Jurassic Stratigraphy of Williston Basin Area

Abstract

Rocks of known Jurassic age in the Williston Basin area are included in five formations which from oldest to youngest are Gypsum Spring, Piper, Rierdon, Swift, and Morrison. A sequence of red silty shales termed the Jura-Triassic redbeds underlies these formations, but is of uncertain age. In the eastern part of the area the Swift, Rierdon, and formations can not be differentiated and strata equivalent to these formations are combined to form a single formation herein identified as the Sundance. An arbitrary cut-off line is used to limit the area in which the Swift, Rierdon, and formations are recognizable units. The Jura-Triassic redbeds are correlated over the entire area and consist essentially of red silty shales, grading into argillaceous siltstone near the base but becoming progressively less silty upward in the section. The Gypsum Spring formation can be traced over the entire area and commonly consists of two members, a basal evaporite member and an upper normal marine member consisting of limestone, dolomite, and shale. In Montana and western Saskatchewan the two members grade into each other and division into members is not possible. A continuous unit serves to indicate the upper boundary of the formation. The Piper, Rierdon, and Swift formations can be traced only over the western half of the area. The can be divided into three members. The lower member is a heterogeneous unit consisting of limestone, sandstone, and shale in which facies changes are abrupt and individual beds can not be traced over great distances. The middle member is of consistent lithologic character, being composed almost entirely of and is commonly known in the area as the Piper limestone. This is confined to the western part of the area and is a different stratigraphic unit from the so-called Piper limestone of North Dakota and Manitoba. The upper member of the formation consists of shales and sandy limestones. A continuous bed of sandy marks the top of this formation. The Rierdon consists primarily of greenish gray calcareous shales which are sandy in the eastern and northern part of the area. End_Page 367------------------------------ The Swift formation is divided into three members. The lower member consists predominantly of greenish gray shale, the middle member is sandstone, and the upper member greenish gray shale locally becoming variegated near the top of the formation. The middle sandstone member does not appear to represent a continuous sheet of sand, and at least two separate sandstone units occur. The Sundance formation represents equivalents of the Swift, Rierdon, and formations in the eastern half of the area and consists of a marine series of shales, sandstones, and limestones. The Morrison formation is a non-marine series of shales and sandstones which overlies the Swift formation in Montana and the Sundance formation in North Dakota.