Marine Jurassic of Northern Rocky Mountains and Williston Basin

Abstract

Regional studies indicate that the pattern of sedimentation within the Jurassic seas of the northern Rocky Mountains was influenced by several associated paleotectonic elements. The major negative-trending units were the Alberta and Twin Creek troughs and the Williston basin. The major positive-trending elements were the Belt island and the Sheridan arch. Regionally the marine Jurassic can be separated into lithogenetic units whose characteristics appear to be directly related to the presence of sedimentary environments, the nature of which was governed by the position and relative activity of the various paleotectonic elements. The Piper and Rierdon formations and their equivalents are each separated into three regional units of member rank. The lithologic nature of these units is believed to be related in a large way to the position of Belt island and its effect upon the salinities, temperature, and current distribution in the waters of the Twin Creek trough and the Williston basin and the intervening Montana-Wyoming shelf area. Much of the misunderstanding concerning the Jurassic stratigraphy of the Williston basin is believed to be caused by an inadequate knowledge of the regional nature of the Jurassic units.