REVISED AGE OF THE CLOVERLY FORMATION (CRETACEOUS, WESTERN INTERIOR, USA)

The Lower Cretaceous Cloverly Formation of Wyoming is important for understanding the development of North American terrestrial landscapes and ecosystems. It records vertebrate faunal turnover, dramatic transgression, initial Sevier tectonism, and the spread and diversification of flowering plants. The underlying Morrison Formation is confidently dated at ca. 150 Ma. However, the Morrison–Cloverly contact has been variably interpreted as a series of brief unconformities or a major single unconformity. In contrast, the age of the uppermost Cloverly Formation is generally regarded as late Aptian–early Albian (ca. 112 Ma) However, published ages have large uncertainties that span much of the Early Cretaceous.

New U-Pb radiometric dates were obtained from euhedral zircons derived from fluvial sandstones and bentonitic claystone layers associated with marginal lacustrine sandstones. The zircons were analyzed using both laser ablation-inductively coupled plasma-mass spectrometry and thermal ionization mass spectrometry. Our results suggest that the Pryor Conglomerate was likely deposited before the Aptian, the Little Sheep Mudstone Member through much of the Aptian–Albian, and portions of the Himes Member, Greybull Sandstone, and overlying the Sykes Mountain Formation and Thermopolis Shale in the late Albian. Our reanalysis of previous biostratigraphic correlations shows that foraminiferal, palynological, and molluscan data independently indicate a late Albian–Cenomanian rather than the Aptian–Albian age for the Himes and Greybull Members that was previously inferred. Faulty biostratigraphic correlations biased subsequent fission track and paleomagnetic studies, in which younger ages tended to be discarded as erroneous. We incorporate refined ages and geological reinterpretations into a new depositional model for the Cloverly Formation.