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

This report summarizes the stratigraphic framework of Lower and Upper
Cretaceous sedimentary rocks in the central and eastern part of Montana, and
was written in support of U.S. Geological Survey studies of energy resources in
Cretaceous rocks. 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. Other rock types, such as
conglomerate and limestone are relatively rare in the section. Bentonite beds,
a result of ash falls from volcanic eruptions to the west, are volumetrically
insignificant, but are important marker beds through much of the Cretaceous
section.

Eighteen formations are described in this report; five formations of Early
Cretaceous age and thirteen of Late Cretaceous age. The thickness of the entire
Cretaceous section ranges between about 3,500 ft and 5,600 ft; within this
range between 500 and 1,000 ft are Lower Cretaceous and the remainder are Upper
Cretaceous. The entire section is thickest on the west, and thins eastward.

[Summary provided by the USGS.]