LASER ABLATION U-PB ZIRCON GEOCHRONOLOGY OF THE CEDAR MOUNTAIN FORMATION AND THE INTIAL EXPRESSION OF FLEXURAL SUBSIDENCE IN THE CORDILLERA OF THE WESTERN UNITED STATES

The Cedar Mountain Formation, located primarily in Utah, is a package of terrestrial sediments deposited in an eastward migrating foreland basin. The formation is significant because it records the Early Cretaceous initiation, migration, and deepening of this economically important basin, which would eventually culminate in the Late Cretaceous, and because it contains several unique and diverse dinosaurian faunas. Understanding the timing of critical tectonic and evolutionary events in the formation has been hindered by the paucity of radiometric ages; well-preserved ash beds only exist in the uppermost, western parts of the formation. To determine the ages of the balance of the formation, we analyzed euhedral detrital zircons from sandstones, siltstones, and mudstones collected from 15-19 horizons in each of three stratigraphic sections using laser ablation multi-collector inductively coupled plasma mass spectrometry. We analyzed zircons from the top of the Morrison through the Cedar Mountain and Dakota Sandstone formations. Recovered age ranges for the Cedar Mountain Formation are as follows: Dinosaur National Monument, 124-109 Ma; north flank of the San Rafael Swell,120-104 Ma; and south flank of the San Rafael Swell, 120 to 92 Ma. At two localities near Moab, Utah, zircons in basal strata of the formation are ~124 Ma and rest unconformably on ~148 Ma Morrison Formation. Thus, (1) the Morrison-Cedar Mountain formational hiatus lasted 24 Ma, (2) following the post-Morrison hiatus, accommodation began to increase over much of the basin between 120 and 124 Ma, and (3) the Cedar Mountain Formation spans about 32 Ma – a huge amount of time for a terrestrial package with an average thickness of <100 m. Apparently, deposition was either extraordinarily slow and uniform or more modest and marked by numerous unconformities. Our studies support the latter model; with the discovery of substantial filled paleovalleys dating from the Middle Aptian through the Cenomanian and an abundance of well-developed paleosols. The fills are difficult to identify without the aid of radiometric ages and indicate that simple “layer-cake” stratal and faunal correlation in this formation must be used with caution.