Stratigraphic, taphonomic, and paleoenvironmental analysis of the Upper Cretaceous Kaiparowits Formation, Grand Staircase-Escalante National Monument, southern Utah

The Kaiparowits Formation is an unusually thick package of Upper Cretaceous strata exposed in Grand Staircase-Escalante National Monument of southern Utah. Radiometric dating of four ash beds produces the first absolute ages for the Kaiparowits Formation, demonstrating a late Campanian (Judithian) age of ∼76.1-74.0 Ma. These new age determinations permit high-resolution correlation with other contemporaneous terrestrial floras and faunas in the Western Interior Basin and provide a new chronological basis for addressing questions relating to vertebrate evolution, biodiversity, and paleobiogeography.; Based on a detailed analysis of the sedimentary facies and alluvial architecture in the Kaiparowits Formation, the formation is herein subdivided into three informal units: upper, middle, and lower. Facies analysis also demonstrates that the formation accumulated in a mosaic of fluvial and floodplain settings. Thick paludal deposits, large channels, and poorly developed, hydromorphic paleosols dominate the sedimentary record, and all are suggestive of a relatively wet alluvial system. This interpretation is supported by taphonomic data, which reveals a high abundance and diversity of aquatic vertebrate and invertebrate fossils preserved within the formation. Taphonomic analysis of vertebrate fossil localities and specimens was also performed during this study and demonstrates that preservation was strongly influenced by a humid/subhumid paleoclimate and a relatively wet paleoenvironment. Finally, during the course of this investigation, a new continental trace fossil, Socialites tumulus , was discovered and described. This new ichnotaxon is interpreted to represent the above ground portion of a social insect nest structure. Synthesis of these findings suggests that the Kaiparowits Formation was part of a complex Late Cretaceous ecosystem and that fossil preservation was strongly influenced by high sediment accumulation rate (41 cm/ka), coupled with deposition in a relatively wet terrestrial environment.