Geology of the Intermountain West

The Wasatch monocline, central Utah—a pre-Basin and Range extensional structure

Shelley Judge — 2024-06-25

The Wasatch monocline is a major structure in the transition zone between the Basin and Range and Colorado Plateau physiographic provinces. The timing of formation of the monocline and its tectonic significance has been the subject of debate because it is an anomalous structural style for central Utah. Constraining the age for flexure and outlining the tectonic regime responsible for Wasatch monocline formation are principal objectives of this research. Field mapping near the southern end of the monocline revealed an unconformity between the older, middle Eocene Crazy Hollow Formation, included in monocline folding, and the younger, middle Eocene (Bartonian) formation of Aurora deposited against the monocline. We report an incremental step-heating and direct single-grain laser fusion age of 38.0 ± 0.2 Ma for biotite from an ash-flow tuff within the formation of Aurora, which constrains monocline formation tO no younger than the mid-Eocene. Structural data from the Wasatch monocline, in the context of regional structural and tectonic analysis, indicate the monocline formed in an extensional regime as a forced fold or a rollover fold of the east-facing Sanpete half-graben, formed during pre-Basin and Range extension, also recorded in the Paleogene-Neogene basins of Utah Valley and Salt Lake Valley regions.

Piecing together a prehistoric puzzle—regional inferences of micro- and macroscopic analyses of possibly one of the last hybrid mammoths in mainland Western North America

Kate Morrison — 2024-08-14

We evaluated the depositional age, taxonomy, diagenetic alteration, and osteology of mammoth skeletal remains from southeastern Idaho. In this study, we identified the first record of M. jeffersonii present in Idaho and only the second record of the species in Western North America that lived 13,586 to 13,444 cal BP. The mammoth remains were preserved in an ancient hot spring deposit and have indicators of possible pre-mortem injuries. The diagenetic processes post-mortem suggest that it was not immediately buried and was gnawed on by small and large carnivores. Our evaluation of the mammoth’s tusks, molars, and limb bones suggest that these remains belonged to a young adult male that had been around 29 years old at its time of death. This specimen lived at a time when mammoths were becoming endangered in western North America before their ultimate disappearance from the fossil record.

The first dinosaur postcranial body fossils from the Lower Jurassic Kayenta Formation of Utah

Adam Marsh — 2024-08-17

The vertebrate assemblage of the Lower Jurassic Kayenta Formation is known for its preservation of post-end Triassic mass extinction lineages, including lissamphibians, lepidosaurs, turtles, mammaliamorphs, crocodylomorphs, pterosaurs, and ornithischian, theropod, and sauropodomorph dinosaurs. Most of the body fossils from the formation are known from its ‘silty facies’ in north-central Arizona and southwestern Utah, whereas the sandier ‘typical facies’ of northeastern Arizona preserves few body fossils, and until recently they were completely absent in the typical facies of southeastern Utah. A 2011 team conducting a paleontological survey of Arches National Park discovered the first body fossils from the typical facies of the Kayenta Formation in Utah, here identified as belonging to a single individual of a saurischian dinosaur, likely a theropod. The fossil elements include a partial centrum articular face, a prezygapophysis, part of a caudal vertebra, the distal end of a left radius, part of the distal end of a left femur, a shaft fragment from the left fibula, the distal end of right metatarsal I, and the proximal portion of left metatarsals III and IV. This specimen from Arches National Park underscores the importance of federally protected land in fossil resource management and suggests that the typical facies of the Kayenta Formation may be undersampled and could preserve more vertebrate bones than previously thought.