IMPROVED SUBSURFACE CORRELATION OF MIDDLE JURASSIC UNITS IN WYOMING AND MONTANA WITH DETAILED OUTCROP DESCRIPTIONS

Detailed outcrop measurements across northern Wyoming and southern Montana provide reference sections for development of a T-R cycle framework for correlation of the hydrocarbon-producing Bajocian and Bathonian Sawtooth, Watrous, Shaunavon, and Gravelbourg formations in the Sweetgrass Arch and Williston Basin of North Dakota, Montana, and Saskatchewan.

The Middle Jurassic units of the northern Western Interior represent a mixed carbonate, evaporite, and siliciclastic succession of complex lateral and vertical facies relationships. Rocks of Middle Jurassic age progressively onlapped a Mississippian to Triassic unconformity during an overall transgression that began during the Early Jurassic. These Jurassic sediments were deposited on a shallow dipping shelf in a large marine embayment interrupted by local erosional topographic relief, which greatly influenced facies geometries during Bajocian time.

Two low-amplitude relative sea level rises are recorded in the Bajocian and Bathonian sediments of Wyoming and Montana. The lower portion of the Gypsum Spring Formation of Wyoming and the equivalent Piper Formation of Montana records the first T-R cycle. The upper section of these same formations record the second T-R cycle. The two cycles can be recognized in well log patterns across the northern Western Interior.