WELL-LOG CORRELATIONS AND SEQUENCE STRATIGRAPHIC INTERPRETATIONS OF THE JURASSIC SUNDANCE FORMATION, NORTHWEST WYOMING

We have previously identified three depositional sequences in the Jurassic Sundance Formation of the Bighorn Basin, Wyoming, where these strata are exposed in a linear belt on the west flank of the Bighorn Mountains. Here, we use well logs, primarily spontaneous potential and resistivity, to extend the correlations into the subsurface. The outcrop belt and accompanying well logs appear to be oriented along a northwest-southeast strike line, based on the limited lateral variation in lithologic signature. Seven distinctive surfaces can be traced throughout the study area, and these correspond to (in ascending order): the basal sequence boundary, a maximum flooding surface and a basal surface of forced regression within the lower Sundance, a sequence boundary at the base of the upper Sundance, two flooding surfaces (including the maximum flooding surface) within the upper Sundance, and a sequence boundary at the base of the Windy Hill. Most units are more easily recognized on resistivity than SP logs. The sequence boundaries at the base of the lower and upper Sundance are each defined by a sharp kick to the left on the resistivity log, corresponding to a combined sequence boundary and transgressive surface. The surface of forced regression in the lower Sundance is marked by a sharp kick to the right on the resistivity log. The sequence boundary at the base of the Windy Hill is placed at a sharp kick to the right on the resistivity log, corresponding to the base of a sandy incised valley succession. Flooding surfaces were recognized based on a sharp kick to the left on the resistivity log, suggesting vertical changes in cementation and pore water content. Oyster shoals are locally correlatable in the upper Sundance, and they are characterized on the resistivity log by a sharp kick to the right at their base and a sharp kick to the left at their top. Future work will focus on extending these correlations along depositional dip.