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The AAPG/Datapages Combined Publications Database

Williston Basin Symposium

Abstract

NDGS/SKGS-AAPG

Fourth International Williston Basin Symposium, October 5, 1982 (SP6)

Pages 295 - 312

THE CRETACEOUS SYSTEM IN THE WILLISTON BASIN — A MODERN APPRAISAL

W. G. E. CALDWELL, Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 0W0

ABSTRACT

Within the Williston Basin, the sedimentary record of the Cretaceous Period takes the form of a prism oidal block of terrigenous clastic rocks, about 1,500 m thick in eastern Alberta and western Montana, and about 600 m thick in the Manitoba Escarpment and its extension into North Dakota. The block is an integral part of a great wedge of the same kind of rocks that occupies the Western Interior Basin of North America and must be considered in that context.

Derived mostly from displaced masses of pre-Cretaceous rocks in the fold-thrust belt of the Columbian Orogen, Cretaceous rocks display significant facies changes across the Williston Basin. Western sequences consist of non-marine to marginal-marine, coarse-grained clastics intertongued with finer-grained clastics of open-marine origin. Eastern sequences contain a higher proportion of marine, fine-grained elastics, some of them notably calcareous, because the eastern basin lay beyond the reach of most coarse-grained detritus.

Five marine cycles, widely recognized throughout the Western Interior and held by many workers to be controlled by tectonoeustasy, are recorded in these Cretaceous sequences. The Greenhorn and Niobrara cycles are best defined within the Williston Basin, where the eastern sequences offer close comparison to those of the Western Interior mid-basin in which the cycles first were elucidated.

Palaeomagnetic studies have shown that the Cretaceous Williston Basin lay between 45° and 55 N latitude. A temperate to tropical climate prevailed; oxygen isotope analyses of fossil shells suggest that water temperatures fluctuated between 15° and 27°C. Rich faunas, including both boreal and Tethyan derivatives, thrived in these waters. Dominated by foraminifers and molluscs,fossils have been used to establish highly refined zonal schemes. More than sixty molluscan range zones have been recognized in the Upper Cretaceous Series of the Western Interior Basin and more than twenty foraminiferal assemblage zones in the same series of the Williston Basin.

Seams of altered volcanic ash (bentonite) in the Cretaceous sequences of the Williston Basin contain minerals that have yielded isotopic ages using the K/Ar and other radiometric methods of dating. These ages have been incorporated into an absolute time-scale for the entire Western Interior Basin. Reconciliation of absolute and relative time-scales has produced an unusually powerful stratigraphic instrument which has been, and is being, used to reconstruct the detailed geological history of the Williston Basin during the Cretaceous Period.

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