THE MORRISON FORMATION IN THE FOUR CORNERS AREA AND IMPLICATIONS FOR REGIONAL STRATIGRAPHY

Exposures near the mouth of Recapture Creek in southeastern Utah show a section of the Morrison Formation that includes, from the base to the top, the upper part of the Bluff Sandstone Member, followed by the Recapture, Westwater Canyon, and Brushy Basin Members. At this locality, the upper part of the Bluff interfingers with the lower part of the Recapture Member. The Salt Wash is missing entirely. Between the exposures at Recapture Creek and the Sand Creek trailhead, located 45 miles to the east, the Salt Wash thins westward by onlap and interfingering with the Bluff-Junction Creek Sandstone Members. At Recapture Creek, the Bluff has replaced the Salt Wash entirely and the Bluff interfingers with the lower part of the Recapture. Previous workers reported that the Salt Wash Member is missing in the area east of the Monument uplift in the southern part of the Blanding basin in the vicinity of Bluff, Utah, six miles west of Recapture Creek. Our interpretation is that the Bluff Sandstone Member has replaced the Salt Wash Member in that area.

Because of (1) the interfingering and onlapping relationships between the Junction Creek and Salt Wash described here, (2) the presence of the J-5 unconformity at the base of the Bluff- Junction Creek Sandstone Members, and (3) the lack of any other unconformities from the base of the Bluff-Junction Creek to the top of the Brushy Basin Member, we consider that the stratigraphic units from the base of the Bluff-Junction Creek Sandstone Members to the top of the Brushy Basin Member are part of a single package of genetically related beds—bounded above and below by unconformities—that constitute the Upper Jurassic Morrison Formation in the Four Corners area.