ABSTRACT
Marine strata of the Upper Jurassic Swift Formation underlie a large area of the Western Interior Plains of the United States and pinch out to an erosional edge in southern Alberta. Two informal members can be distinguished over the study area in southern Alberta and northern Montana: a basal glauconitic shale member, and an upper “ribbon sand” member, composed of interbedded sandstone, siltstone, and shale, with lithotypes ranging from mudstones with lenticular silt beds to flaser-bedded sandstones. An initial marine transgression resulted in deposition of the basal shale, and was followed by gradual regression and episodic deposition of the ribbon sand member in shallow-marine, bar and interbar facies. Erosion during the latest Jurassic and earliest Cretaceous, particularly valley cutting, determined the distribution of the Swift.
Correlation of the Swift with equivalent units throughout the Western Interior of North America provides the basis for a regional reconstruction of Late Jurassic depositional patterns and environments. Insight into erosional patterns and environments during Early Cretaceous time can also be gained.
Although potential reservoir strata are widespread in the Swift, poor predictability of sand body distribution and trapping mechanisms make the Swift a low-priority target for hydrocarbon exploration.
