Time and Tectonism in the Late Jurassic and Early Cretaceous: Evidence from the Morrison and Cedar Mountain Formations

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The Morrison and Cedar Mountain Formations in the Colorado Plateau are key units in understanding Late Jurassic and Early Cretaceous tectonism in western North America. In addition, these units contain a rich fossil record of Mesozoic dinosaurs from numerous quarry sites in several states. The ages and time- correlation of these formations are, however, still poorly understood. This creates problems in understanding regional tectonics and in determining the evolutionary trends of the dinosaurs. A threefold approach is proposed to resolve these problems: 1) develop better techniques for regional correlation using volcanic ashes (which are quite common in many stratigraphic sections worldwide); 2) obtain new isotopic ages to resolve the problems of time and correlation of the dinosaur-bearing strata; and 3) use the correlated ashes and ages to determine the relationships between sedimentation, subsidence, and tectonics for this period of time in the western United States. Samples will be collected of ash beds for dating from sections both along and across the basin in which these sediments were deposited. Fission track 40/39 Ar laser probe techniques on single grains will be employed to avoid detrital contamination and obtain realistic ages. Attempts will be made to correlate ash layers from one section to another by examining zircon morphologies, inclusions, and uranium distributions within the zircons, as well as by examining the trace element chemistry of the ashes, mineralogy of the ashes, and any other distinguishing feature we can find for each ash layer. Three of the sections to be studied are located near dinosaur quarries so that the ages obtained can be used to resolve long-standing controversies about the age of the fossils and the relationships with other sections in western North America.