Stratigraphic Framework of Lower and Upper Cretaceous Rocks in Central and Eastern Montana

The study was undertaken as a part of a larger project, under the direction of J.L. Ridgley, that investigated the occurrence of shallow biogenic gas in Upper Cretaceous rocks in north-central Montana and adjacent areas of Canada.

The approach used here is to describe briefly the rock units and to show their thickness, distribution, and structural configuration using isopach maps, cross sections, and structure contour maps. In the study area, within a central foreland basin setting, most deposition of Lower and Upper Cretaceous rocks was between the lithologic end members of very coarse terrigenous clastics and the open marine chalks. With few exceptions, Cretaceous rocks in the study area consist of fine- to medium-grained sandstone, siltstone, mudstone, and shale.