New 40Ar- 39Ar and detrital zircon U-Pb ages for the Upper Cretaceous Wahweap and Kaiparowits formations on the Kaiparowits Plateau, Utah: implications for regional correlation, provenance, and biostratigraphy

Abstract

In order to better constrain the age and provenance of the Upper Cretaceous Wahweap and Kaiparowits formations in southern Utah, U-Pb SHRIMP ages were obtained for detrital zircons from three sandstone samples, in addition to the first 40Ar- 39Ar age for the Wahweap Formation, obtained from a devitrified volcanic ash horizon (bentonite). The ash horizon, located ∼40 m above the base of the Wahweap Formation, yields an age of 80.1 ± 0.3 Ma. The new radiometric data improve upon previous biostratigraphic age estimates for the Wahweap Formation and indicate that the formation was deposited between approximately 81 and 76 Ma. The youngest population of detrital zircons from the base of the Wahweap Formation clusters around 83–82 Ma, while the youngest population in the capping sandstone near the top of the formation is between 77–81 Ma, consistent with the 40Ar- 39Ar age. Detrital zircons from the base of the overlying Kaiparowits Formation include a younger population clustering around 77–76 Ma, but are otherwise broadly similar to those in the lower Wahweap. Detrital zircon assemblages suggest the lower Wahweap and Kaiparowits sandstones were primarily deposited by longitudinal stream systems sourced in the Cordilleran magmatic arc in southern California or western Nevada, along with Mesozoic volcanics in southern Arizona. The capping sandstone contains detrital zircons that suggest it was proximately sourced from transverse stream systems that drained eastward out of uplifted Mesozoic quartzose sandstones in the Sevier thrust belt to the west. Revised correlations between the Wahweap Formation and coeval strata and faunas across the Western Interior Basin show that the Wahweap Formation is coeval with Judithian age localities including the type-Judithian Judith River Formation. This suggests that the Aquilan and Judithian North American Land Mammal “ages” are in need of recalibration based on recent acquisition of this and other new radiometric data, as well as new faunal data. Moreover, this study provides critical temporal constraint for important mammalian and dinosaurian faunas of the Wahweap Formation.