Ellis, Amsden, and Big Snowy Group, Judith Basin, Montana

Abstract

Strata lying between the top of the middle Mississippian Madison limestone and the base of the Upper Jurassic Morrison formation in an area in central Montana are described and correlated. Sixteen columnar outcrop sections and six well sections are presented on three plates to show the lateral extent, thickness, and lithologic character of the Jurassic Ellis formation, the lower Pennsylvanian Amsden formation, the upper to middle Mississippian Big Snowy group. The Big Snowy group, which is divided into the Heath, Otter, Kibbey and Charles formations in descending order, contains marine, fluvial and estuarine facies. It lies disconformably on the Madison in the southern part of the area. The upper marine limestone strata of the Amsden overlaps successively lower Amsden redbeds and the upper part of the Big Snowy group. The Ellis is a transgressive marine deposit. While it rests on a Jurassic evaporite series in the eastern part of the area, the Ellis also overlies the Amsden, each member of the Big Snowy group, and the Madison at other localities. The regional diastrophic history as revealed by this stratigraphic study includes the following: post-Madison-pre-Big Snowy uplift, post-Big Snowy-pre-Amsden folding, middle Amsden movement, post-Amsden-pre-Gypsum Springs (lower Ellis) uplift, middle Upper Jurassic movement, Laramide folding, Tertiary uplift.