Fission-track ages from a section through the Morrison Formation in central Utah show that most of the Morrison Formation is Late Jurassic or perhaps earliest Cretaceous, ranging in age from 130 to 157 Ma. However, the upper part of the Morrison Formation in the Brushy Basin Member gives ages ranging from 99 to 123 Ma, covering a wide span of Early Cretaceous time. The bentonites dated are stratigraphically below what has been called the basal Buckhorn Conglomerate Member of the Cedar Mountain Formation. This may indicate that (1) the Buckhorn Conglomerate is not the base of the Cedar Mountain age sediments; (2) the Buckhorn Conglomerate has been misidentified in our section and is absent, in which case there would be no clear boundary between the Morrison and Cedar Mountain Formations; or (3) a significant part of the Brushy Basin Member is Cretaceous in age and not Jurassic. These are not necessarily mutually exclusive, and it may be that a combination of these factors is true for our section.
The long-term sediment accumulation rate calculated from the fission-track dates for the lower Brushy Basin, Salt Wash, and Tidwell Members of the Morrison Formation is about 6.2 m/m.y., whereas the long-term rate for the upper Brushy Basin Member is only about 0.6 m/m.y. Some of this difference may be only apparent, related to the differences in lithology. However, the change from relatively rapid to relatively slow sedimentation and subsidence occurs at about 140 Ma. This was about the time of a slowing of plate collision to the west and near the start of a gap in plutonic activity associated with the change in collision rates. Therefore, some of the change in sediment-accumulation rates in our section may be reflecting a reduction in the supply of ash and other sediment into the Morrison basin as a result of the changing tectonic regime to the west.