Abstract

A composite section of approximately 6,000 feet of Upper Cretaceous strata in the Big Hole Mountains of eastern Idaho has been identified by compiling subsurface and surface sections. The author has divided the section, previously called the “Frontier Formation,” into five units based on lithologic and sedimentologic characteristics. The units are from oldest to youngest: The Frontier Formation, the Cody Shale, the Bacon Ridge Formation, the Horseshoe Formation (new name), and an unnamed unit. This section is most similar to the Upper Cretaceous in the Jackson area of Wyoming, as described by Love (1977).

The Frontier Formation is partially a non-marine deposit between 400 and 500 feet thick conformably overlying the Aspen Formation. The lower portion consists of feldspathic sandstone with local pebble conglomerate lenses, and the upper portion is an interbedded shale and sandstone sequence.

The Cody Shale is a marine sequence of shale, silty shale and some sandstone. It is about 2,000 feet thick and conformably overlies the Frontier Formation. The upper portion of the Cody is more sandy. Sandstone in this formation is glauconitic and is often ripple cross-bedded.

Conformably overlying the Cody Shale is the Bacon Ridge Formation, a sequence of marine sandstone interbedded with siltstone, shale, mudstone, coal, and limestone. Thickness is approximately 1,000 feet. The sandstones are lithic arenites or sublitharenites with minor amounts of glauconite and are ripple cross-bedded.

The Horseshoe Formation (new name) is the most diagnostic unit in the study area and is between 1,100 and 1,300 feet thick. This transitional marine sequence contains abundant coal beds, and at least one distinctive fossil bed. The Horseshoe Formation is a major coal interval which appears to be equivalent to the unnamed “coaly sequence” above the Bacon Ridge Formation in the Jackson area, Wyoming.

The unnamed unit is estimated to be 1,500 feet thick within the study area. It is lithologically similar to the Bacon Ridge Formation.