REGIONAL VARIABILITY IN THE STRATIGRAPHY OF THE MARINE JURASSIC OF NORTHERN WYOMING: EVIDENCE FROM RESISTIVITY AND GAMMA LOGS

The marine Jurassic of Wyoming contains seven lithostratigraphic units in ascending order: The Gypsum Springs Formation, Piper Formation, Canyon Springs Sandstone, Stockade Beaver Shale, Hulett Member, Redwater Shale, and Windy Hill Sandstone. Although this lithostratigraphy has implied regional continuity in the stratigraphy, resistivity and gamma well logs from the Bighorn Basin in Wyoming to the Black Hills in South Dakota show considerable lateral variation in stratigraphic architecture.

Incised valley fills are the primary source of variation, such as in the Canyon Springs and Hulett Formations. These fills have a sharp basal kick to high resistivity and vary markedly in thickness, reflecting the local depth of incision. The upper contact of each of these incised valley fills is marked by an abrupt shift to low resistivity, and is interpreted as a major flooding surface overlain by marine shales of the Stockade Beaver Shale and Redwater Shale.

The Windy Hill Formation is an incised valley fill of varying thickness, depending on depth of incision. It maintains a sharp basal contact with the underlying Redwater Shale in most cases, especially near Greybull, but shows a more gradual contact in several other areas that preserve the upward coarsening within the Redwater. These incised valley fills contain two high resistivity peaks, traceable for over 10 kilometers and interpreted as shell beds. The Windy Hill has a sharp upper contact with alluvial plain sediments of the Morrison Formation in all locations.

A second source of local variation occurs in the Gypsum Springs, which can experience solution collapse of its evaporites and local loss of its characteristic high resistivity signal. The Gypsum Springs is also anomalously thin in some locations, primarily near the southern end of the Big Horn Basin.

In contrast, the Stockade Beaver Shale and Redwater Shale marine tongues show little lateral variation. These shale-dominated formations have a low resistivity and high gamma signature. The Redwater Shale contains two high resistivity peaks, which correspond to shell-bed capped parasequences. These are traceable for over 200 miles around the Big Horn Basin, thicken towards the Black Hills, and vanish south of Cody.