to nonmarine, i.e., the Alpine Molasse, deposited as thrusting create large areas of sediment sources. The upper unconformity is cut as thrusting and crustal loading cease, and erosion of the thrust load allows basin uplift.

The stratigraphy of the Alberta basin was analyzed using this model, and six clastic wedges resulting from terrace accretion were identified. The first resembles the model sequence, but later ones differ because of eustatic fluctuations, rapid establishment of sediment sources, and elevation of the basin because of previous accretions. The six clastic wedges show a rough time correlation to the accretions of the six major terranes independently identified in the Canadian Cordillera. We conclude that depositional environments and unconformity-bounded sequences in foreland basins are primarily tectonically controlled, with eustasy a minor factor.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.