The Western Canada Sedimentary Basin

The Western Canada Sedimentary Basin, a simple northeasterly tapering wedge of sedimentary rocks more than 6 km thick, extends southwest from the Canadian Shield into the Cordilleran foreland thrust belt. Its internal structure and the lateral variations in its shape reflect a long and complex history of development involving a foreland basin that was superimposed on a cratonic platform and continental terrace wedge. This history, which is inextricably linked to the evolution of the Canadian Cordillera, can be outlined succinctly with reference to the unconformity-bounded transgressive-regressive stratigraphic sequences established by Sloss (Bull. geol. Soc. Am. 74, 93 (1963)), each of which has a distinctive character in Western Canada. The continental terrace wedge was established with the deposition of the Proterozoic Purcell (1500-1350 Ma) and Windermere (850-600 Ma) sequences, but the first record of the platformal phase is the early Palaeozoic transgressive onlap of the early Proterozoic (> 1750 Ma) crystalline basement by the Sauk sequence. Early Palaeozoic subsidence of the margin of the craton may have been due to cooling of the lithosphere after renewed stretching at the ancient rifted western margin of the Precambrian craton, and to isostatic flexure of the lithosphere under the weight of the sediment that had accumulated at the margin in the oceanward prograding continental terrace wedge. During a subsequent Middle Ordovician to Middle Jurassic phase, the cratonic platform became differentiated into an intersecting network of epeirogenic arches with intervening basins. Development of the basins was as much a result of erosion and uplift of the arches between transgressive-regressive cycles as it was a result of differential subsidence of the basins during the cycles. The cause of the long (> 300 Ma) episode of intermittent epeirogenic movements that produced the basins and arches is a major unsolved problem. The foreland basin developed in two stages, in Middle Jurassic to early Cretaceous and late Cretaceous to Palaeocene time, as a result of collisions between North America and two pieces of a tectonic collage of oceanic terranes that were accreted to its western margin. During these two collisions, the continental terrace wedge, which had accumulated outboard from the rifted margin of the continental craton, was compressed and displaced over the western margin of the craton. Part of the supracrustal cover was scraped off the craton and accreted to the overriding mass to form a wedge of imbricate thrust fault slices that was tectonically prograded over the margin of the continental craton. Isostatic flexure of the continental lithosphere in response to the tectonic loading imposed on it by the displaced continental terrace wedge and the accretionary wedge of thrust slices produced the migrating moat in which the outwash of clastic detritus from the evolving thrust belt was trapped to form the foreland basin. The Palaeohelikian flat-lying unmetamorphosed basin remnants high on the Craton show that the Craton was established long before the commencement of the Phanerozoic cycle of basin development examined in this paper. The first significant event in this history was the development, over attenuated crust, of an Atlantic type down-to-ocean faulted margin in the Neohelikian, upon which was deposited a great wedge of clastic sediments across the structural grain of the Archaean platform, followed after a gap of 500 Ma by renewed faulting and subsidence and further loading in Hadrynian time. The subsidence of the continental margin as a result of this imposed load and thermal contraction extended far into the craton and set the stage for the first cycle of deposition represented by the Sauk sequence. Sauk sequence. The Sauk sequence records progressive onlap with the Upper Cambrian strata extending high on the western side of the craton and the depositional strike running northwesterly, parallel to the probable old continental margin. There is a notable lack of arches or isolated basins. This initial sequence is dominated by clastic sediments from the shield. Tippecanoe sequence. Arches interrupted the linear depositional pattern. Marked sedimentary attenuation over highs created a relative thickening in the area of the Williston Basin. This sequence shows thin seams of fine sand and silt suggesting episodic uplift of the burgeoning arches, but the dominance of carbonate rock indicates that they were for the most part covered. The lack of an onlapping relation, and widespread inundation of the craton indicate a very rapid transgression. Like the Cambrian seas, those of the Ordovician and Silurian transgressed from the western margin. Kaskaskia sequence. The strongly developed pre-Devonian arches surrounding and segmenting the Devonian depositional area resulted in a transgression of over 3000 km from the northwest, rather than the west, over a tectonically disturbed and eroded surface, resulting in complex facies patterns. The Mississippian seas covered almost as large an area as those of the Ordovician, onlapping completely the Western Alberta and Peace River arches. Arches in the southern and western part of the Williston Basin continued to grow into late Kaskaskia time, creating sills with salt deposits in the regressive part of the sequence that culminated with a clastic assemblage derived from the flanking arches. Absaroka sequence. The Sweetgrass Arch isolated the Alberta Basin from the Williston Basin for the first time. These opened to the southwest and west respectively. The Williston Basin had a dominant red bed-evaporite assemblage, the Alberta Basin a marine clastic assemblage. This interval is characterized by periods of non-deposition and erosion, with depositional limits of each system indicating much less encroachment on to the craton, which appears to have progressively increased its rate of dip to the west. The progressively increased rate of westward thickening from the Sauk to Kaskaskia to Absaroka in the undisturbed part of the basin also suggests an increasing westward tilt or subsidence of the basin margin. Zuni sequence. The Zuni reflects the late Jurassic orogenic activity in the west. The convergence of allochthonous terranes with the continental margin resulted in the thrusting of the marginal deposits eastward, tectonically loading the lithosphere and providing the provenance for the continental clastic sediment that filled the foredeep which formed in response to the loading. The continental sediment spread across the basin with marine seaways to the north and south that gradually advanced to join together to form a seaway extending from the Arctic to the Gulf of Mexico in mid-Cretaceous time. A second plate convergence resulted in further tectonic thickening with thrusting extending further east, cannibalizing previously formed deposits and generating a second sequence of terrestrial sediments that covered the entire basin again in the late Cretaceous and Palaeocene. Notably absent during the Zuni was the influence of arches that had previously segmented the basin. Also absent are carbonates and evaporites. Instead the basin fill was entirely of clastic sediments. The present structural configuration of the basin was established during this period as the foredeep encroached onto the craton.