Marine Jurassic Formations of Sweetgrass Arch, Montana

Abstract

The marine Jurassic rocks of the Sweetgrass arch in western north-central Montana have been assigned to the Ellis formation. As a result of measuring and sampling eleven Jurassic sections around the Sweetgrass arch and from examining numerous well cuttings, the writer concludes that the Ellis consists of three major lithologic divisions that should be recognized as formations, and that the Ellis should be raised to group rank. The formations of the Ellis group from oldest to youngest are herein named Sawtooth, Rierdon, and Swift. The Sawtooth formation consists of three members, a basal fine-grained sandstone, a medial dark gray shale containing a few dark limestone beds, and an upper member of light gray calcareous siltstone, fine-grained sandstone, or sandy, oolitic limestone. The Sawtooth is as little as 8 feet in thickness on the Kevin-Sunburst dome. It thickens eastward to 200 feet in the Sweetgrass Hills and westward to about 200 feet along the Rocky Mountain Front. The formation is absent over most of the South arch. The lower two members pass into redbeds east of the Sweetgrass Hills and on the southeast flank of the South arch. The Rierdon formation consists mainly of gray limy shale and nodular limestone. It seems to overlie the Sawtooth formation conformably. It is as little as 72 feet in thickness on the Kevin-Sunburst dome, but thickens eastward to 180 feet in the Sweetgrass Hills and westward to about 200 feet along the Rocky Mountain Front. The formation is absent over most of the South arch. The Swift formation is composed of two members along the Rocky Mountain Front and eastward across the Kevin-Sunburst dome at least as far as the Sweetgrass Hills. The lower member is dark non-calcareous shale, highly glauconitic at the base. The upper member is gray fine-grained flaggy glauconitic sandstone containing abundant black-gray micaceous shale partings. On the South arch the formation consists of fine-grained flaggy sandstone which is locally pebbly at the base. In the vicinity of the Little Belt Mountains the formation is massive fine-grained sandstone containing a prominent basal conglomerate. The Swift attains its maximum thickness of 166 feet in the Pondera field. The formation has been removed by pre-Kootenai erosion from most of the Cut Bank area, and from a small area west of the Whitlash field. All the formations are fossiliferous. According to Imlay, the Sawtooth is upper Bathonian, the Rierdon is upper Bathonian and Callovian, and the Swift is Divesian and Argovian. The South arch was an island through most of the Bathonian and Callovian stages. At the close of the Callovian the marine waters retreated from the area. During the early Divesian the Sawtooth and Rierdon formations were locally thinned by erosion and possibly removed from some areas. Later in the Divesian marine waters transgressed the entire area and remained until the close of the Argovian.