ABSTRACT: Cretaceous Rocks of Jackson Hole, Northwestern Wyoming

J. David Love

Neocomian to Maastrichtian rocks are present in Jackson Hole, northwestern Wyoming. In the northern part, Upper Cretaceous strata are 20,000 ft thick; half nonmarine tan quartzite gold-bearing conglomerate and olive drab to gray sandstone, and half gray siltstone, shale, and coal. In the southern part, 8700 ft thick, conglomerates thin out and sandstones, shales, and coals predominate.

Conglomerates, derived chiefly from Precambrian terrain to the west, were deposited sporadically from middle Niobrara (Santonian) time through Maastrichtian. Some intertongue with and thin eastward into marine strata. Several in the Harebell Formation (Maastrichtian) are nearly 1000 ft thick and intertongue with magnetite-rich sandstones and mafic volcanic debris.

Lower Cretaceous strata in northern Jackson Hole are 650 ft thick and in the southern part 1350 ft. The upper half is black to gray marine shale and sandstone and the lower half is nonmarine gray sandstone and variegated claystone.

A diastem occurs within the Frontier Formation (middle Turonian). Fossils suggest that the Upper-Lower Cretaceous boundary is within the underlying Mowry Shale. Both the top and bottom of the Cretaceous sequence are indefinite. At the base, the lower part of the Cloverly-Morrison (?) Formations, undivided, is unfossiliferous; the upper part contains Lower Cretaceous forms. At the top of the Cretaceous is the Pinyon Conglomerate, 3500 ft thick; the lower part contains dinosaurs and a tuff with a K-Ar age of 67 Ma. Farther south the entire Pinyon is Paleocene.

The Harebell Formation unconformably overlaps northward across 3000 ft of strata down to the Bacon Ridge Sandstone (Santonian) in 30 mi. Another unconformity is at the base of the Pinyon Conglomerate, which overlaps strata down to Paleozoic. The Washakie Range on the east margin of Jackson Hole was uplifted, thrust westward, and eroded to the Paleozoic core before the Pinyon was deposited. In places, the Pinyon overlies the Harebell Formation with a 90° unconformity.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990