Regional Stratigraphic Analysis of Middle and Upper Marine Jurassic in Northern Rocky Mountains-Great Plains

Abstract

Middle and upper marine Jurassic strata are traced throughout Wyoming, Montana, adjoining parts of Idaho, southern Alberta, Saskatchewan, Manitoba, and western North and South Dakota. Lithologic units recognizable in Montana are used as a basis for tracing areally the various stratigraphic units recognized in the area under consideration. The total section is divided into four recognizable operational units which from oldest to youngest are: Piper, Rierdon, Preuss, and Swift. Tectonic and environmental conditions during middle and upper marine Jurassic time are reconstructed from isopach-lithofacies and paleogeologic maps. Clastic ratios and sand-shale ratios are used to define the lithologic variation. The important tectonic elements which are active during middle and upper marine Jurassic sedimentation include: the arcuate Idaho-Utah basin centered in southeastern Idaho; Belt island of central and southwestern Montana; central Montana and Wyoming shelf area; and the basins in southern Saskatchewan and western North Dakota. Normal marine and some evaporite sediments in the lower part are characteristic of the Piper unit. The Rierdon consists predominantly of normal marine clastics and some carbonates. Coarse clastics make a prominent appearance in the Preuss unit associated with a somewhat restricted environment. Normal marine conditions again prevailed with the deposition of coarse and fine clastics in the Swift.