Abstract

The Bacon Ridge Formation of northwest Wyoming was deposited along the western margin of the Late Cretaceous interior seaway. Lying between the offshore Cody Shale and the continental "coaly sequence," the Bacon Ridge Formation represents a marine to continental regressive sequence. The age of the Bacon Ridge Formation is middle to late Coniacian, based on marine invertebrate fauna.

With this precise dating, it is possible to correlate deposition with ongoing uplifts and eustatic sea level fluctuations. During the Coniacian, a worldwide eustatic transgression occurred. However, thrust related uplift in eastern Idaho and western Wyoming resulted in a local regression. As thrusting progressed from west to east, lithic-rich clastic sediments were shed into the interior seaway. Thrusting provided a source for Paleozoic and early Mesozoic sedimentary rock fragments, which are the major components of Bacon Ridge sandstones. Other lithic fragments include volcanics and pyroclastic debris associated with the intrusion of the Idaho Batholith.

The Bacon Ridge Formation contains two facies. The lower, predominantly thickly bedded sandstones, represent a beach and barrier environment. Above are thinly bedded shales, sandstones, and coals deposited in back barrier lagoon, marsh, and estuary environments. Faunal assemblages within each facies support these environmental interpretations.