ABSTRACT

Cambrian, Ordovician and Silurian rocks underlie much of the Great Plains area in northwestern United States and western Canada, mainly in the shallow crustal depression called the Williston Basin. The region described occupies about 350,000 square miles of territory lying east of the Cordillera and southwest of the Canadian Shield.

A stratigraphical enquiry of this scope encounters problems concerning the purpose of stratigraphic nomenclature, leading to special comment on the names of several series and stages, preferred for reasons of adequate documentation, stratigraphical resolving power and wide currency.

Cambrian deposits, mainly sandstones and shales, reach a thickness of about 1,800 feet in east-central Alberta; they pass westward into several thousand feet of strata dominated by limestone. Eastward, in the plains of Saskatchewan and North Dakota they taper to an edge, largely a result of pre-Ordovician erosion, but re-appear in Nebraska. Due to the great Dresbachian transgression, Upper Cambrian deposits occupy most of the Plains region. These strata, in contrast to the Middle Cambrian, display a comparatively low rate of eastward attenuation. The highest beds carry Tremadocian fossils (as they do in the Cambrian System type section) and are overlain by Ordovician at an unconformity of continental magnitude.

Ordovician deposits in the Plains, unlike those of the Cambrian, had centres of maximum accumulation in the Williston Basin area. The initial deposits (Winnipeg Group) were sandstone, succeeded by shale with peripheral sandstone; they reach a thickness of 320 feet. Due to the eccentric position of the presentday Williston Basin with respect to the Winnipeg depositional basin, marginal deposits have survived in central Saskatchewan and Montana. The overlying strata (Bighorn Group) are mainly carbonate rocks. They spread westward far beyond the Winnipeg edge, although they were centred about 100 miles farther east. They comprise at maximum development two sections, the lower of which consists of fossiliferous-fragmental limestone, about 600 feet thick, overlain by

End_Page 455------------------------

about 100 feet of rhythmically deposited evaporitic anhydrites and carbonates. This section belongs to the Trenton-Viola. The upper section of the Bighorn consists of shale (Stony Mountain--lower Utica--Eden) and further evaporitic deposits totalling about 300 feet in thickness; it belongs to the Cincinnati Series. Like the Cambrian strata, Ordovician strata were removed by erosion (mainly pre-Devonian) from the Sioux Arch area.

Silurian deposits, called the Interlake Group, consist mainly of carbonate rocks and range from dolomitized fossiliferous-fragmental limestone to cryptograined dolomite. Several non-sequences interrupt the succession but representatives of the Upper (Cayuga) series, Middle (Niagara) series and probably the Lower series are developed. At its maximum the section reaches 575 feet, but is everywhere affected by pre-Devonian (Caledonian) erosion.